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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 of the 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 the peripheral 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 the spinal 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 adjacent structures. 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 the peripheral 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 and spinal 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).
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