any substance that causes an increase in activity in various parts of the nervous system or directly increases muscle activity. Cerebral, or psychic, stimulants act on the central nervous system and provide a temporary sense of alertness and well-being as well as relief from fatigue. Drugs such as caffeine and the amphetamines belong in this category, and several groups of drugs chemically similar to antihistamines and phenothiazines also act as mild psychic stimulants (see psychopharmacology). Cocaine, besides its effect as a local anesthetic, also stimulates the central nervous system, producing excitement and erratic behavior. The hallucinogenic drugs are also central nervous system stimulants.
A second class of stimulants that affect the medulla and spinal cord includes derivatives of niacinamide (nicotinic acid amide) and other chemically diverse compounds; they are sometimes used to speed the return to wakefulness after anesthesia or to counteract barbiturate poisoning. Ammonia, in smelling salts, is also a medullary stimulant; the alkaloid strychnine is a spinal-cord stimulant.
Other substances act mainly on the autonomic nervous system. Drugs that stimulate the parasympathetic portion of the autonomic nervous system, such as pilocarpine, physostigmine, and neostigmine, cause contracted pupils, salivation and sweating, slowed heartbeat, and lowered blood pressure. Drugs such as norepinephrine, epinephrine, and other catecholamines and synthetic analogs stimulate the sympathetic portion of the autonomic nervous system, resulting in dilated pupils, rapid heartbeat, and increased blood pressure. Because the sympathetic and parasympathetic systems have opposing physiological effects, stimulation of one system amounts to depression of the other. Some of the alkaloids from the ergot fungus act by direct stimulation of smooth muscle, inducing contractions in uterine and intestinal muscle.