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Journal of Neuroscience, Vol 9, 3030-3039, Copyright © 1989 by Society for Neuroscience
Intersegmental interneurons can control the gain of reflexes in adjacent segments of the locust by their action on nonspiking local interneurons
G Laurent and M Burrows
Department of Zoology, University of Cambridge, England.
The gain of local reflexes of one leg of a locust can be altered by
mechanosensory inputs generated by movements of or tactile inputs to an
adjacent leg. Touching the mesothoracic tarsus, for example, increases the
number of spikes that are produced by the metathoracic slow extensor tibiae
motor neuron and enhances the depolarization of flexor tibiae motor neuron
in response to imposed movements of the chordotonal organ in the
ipsilateral hind femur. The sensory information from the middle leg is
conveyed directly to nonspiking interneurons and motor neurons controlling
the movements of the hindleg by a population of mesothoracic intersegmental
interneurons (Laurent and Burrows, 1989). The metathoracic nonspiking
interneurons receive direct inputs from receptors on a hindleg and are,
therefore, a point of convergence for local and intersegmental inputs. We
examine here the role of the connections between mesothoracic
intersegmental interneurons and metathoracic nonspiking interneurons in
controlling metathoracic local reflexes. The amplitude of synaptic
potentials evoked in leg motor neurons by the stimulation of local
afferents can be modulated by altering the membrane potential of an
interposed nonspiking interneuron with current injected through an
intracellular electrode. These imposed voltage changes mimic a mesothoracic
input and show that the state of a nonspiking local interneuron is a
determining factor in the expression of a local reflex. Inputs from
mesothoracic intersegmental interneurons may cause large changes in the
input conductance of nonspiking interneurons that can shunt a local
afferent input. In some nonspiking interneurons, synaptic potentials caused
by mesothoracic interneurons can be recorded, but no underlying conductance
change can be detected at the recording site. Similarly, a particular
nonspiking interneuron may receive synaptic inputs when two distinct
regions of a middle leg are touched, but only one of these intersegmental
inputs may be effective in reducing the amplitude of a synaptic potential
caused by afferents from the hindleg. These results suggest that nonspiking
local interneurons may be compartmentalized, with synaptic inputs and their
associated conductance changes restricted to particular branches. In this
way, an individual nonspiking neuron could contribute simultaneously to
several local circuits. The inputs from different intersegmental
interneurons could then modulate these pathways independently.
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