Therapeutic Overview

The parasympathetic nervous system has a trophotropic function to conserve energy. Stimulation of this system leads to decreased heart rate, blood pressure, and respiration, increased secretions, and miosis. Postganglionic parasympathetic nerves release acetylcholine (ACh), which activates muscarinic cholinergic receptors on exocrine glands, smooth muscle, and cardiac muscle to produce parasympathetic responses. ACh is also released from some postganglionic sympathetic nerves, where it stimulates muscarinic receptors located on sweat glands and some blood vessels. Both parasympathetic and sympathetic ganglia use ACh as the neurotransmitter, as does the splanchnic nerve innervating the adrenal medulla; the receptors that mediate responses at these sites are neuronal nicotinic receptors. The receptors that mediate the responses of ACh at the skeletal neuromuscular junction are muscle nicotinic receptors and differ in composition from the neuronal nicotinic receptors. ACh is also released from neurons throughout the central nervous system (CNS), where it interacts with both muscarinic and neuronal nicotinic receptors.

Drugs affecting the parasympathetic nervous system mimic, antagonize, or prolong the actions of ACh at muscarinic receptors. Drugs that mimic or prolong the actions of ACh at muscarinic receptors are termed parasympathomimetics or cholinomimetics and are used to reduce intraocular pressure in glaucoma and to increase the motility of the gastrointestinal (GI) and urinary tracts.

Drugs that block the actions of ACh at muscarinic receptors are termed muscarinic receptor antagonists, parasympatholytics, or cholinolytics, and are used to treat motion sickness, relieve some of the symptoms of Parkinson’s disease (see Chapter 28), dilate the pupils for ocular examination, reduce motility of the GI and urinary tracts, dilate the airways of the lung in patients with chronic obstructive pulmonary disease (COPD)

(see Chapter 16), and reduce acid secretion in individuals with peptic ulcer disease (see Chapter 18).

Drugs that antagonize the actions of ACh at ganglionic nicotinic receptors are referred to as ganglionic blockers and are used to treat hypertensive emergencies. Drugs that antagonize the actions of ACh at nicotinic receptors at the neuromuscular junction are termed neuromuscular blocking agents and are used to relax skeletal muscle (see Chapter 12).

A summary of the classes and primary uses of compounds that affect the parasympathetic nervous system and autonomic ganglia are in the Therapeutic Overview Box.