Sympathetic nervous system

The sympathetic nervous system (pars sympathica) includes:

1. lateral intermediate (gray) matter (vegetative nucleus) in the lateral (intermediate) columns of the spinal cord from the VIII cervical segment (CVIII) to the II lumbar (LII);
2. sympathetic nerve fibers and nerves running from the cells of the lateral intermediate substance (lateral column) to the nodes of the sympathetic trunk and the autonomic plexuses of the abdominal cavity and pelvis;
3. right and left sympathetic trunks;
4. communicating branches connecting the spinal nerves (CVIII-ThI-LII) with the sympathetic trunk and the sympathetic trunk with all spinal nerves;
5. nodes of the autonomic nerve plexuses located in front of the spine in the abdominal cavity and pelvic cavity, and nerves located in the walls of large vessels (perivascular plexuses);
6. nerves directed from these plexuses to the organs;
7. sympathetic fibers that go as part of the somatic nerves to organs and tissues. Sympathetic preganglionic nerve fibers are usually shorter than postganglionic fibers.

Neurons are present in the thoracic and upper lumbar regions of the spinal cord, their axons are preganglionic fibers that exit with the anterior roots and approach the sympathetic trunk. Preganglionic fibers are also called white connecting fibers, since they have a larger myelin sheath than postganglionic fibers. The most important sympathetic formation is the sympathetic trunk, also called the "sympathetic chain", located on both sides of the spine (hence another name - paravertebral nodes). There are 20-22 nodes in the trunk: 3 cervical (the middle one is sometimes poorly represented, and the lower one, often uniting with the first thoracic node, forms a powerful stellate node), 10-12 thoracic, 3-4 abdominal and 4 pelvic. There are three types of cells in the ganglia, differing in size: large (35-55 µm in diameter), medium (25-32 µm) and small (15-22 µm). They are present in different proportions in different ganglia. Thus, in the superior cervical ganglion their number is 27; 50; 23%, respectively, in the stellate ganglion there are fewer large cells, but more medium (17; 67; 16%).

The preganglionic fibers approaching the ganglia are partially interrupted in the neurons of the nodes, and partially, without interruption, go to the prevertebral ganglia. The vegetative fibers mainly belong to groups B and C; the thickest fibers, rich in myelin sheath, with a diameter of 5-6.5 μm (type A) are rarely observed. The degree of myelination determines the speed of excitation conduction. The highest speed is achieved in cases where a thick fiber interacts with the dendrite of a large cell. Fibers of one neuron can approach several neighboring nodes of the chain (up to 8 nodes). The fact of interaction of the preganglionic fiber with one neuron (high speed) or several (pluricellular innervation) also plays a significant role for the speed of excitation conduction; in this case, the speed of excitation conduction slows down. "One-to-one" transmission is rare. It is possible to observe both the phenomena of spatial summation (when the response to stimulation of two preganglionic nerves exceeds the sum of the responses to their separate stimulation) and the phenomenon of occlusion, or suppression, when the discharge of a postganglionic fiber is less than the sum of the stimulation of several preganglionic nerves. After passing through the sympathetic trunk, the fibers are designated as postganglionic, or gray, connecting fibers, which are on average smaller in size than the preganglionic fibers.

Postganglionic fibers from the superior cervical ganglion, together with the carotid artery, are directed to the brain and face; from the stellate ganglion, in the form of plexuses of the vertebral artery, postganglionic fibers (another name is Frank's nerve) innervate the vessels of the brain that make up the vertebral artery basin.

The fibers that pass through the thoracic, abdominal, and pelvic nodes rush to the next switching station - the prevertebral nodes or plexuses. Unlike the sympathetic trunk, the cellular composition of this formation is more uniform and is represented mainly by medium-sized neurons. The best-known node, which is also one of the most important, is called the solar, and sometimes celiac, plexus, or "abdominal brain". It consists of two nodes (left and right). In the prevertebral nodes, the sympathetic fibers (which are part of the celiac nerve), which did not have contact with the neurons of the sympathetic chain, are interrupted, and parasympathetic neurons also appear (the paravertebral nodes are purely sympathetic formations).

After passing through the prevertebral nodes located in the abdominal cavity or pelvis, the autonomic fibers either go directly to the tissues they innervate (in these cases they are thin nerve fibers that release a chemical substance that affects the tissues by diffusion - diffusion synapses), or to ganglia located in the organs themselves (such intramural ganglia are found in the heart, gastrointestinal tract and other organs).

Sympathetic trunk

The sympathetic trunk (tnincus sympathicus) is a paired formation located on the sides of the spine. It consists of 20-25 nodes connected by interganglionares (rr. interganglionares). The nodes of the sympathetic trunk (ganglia trunci sympathici) are spindle-shaped, ovoid and irregular (polygonal). Only one type of branches approaches the sympathetic trunk - the so-called white communicating branches (rr. communicantes albi). Four types of branches emerge from the sympathetic trunk:

• gray communicating branches (rr. communicantes grisei) to the spinal nerves;
• sympathetic nerves to internal organs;
• sympathetic nerves to blood vessels;
• sympathetic nerves (celiac) to large autonomic plexuses located in the abdominal cavity and in the pelvic cavity.

The white communicating branch is a bundle of preganglionic nerve fibers that branches off from a spinal nerve (at the level of the thoracic and upper lumbar regions) and enters the adjacent ganglion of the sympathetic trunk. The white communicating branches contain preganglionic sympathetic nerve fibers, which are processes of neurons of the lateral intermediate columns (autonomous) of the spinal cord. These fibers pass through the anterior horns of the spinal cord and exit it as part of the anterior roots, and then go in the spinal nerves, from which they branch off as these nerves exit the spinal openings. White communicating branches are present only in the VIII cervical, all thoracic and two upper lumbar spinal nerves and approach all thoracic (including the cervicothoracic) and two upper lumbar nodes of the sympathetic trunk. Preganglionic fibers enter the cervical, lower lumbar, sacral and coccygeal nodes of the sympathetic trunk through the internodal branches of the sympathetic trunk.

Gray connecting branches emerge from the nodes of the sympathetic trunk along their entire length and are directed to the nearest spinal nerve. Gray connecting branches contain postganglionic sympathetic nerve fibers - processes of cells located in the nodes of the sympathetic trunk. As part of the spinal nerves and their branches, these postganglionic sympathetic fibers are directed to the skin, muscles, all organs and tissues, blood and lymphatic vessels, sweat and sebaceous glands, to the muscles that raise the hair, and provide their sympathetic innervation. From the sympathetic trunk, in addition to gray connecting branches, nerves extend to the internal organs and vessels (cardiac, esophageal, aortic, etc.). These nerves also contain postganglionic sympathetic fibers. In addition, sympathetic nerves extend from the sympathetic trunk to the nodes of the vegetative plexuses of the abdominal cavity and pelvis, containing preganglionic fibers that have passed in transit through the nodes of the sympathetic trunk. Topographically, the sympathetic trunk is divided into four sections: cervical, thoracic, lumbar, sacral (pelvic).

The cervical section of the sympathetic trunk is represented by three nodes and the internodal branches connecting them, which are located on the deep muscles of the neck behind the prevertebral plate of the cervical fascia. The preganglionic sympathetic fibers approach the cervical nodes along the internodal branches of the thoracic section of the sympathetic trunk, where they come from the vegetative nuclei of the lateral intermediate (gray) matter of the VIII cervical and six to seven upper thoracic segments of the spinal cord.

The superior cervical ganglion (ganglion cervicale superius) is the largest node of the sympathetic trunk. It is fusiform, its length reaches 2 cm or more (up to 10 cm), thickness - up to 0.5 cm. The superior cervical ganglion is located in front of the transverse processes of the I-III cervical vertebrae. In front of the node are the internal carotid artery, the initial part of the vagus nerve, behind - the long muscle of the head. The following branches containing postganglionic sympathetic fibers extend from the superior cervical sympathetic ganglion: gray communicating branches, internal carotid nerve, external carotid nerves, jugular nerve, laryngeal-pharyngeal branches, superior cervical cardiac nerve.

1. The grey communicating branches (rr. communicantes grisei) go to the cervical spinal nerves.
2. The internal carotid nerve (n. caroticus inteirms) goes to the artery of the same name and along the way forms the internal carotid plexus (plexus caroticus interims). Together with the internal carotid artery, this plexus enters the carotid canal and then into the cranial cavity. In the carotid canal, the carotid-tympanic nerves branch off from the plexus to the mucous membrane of the middle ear. After the internal carotid artery leaves the canal, the deep petrosal nerve (n. petrosus profundus) separates from the internal carotid plexus. It passes through the fibrous cartilage of the lacerated foramen and enters the pterygoid canal of the sphenoid bone, where it joins with the greater petrosal nerve, forming the nerve of the pterygoid canal (n. canalis pterygoidei). The nerve of the pterygoid canal (the more visible nerve), having entered the pterygopalatine fossa, joins the pterygopalatine ganglion. Having passed in transit through the pterygopalatine ganglion, the sympathetic fibers pass along the pterygopalatine nerves into the maxillary nerve and are distributed as part of its branches, providing sympathetic innervation of the vessels, tissues, glands of the mucous membrane of the mouth and nasal cavity, conjunctiva of the lower eyelid and skin of the face. The sympathetic fibers enter the orbit in the form of the periarterial plexus of the ophthalmic artery - a branch of the internal carotid artery. From the ophthalmic plexus, a sympathetic rootlet branches off to the ciliary ganglion. The fibers of this rootlet pass in transit through the ciliary ganglion and as part of the short ciliary nerves reach the eyeball, where they innervate the vessels of the eye and the muscle that dilates the pupil. In the cranial cavity, the internal carotid plexus continues into the perivascular plexus of the branches of the internal carotid artery.
3. The external carotid nerves (nn. carotici externi) in the form of 2-3 trunks are directed to the external carotid artery and form the sympathetic plexus of the same name (plexus carotici externus) along its course. The external carotid plexus extends along the branches of the artery of the same name, providing sympathetic innervation of the vessels, glands, smooth muscle elements and tissues of the organs of the head. The internal carotid plexus (plexus carotici intenuis) is located in the adventitia of the artery of the same name. The sympathetic fibers of this plexus are directed along the branches of this artery to the innervated organs.
4. The jugular nerve (n. jugularis) ascends along the walls of the internal jugular vein to the jugular foramen, where it divides into branches that go to the superior and inferior ganglia of the glossopharyngeal nerve and to the hypoglossal nerve. Due to this, the sympathetic fibers are distributed as part of the branches of the IX, X and XII cranial nerves.
5. The laryngeal-pharyngeal branches (rr. laryngopharyngei) participate in the formation of the laryngeal-pharyngeal plexus, innervate the vessels, mucous membrane of the pharynx and larynx, muscles and other tissues.
6. The superior cervical cardiac nerve (n. cardiacus cervicalis superior) descends parallel to the sympathetic trunk in front of the prevertebral plate of the cervical fascia. The right superior cardiac nerve runs along the brachiocephalic trunk and enters the deep part of the cardiac plexus on the posterior surface of the aortic arch. The left superior cervical cardiac nerve is adjacent to the left common carotid artery, enters the superficial part of the cardiac plexus, located between the aortic arch and the bifurcation of the pulmonary trunk.

The middle cervical ganglion (ganglion cervicale medium) is inconstant and is located anterior to the transverse process of the sixth cervical vertebra. Most often, this ganglion is located at the intersection of the cervical sympathetic trunk and the inferior thyroid artery. The length of the node is 0.75-1.5 cm, the thickness is about 0.4-0.5 cm. The node has an ovoid or triangular shape. The middle cervical ganglion is connected to the superior cervical ganglion by one internodal branch, and to the cervicothoracic (stellate) ganglion by two or three internodal branches. One of these branches passes in front of the subclavian artery, the other - behind, forming the subclavian loop (ansa subclavian or Viessan loop.

The middle sympathetic ganglion gives off gray connecting branches to the V and VI cervical spinal nerves, the middle cervical cardiac nerve (n. cardiacus cervicalis medius). This nerve runs lateral to the superior cervical cardiac nerve. The right middle cervical cardiac nerve runs along the brachiocephalic trunk, and the left one runs along the left common carotid artery. Both nerves enter the deep part of the cardiac plexus. Two or three thin nerves, which participate in the formation of the common carotid plexus and the plexus of the inferior thyroid artery, innervate the thyroid and parathyroid glands, depart from the middle cervical ganglion. In the absence of the middle cervical ganglion, all of the named branches depart from the internodal branches at the level of the transverse process of the VI cervical vertebra, and the postnodal sympathetic fibers enter these branches from the cervicothoracic ganglion.

The cervicothoracic (stellate) ganglion (ganglion cervicothoracicum) lies behind the subclavian artery, at the point where the vertebral artery branches off from it. This ganglion is formed by the fusion of the lower cervical ganglion with the first thoracic ganglion. The cervicothoracic ganglion is flattened in the anteroposterior direction, has an irregular (stellate) shape, and its diameter averages 8 mm.

A number of branches extend from the node:

1. Gray communicating branches (rr. communicantes grisei) are directed to the VI, VII, VIII cervical spinal nerves.
   Several trunks go to the subclavian artery, in the adventitia of which they form the subclavian plexus (plexus subclavicus), continuing to the vessels of the shoulder girdle and upper limb.

Several sympathetic branches join the vagus nerve and its branches, as well as the phrenic nerve.

2. The vertebral nerve (n. vertebralis) approaches the vertebral artery and participates in the formation of the sympathetic vertebral plexus (plexus vertebralis), from which the vessels of the brain and spinal cord are innervated. The inferior cervical cardiac nerve (n. cardiacus cervicalis inferior) passes behind the brachiocephalic trunk on the right, and behind the aorta on the left. The right and left cervical cardiac nerves enter the deep part of the cardiac plexus.

The thoracic section of the sympathetic trunk includes 9-12 thoracic nodes (ganglia thoracica), flattened, fusiform or polygonal. The sizes of the nodes are from 1 to 16 mm, on average 3-5 mm. The upper thoracic nodes up to the level of the sixth thoracic vertebra are located in the intercostal spaces along the line of the heads of the ribs. In the lower thoracic section, the nodes are located on the lateral surface of the vertebral bodies. The thoracic section of the sympathetic trunk is covered by the endothoracic fascia and parietal pleura. The posterior intercostal vessels pass transversely behind the sympathetic trunk. White connecting branches containing preganglionic sympathetic fibers approach the thoracic nodes of the sympathetic trunk from all thoracic spinal nerves. In turn, several types of branches depart from the thoracic nodes of the sympathetic trunk.

The gray rami communicantes, containing postganglionic sympathetic fibers, join adjacent spinal nerves.

The thoracic cardiac nerves (nn. cardiaci thoracici) originate from the second to fifth thoracic nodes, are directed forward and medially, and participate in the formation of the cardiac plexus.

From the thoracic nodes of the sympathetic trunk, thin sympathetic nerves (pulmonary, esophageal, aortic) branch off, which together with the branches of the vagus nerve form the right and left pulmonary plexus (plexus pulmonalis), esophageal plexus (plexus oesophagealis), and thoracic aortic plexus (plexus aorticus thoracicus). The branches of the thoracic aortic plexus continue onto the intercostal vessels and other branches of the thoracic aorta, forming periarterial plexuses along their course. Sympathetic nerves also branch off to the walls of the azygos and hemiazygos veins, the thoracic duct, and participate in their innervation. The largest branches of the sympathetic trunk in the thoracic region are the great and small splanchnic nerves.

The great thoracic splanchnic nerve (n. splanchnicus thoracicus major) is formed from several branches extending from the 5th to 10th thoracic ganglia of the sympathetic trunk. The common trunk of the great thoracic splanchnic nerve is directed downward and medially, adjacent to the bodies of the lower thoracic vertebrae, then penetrates into the abdominal cavity between the muscle bundles of the lumbar part of the diaphragm near the azygos vein on the right and the hemiazygos vein on the left and ends in the nodes of the celiac plexus. At the level of the 12th thoracic vertebra along the course of the great thoracic splanchnic nerve there is a small thoracic splanchnic ganglion (ganglionthoracicus splanchnicum).

The small thoracic splanchnic nerve (n. splanchnicus thoracicus minor) begins with branches extending from the 10th-11th, sometimes the 12th, node of the thoracic sympathetic trunk. The nerve descends lateral to the large thoracic splanchnic nerve, passes between the muscle bundles of the lumbar part of the diaphragm (together with the sympathetic trunk). Some of the fibers of this nerve end in the aortorenal node of the celiac plexus.

The large and small thoracic splanchnic nerves are represented mainly by preganglionic sympathetic fibers, and also contain some postganglionic fibers. These splanchnic nerves contain sensory fibers that conduct impulses from the internal organs to the spinal cord.

Next to the small thoracic splanchnic nerve there is an inconstant inferior thoracic splanchnic nerve (n. splanchnicus thoracicus imus), which starts from the XII (sometimes XI) thoracic ganglion of the sympathetic trunk and ends in the renal plexus.

The lumbar section of the sympathetic trunk is usually represented by 3-5 (from 2 to 7) lumbar nodes and the internodal branches connecting them.

The lumbar nodes (ganglia lumbalia) are fusiform, their size does not exceed 6 mm. The nodes are located on the anterolateral surface of the bodies of the lumbar vertebrae, medial to the psoas major muscle, and are covered by intra-abdominal fascia. The inferior vena cava adjoins the lumbar nodes of the right sympathetic trunk in front. The nodes of the left trunk adjoin the abdominal part of the aorta on the left. The lumbar nodes of the right and left sympathetic trunks are connected by transversely oriented connecting branches lying on the anterior surface of the lumbar vertebrae behind the aorta and inferior vena cava.

From each lumbar node, two types of branches extend:

1. gray rami communicantes containing postganglionic sympathetic fibers directed to the lumbar spinal nerves;
2. lumbar splanchnic nerves (nn. splanchnici lumbales), which have both preganglionic and postganglionic sympathetic nerve fibers directed to the celiac plexus and organ (vascular) autonomic plexuses (splenic, renal, gastric, adrenal).

The sacral section of the sympathetic trunk is formed by four sacral fusiform nodes, each about 5 mm in size, connected by internodal branches. The sacral nodes (ganglia sacralia) lie on the pelvic surface of the sacrum, medial to the pelvic openings. Below, the right and left sympathetic trunks converge and end on the anterior surface of the sacrum with an unpaired node common to these trunks. In the pelvic cavity, in front of the sacral sympathetic nodes, is the rectum, separated from them by a layer of fatty tissue and the parietal leaflet of the pelvic fascia. As in the lumbar region, there are transverse connections between the nodes of the sympathetic trunks of the right and left sides.

Three types of branches depart from the sacral nodes:

1. gray connecting branches, which contain post-nodal sympathetic fibers that are directed to the sacral and coccygeal spinal nerves and further to the areas where these nerves branch;
2. sacral visceral nerves (nn. splanchnici sacrales), which follow to the superior and inferior hypogastric (pelvic) autonomic plexuses;
3. organ branches leading to the organ and vascular plexuses of the small pelvis.