Acetylcholine is a chemical messenger in the process of neurotransmission. Neurotransmission usually involves release of a chemical messenger, called the neurotransmitter, from the presynaptic cell followed by its binding to receptors on the postsynaptic cell (nerve, muscle, or gland). Synapses involving acetylcholine as the neurotransmitter are called cholinergic synapses. In cholinergic synapses, synaptic vesicles, each containing about 10³ to 10⁴ acetylcholine molecules Figure 21.33b are stored. Upon stimulation, acetylcholine is released and moves towards the postsynaptic membrane of the receptor dendrite (Figure 21.33b). Binding of acetylcholine by receptors triggers the opening of ion channels in the postsynaptic membrane (Figure 21.33d), initiating an action potential that can be passed on to the next axon (Figure 21.33e).

Acetylcholine is synthesized from choline and acetyl-CoA, by choline acetyltransferase, in the axonal terminal bulbs of nerve cells. After acetylcholine has been released from vesicles and bound to the receptors, the neurotransmitter is rapidly hydrolyzed by the enzyme acetylcholinesterase, yielding choline, which binds poorly to acetylcholine receptors. Degradation of acetylcholine restores the resting potential in the postsynaptic membrane. After it has been released from vesicles and bound to the receptors, the neurotransmitter is rapidly hydrolyzed by the enzyme acetylcholinesterase, yielding choline, which binds poorly to acetylcholine receptors. Degradation of acetylcholine restores the resting potential in the postsynaptic membrane.

Reactions involving acetylcholine are as follows:

1. Choline + Acetyl-CoA <=> Acetylcholine + CoASH (catalyzed by Choline Acetyltransferase)

2. Acetylcholine + H2O <=> Choline + Acetate (catalyzed by Acetylcholinesterase)

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