The structure and function of the nervous regulatory system of the human body. Regulatory systems of the human body - Dubynin V.A.

Depending on the nature of the innervation of organs and tissues, the nervous system is divided into somatic and vegetative. The somatic nervous system regulates voluntary movements of the skeletal muscles and provides sensitivity. The autonomic nervous system coordinates activity internal organs, glands, of cardio-vascular system and carries out the innervation of all metabolic processes in the human body. The work of this regulatory system is not controlled by consciousness and is carried out thanks to the coordinated work of its two departments: sympathetic and parasympathetic. In most cases, activation of these departments has the opposite effect. Sympathetic influence is most pronounced when the body is in a state of stress or intense work. The sympathetic nervous system is a system of alarm and mobilization of reserves necessary to protect the body from environmental influences. It gives signals that activate brain activity and mobilize defensive reactions(the process of thermoregulation, immune reactions, mechanisms of blood coagulation). Upon activation of the sympathetic nervous system heart rate increases, digestion processes slow down, breathing rate increases and gas exchange increases, the concentration of glucose and fatty acids in the blood increases due to their release by the liver and adipose tissue (Fig. 5).

The parasympathetic division of the autonomic nervous system regulates the work of internal organs at rest, i.e. it is a system of current regulation of physiological processes in the body. The predominance of the activity of the parasympathetic part of the autonomic nervous system creates conditions for rest and restoration of body functions. When it is activated, the frequency and strength of heart contractions decrease, digestion processes are stimulated, and the airway clearance decreases (Fig. 5). All internal organs are innervated by both the sympathetic and parasympathetic divisions of the autonomic nervous system. The skin and the musculoskeletal system have only sympathetic innervation.

Fig.5. Regulation of various physiological processes human body under the influence of the sympathetic and parasympathetic divisions of the autonomic nervous system

The autonomic nervous system has a sensory (sensitive) component represented by receptors (sensitive devices) located in the internal organs. These receptors perceive indicators of the state of the internal environment of the body (for example, concentration carbon dioxide, pressure, concentration of nutrients in the bloodstream) and transmit this information along the centripetal nerve fibers to the central nervous system, where this information is processed. In response to the information received from the central nervous system, signals are transmitted along the centrifugal nerve fibers to the corresponding working organs involved in maintaining homeostasis.

The endocrine system also regulates the activity of tissues and internal organs. This regulation is called humoral and is carried out with the help of special substances (hormones) that are secreted by the endocrine glands into the blood or tissue fluid. Hormones - These are special regulatory substances produced in some tissues of the body, transported with the bloodstream to various organs and affecting their work. While the signals (nerve impulses) that provide nervous regulation propagate at a high speed and it takes fractions of a second to carry out a response from the autonomic nervous system, humoral regulation is much slower, and under its control are those processes of our body that require minutes to regulate. and clock. Hormones are potent substances and cause their effect in very small quantities. Each hormone affects certain organs and organ systems, which are called target organs. Target organ cells have specific receptor proteins that selectively interact with specific hormones. The formation of a complex of a hormone with a receptor protein involves a whole chain of biochemical reactions that determine the physiological action of this hormone. The concentration of most hormones can change in large limits, which ensures the maintenance of the constancy of many physiological parameters with the continuously changing needs of the human body. Nervous and humoral regulation in the body are closely interconnected and coordinated, which ensures its adaptability in a constantly changing environment.

Hormones play a leading role in the humoral functional regulation of the human body. pituitary and hypothalamus. The pituitary gland (lower cerebral appendage) is a part of the brain related to the diencephalon, it is attached with a special leg to another part of the diencephalon, hypothalamus, and is closely related to it. The pituitary gland consists of three parts: anterior, middle and posterior (Fig. 6). The hypothalamus is the main regulatory center of the autonomic nervous system, in addition, this part of the brain contains special neurosecretory cells that combine the properties of a nerve cell (neuron) and a secretory cell that synthesizes hormones. However, in the hypothalamus itself, these hormones are not released into the blood, but enter the pituitary gland, in its posterior lobe ( neurohypophysis) where they are released into the blood. One of these hormones antidiuretic hormone(ADG or vasopressin), predominantly affects the kidney and the walls of blood vessels. An increase in the synthesis of this hormone occurs with significant blood loss and other cases of fluid loss. Under the action of this hormone, the loss of fluid in the body decreases, in addition, like other hormones, ADH also affects brain function. It is a natural stimulant of learning and memory. Lack of synthesis of this hormone in the body leads to a disease called diabetes insipidus, in which the volume of urine excreted by patients sharply increases (up to 20 liters per day). Another hormone released into the blood in the posterior pituitary gland is called oxytocin. The target for this hormone is the smooth muscles of the uterus, muscle cells surrounding the ducts of the mammary glands and testes. An increase in the synthesis of this hormone is observed at the end of pregnancy and is absolutely necessary for the course of childbirth. Oxytocin impairs learning and memory. Anterior pituitary ( adenohypophysis) is an endocrine gland and secretes a number of hormones into the blood that regulate the functions of other endocrine glands (thyroid, adrenal, gonads) and are called tropic hormones. For example, adenocorticotropic hormone (ACTH) acts on the adrenal cortex and under its influence a number of steroid hormones are released into the blood. Thyroid-stimulating hormone stimulates the thyroid gland. growth hormone(or growth hormone) acts on bones, muscles, tendons, internal organs, stimulating their growth. In the neurosecretory cells of the hypothalamus, special factors are synthesized that affect the functioning of the anterior pituitary gland. Some of these factors are called liberals, they stimulate the secretion of hormones by cells of the adenohypophysis. Other factors statins, inhibit the secretion of the corresponding hormones. The activity of the neurosecretory cells of the hypothalamus changes under the influence of nerve impulses coming from peripheral receptors and other parts of the brain. Thus, the connection between the nervous and humoral systems is primarily carried out at the level of the hypothalamus.

Fig.6. Scheme of the brain (a), hypothalamus and pituitary gland (b):

1 - hypothalamus, 2 - pituitary gland; 3 - medulla oblongata; 4 and 5 - neurosecretory cells of the hypothalamus; 6 - pituitary stalk; 7 and 12 - processes (axons) of neurosecretory cells;
8 - posterior pituitary gland (neurohypophysis), 9 - intermediate pituitary gland, 10 - anterior pituitary gland (adenohypophysis), 11 - median elevation of the pituitary stalk.

In addition to the hypothalamic-pituitary system, the endocrine glands include the thyroid and parathyroid glands, the adrenal cortex and medulla, pancreatic islet cells, intestinal secretory cells, sex glands, and some heart cells.

Thyroid- this is the only human organ that is able to actively absorb iodine and include it in biologically active molecules, thyroid hormones. These hormones affect almost all cells of the human body, their main effects are associated with the regulation of growth and development processes, as well as metabolic processes in the body. Thyroid hormones stimulate the growth and development of all body systems, especially the nervous system. When the thyroid gland is not functioning properly, adults develop a disease called myxedema. Its symptoms are a decrease in metabolism and dysfunction of the nervous system: the reaction to stimuli slows down, fatigue increases, body temperature drops, edema develops, the gastrointestinal tract suffers, etc. A decrease in thyroid levels in newborns is accompanied by more severe consequences and leads to cretinism, mental retardation up to complete idiocy. Previously, myxedema and cretinism were common in mountainous areas where there is little iodine in glacial water. Now this problem is easily solved by adding sodium iodine salt to table salt. An overactive thyroid gland leads to a disorder called Graves' disease. In such patients, the basal metabolism increases, sleep is disturbed, the temperature rises, breathing and heartbeat become more frequent. Many patients have bulging eyes, sometimes a goiter is formed.

Adrenals- paired glands located at the poles of the kidneys. Each adrenal gland has two layers: cortical and medulla. These layers are completely different in their origin. The outer cortical layer develops from the middle germ layer (mesoderm), the medulla is a modified node of the autonomic nervous system. The adrenal cortex produces corticosteroid hormones (corticoids). These hormones have a wide range actions: affect water-salt metabolism, fat and carbohydrate metabolism, immune properties of the body, suppress inflammatory reactions. One of the main corticoids, cortisol, is necessary to create a reaction to strong stimuli that lead to the development of stress. Stress can be defined as a threatening situation that develops under the influence of pain, blood loss, fear. Cortisol prevents blood loss, constricts small arterial vessels, and increases the contractility of the heart muscle. With the destruction of the cells of the adrenal cortex develops Addison's disease. Patients have a bronze skin tone in some parts of the body, developing muscle weakness, weight loss, memory loss and mental capacity. Tuberculosis used to be the most common cause of Addison's disease, but nowadays it's autoimmune reactions (faulty production of antibodies to one's own molecules).

Hormones synthesized in the adrenal medulla: adrenalin and norepinephrine. The targets of these hormones are all tissues of the body. Adrenaline and norepinephrine are designed to mobilize all the forces of a person in case of a situation that requires great physical or mental stress, in case of injury, infection, fear. Under their influence, the frequency and strength of heart contractions increase, blood pressure rises, breathing quickens and bronchi expand, and the excitability of brain structures increases.

Pancreas is a gland of a mixed type, it performs both digestive (production of pancreatic juice) and endocrine functions. It produces hormones that regulate carbohydrate metabolism in the body. Hormone insulin stimulates the flow of glucose and amino acids from the blood into the cells of various tissues, as well as the formation in the liver from glucose of the main reserve polysaccharide of our body, glycogen. Another pancreatic hormone glucagon, according to its biological effects, is an insulin antagonist, increasing blood glucose levels. Glucogon stimulates the breakdown of glycogen in the liver. With a lack of insulin develops diabetes, Glucose ingested with food is not absorbed by tissues, accumulates in the blood and is excreted from the body with urine, while tissues are sorely lacking in glucose. The nervous tissue suffers especially strongly: the sensitivity of the peripheral nerves is disturbed, there is a feeling of heaviness in the limbs, convulsions are possible. In severe cases, diabetic coma and death can occur.

The nervous and humoral systems, working together, excite or inhibit various physiological functions, which minimizes deviations of individual parameters of the internal environment. The relative constancy of the internal environment is ensured in humans by regulating the activity of the cardiovascular, respiratory, digestive, excretory systems, and sweat glands. Regulatory mechanisms ensure consistency chemical composition, osmotic pressure, number of blood cells, etc. Very perfect mechanisms ensure the maintenance of a constant temperature of the human body (thermoregulation).

Watching the work of your body, you noticed that after running, the frequency of breathing and heart rate increases. After eating, the amount of glucose in the blood increases. However, after some time, these indicators supposedly acquire their original values ​​on their own. How does this regulation take place?

Humoral regulation

Humoral regulation(lat. humor - liquid) is carried out with the help of substances that affect the metabolic processes in cells, as well as the functioning of organs and the body as a whole. These substances enter the blood, and from it - into the cells. Thus, an increase in the level of carbon dioxide in the blood increases the frequency of respiration.

Some substances, such as hormones, perform their function even if their concentration in the blood is very low. Most hormones are synthesized and released into the blood by the cells of the endocrine glands, which form the endocrine system. Traveling with blood throughout the body, hormones can enter any organ. But the hormone affects the functioning of the organ only if the cells of this organ have receptors for this particular hormone. The receptors are combined with hormones, and this entails a change in the activity of the cell. So, the hormone insulin, joining the receptors of the liver cell, stimulates the penetration of glucose into it and the synthesis of glycogen from this compound.

To prepare for lessons, he advises similar notes and abstracts:

Endocrine system

Endocrine system ensures the growth and development of the body, its individual parts and organs. It is involved in the regulation of metabolism and adapts it to the needs of the body, constantly changing.

Nervous regulation

Unlike the system of humoral regulation, which responds mainly to changes in the internal environment, the nervous system responds to events occurring both inside the body and outside it. With the help of the nervous system, the body responds to any impact very quickly. Such reactions to the action of stimuli are called reflexes. A reflex is carried out due to the work of a chain of neurons that form a reflex arc. Each such arc begins with a sensitive or receptor neuron (receptor neuron). It perceives the action of the stimulus and creates an electrical impulse, which is called a nervous

Impulses arising in the receptor neuron are sent to the nerve centers of the spinal cord and brain, where information is processed. Here a decision is made to which organ should send a nerve impulse in order to respond to the action of the stimulus. After that, the commands are sent along the effector neurons to the organ that responds to the stimulus. Usually such a response is a contraction of a certain muscle or the secretion of a gland. To imagine the speed of signal transmission along the reflex arc, remember how long it takes you to pull your hand away from a hot object.

nerve impulses

nerve impulses are transmitted with the help of special substances - mediators. The neuron in which the impulse originated releases them into the synapse cleft - the junction of neurons. The mediators attach to the receptor proteins of the target neuron, and in response, it generates an electrical impulse and transmits it to the next neuron or another cell.

Immune regulation is provided by the immune system, the task of which is to create immunity - the ability of the body to resist the action of external and internal enemies. They are bacteria, viruses, various substances that disrupt the normal functioning of the body, as well as its cells, dead or reborn. The main fighting forces of the immune regulatory system are certain blood cells and special substances contained in it.

Is the calorie content of foods a decisive factor influencing weight? Let's try to figure this out.

Regulatory system of the body

All that we receive is spent on various needs: the synthesis of enzymes, maintaining body temperature, the work performed, moving in space, thinking and nervous activity etc. The greater the energy consumption, the more intense the metabolism becomes and the process proceeds better (up to a certain point).

An amazing balance is maintained between the intake of energy and its expenditure, the mechanism of self-regulation works.

In the human body, it is carried out at several levels. In the biological body, the process is coordinated by the brain, it can invade the work of any of the systems, up to a single cell.

However, in the conditions of ordinary life, the current tasks in the body are solved by the subconscious, which, in turn, also has several levels of hierarchy, but we will not focus on this. Now the next point is important: if you give a certain setting or program to your subconscious, it is possible to work miracles with your body.

In addition to direct intervention, the subconscious mind influences the body through a complex multi-level system of hormonal regulation. It includes the hypothalamus - the main coordinating center, the pituitary gland - the middle link to which the endocrine glands obey. The metabolism is directly regulated by hormones.

Thus, it turns out that, first of all, a person’s weight is affected by internal causes- installations of the subconscious and hormonal balance. And they, in turn, are affected by health (more precisely, pathologies), genotype and emotions.

American scientists have proven THAT AVERAGE HUMAN WEIGHT DOES NOT DEPEND ON THE CALORIE OF FOOD. Naturally, it means normal conditions when there are no forced food restrictions.

That is, the following situation develops, which, as it were, asserts a certain weight. If there is a slight temporary overeating, then the excess energy increases metabolism and turns into heat until a balance is established. If you deliberately overeat for a long time, then, undoubtedly, fat reserves will begin to replenish. But if a person stops doing this, then the weight will soon begin to return to the original. Of course, such overloads will not pass without a trace, the internal organs will wear out prematurely.

In a situation of malnutrition, the body uses its reserves and exists at their expense. The process of heat generation in order to save is reduced, the metabolism slows down. Hunger arises, which a person seeks to satisfy, and the body's reserves are replenished.

Unfortunately this regulatory system of the body is not what we would like. Nature does not know a lazy life in conditions of abundance. The task of survival requires our body to store a small amount of fat reserves for a rainy day. And if a person eats plentifully and satisfyingly, reserves are gradually formed for “rainy days”, which do not come, and the reserves continue to grow ....

Relationship between food intake and age

In addition, with age, the ratio between the synthesized hormones changes, and the balance begins to shift towards weight accumulation. Some authors (V. Dilman) believe that obesity is a normal consequence of aging.

The fact is that by the age of 22-25, the process of puberty and growth is completed, and the level of metabolic hormones gradually begins to decrease. As a result, the absorption of nutrients decreases by 1-2% annually, and by the age of 50 in relatively healthy people, it is 40-50% of the youthful level and even less in those who are sick.

Although growth has stopped, the cells of the body continue to divide and renew without stopping. The body's need for energy and nutrients increases, because people give birth and raise children, are promoted, etc. In addition, the work of the gastrointestinal tract and the endocrine system in the body worsens, nutritional deficiency is aggravated under the influence of diseases, drugs, smoking, alcohol, stressful situations, various stimulants.

People continue to satisfy the feeling of hunger with the usual amount of food, but cellular level the body experiences hunger due to the assimilation of an ever smaller amount of necessary elements. This deficiency activates protective functions body - fat reserves begin to accumulate in the waist, hips, abdomen, chest and other genetically predisposed places.

A typical reaction of most women and men and women in response to a decrease in the process of assimilation of food, increased stress, increased body weight, lack of energy is a strict diet and exercise. As a result, the body in conditions of deficiency responds with diseases, depressive states, fatigue, and premature aging.

The way out of this situation is, which will ensure health and longevity, but more on that in other articles.

Of course, a person is able to consciously shift the internal balance in the direction he needs. But it requires great job regulatory systems, and for this someone will have to lose weight, increase physical activity, give up sweet cakes and donuts.

Dysregulation is a disease, and a disease cannot be "normal." Indeed, in the "norm" a person has a good constitution, feels vigorous and strong, and when he is thin or fat, then this is already a pathology.

Weight gain can be the cause of self-indulgence in healthy people, however, obesity itself will quickly provoke the development of diseases. In addition, overweight is often the result of congenital or acquired diseases of the body's regulatory system. For example, when with early childhood the child is fed, the body will adapt to this and form new fat cells. That is, parents will doom their child to be complete.

Exhaustion or abnormal thinness is also, as a rule, evidence of some kind of hidden illness - the presence of a nervous or hormonal disorder, gastric or intestinal disease, etc.

Summarizing all of the above, we formulate several provisions:

1. The decisive role in maintaining weight belongs to the regulatory systems of the body, not calories. They coordinate energy consumption, control the feeling of hunger. Obesity or thinness speak of breakdowns in the mechanisms of regulation of a congenital, acquired or age-related nature.

2. To a greater extent, the work of regulatory systems is affected by repeated external influences - nutrition, physical activity, emotions, etc. If there are systematic inconsistencies of any kind, the balance is disturbed. But this position itself gives us the opportunity to consciously influence the regulatory systems of the body.

3. Optimize energy metabolism and weight is possible only with the help of integrated approach-, physical education, mental hygiene. With the help of diet alone, it will be possible to maintain weight for some time, and even then not always. But this disharmony will not give the body health and longevity.

And the most important conclusion: "COUNTING CALORIES IS NOT NEEDED." When the body is able to take food, energy deficiency automatically stimulates a healthy hunger. And satisfying it without overeating is the most reasonable way to eat.

Physiological processes in the human body proceed in a coordinated manner due to the existence of certain mechanisms of their regulation.

Regulation various processes in the body is carried out with the help of nervous and humoral mechanisms.

Humoral regulation carried out with the help of humoral factors ( hormones), which are carried by blood and lymph throughout the body.

nervous regulation is carried out using nervous system.

Nervous and humoral methods of regulation of functions are closely related. The activity of the nervous system is constantly influenced by chemicals brought with the bloodstream, and the formation of the majority chemical substances and their release into the blood is under constant control of the nervous system.

The regulation of physiological functions in the body cannot be carried out with the help of only nervous or only humoral regulation - this is a single complex neurohumoral regulation functions.

AT recent times it has been suggested that there are not two regulation systems (nervous and humoral), but three (nervous, humoral and immune).

Nervous regulation

Nervous regulation- this is the coordinating influence of the nervous system on cells, tissues and organs, one of the main mechanisms of self-regulation of the functions of the whole organism. Nervous regulation is carried out with the help of nerve impulses. Nervous regulation is fast and local, which is especially important in the regulation of movements, and affects all (!) Systems of the body.

The reflex principle underlies the nervous regulation. Reflex is a universal form of interaction of the body with the environment, it is the body's response to irritation, which is carried out through the central nervous system and is controlled by it.

The structural and functional basis of the reflex is a reflex arc - a series-connected chain nerve cells providing a response to irritation. All reflexes are carried out due to the activity of the central nervous system - the brain and spinal cord.

Humoral regulation

Humoral regulation is the coordination of physiological and biochemical processes carried out through the liquid media of the body (blood, lymph, tissue fluid) with the help of biologically active substances (hormones) secreted by cells, organs and tissues in the course of their life.

Humoral regulation arose in the process of evolution earlier than nervous regulation. It became more complex in the process of evolution, as a result of which arose endocrine system(endocrine glands).

Humoral regulation is subordinate to nervous regulation and together with it constitutes a single system of neurohumoral regulation of body functions, which plays an important role in maintaining the relative constancy of the composition and properties of the internal environment of the body (homeostasis) and its adaptation to changing conditions of existence.

immune regulation

Immunity is a physiological function that ensures the body's resistance to the action of foreign antigens. Human immunity makes it immune to many bacteria, viruses, fungi, worms, protozoa, various animal poisons, and protects the body from cancer cells. The task of the immune system is to recognize and destroy all foreign structures.

The immune system is the regulator of homeostasis. This function is carried out through the development autoantibodies, which, for example, can bind excess hormones.

The immunological reaction, on the one hand, is an integral part of the humoral one, since most physiological and biochemical processes are carried out with the direct participation of humoral mediators. However, often the immunological reaction is targeted and thus resembles nervous regulation.

The intensity of the immune response, in turn, is regulated in a neurophilic way. The work of the immune system is corrected by the brain and through the endocrine system. Such nervous and humoral regulation is carried out with the help of neurotransmitters, neuropeptides and hormones. Promediators and neuropeptides reach the organs of the immune system along the axons of the nerves, and hormones are secreted by the endocrine glands unrelatedly into the blood and thus delivered to the organs of the immune system. Phagocyte (cell of immunity), destroys bacterial cells

Form start

Age anatomy and physiology Antonova Olga Alexandrovna

Topic 4. DEVELOPMENT OF REGULATORY SYSTEMS OF THE ORGANISM

4.1. The meaning and functional activity of the elements of the nervous system

The coordination of physiological and biochemical processes in the body occurs through regulatory systems: nervous and humoral. Humoral regulation is carried out through the liquid media of the body - blood, lymph, tissue fluid, nervous regulation - through nerve impulses.

The main purpose of the nervous system is to ensure the functioning of the body as a whole through the relationship between individual organs and their systems. The nervous system perceives and analyzes various signals from the environment and internal organs.

The nervous mechanism of regulation of body functions is more perfect than the humoral one. This, firstly, is explained by the speed of propagation of excitation through the nervous system (up to 100-120 m / s), and secondly, by the fact that nerve impulses come directly to certain organs. However, it should be borne in mind that all the completeness and subtlety of the body's adaptation to environment are carried out with the interaction of both nervous and humoral mechanisms of regulation.

General plan of the structure of the nervous system. In the nervous system, according to the functional and structural principle, the peripheral and central nervous systems are distinguished.

The central nervous system consists of the brain and spinal cord. The brain is located inside the brain region of the skull, and the spinal cord is located in the spinal canal. On a section of the brain and spinal cord, dark-colored areas (gray matter) formed by the bodies of nerve cells (neurons) are distinguished, and white color(white matter), consisting of clusters of nerve fibers covered with a myelin sheath.

The peripheral part of the nervous system is made up of nerves, such as bundles of nerve fibers, that extend beyond the brain and spinal cord and travel to various organs of the body. It also includes any collections of nerve cells outside the spinal cord and brain, such as ganglions or ganglia.

Neuron(from the Greek. neuron - nerve) - the main structural and functional unit of the nervous system. A neuron is a complex highly differentiated cell of the nervous system, the function of which is to perceive irritation, process irritation and transmit it to various organs of the body. A neuron consists of a cell body, one long branching process - an axon, and several short branching processes - dendrites.

Axons are of various lengths: from a few centimeters to 1–1.5 m. The end of the axon branches strongly, forming contacts with many cells.

Dendrites are short, strongly branching processes. From 1 to 1000 dendrites can depart from one cell.

In different parts of the nervous system, the body of a neuron can have a different size (diameter from 4 to 130 microns) and shape (stellate, round, polygonal). The body of a neuron is covered with a membrane and contains, like all cells, cytoplasm, a nucleus with one or more nucleoli, mitochondria, ribosomes, the Golgi apparatus, and the endoplasmic reticulum.

Excitation is transmitted along the dendrites from receptors or other neurons to the cell body, and along the axon, signals arrive at other neurons or working organs. It has been established that from 30 to 50% of nerve fibers transmit information to the central nervous system from receptors. On the dendrites there are microscopic outgrowths that significantly increase the surface of contact with other neurons.

Nerve fibre. Nerve fibers are responsible for conducting nerve impulses in the body. Nerve fibers are:

a) myelinated (pulp); sensory and motor fibers of this type are part of the nerves that supply the sense organs and skeletal muscles, and also participate in the activity of the autonomic nervous system;

b) unmyelinated (non-fleshy), belong mainly to the sympathetic nervous system.

Myelin has an insulating function and has a slightly yellowish color, so the fleshy fibers look light. The myelin sheath in the pulpy nerves is interrupted at intervals of equal length, leaving open areas of the axial cylinder - the so-called intercepts of Ranvier.

Amyelinated nerve fibers do not have a myelin sheath, they are isolated from each other only by Schwann cells (myelocytes).

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