The morphology, ultrastructure, and distribution of synapses on an intersegmental interneurone of the locust

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Abstract
The structure and synaptic relationships of an intersegmental inter-neurone in the mesothoracic ganglion of the locust were studied by intra-ccllular labelling for light and electron microscopy. Before labelling, the interneurone was characterized physiologically. It is depolarized and pro-duces spikes when either of the two forewings are elevated, and by sensory inputs from the tarsi of the middle legs. The gross morphology of the in-terneurone is quite distinctive. Its cell body is on the ventral surface of the ganglion near the midline and its primary neurite lies in the T-tract. Its axon performs a characteristic lateral loop around the lateral dorsal tract before entering an anterior connective and ascending at least as far as the prothoracic ganglion. Within the mesothoracic ganglion, the interneurone gives rise to a profusion of branches. Ultrastructural examination and re-construction of three regions of the interneurone from serial sections shows that input and output synapses are present in approximately equal num-bers, and often are close to each other. There is no evidence for separate input and output regions. Output synapses are characterized by the presence of a presynaptic bar and a population 500-2,000 round, electron-lucent vesicles. Most of the estimated 105-10 6 synapses occur on the smaller di ameter processes or on spines. The spines are of uniform diameter along their lengths, and are pre- or postsynaptic or both, but the postsynaptic spines are of smaller diameter than the others.

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