Central and peripheral neurosecretory pathways to an insect flight motor nerve.

Ultrastructural examination of the IIN1b nerve to the dorsal longitudinal flight muscle of Manduca sexta L. verified the presence of neurosecretory processes. Subspherical and irregular vesicles were found where the nerve enters the muscle, while spherical vesicles were found in the proximal region only. A dorsal unpaired median (DUM) cell, the median nervous system, and two or more peripheral cells are the sources of these neurosecretory inclusions. Light the electron microscopy CoCl2 backfills of the transverse nerve produced intensification of a peripheral neuron (#1) and processes in nerves IIN1a and IIN1b. Similar backfills of nerve IIN1b produced intensification of a DUM cell, a second peripheral neuron (#2), and processes in the transverse nerve and nerve IIN1a. Neuron #1 contained large spherical electron-dense vesicles while neuron #2 contained smaller subspherical vesicles. These cells were situated upon the link and/or transverse nerves. Based on these results, we suspect central and peripheral neurosecretory processes reach nerve IIN1b as follows: the link nerve projects prothoracic median nervous system and neuron #2 processes, nerve IIN1a projects neuron #1 processes, and nerve IIN1 projects mesothoracic DUM cell processes, although this latter pathway was less clear.