Understand the Facts about the Epithelial Tissue
1. What is the function of the skin in humans?
The skin is the external covering of the body. In humans its main functions are protection, perception of information from the environment, control of the body temperature and secretion of substances.
The Epithelium - Image Diversity: skin
2. What are the tissues that form the skin in vertebrates?
The skin of vertebrates is made of epidermis, an external layer of epithelial tissue, and dermis, a layer of connective tissue under the epidermis. One can cite also the hypodermis, a layer of adipose tissue under the dermis.
Skin annexes may exist in some phyla and classes, like hair, sweat glands and sebaceous glands.
The Epithelium - Image Diversity: skin histology
3. Besides the skin what are the other coverings of the body?
Besides the skin there are other covering tissues made of epithelium over other tissue layers. They are the tissues that cover the internal surfaces of hollow organs, like the organs of the digestive tube, the airway, the renal tubules, the ureters, the bladder, the urethra and the blood vessels. The glands and the serous membranes are made ofepithelial tissue too.
4. What are some functions of the epithelium?
The epithelial tissues can perform covering, impermeability and protection against the environment, for example, in the skin, resorption, as in the guts and renal tubules, gas exchange, for example, the amphibian skin, thermal regulation, like sweating, secretion of substances, as in the epithelium of glands. In some animalsthe skin also has the important function of camouflage and mimicry .
5. What is the typical feature of the epithelia? How different is it from the connective tissue?
The typical feature of the epithelium is the absence or almost absence of space between cells. The epithelial cells are compactly positioned side-by-side with the help of specialized structures for cell adhesion like desmosomes and interdigitations. This feature relates to the fact that these tissues are generally exposed to an exterior surround and so they need more resistance and impermeability against the entrance of strange material into the body.
The connective tissue presents opposite features due to its filling function. It has much interstitial material (the matrix) and relatively large space between cells.
6. What are the specialized structures that help the adhesion between cells?
The structures responsible for the union of the epithelial cells are called cell junctions. The main cell junctions are interdigitations, desmosomes, zonula adherens (adherens junction), tight junctions (zonula occludens) and gap junctions.
The Epithelium - Image Diversity: cell junctions
7. Is the epithelium vascularized? How do nutrients and oxygen reach the epithelium? Why is this feature an important evolutionary acquisition?
Epithelia are not vascularized (capillaries do not directly reach their cells). The epithelium exchanges substances by diffusion with theconnective tissue situated under it.
Since the epithelia are not vascularized minuscule skin injuries or scratches that happen all the time do not trigger bleeding and do not expose the blood to contamination from external agents. This is an important protective strategy discovered by evolution.
8. How are the epithelial tissues classified?
The epithelial tissues are classified according to the shape of the cells that form it (epithelial cells may be cuboidal, columnar, or squamous) and according to the number of layers in which those cells are placed in the tissue (into simple or stratified).
The main types of epithelial tissues are simple cuboidal, simple columnar, simple squamous, stratified squamous and pseudostratified columnar (resembling more than one layer but actually having only one). There are also stratified cuboidal and stratified columnar epithelia (rare).
The Epithelium - Image Diversity: types of epithelial tissue
9. How different is the simple cuboidal epithelium from the columnar epithelium? Where can these epithelia be found in the human body?
The simple cuboidal epithelium is made of a single layer of cuboidal epithelial cells. The simple columnar epithelium is made of a single layer of prismatic cells.
The simple cuboidal epithelium can be found, for example, in the renal tubules and in the walls of the thyroid follicles. The simple columnar is the epithelium that covers internally the intestines, the stomach and the gallbladder, for example.
10. How different is the simple squamous epithelium from the stratified squamous epithelium? Where can these epithelia be found in the human body?
The simple squamous epithelium is made of a single layer of flat (squamous) cells. The stratified squamous epithelium is made of the same type of flat cells placed in several superimposed layers.
The simple squamous epithelium is found in the pulmonary alveoli. The stratified squamous epithelium can be found in the moist mucosae, like the mucosae of the mouth, esophagus and vagina, and it is the epithelium of the skin.
11. What is the function of keratin in the epidermis?
The epidermis is the outer layer of the skin made of epithelial tissue. In the epidermis there are keratin-secreting cells (keratinocytes). Keratin is an insoluble protein that impregnates the surface ofthe skin providing protection and impermeability. In mammals keratin also forms the hairs.
The keratinized cells of the skin surface form the corneal layer. These cells die and are continuously replaced by others.
12. How different is the fish epidermis from the amphibian epidermis?
The fish epidermis is very thin and contains mucus-secreting cells. The fish skin does not present keratin. The mucus has a protective function and it also helps the sliding of the animal under water. (The fish scales originate from the dermis and not from the epidermis.)
In amphibians there is already a slight keratinization of the skin, probably an additional adaptation to the terrestrial environment. Amphibians have smooth and wet epidermis without scales. These features facilitate their cutaneous respiration.
13. Which are the glands present in the epidermis of mammals, birds and reptiles?
In the epidermis of birds and reptiles there are practically no glands. In mammals there are sweat glands and sebaceous glands.
The Epithelium - Image Diversity: gland histology
14. What are melanocytes?
Melanocytes are epithelial cells of the skin specialized in secretion of melanin. Melanin is a pigment that besides coloring the skin, the iris of the eye and the hair, also works as a filter against the ultraviolet radiation of the sun thus protecting the body against the harmful effects of this radiation (mainly burns and carcinogenic mutations).
Melanocytes are the cells affected in one of the more deadly skin cancers: melanoma.
The Epithelium - Image Diversity: melanocytes
Learn Cartilages, Bones and Muscles
1. Which are the organs that are part of the musculoskeletal system?
The main organs and tissues that are part of the musculoskeletal system in humans are the cartilages, the bones and the muscles.
Musculoskeletal System - Image Diversity: musculoskeletal system
2. What are the functions of the musculoskeletal system?
The musculoskeletal system has the functions of supporting and protecting organs, maintenance of the body spatial conformation, motion of organs, limbs and bodily portions and nutrient storage (glycogen in muscles, calcium and phosphorus in bones).
3. Which type of tissue are the cartilaginous and the osseous tissue?
The cartilaginous and the osseous tissues are considered connective tissues since they are tissues in which the cells are relatively distant from others with a great amount of extracellular matrix in the interstitial space.
4. What are the cells that form the cartilaginous tissue?
The main cells of the cartilages are the chondrocytes, originated from the chondroblasts that secrete the intersticial matrix. There are also chondroclasts, cells with many lisosomes and responsible for the digestion and remodelation of the cartilaginous matrix.
Musculoskeletal System - Image Diversity: cartilaginous tissue
5. What is the constitution of the cartilaginous matrix?
The cartilaginous matrix is made of collagen fibers, mainly collagen type II, and of proteoglycans, proteins associated to glycosaminoglycans, chiefly hyaluronic acid. The proteoglycans provide the typical rigidity of the cartilages.
6. What are some functions of the cartilages in the human body?
Cartilages are responsible for the structural support of the nose and ears. The trachea and the bronchi are also organs with cartilaginous structures that prevent the closing of these tubes. In joints there are cartilages that cover the bones providing a smooth surface to reduce the friction of the joint movement. In the formation of bones the cartilages act as a mold and they are gradually substituted by theosseous tissue.
Musculoskeletal System - Image Diversity: human cartilages
7. What are the three main cell types that form the osseous tissue? What are their functions?
The three main cell types of the osseous tissue are the osteoblasts, the osteocytes and the osteoclasts.
Osteoblasts are known as bone-forming cells since they are the cells that secrete the proteinaceous part of the bone matrix (collagen, glycoproteins and proteoglycans). The bone matrix is the intercellular space where the mineral substances of the bones are deposited.
Osteocytes are differentiated mature osteoblasts formed after these cells are completely surrounded by the bone matrix. Osteocytes have the function of supporting the tissue.
Osteoclasts are the giant multinucleate cells that remodelate the osseous tissue. They are originated from monocytes and they contain many lisosomes. Osteoblasts secrete enzymes that digest the osseous matrix creating canals throughout the tissue.
Musculoskeletal System - Image Diversity: osseous tissue osteoblasts osteocytes osteoclasts
8. What is the bone matrix? What are its main components?
Bone matrix is the content that fills the intercellular space of the osseous tissue. The bone matrix is made of mineral substances (about 5%), mainly phosphorus and calcium salts, and organic substances (95%), mainly collagen, glycoproteins and proteoglycans.
Musculoskeletal System - Image Diversity: bone matrix
9. What are the Haversian canals and the Volkmann’s canals of the bones? Is the osseous tissue vascularized?
The Haversian canals are longitudinal canals present in the osseous tissue within which blood vessels and nerves pass. The osseous tissue distributes itself in a concentric manner around these canals. The Volkmann’s canals are communications between the Harvesian canals.
The osseous tissue is highly vascularized in its interior.
Musculoskeletal System - Image Diversity: bone anatomy
10. What are the functions of the osseous tissue?
The main functions of the osseous tissue are: to provide structural rigidity to the body and to delineate the spatial positioning of the other tissues and organs; to support the body weight; to serve as a site for mineral storage, mainly of calcium and phosphorus; to form protective structures for important organs like the brain, the spinal cord, the heart and the lungs; to work as a lever and support for the muscles, providing movement; to contain the bone marrow where hematopoiesis occurs.
11. What are the flat bones and the long bones?
The main bones of the body may be classified as flat or long bones (there are bones not classified into these categories). Examples of flat bones are the skull, the ribs, the hipbones, the scapulae and the sternum. Examples of long bones are the humerus, the radius, the ulna, the femur, the tibia and the fibula.
Musculoskeletal System - Image Diversity: flat bones long bones
12. What are the types of muscle tissues? What are the morphological features that differentiate those types?
There are three types of muscle tissue: the skeletal striated muscle tissue, the cardiac striated muscle tissue and the smooth muscle tissue.
The striated muscles present under microscopic view transversal stripes and their fibers (cells) are multinucleate (in the skeletal) or may have more than one nucleus (in the cardiac). The smooth muscle does not present transversal stripes and it has spindle-shaped fibers each with only one nucleus.
Musculoskeletal System - Image Diversity: muscle tissue
13. Which is the type of muscle tissue that moves the bones?
The bones are moved by the skeletal striated muscles. These muscles are voluntary (controlled by volition).
Musculoskeletal System - Image Diversity: skeletal striated muscle tissue
14. Which is the type of muscle tissue that contracts and relaxes the heart chambers?
The myocardium of the heart is made of cardiac striated muscle tissue.
Musculoskeletal System - Image Diversity: cardiac striated muscle tissue
15. Which is the type of muscle tissue that performs the peristaltic movements of the intestines?
The smooth muscle tissue is responsible for the peristaltic movements of the intestines. The smooth muscles are not controlled by volition.
Musculoskeletal System - Image Diversity: smooth muscle tissue
16. Which is the type of muscle tissue that helps to push the food down through the esophagus?
The esophageal wall in its superior portion is made of skeletal striated muscle. The inferior portion is made of smooth muscle. In the intermediate portion there are skeletal striated and smooth muscles. All of these muscles are important to push the food down towards the stomach.
17. How is the striped pattern of the striated muscle cells formed?
The functional units of the muscle fibers are the sarcomeres. Within the sarcomeres blocks of actin and myosin molecules are posed in organized manner. The sarcomeres align in sequence forming myofibrils that are longitudinally placed in the cytoplasm of the muscle fibers (cells). The grouping of consecutive blocks of actin and myosin in parallel filaments creates the striped pattern of the striated muscle tissue seen under the microscope.
Musculoskeletal System - Image Diversity: muscle cell
18. What are sarcomeres?
Sarcomeres are the contractile units of the muscle tissue formed of alternating actin blocks (thin filaments) and myosin blocks (thick filaments). Several sarcomeres placed in linear sequence form a myofibril. Therefore one muscle fiber (cell) has many myofibrils made of sacomeres.
The compartments where myofibrils are inserted are delimited by an excitable membrane known as sarcolemma. The sarcolemma is the plasma membrane of the muscle cell.
Musculoskeletal System - Image Diversity: sarcomere
19. What are the main proteins that constitute the sarcomere? What is the function of those molecules in the muscle cells?
In the sarcomere there are organized actin and myosin blocks. Troponin and tropomyosin also appear associated to actin.
The actin molecules when activated by calcium ions liberated in the proximities of the sarcomere are pulled by myosin molecules. This interaction between actin and myosin shortens the myofibrils originating the phenomenon of muscle contraction.
20. What are the positions of actin and myosin molecules in the sarcomere before and during the muscle contraction?
Schematically actin filaments attached perpendicularly to both sarcomere extremities (longitudinal sides) make contact with myosin filaments positioned in the middle of the sarcomere and in parallel to the actin filaments.
Before the contraction the sarcomeres are extended (relaxed) since the contact between actin and myosin filaments is only made by their extremities. During contraction actin filaments slide along the myosin filaments and the sarcomeres shorten.
Musculoskeletal System - Image Diversity: actin-myosin interaction
21. How do calcium ions participate in muscle contraction? Why do both muscle contraction and muscle relaxation spend energy?
In the muscle cells calcium ions are stored within the sarcoplasmic reticulum. When a motor neuron emits stimulus for the muscle contraction neurotransmitters called acetylcholine are released in the neuromuscular junction and the sarcolemma is excited. The excitation is conduced to the sarcoplasmic reticulum that then realeases calcium ions into the sarcomeres.
In the sarcomeres the calcium ions bind to troponin molecules associated to actin activating myosin binding sites of actin. The myosin, then able to bind to actin, pulls this protein and the sarcomere shortens. The summation of simultaneous contraction of sarcomeres and myofibrils constitutes the muscle contraction. During muscle relaxation the calcium ions return back to the sarcoplasmic reticulum.
For myosin to bind to actin, and thus for the contraction to occur, hydrolysis of one ATP molecule is necessary. During relaxation the return of calcium ions to the sarcoplasmic reticulum is an active process that spends ATP too. So both muscle contraction and relaxation are energy-spending processes.
Musculoskeletal System - Image Diversity: muscle contraction
22. What is myoglobin? What is the function of this molecule in the muscle tissue?
Myoglobin is a pigment similar to hemoglobin and present in muscle fibers. Myoglobin has a great affinity for oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. So myoglobin acts as an oxygen reserve for the muscle cell.
23. How does phosphocreatine act in the muscle contraction and relaxation?
Phosphocreatine is the main means of energy storage of the muscle cells.
During relaxed periods ATP molecules made by the aerobic cellular respiration transfer highly energized phosphate groups to creatine forming phosphocreatine. In exercise periods phosphocreatine and ADP resynthesize ATP to dispose energy for the muscle contraction.
24. What happens when the oxygen supply is insufficient to maintain aerobic cellular respiration during muscle exercise?
If oxygen from hemoglobin or myoglobin is not enough for the energy supply of the muscle cell the cell then begins to do lactic fermentation in an attempt to compensate the deficiency.
The lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps.
25. What is the neurotransmitter of the neuromuscular junction? How does the nervous system trigger muscle contraction?
The nervous cells that trigger the muscle contraction are the motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited the depolarizing current flows along the membrane of its axon until reaching the synapse at the neuromuscular junction (the neural impulse passage zone between the axon extremity and the sarcolemma). Near the axonal extremity the depolarization allows the entrance of calcium ions into the axon (note that calcium also has a relevant role here). The calcium ions stimulate the neuron to release acetylcholine in the synapse.
Acetylcholine then binds to special receptors in the outer surface of the sarcolemma, the permeability of this membrane is altered and an action potential is created. The depolarization is then conduced along the sarcolemma to the sarcoplasmic reticulum that thus releases calcium ions for the sarcomere contraction.
Musculoskeletal System - Image Diversity: neuromuscular junction
26. To increase the strength of the muscle work is the muscle contraction intensely increased?
An increase in the strength of the muscle work is not achieved by increase in the intensity of the stimulation of each muscle fiber. The muscle fiber obeys an all-or-nothing rule, i.e., its contraction strength is only one and cannot be increased.
When the body needs to increase the strength of the muscle work a phenomenon known as spatial summation occurs: new muscle fibers are recruited in addition to the fibers already in action. So the strength of the muscle contraction increases only when the number of active muscle cells increases.
27. What is the difference between spatial summation and temporal summation of muscle fibers? What is tetany?
Spatial summation is the recruiting of new muscle fibers to increase the muscle strength. Temporal summation occurs when a muscle fiber is continuously stimulated to contract without being able to conclude relaxation.
The permanence of a muscle fiber under a continuous state of contraction by temporal summation is known as tetany (e.g., the clinical condition of patients contaminated by the toxin of the tetanus bacteria). Tetany ends when all available energy for contraction is spent or when the stimulus ceases.
The Nervous System - Questions and Answers
1. What are the physiological systems known as integrative systems? Why is this designation justified?
The integrative systems are the nervous system and the endocrine system. The designation is justified since both systems control and regulate biological functions and act at distance receiving information from organs and tissues and sending effector commands (nervous impulses or hormones) to organs and tissues thus integrating the body.
2. Which are the structures that are part of the nervous system?
The structures that form the nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
The organs of the CNS are the brain (cerebrum, brainstem and cerebellum) and spinal cord. The PNS is made of nerves and neural ganglia. Besides these organs the meninges (dura-mater, arachnoid and pia-mater) are part ofthe nervous system too since they cover and protect the encephalon and the spinal cord.
Image Diversity: human nervous system CNS PNS
3. Which are the main cells of the nervous system?
The main cells of the nervous system are the neurons. Besides the neurons the nervous system is also constituted of glial cells.
Image Diversity: neurons glial cells
4. What are the functional differences between neurons and glial cells?
Glial cells and neurons are the cells that form the nervous system. Neurons are cells that have the function of receiving and transmitting the neural impulses and glial cells (astrocytes, microgliacytes, ependymal cells and oligodendrocytes) are the cells that support, feed and insulate (electrically) the neurons. The Schwann cells that produce the myelin sheath of theperipheral nervous system can also be considered glial cells.
5. What are the three main parts into which a neuron can be divided? What are their respective functions?
The three mains parts into which a neuron can be didactically divided are: dendrites, cell body and axon.
Dendrites are projections of the plasma membrane that receive the neural impulse from other neurons. The cell body is where the nucleus and the main cellular organelles are located. Axon is the long membrane projection that transmits the neural impulse at distance to other neurons, to muscle cells and to other effector cells.
Image Diversity: neuron structure
6. What is the name of the terminal portion of the axon?
The terminal portion of the axon is called presynaptic membrane. Through this membrane neurotransmitters are released into the synaptic junction.
Image Diversity: synapse
7. What are synapses?
Synapses are the structures that transmit the neural impulse between two neurons.
When the electric impulse arrives the presynaptic membrane of the axon releases neurotransmitters that bind to postsynaptic receptors of the dendrites of the next cell. The activated state of these receptors alters the permeability of the dendritic membrane and the electric depolarization propagates along the neuron plasma membrane to its axon.
8. What is an example of a situation in which the neuron cell body is located in a part of the body and its axonal terminal portion is in another distant part of the body? Why does this happen?
Most of the neurons are situated within the brain and the spinal cord (central nervous system) in places known as neural nuclei. Neural ganglia, or simply ganglia, are structures of the peripheral nervous system located beside the spinal column or near some organs where neuron cell bodies are also located.
Neurons situated at specific points can present distant axonal terminations and they also can receive impulses from axons of distant neurons. The inferior motor neurons situated in thespinal cord are examples since their axons can transmit information to the extremities of the inferior limbs triggering contractions of the foot.
9. According to the function of the transmitted neural impulse which are the types of neurons? How different are the concepts of afference and efference of the neural impulse transmission?
There are three types of neurons: afferent neurons, efferent neurons and interneurons. Afferent neurons are those that only transmit sensory information from the tissues to neural nuclei and ganglia (where they make connection with interneurons or effector neurons). Efferent neurons are those that transmit commands to tasks performed in several parts of the body. Interneurons, also known as association neurons or relay neurons, serve as connection between two other neurons.
Afference is the conduction of sensory impulses and efference is the conduction of effector impulses (impulses that command some body action).
10. What are nerves?
Axons extend throughout the body inside nerves. Nerves are axon-containing structures presenting many axons and covered by connective tissue. The nerves connect neural nuclei and ganglia with the tissues.
Nerves may contain only sensory axons (sensory nerves), only motor axons (motor neurons) or both types of axons (mixed nerves).
Image Diversity: nerves
11. What are ganglia?
Ganglia (singular ganglion), or neural ganglia, are structures located outside the central nervous system (for example, beside the spinal column or near viscera) made of concentration of neuron bodies.
Examples of neural ganglia are the ganglia that concentrate cell bodies of sensory neurons in the dorsal roots of the spinal cord and the ganglia of the myenteric plexus responsible for the peristaltic movements of the digestive tube.
In the central nervous system (CNS) the concentrations of neuron bodies are called nuclei and not ganglia.
Image Diversity: ganglia
12. What is meant by the peripheral nervous system (PNS)?
The peripheral nervous system comprehends the nerves and ganglia of the body.
13. What is the function of the myelin sheath? Do all axons present a myelin sheath?
The function of the myelin sheath is to improve the safety and speed of the neural impulse transmission along the axon. The myelin sheath serves as an electrical insulator preventing the dispersion of the impulse to other adjacentstructures . Since the myelin sheath has gaps called Ranviers’ nodes in its length, the neural impulse “jumps” from one node to another thus increasing the speed of the neural transmission.
Not all neurons have a myelin sheath. There are myelinated axonal fibers and unmyelinated ones.
Image Diversity: myelin sheath
14. What are the cells that produce the myelin sheath? Of which substance is the myelin sheath formed?
In the central nervous system (CNS) the myelin sheath is made by apposition of oligodendrocyte membranes. Each oligodendrocyte can cover portions of axons of several different neurons. In theperipheral nervous system (PNS) the myelin sheath is made by consecutive Schwann cell membranes covering segments of a single axon. The Ranviers’ nodes appear in the intercellular space between these cells.
The myelin sheath is rich in lipids but it also contains proteins.
Image Diversity: oligodendrocytes Schwann cells
15. What are some diseases characterized by progressive loss of the axonal myelin sheath?
Multiple sclerosis is a severe disease caused by progressive destruction of the myelin sheath in the central nervous system. The Guillain-Barré disease is due to destruction of the myelin sheath in the peripheral nervous system caused by autoimmunity (attack by the own immune system). The genetic deficiency in the formation or preservation of the myelin sheath is an X-linked inheritance called adrenoleukodystrophy. The movie “Lorenzo’s Oil” featured a boy with this disease and his father's dramatic search for treatment.
16. What are meninges and cerebrospinal fluid?
Meninges are the membranes that enclose and protect the central nervous system (CNS). Cerebrospinal fluid is the fluid that separates the three layers that form the meninges and it has the functions of nutrient transport, defense and mechanical protection for the CNS.
The cerebrospinal fluid fills and protects cavities of the brain and the spinal cord.
Image Diversity: meninges
17. What is the difference between brain and cerebrum? What are the main parts of these structures?
The concept of brain, or encephalon, comprehends the cerebrum (mostly referred to as the hemispheres, but actually the concept also includes the thalamus and the hypothalamus), the brainstem (midbrain, pons and medulla) and the cerebellum. Brain andspinal cord form the central nervous system (CNS).
18. How is the cerebrum anatomically divided?
The cerebrum is divided into two cerebral hemispheres, the right and the left. Each hemisphere is made of four cerebral lobes: frontal lobe, parietal lobe, temporal lobe and occipital lobe.
Each cerebral lobe contains the gray matter and the white matter. The gray matter is the outer portion and it is made of neuron bodies; the gray matter is also known as the cerebral cortex. The white matter is the inner portion and it is white because it is in the region where axons of the cortical neurons pass.
Image Diversity: brain structure
19. Which is the brain region responsible for the coordination and equilibrium of the body?
In the central nervous system the cerebellum is the main controller of the motor coordination and equilibrium of the body. (Do not confuse this with muscle command, performed by the cerebral hemispheres).
Image Diversity: cerebellum
20. Why is the cerebellum more developed in mammals that jump or fly?
The cerebellum is the main brain structure that coordinates the movement and the equilibrium of the body. For this reason it appears more developed in mammals that jump or fly (like bats). The cerebellum is also very important for the flight of birds.
21. Which is the brain region responsible for the regulation of breathing and blood pressure?
The neural regulation of breathing, blood pressure and other physiological parameters like heartbeat, digestive secretions, peristaltic movements and transpiration is performed by the medulla.
The medulla, together with the pons and the midbrain, is part of the brainstem.
22. Which is the brain region that receives conscious sensory information? Which is the brain region that triggers the voluntary motor activity?
In the brain conscious sensory information is received by the neurons situated in a special region called postcentral gyrus (or sensory gyrus). Gyri are the convolutions of the cerebrum. Each of the two postcentral gyri are located in one of the parietal lobes of the cerebrum.
The voluntary motor activity (voluntary muscle movement) is commanded by neurons situated in the precentral gyrus (or motor gyrus). Each of the two precentral gyri are located in one of the frontal lobes of the cerebrum.
The names post- and pre-central refer to the fact that the motor and sensory gyri are spaced apart in each cerebral hemisphere by the sulcus centralis, a fissure that separates the parietal and frontal lobes.
Image Diversity: sensory gyri motor gyri
23. What is the spinal cord? Of which elements is the spinal cord constituted?
The spinal cord is the dorsal neural cord of vertebrates. It is the part of the central nervous system that continues in the trunk to facilitate the nervous integration of the whole body.
The spinal cord is made of groups of neurons situated in its central portion forming the gray matter and of axon fibers in its exterior portion forming the white matter. Neural bundles connect to both lateral sides of the spinal cord segments to form the dorsal and ventral spinal roots that join to form the spinal nerves. The dorsal spinal roots present a ganglion with neurons that receive sensory information; the ventral spinal roots contain motor fibers. Therefore the dorsal roots are sensory roots and the ventral roots are motor roots.
Image Diversity: spinal cord
24. Which are the brain regions associated with memory?
According to researchers some of the main regions of the nervous system associated with the memory phenomenon are the hippocampus, situated in the interior portion of the temporal lobes, and the frontal lobe cortex, both part of the cerebral hemispheres.
25. How is it structurally explained that the motor activity of the left side of the body is controlled by the right cerebral hemisphere and the motor activity of the right side of the body is controlled by the left cerebral hemisphere?
In the cerebral hemispheres there are neurons that centrally command and control muscle movements. These neurons are called superior motor neurons and they are located in a special gyrus of both frontal lobes known as motor gyrus (or precentral gyrus). The superior motor neurons send axons that transmit impulses to the inferior motor neurons of the spinal cord (for neck, trunk and limb movements) and to the motor nuclei of the cranial nerves (for face, eyes and mouth movements).
The fibers cross to the other side in specific areas of those axon paths. About 2/3 of the fibers that go down the spinal cord cross at the medullar level forming a structure known as pyramidal decussation. The other (1/3) of fibers descend in the same side of their original cerebral hemisphere and cross only within the spinal cord at the level where their associated motor spinal root exit. The fibers that command the inferior motor neurons of the cranial nerves cross to the other side just before the connection with the nuclei of these nerves.
The motor fibers that descend from the superior motor neurons to the inferior motor neurons of the spinal cord form the pyramidal tract. Injuries in this tract, for example, caused by spinal sections or by central or spinal tumors may lead to paraplegia and tetraplegia.
Image Diversity: pyramidal tract
26. What is meant by the arch reflex?
In some situations the movement of the skeletal striated muscles does not depend upon commands of the superior motor neurons, i.e., it is not triggered by volition.
Involuntary movements of those muscles may happen when sensory fibers that make direct or indirect connection with inferior motor neurons are unexpectedly stimulated in situations that suggest danger to the body. This happens, for example, in the patellar reflex, or knee jerk reflex, when a sudden percussion on the knee patella (kneecap) triggers an involuntary contraction of the quadriceps (the extension muscle of the thigh). Another example of the arch reflex occurs when someone steps on a sharp object: one leg retracts and the other, by the arch reflex, distends to maintain the equilibrium of the body.
Image Diversity: arch reflex
27. Which are the types of neurons that participate in the spinal arch reflex? Where are their cell bodies situated?
In the arch reflex first a sensory neuron located in the ganglion of a dorsal spinal root collects the stimulus information from the tissues. This sensory neuron makes direct or indirect (through interneurons) connection with inferior motor neurons of the spinal cord. These motor neurons then command the reflex reaction. So sensory neurons, interneurons and inferior motor neurons participate in the arch reflex.
28. What are the respective constituents of the gray matter and of the white matter of the spinal cord?
The gray matter, or gray substance, of the spinal cord contains predominantly neuron bodies (inferior motor neurons, secondary sensory neurons and interneurons). The white matter is mainly made of axons that connect neurons of the brain with spinal neurons.
29. Is the neural impulse generated by the stimulus that triggers the arch reflex restricted within the neurons of this circuit?
The sensory fiber that first conducts the arch reflex connects with neurons of the arch reflex but it also connects with secondary sensory neurons of the spinal cord that transmit information upwards to other neurons of the brain. This is obvious since the person that received the initial stimulus (e.g., the percussion on his/her kneecap) perceives it (meaning that the brain became conscious of the fact).
30. How is it explained that a person with the spinal cord sectioned at the cervical level is still able to perform the patellar reflex?
The arch reflex depends only on the integrity of the fibers at a single spinal level. In the arch reflex the motor response to the stimulus is automatic and involuntary and does not depend upon the passage of information to the brain. So it happens even if the spinal cord is damaged at other levels.
31. How does poliomyelitis affect the neural transmission in the spinal cord?
The poliovirus parasites and destroys spinal motor neurons causing paralysis of the muscles that depend on these neurons.
32. Concerning volition of the individual how can the reactions of the nervous system be classified?
The efferences (reactions) of the nervous system can be classified into voluntary, when controlled by the will, and involuntary, those not consciously controlled. Examples of reactions triggered by volition are the movements of the limb, tongue and respiratory muscles. Examples of involuntary efferences are those that command the peristaltic movements, the heartbeat and the arterial wall muscles. The skeletal striated muscles are voluntarily contracted; the cardiac striated and the smooth muscles are involuntarily contracted.
33. What are the functional divisions of the nervous system?
Functionally the nervous system can be divided into the somatic nervous system and visceral nervous system.
The somatic nervous system includes the central and peripheral structures that make voluntary control of efferences. Central and peripheral structures that participate in the control of the vegetative (unconscious) functions of the body are included in the concept of visceral nervous system.
The efferent portion of the visceral nervous system is called the autonomic nervous system.
34. What are the two divisions of the autonomic nervous system?
The autonomic nervous system is divided into the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system comprehends the nerves that come out from the ganglia of the neural chains lateral to the spinal column (near the spinal cord) and thus are distant from the tissues they innervate. The central and peripheral neurons associated to those neurons are also part of the sympathetic.
The parasympathetic nervous system is made of nerves and central or peripheral neurons related to the visceral ganglia, neural ganglia situated near the tissues they innervate.
Image Diversity: the sympathetic the parasympathetic
35. What is the antagonism between the sympathetic and the parasympathetic neural actions?
In general the actions of the sympathetic and the parasympathetic are antagonistic, i.e., while one stimulates something the other inhibits and vice versa. The organs, with few exceptions, get efferences from these two systems and the antagonism between them serves to modulate their effects. For example, the parasympathetic stimulates salivation while the sympathetic inhibits it; the parasympathetic constricts pupils while the sympathetic dilates it; the parasympathetic contracts the bronchi while the sympathetic relaxes them; the parasympathetic excites the genital organs while the parasympathetic inhibits the excitation.
36. Using examples of invertebrate nervous systems how can the process of evolutionary cephalization be described?
Considering the example of invertebrates it is observed that evolution makes the increasing of the complexity of the organisms to be accompanied by convergence of nervous cells to special structures for controlling and commanding: the ganglia and the brain. In simple invertebrates, like cnidarians, the nervous cells are not concentrated but they are found dispersed in the body. In platyhelminthes a beginning of cephalization with the anterior ganglion concentrating neurons is already verified. In annelids and arthropods the existence of a cerebral ganglion is evident. In cephalopod molluscs the cephalization is even greater and the brain commands the nervous system.
37. What are some main differences of the vertebrate nervous systems comparing to invertebrates?
In vertebrates the nervous system is well-characterized, having the brain and dorsal neural cord protected by rigid skeletal structures. In most invertebrates the nervous system is predominantly ganglial, with ventral neural cords.
38. What are the protective structures of the central nervous system present in vertebrates?
In vertebrates the brain and the spinal cord are protected by membranes, the meninges, and by osseous structures, respectively the skull and the vertebral column. These protections are fundamental for the integrity of those important organs that command the functioning of the body.
39. What is the nature of the stimulus received and transmitted by the neurons?
Neurons receive and transmit chemical stimuli through neurotransmitters released in the synapses. Along the neuron body however the impulse transmission is electrical. So neurons conduct electric and chemical stimuli.
40. What are the two main ions that participate in the electrical impulse transmission in neurons?
The two main ions that participate in the electrical impulse transmission in neurons are the sodium cation (Na+) and the potassium cation (K+).
41. Which is the normal sign of the electric charge between the two sides of the neuron plasma membrane? What is the potential difference (voltage) generated between these two sides? What is that voltage called?
As in most cells the region just outside the surface of the neuron plasma membrane presents a positive electrical charge in relation to the region just inside that thus is negative.
The normal (at rest) potential difference across the neuron membrane is about –70 mV (millivolts). This voltage is called the resting potential of the neuron.
Image Diversity: neuron resting potential
42. How do the sodium and potassium ions maintain the resting potential of the neuron?
The plasma membrane of the neuron when at rest maintains an electric potential difference between its external and internal surfaces. This voltage is called resting potential. The resting potential about –70 mV indicates that the interior is more negative than the exterior (negative polarization). This condition is maintained by transport of sodium and potassium ions across the plasma membrane.
The membrane is permeable to potassium ions but not to sodium ions. At rest the positive potassium ions exit the cell in favor of the concentration gradient since within the cell the potassium concentration is higher than in the extracellular space. The positive sodium ions cannot however go into the cell. As positive potassium ions exit the cell with not enough compensation of positive ions entering the cell, the intracellular space becomes more negative and the cell stays polarized.
43. How is the depolarization of the neuronal plasma membrane generated? How does the cell return to its original rest?
When the neuron receives a stimulus by the binding of neurotransmitters to specific receptors sodium channels open and the permeability of the plasma membrane in the postsynaptic region is altered. Sodium ions then go into the cell causing lowering (less negative) of the membrane potential. If this reduction of the membrane potential reaches a level called the excitation threshold, or threshold potential, about –50 mV, the action potential is generated, i.e., the depolarization intensifies until reaching its maximum level and the depolarization current is transmitted along the remaining length of the neuronal membrane.
If the excitation threshold is reached voltage-dependent sodium channels in the membrane open allowing more sodium ions to enter the cell in favor of the concentration gradient and an approximate –35 mV level of positive polarization of the membrane is achieved. The voltage-dependent sodium channels then close and more voltage-dependent potassium channels open. Potassium ions then exit the cell in favor of the concentration gradient and the potential difference of the membrane decreases, a process called repolarization.
The action potential triggers the same electrical phenomenon in neighboring regions of the plasma membrane and the impulse is thus transmitted from the dendrites to the terminal region of the axon.
Image Diversity: neuron depolarization
44. What is the excitation threshold of a neuron? How does this threshold relate to the “all-or-nothing” rule of the neural transmission?
The excitation threshold of a neuron is the depolarization level that must be caused by a stimulus to be transmitted as a neural impulse. This value is about –50 mV.
The transmission of the neural impulse along the neuronal membrane obeys an all-or-nothing rule: or it happens with maximum intensity or nothing happens. Always and only when the excitation threshold is reached the depolarization continues and the membrane reaches its maximum possible positive polarization, about +35 mV. If the excitation threshold is not reached nothing happens.
45. How does the depolarization of the neuronal membrane start?
The primary cause of the neuronal depolarization is the binding of neurotransmitters released in the synapse (by the axon of the neuron that sent the signal) to specific receptors in the membrane of the neuron that is receiving the stimulus. The binding of neurotransmitters to those receptors is a reversible phenomenon that alters the membrane permeability of the region since the binding causes sodium channels to open. When positive sodium ions enter the cell in favor of their concentration gradient, the membrane voltage increases, thus lessening the negative polarization. If this depolarization reaches the excitation threshold (about –50 mV) the depolarization continues, the action potential is reached and the impulse is transmitted along the cell membrane.
46. How different are the concepts of action potential, resting potential and excitation threshold concerning neurons?
Action potential is the maximum positive voltage level achieved by the neuron in the process of neuronal activation, around + 35 mV. The action potential triggers the depolarization of the neighboring regions of the plasma membrane and thus the propagation of the impulse along the neuron.
Resting potential is the membrane voltage when the cell is not excited, about –70 mV.
Excitation threshold is the voltage level, about –50 mV, that the initial depolarization must reach for the action potential to be attained.
47. In chemical terms how is the neuronal repolarization achieved?
Repolarization is the return of the membrane potential from the action potential (+35 mV) to the resting potential (-70 mV).
When the membrane reaches its action potential voltage-gated sodium channels close and voltage-gated potassium channels open. So sodium stops entering into the cell and potassium starts to exit. Therefore the repolarization is due to exiting of potassium cations from the cell.
The repolarization causes the potential difference temporarily to increase under –70 mV, below the resting potential, a phenomenon known as hyperpolarization.
Image Diversity: neuron repolarization
48. What is the mechanism by which the neural impulse is transmitted along the axon?
The neural impulse is transmitted along the neuronal membrane through depolarization of consecutive neighboring regions. When a region in the internal surface of the membrane is depolarized it becomes more positive in relation to the neighboring internal region. So positive electrical charges (ions) move towards this more negative region and voltage-gated sodium channels are activated and open. The action potential then linearly propagates along the membrane until near the presynaptic region of the axon.
Image Diversity: axonal impulse transmission
49. What is the structure through which the neural impulse is transmitted from one cell to another? What are its parts?
The structure through which the neural impulse passes from one cell to another is the synapse. The synapse is composed by the presynaptic membrane in the terminal portion of the axon of the transmitter cell, the synaptic cleft (or synaptic space) and the postsynaptic membrane in the dendrite of the receptor cell.
50. How does synaptic transmission between neurons take place?
The propagation of the action potential along the axon reaches the region immediately anterior to the presynaptic membrane causing its permeability to calcium ions to change and these ions to enter the cell. In the presynaptic area of the axon there are many neurotransmitter-repleted vesicles that by means of exocytosis activated by the calcium influx release the neurotransmitters into the synaptic cleft. The neurotransmitters then bind to specific receptors of the postsynaptic membrane. (The binding of neurotransmitters to their receptors is reversible, i.e., the neurotransmitters are not consumed after the process.) With the binding of neurotransmitters to the postsynaptic receptors the permeability of the postsynaptic membrane is altered and the depolarization that will lead to the first action potential of the postsynaptic cell begins.
Image Diversity: synaptic transmission
51. What are some important neurotransmitters?
The following are some neurotransmitters: adrenaline (epinephrine), noradrenaline (norepinephrine), acetylcholine, dopamine, serotonin, histamine, gaba (gamma aminobutyric acid), glycine, aspartate, nitric oxide.
52. Since neurotransmitters are not consumed in the synaptic process, what are the mechanisms to reduce their concentrations in the synaptic cleft after they have been used?
Since the binding of neurotransmitters to the postsynaptic receptors is reversible, after these neurochemicals perform their role they must be eliminated from the synaptic cleft. Neurotransmitters can then bind to specific proteins that carry them back to the axon they came from in a process called neurotransmitter re-uptake. They can also be destroyed by specific enzymes, like acetylcholinesterase, an enzyme that destroys acetylcholine. Or they can simply diffuse out of the synaptic cleft.
Image Diversity: neurotransmitter re-uptake
53. Fluoxetine is an antidepressant drug that presents an action mechanism related to the synaptic transmission. What is that mechanism?
Fluoxetine is a substance that inhibits the re-uptake of serotonin, a neurotransmitter that acts mainly in the central nervous system. By inhibiting the re-uptake of the neurotransmitter the drug increases its availability in the synaptic cleft thus improving the neuronal transmission.
54. What is the neuromuscular synapse?
Neuromuscular synapse is the structure through which the neural impulse passes from the axon of a motor neuron to the muscle cell. This structure is also known as neuromuscular junction, or motor end plate.
As in the nervous synapse, the axonal terminal membrane releases the neurotransmitter acetylcholine in the cleft between the two cells. Acetylcholine binds to specific receptors of the muscle membrane, dependent sodium channels then open and the depolarization of the muscle membrane begins. The impulse is then transmitted to the sarcoplasmic reticulum that releases calcium ions into the sarcomeres of the myofibrils thus triggering contraction.
Image Diversity: neuromuscular synapse
55. How does the nervous system get information about the external environment, the organs and the tissues?
Information about the conditions of the external and internal environments, like temperature, pressure, touch, spatial position, pH, metabolite levels (oxygen, carbon dioxide, etc.), light, sounds, etc., are collected by specific neural structures (each for each type of information) called sensory receptors. Sensory receptors are distributed throughout the tissues according to their specific roles. The receptors get that information and transmit them through their own axons or through dendrites of neurons that connect to them. The information reaches the central nervous system that interprets and uses it to control and regulate the body.
56. What are sensory receptors?
Sensory receptors are structures specialized in the acquiring of information, like temperature, mechanical pressure, pH, chemical environment and luminosity, transmitting them to the central nervous system. Sensory receptors may be specialized cells, e.g., the photoreceptors of the retina, or specialized interstitial structures, for example the vibration receptors of the skin. In this last case they transmit information to dendrites of sensory neurons connected to them. There are also sensory receptors that are specialized terminations of neuronal dendrites (e.g., the olfactory receptors).
Image Diversity: sensory receptors
57. According to the stimuli they collect how are the sensory receptors classified?
The sensory receptors are classified according to the stimuli they get: mechanoreceptors are stimulated by pressure (e.g., touch or sound); chemoreceptors respond to chemical stimuli (olfactory, taste, pH, metabolite concentration, etc.); thermoreceptors are sensitive to temperature changes; photoreceptors are stimulated by light; nocireceptors send pain information; proprioceptors are sensitive to the spatial position of muscles and joints (they generate information for the equilibrium of the body).
The Eyes - Study the Visual System Here
1. What is vision? Why is vision important for life on earth?
Vision is the ability of some living beings to perceive, to distinguish and to interpret luminous stimuli.
Vision is important on earth mainly in the terrestrial and in the superficial aquatic habitats because our planet is intensely exposed to sunlight and thus light and colors become distinguishing factors of objects present in the environment, even at distance. This distinction provided new survival strategies for the organisms, new protection mechanisms against external dangers, new ways to find food and to communicate with other individuals, new types of courting and reproduction behaviors, etc. That is, it created new possibilities of interaction with the surrounds and increased capacity to explore new ecological niches.
2. How does photosensitivity in cnidarians, annelids and worms differ from insects, cephalopods and vertebrates?
In the first mentioned group of animals there are photoreceptor cells organized in ocelli or diffusely dispersed in the body. These animals do not form images.
In the animals of the second group the photoreceptor cells are part of more sophisticated structures, the eyes, able to form images and to send them to the nervous system.
3. What are the structures that compose the human vision apparatus?
The organs of the human visual apparatus are the eyes, the optical nerves and the visual areas of the brain (located in the occipital lobes of both hemispheres).
Image Diversity: human eye eye anatomy
4. What are the main structures of the human eye?
The main structures of the human eye are the cornea, the iris, the pupil, the ciliary muscles, the crystalline lens and the retina (the space between the crystalline lens and the retina within the eyeball is filled with vitreous humor).
5. What is the function of the iris and of the pupil?
The iris works like the diaphragm of a photographic camera since it has muscles that contract or relax varying the pupil diameter. When the luminous intensity heightens the parasympathetic nervous system commands the contraction of the pupil; when there is shortage of light the sympathetic nervous system stimulates the dilation of the pupils. These movements depend upon the muscles of the iris.
6. Which is the part of the human visual system where the receptors that sense light, i.e., the photoreceptor cells, are located? How do those cells work?
The photoreceptor cells form the retina, a lamina that covers the internal posterior region of the eyeball. The photosensitive cells of the retina are divided into two types: the cone cells and the rod cells. These cells have pigments that sense specific light wave ranges (frequencies) and trigger action potentials conducted by the optical nerves to the visual area of the brain.
Image Diversity: retina histology cone cells rod cells
7. Since the visual images are projected in an inverted manner on the retina why don't we see things upside down?
Since the crystalline lens is a convex spherical lens it forms inverted images on the retina (every converging lens forms inverted images). The inverted information follows through the optical nerves until the occipital cerebral cortex that contains the visual area of the brain. In the brain the interpretation of the image takes place and the inverted information is reverted.
Image Diversity: vision pathway
8. What type of structure is the crystalline lens? What is its function?
The crystalline is a converging spherical lens. This natural lens has the function to project images of objects onto the retina.
9. What is visual accommodation?
Visual accommodation is the phenomenon of varying the curvature of the crystalline lens to make possible the variation of its refractivity to adjust the images of objects exactly onto the retina. The visualaccommodation is accomplished by the action of the ciliary muscles.
The nitid vision depends on the visual accommodation since, if the images are not projected onto the retina but in front or behind it, they will appear blurred. The closer an object is more the ciliary muscles must compress the crystalline lens (increasing its curvature); the more distant an object is more the ciliary muscles must relax.
10. What are the near point and the far point of the vision?
The near point is the closest distance between an object and the eye that makes possible the formed image to be focused, i.e., it is the point in which the ciliary muscles are in their maximum contraction. Thefar point is the most distant point from the eye in which an object can be placed and its image is still focused, i.e., it is the situation of maximum relaxation of the ciliary muscles. The zone between the near point and thefar point is called the accommodation zone.
11. How can the visual deficiencies known as myopia and hypermetropia be optically explained?
Myopia is the visual condition in which the images are formed before (in front of) the retina. Hypermetropia is the visual condition in which the point of image formation is beyond (behind) the retina. Actually myopia is due to an increase in the distance between the retina and the crystalline lens, mainly caused by a slight flattening of the eyeball. Inhypermetropia the retina is too close to the crystalline lens due to slight shortening of the eyeball.
In myopia the near point and the far point of vision come closer (the refractivity of the crystalline lens that corresponds to the maximum distension capacity of the ciliary muscles is not enough to provide visualaccommodation). In hypermetropia the ciliary muscles are not able to contract more to compensate the inadequate position of the retina, i.e., the near point becomes more distant.
Image Diversity: myopia hypermetropia
12. What are presbyopia and astigmatism?
Presbyopia is the visual impairment in which there is loss of the cililary muscle strength thus reducing the capability of the crystalline lens to adjust images of near objects onto the retina. Inpresbyopia the near point of vision becomes more distant. The disease generally occurs in old people.
Astigmatism is caused by irregular shape of the refractive structures, mainly the cornea. In astigmatism a single object-point may produce more than one image onto the retina and so the vision becomes distorted.
Image Diversity: presbyopia
Learn About Your Hearing System
1. What are the structures that participate in the human auditory sensitivity?
The structures of the human auditory sensitivity are the ears (external, middle and internal), the vestibulocochlear nerves and the auditory areas of the brain (located in the temporal lobes of both hemispheres).
2. What are the main parts of the human ear?
The human ear is divided into three mains parts: the external ear, the middle ear and the internal ear.
Hearing Review - Image Diversity: human ear
3. What are the structures that form the external ear? What is its function?
The internal ear comprises the pinna, or auricle, and the auditory canal. Its function is to conduct the sound waves to the tympanum.
4. What are the elements that form the middle ear? What are the names of the three middle ear ossicles that participate in the phonosensitivity?
The middle ear is formed by the tympanum, the ossicular chain and the oval window. The functional ossicles of the middle ear are the hammer (malleus), the incus and the stapes.
Hearing Review - Image Diversity: middle ear ossicles
5. What is the tympanum? In which part of the ear is it located and what is its function?
The tympanum (or ear drum) is a membrane located in the middle ear just after the auditory canal and so it separates the middle ear from the external ear. The function of the tympanum is to vibrate with the same frequency of the sound waves that reach it.
6. How is the sound vibration captured by the tympanum transmitted through the ossicular chain of the middle ear?
The acoustic transmission from the external to the middle ear (and to the internal ear too) is entirely mechanical. The vibration of the tympanic membrane triggers the vibration of the hammer that then causes the incus to vibrate. The incus then causes the stapes to vibrate.
7. What are the elements that constitute the internal ear? What are the functions of those structures?
In the internal ear there are the cochlea and the semicircular canals. The fluid that fills the cochlea receives vibration from the ossicular chain of themiddle ear and transmits the pressure to the semicircular canals. Within the semicircular canals the pressure variation of the filling fluid moves cilia of the hair cells of these structures. The hair cells then generate action potentials that are transmitted to the brain through the auditory nerves.
Hearing Review - Image Diversity: internal ear auditory pathway
8. Why is there a sense of pressure change inside the ear when someone goes down a mountain?
The pressure inside the middle ear is maintained equal to the external ear (so to the exterior too) due to a communicating duct between the middle ear and the pharynx called the auditory tube, or Eustachian tube. When someone goes down a mountain the air pressure upon the middle ear increases and it is necessary to do some exercises like fake swallowing to force the opening of the pharyngeal orifice of the auditory tube to equalize the pressure again.
9. What is the vestibular system? How does it operate?
The vestibular system is the part of the ear that participates in the control and regulation of the equilibrium of the body (balance).
The semicircular canals of the inner (internal) ear are perpendicularly placed and detect changes in the gravitational position of the head (this is another sensorial function of the inner ear, besides auditory perception). When the head rotates the pressure of the fluid within the canals upon the cilia of specific receptor cells varies and these cells generate action potentials transmitted by the vestibulocochlear nerve. The neural impulse is then interpreted by the brain as information about the gravitational position of the head.
Hearing Review - Image Diversity: vestibular system
The Endocrine System - Questions and Answers
1. What is the difference between the endocrine gland and the exocrine gland?
Endocrine gland is a gland whose secretions (called hormones) are collected by the blood and reach the tissues through the circulation. The hypophysis (pituitary) and the adrenals are examples of endocrine glands. Exocrine gland is a gland whose secretions are released externally through ducts (into the skin, intestinal lumen, mouth, etc.). The sebaceous glands and the salivary glands are examples of exocrine glands.
The Endocrine System - Image Diversity: exocrine glands
2. What is the constitution of the endocrine system?
The endocrine system is constituted by the endocrine glands and the hormones they secrete.
The Endocrine System - Image Diversity: endocrine glands
3. What is the histological nature of the glands? How are they formed?
The glands are epithelial tissues. They are made of epithelium that during the embryonic development invaginated into other tissues.
In the exocrine glands the invagination has preserved secretion ducts. In the endocrine glands the invagination is complete and there are no secretion ducts.
The Endocrine System - Image Diversity: gland formation
4. Why is the endocrine system considered one of the integrative systems of the body? What is the other physiological system that also has this function?
The endocrine system is said to have integrative character since the hormones produced by the endocrine glands are substances that act at a distance and many of them act in different organs of the body. So the endocrine glands receive information from some regions of the body and they can produce effects in other regions providing functional integration for the body.
Besides the endocrine system, the other physiological system that also has integrative function is the nervous system. The nervous system integrates the body through a network of nerves connected to central and peripheral neurons.The endocrine system integrates the body through hormones that travel through the circulation and are produced by the endocrine glands.
5. What are hormones?
Hormones are substances secreted by the endocrine glands and collected by the circulation that act to produce effects upon specific organs and tissues.
Hormones are effectors of the endocrine system.
6. What are target organs of the hormones?
Target organs, target tissues and target cells are those specific organs, tissues and cells upon which each hormone acts and produces its effects. Hormones selectively act upon their targets due to specific receptor proteins present in these targets.
7. How does the circulatory system participate in the functioning of the endocrine system?
The circulatory system is fundamental for the functioning of the endocrine system. The blood collects the hormones made by the endocrine glands and through the circulation these hormones reach their targets. Without the circulatory system the 'action at distance' characteristic ofthe endocrine system would not be possible.
8. Are hormones only proteins?
Some hormones are proteins, like insulin, glucagon and ADH, others are derived from proteins (modified amino acids), like adrenaline and noradrenaline, other are steroids, like the corticosteroids and estrogen.
9. What are the main endocrine glands of the human body?
The main endocrine glands of the human body are the pineal gland (or pineal body), the hypophysis (or pituitary), the thyroid, the parathyroids, the endocrine part of the pancreas, the adrenals and the gonads (testicles or ovaries).
Other organs like the kidneys, the heart and the placenta also have endocrine functions.
10. What is the pineal gland?
The pineal gland, also known as pineal body or epiphysis, is situated centrally in the head. It secretes the hormone melatonin, a hormone produced at night and related to the regulation of the circadian rhythm (or circadian cycle, the wakefulness-sleep cycle). Melatonin possibly regulates many body functions related to the night-day alternation.
The Endocrine System - Image Diversity: pineal gland
11. What is the osseous cavity where the pituitary gland is located?
The pituitary gland, or hypophysis, is located in the sella turcica of the sphenoid bone (one of the bones in the base of the skull). So the gland is situated within the head.
The Endocrine System - Image Diversity: hypophysis
12. What are the main divisions of the hypophysis? What are their functions?
The hypophysis is divided into two portions: the adenohypophysis, or anterior hypophysis, and the neurohypophysis, or posterior hypophysis.
In the adenohypophysis two hormones that act directly, the growth hormone (GH) and the prolactin, and four tropic hormones, i.e., hormones that regulate otherendocrine glands , the adrenocorticotropic hormone (ACTH), the thyroid-stimulating hormone (TSH), the luteinizing hormone (LH) and the follicle-stimulating hormone (FSH) are produced.
The neurohypohysis stores and releases two hormones produced in the hypothalamus, oxytocin and the antidiuretic hormone (ADH, or vasopressin).
13. What is the relation between the hypothalamus and the hypophysis?
The hypothalamus is a part of the brain situated just above the hypophysis. The hypothalamus gets peripheral and central neural impulses that trigger response of its neurosecretory cells. The axons of these cells go down to the adenohypophysis to regulate the hipophyseal secretions by means of negative feedback. When the plasma levels of adenohypophyseal hormones are too high the hypothalamus detects this information and commands the interruption of the production of the hormone. When the blood level of an adenohypophyseal hormone is low the hypothalamus stimulates the secretion of the hormone.
The hypothalamic cells produce the hormones released by the neurohypophysis. These hormones are transported by their axons to the hypophysis and then released in the circulation.
The Endocrine System - Image Diversity: hypothalamus
14. What are the hormones secreted by the adenohypophysis? What are their respective functions?
The adenohypophisys secretes GH (growth hormone), prolactin, ACTH (adrenocorticotropic hormone), TSH (thyroid-stimulating hormone), FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
GH, also known as somatotropic hormone (STH), acts upon bones, cartilages and muscles promoting the growth of these tissues. Prolactin is the hormone that in women stimulates the production and secretion of milk by the mammary glands. The ACTH is the hormone that stimulates the cortical portion of the adrenal gland to produce and secrete the cortical hormones (glucocorticoids). The TSH is the hormone that stimulates the activity of the thyroid gland increasing the production and secretion of its hormones T3 and T4. The FSH is a gonadotropic hormone, i.e., it stimulates the gonads and in women it acts upon the ovaries inducing the growth of follicles, in men it stimulates spermatogenesis. The LH is also agonadotropic hormone that acts upon the ovaries of women to stimulate ovulation and the formation of the corpus luteum (that secretes estrogen) and in men upon the testicles to stimulate the production of testosterone.
15. What is the relation between the thyroid and the hypophysis?
The hypophysis secretes TSH, thyroid-stimulating hormone. This hormone hastens the secretion of thyroid hormones (triiodothyronine and thyroxine, or T3 and T4).
When the plasma concentration of thyroid hormones is high this information is detected by the hypothalamus and by the hypophysis and this gland reduces the TSH secretion. When the thyroid hormone levels are low the TSH secretion increases. It is thus a negative feedback.
Injuries of the hypophysis that cause TSH hyposecretion (for example, in case of tissue destruction) or hypersecretion (in case, e,g., of excessive cell proliferation or cancer) can change the functioning of the thyroid gland completely.
16. What are some diseases caused by abnormal GH secretion by the hypophysis?
In childhood deficient GH secretion may lead to delayed growth and in severe cases to nanism (dwarfism). Excessive production of GH in children may cause exaggerated osseous growth and gigantism. In adults GH excess (for example, in hypophiseal cancer or in people that wrongly ingest GH as a nutritional supplement) may lead to acromegaly, excessive and disproportional growth of the bone extremities, like the skull, the maxillaries, the hands and the feet.
17. What are the target tissues and target organs of each adenohypophyseal hormone?
GH: bones, cartilages and muscles. Prolactin: mammary glands. ACTH: the cortical portion of the adrenals. TSH: thyroid gland. FSH and LH: ovaries and testicles.
18. What are the hormones secreted by the neurohypophysis? What are their respective functions?
The neurohypophysis secretes oxytocin and the antidiuretic hormone (ADH).
Oxytocin is secreted in women during delivery to increase the strength and frequency of the uterine contractions and thus to help the baby’s birth. During the lactation period the infant’s sucking action on the mother’s nipples stimulates the production of oxytocin that then increases the secretion of milk by the mammary glands.
Vasopressin, or ADH, participates in the water regulation of the body and thus in the control of the blood pressure since it allows the resorption of free water through the renal tubules. As water goes back to the circulation the blood volume increases.
19. What is the difference between diabetes mellitus and diabetes insipidus? What are the characteristic signs of diabetes insipidus?
Diabetes mellitus is the disease caused by deficient insulin secretion by the pancreas or by impaired capturing of this hormone by the cells. Diabetes insipidus is the disease caused by deficient ADH secretion by the pituitary (hypophysis) or also by impaired sensitivity of the kidneys to this hormone.
In diabetes insipidus the blood lacks ADH and so tubular resorption of water in the kidneys is reduced and a great volume of urine is produced. The patient urinates a lot and many times a day, a sign also accompanied by polydipsia (increased thirst and exaggerated ingestion of water) and sometimes by dehydration.
20. Why does the urinary volume increase when alcoholic beverages are ingested?
Alcohol inhibits the ADH (antidiuretic hormone) secretion by the hypophysis. Low ADH reduces the tubular resorption of water in the kidneys and thus the urinary volume increases.
21. Which are the target organs and target tissues of the neurohypophysis?
The target organs of oxytocin are the uterus and the mammary glands. The target organs of ADH are the kidneys.
22. Where in the body is the thyroid gland located?
The thyroid is located in the anterior cervical region (frontal neck), in front of the trachea and just below the larynx. It is a bilobated mass below the Adam’s apple.
The Endocrine System - Image Diversity: thyroid
23. What are the hormones secreted by the thyroid gland? What are their functions?
The thyroid secretes the hormones thyroxine (T4), triiodothyronine (T3) and calcitonin.
T3 and T4 are iodinated substances derived from the amino acid tyrosine. They act to increase the cellular metabolic rate of the body (cellular respiration, metabolism of proteins and lipids, etc.). Calcitonin inhibits the release of calcium cations by the bones thus controlling the blood level of calcium.
The Endocrine System - Image Diversity: thyroxine molecule
24. Why is the dietary obtainment of iodine so important for thyroid functioning?
The obtainment of iodine from the diet is important for the thyroid because this chemical element is necessary for the synthesis of the thyroid hormones T3 and T4. The iodine supply often comes from the diet.
25. What is a goiter? What is endemic goiter? How is this problem socially solved?
Goiter is the abnormal enlargement of the thyroid gland. The goiter appears as a tumor in the anterior neck and it may be visible or sometimes not visible but palpable. Goiter can occur in hypothyroidism or in hyperthyroidism.
Endemic goiter is the goiter caused by deficient iodine ingestion (deficiency of iodine in the diet). The endemic character of the disease is explained because the iodine content of the diet is often a social or cultural condition affecting many people of some geographical regions. The hypothyroidism caused by deficient iodine ingestion is more frequent in regions far from the sea coast (since sea food is rich in iodine).
Nowadays the problem is often solved by obligatory addition of iodine in table salt. As table salt is a widely used condiment the supply of iodine in diet is almost assured by this method.
The Endocrine System - Image Diversity: goiter
26. What happens to the TSH (thyroid-stimulating hormone) blood level in hypothyroidism? Why is there enlargement of the thyroid in the endemic goiter disease?
When there is low T3 and T4 secretion by the thyroid the TSH secretion by the hypophysis is very stimulated and the TSH blood level increases. The increase in the TSH availability promotes the enlargement of the thyroid gland.
The thyroid enlargement is a reaction of a tissue that tries to compensate the functional deficiency by making the gland increase its size.
27. What are some signs and symptoms found in patients with hyperthyroidism?
The hormones made by the thyroid gland stimulate the basal metabolism of the body. In hyperthyroidism there is abnormally high production and secretion of T3 and T4 so the basal metabolic rate is increased. The signs of this condition may be tachycardia (abnormally high heart rate), weight loss, excessive heat sensation, excessive sweating, anxiety, etc. One of the typical signs of hyperthyroidism is exophthalmos (protrusion of the eyeballs). Generally the patient also presents goiter.
The Endocrine System - Image Diversity: exophthalmos
28. What are some signs and symptoms found in patients with hypothyroidism?
In hypothyroidism the production and secretion of T3 and T4 are impaired. Since these thyroid hormones stimulate the basal metabolism of the body (cellular respiration, fat acid and protein metabolism, etc.) the patient with hypothyroidism may present bradycardia (low heart rate), low respiratory rate, excessive tiredness, depression, cold intolerance and weight gain. Hypothyroidism is normally accompanied by goiter (enlargement of the thyroid in the neck).
29. What is the physiological cause of the syndrome known as cretinism?
Cretinism is caused by chronic deficiency of the thyroid hormones (T3 and T4) during childhood. The chronic hypothyroidism during childhood may cause retardation and low stature due to the low basal metabolic rate in a period of life when growth and development of mind faculties occur.
30. What are the parathyroids? Where are they located and what are the hormones secreted by these glands?
The parathyroids are four small glands embedded two in each posterior face of one thyroid lobe. The parathyroids secrete parathormone, a hormone that together with calcitonin and vitamin D regulates the calcium blood level.
The Endocrine System - Image Diversity: parathyroids
31. What is the relation between secretion of parathormone and the calcium blood level?
The parathormone increases the calcium blood level since it stimulates the resorption (remodelation) of the osseous tissue. When osteoclasts remodel bones calcium is released in the circulation.
Parathormone also acts increasing the calcium absorption in the intestines by vitamin D activation. It acts in the kidneys promoting tubular calcium resorption too.
32. What is a mixed gland? Why is the pancreas considered a mixed gland?
Mixed gland is a gland that produces endocrine and exocrine secretions.
The pancreas is an example of a mixed gland because it secretes hormones in the circulation, like insulin and glucagon, but it also releases an exocrine secretion, the pancreatic juice.
The Endocrine System - Image Diversity: endocrine pancreas
33. What are the pancreatic tissues involved respectively in the exocrine and endocrine secretions? What are their respective hormones and enzymes?
The exocrine secretion of the pancreas is produced in the pancreatic acini, aggregates of secretory cells that surround small exocrine ducts. The exocrine pancreas secretes digestive enzymes of the pancreatic juice: amylase, lipase, trypsin, chymotrypsin, carboxypeptidase, ribonuclease, deoxyribonuclease, elastase and gelatinase.
The endocrine secretion of the pancreas is produced and secreted by small groups of cells dispersed throughout the organ called islets of Langerhans. The pancreatic islets make insulin, glucagon and somatostatin.
The Endocrine System - Image Diversity: islets of Langerhans
34. What is the importance of the glucose blood level for human health?
The glucose blood level (glycemia) must be kept normal. If it is abnormally low there is not enough glucose to supply the energetic metabolism of the cells. If it is abnormally and chronically high it causes severe harm to the peripheral nerves, the skin, the retina, the kidneys and other important organs and it may predispose to cardiovascular diseases (acute myocardial infarction, strokes, thrombosis, etc). If it is acutely too high medical emergencies like diabetic ketoacidosis and the hyperglycemic hyperosmolar state may occur.
35. What are the functions of insulin and glucagon for the blood glucose control?
Glucagon increases glycemia and insulin reduces it. They are antagonistic pancreatic hormones. Glucagon acts stimulating glycogenolysis and thus forming glucose from glycogen breaking. Insulin is the hormone responsible for the entrance of glucose from the blood into the cells.
When glycemia is low, for example, during fasting, glucagon is secreted and insulin is inhibited. When glycemia is high, as after meals, there are inhibition of glucagon and more secretion of insulin.
The Endocrine System - Image Diversity: insulin molecule glucose uptake
36. What are the target organs upon which insulin and glucagon act?
Glucagon mainly acts upon the liver. Insulin acts in general upon all cells. Both also act upon the adipose tissue respectively stimulating (glucagon) and inhibiting (insulin) the use of fatty acids in the energetic metabolism (an alternate path of the energetic metabolism is activated when there is shortage of glucose).
37. What are the effects of somatostatin for the pancreatic hormonal secretion?
Somatostatin inhibits both insulin and glucagon secretions.
38. What is diabetes mellitus?
Diabetes mellitus is the disease caused by deficient production or action of insulin and the consequent low glucose uptake by the cells and high blood glucose level.
39. What are the three main signs of diabetes?
The three main signs of diabetes mellitus are known as the diabetic triad: polyuria, polydipsia and polyphagia.
Polyuria is the excessive elimination of urine; in diabetes it is caused by reduced water resorption in the renal tubules due to increased osmolarity of the glomerular filtrate (caused by excessive glucose). Polydipsia is the exaggerated ingestion of water; the thirst is due to the excessive water loss in the urine. Polyphagia is the exaggerated ingestion of food caused by deficiency in energy generation by glucose-lacking cells.
40. Why do diabetic patients often undergo dietary sugar restriction? What are the main complications of diabetes mellitus?
Diabetic patients are often advised to ingest less carbohydrates since these substances are degraded into glucose and this molecule is absorbed in the intestines. The dietary sugar restriction goal is to control glycemia to maintain it at normal levels.
The main complications of diabetes are tissue injuries that occur in vaious organs caused by the chronic increased blood osmolarity: in the peripheral nerves (diabetic neuropathy), resulting in sensitivity loss, increased wounds (the person does not feel that the tissue is being wounded and the wound expands) and muscle fatigue; in the kidneys (diabetic nephropathy), causing glomerular lesions that may lead to renal failure; in the retina (diabetic retinopathy), leading to vision impairment and blindness; in the skin, as a consequence of the neuropathy. Diabetes mellitus also is one of the major risk factors for cardiovascular diseases like embolism, myocardial infarction and stroke.
41. What is the difference between type I diabetes mellitus and type II diabetes mellitus?
Type I diabetes, also known as juvenile diabetes, or insulin-dependent diabetes (this name is not adequate as type II diabetes may become insulin-dependent), is the impaired production of insulin by the pancreas believed to be caused by destruction of cells of the islets of Langerhans by autoantibodies (autoimmunity).
Type II diabetes occurs in the adult individual and it is often diagnosed in people of more advanced age. In type II diabetes there is normal or low secretion of insulin by the pancreas but the main cause of the high glycemia is the peripheral resistance of the cells to the action of the hormone.
42. In ancient Greece the father of Medicine, Hypocrates, described a method of diagnosing diabetes mellitus by tasting the patient's urine. What is the physiological explanation for this archaic method?
Under normal conditions the glucose filtered by the renal glomeruli is almost entirely resorbed in the nephron tubules and not excreted in urine. With the elevated glucose blood level the renal tubules cannot resorb all the filtered glucose and some amount of the substance appears in the urine. This amount is enough to provide the sweet taste that helped Hypocrates to diagnose diabetes and to differentiate it from other diseases accompanied by polyuria. Nowadays the method is inconceivable due to the danger of contamination of the tester by disease agents possibly present in the patient's urine.
The Endocrine System - Image Diversity: Hypocrates
43. What are the main treatments of diabetes mellitus?
The general goal of the diabetes treatment is to maintain normal glycemic levels.
Type I diabetes is treated with parenteral administration of insulin. Insulin must be administered intravenously or intramuscularly because as a protein it would be digested if ingested orally. In type II diabetes treatment is done with oral drugs that regulate the glucose metabolism or in more severe cases with parenteral administration of insulin. The moderation of carbohydrate ingestion is an important aid to diabetes treatment.
The diabetes treatment with the use of hypoglycemic agents, like insulin or oral medicines, must be carefully and medically supervised since if wrongly used these drugs may abruptly decrease the glucose blood level, cause hypoglycemia and even death.
Many other forms of diabetes treatment are under research worldwide.
44. How can bacteria produce human insulin on an industrial scale? What are the other forms of insulin made available by the pharmaceutical industry?
Bacteria do not naturally synthesize insulin. It is possible however to implant human genetic material containing the insulin gene into the bacterial DNA. The mutant bacteria then multiply and produce human insulin. The insulin is isolated and purified for later commercialization. This biotechnology is known as the recombinant DNA technology.
Besides human insulin the pharmaceutical industry also produces insulin to be used by humans made from the pancreas of pigs and cows.
45. Where are the adrenal glands located? How many are they and what are their portions?
Each adrenal gland is located on the top of each kidney (forming a hat-like structure for the kidneys), so there are two glands. The adrenal parenchymal structure is divided into two portions: the most peripheral is the cortical portion, or adrenal cortex, and the central is the medullary portion, or adrenal medulla.
The Endocrine System - Image Diversity: adrenals
46. What are the hormones secreted by the adrenal medulla? What are their respective functions?
The medullary portion of the adrenals secretes hormones of the catecholamine group: adrenaline (also known as epinephrine) and noradrenaline (also known as norepinephrine). Besides their hormonal function, adrenaline and noradrenaline act as neurotransmitters too. The neurons that use them as neurotransmitters are called adrenergic neurons.
Adrenaline increases the glycogen breaking into glucose (glycogenolysis) thus increasing glycemia and the basal metabolic rate of the body. Adrenaline and noradrenaline are released during situations of danger (fightfight or flight response) and they intensify the strength and rate of the heartbeat and selectively modulate the blood irrigation in some tissues by selective vasodilation and selective vasoconstriction. By vasodilation they increase the blood supply to the brain, the muscles and the heart and by vasoconstriction they reduce the blood supply to the kidneys, the skin and the gastrointestinal tract.
Substances like adrenaline and noradrenaline that promote vasodilation or vasoconstriction are called vasoactive substances.
47. What are the hormones secreted by the adrenal cortex? What are their respective functions?
The cortical portion of the adrenals secretes hormones of the corticoid (or corticosteroid) group, derived from cholesterol: glucocorticoid, mineralocorticoids and cortical sex hormones.
The glucocorticoids are cortisol and cortisone. The glucocorticoids stimulate the formation of glucose from the degradation of proteins of the muscle tissue (gluconeogenesis) and so they help to increase glycemia. These hormones play an important immunosuppressant role, i.e., they reduce the action of the immune system and for this reason they are used as medicine to treat inflammatory and autoimmune diseases and rejection of transplanted organs.
The mineralocorticoids aldosterone and deoxycorticosterone regulate the sodium and potassium blood concentration and thus they control the water level of the extracellular space. Aldosterone increases the sodium resorption and thus the water resorption in the renal tubules and it also stimulates the renal excretion of potassium and hydrogen.
The adrenal cortical sex hormones are androgens, male sex hormones present in men and women. In men their main site of production is the testicle and they promote the appearance of secondary male sex characteristics, like body hair and beard, deep voice, the male pattern of fat distribution and maturation of the genitalia. If abnormally high in women they cause inhibited maturation of the female genitalia and disturbances of the menstrual cycle.
The Endocrine System - Image Diversity: glucocorticoid molecules
48. Why are glucorticoids used in transplant patients?
Patients with transplanted organs are prone to host versus graft rejection since their own immune system tends to attack the grafted organ because of recognition of the grafted tissue as foreign matter. In the prevention and treatment of this common problem patients are given glucorticoids or other immunosupressants. Glucocorticoids have an immunosuppressant action and so they reduce the aggression of the immune system against the graft.
The immune action however is also very important for the individual. The immune system defends the body against invasion and infection by pathogenic agents (virus, bacteria, toxins) besides being fundamental for the elimination of modified cells that may proliferate and cause cancer. Patients receiving immunosuppressants like glucocorticoids are thus under increased risk of infectious and neoplastic diseases.
49. What are the hormones produced by the testicles and the ovaries?
The testicles make androgenic hormones, the main of them being testosterone. The ovaries produce estrogen and progesterone.
The Endocrine System - Image Diversity: testicles and ovaries
50. What is the endocrine function of the placenta?
The placenta is not a permanent gland of the endocrine system but it also has endocrinal function. The placenta produces estrogen and progesterone. It also secretes human chorionic gonadotropin (HCG, that acts similarly to the hypophyseal LH), human placental lactogen, similar to prolactin and stimulant of the mammary glands, and a series of hormonal peptides similar to the hormones of the hypothalamus-hypophysis axis.
Understand the Immune Defense System
1. What is the function of the immune system?
The immune system performs specific defense against agents, the antigens, that are foreign or harmful to the body.
Exogenous antigens are often in contact with the skin or entering the airway, the digestive tube and the genital orifices and mucosae. They can also penetrate the circulation directly through wounds.
2. What are the two groups of defense mechanisms of the body against foreign or harmful agents? What is the difference between them?
The body has many defense mechanisms against foreign pathogenic agents. These mechanisms are divided into two groups: the specific mechanisms and the unspecific mechanisms. The specific mechanisms are part ofthe immune system and comprehend the humoral immune response and the cellular immune response that respectively produce antibodies and defense cells against specific antigens. The unspecific mechanisms fight in a general manner any type of antigen (they do not have specificity) and in them a series of defense means are included, like the skin barrier against foreign agents, the mucous and ciliated epithelium of the airway, inflammation (the inflammatory response) and the action of unspecific proteins and defense cells (e.g., interferons and macrophages).
3. What is inflammation?
Inflammation is the initial response of the unspecific defense system versus aggressions against the body (the aggressions may be caused by infectious parasites, chemical contamination, trauma, physical agents like heat and fire, autoimmunity, etc.). During inflammation a series of unspecific leukocytes present in the circulation are attracted to the injury site in an attempt to destroy harmful agents and to isolate the affected region of the tissue.
4. How does the inflammation mechanism work?
When some tissue injury occurs histamine and other vasoactive substances (called mediators of inflammation) are released, they cause vasodilation and the blood flow to the affected site increases. Granulocyte leukocytes present in the blood are attracted to the site of the injury by substances known as chemotactic factors also released by the injured tissue and by the active granulocytes in the area. The granulocytes exit the capillaries by diapedesis, i.e., using pseudopods. Macrophages present in the region are activated too. These cells flood the extracellular space of the affected area trying to kill or eliminate harmful agents, to prevent tissue necrosis and to isolate the damaged tissue.
The Immune System - Image Diversity: inflammation mechanism
5. What is pus?
Pus is a residual of the inflammatory reaction. It contains a mixture of fragments of dead leukocytes, infectious agents (generally bacteria) and tissues.
6. What is the association between inflammation and fever?
In the tissue region where inflammation occurs bacterial toxins, cytokines, prostaglandins, interleukins and endothelins are released. These substances gain the circulation and reach the central nervous system which then commands the increase of the body temperature.
7. Which type of defense cell do bacteria attract and cause to multiply during the inflammation process? What is the name given to the waste material produced by the inflammation triggered by bacterial infection?
The main leukocytes that generally multiply and participate in the inflammation reaction against bacterial infections are the neutrophils. In this type of inflammation the blood level of these cells are increased, a clinical condition known as neutrophilia.
In the bacterial inflammation fragments of dead bacteria, dead neutrophils and tissues form the pus.
8. Of which type of defense cell do worm infections stimulate the multiplication?
The main leukocytes that generally multiply and participate in the defense against worm infections are the eosinophils. In this type of inflammation the blood level of these cells are increased, a clinical condition known as eosinophilia.
Eosinophils are also increased in allergic conditions.
9. Of which type of defense cell do viral infections stimulate the multiplication?
The main leukocytes that generally multiply and participate in the defense against viral infections are the lymphocytes. In this type of inflammation the blood level of these cells are increased, a clinical condition known as lymphocytosis.
The Immune System - Image Diversity: lymphocytes
10. What is the defense mechanism that begins to work when inflammation fails to stop an infection?
If the inflammatory attack is not enough to halt the infectious process the body still relies on a specific defense, the immune response proper (humoral and cellular) performed by the lymphocytes.
11. What is the difference between humoral specific immune response and cellular specific immune response?
Humoral specific immune response is the defense system by means of antibodies, defense proteins secreted by lymphocytes that attack foreign agents with high specificity. Cellular specificimmune response is the defense system by means of specific lymphocytes (cells) that directly attack other foreign cells and agents.
12. What is an antigen?
Antigen is any substance, particle or infectious agent recognized as foreign to the body. The contact of the antigen with the body promotes a defense reaction against the antigen (unspecific, specific or both).
13. What are the cells responsible for the production of antibodies?
The cells that produce antibodies, i.e., the cells of the humoral immune system, are the B lymphocytes (B cells).
The Immune System - Image Diversity: B cells
14. What are immunoglobulins?
Immunoglobulin is the alternate name given to antibody. Immunoglobulins are complex proteins containing an invariable portion and a variable portion and made of four polypeptide chains. The variable portion of each immunoglobulin is responsible for the high specificity of the antigen-antibody bond.
The Immune System - Image Diversity: immunoglobulin molecule
15. How do antibodies work to neutralize antigens?
The antibodies, or immunoglobulins, act to facilitate the destruction of antigens: they attract phagocytic leukocytes, they trigger the attack of specific defense molecules (activation of the complement system) and they directly neutralize the toxicity of some antigens.
16. How can an organism that once underwent contact with an antigen be immunized against future infections by the same agent?
This phenomenon is called immune memory. When an antigen makes contact for the first time with cells of the humoral immune system, B lymphocytes that are producers of specific immunoglobulins against that antigen multiply and in days synthesize their antibodies. This is called primary response. Some of these specific B lymphocytes remain in the circulation for a long time, sometimes during the entire life of the individual, and they become the memory cells ofthe immune system. When the body is exposed in the future to the same antigen the production of antibodies will be faster and more intense since the immune system is already prepared to react against that antigen. This is called the secondary response.
17. How can the immune memory lead to the efficacy of vaccines and also produce allergies?
Vaccines are controlled inoculations of fragments of infectious agents or of inactive infectious agents to induce the primary immune response, the formation of specific memory B lymphocytes against the antigen. Therefore the organism produces immunoglobulins and becomes prepared to destroy antigens when exposed to new infections by those agents.
In allergies the humoral immune system is sensitized (makes antibodies and specific memory B lymphocytes) against some common environmental substances wrongly recognized as antigens. For example, pollen-derived substances, dust particles, compounds present in foods or in medicines, etc. may be recognized as antigens triggering the primary response and creating an immune memory against them that then become causes of allergy. The more the individual is exposed to those substances the more intense is the immune reaction.
The IgE antibodies that cause allergy bind to receptors of leukocytes called mastocytes whose cytoplasm is full of histamine granules. The antibody-mastocyte bond causes these cells to release a great amount of histamine in the circulation, stimulating inflammation and generating the allergic symptoms and signs. For this reason allergy is treated with antihistamines, drugs that block the histaminic reaction. Exacerbated allergic reactions, for example, in hypersensitivity to some medicines like penicillin and sulfas, may cause anaphylactic shock, a severe clinical condition that sometimes leads to death.
18. How different are the actions of antibodies against bacteria and against virus? Why is the cellular immune response activated in case of chronic viral infection?
The antibodies of the humoral immune system act against extracellular agents, like toxins or bacteria, but they are not active in the intracellular space and they cannot fight virus efficiently.
In case of viral infection (and also of cancerous or precancerous cells) the immune attack is made by the cellular immune system, mediated by T and NK (natural killers) lymphocytes that destroy specific cells and virus.
19. How does the cellular immune response take place?
The lymphocytes that participate in the cellular immune response are the T lymphocytes. T lymphocytes differentiate into three main types: cytotoxic T lymphocytes (cytotoxic T cell), helper T lymphocytes (helper cell) and suppressor T lymphocytes. The cytotoxic cells are the effectors of the system, i.e., they directly attack other cells recognized as foreign (for example, fungi cells, cells infected by virus, neoplastic cells, graft cells, etc.). The helper cells and the suppressor T lymphocytes act as regulators of the system releasing substances that respectively stimulate and inhibit the immune action of T and B lymphocytes. After the primary immune response memory T lymphocytes also remain in the circulation to provide faster and more effective reaction in case of future infections.
The Immune System - Image Diversity: T cells
20. What are the antigen-presenting cells of the immune system?
The antigen-presenting cells of the immune system, also known as APC cells, are cells that do phagocytosis and digestion of foreign (to the body) microorganisms and later expose antigens derived from these microorganisms in the outer side of their plasma membrane. These processed antigens are then recognized by lymphocytes that activate the immune response. Several types of cells, like the macrophages, can act as antigen-presenting cells.
The Immune System - Image Diversity: antigen-presenting cells
21. What are passive and active immunization? According to the duration of the protection how do these types of immunization differ?
Active immunization is that in which an antigen penetrates the body triggering the primary immune response and the production of memory lymphocytes and antibodies that provide faster and more effective immune defense in future infections by the same antigen. Passive immunization is that in which immunoglobulins against an antigen are inoculated in the body to provide protection in case the body becomes infected by the antigen.
Active immunization tends to be longer lasting than passive immunization since in the active type as well as antibodies, specific memory lymphocytes remain in the circulation. In the passive immunization the duration of the protection is that of the duration of the antibodies in the circulation.
22. Why is maternal milk important for the immune protection of the baby?
Besides being nutritionally important, maternal milk participates in the defense of the baby against infectious agents. Soon after delivery the mother produces a more fluid milk called colostrum that is rich in immunoglobulins (antibodies). These antibodies are not absorbed by the baby’s circulation but they cover the internal surface of the baby’s bowels thus attacking possible antigens and making more difficult the proliferation of pathogenic bacteria within the organ.
23. How are antivenoms produced? Why are antivenoms an example of passive immunization?
Antivenoms are obtained by the following process: the venom (antigen) is inoculated into other mammals, e.g., in horses; these animals make specific antibodies against the antigen; blood from the animals is collected and purified to get the antibodies; this antibody-containing material is the antivenom. When a human being is infected by the antigen the specific antivenom is given to him/her and the action against the antigen occurs.
Antivenoms may also be administered as a preventive measure and, since it is basically made of specific immunoglobulins against some antigen, the process is an example of passive immunization.
24. What is the difference between homologous and heterologous immunoglobulins?
Homologous immunoglobulin is the human (from the same species) immunoglobulin. In case of inoculation in animals as in veterinary procedures homologous immunoglobulin is that from the blood of animals of the same species of the animal undergoing treatment. Heterologous immunoglobulin is that obtained from animals of different species from the individual into which it will be inoculated.
The homologous immunoglobulin is safer since it is collected from beings of the same species of the individual in which it will be inoculated and thus the risk of the antibodies to be recognized as foreign and to trigger an immune response is lower. Heterologous immunoglobulins are more prone to being destroyed by the own antibodies of the individual.
25. What are natural active immunization and artificial active immunization?
Natural active immunization is that in which a previous natural infection induces the primary immune response, specific memory cells are produced and the individual becomes immune to new infections with the antigen. This is what happens in diseases that affect people only once in life, like mumps and chickenpox.
Artificial active immunization is that in which the primary immune response is caused by the inoculation into an individual of specially prepared antigens. This is the case with vaccines.
26. Why are vaccines made of the own disease agent or of fragments of it?
The goal of vaccines is to artificially induce a specific primary immune response (and the consequent formation of antibodies and memory cells) concerning a given infection or disease in order to immunize the individual against infections by the pathogenic agent in the future.
Since each antibody does not act against a variety of antigens but instead it acts only against its specific antigen, it is necessary for the immune system to make contact in some way with the antigen against which the immunization is wanted. The reconnaissance of specific molecular portions of each antigen causes the immune system to produce the specific variable portion of the immunoglobulins to attack that antigen. Therefore to induce the active immunization it is necessary to inoculate into the body small parts of the infectious agent or the agent entirely (dead or inactivated).
27. What are the types of antigenic agents that may constitute vaccines?
Vaccines can be constituted of dead agents of disease, of inactivated agents of disease, of inactivated toxins or of fragments of the infectious agent.
Examples of some vaccines and their type of antigenic agents are: BCG, inactivated tuberculosis bacilli; antitetanic vaccine, inactivated toxin; antidiphtheric, inactivated toxin; antipolio Salk, dead poliovirus; antipolio Sabin, attenuated (inactivated) poliovirus.
28. Why doesn't a long lasting vaccine against common cold exist yet?
Viruses that present a high mutation rate like the virus that causes the common cold escape easily from the action of vaccines against them. After a primary immune response (natural or artificially induced) against the virus in the next season of infection new mutant resistant strains appear and the protection obtained with the immune response of the last season is lost. (One could say that the high mutation rate is a form of “immunization” found by these viruses.)
29. Why are vaccines used in the prevention but not in the treatment of infections? Why can antivenom serums be used in prevention and treatment?
Vaccines are not used in the treatment of infections because they depend on the primary immune response that takes about a week to occur and is not so intense and effective. Antivenom serums however are inoculated into the circulation and used as an immediate treatment because they are made of a great amount of immunoglobulin (antibodies) which is potent against their respective specific venom.
30. What is the DNA vaccine?
The DNA vaccine, or DNA vaccination, is a vaccination technology based on genetic engineering. In DNA vaccination a recombinant plasmid (vector) containing the gene of a specific antigen that is part of a given pathogenic agent is inserted into cells of the individual to be immunized. These cells then begin to produce the antigen that triggers the primary immune response and theoretically the individual becomes immunized against that antigen.
31. What is the name given to conditions in which the own immune system of the individual is the agent of diseases? What are some examples of these conditions?
Diseases caused by the action of the own immune system of the individual are called autoimmune diseases.
The autoimmune diseases appear when the immune system makes antibodies or defense cells that attack cells, tissues and organs of its own body. The attacked cells or tissues are wrongly recognized as antigens by the immune system. Rheumatoid arthritis, lupus, scleroderma, vitiligo, pemphigus, type I diabetes mellitus, Crohn's disease (chronic inflammation of the gut), myasthenia gravis, Graves disease, Hashimoto's disease, etc., are all examples of autoimmune diseases.
Easily Understand Gametogenesis
1. What are gametes?
Gametes are cells specialized in sexual reproduction. They contain half of the maximum number of chromosomes of the species and unite with another gamete giving birth to a zygote with double of the number of chromosomes of the gametic cells.
In humans gametes are formed by meiosis; the male gametes are the sperm cells and the female gametes are the egg cells.
Gametogenesis - Image Diversity: sperm cells egg cells
2. What is the type of cell division that allows sexual reproduction? What is gametogenesis?
Meiosis is the type of cell division that allows sexual reproduction since it reduces to a half the number of chromosomes of the species making possible the combination of two gametes to form a new individual. (In some beings meiosis creates haploid gametophytes that by means of mitosis generate gametes. Even in this case the function of meiosis is the same: to provide cells with half ofthe number of chromosomes of the species with separation of the homologous.)
Gametogenesis is the name given to the process of gamete production.
Gametogenesis - Image Diversity: meiosis
3. What is the name of the cells capable of making gametes? What is the ploidy of these gamete-forming cells?
The cells that form gametes are the germ cells as opposed to the somatic cells. The ploidy (number of chromosomes) of the germ cells is the same as the somatic cells (only during the formation of gametes meiosis occurs andthe number of chromosomes is reduced to half).
Gametogenesis - Image Diversity: germ cells
4. What are gonads? What are the male and the female gonads in humans?
Gonads are the organs that produce gametes. They contain the germ cells that undergo division and generate gametes. In males the gonads are the testicles. In females the gonads are the ovaries.
Gametogenesis - Image Diversity: gonads
5. Indicating the name and respective ploidy of each involved cell how can the formation of sperm cells from germ cells be described?
The formation of sperm cells, or spermatogenesis, begins with a germ cell called spermatogonium (2n) that suffers mitosis and gives birth to the spermatocyte I (2n). The spermatocyte I undergoes meiosis I and generates two spermatocyte II (n) that then undergo meiosis II and produce four spermatids (n). Each spermatid undergoes a maturation process called spermiogenesis and four sperm cells appear.
Gametogenesis - Image Diversity: spermatogenesis
6. What is the difference between spermatogonium and spermatocyte I?
The male germ cells are the spermatogonia (diploid cells, 2n) situated in the testicles. They mature and by means of mitosis give birth to spermatocytes I (2n) that will undergo meiosis.
7. What is the difference between spermatocyte I and spermatocyte II?
The spermatocyte I (2n) undergoes the first division of meiosis (meiosis I) originating two spermatocyte II (haploid, n).
8. What is the difference between spermatocyte II and spermatid?
The spermatids (n) are the products of the second division of meiosis (meiosis II) in the male gametogenesis. Each spermatocyte II originates two spermatids totaling four spermatids for each spermatocyte I that enter meiosis.
9. What is the difference between spermatids and sperm cells? What is the name of the transformation of spermatids into sperm cells?
Sperm cells (the male gametes) are matured spermatids that have already undergone differentiation (appearance of the flagellum, reduction of the cytoplasm, formation of the acrosome, increase in the number of mitochondria). This differentiation process is called spermiogenesis.
Gametogenesis - Image Diversity: spermiogenesis
10. What is the acrosome of the sperm cell? How is it formed?
The acrosome is a structure that contains a great number of digestive enzymes, it is located in the anterior end of the sperm cell and it is formed by the union of Golgi apparatus vesicles. The function of theacrosome is to release its enzymes when the sperm cell meets the egg cell to break the external covering of the female gamete thus making fecundation possible.
11. What is the function of the flagellum of the sperm cell? How is it formed?
The flagellum of the sperm cell is made by the centrioles that migrate to the region posterior to the nucleus. Its function is to promote locomotion towardsthe egg cell.
12. Why is the cytoplasm of sperm cells very reduced? Why do mitochondria of sperm cells concentrate in the base of the flagellum?
The reduced cytoplasm of sperm cells decreases the cell weight and provides a more hydrodynamic shape for the locomotion in fluids.
The high concentration of mitochondria at the base of the flagellum of the sperm cell is necessary for the energetic supply of the flagellum (for it to beat and move the sperm cell).
13. Concerning events during the periods of life how different is the gametogenesis in women and in men?
The formation of spermatogonia in men takes place during the embryonic period. The formation of sperm cells however is a continuous process that begins in puberty and goes on until old age and sometimes during all the remaining life of the man.
In women all oogonia are formed before birth. The oogonia turn into oocytes I that enter the first division of meiosis (meiosis I). This division however is interrupted at prophase and continues only in puberty. After the beginning of menses an egg cell is released during each period and, if fecundated, it finishes the meiotic division. The oogenesis stops after menopause (cessation of the menstrual activity) and the climacteric period of life begins.
14. Indicating the name and respective ploidy of each involved cell how can the formation of egg cells from germ cells be described?
The formation of egg cells begins with a germ cell called oogonium (2n) that undergoes mitosis and gives birth to the oocyte I (2n). The oocyte I undergoes meiosis I that however is interrupted at prophase. After puberty during each menstrual cycle an oocyte I finishes the meiosis I and generate one oocyte II (n) and the first polar body (n). With fecundation the oocyte II then undergoes meiosis II and produces the mature egg cell (n) and the second polar body (n).
Gametogenesis - Image Diversity: oogenesis
15. What is the first polar body? How different is it from the oocyte II?
In oogenesis the oogonium differentiates into oocyte I (2n) and this cell enters meiosis. After finishing the first meiotic division (meiosis I) the oocyte I forms two cells: the oocyte II (n) and the first polar body. The oocyte II is bigger because it gets almost all the cytoplasm and the cytoplasmic structures of the oocyte I as a strategy for metabolite and nutrient storage. The oocyte II cell goes then to the second meiotic division. The first polar body is very small and almost lacks cytoplasm; it disintegrates or stays attached to the oocyte II.
16. What is the relation between fecundation and the end of the meiotic process during oogenesis?
The oocyte II only completes the second meiotic division (interrupted at metaphase) if fecundation by a male gamete occurs. (One can say therefore that in fact the female gamete is the oocyte II).
17. What is the second polar body?
After termination of the second meiotic division of the oocyte II two cells are generated: the egg cell proper and the second polar body. The second polar body is a very small cell that almost lacks cytoplasm and stays adnexal tothe egg cell. The entire cytoplasmic content of the oocyte II passes to the egg cell.
18. What is the relationship between the menstrual cycle and ovulation?
Ovulation is the releasing of the female gamete from the ovary. Ovulation is a periodical event that occurs during each menstrual cycle. Considering as the first day of the menstrual cycle the day when menses begins, the ovulation occurs around the 14th day when the concentrations of the hormones LH and FSH reach high levels.
19. How does the male gamete penetrate the egg cell? How does the female gamete protect itself from the entrance of more gametes after the entrance of the first sperm cell?
The sperm cell that reaches the egg cell triggers the acrosome reaction, a process in which hydrolytic enzymes of the acrosome are released on the external surface of the zona pellucida (the protective layer that surrounds the egg cell). A portion of this layer is digested by the acrosomal enzymes allowing the sperm cell to reach the plasma membrane of the egg cell carrying out fecundation.
At the moment that the sperm cell makes contact with the egg cell membrane a chemical alteration of this membrane occurs. Enzymes secreted by exocytosis (cortical reaction) make the zona pellucida unable to bind to other sperm cells (zonal reaction) and other male gametes cannot enter the egg cell.
Gametogenesis - Image Diversity: acrosome reaction
20. What are the female pronucleus and the male pronucleus?
The female pronucleus is the proper haploid nucleus of the egg cell. Male pronucleus is the haploid nucleus of the sperm cell that has fecundated the egg cell. After fecundation both pronuclei fuse forming the nucleus of the diploid zygote.
21. Concerning their size and basic morphology how and why do the male and the female gametes differentiate from each other?
The female gametes are big cells full of vitellus (nutritive material). The male gametes are small, mobile and agile flagellate cells.
Those features are related to their respective biological functions. While the female gametes have the basic functions of receiving the sperm cell nucleus and of storing nutrients for the zygote, the male gametes have the function of active movement towards the egg cell.
The Reproductive System
1. What are the organs that are part of the male genital system?
The organs that comprise the male genital system are the testicles, the epididymides, the vas deferens, the seminal vesicles, the ejaculatory duct, the prostate, the bulbourethral glands, the urethra and the penis.
Genital System - Image Diversity: male genital system
2. Concerning reproduction what is the function of the testicles?
The testicles are the male gonads, i.e., the organs where the production of gametes takes place. In human beings the gametes are made by meiosis that occur in the testicles.
3. After passing the epididymides through which structures do sperm cells go until exteriorization?
After leaving the epididymis in the testicle sperm cells enter the vas deferens, after that they receive secretions from the seminal vesicles and gather (from right and left sides) in the ejaculatory duct that passes insidethe prostate. They also get secretions from the prostate and the bulbourethral glands and then go through the urethra, inside the penis, to the exterior.
4. What is the function of the secretions of the prostate, seminal vesicle and bulbourethral glands in reproduction?
These secretions along with sperm cells from the testicles form the semen. The secretions have the function of nourishing the sperm cells and serving them as a fluid means of propagation. The basic pH of the seminal fluid also neutralizes the acid secretions of the vagina allowing the survival of sperm cells in the vaginal environment after copulation.
5. What are the endocrine glands that regulate sexual activity in males? How does this regulation work and what are the involved hormones?
In males the sexual activity is regulated by the endocrine glands hypophysis (pituitary), adrenals and gonads (testicles).
The FSH (follicle-stimulating hormone) secreted by the adenohypophysis acts upon the testicles stimulating the spermatogenesis. The LH (luteinizing hormone), another adenohypophyseal hormone, stimulates the production of testosterone by the testicles too. Testosterone, whose production intensifies after the beginning of puberty, acts in several organs of the body and it is responsible for the appearing of the male secondary sex characteristics (beard, body hair, deep voice, increase of the muscle and osseous mass, maturation of genitalia, etc.) Testosterone also stimulates spermatogenesis.
Reproductive System - Image Diversity: hypophysis adrenals
6. What are the organs that are part of the female reproductive system?
The organs that constitute the female reproductive system are the ovaries, the Fallopian tubes (or uterine tubes), the uterus, the vagina and the vulva.
Reproductive System - Image Diversity: female reproductive system
7. In which period of life does the formation of gametes begin in women?
The meiosis that forms female gametes begins in the cells of the ovarian follicles before birth. After the beginning of puberty, under hormonal stimuli, during each menstrual cycle one of the cells is released on the surface of the ovary and meiosis resumes. The meiotic process is only concluded however if fecundation happens.
8. What is the organ that releases the female gamete under formation? How is this release triggered? What is the organ that collects the released gametes?
The organ that liberates the female gamete is the ovary, the female gonad. The releasing of the oocyte is a response to hormonal stimuli. The immature egg cell (still an oocyte) falls into the abdominal cavity and is picked up by the Fallopian tube (uterine tube, or oviduct), a tubular structure that connects the ovary with the uterus.
9. What are the anatomical relationships between the organs of the female reproductive system from the external vulva to the ovaries?
The external female genitalia is called the vulva. The vulva is the external opening of the vaginal canal, or vagina. The vagina is the copulation organ of the females and its posterior extremity communicates with the uterus through the uterine cervix. The uterus is divided into two portions: the cervix and the uterine cavity. The lateral walls of the uterine fundus communicate with the Fallopian tubes. The other extremity of each Fallopian tube ends in fimbria forming fringes in the abdominal cavity. Between the uterine tube and the ovary there is still intra-abdominal space.
10. What is the menstrual cycle?
The menstrual cycle is the periodic succession of interactions between hormones and the organs of the female reproductive system that, after the beginning of puberty, regulates the release of the female gametes and prepares the uterus for fecundation and pregnancy.
Reproductive System - Image Diversity: menstrual cycle
11. What are the endocrine glands involved in the menstrual cycle? What are the hormones in action?
The endocrine glands that secrete hormones involved in the menstrual cycle are the hypophysis (pituitary) and the ovaries.
The hormones from adenohypophysis are FSH (follicle-stimulating hormone) and LH (luteinizing hormone) and the hormones from the ovaries are estrogen and progesterone.
Reproductive System - Image Diversity: hormone levels during menstrual cycle
12. What event marks the beginning of the menstrual cycle? What is the blood concentration of FSH, LH, estrogen and progesterone in this phase of the cycle?
By convention the menstrual cycle begins at the day that menses begins. (Menses is the endometrial hemorrhage excreted through the vaginal canal.) At these days the hormones FSH, LH, estrogens and progesterone are in low concentration.
13. After menses what is the hormone that influences the maturation of the ovarian follicles?
The maturation of the ovarian follicles after menses is stimulated by the action of FSH (follicle-stimulating hormone).
Reproductive System - Image Diversity: ovarian follicles
14. What is the hormone secreted by the growing ovarian follicles? What is the action of that hormone upon the uterus?
The follicles that are growing after menses secrete estrogen. These hormones act upon the uterus stimulating the thickening of the endometrium (the internal mucosa of the uterus).
15. What is the relationship between the estrogen level and the LH level in the menstrual cycle? What is the function of LH in the menstrual cycle and when does its blood concentration reach a peak?
The increase in the blood concentration of estrogen with the growing of the ovarian follicle causes the hypophysis to secrete LH. In this phase LH acts together with FSH promoting the maturation of the follicle that at the 14th day ruptures releasing the female gamete (ovulation). After the release of the ovum LH acts stimulating the formation of the corpus luteum, a structure made from the remaining follicular mass. The LH concentration is at maximum at the 14th day of the cycle.
Reproductive System - Image Diversity: corpus luteum
16. What are the hormones that promote the release of the female gamete from the follicle and at which day of the menstrual cycle does this phenomenon happen? What is this event called?
The hormones that promote the release of the ovum from the follicle are FSH and LH, hormones found in maximum blood concentration around the 14th day of the cycle. The release of the female gamete from the ovary is called ovulation. Ovulation happens at (around) the 14th day of the menstrual cycle.
17. How does the female gamete move from the ovary to the uterus?
The female gamete released from the ovary falls into the surrounding abdominal cavity and is collected by the Fallopian tube. The internal epithelium of the uterine tubes has ciliated cells that move the ovum or the fecundated egg cell towards the uterus.
Reproductive System - Image Diversity: Fallopian tube
18. How long after ovulation must fecundation occur to be effective?
If fecundation does not occur approximately 24 hours after ovulation the released ovum often dies.
19. What is the structure into which the follicle is transformed after ovulation? What is the importance of that structure in the menstrual cycle?
The follicle that released the ovum suffers the action of LH and is transformed into the corpus luteum. The corpus luteum is very important because it secretes estrogen and progesterone.
These hormones prepare the uterine mucosa, also known as endometrium, for nidation (implantation of the zygote in the uterine wall) and embryonic development since they stimulate the thickening of the mucous tissue, increase its vascularity and make the appearing of uterine glycogen-producing glands.
20. What is the importance of the uterine glycogen-producing glands?
The uterine glands produce glycogen that can be degraded into glucose to nourish the embryo before the complete development of the placenta.
21. How does the hypophysis-corpus luteum negative feedback work? What is the name given to the atrophied corpus luteum after this feedback process?
After ovulation the estrogen and progesterone secretions from the corpus luteum inhibit the hypophyseal FSH and LH secretions (this happens by inhibition of GnRH, gonadotropin-releasing hormone, a hypothalamic hormone). The blood concentration of these adenohypophyseal hormones falls to basal levels again. As LH lowers the corpus luteum (luteum means “yellow”) becomes atrophic and turns into the corpus albicans (“white”). With the regression of the corpus luteum the production of estrogen and progesterone ceases.
Reproductive System - Image Diversity: corpus albicans
22. In hormonal terms why does menses occur?
Menses is the endometrial monthly desquamation that occurs as the estrogen and progesterone levels fall after the regression of the corpus luteum because these hormones, mainly progesterone, can no longer support and maintain the thickening of the endometrium.
Reproductive System - Image Diversity: endometrium
23. What is the explanation for the bleeding that accompanies menses?
The hemorrhage that accompanies menses occurs because the endometrium is a richly vascularized tissue. The rupture of blood vessels of the uterine mucosa during the menstrual desquamation causes the bleeding.
24. Which are the phases of the menstrual cycle?
The menstrual cycle is divided into two main phases: the follicular (or menstrual) phase and the luteal (or secretory) phase.
The menstrual phase begins at the first day of menses and lasts until ovulation (around the 14th day). The luteal phase begins after ovulation and ends when menses begins (around the 28th day).
25. Including main events and hormonal changes how can the menstrual cycle be described?
One can imagine a cycle like an analog clock at which at 0 o’clock is the beginning and the end of the menstrual cycle and that 6 o’clock corresponds to the 14h day of the cycle.
At 0 o’clock the menses and so the menstrual cycle begins and FSH blood level begins to increase. Around 2 o’clock the maturing follicles under FSH action are already secreting estrogen and the endometrium is thickening. Around 3 o’clock estrogen is intensely stimulating the increase of LH blood level. At 6 o’clock (the 14th day) LH is at its maximum concentration and FSH also at high levels to promote ovulation, LH then stimulates the formation of the corpus luteum. Around 7 o’clock the corpus luteum is already secreting a great amount of estrogen and progesterone and the endometrium thickens even more, concomitant lowering of FSH and LH occurs with the increasing of the ovarian hormones. Around 11 o’clock the reduced LH and FSH levels make the corpus luteum turn into the corpus albicans, the production of estrogen and progesterone ceases and the endometrium regresses. At 0 o’clock again (28th day) the endometrium desquamates and a new menstrual cycle begins.
26. In general what is the phase of the menstrual cycle when copulation may lead to fecundation?
Although this is not a rule, to be effective fecundation in general must occur within about 24 hours after ovulation (that occurs around the 14th day of the menstrual cycle). Fecundation may occur even if copulation took place up to 3 days before ovulation since the male gametes remain viable for about 72 hours within the female reproductive system.
The fertile period of the women however is considered the period from 7 days before ovulation to 7 days after ovulation.
27. What is the part of the female reproductive system where fecundation occurs?
Fecundation generally occurs in the Fallopian tubes but it can also take place within the uterus. There are cases when fecundation may occur even before the ovum enters the uterine tube, a fact that may lead to a severe medical condition known as abdominal pregnancy.
28. How does the sexual arousal mechanism in women facilitate fecundation?
During sexual arousal in women the vagina secretes substances to neutralize its acidity thus allowing the survival of sperm cells within it. During the female fertile period hormones make the mucus that covers the internal surface of the uterus less viscous to help the passage of sperm cells to the uterine tubes. During copulation the uterine cervix advances inside the vagina to facilitate the entering of male gametes through the cervical canal.
29. What is nidation? In which phase of the menstrual cycle does nidation occur?
Nidation is the implantantion of the embryo in the uterus. Nidation occurs around the 7th day after fecundation, i.e., 7 to 8 days after ovulation (obviously, it occurs only if fecundation also occurs). Since it occurs in the luteal phase the progesterone level is high and the endometrium is in its best condition to receive the embryo.
Reproductive System - Image Diversity: nidation
30. What is tubal pregnancy?
Many times fecundation takes place in the Fallopian tubes. Generally the newly formed zygote is taken to the uterus where nidation and the embryonic development occur. In some cases however the zygote cannot go down to the uterus and the embryo implants itself in the uterine tube tissue, characterizing the tubal pregnancy. Tubal pregnancy is a severe clinical condition since often the tube ruptures during gestation causing hemorrhage and even death of the woman. The most common treatment for tubal pregnancy has been surgery.
31. How do hormonal tests to detect pregnancy work?
Laboratory tests to detect pregnancy commonly test for human chorionic gonadotropin (HCG) concentration in blood or urine samples. If the level of this hormone is abnormally high, pregnancy is likely.
32. Does the hypophysis-ovaries endocrine axis work in the same way during pregnancy as in non-pregnant women? If pregnancy does not occur how does another menstrual cycle begin?
The functioning of the hypophysis is altered during pregnancy. Since estrogen and progesterone levels remain elevated during the gestational period the production of GnRH (gonadotropin-releasing hormone) from the hypothalamus is inhibited. The lack of GnRH thus inhibits the secretion of FSH and LH from the hypophysis and a new menstrual cycle does not begin.
If pregnancy does not occur the lowering of estrogen and progesterone levels stimulates the production of GnRH by the hypothalamus. This hormone then hastens the adenohypophyseal secretion of FHS and LH that in their turn stimulate the maturation of follicles and the beginning of a new menstrual cycle.
33. What is the endocrine function of the placenta?
The placenta besides being the organ through which the exchange of substances between the mother and the fetus is done also has the function of secreting estrogen and progesterone to keep a high level of these hormones during pregnancy. (The placenta still secretes other hormones like human placental lactogen, that act similarly to the hypophyseal hormones that regulate reproduction, and HCG, human chorionic gonadotropin.)
34. How do contraceptive pills generally work?
Contraceptive pills generally contain the hormones estrogen and progesterone. If taken daily from the 4th day after menses the abnormal elevation of these hormones acts upon the hypophysis-hypothalamus endocrine axis inhibiting the FSH and LH secretions. Since these hormones then do not reach their normal high levels during the menstrual cycle ovulation does not occur.
(Treatment with contraceptive pills must be initiated under medical supervision.)
35. What are the common contraindications of the contraceptive pills?
There are medical reports associating the use of contraceptive pills with vomiting, nausea, vertigo, headaches, hypertension and other pathological conditions. Some research has attempted to relate the medical ingestion of estrogen and progesterone with increased propensity to cardiovascular diseases (like infarction, strokes and thrombosis) and to malignant neoplasias (cancers). Doctors must always be asked about the risks and benefits of the contraceptive pill prior to use.
36. What are the most common methods of male and female surgical sterilization?
Vasectomy is the most common method of surgical sterilization in men. In vasectomy the vas deferens inside the scrotum are sectioned and closed at a section which will forbid the sperm cells to follow to the ejaculatory duct but still allowing the release of seminal fluid during ejaculation.
Surgical sterilization of women is often done by bilateral tubal ligation. With tubal ligation the ovum does not pass to the uterus so the sperm cells cannot reach it.
Reproductive System - Image Diversity: vasectomy tubal ligation
37. How does the contraceptive diaphragm work? What are the limitations of this contraceptive method?
The contraceptive diaphragm is an artifact made of latex or plastic that when placed on the vaginal fundus covers the uterine cervix forbidding the passage of sperm cells through the cervical canal. To be more effective the diaphragm needs to be used together with spermicide. This method however does not prevent sexually transmitted diseases (STDs).
Reproductive System - Image Diversity: contraceptive diaphragm
38. Why is the use of condoms not just a contraceptive method but also a health protection behavior?
The use of condoms besides being an efficient contraceptive method also helps the prevention of diseases caused by sexually transmitted agents (STDs), like syphilis, gonorrhea, HPV (human papilloma virus that may lead to genital cancers) infestation, HIV infection, etc.
Reproductive System - Image Diversity: condom
39. What is the normal duration of the menstrual cycle? How does the calendar contraceptive method work?
The normal duration of the menstrual cycle is 28 days but it can vary among different women or in different cycles of the same woman.
In the calendar contraceptive method the date n-14 (n minus 14) is taken considering n the number of days of the normal menstrual cycle of the woman (generally n=28). The safety margin +3 or –3 refers to the days around n-14 that intercourse should be avoided to prevent pregnancy. (This method is not exempt from failures. A doctor must always be consulted before relying on any contraceptive method.)
40. How is the ovulation date estimated with the control of the woman's body temperature?
One method to estimate the exact ovulation day is daily control of the body temperature taken always under same conditions. At the ovulation day the body temperature often increases about 0.5 degrees centigrade.
41. What is the contraceptive mechanism of the IUD?
The IUD (intrauterine device) is a piece of plastic coated with copper that is inserted within the uterus by a doctor. Copper is then gradually released (IUD may last 5 to 10 years) and since it has a spermicidal action sperm cells are destroyed before fecundation. Besides this mechanism the movement of the IUD inside the uterus causes slight endometrial inflammation that helps to prevent nidation.
Reproductive System - Image Diversity: IUD
42. Generally how does a male animal realize that the female is receptive to copulation?
In most vertebrate species with internal fecundation the females have reproductive cycles with fertile periods. During this period the female secretes pheromones (odoriferous substances that attract the male of the species) from the skin and mucosae. The presence of the male individual and his pheromones also stimulates the release of pheromones by the female. (Many animals also use pheromones for territorial demarcation and for signal transmission between individuals about the location of dangers and food.)
43. What is parthenogenesis?
Parthenogenesis is the reproduction or formation of a new individual from the egg cell but without fecundation by the male gamete. According to the species, individuals born by parthenogenesis may be male or female, or of any sex.
In bees the drone (the single male bee) is haploid and born by parthenogenesis while the females (queen and workers) are diploid.
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