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Overview of the Autonomic Nervous System

By

Phillip Low

, MD, College of Medicine, Mayo Clinic

Last full review/revision Sep 2021| Content last modified Sep 2021
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The autonomic nervous system regulates physiologic processes. Regulation occurs without conscious control, ie, autonomously. The 2 major divisions are the

  • Sympathetic system

  • Parasympathetic system

Disorders of the autonomic nervous system cause autonomic insufficiency or failure and can affect any system of the body.

Anatomy of the Autonomic Nervous System

The autonomic nervous system receives input from parts of the central nervous system (CNS) that process and integrate stimuli from the body and external environment. These parts include the hypothalamus, nucleus of the solitary tract, reticular formation, amygdala, hippocampus, and olfactory cortex.

The sympathetic and parasympathetic systems each consist of 2 sets of nerve bodies:

The autonomic nervous system

The autonomic nervous system

Sympathetic

The preganglionic cell bodies of the sympathetic system are located in the intermediolateral horn of the spinal cord between T1 and L2 or L3.

The sympathetic ganglia are adjacent to the spine and consist of the vertebral (sympathetic chain) and prevertebral ganglia, including the superior cervical, celiac, superior mesenteric, inferior mesenteric, and aorticorenal ganglia.

Long fibers run from these ganglia to effector organs, including the following:

  • Smooth muscle of blood vessels, viscera, lungs, scalp (piloerector muscles), and pupils

  • Heart

  • Glands (sweat, salivary, and digestive)

Parasympathetic

The preganglionic cell bodies of the parasympathetic system are located in the brain stem and sacral portion of the spinal cord. Preganglionic fibers exit the brain stem with the 3rd, 7th, 9th, and 10th (vagus) cranial nerves and exit the spinal cord at S2 and S3; the vagus nerve contains about 75% of all parasympathetic fibers.

Parasympathetic ganglia (eg, ciliary, sphenopalatine, otic, pelvic, and vagal ganglia) are located within the effector organs, and postganglionic fibers are only 1 or 2 mm long. Thus, the parasympathetic system can produce specific, localized responses in effector organs, such as the following:

  • Blood vessels of the head, neck, and thoracoabdominal viscera

  • Lacrimal and salivary glands

  • Smooth muscle of glands and viscera (eg, liver, spleen, colon, kidneys, bladder, genitals)

  • Muscles of the pupil

Physiology of the Autonomic Nervous System

The autonomic nervous system controls blood pressure (BP), heart rate, body temperature, weight, digestion, metabolism, fluid 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 may receive input from both; occasionally, functions are reciprocal (eg, sympathetic input increases heart rate; parasympathetic decreases it).

The sympathetic nervous system is catabolic; it activates fight-or-flight responses.

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Two major neurotransmitters in the autonomic nervous system are

  • Acetylcholine: Fibers that secrete acetylcholine (cholinergic fibers) include all preganglionic fibers, all postganglionic parasympathetic fibers, and some postganglionic sympathetic fibers (those that innervate piloerectors, sweat glands, and blood vessels).

  • Norepinephrine: Fibers that secrete norepinephrine (adrenergic fibers) include most postganglionic sympathetic fibers. Sweat glands on the palms and soles also respond to adrenergic stimulation to some degree.

Etiology of Autonomic Insufficiency

Disorders causing autonomic insufficiency or failure can originate in the peripheral or central nervous system and may be primary or secondary to other disorders.

The most common causes of autonomic insufficiency are

Other causes include

Autonomic insufficiency that occurs with COVID-19 typically develops after recovery from the respiratory and other acute systemic symptoms of COVID. The usual manifestation is the postural orthostatic tachycardia syndrome (POTS), which is characterized by abnormal autonomic responses (eg, light-headedness) when upright (orthostatic intolerance). Whether the mechanism is viral or immune-mediated is unknown.

Evaluation of Autonomic Insufficiency

History

Symptoms suggesting autonomic insufficiency include

  • Orthostatic intolerance (development of autonomic symptoms such as light-headedness that is relieved by sitting down) due to orthostatic hypotension or postural orthostatic tachycardia syndrome

  • Heat intolerance

  • Loss of bladder and bowel control

  • Erectile dysfunction (an early symptom)

Other possible symptoms include dry eyes and dry mouth, but they are less specific.

Physical examination

Important parts of the examination include the following:

Laboratory testing

If patients have symptoms and signs suggesting autonomic insufficiency, sudomotor, cardiovagal, and adrenergic testing is usually done to help determine severity and distribution of the insufficiency.

Sudomotor testing includes the following:

  • Quantitative sudomotor axon-reflex test: This test evaluates integrity of postganglionic fibers. The fibers are activated by iontophoresis using acetylcholine. Standard sites on the leg and wrist are tested, and the volume of sweat is then measured. The test can detect decreased or absent sweat production.

  • Thermoregulatory sweat test: This test evaluates both preganglionic and postganglionic pathways. After a dye is applied to the skin, patients enter a closed compartment that is heated to cause maximal sweating. Sweating causes the dye to change color, so that areas of anhidrosis and hypohidrosis are apparent and can be calculated as a percentage of body surface area (BSA).

Cardiovagal testing evaluates heart rate response (via ECG rhythm strip) to deep breathing and to the Valsalva maneuver. If the autonomic nervous system is intact, heart rate varies with these maneuvers; normal responses to deep breathing and the Valsalva ratio vary by age.

Adrenergic testing evaluates response of beat-to-beat BP to the following:

With the head-up tilt test and Valsalva maneuvers, the pattern of responses is an index of adrenergic function.

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