The morphology of a population of thoracic intersegmental interneurones in the locust

A population of intersegmental interneurones with axons extending from the meso- to the metathoracic ganglion of the locust is described. They receive specific mechanosensory inputs from one mesothoracic leg. Their cell bodies are in group at the posterior of the mesothoracic ganglion, lying over the lateral base of each connective, and their primary neurites emerge in one of four bundles. Their mesothoracic branches are ipsilateral to the cell bodies and the leg from which they receive inputs. Each interneurone has two to six mesothoracic secondary neurites that divide and form a dense field of arborizations in specific regions of the neuropil so that each individual interneurone has a characteristic shape that is an elaboration of a basic and common plan. An interneurone excited by tibial campaniform sensilla and tarsal hair afferents branches in the intermediate neuropil and the ventral association center where the afferents from these receptors also project. An interneurone excited by proprioceptive inputs from the tarsus arborizes in the dorsal and intermediate neuropils, lateral to the ventral intermediate and ventral median tracts, in the same area as the proprioceptors afferents. An interneurone inhibited by proprioceptive inputs from the tibia (and wing) arborizes only in the dorsal neuropil, where there are no afferent projections. Some interneurones have one to three axonal branches with sparse and varicose side branches in the mesothoracic ganglion, which resemble the metathoracic axonal branches. The metathoracic axonal branches are mostly restricted to the dorsal neuropil and the dorsal part of the intermediate neuropil where local non-spiking interneurones and motor neurones controlling movements of the hind leg also project. The overlap between the branches of the sensory afferents and the intersegmental interneurones in the mesothoracic ganglion and between those of the nonspiking local interneurones or motor neurones and intersegmental interneurones in the metathoracic ganglion suggest that these interneurones are responsible for transferring information about the action of one leg to an adjacent leg.