The Ultrastructure of Autonomic Cholinergic Nerves and Junctions

Abstract

Publisher Summary This chapter discusses the ultrastructure of autonomic cholinergic nerves and junctions. Cholinergic neurones are widely distributed in the autonomic nervous system. Terminal varicosities of cholinergic autonomic nerves contain a predominance of small agranular vesicles and a small number of large granular vesicles, which differ in some respect from those found in adrenergic nerves and from “large opaque vesicles” found in non-adrenergic, non-cholinergic “purinegric” or “peptidergic” nerves. In the gastrointestinal tract, some nerve profiles contain flattened agranular vesicles that may represent another type of cholinergic nerve; some profiles contain mixtures of small agranular vesicles and other vesicle types perhaps indicating co-transmitters. The chapter also describes a generalized model of the cholinergic neuromuscular junction, which emphasizes “en passage” release of transmitter from extensive terminal, varicose fibres and electrotonic spread of activity via “gap junctions” between neighboring smooth muscle cells within effector bundles. The minimum width of the junctional cleft between cholinergic varicosities and effector cells varies between 15-20 μm in densely-innervated tissues, such as vas deferens and iris and 1-2 pm in large elastic arteries. In densely innervated tissues, sub-synaptic cysternae in smooth muscle are often found at close neuromuscular junctions.