Motor and sensory flight neurones in a locust demonstrated using cobalt chloride

Abstract

The central projections of motor and sensory neurons involved in the control of the wing beat in locusts have been analysed using cobalt chloride introduced axonally. From sections fo cobalt material, components of particular neuropile regions can be identified. The cobalt method offers an important advance in the analysis of insect neuropile. The flight motor neurones in the mesothoracic ganglion are all basically similar in shape, with no clear distinction between elevator and depressor neurones. All have a highly branched dendritic tree which ramifies extensively through the dorsal neuropile. A neurone innervating a particular muscle can be recognised in different individuals by characteristic features but the pattern of dendritic branching is variable. Neurones involved in both flight and walking have some branches in a deeper layer of neuropile than those concerned only with flight, where the branches lie superficially. Sensory projections from the flight sense organs are complex but compact and their positions in the neuropile are very predictable. They have large interganglionic components. The stretch receptors and wing sense organ projections lie mainly in the dorsal neuropile, whereas the other thoracic sensory receptors project ventrally, with branches to the dorsal neuropile at a few specific points. Associations between certain branches of sensory and motor neurones are demonstrated and their implications for the organisation of the flight reflexes discussed. It is suggested that parts of the motor neurone dendritic tree may be functionally equivalent to interneurones.