Normal function of all components of the ANS is not required to maintain life, as long as environmental conditions are a constant and optimum. Abnormal autonomic functions, however, markedly affect the individual's ability to respond to changing conditions. This can be demonstrated by sympathectomy, the removal of sympathetic ganglia. An animal becomes highly sensitive to heat, cold, or other forms of stress following sympathectomy. In a hot environment the animal's ability to lose heat by increasing blood flow to the skin and by sweating is decreased. When exposed to the cold, the animal is less able to reduce blood flow to the skin and conserve heat. Sympathectomy also results in low blood pressure caused by dilation of peripheral blood vessels and results in the inability to increase blood pressure during periods of physical activity.

Orthostatic hypotension is a drop in blood pressure that occurs when a person who was sitting or lying down suddenly stands up. It is sometimes caused by a CNS disorder that decease's the frequency of action potentials in sympathetic nerves innervating blood vessels. Consequently, on standing, blood pools in dilated blood vessels in the lower extremities, a small amount of blood returns to the heart, and the amount of blood the heart pumps decreases. Blood pressure decreases, resulting in reduced blood flow to the brain, which causes fainting because of a lack of oxygen.

Raynaud's disease involves the spasmodic contraction of blood vessels in the periphery, especially in the digits, and results in pale, cold hands that are prone to ulcerations and gangrene because of poor circulation. This condition can be caused by exaggerated sensitivity of blood vessels to sympathetic innervation. Preganglionic denervation (cutting the preganglionic neurons) is occasionally performed to alleviate the condition.

Hyperhidrosis (hi'per-hi-dro'sis) or excessive sweating, is caused by exaggerated sympathetic innervation of the sweat glands.

Achalasia (a-kal-a'ze-ah) is characterized by difficulty in swallowing and in controlling contraction of the esophagus where it enters the stomach. Therefore normal peristaltic contractions of the esophagus are interrupted. The swallowing reflex is controlled partly by somatic motor reflexes and partly by parasympathetic reflexes. The cause of achalasia can be abnormal parasympathetic regulation of the swallowing reflex. The condition is aggravated by emotions.

Dysautonomia (dis-aw-to-no'me-ah), an inherited condition involving an autosomal recessive gene, causes reduced tear gland secretion, poor vasomotor control, trouble in swallowing, and other symptoms. It is the result of poorly controlled autonomic reflexes.

Hirschsprung's disease, or megacolon, is caused by a functional obstruction in the lower colon and rectum. Ineffective parasympathetic innervation and a predominance of sympathetic innervation of the colon inhibits peristaltic contractions, causing feces to accumulate above the inhibited area. The resulting dilation of the colon can be so great that surgery is required to alleviate the condition.

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