Sensory inhibition of flexion producing interneurons in lobster abdomen.

Mechanosensory stimulation of an abdominal swimmeret initiates a fictive extension which includes flexion inhibition. The role of flexion producing interneurons (FPIs) in this motor program was examined by recording from a pair of FPIs which excite flexor motor neuron f3. The afferent-FPI-f3 pathway includes at least 5 levels of neural activity. Since swimmeret stimulation hyperpolarizes the FPIs, and mechanosensory afferents are not known to be inhibitory, sensory interneurons must connect the afferent with the FPIs. The generalized receptive fields and long latencies of the FPI response support polysynaptic afferent-FPI connections. The poor correlation between f3FPI spikes and f3 EPSPs they evoke, and the 15 ms delay for initiation of these EPSPs, suggest that additional premotor interneurons are interposed between f3FPI and f3. Since restricted stimulation of swimmeret sensilla generates IPSPs in f3 without affecting f3FPI activity, f3FPI and f3 must be inhibited by different interneurons. Sensory evoked FPI inhibition contributes to the flexion inhibition component of the swimmeret evoked responses since hyperpolarization of f3FPI to block f3FPI spiking during spontaneous flexion activity decreases ongoing f3 spike discharge. Coupling between f3FPI and f3 activities during spontaneously initiated postural flexions supports this conclusion.