Responses of spiking local interneurones in the locust to proprioceptive signals from the femoral chordotonal organ

Summary The responses of spiking local interneurones of a ventral midline population in the metathoracic ganglion of the locust,Schistocerca gregaria, to controlled movements of a proprioceptor, the femoral chordotonal organ (FCO) in a hindleg, were revealed by intracellular recording. Afferents from the FCO which signal specific features of the movement or angle of the femoro-tibial joint, can make direct excitatory synapses with particular interneurones in this population (Burrows 1987a).Some interneurones in this population are excited only by flexion, some only by extension, but others by both flexion and extension movements of the femoro-tibial joint. Interneurones excited by one direction of movement may be either unaffected, or inhibited by the opposite movement. The balance between excitation and inhibition is determined by the range over which the movement occurs, and can increase the accuracy of a representation of a movement.The response of some interneurones has tonic components, so that the angle of the joint over a certain range is represented in the frequency of their spikes. Different interneurones respond within different ranges of femoro-tibial angles so that information about the position of the joint is fractionated amongst several members of the population. These interneurones respond to repetitive movements, similar to those used by the locust during walking, with bursts of spikes whose number and frequency are determined by the repetition rate and amplitude of the movement. A brief movement of the FCO may induce effects which persist for many seconds and outlast the changed pattern of afferent spikes. The sign of such an effect depends upon the preceding history of stimulation.Other interneurones respond only to movement so that their response is more phasic. The velocities to which they respond fall within the range of those generated by twitches of the flexor and extensor tibiae muscles and the movements of the tibia during locomotion. Some interneurones respond only to a specific range of velocities because they are inhibited by all other movements. Some interneurones respond to repetitive movements with reliable bursts of spikes, whilst in others the frequency of spikes may be raised but may contain no cyclical information. All, however, produce the largest number of spikes during the first cycle of a repetitive movement.Inputs from the FCO may sum either with excitation generated by direct inputs from exteroceptors or with inhibition produced by other local interneurones as a result of afferent signals.These spiking local interneurones are essential elements in the integration of local reflexes initiated by signals from the FCO. For example, one ensures that the levator tarsi motor neurone is reflexly inhibited when the FCO signals an extension movement. Exteroceptive inputs from the ventral tarsus suppress the spikes in this interneurone and would prevent expression of the reflex when the tarsus is in contact with ground.Abbreviation FCO femoral chordotonal organ