Autonomic nervous system

The autonomic nervous system (systema nervosum autonomicum) is a part of the nervous system that controls the functions of internal organs, glands, and blood vessels, and has an adaptive and trophic effect on all human organs. The autonomic nervous system maintains the constancy of the internal environment of the body (homeostasis). The function of the autonomic nervous system is not controlled by human consciousness, but it is subordinate to the spinal cord, cerebellum, hypothalamus, basal nuclei of the endbrain, limbic system, reticular formation, and cerebral cortex.

The distinction between the vegetative (autonomous) nervous system is determined by some of its structural features. These features include the following:

1. focal location of vegetative nuclei in the central nervous system;
2. accumulation of bodies of effector neurons in the form of nodes (ganglia) as part of the peripheral autonomic plexuses;
3. two-neuronal nature of the nerve pathway from the nuclei in the central nervous system to the innervated organ;
4. preservation of features reflecting a slower evolution of the autonomic nervous system (in comparison with the animal nervous system): smaller caliber of nerve fibers, lower speed of conduction of excitation, absence of a myelin sheath in many nerve conductors.

The autonomic nervous system is divided into central and peripheral sections.

The central department includes:

1. parasympathetic nuclei of the III, VII, IX and X pairs of cranial nerves located in the brain stem (midbrain, pons, medulla oblongata);
2. parasympathetic sacral nuclei located in the gray matter of the three sacral segments of the spinal cord (SII-SIV);
3. vegetative (sympathetic) nucleus located in the lateral intermediate column [lateral intermediate (gray) matter] of the VIII cervical, all thoracic and two upper lumbar segments of the spinal cord (CVIII-ThI-LII).

The peripheral part of the autonomic nervous system includes:

1. vegetative (autonomic) nerves, branches and nerve fibers emerging from the brain and spinal cord;
2. vegetative (autonomous) visceral plexuses;
3. nodes of the vegetative (autonomous, visceral) plexuses;
4. sympathetic trunk (right and left) with its nodes, internodal and connecting branches and sympathetic nerves;
5. nodes of the parasympathetic part of the autonomic nervous system;
6. vegetative fibers (parasympathetic and sympathetic) that go to the periphery (to organs, tissues) from the vegetative nodes that are part of the plexuses and located in the thickness of the internal organs;
7. nerve endings involved in autonomic reactions.

Neurons of the nuclei of the central part of the autonomic nervous system are the first efferent neurons on the paths from the CNS (spinal cord and brain) to the innervated organ. The fibers formed by the processes of these neurons are called preganglionic nerve fibers, since they go to the nodes of the peripheral part of the autonomic nervous system and end in synapses on the cells of these nodes.

The vegetative nodes are part of the sympathetic trunks, large vegetative plexuses of the abdominal cavity and pelvis, and are also located in the thickness of or near the organs of the digestive, respiratory systems and genitourinary system, which are innervated by the autonomic nervous system.

The size of the vegetative nodes is determined by the number of cells located in them, which ranges from 3000-5000 to many thousands. Each node is enclosed in a connective tissue capsule, the fibers of which, penetrating deep into the node, divide it into lobes (sectors). Between the capsule and the body of the neuron are satellite cells - a type of glial cells.

Glial cells (Schwann cells) include neurolemmocytes, which form the sheaths of peripheral nerves. Neurons of the autonomic ganglia are divided into two main types: Dogel cells of type I and type II. Dogel cells of type I are efferent, and preganglionic processes end on them. These cells are characterized by a long, thin, unbranched axon and many (from 5 to several dozen) dendrites branching near the body of this neuron. These cells have several slightly branched processes, among which there is an axon. They are larger than Dogel neurons of type I. Their axons enter into synaptic connection with efferent Dogel neurons of type I.

Preganglionic fibers have a myelin sheath, which is why they are whitish. They exit the brain as part of the roots of the corresponding cranial and spinal nerves. The nodes of the peripheral part of the autonomic nervous system contain the bodies of the second efferent (effector) neurons lying on the paths to the innervated organs. The processes of these second neurons, which carry the nerve impulse from the autonomic nodes to the working organs (smooth muscles, glands, vessels, tissues), are postganglionic nerve fibers. They do not have a myelin sheath, and therefore they are gray.

The speed of impulse conduction along sympathetic preganglionic fibers is 1.5-4 m/s, and parasympathetic - 10-20 m/s. The speed of impulse conduction along postganglionic (unmyelinated) fibers does not exceed 1 m/s.

The bodies of the afferent nerve fibers of the autonomic nervous system are located in the spinal (intervertebral) nodes, as well as in the sensory nodes of the cranial nerves; in the proper sensory nodes of the autonomic nervous system (Dogel cells type II).

The structure of the reflex autonomic arc differs from the structure of the reflex arc of the somatic part of the nervous system. The reflex arc of the autonomic nervous system has an efferent link consisting of two neurons rather than one. In general, a simple autonomic reflex arc is represented by three neurons. The first link of the reflex arc is a sensory neuron, the body of which is located in the spinal ganglia or ganglia of the cranial nerves. The peripheral process of such a neuron, which has a sensitive ending - a receptor, originates in organs and tissues. The central process as part of the posterior roots of the spinal nerves or sensory roots of the cranial nerves is directed to the corresponding vegetative nuclei of the spinal cord or brain. The efferent (outgoing) path of the autonomic reflex arc is represented by two neurons. The body of the first of these neurons, the second in a simple autonomic reflex arc, is located in the autonomic nuclei of the central nervous system. This neuron can be called intercalary, since it is located between the sensory (afferent, afferent) link of the reflex arc and the third (efferent, efferent) neuron of the efferent pathway. The effector neuron is the third neuron of the autonomic reflex arc. The bodies of effector neurons are located in the peripheral nodes of the autonomic nervous system (sympathetic trunk, autonomic nodes of the cranial nerves, nodes of extra- and intraorgan autonomic plexuses). The processes of these neurons are directed to organs and tissues as part of organ autonomic or mixed nerves. Postganglionic nerve fibers end in smooth muscles, glands, in the walls of blood vessels and in other tissues with corresponding terminal nerve apparatuses.

Based on the topography of the autonomic nuclei and nodes, differences in the length of the first and second neurons of the efferent pathway, as well as the features of the functions, the autonomic nervous system is divided into two parts: sympathetic and parasympathetic.

Physiology of the autonomic nervous system

The autonomic nervous system controls blood pressure (BP), heart rate (HR), body temperature and weight, digestion, metabolism, water and electrolyte balance, sweating, urination, defecation, sexual response, and other processes. Many organs are controlled primarily by either the sympathetic or parasympathetic system, although they can receive input from both parts of the autonomic nervous system. More often, the action of the sympathetic and parasympathetic systems on the same organ is directly opposite, for example, sympathetic stimulation increases the heart rate, and parasympathetic stimulation decreases it.

The sympathetic nervous system promotes intensive activity of the body (catabolic processes) and hormonally provides the "fight or flight" phase of the stress response. Thus, sympathetic efferent signals increase the heart rate and myocardial contractility, cause bronchodilation, activate glycogenolysis in the liver and the release of glucose, increase the basal metabolic rate and muscle strength; and also stimulate sweating on the palms. Less important life-supporting functions in a stressful environment (digestion, renal filtration) are reduced under the influence of the sympathetic autonomic nervous system. But the process of ejaculation is completely under the control of the sympathetic division of the autonomic nervous system.

The parasympathetic nervous system helps restore the body's resources, i.e. ensures anabolism processes. The parasympathetic autonomic nervous system stimulates the secretion of digestive glands and the motility of the gastrointestinal tract (including evacuation), reduces the heart rate and blood pressure, and ensures erection.

The functions of the autonomic nervous system are provided by two main neurotransmitters - acetylcholine and norepinephrine. Depending on the chemical nature of the mediator, nerve fibers that secrete acetylcholine are called cholinergic; these are all preganglionic and all postganglionic parasympathetic fibers. Fibers that secrete norepinephrine are called adrenergic; these are most postganglionic sympathetic fibers, with the exception of those innervating blood vessels, sweat glands and the arectores pilorum muscles, which are cholinergic. The palmar and plantar sweat glands partially respond to adrenergic stimulation. Subtypes of adrenergic and cholinergic receptors are distinguished depending on their localization.

Evaluation of the autonomic nervous system

Autonomic dysfunction may be suspected in the presence of symptoms such as orthostatic hypotension, lack of tolerance to high temperatures, and loss of bowel and bladder control. Erectile dysfunction is one of the early symptoms of autonomic dysfunction. Xerophthalmia and xerostomia are not specific symptoms of autonomic dysfunction.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ], [ 8 ]

Physical examination

A sustained decrease in systolic blood pressure by more than 20 mm Hg or diastolic by more than 10 mm Hg after assuming a vertical position (in the absence of dehydration) suggests the presence of autonomic dysfunction. Attention should be paid to changes in heart rate (HR) during breathing and when changing body position. The absence of respiratory arrhythmia and insufficient increase in HR after assuming a vertical position indicate autonomic dysfunction.

Miosis and moderate ptosis (Horner's syndrome) indicate damage to the sympathetic division of the autonomic nervous system, and a dilated pupil that does not react to light (Adie's pupil) indicates damage to the parasympathetic autonomic nervous system.

Abnormal urogenital and rectal reflexes may also be symptoms of autonomic nervous system insufficiency. The examination includes assessment of the cremasteric reflex (normally, stroking the skin of the thigh results in elevation of the testicles), anal reflex (normally, stroking the perianal skin results in contraction of the anal sphincter), and bulbocavernous reflex (normally, compression of the glans penis or clitoris results in contraction of the anal sphincter).

Laboratory research

In the presence of symptoms of autonomic dysfunction, in order to determine the severity of the pathological process and an objective quantitative assessment of the autonomic regulation of the cardiovascular system, a cardiovagal test, tests for the sensitivity of peripheral α-drenergic receptors, and a quantitative assessment of sweating are performed.

The quantitative sudomotor axon reflex test is used to check the function of postganglionic neurons. Local sweating is stimulated by acetylcholine iontophoresis, electrodes are placed on the shins and wrists, the intensity of sweating is recorded by a special sudometer that transmits information in analog form to a computer. The test result may be a decrease in sweating, or its absence, or continued sweating after stimulation stops. The thermoregulatory test is used to assess the condition of the preganglionic and postganglionic conduction pathways. Dye tests are used much less often to assess the function of sweating. After applying dye to the skin, the patient is placed in a closed room that is heated until maximum sweating is achieved; sweating leads to a change in the color of the dye, which reveals areas of anhidrosis and hypohidrosis and allows for their quantitative analysis. The absence of sweating indicates damage to the efferent part of the reflex arc.

Cardiovagal tests evaluate the response of the heart rate (ECG recording and analysis) to deep breathing and the Valsalva maneuver. If the autonomic nervous system is intact, the maximum increase in heart rate is noted after the 15th heartbeat and a decrease after the 30th. The ratio between the RR intervals at the 15th to 30th beats (i.e. the longest interval to the shortest) - the ratio 30:15 - is normally 1.4 (Valsalva ratio).

Peripheral adrenoreceptor sensitivity tests include heart rate and blood pressure testing in the tilt test (passive orthostatic test) and the Valsalva test. During the passive orthostatic test, blood volume is redistributed to the underlying body parts, causing reflex hemodynamic responses. The Valsalva test evaluates changes in blood pressure and heart rate as a result of increased intrathoracic pressure (and decreased venous inflow), causing characteristic changes in blood pressure and reflex vasoconstriction. Normally, changes in hemodynamic parameters occur over 1.5-2 minutes and have 4 phases, during which blood pressure increases (phases 1 and 4) or decreases after rapid recovery (phases 2 and 3). Heart rate increases in the first 10 seconds. If the sympathetic division is affected, a blockade of the response occurs in the 2nd phase.