A PRESYNAPTIC GAIN-CONTROL MECHANISM AMONG SENSORY NEURONS OF A LOCUST LEG PROPRIOCEPTOR

The chordotonal organ at the femorotibial joint of a locust hind leg m onitors extension and flexion movements of the tibia. During evoked or imposed movements of this joint the central terminals of afferent neu rons from the chordotonal organ receive depolarizing, inhibitory synap tic inputs. The afferent spikes are therefore superimposed on these de polarizing IPSPs, which are generated indirectly by other afferents fr om the same organ that respond to the same movement. Each afferent spi kes preferentially to particular features of a joint movement, and its synaptic input is typically greatest at the joint position or during the movement that generates its best response. Afferents that respond to only one direction of movement receive synaptic inputs either durin g movements in both directions, or only during movements in their pref erred direction. Phasic velocity-sensitive afferents receive either ph asic inputs during movements, or tonic inputs at new sustained joint p ositions, or both. The spikes of tonic position-sensitive afferents ar e superimposed on synaptic inputs that are dependent on joint position . The synaptic inputs sum but do not themselves evoke antidromic spike s in the afferent terminals. They reduce the amplitude of orthodromic afferent spikes by 12-28%, and this is accompanied by a reduction of u p to 50% in the amplitude of monosynaptic EPSPs evoked by an afferent in postsynaptic leg motor neurons. These interactions suggest that a l ocal gain control mechanism operates between the afferents of this pro prioceptor. Thus, the effectiveness of the output synapses of an indiv idual afferent is regulated by the network action of other chordotonal afferents that respond to the same movement.