Anatomy of the innervation and neuromuscular junctions of the radular protractor muscle of the whelk, Busycon canaliculatum (L.)

A detailed and integrated picture of the whole buccal innervation of Busycon canaliculatum is presented. Each radular protractor muscle is innervated by a nerve arising from the cerebrobuccal connective. The nerve trunk enters the proximal end of the muscle, and runs parallel to the long axis of the muscle bundle. A layer of connective tissue and epineural muscle cells surrounds the trunk. Subdivisions of the main nerve trunk branch laterally into the muscle bundle, losing their epineural muscle coat. Further subdivisions of the branches produce single axons which can be observed among the muscle cells, but no specialized motor nerve endings were observed. Within the nerve endings are two types of synaptic vesicles: agranular (clear) and granular (dense). The granular vesicles are larger, ranging in diameter from 800 to 1350 Å (mean 970 Å). The clear vesicles vary in diameter from 500 to 1000 Å (mean 630 Å). The ratio of agranular to granular vesicles within a single nerve ending varies widely. When one type of vesicle predominates in an ending, then that ending comes to have a resemblance to a cholinergic or to a serotoninergic nerve ending.