 |
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 can affect any system of the body. They can originate in the peripheral or central nervous system and may be primary or secondary to other disorders.
Anatomy
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.
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, 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).
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; the vagus nerve contains about 75% of all parasympathetic fibers. Parasympathetic 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, including blood vessels of the head, neck, and thoracoabdominal viscera; lacrimal and salivary glands; smooth muscle of viscera and glands (eg, liver, spleen, colon, kidneys, bladder, genitals); and ocular muscles.
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.
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 and activates fight-or-flight responses. Thus, sympathetic output increases heart rate and contractility, bronchodilation, hepatic glycogenolysis and glucose release, BMR, and muscular strength; it also causes sweaty palms. Less immediately life-preserving functions (eg, digestion, renal filtration) are decreased. Ejaculation is a sympathetic function.
The parasympathetic nervous system is anabolic; it conserves and restores. GI secretions and motility (including evacuation) are stimulated, heart rate is slowed, and BP decreases. Erection is a parasympathetic function.
Two major neurotransmitters in the ANS are acetylcholine and norepinephrine . Fibers that secrete acetylcholine are termed cholinergic; they include all preganglionic fibers and all postganglionic parasympathetic fibers. Fibers that secrete norepinephrine are termed adrenergic; they include most postganglionic sympathetic fibers, except for those that innervate piloerectors, sweat glands, and blood vessels, which are cholinergic. However, sweat glands on the palms and soles also respond to adrenergic stimulation to some degree. There are different subtypes of adrenergic (see Neurotransmission: Norepinephrine) and cholinergic (see Neurotransmission: Acetylcholine) receptors, varying by location.
Evaluation
History:
Symptoms suggesting autonomic dysfunction include orthostatic hypotension, heat intolerance, and loss of bladder and bowel control. Erectile dysfunction is an early symptom. Other possible symptoms include dry eyes and dry mouth, but they are nonspecific.
Physical
examination:
In a normally hydrated patient, a sustained decrease of > 20 mm Hg in systolic BP or a decrease of > 10 mm Hg in diastolic BP with standing suggests autonomic dysfunction. 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 dysfunction.
Miosis and mild ptosis (Horner's syndrome) suggest a sympathetic lesion. A dilated, unreactive pupil (Adie's pupil) suggests a parasympathetic lesion.
Abnormal GU and rectal reflexes may indicate ANS deficits. Testing includes the cremasteric reflex (normally, stroking the 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 dysfunction, sudomotor, cardiovagal, and adrenergic tests are usually done to help determine severity and distribution of the dysfunction.
The quantitative sudomotor axon-reflex test evaluates integrity of postganglionic neurons using iontophoresis; electrodes filled with acetylcholine are placed on the legs and wrist to stimulate sweat glands, and the volume of sweat is then measured. The test can detect decreased, absent, or persistent (after stimulus discontinuation) sweat production. The thermoregulatory sweat 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; the ratio of longest to shortest R-R interval (Valsalva ratio) should be ≥ 1.4.
Adrenergic testing evaluates response of beat-to-beat BP to the head-up tilt and Valsalva maneuver. The head-up tilt shifts blood to dependent parts, causing reflex responses. The Valsalva maneuver increases intrathoracic pressure and reduces venous return, causing BP changes and reflex vasoconstriction. In both tests, the pattern of responses is an index of adrenergic function.
Last full review/revision November 2005
Content last modified November 2005
| |
