Correlated physiological and ultrastructural studies on specialised muscles. Ic. Neuromuscular junctions in the eyestalk levator muscles of Podophthalmus vigil (Weber)

Abstract The ultrastructure of physiologically different motor nerve terminals on fibers of the eye‐raising muscle of the crab Podophthalmus vigil is described. The terminals occur wherever branching axons lose part or all of their Schwann cell cover, either in simple grooves on the surface, in invaginated clefts, or even deep within the fiber in invaginated fingers lined by extension from the surface plasma membrane of the muscle fiber. Dense innervation of the surface of the fiber is associated with the occurrence of large, non‐facilitating e.p.s.ps., i.e., with “fast” axons. Less dense innervation, with deeply invaginated terminals is associated with small‐sized e.p.s.ps. and with facilitation typical of “slow” axons. No differences in either vesicle content, vesicle shape, mitochondrial density or density of other particles could be found in terminals having different associated biophysical properties.