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Overview of the Autonomic Nervous System

by Phillip Low, MD

The autonomic nervous system (ANS) regulates physiologic processes. Regulation occurs without conscious control, ie, autonomously. The 2 major divisions are the sympathetic and parasympathetic systems.

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

Anatomy

The ANS receives input from parts of the 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: one set (called preganglionic) in the CNS, with connections to another set in ganglia outside the CNS. Efferent fibers from the ganglia (postganglionic fibers) lead to effector organs (see see Figure: 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 smooth muscle of blood vessels, viscera, lungs, scalp (piloerector muscles), and pupils; the heart; and 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 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); and ocular muscles.


Physiology

The ANS controls 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. The parasympathetic nervous system is anabolic; it conserves and restores (see Divisions of the Autonomic Nervous System).

Divisions of the Autonomic Nervous System

Division

Effects

Sympathetic

Increases the following:

  • Heart rate and contractility

  • Bronchodilation

  • Hepatic glycogenolysis and glucose release

  • BMR

  • Muscular strength

Causes sweaty palms

Decreases less immediately life-preserving functions (eg, digestion)

Controls ejaculation

Parasympathetic

Stimulates GI secretions and motility (including evacuation)

Slows heart rate

Reduces BP

Controls erection

Two major neurotransmitters in the ANS 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.

There are different subtypes of adrenergic receptors (see Neurotransmission:Norepinephrine) and cholinergic receptors (see Neurotransmission:Acetylcholine), which vary by location.

Etiology

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

  • Peripheral neuropathies

  • Aging

  • Parkinson disease

Other causes include

  • Autoimmune autonomic neuropathy

  • Multiple system atrophy

  • Spinal cord disorders

  • Drugs

  • Disorders of the neuromuscular junction (eg, botulism, Lambert-Eaton syndrome)

Evaluation

History

Symptoms suggesting autonomic insufficiency include

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

  • 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:

  • Postural BP and heart rate: In a normally hydrated patient, a sustained (eg, > 1 min) decrease of 20 mm Hg in systolic BP or a decrease of 10 mm Hg in diastolic BP with standing suggests autonomic insufficiency (see Orthostatic Hypotension). Heart rate change with respiration and standing should be noted; absence of physiologic sinus arrhythmia and failure of heart rate to increase with standing indicate autonomic insufficiency. In contrast, patients with postural tachycardia syndrome, a benign disorder, typically have postural tachycardia without hypotension.

  • Eye examination: Miosis and mild ptosis (Horner syndrome) suggest a sympathetic lesion. A dilated, unreactive pupil (Adie pupil) suggests a parasympathetic lesion.

  • GU and rectal reflexes: Abnormal GU and rectal reflexes may indicate ANS deficits. Testing includes the cremasteric reflex (normally, stroking the upper inner thigh results in retraction of the testes), anal wink reflex (normally, stroking perianal skin results in contraction of the anal sphincter), and bulbocavernosus reflex (normally, squeezing the glans penis or clitoris results in contraction of the anal sphincter).


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 BSA.

Cardiovagal testing evaluates heart rate response (via ECG rhythm strip) to deep breathing and to the Valsalva maneuver. If the ANS 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:

  • Head-up tilt: Blood is shifted to dependent parts, causing reflex responses in BP and heart rate. This test helps differentiate autonomic neuropathies from postural tachycardia syndrome.

  • Valsalva maneuver: This maneuver increases intrathoracic pressure and reduces venous return, causing BP changes and reflex vasoconstriction.

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

Plasma norepinephrine levels can be measured with patients supine and then after they stand for > 5 min. Normally, levels increase after standing. If patients have autonomic insufficiency, levels may not increase with standing and may be low in the supine position, particularly in postganglionic disorders (eg, autonomic neuropathy, pure autonomic failure).


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