Thoracic intersegmental interneurones in the locust with mechanoreceptive inputs from a leg

Summary1.In the mesothoracic ganglion of the locust,Schistocerca gregaria, are a discrete population of intersegmental interneurones which receive inputs from one middle leg and project to the metathoracic ganglion controlling the hind legs.2.Their small somata (15–25 μm) are lateral to a posterior connective. They arborize ipsilaterally and send an axon to the metathoracic ganglion where they also branch ipsilaterally.3.These interneurones show specific responses to inputs from joint receptors or exteroreceptors of one mesothoracic leg. One interneurone responds to inputs from both middle legs but none respond to pro- or metathoracic receptors. None are sensitive to visual and auditory stimuli.4.Several interneurones respond to proprioceptive inputs from one joint. Some for example, are excited phasically or phasotonically by imposed and voluntary movements of the tibia, with a directional sensitivity. Some are sensitive to both passive and active movements of the tarsal segments only, while others are excited by inputs from proprioceptors of both femoro-tibial and tibio-tarsal joints. One is excited by movements of the tarsus, and inhibited by a voluntary flexion of the tibia.5.Within the same population are interneurones excited by the stimulation of external mechanoreceptors on the tibia.6.The function of these interneurones is most easily explained in the context of interlimb coordination during locomotion. Their behavioural significance is emphasised by the fact that they are also active during voluntary movement.