Motor neuronal receptive fields delimit patterns of motor activity during locomotion of the locust

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.