Journal of Neuroscience, Vol 9, 3019-3029, Copyright © 1989 by Society for Neuroscience
Distribution of intersegmental inputs to nonspiking local interneurons and motor neurons in the locust
G Laurent and M Burrows
Department of Zoology, University of Cambridge, England.
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.