Motor innervation of the supercontracting longitudinal ventro-lateral muscles of the blowfly larva

The gross motor innervation of the abdominal longitudinal ventro-lateral muscles of the larva of Calliphora erythrocephala is described. Two of these muscles, 6A and 7A, are innervated by the same two multiterminally-ending axons, and thus comprise a single motor unit. No difference is found between the axon diameters in the main nerve trunks, but there is a difference where the axons run over the muscle surface. Only the dorsal, inner, surface of the muscle is innervated. Electro-physiological results show two sizes of EPSP: the large fast EPSP presumably corresponds to the thicker axon and the small slow one to the thinner axon. Preliminary work indicates that it is not possible to distinguish between the two axons with electron microscopy; the presynaptic regions possess both ‘classical’ synaptic and ‘neurosecretory’ type vesicles, and have no glial cell covering.