From preganglionic neurons to postganglionic and from them to effector organs, excitation is transmitted through mediators. Mechanisms of mediator transmission in autonomic synapses nervous system in general, the same as in the neuromuscular plate and central synapses, but the nature of the synapses of the autonomic nervous system, their variability and density will be different. There is also a specificity in mediator transmission in the ganglia. At the endings of all preganglionic parasympathetic fibers, the mediator acetylcholine is released. Acetylcholine acts on receptors on the postsynaptic membrane and causes excitation of postganglionic fibers. Since ganglionic transmission was first reproduced using nicotine, the corresponding receptors were called nicotine-like (N-cholinergic receptors).
In the ganglia, to which the preganglionic sympathetic fibers are connected, mediator transmission is reproduced by both acetylcholine and norepinephrine. The validity of this provision is confirmed by experiments with the use of ganglionic blockers. Under the influence of benzohexonium, pyrilene, temekhin, higronium, blockade of H-cholinergic receptors occurs, under the influence of obzidan, prazosin - adrenoreceptors, which leads to inhibition of the transmission of nervous excitation from preganglionic to postganglionic fibers of the autonomic nerves.
The action of postganglionic nerve fibers on the effector is ensured by the release of mediators into the synaptic cleft, which affect the postsynaptic membrane - the membrane of the cell of the working organ. Postganglionic parasympathetic fibers secrete acetylcholine, which binds to M-cholinergic receptors, i.e. muscari-
but similar to receptors (M - XP). M-XR blockers that prevent parasympathetic effects are atropine, scopolamine, platifillin.
Gyostganglionic sympathetic transmission of information is carried out with the participation of two types of receptors - a- and B-adrenergic receptors (AR). Blockade of ct-AP is carried out with the help of phentolamine, tropafen, etc.; blockade of B-AR - anaprilin (inderal, obzidan), etc., which inhibit the influence of the sympathetic nervous system.
Catecholamines are secreted not only by sympathetic nerve endings, but by the adrenal medulla. The adrenal glands (medulla), which are homologous to sympathetic postganglionic neurons, secrete into the blood mainly epinephrine (about 80%) and norepinephrine (20%).
Catecholamines of sympathetic nerve endings and adrenal glands act on adrenergic receptors. There are a1 and a2, B1 and B2-adrenergic receptors. Little is known about their molecular structure. Stimulation of a-AR causes constriction of blood vessels, contraction of the sphincters of the stomach, intestines, ureters, uterus, dilation of the pupils.
Stimulation of RgAR causes an increase in the frequency and strength of heart contractions, stimulates lipolysis, etc.. "
Activation of fb-AP causes expansion of some vessels (for example, coronary), relaxation of the muscles of the intestine, gallbladder, uterus, bronchial dilation, and glycogenolysis is enhanced.
The action of sympathetic adrenergic neurons is reproduced by sympathomimetic substances, or sympatholytic, which block their influence.
Most organs that respond to catecholamines contain both a- and p-adrenergic receptors, and the reaction of one or another organ depends on what prevails - a- or B-adrenergic receptors. More often, the effects of excitation of these types of receptors are opposite. So, the excitation of a-adrenergic receptors leads to a narrowing of the vessels of the skin and mucous membranes, and the excitation of p-adrenergic receptors leads to their expansion. Norepinephrine causes a strong excitation of B-adrenergic receptors of the myocardium, but slightly affects B-adrenergic receptors of the smooth muscles of the vessels of the bronchi and trachea.
In addition to acetylcholine and norepinephrine, ATP, substance P, angiotensin and other polypeptides, prostaglandin E, serotonin and histamine also belong to the mediators of the autonomic nervous system. In the peripheral part of the autonomic nervous system, pre- and postsynaptic receptors for dopamine, histamine (No and H2), opiates, angiotensin and other polypeptides, prostaglandin E were also identified.
Named cytoreceptors are very important for drug therapy. So, drugs that belong to p-blockers are widely used in cardiology practice.
(For the treatment

Sympathetic synapses are formed not only in the region of numerous terminal branches of the sympathetic nerve, as in all other nerve fibers, but also in membranes. varicose veins - numerous extensions of the peripheral sections of sympathetic fibers in the region of innervated tissues. Varicose veins also contain synaptic vesicles with a mediator, although in lower concentrations than the terminal endings.

The main mediator of sympathetic synapses is norepinephrine and such synapses are called adrenergic. The receptors that bind the adrenergic neurotransmitter are called adrenoreceptors. There are two types of adrenergic receptors - alpha And beta, each of which is divided into two subtypes - 1 and 2. A small part of the sympathetic synapses uses the mediator acetylcholine and such synapses are called cholinergic, and the receptors cholinergic receptors. Cholinergic synapses of the sympathetic nervous system are found in the sweat glands. In adrenergic synapses, in addition to norepinephrine, adrenaline and dopamine, also related to catecholamines, are contained in much smaller amounts, so the mediator substance in the form of a mixture of three compounds was previously called sympathin.

The action of postganglionic nerve fibers on the effector is ensured by the release of mediators into the synaptic cleft, which affect the postsynaptic membrane - the membrane of the cell of the working organ. Postganglionic parasympathetic fibers secrete acetylcholine, which binds to M-cholinergic receptors, i.e. muscarinic-like receptors (M-XR).

33. Parasympathetic synapse

Parasympathetic postganglionic or peripheral synapses use acetylcholine as a mediator, which is located in the axoplasm and synaptic vesicles of presynaptic terminals in three main pools or funds. This, Firstly, stable, strongly associated with the protein, not ready to release the mediator pool; Secondly, mobilization, less firmly bound and suitable for release, pool; third A ready to be deallocated spontaneously or actively allocated pool. In the presynaptic ending, pools are constantly moving in order to replenish the active pool, and this process is also carried out by moving synaptic vesicles to the presynaptic membrane, since the mediator of the active pool is contained in those vesicles that are directly adjacent to the membrane. The release of the mediator occurs in quanta, the spontaneous release of single quanta is replaced by an active one upon receipt of excitation impulses that depolarize the presynaptic membrane. The process of release of neurotransmitter quanta, as well as in other synapses, is calcium-dependent.

34. Reflexes of the autonomic nervous system

1) viscero-visceral, when both afferent and efferent links, i.e. the beginning and effect of the reflex refers to the internal organs or the internal environment (gastro-duodenal, gastrocardial, angiocardial, etc.);

2) viscero-somatic, when the reflex, which begins with irritation of the interoceptors, is realized in the form of a somatic effect due to the associative connections of the nerve centers. For example, when the chemoreceptors of the carotid sinus are irritated by an excess of carbon dioxide, the activity of the respiratory intercostal muscles increases and breathing becomes more frequent;

3) viscero-sensory, - change in sensory information from exteroceptors when stimulating interoceptors. For example, during oxygen starvation of the myocardium, there are so-called reflected pains in areas of the skin (Head's zones) that receive sensory conductors from the same segments of the spinal cord;

4) somato-visceral, when, with stimulation of the afferent inputs of the somatic reflex, the vegetative reflex is realized. For example, during thermal irritation of the skin, the skin vessels expand and the vessels of the organs narrow. abdominal cavity. Somatovegetative reflexes also include the Ashner-Dagnini reflex - a decrease in the pulse with pressure on the eyeballs.

Vegetative reflexes are also divided into segmental, those. implemented by the spinal cord and brain stem structures, and suprasegmental, the implementation of which is provided by the higher centers of autonomic regulation located in the suprasegmental structures of the brain.

Neurons are divided into three main types: afferent, efferent and intermediate. Afferent neurons(sensitive-gel, or centripetal) transmit information from receptors to the central nervous system. The bodies of these neurons located outside the central nervous system - in the spinal nodes and in the nodes cranial nerves. Afferent neuron they have a long process - a dendrite, which contacts on the periphery with a receiving formation - a receptor or forms a receptor itself, as well as a second process - an axon that enters the spinal cord through the posterior horns. Efferent neurons(centrifugal) are associated with the transfer of downward influences from the higher levels of the nervous system to the book-lying ones or to the working organs. For example, downward influences from pyramidal neurons cerebral cortex or from other
motor centers c.n.s. follow to the neurons of the spinal cord (motor neurons), from which fibers go to the skeletal muscles. In the lateral horns of the spinal cord there are cells of the autonomic nervous system, from which paths go to the internal organs. For efferent neurons characterized by a branched network of short processes - dendrites and one long process-axon. Interneurons (interneurons), or intercalary, are, as a rule, smaller cells that communicate between different (in particular, afferent and efferent) neurons. They transmit nerve influences in horizontal (for example, within one segment of the spinal cord) and in vertical directions (for example, from one segment of the spinal cord to other higher or lower segments). Due to the numerous branches of the axon intermediate neurons can simultaneously excite other neurons (such, for example, stellate cells of the cortex).

Autonomic ganglia subdivided into paravertebral, prevertebral and intramural. The first 2 types are characteristic of the sympathetic division of the autonomic nervous system, and the intramural ganglia are characteristic of the parasympathetic.

The autonomic ganglion has a connective tissue capsule and stroma. The ganglion contains multipolar neurons with eccentrically located rounded nuclei and large nucleoli. Multipolar neurons are motor neurons. They are surrounded by mantle glia, but it is located less densely than in the spinal ganglion. Paravertebral ganglia are located on both sides of the spine, forming sympathetic chains (sympathetic trunk). Prevertebral ganglia are in front of the aorta, forming the abdominal plexus, which consists of 3 types of nodes : celiac (solar), superior mesenteric, inferior mesenteric. Their multipolar neurons have many dendrites that branch profusely. Axons, on the other hand, form postganglionic non-myelinated fibers that go deep into the innervated organs and form axosomatic synapses there. Most of the neurons of the sympathetic ganglia contain catecholamines in small vesicles. The latter are detected by the Haik method. In addition to multipolar neurons, there are MYTH cells in the nerve ganglia, that is, small intensely fluorescent cells. They have a small perikaryon and short processes. MYTH cells secrete catecholamines, inhibit the transmission of impulses from preganglionic nerve fibers to peripheral neurons of the ganglion. The neurons of the intramural ganglia of the parasympathetic department are cholinergic. They are detected by the Kulle method (reaction to acetylcholine esterase). Intramural ganglia are located in the wall of organs and form plexuses, most clearly identified in the gastrointestinal tract: Meissner's submucosal plexus, Auerbach's intermuscular plexus, and Vorobyov's subserous plexus. The neurons of the intramural ganglia are heterogeneous. The classification of these neurons according to Dogel is presented below. Type I Dogel cell: long-axon effector neuron. The perikaryon is flattened, there are many short dendrites with an expanded base, 1 long axon. The axon terminates on target cells, such as smooth myocytes. Type II Dogel cell: Equal outgrowth afferent neurons (sensory). Pericaryon oval with smooth surface, processes are equal in length, axons form synapses with Type I Dogel cells, forming a local reflex arc. Type III Dogel cell: Associative neurons that contact neighboring ganglia. The bodies are oval or polygonal in shape, have 1 axon and many dendrites. These cells synthesize various neurotransmitters.

The vegetative (from Latin vegetare - to grow) activity of the body is understood as the work of internal organs, which provides energy and other components necessary for existence to all organs and tissues. IN late XIX century, the French physiologist Claude Bernard (Bernard C.) came to the conclusion that "the constancy of the internal environment of the body is the key to its free and independent life." As he noted back in 1878, the internal environment of the body is subject to strict control, keeping its parameters within certain limits. In 1929, the American physiologist Walter Cannon (Cannon W.) proposed to designate the relative constancy of the internal environment of the body and some physiological functions by the term homeostasis (Greek homoios - equal and stasis - state). There are two mechanisms for maintaining homeostasis: nervous and endocrine. This chapter will deal with the first of these.

11.1. autonomic nervous system

The autonomic nervous system innervates the smooth muscles of the internal organs, the heart and exocrine glands (digestive, sweat, etc.). Sometimes this part of the nervous system is called visceral (from Latin viscera - insides) and very often - autonomous. The last definition emphasizes an important feature of autonomic regulation: it occurs only reflexively, i.e. is not realized and is not subject to voluntary control, thereby fundamentally different from the somatic nervous system that innervates the skeletal muscles. In the English-language literature, the term autonomic nervous system is usually used, in the domestic literature it is often called the autonomic nervous system.

At the very end of the 19th century, the British physiologist John Langley (Langley J.) subdivided the autonomic nervous system into three sections: sympathetic, parasympathetic and enteral. This classification remains generally accepted at the present time (although in the domestic literature, the enteric section, consisting of neurons of the intermuscular and submucosal plexuses of the gastrointestinal tract, is quite often called metasympathetic). This chapter deals with the first two divisions of the autonomic nervous system. Cannon drew attention to their different functions: the sympathetic controls the reactions of fight or flight (in the English rhyming version: fight or flight), and the parasympathetic is necessary for rest and digestion of food (rest and digest). The Swiss physiologist Walter Hess (Hess W.) proposed calling the sympathetic department ergotropic, i.e. contributing to the mobilization of energy, intense activity, and the parasympathetic - trophotropic, i.e. regulating the nutrition of tissues, recovery processes.

11.2. Peripheral division of the autonomic nervous system

First of all, it should be noted that the peripheral part of the autonomic nervous system is exclusively efferent; it serves only to conduct excitation to effectors. If in the somatic nervous system only one neuron (motoneuron) is needed for this, then in the autonomic nervous system two neurons are used, connecting through a synapse in a special autonomic ganglion (Fig. 11.1).

The bodies of preganglionic neurons are located in the brainstem and spinal cord, and their axons go to the ganglia, where the bodies of postganglionic neurons are located. The working organs are innervated by axons of postganglionic neurons.

The sympathetic and parasympathetic divisions of the autonomic nervous system differ primarily in the location of the preganglionic neurons. The bodies of sympathetic neurons are located in the lateral horns of the thoracic and lumbar (two or three upper segments) sections. The preganglionic neurons of the parasympathetic division are, firstly, in the brainstem, from where the axons of these neurons emerge as part of four cranial nerves: oculomotor (III), facial (VII), glossopharyngeal (IX) and vagus (X). Second, parasympathetic preganglionic neurons are found in the sacral spinal cord (Fig. 11.2).

Sympathetic ganglia are usually divided into two types: paravertebral and prevertebral. Paravertebral ganglia form the so-called. sympathetic trunks, consisting of nodes connected by longitudinal fibers, which are located on both sides of the spine, extending from the base of the skull to the sacrum. In the sympathetic trunk, most axons of preganglionic neurons transmit excitation to postganglionic neurons. A smaller part of the preganglionic axons passes through the sympathetic trunk to the prevertebral ganglia: cervical, stellate, celiac, superior and inferior mesenteric - in these unpaired formations, as well as in the sympathetic trunk, there are sympathetic postganglionic neurons. In addition, part of the sympathetic preganglionic fibers innervates the adrenal medulla. The axons of preganglionic neurons are thin and, despite the fact that many of them are covered with a myelin sheath, the speed of excitation conduction along them is much less than along the axons of motor neurons.

In the ganglia, the fibers of the preganglionic axons branch and form synapses with the dendrites of many postganglionic neurons (a phenomenon of divergence), which, as a rule, are multipolar and have an average of about a dozen dendrites. There are on average about 100 postganglionic neurons per preganglionic sympathetic neuron. At the same time, in the sympathetic ganglia, the convergence of many preganglionic neurons to the same postganglionic neurons is also observed. Due to this, the summation of excitation occurs, which means that the reliability of signal transmission increases. Most of the sympathetic ganglia are located quite far from the innervated organs, and therefore the postganglionic neurons have rather long axons that are devoid of myelin coverage.

In the parasympathetic division, preganglionic neurons have long fibers, some of which are myelinated: they end near the innervated organs or in the organs themselves, where the parasympathetic ganglia are located. Therefore, in postganglionic neurons, the axons are short. The ratio of pre- and postganglionic neurons in the parasympathetic ganglia differs from the sympathetic ones: it is only 1: 2 here. Most internal organs have both sympathetic and parasympathetic innervation, an important exception to this rule is the smooth muscles of the blood vessels, which are regulated only by the sympathetic department. And only the arteries of the genital organs have a double innervation: both sympathetic and parasympathetic.

11.3. Autonomic nerve tone

Many autonomic neurons show background spontaneous activity, i.e. the ability to spontaneously generate action potentials at rest. This means that the organs innervated by them, in the absence of any irritation from the external or internal environment, still receive excitation, usually at a frequency of 0.1 to 4 impulses per second. This low frequency stimulation appears to maintain a constant slight contraction (tone) of the smooth muscles.

After cutting or pharmacological blockade of certain autonomic nerves, the innervated organs are deprived of their tonic influence and such a loss is immediately detected. Thus, for example, after unilateral transection of the sympathetic nerve that controls the vessels of the rabbit's ear, a sharp expansion of these vessels is detected, and after transection or blockade of the vagus nerves in the experimental animal, heart contractions become more frequent. Removing the blockade restores the normal heart rate. After cutting the nerves, the heart rate and vascular tone can be restored if the peripheral segments are artificially irritated with an electric current, choosing its parameters so that they are close to the natural rhythm of the impulse.

As a result of various influences on the vegetative centers (which is yet to be considered in this chapter), their tone may change. So, for example, if 2 impulses per second pass through the sympathetic nerves that control the smooth muscles of the arteries, then the width of the arteries is typical for a state of rest, and then normal blood pressure is recorded. If the tone of the sympathetic nerves increases and the frequency of nerve impulses entering the arteries increases, for example, up to 4-6 per second, then the smooth muscles of the vessels will contract more strongly, the lumen of the vessels will decrease, and blood pressure will increase. And vice versa: with a decrease in sympathetic tone, the frequency of impulses entering the arteries becomes less than usual, which leads to vasodilation and a decrease in blood pressure.

The tone of the autonomic nerves is extremely important in the regulation of the activity of internal organs. It is maintained due to the supply of afferent signals to the centers, the action of various components of cerebrospinal fluid and blood on them, as well as the coordinating influence of a number of brain structures, primarily the hypothalamus.

11.4. Afferent link of autonomic reflexes

Vegetative reactions can be observed upon stimulation of almost any receptive area, but most often they occur in connection with shifts in various parameters of the internal environment and activation of interoreceptors. For example, activation of mechanoreceptors located in the walls of hollow internal organs (blood vessels, digestive tract, bladder, etc.) occurs when pressure or volume changes in these organs. Excitation of the chemoreceptors of the aorta and carotid arteries occurs due to an increase in the arterial blood pressure of carbon dioxide or the concentration of hydrogen ions, as well as a decrease in oxygen tension. Osmoreceptors are activated depending on the concentration of salts in the blood or in the cerebrospinal fluid, glucoreceptors - depending on the concentration of glucose - any change in the parameters of the internal environment causes irritation of the corresponding receptors and a reflex reaction aimed at maintaining homeostasis. There are also pain receptors in the internal organs, which can be excited with a strong stretching or contraction of the walls of these organs, with their oxygen starvation, with inflammation.

Interoreceptors can belong to one of two types of sensory neurons. Firstly, they can be sensitive endings of neurons of the spinal ganglia, and then excitation from the receptors is carried out, as usual, to the spinal cord and then, with the help of intercalary cells, to the corresponding sympathetic and parasympathetic neurons. Switching of excitation from sensitive to intercalary, and then efferent neurons often occurs in certain segments of the spinal cord. With a segmental organization, the activity of the internal organs is controlled by autonomic neurons located in the same segments of the spinal cord, which receive afferent information from these organs.

Secondly, the propagation of signals from interoreceptors can be carried out along sensory fibers that are part of the autonomic nerves themselves. So, for example, most of the fibers that form the vagus, glossopharyngeal, and celiac nerves do not belong to vegetative, but to sensory neurons, whose bodies are located in the corresponding ganglia.

11.5. The nature of sympathetic and parasympathetic influence on the activity of internal organs

Most organs have a double, i.e. sympathetic and parasympathetic innervation. The tone of each of these sections of the autonomic nervous system can be balanced by the influence of another section, but in certain situations, increased activity is detected, the predominance of one of them, and then the true nature of the influence of this section appears. Such an isolated action can also be found in experiments with cutting or pharmacological blockade of sympathetic or parasympathetic nerves. After such an intervention, the activity of the working organs changes under the influence of the department of the autonomic nervous system that has retained its connection with it. Another way of experimental study is to alternately stimulate the sympathetic and parasympathetic nerves with specially selected parameters of the electric current - this simulates an increase in sympathetic or parasympathetic tone.

The influence of the two divisions of the autonomic nervous system on the controlled organs is most often opposite in the direction of shifts, which even gives reason to speak of the antagonistic nature of the relationship between the sympathetic and parasympathetic divisions. So, for example, when the sympathetic nerves that control the work of the heart are activated, the frequency and strength of its contractions increase, the excitability of the cells of the conduction system of the heart increases, and with an increase in the tone of the vagus nerves, opposite shifts are recorded: the frequency and strength of heart contractions decrease, the excitability of the elements of the conduction system decreases . Other examples of the opposite influence of sympathetic and parasympathetic nerves can be seen in table 11.1

Despite the fact that the influence of the sympathetic and parasympathetic divisions on many organs is opposite, they act as synergists, i.e. friendly. With an increase in the tone of one of these departments, the tone of the other decreases synchronously: this means that physiological shifts of any direction are due to coordinated changes in the activity of both departments.

11.6. Transmission of excitation in the synapses of the autonomic nervous system

In the vegetative ganglia of both the sympathetic and parasympathetic divisions, the mediator is the same substance - acetylcholine (Fig. 11.3). The same mediator serves as a chemical mediator for the transmission of excitation from parasympathetic postganglionic neurons to the working organs. The main mediator of sympathetic postganglionic neurons is norepinephrine.

Although the same mediator is used in the autonomic ganglia and in the transmission of excitation from parasympathetic postganglionic neurons to the working organs, the cholinergic receptors interacting with it are not the same. In the autonomic ganglia, nicotine-sensitive or H-cholinergic receptors interact with the mediator. If in the experiment the cells of the autonomic ganglia are moistened with a 0.5% solution of nicotine, then they cease to conduct excitation. The introduction of a nicotine solution into the blood of experimental animals leads to the same result, thereby creating a high concentration of this substance. At low concentrations, nicotine acts like acetylcholine, i.e. excites this type of cholinergic receptors. Such receptors are associated with ionotropic channels, and when they are excited, sodium channels of the postsynaptic membrane open.

Cholinergic receptors located in the working organs and interacting with acetylcholine of postganglionic neurons belong to a different type: they do not respond to nicotine, but they can be excited by a small amount of another alkaloid - muscarine or blocked by a high concentration of the same substance. Muscarin-sensitive or M-cholinergic receptors provide metabotropic control, which involves secondary messengers, and mediator-induced reactions develop more slowly and last longer than with ionotropic control.

The mediator of sympathetic postganglionic neurons, noradrenaline, can be bound by two types of metabotropic adrenoreceptors: a- or b, the ratio of which in different organs is not the same, which determines various physiological reactions to the action of norepinephrine. For example, β-adrenergic receptors predominate in the smooth muscles of the bronchi: the action of the mediator on them is accompanied by muscle relaxation, which leads to the expansion of the bronchi. In the smooth muscles of the arteries of the internal organs and skin, there are more a-adrenergic receptors, and here the muscles contract under the action of norepinephrine, which leads to a narrowing of these vessels. The secretion of sweat glands is controlled by special, cholinergic sympathetic neurons, the mediator of which is acetylcholine. There is also evidence that skeletal muscle arteries also innervate sympathetic cholinergic neurons. According to another point of view, skeletal muscle arteries are controlled by adrenergic neurons, and norepinephrine acts on them through a-adrenergic receptors. And the fact that during muscular work, which is always accompanied by an increase in sympathetic activity, the arteries of skeletal muscles expand, is explained by the action of the adrenal medulla hormone adrenaline on β-adrenergic receptors.

With sympathetic activation, adrenaline is released in large quantities from the adrenal medulla (attention should be paid to the innervation of the adrenal medulla by sympathetic preganglionic neurons), and also interacts with adrenoceptors. This enhances the sympathetic response, since the blood brings adrenaline to those cells near which there are no endings of sympathetic neurons. Norepinephrine and epinephrine stimulate the breakdown of glycogen in the liver and lipids in adipose tissue, acting there on b-adrenergic receptors. In the heart muscle, b-receptors are much more sensitive to norepinephrine than to adrenaline, while in the vessels and bronchi they are more easily activated by adrenaline. These differences formed the basis for the division of b-receptors into two types: b1 (in the heart) and b2 (in other organs).

Mediators of the autonomic nervous system can act not only on the postsynaptic, but also on the presynaptic membrane, where there are also corresponding receptors. Presynaptic receptors are used to regulate the amount of neurotransmitter released. For example, with an increased concentration of noradrenaline in the synaptic cleft, it acts on presynaptic a-receptors, which leads to a decrease in its further release from the presynaptic ending (negative feedback). If the concentration of the mediator in the synaptic cleft becomes low, predominantly b-receptors of the presynaptic membrane interact with it, and this leads to an increase in the release of norepinephrine (positive feedback).

By the same principle, i.e. with the participation of presynaptic receptors, the regulation of the release of acetylcholine is carried out. If the endings of sympathetic and parasympathetic postganglionic neurons are close to each other, then reciprocal influence of their mediators is possible. For example, the presynaptic endings of cholinergic neurons contain a-adrenergic receptors and, if norepinephrine acts on them, the release of acetylcholine will decrease. In the same way, acetylcholine can reduce the release of norepinephrine if it joins the M-cholinergic receptors of the adrenergic neuron. Thus, the sympathetic and parasympathetic divisions compete even at the level of postganglionic neurons.

A lot of drugs act on the transmission of excitation in the autonomic ganglia (ganglioblockers, a-blockers, b-blockers, etc.) and therefore are widely used in medical practice to correct various kinds of autonomic regulation disorders.

11.7. Centers of autonomic regulation of the spinal cord and trunk

Many preganglionic and postganglionic neurons are able to fire independently of each other. For example, some sympathetic neurons control sweating, while others control skin blood flow, some parasympathetic neurons increase the secretion of the salivary glands, and others increase the secretion of the glandular cells of the stomach. There are methods for detecting the activity of postganglionic neurons that make it possible to distinguish vasoconstrictor neurons in the skin from cholinergic neurons that control skeletal muscle vessels or from neurons that act on hairy muscles of the skin.

Topographically organized input of afferent fibers from different receptive areas to certain segments of the spinal cord or different areas of the trunk excites intercalary neurons, and they transmit excitation to preganglionic autonomic neurons, thus closing the reflex arc. Along with this, the autonomic nervous system is characterized by integrative activity, which is especially pronounced in the sympathetic department. Under certain circumstances, for example, when experiencing emotions, the activity of the entire sympathetic department can increase, and, accordingly, the activity of parasympathetic neurons decreases. In addition, the activity of autonomic neurons is consistent with the activity of motor neurons, on which the work of skeletal muscles depends, but their supply with the glucose and oxygen necessary for work is carried out under the control of the autonomic nervous system. The participation of vegetative neurons in integrative activity is provided by the vegetative centers of the spinal cord and trunk.

In the thoracic and lumbar regions of the spinal cord are the bodies of sympathetic preganglionic neurons, which form the intermediate-lateral, intercalary and small central autonomic nuclei. Sympathetic neurons that control the sweat glands, blood vessels of the skin, and skeletal muscles are located lateral to the neurons that regulate the activity of internal organs. By the same principle, parasympathetic neurons are located in the sacral spinal cord: laterally - innervating the bladder, medially - the large intestine. After separation of the spinal cord from the brain, vegetative neurons are able to discharge rhythmically: for example, sympathetic neurons of twelve segments of the spinal cord, united by intraspinal pathways, can, to a certain extent, reflexively regulate the tone of blood vessels. However, in spinal animals the number of discharged sympathetic neurons and the frequency of discharges are less than in intact ones. This means that the spinal cord neurons that control vascular tone are stimulated not only by the afferent input, but also by the centers of the brain.

The brain stem contains the vasomotor and respiratory centers, which rhythmically activate the sympathetic nuclei of the spinal cord. Afferent information from baro- and chemoreceptors is continuously supplied to the trunk, and, in accordance with its nature, autonomic centers determine changes in the tone of not only sympathetic, but also parasympathetic nerves that control, for example, the work of the heart. This is a reflex regulation, in which the motor neurons of the respiratory muscles are also involved - they are rhythmically activated by the respiratory center.

In the reticular formation of the brain stem, where the vegetative centers are located, several mediator systems are used that control the most important homeostatic indicators and are in difficult relationship between themselves. Here, some groups of neurons can stimulate the activity of others, inhibit the activity of others, and at the same time experience the influence of both of them on themselves. Along with the centers for regulating blood circulation and respiration, there are neurons here that coordinate many digestive reflexes: salivation and swallowing, secretion of gastric juice, gastric motility; a protective gag reflex can be mentioned separately. Different centers constantly coordinate their activities with each other: for example, when swallowing, the entrance to the respiratory tract reflexively closes and, thanks to this, inhalation is prevented. The activity of the stem centers subordinates the activity of the autonomic neurons of the spinal cord.

11. 8. The role of the hypothalamus in the regulation of autonomic functions

The hypothalamus accounts for less than 1% of the brain volume, but it plays a decisive role in the regulation of autonomic functions. This is due to several factors. First, the hypothalamus promptly receives information from interoreceptors, the signals from which come to it through the brainstem. Secondly, information comes here from the surface of the body and from a number of specialized sensory systems (visual, olfactory, auditory). Thirdly, some neurons of the hypothalamus have their own osmo-, thermo- and glucoreceptors (such receptors are called central). They can respond to shifts in osmotic pressure, temperature, and glucose levels in CSF and blood. In this regard, it should be recalled that in the hypothalamus, in comparison with the rest of the brain, the properties of the blood-brain barrier are manifested to a lesser extent. Fourth, the hypothalamus has bilateral connections with the limbic system of the brain, the reticular formation, and the cerebral cortex, which allows it to coordinate autonomic functions with certain behavior, for example, with the experience of emotions. Fifth, the hypothalamus forms projections on the vegetative centers of the trunk and spinal cord, which allows it to directly control the activity of these centers. Sixth, the hypothalamus controls the most important mechanisms of endocrine regulation (See Chapter 12).

The most important switching for autonomic regulation is carried out by neurons of the nuclei of the hypothalamus (Fig. 11.4), in different classifications they number from 16 to 48. In the 40s of the twentieth century, Walter Hess (Hess W.) consistently irritated different areas hypothalamus in experimental animals and found different combinations of vegetative and behavioral responses.

When the posterior region of the hypothalamus and the gray matter adjacent to the water supply were stimulated, the blood pressure in the experimental animals increased, the heart rate increased, breathing quickened and deepened, the pupils dilated, and the hair rose, the back curved in a hump and the teeth bared, i.e. vegetative shifts indicated the activation of the sympathetic department, and the behavior was affective-defensive. Irritation of the rostral parts of the hypothalamus and the preoptic region caused feeding behavior in the same animals: they began to eat, even if they were fed to the full, while salivation increased and motility of the stomach and intestines increased, while the heart rate and breathing decreased, and muscle blood flow also became smaller. , which is quite typical for an increase in parasympathetic tone. With a light hand of Hess, one region of the hypothalamus began to be called ergotropic, and the other - trophotropic; they are separated from each other by some 2-3 mm.

From these and many other studies, the idea gradually emerged that the activation of different areas of the hypothalamus triggers an already prepared complex of behavioral and autonomic reactions, which means that the role of the hypothalamus is to evaluate the information coming to it from different sources and, based on it, choose one or another option that combines behavior with a certain activity of both parts of the autonomic nervous system. The very same behavior can be considered in this situation as an activity aimed at preventing possible shifts in the internal environment. It should be noted that not only the deviations of homeostasis that have already occurred, but also any event potentially threatening homeostasis can activate the necessary activity of the hypothalamus. So, for example, with a sudden threat, vegetative changes in a person (an increase in the frequency of heart contractions, an increase in blood pressure, etc.) occur faster than he takes to flight, i.e. such shifts already take into account the nature of subsequent muscle activity.

The direct control of the tone of the autonomic centers, and hence the output activity of the autonomic nervous system, is carried out by the hypothalamus with the help of efferent connections with three most important areas (Fig. 11.5):

one). The nucleus of the solitary tract in the upper part of the medulla oblongata, which is the main recipient of sensory information from the internal organs. It interacts with the nucleus of the vagus nerve and other parasympathetic neurons and is involved in the control of temperature, circulation, and respiration. 2). Rostral ventral region of the medulla oblongata, which is crucial in increasing the overall output activity of the sympathetic division. This activity is manifested in an increase in blood pressure, an increase in the heart rate, secretion of sweat glands, dilation of the pupils and contraction of the muscles that raise the hair. 3). Autonomic neurons of the spinal cord, which can be directly influenced by the hypothalamus.

11.9. Vegetative mechanisms of blood circulation regulation

In a closed network of blood vessels and the heart (Fig. 11.6), blood is constantly moving, the volume of which averages 69 ml/kg of body weight in adult men and 65 ml/kg of body weight in women (i.e., with a body weight of 70 kg, it will be 4830 ml and 4550 ml, respectively). At rest, from 1/3 to 1/2 of this volume does not circulate through the vessels, but is located in the blood depots: capillaries and veins of the abdominal cavity, liver, spleen, lungs, and subcutaneous vessels.

During physical work, emotional reactions, stress, this blood passes from the depot into the general circulation. The movement of blood is provided by rhythmic contractions of the ventricles of the heart, each of which expels approximately 70 ml of blood into the aorta (left ventricle) and pulmonary artery (right ventricle), and with heavy physical exertion in well-trained people, this indicator (it is called systolic or stroke volume) can increase up to 180 ml. The heart of an adult is reduced at rest approximately 75 times per minute, which means that during this time over 5 liters of blood (75x70 = 5250 ml) must pass through it - this indicator is called the minute volume of blood circulation. With each contraction of the left ventricle, the pressure in the aorta, and then in the arteries, rises to 100-140 mm Hg. Art. (systolic pressure), and by the beginning of the next contraction it drops to 60-90 mm (diastolic pressure). In the pulmonary artery, these figures are less: systolic - 15-30 mm, diastolic - 2-7 mm - this is due to the fact that the so-called. the pulmonary circulation, starting from the right ventricle and delivering blood to the lungs, is shorter than the large one, and therefore has less resistance to blood flow and does not require high pressure. Thus, the main indicators of the function of blood circulation are the frequency and strength of heart contractions (the systolic volume depends on it), systolic and diastolic pressure, which are determined by the volume of fluid in a closed circulatory system, the minute volume of blood flow and the resistance of vessels to this blood flow. The resistance of the vessels changes due to the contractions of their smooth muscles: the narrower the lumen of the vessel becomes, the greater the resistance to blood flow it provides.

The constancy of the volume of fluid in the body is regulated by hormones (See Chapter 12), but what part of the blood will be in the depot, and what part will circulate through the vessels, what resistance the vessels will provide to the blood flow - depends on the control of the vessels by the sympathetic department. The work of the heart, and hence the magnitude of blood pressure, primarily systolic, is controlled by both sympathetic and vagus nerves (although endocrine mechanisms and local self-regulation also play here important role). The mechanism for monitoring changes in the most important parameters of the circulatory system is quite simple, it comes down to continuous recording by baroreceptors of the degree of stretching of the aortic arch and the place where the common carotid arteries are divided into external and internal (this area is called the carotid sinus). This is sufficient, since the stretching of these vessels reflects the work of the heart, and vascular resistance, and blood volume.

The more the aorta and carotid arteries are stretched, the more often nerve impulses propagate from the baroceptors along the sensitive fibers of the glossopharyngeal and vagus nerves to the corresponding nuclei of the medulla oblongata. This leads to two consequences: an increase in the influence of the vagus nerve on the heart and a decrease in the sympathetic effect on the heart and blood vessels. As a result, the work of the heart decreases (the minute volume decreases) and the tone of the vessels that resist blood flow decreases, and this leads to a decrease in the stretching of the aorta and carotid arteries and a corresponding decrease in impulses from baroreceptors. If it begins to decrease, then there will be an increase in sympathetic activity and a decrease in the tone of the vagus nerves, and as a result, the proper value of the most important parameters of blood circulation will be restored again.

The continuous movement of blood is necessary, first of all, in order to deliver oxygen from the lungs to the working cells, and carry the carbon dioxide formed in the cells to the lungs, where it is excreted from the body. The content of these gases in the arterial blood is maintained at a constant level, which reflects the values ​​of their partial pressure (from Latin pars - part, i.e. partial of the whole atmospheric pressure): oxygen - 100 mm Hg. Art., carbon dioxide - about 40 mm Hg. Art. If the tissues begin to work more intensively, they will begin to take more oxygen from the blood and release more carbon dioxide into it, which will lead to a decrease in the oxygen content and an increase in carbon dioxide in the arterial blood, respectively. These shifts are captured by chemoreceptors located in the same vascular regions as baroreceptors, i.e. in the aorta and bifurcations of the carotid arteries that feed the brain. The arrival of more frequent signals from chemoreceptors to the medulla oblongata will lead to the activation of the sympathetic department and a decrease in the tone of the vagus nerves: as a result, the work of the heart will increase, the tone of the vessels will increase and, under high pressure, the blood will circulate faster between the lungs and tissues. At the same time, the increased frequency of impulses from the chemoreceptors of the vessels will lead to an increase and deepening of breathing, and the rapidly circulating blood will become faster saturated with oxygen and freed from excess carbon dioxide: as a result, the blood gas composition will normalize.

Thus, the baroreceptors and chemoreceptors of the aorta and carotid arteries immediately respond to shifts in hemodynamic parameters (manifested by an increase or decrease in the stretching of the walls of these vessels), as well as to changes in blood saturation with oxygen and carbon dioxide. The vegetative centers that received information from them change the tone of the sympathetic and parasympathetic divisions in such a way that their influence on the working organs leads to the normalization of parameters that have deviated from homeostatic constants.

Of course, this is only part complex system regulation of blood circulation, in which, along with the nervous, there are also humoral and local mechanisms of regulation. For example, any particularly intensively working organ consumes more oxygen and forms more under-oxidized metabolic products, which themselves are able to expand the vessels that supply the organ with blood. As a result, he begins to take more from the general blood flow than he took before, and therefore in the central vessels, due to the decreasing volume of blood, the pressure decreases and it becomes necessary to regulate this shift with the help of nervous and humoral mechanisms.

During physical work, the circulatory system must adapt to muscle contractions, and to increased oxygen consumption, and to the accumulation of metabolic products, and to the changing activity of other organs. With various behavioral reactions, during the experience of emotions, complex changes occur in the body, which are reflected in the constancy of the internal environment: in such cases, the whole complex of such changes that activate different areas of the brain will certainly affect the activity of hypothalamus neurons, and it already coordinates the mechanisms of autonomic regulation with muscle work , emotional state or behavioral reactions.

11.10. The main links in the regulation of breathing

With calm breathing, about 300-500 cubic meters enter the lungs during inhalation. cm of air and the same volume of air when exhaled goes into the atmosphere - this is the so-called. respiratory volume. After a quiet breath, you can additionally inhale 1.5-2 liters of air - this is the inspiratory reserve volume, and after a normal exhalation, you can expel another 1-1.5 liters of air from the lungs - this is the expiratory reserve volume. The sum of respiratory and reserve volumes is the so-called. lung capacity, which is usually measured with a spirometer. Adults breathe on average 14-16 times per minute, ventilating 5-8 liters of air through the lungs during this time - this is the minute volume of breathing. With an increase in the depth of breathing due to reserve volumes and a simultaneous increase in the frequency of respiratory movements, it is possible to increase the minute ventilation of the lungs several times (on average, up to 90 liters per minute, and trained people can double this figure).

Air enters the alveoli of the lungs - air cells densely braided with a network of blood capillaries that carry venous blood: it is poorly saturated with oxygen and excess with carbon dioxide (Fig. 11.7).

Very thin walls of the alveoli and capillaries do not interfere with gas exchange: along the partial pressure gradient, oxygen from the alveolar air passes into the venous blood, and carbon dioxide diffuses into the alveoli. As a result, arterial blood flows from the alveoli with a partial pressure of oxygen in it of about 100 mm Hg. Art., and carbon dioxide - no more than 40 mm Hg. lung ventilation constantly renews the composition of alveolar air, and continuous blood flow and diffusion of gases through the lung membrane allow you to constantly turn venous blood into arterial blood.

Inhalation occurs due to contractions of the respiratory muscles: external intercostal and diaphragm, which are controlled by motor neurons of the cervical (diaphragm) and thoracic spinal cord (intercostal muscles). These neurons are activated by pathways descending from the respiratory center of the brainstem. The respiratory center is formed by several groups of neurons in the medulla oblongata and the pons, one of them (the dorsal inspiratory group) is spontaneously activated at rest 14-16 times per minute, and this excitation is conducted to the motor neurons of the respiratory muscles. In the lungs themselves, in the pleura covering them and in the airways, there are sensitive nerve endings that are excited when the lungs are stretched and air moves through the airways during inspiration. Signals from these receptors are sent to the respiratory center, which, based on them, regulates the duration and depth of inspiration.

With a lack of oxygen in the air (for example, in the rarefied air of mountain peaks) and during physical work, oxygen saturation of the blood decreases. During physical work, at the same time, the content of carbon dioxide in the arterial blood increases, since the lungs, working in the usual mode, do not have time to purify the blood from it to the required condition. Chemoreceptors of the aorta and carotid arteries respond to the shift in the gas composition of arterial blood, signals from which are sent to the respiratory center. This leads to a change in the nature of breathing: inhalation occurs more often and becomes deeper due to reserve volumes, exhalation, usually passive, becomes forced under such circumstances (the ventral group of neurons of the respiratory center is activated and the internal intercostal muscles begin to act). As a result, the minute volume of respiration increases and greater ventilation of the lungs with a simultaneously increased blood flow through them allows you to restore the gas composition of the blood to the homeostatic standard. Immediately after intense physical work, a person has shortness of breath and a rapid pulse, which stop when the oxygen debt is paid off.

The activity rhythm of the neurons of the respiratory center also adapts to the rhythmic activity of the respiratory and other skeletal muscles, from whose proprioceptors it continuously receives information. The coordination of the respiratory rhythm with other homeostatic mechanisms is carried out by the hypothalamus, which, interacting with the limbic system and the cortex, changes the breathing pattern during emotional reactions. The cerebral cortex can have a direct effect on the function of breathing, adapting it to talking or singing. Only the direct influence of the cortex makes it possible to arbitrarily change the nature of breathing, deliberately delay it, slow it down, or speed it up, but all this is possible only to a limited extent. So, for example, the arbitrary holding of breath in most people does not exceed a minute, after which it involuntarily resumes due to the excessive accumulation of carbon dioxide in the blood and the simultaneous decrease in oxygen in it.

Summary

The constancy of the internal environment of the organism is the guarantor of its free activity. The rapid recovery of displaced homeostatic constants is carried out by the autonomic nervous system. It is also able to prevent possible shifts in homeostasis associated with changes in the external environment. Two departments of the autonomic nervous system simultaneously control the activity of most internal organs, exerting an opposite effect on them. An increase in the tone of sympathetic centers is manifested by ergotropic reactions, and an increase in parasympathetic tone is manifested by trophotropic ones. The activity of the vegetative centers is coordinated by the hypothalamus, it coordinates their activity with the work of the muscles, emotional reactions and behavior. The hypothalamus interacts with the limbic system of the brain, the reticular formation and the cerebral cortex. Vegetative mechanisms of regulation play leading role in the implementation of the vital functions of blood circulation and respiration.

Questions for self-control

165. In what part of the spinal cord are the bodies of parasympathetic neurons located?

A. Sheyny; B. Thoracic; B. Upper segments of the lumbar; D. Lower segments of the lumbar; D. Sacred.

166. What cranial nerves do not contain fibers of parasympathetic neurons?

A. Trinity; B. Oculomotor; B. Facial; G. Wandering; D. Glossopharyngeal.

167. Which ganglia of the sympathetic department should be classified as paravertebral?

A. Sympathetic trunk; B. Neck; B. Starry; G. Chrevny; B. Inferior mesenteric.

168. Which of the following effectors mainly receives only sympathetic innervation?

A. Bronchi; B. Stomach; B. Intestine; D. Blood vessels; D. Bladder.

169. Which of the following reflects an increase in the tone of the parasympathetic division?

A. Pupil dilation; B. Bronchial dilatation; B. Increased heart rate; G. Increased secretion of the digestive glands; D. Increased secretion of sweat glands.

170. Which of the following is characteristic of an increase in the tone of the sympathetic department?

A. Increased secretion of bronchial glands; B. Increased motility of the stomach; B. Increased secretion of the lacrimal glands; D. Contraction of the muscles of the bladder; D. Increased breakdown of carbohydrates in cells.

171. The activity of what endocrine gland is controlled by sympathetic preganglionic neurons?

A. Adrenal cortex; B. Adrenal medulla; B. Pancreas; G. Thyroid gland; D. Parathyroid glands.

172. What neurotransmitter is used to transmit excitation in the sympathetic vegetative ganglia?

A. Adrenaline; B. Norepinephrine; B. Acetylcholine; G. Dopamine; D. Serotonin.

173. With what mediator do parasympathetic postganglionic neurons usually act on effectors?

A. Acetylcholine; B. Adrenaline; B. Norepinephrine; G. Serotonin; D. Substance R.

174. Which of the following characterizes H-cholinergic receptors?

A. Belong to the postsynaptic membrane of the working organs regulated by the parasympathetic division; B. Ionotropic; B. Activated by muscarine; G. Relate only to the parasympathetic department; D. They are located only on the presynaptic membrane.

175. What receptors must bind to the mediator in order for the increased breakdown of carbohydrates to begin in the effector cell?

A. a-adrenergic receptors; B. b-adrenergic receptors; B. N-cholinergic receptors; G. M-cholinergic receptors; D. Ionotropic receptors.

176. What brain structure coordinates vegetative functions and behavior?

A. spinal cord; B. medulla oblongata; B. Midbrain; G. Hypothalamus; D. The cerebral cortex.

177. What homeostatic shift will have a direct effect on the central receptors of the hypothalamus?

A. Increased blood pressure; B. Increase in blood temperature; B. Increase in blood volume; G. Increase in partial pressure of oxygen in arterial blood; D. Decreased blood pressure.

178. What is the value of the minute volume of blood circulation, if the stroke volume is 65 ml, and the heart rate is 78 per minute?

A. 4820 ml; B. 4960 ml; B. 5070 ml; D. 5140 ml; D. 5360 ml.

179. Where are the baroreceptors located that supply information to the vegetative centers of the medulla oblongata, which regulate the work of the heart and blood pressure?

A. Heart; B. Aorta and carotid arteries; B. Large veins; G. Small arteries; D. Hypothalamus.

180. In a lying position, a person reflexively decreases the frequency of contractions of the heart and blood pressure. Activation of what receptors causes these changes?

A. Intrafusal muscle receptors; B. Golgi tendon receptors; B. Vestibular receptors; D. Mechanoreceptors of the aortic arch and carotid arteries; D. Intracardiac mechanoreceptors.

181. What event is most likely to occur as a result of an increase in the tension of carbon dioxide in the blood?

A. Reducing the frequency of breathing; B. Reducing the depth of breathing; B. Decreased heart rate; D. Decrease in the strength of contractions of the heart; D. Increased blood pressure.

182. What is the vital capacity of the lungs if the tidal volume is 400 ml, the inspiratory reserve volume is 1500 ml, and the expiratory reserve volume is 2 liters?

A. 1900 ml; B. 2400 ml; B. 3.5 l; D. 3900 ml; E. It is impossible to determine the vital capacity of the lungs from the available data.

183. What can happen as a result of short-term voluntary hyperventilation of the lungs (frequent and deep breathing)?

A. Increased tone of the vagus nerves; B. Increased tone of sympathetic nerves; B. Increased impulses from vascular chemoreceptors; D. Increased impulses from vascular baroreceptors; D. Increased systolic pressure.

184. What is meant by the tone of the autonomic nerves?

A. Their ability to be excited by the action of a stimulus; B. Ability to conduct excitation; B. Presence of spontaneous background activity; D. Increasing the frequency of conducted signals; E. Any change in the frequency of transmitted signals.

A synapse is a certain contact zone of the processes of nerve cells and other non-excitable and excitable cells that provide the transmission of an information signal. The synapse is morphologically formed by contacting membranes of 2 cells. The membrane related to the process is called the presynaptic membrane of the cell into which the signal enters, its second name is postsynaptic. Together with belonging to the postsynaptic membrane, the synapse can be interneuronal, neuromuscular and neurosecretory. The word synapse was introduced in 1897 by Charles Sherrington (English physiologist).

What is a synapse?

A synapse is a special structure that ensures the transmission of a nerve impulse from a nerve fiber to another nerve fiber or nerve cell, and in order for the nerve fiber to be affected by a receptor cell (the area where nerve cells and another nerve fiber come into contact with each other), two nerve cells are required .

A synapse is a small section at the end of a neuron. With its help, information is transmitted from the first neuron to the second. The synapse is located in three areas of nerve cells. Also, synapses are located in the place where the nerve cell comes into contact with various glands or muscles of the body.

What is a synapse made of?

The structure of the synapse is a simple circuit. It is formed from 3 parts, in each of which certain functions are carried out during the transmission of information. Thus, such a structure of the synapse can be called suitable for transmission. Two main cells directly affect the process: the perceiving and transmitting. At the end of the axon of the transmitting cell is the presynaptic ending (the initial part of the synapse). It can affect the launch of neurotransmitters in the cell (this word has several meanings: mediators, mediators or neurotransmitters) - determined by which an electrical signal is transmitted between 2 neurons.

The synaptic cleft is the middle part of the synapse - this is the gap between 2 interacting nerve cells. Through this gap, an electrical impulse comes from the transmitting cell. The final part of the synapse is considered to be the perceiving part of the cell, which is the postsynaptic ending (the contacting fragment of the cell with different sensitive receptors in its structure).

Synapse mediators

Mediator (from the Latin Media - transmitter, intermediary or middle). Such synapse mediators are very important in the transmission process.

The morphological difference between inhibitory and excitatory synapses is that they do not have a mediator release mechanism. The mediator in the inhibitory synapse, motor neuron, and other inhibitory synapses is considered to be the amino acid glycine. But the inhibitory or excitatory nature of the synapse is determined not by their mediators, but by the property of the postsynaptic membrane. For example, acetylcholine gives an excitatory effect in the neuromuscular synapse of the terminals (vagus nerves in the myocardium).

Acetylcholine serves as an excitatory mediator in cholinergic synapses (the end of the spinal cord of a motor neuron plays the presynaptic membrane in it), in a synapse on Ranshaw cells, in the presynaptic terminal of the sweat glands, the adrenal medulla, in the intestinal synapse and in the ganglia of the sympathetic nervous system. Acetylcholinesterase and acetylcholine were also found in the fraction of different parts of the brain, sometimes in large quantities, but apart from the cholinergic synapse on Ranshaw cells, they have not yet been able to identify other cholinergic synapses. According to scientists, the mediator excitatory function of acetylcholine in the central nervous system is very likely.

The catelchomines (dopamine, norepinephrine, and epinephrine) are considered adrenergic neurotransmitters. Adrenaline and norepinephrine are synthesized at the end of the sympathetic nerve, in the cell of the head substance of the adrenal gland, spinal cord and brain. Amino acids (tyrosine and L-phenylalanine) are considered the starting material, and adrenaline is the final product of the synthesis. The intermediate substance, which includes norepinephrine and dopamine, also function as mediators in the synapse created at the endings of sympathetic nerves. This function can be either inhibitory (intestinal secretory glands, several sphincters, and smooth muscle of the bronchi and intestines) or excitatory (smooth muscles of certain sphincters and blood vessels, norepinephrine in the myocardial synapse, dopamine in the subcutaneous nuclei of the brain).

When the synaptic mediators complete their function, catecholamine is absorbed by the presynaptic nerve ending, and transmembrane transport is switched on. During the absorption of neurotransmitters, the synapses are protected from premature depletion of the supply during a long and rhythmic work.

Synapse: main types and functions

Langley in 1892 suggested that synaptic transmission in the vegetative ganglion of mammals is not of an electrical nature, but of a chemical one. After 10 years, Eliott found out that adrenaline is obtained from the adrenal glands from the same effect as stimulation of the sympathetic nerves.

After that, it was suggested that adrenaline is able to be secreted by neurons and, when excited, be released by the nerve ending. But in 1921, Levi made an experiment in which he established the chemical nature of transmission in the autonomic synapse among the heart and vagus nerves. He filled the vessels with saline and stimulated the vagus nerve, creating a slow heart rate. When the fluid was transferred from the inhibited stimulation of the heart to the unstimulated heart, it beat more slowly. It is clear that stimulation of the vagus nerve caused the release of the inhibitory substance into the solution. Acetylcholine fully reproduced the effect of this substance. In 1930, the role in the synaptic transmission of acetylcholine in the ganglion was finally established by Feldberg and his collaborators.

Synapse chemical

The chemical synapse is fundamentally different in the transmission of irritation with the help of a mediator from the presynapse to the postsynapse. Therefore, differences are formed in the morphology of the chemical synapse. The chemical synapse is more common in the vertebral CNS. It is now known that a neuron is capable of isolating and synthesizing a pair of mediators (coexisting mediators). Neurons also have neurotransmitter plasticity - the ability to change the main mediator during development.

neuromuscular junction

This synapse carries out the transmission of excitation, but this connection can be destroyed by various factors. The transmission ends during the blockade of the ejection of acetylcholine into the synaptic cleft, as well as during an excess of its content in the zone of postsynaptic membranes. Many poisons and drugs affect the capture, output, which is associated with the cholinergic receptors of the postsynaptic membrane, then the muscle synapse blocks the transmission of excitation. The body dies during suffocation and stops the contraction of the respiratory muscles.

Botulinus is a microbial toxin in the synapse; it blocks the transmission of excitation by destroying the syntaxin protein in the presynaptic terminal, which is controlled by the release of acetylcholine into the synaptic cleft. Several poisonous warfare agents, pharmacological drugs (neostigmine and neostigmine), and insecticides block the conduction of excitation to the neuromuscular junction by inactivating acetylcholinesterase, an enzyme that destroys acetylcholine. Therefore, acetylcholine accumulates in the zone of the postsynaptic membrane, the sensitivity to the mediator decreases, the postsynaptic membranes are released and the receptor block is immersed in the cytosol. The acetylcholine will be ineffective and the synapse will be blocked.

Synapse nerve: features and components

A synapse is a connection between a contact point between two cells. Moreover, each of them is enclosed in its own electrogenic membrane. The synapse is made up of three main components: the postsynaptic membrane, the synaptic cleft, and the presynaptic membrane. The postsynaptic membrane is a nerve ending that passes to the muscle and descends into the muscle tissue. In the presynaptic region there are vesicles - these are closed cavities that have a neurotransmitter. They are always on the move.

Approaching the membrane of nerve endings, the vesicles merge with it, and the neurotransmitter enters the synaptic cleft. One vesicle contains a quantum of the mediator and mitochondria (they are needed for the synthesis of the mediator - the main source of energy), then acetylcholine is synthesized from choline and, under the influence of the enzyme acetylcholine transferase, is processed into acetyl-CoA).

Synaptic cleft among post- and presynaptic membranes

In different synapses, the size of the gap is different. filled with intercellular fluid, which contains a neurotransmitter. The postsynaptic membrane covers the site of contact of the nerve ending with the innervated cell in the myoneural synapse. In certain synapses, the postsynaptic membrane creates a fold, the contact area increases.

Additional substances that make up the postsynaptic membrane

The following substances are present in the zone of the postsynaptic membrane:

Receptor (cholinergic receptor in myoneural synapse).

Lipoprotein (has a great similarity with acetylcholine). This protein has an electrophilic end and an ionic head. The head enters the synaptic cleft and interacts with the cationic head of acetylcholine. Because of this interaction, the postsynaptic membrane changes, then depolarization occurs, and potentially dependent Na-channels open. Membrane depolarization is not considered a self-reinforcing process;

Gradual, its potential on the postsynaptic membrane depends on the number of mediators, that is, the potential is characterized by the property of local excitations.

Cholinesterase - is considered a protein that has an enzymatic function. In structure, it is similar to the cholinergic receptor and has similar properties with acetylcholine. Cholinesterase destroys acetylcholine, initially the one that is associated with the cholinergic receptor. Under the action of cholinesterase, the cholinergic receptor removes acetylcholine, repolarization of the postsynaptic membrane is formed. Acetylcholine breaks down to acetic acid and choline, which is necessary for the trophism of muscle tissue.

With the help of the existing transport, choline is displayed on the presynaptic membrane, it is used to synthesize a new mediator. Under the influence of the mediator, the permeability in the postsynaptic membrane changes, and under cholinesterase, the sensitivity and permeability returns to the initial value. Chemoreceptors are able to interact with new mediators.