Parasympathetic nervous system

The parasympathetic part of the nervous system is divided into the head and sacral divisions. The head part (pars cranialis) includes vegetative nuclei and parasympathetic fibers of the oculomotor (III pair), facial (VII pair), lingopharyngeal (IX pair) and vagus (X pair) nerves, as well as ciliary, pterygoporous, submandibular, sublingual, ear and other parasympathetic nodes and their branches. The sacral (pelvic) section of the parasympathetic part is formed by the sacral parasympathetic nuclei (nuclei parasympathic sacrales) of the II, III and IV sacral segments of the spinal cord (SII-SIV), the pelvic nerve (nn. Splanchnici pelvini), the parasympathetic pelvic nodes (gariglia pelvina) branches.

1. The parasympathetic part of the oculomotor nerve is represented by the additional (parasympathetic) nucleus (nucleus oculomotorius accessorius, the Yakubovich-Edinger-Westfal nucleus), the ciliary node and the processes of the cells whose bodies lie in this nucleus and node. The axons of the cells of the additional nucleus of the oculomotor nerve, which lies in the midbrain, pass through this cranial nerve as preganglionic fibers. In the cavity of the orbit, these fibers separate from the lower branch of the oculomotor nerve in the form of the oculomotor root (radix oculomotoria [parasympathetica], short spine of the ciliary node) and enter the ciliary node in the posterior part of it, ending on its cells.

The cervical node (ganglion ciliare)

Flat, with a length and thickness of about 2 mm, is located near the upper glandular gap in the thickness of fat tissue in the lateral semicircle of the optic nerve. This node is formed by the accumulation of the bodies of the second neurons of the parasympathetic part of the autonomic nervous system. Preganglionic parasympathetic fibers that have come to this node in the oculomotor nerve end with synapses on the cells of the ciliary node. Postganglionic nerve fibers in the composition of three to five short ciliary nerves come out from the anterior part of the ciliary node, are directed to the back of the eyeball and penetrate into it. These fibers innervate the ciliary muscle and the sphincter of the pupil. Through the ciliary junction, fibers passing through the lumbosacon conduct a common sensitivity (the branches of the nosorematic nerve) forming a long (sensitive) spine of the ciliary knot. Transit through the node go and sympathetic postganglionic fibers (from the internal carotid plexus).

2. The parasympathetic part of the facial nerve consists of the upper salivary nucleus, the pterygoid, submandibular, sublingual nodes and parasympathetic nerve fibers. Axons of cells of the upper salivary nucleus lying in the bridge cover, in the form of preganglionic parasympathetic fibers, pass through the facial (intermediate) nerve. In the area of the lap of the facial nerve, a part of the parasympathetic fibers is separated in the form of a large stony nerve (n. Petrosus major) and leaves the facial canal. The large stony nerve lies in the eponymous furrow of the temporal bone pyramid, then the fibrous cartilage filling the laceration in the base of the skull perforates, and enters the pterygoid canal. In this channel, a large stony nerve, along with a sympathetic deep stony nerve, forms the nerve of the pterygoid canal, which opens into the pterygopalatine fossa and is directed to the ves- pel node.

The pterygopalatine (gangion pterygopalatinum)

The size of 4-5 mm, irregular in shape, is located in the pterygoid fossa, below and medial to the maxillary nerve. The processes of the cells of this node - postganglionic parasympathetic fibers attach to the maxillary nerve and then follow in its branches (nasonephalic, large and small palatine, nasal nerves and pharyngeal branch). From the zygomatic nerve, the parasympathetic nerve fibers pass into the lacrimal nerve through its connecting branch with the zygomatic nerve and innervate the lacrimal gland. In addition, nerve fibers from the wing-palatal node through its branches: nasonephalic nerve (n. Nasopalatine), large and small palatine nerves (nn. Palatini major et minores), posterior, lateral and medial nasal nerves (nn. Nasales posteriores, laterales et mediates), the pharyngeal branch (r. Pharyngeus) - are sent to innervate the glands of the mucous membrane of the nasal cavity, palate and pharynx.

That part of the preganglionic parasympathetic fibers that did not form part of the stony nerve departs from the facial nerve as part of its other branch, the drum string. After joining the tympanic strings to the lingual nerve, preganglionic parasympathetic fibers go in its composition to the submandibular and sublingual node.

The submandibular node (ganglion submandibulare)

An irregular shape, measuring 3.0-3.5 mm, is located under the trunk of the lingual nerve on the medial surface of the submandibular salivary gland. In the submandibular node lie the bodies of parasympathetic nerve cells, the processes of which (postganglionic nerve fibers) in the glandular branches are directed to the submaxillary salivary gland for secretion of its innervation.

The sympathetic branch (r. Sympathicus) from the plexus located around the facial artery is suitable for the submandibular node, in addition to the indicated preganglionic fibers of the lingual nerve. In the glandular branches there are also sensitive (afferent) fibers, the receptors of which lie in the gland itself.

The sublingual node (ganglion sublinguale)

Unstable, located on the outer surface of the sublingual salivary gland. It has smaller dimensions than the submaxillary node. The preganglionic fibers (nodal branches) from the lingual nerve approach the sublingual node, and the glandular branches from it to the salivary gland of the same name come from it.

3. The parasympathetic part of the glossopharyngeal nerve is formed by the lower salivary nucleus, the ear node and the outgrowths of the cells lying in them. Axons of the lower salivary nucleus, which is located in the medulla oblongata, form the glossopharyngeal nerve from the cavity of the skull through the jugular opening. At the level of the lower edge of the jugular foramen, the foreground parasympathetic nerve fibers branch off into the drum nerve (n. Tympanicus), which penetrates into the tympanic cavity, where it forms a plexus. Then these preganglionic parasympathetic fibers leave the tympanic cavity through the cleft of the channel of the small stony nerve in the form of the nerve of the same name, the small stony nerve (n. Petrosus minor). This nerve leaves the cranial cavity through the cartilage of the lacerated hole and approaches the ear node, where preganglionic nerve fibers terminate on the cells of the ear node.

Earplant (ganglion oticum)

Rounded, 3-4 mm in size, is attached to the medial surface of the mandibular nerve under the oval aperture. This node is formed by the bodies of parasympathetic nerve cells, the postganglionic fibers of which are directed to the parotid salivary gland in the parotid branches of the ear-temporal nerve.

1. The parasympathetic part of the vagus nerve consists of the posterior (parasympathetic) nucleus of the vagus nerve, the numerous nodes that make up the organ vegetative plexuses, and the processes of the cells located in the nucleus and these nodes. Axons of the cells of the posterior nucleus of the vagus nerve, which is located in the medulla oblongata, go in the composition of its branches. Preganglionic parasympathetic fibers reach the parasympathetic nodes of the near- and intragonal vegetative plexuses [cardiac, esophageal, pulmonary, gastric, intestinal and other vegetative (visceral) plexuses]. In parasympathetic nodes (ganglia parasympathica) near- and intraorganic plexuses, cells of the second neuron of the efferent path are located. The processes of these cells form bundles of postganglionic fibers innervating the smooth muscles and glands of the internal organs, neck, chest and abdomen.
2. The sacral part of the parasympathetic part of the autonomic nervous system is represented by the sacral parasympathetic nuclei located in the lateral intermediate of the II-IV sacral segments of the spinal cord, as well as by the pelvic parasympathetic nodes and outgrowths of the cells located in them. Axons of the sacral parasympathetic nuclei emerge from the spinal cord as part of the anterior roots of the spinal nerves. Then these nerve fibers go in the composition of the anterior branches of the sacral spinal nerves and after leaving them through the anterior pelvic sacral orifices branch off, forming the pelvic internal nerves (nn. Splanchnici pelvici). These nerves approach the parasympathetic nodes of the lower hypogastric plexus and the nodes of the vegetative plexuses located near the internal organs or in the thickness of the organs themselves in the cavity of the small pelvis. The cells of these nodes terminate preganglionic fibers of the pelvic internal nerves. The processes of the cells of the pelvic nodes are postganglionic parasympathetic fibers. These fibers are directed to the pelvic organs and innervate their smooth muscles and glands.

Neurons originate in the lateral horns of the spinal cord at the sacral level, as well as in the autonomic nuclei of the brain stem (the nucleus of IX and X cranial nerves). In the first case preganglionic fibers approach the prevertebral plexuses (ganglia), where they are interrupted. From here begin postganglionic fibers, which are directed to tissues or intramural ganglia.

At the present time, the intestinal nervous system is also isolated (as J. Langley pointed out in 1921), whose difference from the sympathetic and parasympathetic systems, except for the location in the intestine, is as follows:

1. intestinal neurons are histologically distinct from neurons of other vegetative ganglia;
2. in this system there are independent reflex mechanisms;
3. Ganglia do not contain connective tissue and vessels, and glial elements resemble astrocytes;
4. have a wide range of mediators and modulators (angiotensin, bombesin, cholecystokinin-like substance, neurotensin, pancreatic polypeptide, enfakalin, substance P, vasoactive intestinal polypeptide).

Adrenergic, cholinergic, serotonergic mediation or modulation is discussed, the role of ATP as a mediator (purinergic system) is shown. AD Nozdrachev (1983), which designates this system as metasympathetic, believes that its microganglia are located in the walls of internal organs that have motor activity (heart, digestive tract, ureter, etc.). The function of the metasympathetic system is considered in two aspects:

1. transmitter of central influences to tissues and
2. independent integrative education, including local reflex arcs that can function with full decentralization.

Clinical aspects of studying the activity of this department of the autonomic nervous system are difficult to isolate. There are no adequate methods of studying it, except for the study of the biopsy material of the large intestine.

So the efferent part of the segmental vegetative system is constructed. The situation is more complicated with the afferent system, the existence of which, in essence, was denied by J. Langley. Vegetative receptors of several types are known:

1. Response to pressure and expansion of the type of Faterpachiniae corpuscles;
2. chemoreceptors, perceiving chemical shifts; Thermo-and osmoreceptors are less common.

From the receptor, the fibers go, without interruption, through the prevertebral plexus, the sympathetic trunk to the intervertebral node, where the afferent neurons are located (along with somatic sensory neurons). Then the information goes along two paths: together with the spinotalamic tract to the visual hillock on the thin (fiber C) and medium (fiber B) conductors; the second way - together with the conductors of deep sensitivity (fiber A). At the level of the spinal cord, it is not possible to differentiate sensory animatic and sensory vegetative fibers. Undoubtedly, information from internal organs reaches the cortex, but in normal conditions it is not realized. Experiments with irritation of the visceral formations suggest that the evoked potentials can be detected in various regions of the cortex of the cerebral hemispheres. It is not possible to detect pain-bearing conductors in the system of the vagus nerve. Most likely they go on sympathetic nerves, therefore it is fair that vegetative pains are not vegetative, but sympathetic.

It is known that sympathalgia differ from somatic pains by greater diffusion and affective support. The explanation of this fact can not be found in the spread of pain signals along the sympathetic chain, since the sensory pathways pass through the sympathetic trunk without interruption. Apparently, the absence in the vegetative afferent systems of receptors and conductors bearing tactile and deep sensitivity, as well as the leading role of the visual hillock as one of the final points of sensory information input from the visceral systems and organs, are important.

It is obvious that vegetative segmental devices have a certain autonomy and automatism. The latter is determined by the periodic occurrence of an excitatory process in the intramural ganglia on the basis of current metabolic processes. A convincing example is the activity of the intramural ganglia of the heart in the conditions of its transplantation, when the heart is practically deprived of all neurogenic extracardiac influences. Autonomy is also determined by the presence of the axon reflex, when the transmission of excitation occurs in a single axon system, and also by the mechanism of spinal viscero- somatic reflexes (through the anterior horns of the spinal cord). Recently, data have also appeared on nodular reflexes, when the closure occurs at the level of prevertebral ganglia. This assumption is based on the morphological data on the presence of a two-neuron chain for sensitive vegetative fibers (the first sensitive neuron is located in the prevertebral ganglia).

As for the generality and differences in the organization and structure of the sympathetic and parasympathetic divisions, there are no differences between them in the structure of neurons and fibers. The differences concern the grouping of sympathetic and parasympathetic neurons in the central nervous system (the thoracic spinal cord for the first, the brainstem and the sacral region of the spinal cord for the second), and the location of the ganglia (parasympathetic neurons predominate at nodes close to the working organ, and sympathetic in remote ). The latter circumstance leads to a shorter preganglionic fiber in the sympathetic system and longer postganglionic fibers, and vice versa in the parasympathetic system. This feature has a significant biological meaning. The effects of sympathetic stimulation are more diffuse and generalized, parasympathetic - less global, more local. The sphere of action of the parasympathetic nervous system is relatively limited and concerns mainly the internal organs, while at the same time there are no tissues, organs, systems (including the central nervous system), wherever the fibers of the sympathetic nervous system penetrate. The next significant difference is different mediation at the endings of postganglionic fibers (the mediator of preganglionic as sympathetic and parasympathetic fibers is acetylcholine, the action of which is potentiated by the presence of potassium ions). At the end of the sympathetic fibers, sympathy (a mixture of adrenaline and norepinephrine) that exerts a local influence, and after absorption into the bloodstream, is common. The mediator of parasympathetic postganglionic fibers acetylcholine causes predominantly local effects and is rapidly destroyed by cholinesterase.

Representations of synaptic transmission have now become more complicated. First, in the sympathetic and parasympathetic ganglia, not only cholinergic, but also adrenergic (in particular, dopaminergic) and peptidergic (in particular, VKP - vasoactive intestinal polypeptide) are found. Secondly, the role of presynaptic formations and postsynaptic receptors in the modulation of various forms of reactions (beta-1-, a-2-, a-1- and a-2-adrenergic receptors) is shown.

The idea of the generalized nature of sympathetic reactions that occur simultaneously in different systems of the body has become very popular and has brought to life the term "sympathetic tone". If we use the most informative method for studying the sympathetic system-measuring the amplitude of total activity in the sympathetic nerves, then this idea should be somewhat supplemented and modified, since a different degree of activity is detected in individual sympathetic nerves. This indicates a differentiated regional control of sympathetic activity, that is, against the background of general generalized activation, certain systems have their own level of activity. So, at rest and under loads, a different level of activity is found in skin and muscle sympathetic fibers. Inside certain systems (skin, muscles), there is a high parallelism in the activity of the sympathetic nerves in various muscles or in the skin of the feet and hands.

This indicates a homogeneous supraspinal control of certain populations of sympathetic neurons. All this indicates a certain relativity of the concept of "general sympathetic tone".

Another important method for assessing sympathetic activity is the level of plasma norepinephrine. This is understandable in connection with the isolation of this mediator in postganglionic sympathetic neurons, its increase with electrical stimulation of sympathetic nerves, as well as in stressful situations and certain functional loads. The level of plasma norepinephrine varies in different people, but in a certain person it is relatively constant. In older people, it is slightly higher than that of young people. A positive correlation was established between the frequency of volleys in the sympathetic muscle nerves and the plasma concentration of norepinephrine in the venous blood. There are two reasons for this:

1. the level of sympathetic activity in muscles reflects the level of activity in other sympathetic nerves. However, we have already spoken about the different activity of nerves supplying muscles and skin;
2. muscles make 40% of the total mass and contain a large number of adrenergic endings, so the release of adrenaline from them and will determine the level of concentration of norepinephrine in the plasma.

At that time, it is impossible to detect a certain interrelation of blood pressure with the level of plasma norepinephrine. Thus, modern vegetology is constantly on the path of accurate quantitative assessments instead of general provisions on sympathetic activation.

When considering the anatomy of the segmental vegetative system, it is advisable to take into account embryology data. The sympathetic chain is formed as a result of displacement of neuroblasts from the medullary tube. In the embryonic period, vegetative structures develop mainly from a nerve roller (crista neuralis), in which a certain regionalization can be traced; cells of sympathetic ganglia are formed from elements located along the entire length of the nervous cushion, and migrate in three directions: paravertebral, prevertebral, and pre-vascular. Paravertebral clusters of neurons with vertical links form a sympathetic chain, the right and left chains can have transverse connections on the lower-cervical and lumbosacral level.

Prevertebral migrating cell masses at the level of the abdominal aorta form the prevertebral sympathetic ganglia. Prevesertal sympathetic ganglia are found near the pelvic organs or in their wall - the pre-vascular sympathetic ganglia (referred to as the "small adrenergic system"). In the later stages of embryogenesis, preganglionic fibers (from the cells of the spinal cord) approach the peripheral vegetative ganglia. Completion of myelination of preganglionic fibers occurs after birth.

The main part of the intestinal ganglia originates from the "vagal" level of the nervous cushion, from where the neuroblasts migrate in the ventral direction. The precursors of the intestinal ganglia are included in the formation of the wall of the anterior part of the digestive canal. Later they migrate caudally along the intestine and form the plexuses of Meissner and Auerbach. From the lumbosacral section of the nervous cushion, the parasympathetic ganglions of Remak and some of the ganglia of the lower intestine are formed.

The vegetative peripheral nodes of the face (ciliary, wing-palatine, ear) are also formations of a partially medullary tube, partly a trigeminal node. These data allow us to visualize these formations as parts of the central nervous system that have been brought to the periphery, the original anterior horns of the vegetative system. Thus, preganglionic fibers are elongated intermediate neurons well described in the somatic system, therefore vegetative two-neuronality in the peripheral link is only apparent.

This is the general outline of the structure of the autonomic nervous system. Only segmental apparatuses are truly specifically vegetative with functional and morphological positions. In addition to the features of the structure, the slow speed of impulses, mediator differences, it is important to note the presence of double innervation of organs by sympathetic and parasympathetic fibers. From this position there are exceptions: only the sympathetic fibers are suitable for the adrenal medulla (this is explained by the fact that in its essence this formation is a re-formed sympathetic node); to the sweat glands are also suitable only sympathetic fibers, at the end of which, however, acetylcholine is released. According to modern ideas, the vessels also have only sympathetic innervation. In this case, sympathetic vasoconstrictive fibers are distinguished. The few exceptions just confirm the rule about the presence of double innervation, and the sympathetic and parasympathetic systems exert the opposite influence on the working organ. Expansion and contraction of blood vessels, acceleration and slowing of the rhythm of the heart, changes in the lumen of the bronchi, secretion and peristalsis in the gastrointestinal tract - all these changes are determined by the nature of the influence of various parts of the autonomic nervous system. The presence of antagonistic influences, which are the most important mechanism for adapting the organism to the changing environmental conditions, formed the basis for a misconception about the functioning of the vegetative system according to the principle of weights [Eppinger H., Hess L., 1910].

In accordance with this, it seemed that increasing the activity of sympathetic apparatus should lead to a decrease in the functional capabilities of the parasympathetic department (or, conversely, parasympathetic activation causes a decrease in sympathetic apparatus activity). In fact, there is a different situation. Strengthening the functioning of one department in normal physiological conditions leads to compensatory tension in the apparatus of another department, which return the functional system to homeostatic indices. The most important role in these processes is played by both supra-segmental formations and segmental vegetative reflexes. In a state of relative rest, when there are no disturbing influences and no active work of any kind is present, the segmental vegetative system can ensure the existence of the organism by carrying out automated activities. In real life situations adaptation to changing environmental conditions, adaptive behavior is carried out with a pronounced participation of supra-segmental apparatuses that use the segmental vegetative system as an apparatus for rational adaptation. The study of the functioning of the nervous system provides sufficient justification for the position that specialization is achieved at the expense of loss of autonomy. The existence of vegetative apparatuses only confirms this idea.

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