Spatial segregation of synaptic inputs and outputs in a locust auditory interneurone

Using intracellular injection of cobalt, processing for electron microscopy, and using on-section silver intensification, we have investigated the distribution of synapses on the metathoracic arborization of the TN1 intersegmental interneurone in the auditory system of the locust. Previous studies have indicated that inputs to this neurone from tympanal afferents are spatially segregated from its outputs to higher-order inhibitory interneurones. Branches in the area of input are characterised by a smooth appearance in Lucifer Yellow preparations, whereas branches in the main output area appear beaded. Sampling different areas of the TN1 arborization, we have found marked differences in the respective densities of input and output synapses. Labelled profiles in areas of beaded branching have an extremely high density of presynaptic sites, with a much smaller number of synaptic inputs. Smooth branches bear many input synapses, but we have found no output synapses on them. Our results indicate a good correlation between branch beading and presynaptic function and between smooth branches and postsynaptic function. These correlations enable us to predict synapse distribution on other insect neurones with a similar division in branch morphology and also extend our knowledge of the functioning of TN1 itself.