Journal of Neuroscience, Vol 8, 4349-4366, Copyright © 1988 by Society for Neuroscience
Motor neuronal receptive fields delimit patterns of motor activity during locomotion of the locust
G Laurent and R Hustert
Department of Zoology, University of Cambridge, England.
During walking, the muscles of a leg undergo a typical sequence of
activity, which is partly shaped by phasic sensory feedback. To assess the
role played by such feedback, we characterized intracellularly the
receptive fields of tarsal motor neurons in the locust Schistocerca
gregaria and considered these receptive fields within the context of a step
cycle. The depressor motor neurons, active during the stance phase, are
excited by ventral tarsal contact or an imposed levation and are inhibited
by dorsal contact or an imposed depression. Partial deafferentation of the
anterior tarsus reduces this stance phase depressor activity. The levator
motor neuron, active during the swing phase, has the opposite receptive
field. The retractor unguis motor neurons, synergistic to the depressors,
are, like them, excited by ventral contact but, like the levator, are
inhibited by afferents which can signal the end of the stance phase. The
inhibition of the retractors could constitute a preparation for the swing
phase, by reducing the grip on the substrate. The motor neuronal receptive
fields thus appear to support the patterns of muscular activity recorded
during walking. Excitation of the motor neurons by extero- and
proprioceptors is usually direct: hair, canal, campaniform, and chordotonal
afferents all evoke 1:1 EPSPs in motor neurons after a central latency of
1-1.5 msec. Inhibition is indirect, as IPSPs occur at least 2 msec later
than the EPSPs. The motor neurons of one pool have overlapping but not
necessarily identical, receptive fields. Parallel, supplementary excitatory
and inhibitory pathways involving nonspiking local interneurons also exist,
which can allow gain control of a specific reflex. The weight given to a
reflex response will thus depend, first, on the number of motor neurons in
a pool affected by the stimulus and, second, on the existence and state of
intercalated interneurons.
Previous Article | Next Article
