Cholinergic Receptors

Cholinergic Receptors

Sympathetic and parasympathetic preganglionic fibers secrete Ach, and are therefore called cholinergic fibers. This is in contrast to most sympathetic ­postganglionic axons, which release NE, and are called adrenergic fibers. The effects of ACh and NE upon their effectors are not always excitatory or inhibitory, because the action of each neurotransmitter is based on the receptor to which it binds. Each autonomic ­neurotransmitter binds with two or more receptor types, which allows it to activate or inhibit various body targets. TABLE 14-2 summarizes adrenergic and cholinergic receptors and their effects on organs. The two types of cholinergic receptors are nicotinic recep-tors and muscarinic receptors.

Nicotinic Receptors

Nicotinic receptors respond to nicotine, whereas muscarinic receptors respond to the toxin known as muscarine, which is derived from mushrooms. All ACh receptors fall into these two categories. Nicotinic receptors are present on all postganglionic neurons, whether they are sympathetic or parasympathetic. They are also present on the hormone-producing cells in the adrenal medulla and on the sarcolemma of ­skeletal muscle cells at neuromuscular junctions. ACh binding to nicotinic receptors always causes stimula-tion, directly opening ion channels and depolarizing postsynaptic cells.

Muscarinic Receptors

Muscarinic receptors are present on all parasympa-thetic target organs and certain sympathetic targets, such as the eccrine sweat glands. They are activated by ACh. All these effector cells are stimulated by post-ganglionic cholinergic fibers. ACh binding to musca-rinic receptors causes either inhibitory or stimulatory effects, based on the type of muscarinic receptor on the target organ. When ACh binds to cardiac muscle receptors, heart activity is slowed, and when it binds to smooth muscle receptors in the gastrointestinal tract, motility is increased.