The role of premotor interneurons in phase-dependent modulation of a cutaneous reflex during swimming in Xenopus laevis embryos.

Phase-dependent reflex modulation during fictive "swimming" in Xenopus laevis embryos has been examined with intracellular recordings from rhythmically active spinal neurons. (1) At rest, cutaneous trunk or tail skin stimulation evokes EPSPs in motoneurons and premotor excitatory and inhibitory interneurons of the opposite motor system. During swimming, these EPSPs can only be evoked during the depolarized phase of activity and can then produce extra action potentials that lead to phase-dependent reflexes in ventral roots. On the stimulated side, IPSPs are evoked in rhythmic neurons that can block centrally generated action potentials if the stimulus coincides with the inhibited phase of the swimming cycle. This inhibition suppresses ventral root discharge in a phase-dependent manner. (2) The presence of premotor interneurons in the crossed reflex pathway suggests two parallel routes for cutaneous excitation to reach the motoneurons, one direct and the other indirect through excitatory premotor interneurons. During swimming, the crossed excitation through both routes is gated by the rhythm-generating circuit to allow summation in motoneurons only during the depolarized phase of the swim cycle. (3) Following phase-dependent reflexes, the frequency of swimming is raised for several cycles, a phenomenon that requires sensory activation of premotor rhythm-generating interneurons. The results provide evidence on the role of identified premotor spinal interneurons in phase-dependent reflex modulation.