Parallel effects of joint receptors on motor neurones and intersegmental interneurones in the locust

At the distal end of a mesothoracic tibia of the locust, Schistocerca gregaria, is a chordotonal organ which monitors the position and movement of the tarsus relative to the tibia. It contains approximately 35 receptors that variously encode different spatial and temporal parameters (position, velocity and direction of movement). Some excite intersegmental interneurones that respond phasically or tonically, with directional sensitivity to active or imposed movements of the tarsus. Some of these interneurones are also excited by intrinsic movements of the tarsal segments. Others, besides being excited by tarsal proprioceptive inputs, are also excited by exteroreceptors on the tarsus. When stimulated mechanically or electrically, chordotonal afferents evoke excitatory postsynaptic potentials with a central latency of between 0.9 and 1.4 ms simultaneously in the intersegmental interneurones and in tarsal motor neurones. The central arborizations of the afferents, the intersegmental interneurones and the tarsal motor neurones overlap in certain neuropilar regions of the mesothoracic ganglion. Other afferents cause an inhibition of the motor neurones, with a longer and non-consistent latency suggesting the involvement of other intercalated interneurones. These results indicate that proprioceptive inputs from the tarsal joint receptors are transmitted in parallel and monosynaptically to tarsal motor neurones and to the intersegmental interneurones.