Regional specialisation for synaptic input and output on a locust intersegmental interneurone with multiple spike-initiating zones.

A4I1 is a long intersegmental interneurone whose soma lies in the fourth abdominal ganglion and whose axon ascends through, and branches in, the thoracic and suboesophageal ganglia and the brain. It receives sensory input in the prothoracic ganglion from wind-sensitive hairs on the head and prosternum and in the fourth abdominal ganglion from cercal afferents (Pflüger: J. Comp. Neurol. 222:343-357, '84). Spikes can be initiated from both of these ganglia. In the present study the neurone was stained by intracellular injection of cobalt salts for light microscopy and of horseradish peroxidase for electron microscopy. The morphology of its neurites in the thoracic and fourth abdominal ganglia and the distribution of synapses upon them could then be observed and correlated. In the fourth abdominal ganglion the neurone receives input synapses but makes no outputs. In each thoracic ganglion the neurone has fine, varicose ventral neurites and thicker dorsal neurites. The ventral neurites are most abundant in the prothoracic ganglion where they receive only input synapses. In the meso- and metathoracic ganglia the ventral neurites also receive inputs, but in addition they make output synapses. In all three thoracic ganglia output synapses are abundant on the dorsal branches, but in the meso- and metathoracic ganglia a small number of inputs are also received here. The possibility that the function of the interneurone may not be the same in all of the thoracic ganglia is discussed in the light of its known physiological properties.