Cholinergic Transmission

The neurotransmitter of the parasympathetic system is Acetylcholine, which is synthesized from choline and acetyl-coenzyme A in the cholinergic neurons with the help of choline-acetyltransferase enzyme. It is stored in the vesicles in the axonal terminals of the cholinergic neurons and released in response to impulses from the central nervous system. After release it diffuses in the synaptic space and acts on the specific receptors (muscarinic &/or nicotinic) on the target sites. Then it is rapidly metabolized (within few milliseconds) to choline and acetate by acetyl-cholinesterase enzyme (Fig. 2.1.2). Choline is actively taken up again by the corresponding neuron and reutilized for the synthesis of Acetylcholine.

Fig. 2.1.2. (a) Synthesis of acetylcholine from choline and acetyl-coenzyme A and metabolism to choline and acetate. (b) Cholinergic neuron, showing storage and release of acetylcholine at its axonal terminal and the target site.