Parasympathetic nervous system

The parasympathetic part of the nervous system is divided into the cephalic and sacral sections. The cephalic section (pars cranialis) includes the autonomic nuclei and parasympathetic fibers of the oculomotor (III pair), facial (VII pair), glossopharyngeal (IX pair) and vagus (X pair) nerves, as well as the ciliary, pterygopalatine, submandibular, hypoglossal, auricular and other parasympathetic nodes and their branches. The sacral (pelvic) section of the parasympathetic part is formed by the sacral parasympathetic nuclei (nuclei parasympathici sacrales) of the II, III and IV sacral segments of the spinal cord (SII-SIV), the visceral pelvic nerves (nn. splanchnici pelvini), and the parasympathetic pelvic nodes (gariglia pelvina) with their branches.

1. The parasympathetic part of the oculomotor nerve is represented by the accessory (parasympathetic) nucleus (nucleus oculomotorius accessorius; Yakubovich-Edinger-Westphal nucleus), the ciliary ganglion and processes of the cells whose bodies lie in this nucleus and ganglion. The axons of the cells of the accessory nucleus of the oculomotor nerve, located in the tegmentum of the midbrain, pass as part of this cranial nerve in the form of preganglionic fibers. In the orbital cavity, these fibers separate from the lower branch of the oculomotor nerve in the form of the oculomotor rootlet (radix oculomotoria [parasympathetica]; short rootlet of the ciliary ganglion) and enter the ciliary ganglion in its posterior part, ending on its cells.

Ciliary ganglion (ganglion ciliare)

Flat, about 2 mm long and thick, located near the superior orbital fissure in the thickness of the fatty tissue at the lateral semicircle of the optic nerve. This ganglion is formed by the accumulation of bodies of the second neurons of the parasympathetic part of the autonomic nervous system. Preganglionic parasympathetic fibers that come to this ganglion as part of the oculomotor nerve end in synapses on the cells of the ciliary ganglion. Postganglionic nerve fibers as part of three to five short ciliary nerves exit from the anterior part of the ciliary ganglion, go to the back of the eyeball and penetrate it. These fibers innervate the ciliary muscle and the sphincter of the pupil. Fibers conducting general sensitivity (branches of the nasociliary nerve) pass through the ciliary ganglion in transit, forming the long (sensory) rootlet of the ciliary ganglion. Sympathetic postganglionic fibers (from the internal carotid plexus) also pass through the node in transit.

2. The parasympathetic part of the facial nerve consists of the superior salivary nucleus, pterygopalatine, submandibular, hypoglossal ganglia and parasympathetic nerve fibers. The axons of the cells of the superior salivary nucleus, located in the tegmentum of the bridge, pass as preganglionic parasympathetic fibers in the facial (intermediate) nerve. In the area of the genu of the facial nerve, part of the parasympathetic fibers separates in the form of the greater petrosal nerve (n. petrosus major) and exits the facial canal. The greater petrosal nerve lies in the groove of the same name in the pyramid of the temporal bone, then pierces the fibrous cartilage filling the lacerated opening in the base of the skull and enters the pterygoid canal. In this canal, the greater petrosal nerve, together with the sympathetic deep petrosal nerve, forms the nerve of the pterygoid canal, which exits into the pterygopalatine fossa and goes to the pterygopalatine ganglion.

Pterygopalatine ganglion (gangion pterygopalatinum)

4-5 mm in size, irregular in shape, located in the pterygoid fossa, below and medial to the maxillary nerve. The processes of the cells of this node - postganglionic parasympathetic fibers join the maxillary nerve and then follow as part of its branches (nasopalatine, greater and lesser palatine, nasal nerves and pharyngeal branch). From the zygomatic nerve, 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 pterygopalatine ganglion through its branches: the nasopalatine nerve (n. nasopalatine), the greater and lesser palatine nerves (nn. palatini major et minores), the posterior, lateral and medial nasal nerves (nn. nasales posteriores, laterales et mediates), the pharyngeal branch (r. pharyngeus) - are directed to innervate the glands of the mucous membrane of the nasal cavity, palate and pharynx.

The part of the preganglionic parasympathetic fibers that are not included in the petrosal nerve departs from the facial nerve as part of its other branch, the chorda tympani. After the chorda tympani joins the lingual nerve, the preganglionic parasympathetic fibers go as part of it to the submandibular and sublingual ganglion.

Submandibular ganglion (ganglion submandibulare)

Irregularly shaped, 3.0-3.5 mm in size, located under the trunk of the lingual nerve on the medial surface of the submandibular salivary gland. The submandibular ganglion contains the bodies of parasympathetic nerve cells, the processes of which (postganglionic nerve fibers) as part of the glandular branches are directed to the submandibular salivary gland for its secretory innervation.

In addition to the preganglionic fibers of the lingual nerve, the sympathetic branch (r. sympathicus) approaches the submandibular ganglion from the plexus located around the facial artery. The glandular branches also contain sensory (afferent) fibers, the receptors of which are located in the gland itself.

Sublingual ganglion (ganglion sublinguale)

Inconstant, located on the outer surface of the sublingual salivary gland. It is smaller in size than the submandibular node. Preganglionic fibers (nodal branches) from the lingual nerve approach the sublingual node, and glandular branches depart from it to the salivary gland of the same name.

3. The parasympathetic part of the glossopharyngeal nerve is formed by the inferior salivary nucleus, the otic ganglion, and processes of the cells located in them. Axons of the inferior salivary nucleus, located in the medulla oblongata, as part of the glossopharyngeal nerve, exit the cranial cavity through the jugular foramen. At the level of the lower edge of the jugular foramen, the preganglionic parasympathetic nerve fibers branch off as part of the tympanic nerve (n. tympanicus), penetrating the tympanic cavity, where it forms a plexus. Then these preganglionic parasympathetic fibers exit the tympanic cavity through the cleft of the canal of the lesser petrosal nerve in the form of the nerve of the same name - the lesser petrosal nerve (n. petrosus minor). This nerve leaves the cranial cavity through the cartilage of the lacerated foramen and approaches the otic ganglion, where the preganglionic nerve fibers end on the cells of the otic ganglion.

Otic ganglion (ganglion oticum)

Round, 3-4 mm in size, adjacent to the medial surface of the mandibular nerve under the oval opening. This node is formed by the bodies of parasympathetic nerve cells, the postganglionic fibers of which are directed to the parotid salivary gland as part of the parotid branches of the auriculotemporal nerve.

1. The parasympathetic part of the vagus nerve consists of the posterior (parasympathetic) nucleus of the vagus nerve, numerous nodes that are part of the organ autonomic plexuses, and processes of cells located in the nucleus and these nodes. The axons of the cells of the posterior nucleus of the vagus nerve, located in the medulla oblongata, go as part of its branches. Preganglionic parasympathetic fibers reach the parasympathetic nodes of the peri- and intraorgan autonomic plexuses [cardiac, esophageal, pulmonary, gastric, intestinal and other autonomic (visceral) plexuses]. In the parasympathetic nodes (ganglia parasympathica) of the peri- and intraorgan plexuses are located the cells of the second neuron of the efferent pathway. The processes of these cells form bundles of postganglionic fibers that innervate the smooth muscles and glands of the internal organs, neck, chest and abdomen.
2. The sacral section of the parasympathetic part of the autonomic nervous system is represented by the sacral parasympathetic nuclei located in the lateral intermediate substance of the II-IV sacral segments of the spinal cord, as well as the pelvic parasympathetic nodes and processes of the cells located in them. The axons of the sacral parasympathetic nuclei exit the spinal cord as part of the anterior roots of the spinal nerves. Then these nerve fibers go as part of the anterior branches of the sacral spinal nerves and after their exit through the anterior pelvic sacral openings, they branch off, forming the pelvic visceral nerves (nn. splanchnici pelvici). These nerves approach the parasympathetic nodes of the inferior hypogastric plexus and the nodes of the autonomic plexuses located near the internal organs or in the thickness of the organs themselves, located in the pelvic cavity. The preganglionic fibers of the pelvic visceral nerves terminate on the cells of these nodes. 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 brainstem (nuclei of the IX and X cranial nerves). In the first case, preganglionic fibers approach the prevertebral plexuses (ganglions), where they are interrupted. From here, postganglionic fibers begin, directed to tissues or intramural ganglia.

Currently, an enteric nervous system is also distinguished (this was pointed out back in 1921 by J. Langley), the difference between which and the sympathetic and parasympathetic systems, in addition to its location in the intestine, is as follows:

1. enteric neurons are histologically distinct from neurons of other autonomic 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, enfecalins, substance P, vasoactive intestinal polypeptide).

Adrenergic, cholinergic, serotonergic mediation or modulation is discussed, the role of ATP as a mediator (purinergic system) is shown. A. D. Nozdrachev (1983), who designates this system as meta-sympathetic, 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 meta-sympathetic system is considered in two aspects:

1. transmitter of central influences to tissues and
2. an independent integrative formation that includes local reflex arcs capable of functioning with complete decentralization.

The clinical aspects of studying the activity of this section of the autonomic nervous system are difficult to isolate. There are no adequate methods for studying it, except for studying biopsy material from the large intestine.

This is how the efferent part of the segmental vegetative system is constructed. The situation is more complicated with the afferent system, the existence of which was essentially denied by J. Langley. Several types of vegetative receptors are known:

1. pressure and stretch-responsive structures such as Vaterpacinian corpuscles;
2. chemoreceptors that sense chemical shifts; less common are thermo- and osmoreceptors.

From the receptor, the fibers go without interruption through the prevertebral plexuses, the sympathetic trunk to the intervertebral ganglion, where the afferent neurons are located (along with somatic sensory neurons). Then the information goes along two paths: along the spinothalamic tract to the thalamus via thin (C fibers) and medium (B fibers) conductors; the second path is along the conductors of deep sensitivity (A fibers). At the level of the spinal cord, it is not possible to differentiate sensory animal and sensory vegetative fibers. Undoubtedly, information from the internal organs reaches the cortex, but under normal conditions it is not realized. Experiments with irritation of visceral formations indicate that evoked potentials can be recorded in various areas of the cerebral cortex. It is not possible to detect conductors in the vagus nerve system that carry the feeling of pain. Most likely they go along the sympathetic nerves, so it is fair that vegetative pains are called sympathalgias, not vegetalgias.

It is known that sympathalgias differ from somatic pains by their greater diffusion and affective accompaniment. An explanation for this fact cannot be found in the distribution of pain signals along the sympathetic chain, since the sensory pathways pass through the sympathetic trunk without interruption. Apparently, the absence of receptors and conductors in the vegetative afferent systems that carry tactile and deep sensitivity, as well as the leading role of the thalamus as one of the final points of receipt of sensory information from the visceral systems and organs, are important.

It is obvious that the vegetative segmental apparatuses have a certain autonomy and automatism. The latter is determined by the periodic occurrence of the excitatory process in the intramural ganglia based on current metabolic processes. A convincing example is the activity of the intramural ganglia of the heart under conditions of its transplantation, when the heart is practically deprived of all neurogenic extracardiac influences. Autonomy is also determined by the presence of an axon reflex, when the transmission of excitation is carried out in the system of one axon, as well as by the mechanism of spinal viscerosomatic reflexes (through the anterior horns of the spinal cord). Recently, data have appeared on nodal reflexes, when the closure is carried out at the level of the prevertebral ganglia. Such an assumption is based on 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 commonality and differences in the organization and structure of the sympathetic and parasympathetic divisions, there are no differences in the structure of neurons and fibers between them. The differences concern the grouping of sympathetic and parasympathetic neurons in the central nervous system (the thoracic spinal cord for the former, the brainstem and sacral spinal cord for the latter) and the location of the ganglia (parasympathetic neurons predominate in nodes located close to the working organ, and sympathetic neurons - in distant ones). The latter circumstance leads to the fact that in the sympathetic system, preganglionic fibers are shorter and postganglionic fibers are longer, and vice versa in the parasympathetic system. This feature has a significant biological meaning. The effects of sympathetic irritation are more diffuse and generalized, while those of parasympathetic irritation are less global and more local. The sphere of action of the parasympathetic nervous system is relatively limited and concerns mainly internal organs, at the same time there are no tissues, organs, systems (including the central nervous system), where the fibers of the sympathetic nervous system would not penetrate. The next essential difference is the different mediation at the endings of postganglionic fibers (the mediator of preganglionic both sympathetic and parasympathetic fibers is acetylcholine, the effect of which is potentiated by the presence of potassium ions). At the endings of sympathetic fibers, sympathine (a mixture of adrenaline and noradrenaline) is released, which has a local effect, and after absorption into the bloodstream - a general effect. The mediator of parasympathetic postganglionic fibers, acetylcholine, causes mainly a local effect and is quickly destroyed by cholinesterase.

The concepts of synaptic transmission have become more complex at present. Firstly, in the sympathetic and parasympathetic ganglia not only cholinergic, but also adrenergic (in particular, dopaminergic) and peptidergic (in particular, VIP - vasoactive intestinal polypeptide) are found. Secondly, the role of presynaptic formations and postsynaptic receptors in modulating various forms of reactions (beta-1-, a-2-, a-1- and a-2-adrenoreceptors) has been demonstrated.

The idea of the generalized nature of sympathetic reactions occurring simultaneously in various systems of the body has gained wide popularity and has given rise to the term "sympathetic tone". If we use the most informative method of studying the sympathetic system - measuring the amplitude of the general activity in the sympathetic nerves, then this idea should be somewhat supplemented and modified, since different levels of activity are detected in individual sympathetic nerves. This indicates differentiated regional control of sympathetic activity, i.e. against the background of generalized activation, certain systems have their own level of activity. Thus, at rest and under load, different levels of activity are established in the skin and muscle sympathetic fibers. Within certain systems (skin, muscles), high parallelism of sympathetic nerve activity is noted in various muscles or skin of the feet and hands.

This indicates a homogeneous supraspinal control of certain populations of sympathetic neurons. All this speaks to the well-known 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 release of this mediator in postganglionic sympathetic neurons, its increase during electrical stimulation of sympathetic nerves, as well as during stressful situations and certain functional loads. The level of plasma norepinephrine varies in different people, but in a given person it is relatively constant. In older people it is somewhat higher than in young people. A positive correlation has been established between the frequency of volleys in sympathetic muscle nerves and the plasma concentration of norepinephrine in venous blood. This can be explained by two circumstances:

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

At that time, it was impossible to detect a definite relationship between arterial pressure and plasma norepinephrine levels. Thus, modern vegetology is constantly moving towards precise 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 the data of embryology. The sympathetic chain is formed as a result of the displacement of neuroblasts from the medullary tube. In the embryonic period, the vegetative structures develop mainly from the neural crest (crista neuralis), in which a certain regionalization can be traced; the cells of the sympathetic ganglia are formed from elements located along the entire length of the neural crest, and migrate in three directions: paravertebrally, prevertebrally, and previscerally. Paravertebral clusters of neurons form the sympathetic chain with vertical connections; the right and left chains can have cross connections at the lower cervical and lumbosacral levels.

Prevertebral migrating cell masses at the level of the abdominal aorta form prevertebral sympathetic ganglia. Previsceral sympathetic ganglia are found near the pelvic organs or in their wall - previsceral sympathetic ganglia (designated as the "minor adrenergic system"). At later stages of embryogenesis, preganglionic fibers (from spinal cord cells) approach the peripheral autonomic ganglia. Completion of myelination of the preganglionic fibers occurs after birth.

The main part of the intestinal ganglia originates from the "vagal" level of the neural crest, from where neuroblasts migrate ventrally. The precursors of the intestinal ganglia are involved in the formation of the wall of the anterior part of the digestive tract. They then migrate caudally along the intestine and form the plexuses of Meissner and Auerbach. The parasympathetic ganglia of Remak and some ganglia of the lower intestine are formed from the lumbosacral part of the neural crest.

The vegetative peripheral facial ganglia (ciliary, pterygopalatine, auricular) are also formations partly of the medullary tube, partly of the trigeminal ganglion. The data presented allow us to imagine these formations as parts of the central nervous system, brought out to the periphery - a kind of anterior horns of the vegetative system. Thus, preganglionic fibers are elongated intermediate neurons, well described in the somatic system, therefore the vegetative two-neuronality in the peripheral link is only apparent.

This is the general structure of the autonomic nervous system. Only segmental apparatuses are truly specifically autonomic from the functional and morphological positions. In addition to the structural features, slow speed of impulse conduction, and mediator differences, the position on the presence of dual innervation of organs by sympathetic and parasympathetic fibers remains important. There are exceptions to this position: only sympathetic fibers approach the adrenal medulla (this is explained by the fact that this formation is essentially a re-formed sympathetic ganglion); only sympathetic fibers also approach the sweat glands, at the end of which, however, acetylcholine is released. According to modern concepts, blood vessels also have only sympathetic innervation. In this case, sympathetic vasoconstrictor fibers are distinguished. The few exceptions cited only confirm the rule on the presence of dual innervation, with the sympathetic and parasympathetic systems exerting opposite effects on the working organ. The expansion and contraction of blood vessels, the acceleration and deceleration of the heart rhythm, changes in the lumen of the bronchi, secretion and peristalsis in the gastrointestinal tract - all these shifts 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 the body's adaptation to changing environmental conditions, formed the basis of the incorrect idea of the functioning of the autonomic system according to the principle of scales [Eppinger H., Hess L., 1910].

Accordingly, it was thought that increased activity of the sympathetic apparatus should lead to a decrease in the functional capabilities of the parasympathetic division (or, conversely, parasympathetic activation causes a decrease in the activity of the sympathetic apparatus). In reality, a different situation arises. Increased functioning of one division under normal physiological conditions leads to compensatory stress in the apparatuses of another division, returning the functional system to homeostatic indicators. Both suprasegmental formations and segmental vegetative reflexes play a key role in these processes. In a state of relative rest, when there are no disturbing effects and no active work of any kind, the segmental vegetative system can ensure the existence of the organism by carrying out automated activity. In real life situations, adaptation to changing environmental conditions and adaptive behavior are carried out with the pronounced participation of suprasegmental apparatuses, using 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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