Distribution of intersegmental inputs to nonspiking local interneurons and motor neurons in the locust

Abstract

Intersegmental interneurons in a mesothoracic population that receive inputs from extero- and proprioceptors on a middle leg of a locust (Laurent, 1987a) make direct synaptic connections with nonspiking local interneurons and motor neurons controlling the movements of the ipsilateral hindleg. Of 25 direct connections that were established, 80% are excitatory, and 60% are made with nonspiking interneurons. Two or more intersegmental interneurons with outputs of either polarity and with different or overlapping receptive fields can converge onto one metathoracic nonspiking interneuron. Direct connections were found with motor neurons of the coxotrochanteral, femorotibial, and tibiotarsal joints of the hindleg. An intersegmental interneuron can make divergent connections with several motor neurons of the same pool or with a motor neuron and a nonspiking interneuron presynaptic to it. By virtue of the lateral inhibitory connections between nonspiking interneurons controlling different pools of motor neurons (Burrows, 1979), an intersegmental interneuron can excite directly one pool and disynaptically inhibit the antagonistic pool. Metathoracic nonspiking interneurons also receive inputs from afferents of mechanoreceptors on a hindleg. They thus have 2 receptive fields: one on the hindleg whose motor neurons they control and one on the ipsilateral middle leg, provided by inputs from the mesothoracic intersegmental interneurons. Nonspiking local interneurons are thus a point of convergence of local and intersegmental pathways. The effectiveness of the intersegmental pathways can thus be modified by local inputs to the nonspiking interneurons.