Acetylcholine and Norepinephrine Have Different Postsynaptic Receptors

The neurotransmitters secreted by the ANS all stimulate their target organ by first binding with a postsynaptic receptor. These receptorsare proteins in the cell membrane. W^rhen the transmitter binds with the postsynaptic receptor, the membrane’s permeabil­ity to selected ions is changed, and the postsynaptic membrane potential either increases or decreases, with a resulting change in the probability' of action potentials in the postsynaptic cell.

Acetydcholine stimulates two different types of receptors (Figure 13-4). Muscarinic acetylcholine receptors are found on all the target cells stimulated by postganglionic parasym­pathetic neurons and cholinergic postganglionic neurons of the sympathetic nervous system. Nicotinic receptors are found at all synapses between autonomic preganglionic and postganglionic neurons and at the somatic neuromuscular junction.

Muscarinic receptors were named because they are stimulated by muscarine, a toadstool poison. Muscarine does not stimulate nicotinic receptors. Nicotine stimulates the nicotinic receptors but not muscarinic receptors. Acetyl­choline stimulates both, and different drugs block each receptor. For example, atropine blocks muscarinic receptors, whereas curare blocks nicotinic receptors.

There are two major types of adrenergic receptors, called alpha (α) and beta (β) receptors. The β receptors have been Iurthersubdivided into β∣ and β₂ receptors, on the basis of the effect ofadrenergic blocking and stimulating drugs. There is now evidence for a third class of β receptor (β₃) and for two classes of α receptor (a₁ and α₂).