What is the human autonomic nervous system responsible for? Autonomic (autonomic) nervous system
Autonomic nervous system
Some general principles of organization of sensory and motor systems will be very useful to us when studying systems of internal regulation. All three sections of the vegetative (autonomous) nervous system have “sensory” and “motor” components. While the former record indicators of the internal environment, the latter enhance or inhibit the activity of those structures that carry out the regulation process itself.
Intramuscular receptors, along with receptors located in tendons and some other places, respond to pressure and stretch. Together they make up a special kind of internal sensory system that helps control our movements.
Receptors involved in homeostasis act in a different way: they perceive changes in chemical composition blood pressure or blood pressure fluctuations vascular system and in hollow internal organs such as the digestive tract and bladder. These sensory systems systems that collect information about the internal environment are very similar in organization to systems that receive signals from the surface of the body. Their receptor neurons form the first synaptic switches within spinal cord. Along the motor pathways of the autonomic system, commands go to the organs that directly regulate the internal environment. These pathways begin with special autonomic preganglionic neurons in the spinal cord. This organization is somewhat reminiscent of the organization of the spinal level of the motor system.
The main focus of this chapter will be on those motor components of the autonomic system that innervate the muscles of the heart, blood vessels and intestines, causing their contraction or relaxation. The same fibers innervate the glands, causing the process of secretion.
The autonomic nervous system consists of two large sections - sympathetic And parasympathetic. Both divisions share a structural feature that we have not encountered before: the neurons that control the muscles of the internal organs and glands lie outside the central nervous system, forming small encapsulated clusters of cells called ganglia. Thus, in the autonomic nervous system there is an additional link between the spinal cord and the end working organ (effector).
Autonomic neurons in the spinal cord integrate sensory information from internal organs and other sources. On this basis, they then regulate the activity of autonomic ganglion neurons. The connections between the ganglia and the spinal cord are called preganglionic fibers. The neurotransmitter used to transmit impulses from the spinal cord to ganglion neurons in both the sympathetic and parasympathetic divisions is almost always acetylcholine, the same transmitter that the spinal cord motor neurons directly control skeletal muscles. As in the fibers innervating skeletal muscles, the action of acetylcholine can be enhanced in the presence of nicotine and blocked by curare. Axons coming from neurons of the autonomic ganglia, or postganglionic fibers, then go to the target organs, forming many branches there.
Rice. 63.The sympathetic and parasympathetic divisions of the autonomic nervous system, the organs they innervate, and their effects on each organ.
The sympathetic and parasympathetic divisions of the autonomic nervous system differ from each other 1) in the levels at which preganglionic fibers exit the spinal cord; 2) according to the proximity of the ganglia to the target organs; 3) by neurotransmitter, which is used by postganglionic neurons to regulate the functions of these target organs. We will now consider these features.
Sympathetic nervous system
IN sympathetic systems e preganglionic fibers emerge from breast And lumbar parts of the spinal cord. Its ganglia are located quite close to the spinal cord, and very long postganglionic fibers extend from them to the target organs (see Fig. 63). The main transmitter of the sympathetic nerves is norepinephrine, one of the catecholamines, which also serves as a mediator in the central nervous system.
To understand what organs the sympathetic nervous system affects, it is easiest to imagine what happens to an excited animal ready for a fight-or-flight response. The pupils dilate to let in more light; The heart rate increases and each contraction becomes more powerful, which leads to increased overall blood flow. Blood flows from the skin and internal organs to the muscles and brain. The motility of the gastrointestinal system weakens, digestion processes slow down. The muscles along the airways leading to the lungs relax, allowing the breathing rate to increase and gas exchange to increase. Liver and adipose tissue cells release more glucose into the blood and fatty acids- high-energy fuel, and the pancreas is commanded to produce less insulin. This allows the brain to receive a larger share of the glucose circulating in the bloodstream, since, unlike other organs, the brain does not require insulin to utilize blood sugar. The mediator of the sympathetic nervous system, which carries out all these changes, is norepinephrine.
There is an additional system that has an even more generalized effect to more accurately ensure all these changes. The adrenal glands sit on the tops of the kidneys, like two small caps. In their inner part - the medulla - there are special cells innervated by preganglionic sympathetic fibers. During embryonic development, these cells are formed from the same neural crest cells from which the sympathetic ganglia are formed. Thus, the medulla is a component of the sympathetic nervous system. When activated by preganglionic fibers, medullary cells release their own catecholamines (norepinephrine and epinephrine) directly into the blood for delivery to target organs (Fig. 64). Circulating hormone mediators serve as an example of how regulation is carried out by endocrine organs (see p. 89).
Rice. 64.When sympathetic nerve activity causes the adrenal medulla to release catecholamines, these signaling substances are carried into the blood and influence the activity of various target tissues; thus, they ensure a coordinated response from organs that are distant from each other.
Parasympathetic nervous system
In the parasympathetic division, preganglionic fibers come from brain stem(“cranial component”) and from the lower, sacral segments of the spinal cord (see Fig. 63 above). They form, in particular, a very important nerve trunk called vagus nerve, whose numerous branches carry out the entire pair sympathetic innervation heart, lungs and intestinal tract. (The vagus nerve also relays sensory information from these organs back to the central nervous system.) Preganglionic parasympathetic axons are very long because their ganglia are typically located near or within the tissues they innervate.
A transmitter is used at the endings of the fibers of the parasympathetic system acetylcholine. The response of the corresponding target cells to acetylcholine is insensitive to the effects of nicotine or curare. Instead, acetylcholine receptors are activated by muscarine and blocked by atropine.
The predominance of parasympathetic activity creates conditions for “rest and restoration” of the body. In its extreme manifestation, the general pattern of parasympathetic activation resembles the state of rest that occurs after a satisfying meal. Increased blood flow to the digestive tract speeds up the movement of food through the intestines and increases the secretion of digestive enzymes. The frequency and strength of heart contractions decrease, the pupils narrow, respiratory tract decreases, and the formation of mucus in them increases. The bladder contracts. Taken together, these changes return the body to the peaceful state that preceded the fight-or-flight response. (All this is presented in Fig. 63; see also Chapter 6.)
Comparative characteristics of the parts of the autonomic nervous system
The sympathetic system, with its extremely long postganglionic fibers, is very different from the parasympathetic system, in which, on the contrary, the preganglionic fibers are longer and the ganglia are located near or inside the target organs. Many internal organs such as lungs, heart, salivary glands, bladder, gonads, receive innervation from both parts of the autonomic system (they have, as they say, “double innervation”). Other tissues and organs, such as muscle arteries, receive only sympathetic innervation. In general, we can say that the two departments work alternately: depending on the activity of the body and on the commands of higher vegetative centers first one, then the other dominates.
This characterization, however, is not entirely correct. Both systems are constantly in a state of varying degrees of activity. The fact that target organs such as the heart or iris can respond to impulses from both parts simply reflects their complementary roles. For example, when you are very angry, your blood pressure rises, which excites the corresponding receptors located in the carotid arteries. These signals are perceived by the integrating center of cardio-vascular system, located in the lower part of the brain stem and known as nuclei of the solitary tract. Excitation of this center activates preganglionic parasympathetic fibers vagus nerve, which leads to a decrease in the frequency and strength of heart contractions. At the same time, under the influence of the same coordinating vascular center, sympathetic activity is suppressed, counteracting the increase in blood pressure.
How important is the functioning of each department for adaptive reactions? Surprisingly, not only animals, but also people can tolerate almost complete shutdown of the sympathetic nervous system without visible bad consequences. This switch-off is recommended for some forms of persistent hypertension.
But it’s not so easy to do without the parasympathetic nervous system. People who have undergone such an operation and find themselves outside the protective conditions of a hospital or laboratory adapt very poorly to the environment. They cannot regulate body temperature when exposed to heat or cold; when they lose blood, their blood pressure regulation is disrupted, and fatigue quickly develops with any intense muscle activity.
Diffuse nervous system of the intestine
Recent research has revealed the existence of a third important division of the autonomic nervous system - diffuse nervous system of the intestine. This department is responsible for the innervation and coordination of the digestive organs. Its work is independent of the sympathetic and parasympathetic systems, but can be modified under their influence. This is an additional link that connects the autonomic postganglionic nerves with the glands and muscles gastrointestinal tract.
The ganglia of this system innervate the intestinal walls. Axons from these ganglion cells cause contractions of the circular and longitudinal muscles that push food through the gastrointestinal tract, a process called peristalsis. Thus, these ganglia determine the characteristics of local peristaltic movements. When the food mass is inside the intestine, it slightly stretches its walls, which causes a narrowing of the area located slightly higher along the intestine and relaxation of the area located just below. As a result, the food mass is pushed further. However, under the influence of parasympathetic or sympathetic nerves, the activity of the intestinal ganglia can change. Activation of the parasympathetic system increases peristalsis, and the sympathetic system weakens it.
The mediator that excites the smooth muscles of the intestine is acetylcholine. However, the inhibitory signals leading to relaxation appear to be transmitted by a variety of substances, of which only a few have been studied. Among the intestinal neurotransmitters, there are at least three that also act in the central nervous system: somatostatin(see below), endorphins and substance P (see Chapter 6).
Central regulation of the functions of the autonomic nervous system
The central nervous system exerts much less control over the autonomic system than over the sensory or skeletal system motor system. The areas of the brain most associated with autonomic functions are hypothalamus And brain stem, especially that part of it that is located directly above the spinal cord - medulla. It is from these areas that the main pathways to the sympathetic and parasympathetic preganglionic autonomic neurons at the spinal level come.
Hypothalamus. The hypothalamus is one of the regions of the brain, the general structure and organization of which is more or less similar in representatives of different classes of vertebrates.
In general, it is generally accepted that the hypothalamus is the focus of visceral integrative functions. Signals from the neural systems of the hypothalamus directly enter networks that excite the preganglionic portions of the autonomic nerve pathways. In addition, this region of the brain exercises direct control over the entire endocrine system through specific neurons that regulate the secretion of hormones from the anterior pituitary gland, and the axons of other hypothalamic neurons terminate in the posterior pituitary gland. Here these endings release mediators that circulate in the blood as hormones: 1) vasopressin, which increases blood pressure in emergency cases when fluid or blood loss occurs; it also reduces the excretion of water in the urine (this is why vasopressin is also called antidiuretic hormone); 2) oxytocin, stimulating uterine contractions at the final stage of labor.
Although there are several clearly demarcated nuclei among the clusters of hypothalamic neurons, most of the hypothalamus is a collection of zones with blurred boundaries (Fig. 65). However, in three zones there are quite pronounced nuclei. We will now consider the functions of these structures.
1. Periventricular zone directly adjacent to the third cerebral ventricle, which passes through the center of the hypothalamus. The cells lining the ventricle convey information to the neurons of the periventricular zone about important internal parameters that may require regulation, for example, temperature, salt concentration, levels of hormones secreted thyroid gland, adrenal glands or gonads in accordance with instructions from the pituitary gland.
2. Medial zone contains most of the pathways through which the hypothalamus exerts endocrine control through the pituitary gland. Very roughly, we can say that the cells of the periventricular zone control the actual execution of commands given to the pituitary gland by the cells of the medial zone.
3. Through cells lateral zone The hypothalamus is controlled by higher levels of the cerebral cortex and limbic system. It also receives sensory information from the centers of the medulla oblongata, which coordinate respiratory and cardiovascular activity. The lateral zone is the place where higher brain centers can make adjustments to the hypothalamus' reactions to changes in the internal environment. In the cortex, for example, there is a comparison of information coming from two sources - the internal and external environment. If, say, the cortex judges that the time and circumstances are inappropriate for eating, the sensory report of low blood sugar and an empty stomach will be put aside until a more favorable moment. The hypothalamus is less likely to be ignored by the limbic system. Rather, this system may add emotional and motivational overtones to the interpretation of external sensory signals or compare the representation of the environment based on these signals with similar situations that occurred in the past.
Rice. 65. Hypothalamus and pituitary gland. The main functional areas of the hypothalamus are shown schematically.
Together with the cortical and limbic components, the hypothalamus also performs many routine integrating actions, and over much longer periods of time than when carrying out short-term regulatory functions. The hypothalamus “knows” in advance what needs the body will have during the normal daily rhythm of life. For example, it brings the endocrine system into full readiness for action as soon as we wake up. He also monitors the hormonal activity of the ovaries throughout menstrual cycle; takes measures to prepare the uterus for the arrival of a fertilized egg. In migratory birds and hibernating mammals, the hypothalamus, with its ability to determine the length of daylight hours, coordinates the body's vital functions during cycles lasting several months. (About these aspects of centralized regulation internal functions will be discussed in chapters 5 and 6.)
Rice. 66.Here is a schematic representation of the various functions of the medulla oblongata. Shows connections coming from various internal organs to the brain stem and reticular formation. Sensory signals emanating from these organs regulate the degree of activity and attention with which the brain responds to external events. Such signals also trigger specific behavioral programs with the help of which the body adapts to changes in the internal environment.
Medulla. The hypothalamus makes up less than 5% of the total brain mass. However, this small amount of tissue contains centers that support all body functions, with the exception of spontaneous breathing movements, regulation of blood pressure and heart rhythm. These latter functions depend on the medulla oblongata (see Fig. 66). In traumatic brain injury, so-called “brain death” occurs when all signs of electrical activity in the cortex disappear and control by the hypothalamus and medulla oblongata is lost, although with the help of artificial respiration It is still possible to maintain sufficient saturation of the circulating blood with oxygen.
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Autonomic nervous system(from lat. vegetatio - excitement, from lat. vegetativus - plant), VNS, autonomic nervous system, ganglion nervous system(from Lat. ganglion - nerve ganglion), visceral nervous system (from Lat. viscera - insides), organ nervous system, splanchnic nervous system, systema nervosum autonomicum(PNA) is part of the body’s nervous system, a complex of central and peripheral cellular structures that regulate the functional level of the body, necessary for the adequate response of all its systems.
The autonomic nervous system is a section of the nervous system that regulates the activity of internal organs, endocrine and exocrine glands, blood and lymphatic vessels. Plays a leading role in maintaining the constancy of the internal environment of the body and in the adaptive reactions of all vertebrates.
Anatomically and functionally, the autonomic nervous system is divided into sympathetic, parasympathetic and metasympathetic. The sympathetic and parasympathetic centers are under the control of the cortex cerebral hemispheres and hypothalamic centers.
The sympathetic and parasympathetic divisions have central and peripheral parts. The central part is formed by the bodies of neurons lying in the spinal cord and brain. These clusters of nerve cells are called vegetative nuclei. Fibers extending from the nuclei, autonomic ganglia lying outside the central nervous system, and nerve plexuses in the walls of internal organs form the peripheral part of the autonomic nervous system.
The sympathetic nuclei are located in the spinal cord. Departing from him nerve fibers end outside the spinal cord in the sympathetic ganglia, from which the nerve fibers originate. These fibers are suitable for all organs.
The parasympathetic nuclei lie in the midbrain and medulla oblongata and in the sacral part of the spinal cord. Nerve fibers from the nuclei of the medulla oblongata are part of the vagus nerves. From the nuclei of the sacral part, nerve fibers go to the intestines and excretory organs.
The sympathetic nervous system enhances metabolism, increases the excitability of most tissues, and mobilizes the body's forces for vigorous activity. The parasympathetic system helps restore spent energy reserves and regulates the functioning of the body during sleep.
The organs of circulation, respiration, digestion, excretion, reproduction, as well as metabolism and growth are under the control of the autonomous system. In fact, the efferent section of the ANS carries out nervous regulation of the functions of all organs and tissues, except for skeletal muscles, which are controlled by the somatic nervous system.
Location of ganglia and structure of pathways
Neurons nuclei of the central part of the autonomic nervous system are the first efferent neurons on the way from the central nervous system (spinal cord and brain) to the innervated organ. The nerve fibers formed by the processes of these neurons are called prenodal (preganglionic) fibers, since they go to the nodes of the peripheral part of the autonomic nervous system and end with synapses on the cells of these nodes. Preganglionic fibers have a myelin sheath, which makes them whitish in color. They leave the brain as part of the roots of the corresponding cranial nerves and the anterior roots of the spinal nerves.
Reflex arc
The structure of the reflex arcs of the autonomic part differs from the structure of the reflex arcs of the somatic part of the nervous system. In the reflex arc of the autonomic part of the nervous system, the efferent link consists not of one neuron, but of two, one of which is located outside the central nervous system. In general, a simple autonomic reflex arc is represented by three neurons.
First link reflex arc is a sensitive neuron, the body of which is located in the spinal ganglia and in the sensory ganglia of the cranial nerves. The peripheral process of such a neuron, which has a sensitive ending - a receptor, originates in organs and tissues. The central process, as part of the dorsal roots of the spinal nerves or sensory roots of the cranial nerves, is directed to the corresponding nuclei in the spinal cord or brain.
The second link of the reflex arc is efferent, since it carries impulses from the spinal cord or brain to the working organ. This efferent pathway of the autonomic reflex arc is represented by two neurons. The first of these neurons, the second in a simple autonomic reflex arc, is located in the autonomic nuclei of the central nervous system. It can be called intercalary, since it is located between the sensitive (afferent) link of the reflex arc and the second (efferent) neuron of the efferent pathway.
The effector neuron is the third neuron of the autonomic reflex arc. The bodies of effector (third) neurons lie in the peripheral nodes of the autonomic nervous system (sympathetic trunk, autonomic ganglia of cranial nerves, nodes of extraorgan and intraorgan autonomic plexuses). The processes of these neurons are directed to organs and tissues as part of organ autonomic or mixed nerves. Postganglionic nerve fibers end on smooth muscles, glands and other tissues with the corresponding terminal nerve apparatus.
Physiology
General importance of autonomic regulation
The autonomic nervous system adapts the functioning of internal organs to changes environment. The ANS ensures homeostasis (constancy of the internal environment of the body). The ANS is also involved in many behavioral acts carried out under the control of the brain, influencing not only physical, but also mental activity of a person.
The role of the sympathetic and parasympathetic departments
The sympathetic nervous system is activated during stress reactions. It is characterized by a generalized effect, with sympathetic fibers innervating the vast majority of organs.
It is known that parasympathetic stimulation of some organs has an inhibitory effect, while others have an exciting effect. In most cases, the action of the parasympathetic and sympathetic systems is opposite.
The influence of the sympathetic and parasympathetic departments on individual organs
Influence of the sympathetic department:
Influence of the parasympathetic department:
- On the heart - reduces the frequency and strength of heart contractions.
- On arteries - does not affect most organs, causes dilation of the arteries of the genitals and brain, narrowing coronary arteries and pulmonary arteries.
- On the intestines - enhances intestinal motility and stimulates the production of digestive enzymes.
- On the salivary glands - stimulates salivation.
- On the bladder - contracts the bladder.
- On the bronchi and breathing - narrows the bronchi and bronchioles, reduces ventilation of the lungs.
- On the pupil - constricts the pupils.
Neurotransmitters and cellular receptors
The sympathetic and parasympathetic departments have different, in some cases opposite, effects on various organs and tissues, and also cross-influence each other. The different effects of these sections on the same cells are associated with the specificity of the neurotransmitters they secrete and with the specificity of the receptors present on the presynaptic and postsynaptic membranes of neurons of the autonomic system and their target cells.
Preganglionic neurons of both parts of the autonomic system secrete acetylcholine as the main neurotransmitter, which acts on nicotinic acetylcholine receptors on the postsynaptic membrane of postganglionic (effector) neurons. Postganglionic neurons of the sympathetic department, as a rule, release norepinephrine as a transmitter, which acts on adrenergic receptors of target cells. On the target cells of sympathetic neurons, beta-1 and alpha-1 adrenergic receptors are mainly concentrated on the postsynaptic membranes (meaning that in vivo they are mainly affected by norepinephrine), and al-2 and beta-2 receptors are on extrasynaptic areas of the membrane (they are mainly affected by blood adrenaline). Only some postganglionic neurons of the sympathetic division (for example, those acting on the sweat glands) release acetylcholine.
Postganglionic neurons of the parasympathetic division release acetylcholine, which acts on muscarinic receptors on target cells.
On the presynaptic membrane of postganglionic neurons of the sympathetic division, two types of adrenergic receptors predominate: alpha-2 and beta-2 adrenergic receptors. In addition, the membrane of these neurons contains receptors for purine and pyrimidine nucleotides (P2X ATP receptors, etc.), nicotinic and muscarinic cholinergic receptors, neuropeptide and prostaglandin receptors, and opioid receptors.
When norepinephrine or blood adrenaline acts on alpha-2 adrenoreceptors, the intracellular concentration of Ca 2+ ions drops, and the release of norepinephrine at the synapses is blocked. A negative feedback loop arises. Alpha-2 receptors are more sensitive to norepinephrine than to epinephrine.
When norepinephrine and epinephrine act on beta-2 adrenergic receptors, the release of norepinephrine usually increases. This effect is observed during normal interaction with the G s protein, during which the intracellular concentration of cAMP increases. Beta two receptors are more sensitive to adrenaline. Since adrenaline is released from the adrenal medulla under the influence of norepinephrine from the sympathetic nerves, a positive feedback loop occurs.
However, in some cases, activation of beta-2 receptors can block the release of norepinephrine. It has been shown that this may be a consequence of the interaction of beta-2 receptors with G i / o proteins and their binding (sequestration) of G s proteins, which, in turn, prevents the interaction of G s proteins with other receptors.
When acetylcholine acts on muscarinic receptors of sympathetic neurons, the release of norepinephrine in their synapses is blocked, and when it acts on nicotinic receptors, it is stimulated. Because muscarinic receptors predominate on the presynaptic membranes of sympathetic neurons, activation of the parasympathetic nerves typically reduces the level of norepinephrine released from the sympathetic nerves.
Alpha-2 adrenergic receptors predominate on the presynaptic membranes of postganglionic neurons of the parasympathetic department. When norepinephrine acts on them, the release of acetylcholine is blocked. Thus, the sympathetic and parasympathetic nerves mutually inhibit each other.
Development in embryogenesis
- Development of the peripheral (somatic) and autonomic nervous system. The peripheral (somatic) and autonomic nervous system develops from the outer germ layer - the ectoderm. Cranial and spinal nerves in the fetus are formed very early (5-6 weeks). Myelination of nerve fibers occurs later (for the vestibular nerve - 4 months; for most nerves - at 6-7 months).
Spinal and peripheral autonomic ganglia are formed simultaneously with the development of the spinal cord. The starting material for them is the cellular elements of the ganglion plate, its neuroblasts and glioblasts, from which the cellular elements of the spinal ganglia are formed. Some of them are shifted to the periphery to the localization of the autonomic nerve ganglia
Comparative anatomy and evolution of the autonomic nervous system
Insects have a so-called sympathetic or stomodeal nervous system. It includes the frontal ganglion, which is located in front of the brain and is connected by paired connectives to the tritocerebrum. An unpaired frontal nerve departs from it, stretching along the dorsal side of the pharynx and esophagus. This nerve connects to several nerve ganglia; the nerves extending from them innervate the foregut, salivary glands and aorta.
The somatic and autonomic nervous systems are two equal parts of the general nervous system. The first of them covers those parts that innervate the skeletal muscles and sensory organs.
First, information about the state of the internal and external environment is received from the receptors. It is selected and carefully processed. And on the basis of this data, a specific movement program is selected that will satisfy the needs to the maximum and will contribute to achieving the goal. The autonomic nervous system is responsible for controlling the activity of glands, internal organs, lymphatic and blood vessels, and some muscles. It is also called involuntary, since all the functions that it controls cannot be called or stopped on purpose. The autonomic nervous system is divided into two types: sympathetic and parasympathetic. This division is to a certain extent arbitrary, but nevertheless it exists. Each of them performs its own functions. And their actions are controlled by the central vegetative apparatus. Their location is the brain.
Autonomic nervous system: sympathetic division
The central part of the spinal cord is located. And the components of the peripheral part are nerve fibers and sympathetic nerve ganglia. Together with the spinal nerves (their anterior roots), they exit the spinal cord. From there they are sent to the corresponding nodes of the nervous system. There they switch to its other neurons. These processes innervate their corresponding organs.
Autonomic nervous system: parasympathetic division
Its central part is located in the nuclei of both the midbrain and medulla oblongata, as well as in the spinal cord (in the area of the spine). And the peripheral part of this department consists of the internal sacral nerves, as well as nodes and fibers that enter the cranial nerves (but not all). The axons of the first neurons end in the parasympathetic nerve ganglia. They are located directly near the organs that they innervate, or even inside.
Autonomic nervous system: role
Its main purpose is to act so that the internal environment of the human body remains stable. At the same time, its sympathetic department intensifies functioning in conditions that require the mobilization of physical forces. The parasympathetic one ensures the restoration of resources expended during hard work. Most organs are innervated by both departments, which act on them from both sides. So, the sympathetic department, for example, dilates the pupils, inhibits the secretion of the gastric glands, and intestinal motility. But the parasympathetic does exactly the opposite. It constricts the pupils, slows the heart rate, and stimulates peristalsis. Both sections of this system always work harmoniously thanks to its centers, which are located in the subcortical structures of the nervous system. And the regulation of all functions, the highest control over them, is exercised directly by the cerebral cortex.
AUTONOMIC NERVOUS SYSTEM, part of the nervous system of vertebrates and humans, which regulates the activities of the circulatory, digestive, respiratory, excretory, reproduction, metabolism and growth of the body; plays a leading role in maintaining homeostasis and in the adaptive reactions of the body. The term “autonomic nervous system” was introduced in 1800 by M. Bichat, based on the fact that this part of the nervous system regulates processes characteristic not only of animals, but also of other organisms. Since the functions of the autonomic nervous system cannot be voluntarily evoked or deliberately terminated, the English physiologist J. Langley called it autonomous.
Anatomically and functionally, the autonomic nervous system is divided into the sympathetic nervous system (SNS), the parasympathetic nervous system (PNS), and the metasympathetic nervous system (MNS). In the SNS and PNS, the efferent pathways emanating from the central nervous system (CNS) consist of two neurons connected in series. The cell bodies of the first neurons of the SNS lie in the thoracic and lumbar regions spinal cord, and the PNS - in the midbrain and medulla oblongata and in the sacral spinal cord. The second neurons (located outside the CNS) form ganglia near the spine, on the way to the organs (in the SNS), near the innervated organ or directly in it (in the PNS). The influence of the PNS on the functioning of many organs (heart, kidneys, etc.) is provided mainly through the vagus nerve. Nerve fibers of the autonomic nervous system are characterized by a low speed of signal transmission compared to the central nervous system. In the ganglia of the SNS and PNS, acetylcholine serves as a signal transmitter; it is also released from postganglionic fibers of the PNS. In the SNS, norepinephrine (rarely acetylcholine) plays this role. Other neurotransmitters may be used in conjunction with norepinephrine and acetylcholine.
The influence of the SNS and PNS on organs is often opposite. Thus, activation of the SNS leads to dilation of the bronchi, an increase in the strength and frequency of heart contractions, dilation of the pupils, inhibition of gastrointestinal motility and secretion of digestive juices, relaxation of the bladder, and activation of the PNS causes the opposite effect. The SNS and PNS are characterized by tonic (maintained) activity: for example, an increase in heart rate can be achieved by activation of the SNS or inhibition of the PNS. The effects may have the same direction, but differ in the characteristics of their manifestation: for example, the PNS causes abundant secretion of liquid saliva, and the SNS causes moderate secretion of viscous saliva. For a number of functions, the effects of the two departments may be additive; Thus, the PNS stimulates erection, and the SNS stimulates ejaculation. Some functions are regulated only by the PNS (for example, the work of the lacrimal glands) or the SNS (the breakdown of glycogen and fats, increasing the performance of skeletal muscles, the work of sweat glands). In many organs (except the brain, tongue, digestive glands, genitals), vascular tone is also maintained only by the SNS. In general, the PNS is responsible for restoring the resources expended by the body, and the SNS ensures its adaptation to extreme conditions.
The MNS (the term was introduced by A.D. Nozdrachev) innervates internal organs endowed with their own motor activity: the stomach and intestines (Auerbach’s plexus, Meissner’s plexus), bladder, heart, etc. It has its own sensory and interneurons and is extremely diverse in its set of mediators . After damage to the MHC, organs lose the ability to coordinate rhythmic contractions.
The work of the MNS is autonomous, but is regulated by the SNS and PNS. The activity of the SNS and PNS is controlled by the nerve centers (respiratory, cardiovascular, salivary, etc.), which are located in the medulla oblongata. At this level, the work of centers can change reflexively and independently of others. Such reflexes are under the control of the hypothalamus. Signals coming from the cerebral cortex also change the activity of the autonomic nervous system, which ensures the body's holistic response to stimuli.
The parts of the nervous system that coordinate the work of internal organs in invertebrates are called visceral. Their elements are found in lower worms as formations associated with the intestinal tube, and starting with nemerteans and annelids, independent ganglia are formed. In arthropods, a system of ganglia and nerve trunks leading to the heart and stomach muscles is quite clearly identified, but only in insects are there separate head and tail sections, sometimes compared to the PNS of vertebrates, and a trunk section, comparable to the SNS.
Lit.: Nozdrachev A.D. Physiology of the autonomic nervous system. L., 1983.
O. L. Vinogradova, O. S. Tarasova.
It is the material basis of thinking and speech. In a single nervous system, it is customary to distinguish between the central nervous system (CNS), which includes the spinal cord and brain, and the peripheral nervous system, formed by nerves connecting the brain and spinal cord with all organs.
Functional division of the nervous system
In functional terms, the nervous system is divided into somatic and autonomic. The somatic nervous system perceives irritations from the external environment and regulates the functioning of skeletal muscles, i.e. responsible for the movements of the body and its movement in space. The autonomic nervous system (ANS) regulates the functions of all internal organs, glands and blood vessels, and its activity is practically independent of human consciousness, which is why it is also called autonomous.
The nervous system is a huge collection of neurons (nerve cells), consisting of a body and processes. With the help of processes, neurons connect with each other and with innervated organs. Any information from the external environment or from the body and internal organs is transmitted along chains of neurons to the nerve centers of the central nervous system in the form of a nerve impulse. After analysis in the nerve centers, the corresponding commands are also sent along chains of neurons to the working bodies for implementation required action, for example, contraction of skeletal muscles or increased production of juices digestive glands. The transmission of a nerve impulse from one neuron to another or to an organ occurs at synapses (translated from Greek as connection) with the help of special chemical substances- mediators. The nerves connecting the central nervous system and organs are large clusters of neuronal processes (nerve fibers) surrounded by special sheaths.
Differences between the autonomic and somatic nervous systems
Although the autonomic and somatic nervous systems have a common origin, not only functional but also structural differences have been established between them. Thus, somatic nerves emerge from the brain and spinal cord evenly throughout their entire length, while autonomic nerves emerge only from several sections. Somatic motor nerves go from the central nervous system to the organs without interruption, while autonomic ones are interrupted in the ganglia (nerve nodes), and therefore their entire path to the organ is usually divided into preganglionic (prenodal) and postganglionic (postnodal) fibers. In addition, autonomic nerve fibers are thinner than somatic ones, since they lack a special sheath that increases the speed of nerve impulse transmission.
When the autonomic nerves are excited, the effect occurs slowly, lasts a long time and disappears gradually, causing a monotonous calm rhythm of the internal organs. The speed of nerve impulse transmission through somatic nerves is tens of times higher, which ensures fast and expedient movements of skeletal muscles. In many cases, impulses from the internal organs, bypassing the central nervous system, are sent directly to the autonomic ganglion, which contributes to the autonomy of the functioning of the internal organs.
The role of the autonomic nervous system
The ANS provides regulation of the activity of internal organs, which include smooth muscle and glandular tissue. Such organs include all organs of the digestive, respiratory, urinary, reproductive systems, heart and blood vessels (blood and lymphatic), and endocrine glands. The ANS also takes part in the work of skeletal muscles, regulating metabolism in the muscles. The role of the ANS is to maintain a certain level of functioning of organs, to strengthen or weaken their specific activity depending on the needs of the body. In this regard, the ANS has two parts (sympathetic and parasympathetic), which have opposite effects on the organs.
Structure of the autonomic nervous system
There are also differences in the structure of the two parts of the ANS. The centers of its sympathetic part are located in the thoracic and lumbar parts of the spinal cord, and the centers of the parasympathetic part are in the brain stem and sacral part of the spinal cord (see figure).
The highest centers that regulate and coordinate the work of both parts of the ANS are the hypothalamus and the cortex of the frontal and parietal lobes of the cerebral hemispheres. Autonomic nerve fibers emerge from the brain and spinal cord as part of the cranial and spinal nerves and are directed to the autonomic ganglia. The ganglia of the sympathetic part of the ANS are located near the spine, and the parasympathetic part is located in the walls of the internal organs or near them. Therefore, the preganglionic and postganglionic sympathetic fibers are almost the same length, and the parasympathetic preganglionic fiber is much longer than the postganglionic fiber. After passing the ganglion, autonomic fibers, as a rule, are directed to the innervated organ along with blood vessels, forming plexuses in the form of a network on the wall of the vessel.
The paravertebral ganglia of the sympathetic part of the ANS are combined into two chains, which are located symmetrically on both sides of the spinal column and are called sympathetic trunks. In each sympathetic trunk, consisting of 20-25 ganglia, the cervical, thoracic, lumbar, sacral and coccygeal sections are distinguished.
From the 3 cervical ganglia of the sympathetic trunk, nerves arise that regulate the activity of the organs of the head and neck, as well as the heart. These nerves form plexuses on the wall of the carotid arteries and, together with their branches, reach the lacrimal gland and salivary glands, glands of the mucous membrane of the oral and nasal cavities, larynx, pharynx and muscle that dilates the pupil. The cardiac nerves, arising from the cervical ganglia, descend into the chest cavity and form a plexus on the surface of the heart.
From 10-12 thoracic ganglia of the sympathetic trunk, nerves extend to the organs chest cavity(heart, esophagus, lungs), as well as the large and small splanchnic nerves heading into the abdominal cavity to the ganglia of the celiac (solar) plexus. The solar plexus is formed by autonomic ganglia and numerous nerves and is located in front abdominal aorta on the sides of its large branches. The celiac plexus supplies the innervation of the abdominal organs - the stomach, small intestine, liver, kidneys, pancreas.
From the 4 lumbar ganglia of the sympathetic trunk depart the nerves involved in the formation of the celiac plexus and other autonomic plexuses of the abdominal cavity, which provide sympathetic innervation to the intestines and blood vessels.
The sacrococcygeal section of the sympathetic trunk consists of lying on inner surface sacrum and coccyx, four sacral ganglia and one unpaired coccygeal ganglia. Their branches participate in the formation of the vegetative plexuses of the pelvis, which provide sympathetic innervation to the organs and vessels of the pelvis (rectum, bladder, internal genital organs), as well as the external genitalia.
Nerve fibers of the parasympathetic part of the ANS exit the brain in composition III, VII, IX and X cranial nerves (in total, 12 pairs of cranial nerves depart from the brain), and from the spinal cord as part of the II-IV sacral nerves. The parasympathetic ganglia in the head area are located near the glands. Postganglionic fibers are sent to the organs of the head along the branches of the trigeminal nerve (V cranial nerve). Parasympathetic innervation is received by the lacrimal and salivary glands, glands of the mucous membrane of the oral and nasal cavities, as well as the muscle that constricts the pupil and the ciliary muscle (provides accommodation - adapting the eye to seeing objects at different distances).
The most a large number of parasympathetic fibers pass as part of the vagus nerve (X cranial nerve). The branches of the vagus nerve innervate the internal organs of the neck, chest and abdominal cavities - the larynx, trachea, bronchi, lungs, heart, esophagus, stomach, liver, spleen, kidneys and most of the intestines. In the chest and abdominal cavities, the branches of the vagus nerve are part of the autonomic plexuses (in particular, the celiac plexus) and together with them reach the innervated organs. Pelvic organs receive parasympathetic innervation from the splanchnic pelvic nerves emerging from sacral region spinal cord. Parasympathetic ganglia are located in or near the wall of the organ.
The importance of the autonomic nervous system
The activity of most internal organs is regulated by both parts of the ANS, which, as already noted, have different, sometimes opposite, effects due to the action of mediators.
The main transmitter of the sympathetic part of the ANS is norepinephrine, and the parasympathetic part is acetylcholine. The sympathetic part of the ANS mainly provides activation of trophic functions (increased metabolic processes, breathing, cardiac activity), and parasympathetic - their inhibition (decrease in heart rate, decrease in respiratory movements, bowel movements, bladder, etc.). Irritation of the sympathetic nerves causes dilation of the pupils, bronchi, arteries of the heart, increased heart rate and intensification, but inhibition of intestinal motility, suppression of the secretion of glands (except sweat glands), narrowing of skin vessels and abdominal vessels.
Irritation of the parasympathetic nerves leads to constriction of the pupils, bronchi, arteries of the heart, slowing and weakening of the heartbeat, but increased intestinal motility and opening of the sphincters, increased secretion of the glands, and dilation of peripheral vessels.
In general, the sympathetic portion of the ANS is associated with the body's fight-or-flight response, which increases the delivery of oxygen and nutrients to the muscles and heart, causing them to contract more. The predominance of activity of the parasympathetic part of the ANS causes reactions such as “rest and recovery,” which leads to the accumulation of vitality by the body. Normally, body functions are ensured by the coordinated action of both parts of the ANS, which is controlled by the brain.