Journal of Neuroscience, Vol 7, 1064-1080, Copyright © 1987 by Society for Neuroscience
Parallel processing of proprioceptive signals by spiking local interneurons and motor neurons in the locust
M Burrows
The connections made by afferents from a proprioceptor at the femorotibial
joint in a hind leg of a locust, the femoral chordotonal organ (FCO), were
determined by making intracellular recordings from motor neurons and
spiking local interneurons in the central nervous system and from afferent
cell bodies in the periphery. Staining the central projections of the
afferent neurons with dye introduced into their axons at the receptor, and
the intracellular injection of dye into motor neurons and interneurons,
shows that the branches of all 3 types of neuron overlap in specific
regions of neuropile. Afferents excited by a movement of the receptor
apodeme that is equivalent to an imposed extension of the femorotibial
joint excite flexor tibiae motor neurons and some spiking local
interneurons with cell bodies at the ventral midline of the metathoracic
ganglion. The opposite movement excites extensor tibiae motor neurons and a
different set of spiking local interneurons. Spikes in afferents that
excite flexor motor neurons evoke depolarizing potentials that follow each
spike with a consistent central latency of approximately 1.5 msec. The
amplitude of the depolarizing potentials is dependent upon the membrane
potential of the motor neuron. This evidence points to the connection being
direct and to the potentials' being EPSPs. Simultaneous recordings from
certain spiking local interneurons and certain flexor motor neurons show
that they receive many synaptic potentials in common and are driven in a
parallel fashion by movements of the receptor apodeme. Spikes of some
afferents evoke EPSPs in both neurons with the same consistency and
latency. An afferent can therefore synapse directly upon a motor neuron and
a spiking local interneuron. Each afferent synapses on several motor
neurons and possibly upon several interneurons. In turn, each motor neuron
and each interneuron receives inputs from several afferents.
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