Tonic and phasic synaptic input to spinal cord motoneurons during fictive locomotion in frog embryos.

1. In curarized, late developmental stage Xenopus embryos, episodes of rhythmic motor root discharge, termed fictive swimming (17), may be evoked by touch or by dimming the lights, as in unparalyzed animals. Motoneurons are tonically depolarized throughout each episode, are phasically excited to fire 1 spike per cycle, and receive a midcycle inhibitory postsynaptic potential (IPSP) in phase with motor root activity on the opposite side. 2. Rostral hemisection of the spinal cord abolishes motor root discharge on the operated side caudal to the cut but leaves activity on the intact side unaffected. In motoneurons, the tonic depolarization is abolished on the hemisected side but is still present on the intact side. This is evidence that the tonic depolarization is a descending drive. 3. Midcycle IPSPs normally seen in motoneurons during fictive swimming are abolished by rostral hemisection of the opposite side of the cord but are still recorded on the cut side. The simplest conclusion is that the inhibitory interneurons responsible lie on the opposite side of the spinal cord to the motoneurons they inhibit, and so represent a reciprocal inhibitory pathway. 4. The phasic excitatory postsynaptic potentials (EPSPs), which drive motoneuron spikes during swimming, are still present on the intact side of a rostrally hemisected cord but are abolished on the operated side. We conclude that the excitatory interneurons responsible lie on the same side of the cord as the motoneurons they excite.