NEUROPHARMACOLOGIC DRUGS

These drugs act on the nervous system. There are two major types of neuropharmacologic drugs: autonomic drugs and central nervous system drugs.

Autonomic Drugs. These drugs influence the body in a manner simi­lar to the action of the parasympathetic and sympathetic nerves of the autonomic nervous system.

The function of the sympathetic nerve network in the body is (l) to stimulate the flow of epinephrine from the adrenal gland, (2) to increase heart rate, (3) to constrict blood vessels, and (4) to dilate air passages.

Drugs which mimic the action of sympathetic nerves are called sympathomimetic or adrenergic agents. They stimulate the flow of epi­nephrine, increase heart rate, constrict blood vessels, and dilate air pass­ages. Examples of sympathomimetic drugs are epinephrine (adrenaline) and norepinephrine (noradrenaline). These drugs are the same chemicals which are naturally released from the sympathetic nerve endings and ad­renal glands during times of stress or

emergency.

Drugs which mimic the action of parasympathetic nerves are called parasympathomimetic or cholinergic agents. These drugs oppose the ac­tions of the sympathomimetic (adrenergic) drugs: they slow down heart rate, constrict air passages, and stimulate involuntary muscles in the digestive tract and other organs. The parasympathetic agent which is produced normally by parasympathetic nerve endings is called acetylcholine.

Acetylcholine, unlike a drug such as epi­nephrine (adrenaline), cannot be administered to patients. This is because there are enzymes in the body called holinesterases which inactivate acetylcholine almost as quickly as it is given. Other cholinergic drugs are, therefore, chosen as exogenous agents. One example of a cholinergic drug similar to acetylcholine in effect but longer lasting in the body is bethanechol. Bethanechol is used in postoperative urinary retention to include the constriction of the urinary bladder, aiding uri­nation.

Other autonomic drugs are parasympatholytic agents which oppose the effect of parasympathetic nerve stimulation. Examples of these drugs are atropine and belladonna, which are also known as antispasmodic drugs because they act to relax the muscles in the gastrointestinal tract and decrease peristalsis.

Sympatholytic agents,which block the action of the sympathetic nerv­ous system, include reserpine, guanethidine, and phentolamine. These drugs are used to decrease blood pressure and protect against the excess epinephrine secretion liberated by tumors of the adrenal gland.

Various types of autonomic drugs

Sympathomimetic (adrenergic):

Parasympathomimetic (cholinergic):

Sympatholytic

Parasympatholytic

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