Cholinergic, adrenergic, and purinergic neuromuscular transmission.

A general model of the autonomic neuromuscular junction is proposed which emphasizes muscle effector bundles with gap junctions (or 'nexuses') forming the low resistance pathways allowing electrotonic coupling between neighboring cells, and extensive terminal varicose nerve fibers with 'en passage' release of transmitter. Some variations in autonomic neuromuscular geometry are discussed. Junctional clefts vary from 15nm in densely-innervated tissues such as vas deferens and iris to 2,000 nm in some large elastic arteries. Postjunctional specializations take the form of subsynaptic cysternae (in vas deferens and iris) and aggregations of plasmalemmal vesicles (in circular intestinal muscle). Current views of the synthesis, storage, release, and inactivation of transmitter during cholinergic, adrenergic, and purinergic transmission are summarized.