Two distinct rhythmic motor patterns are driven by common premotor and motor neurons in a simple vertebrate spinal cord.

Xenopus embryos show two distinct rhythmic motor patterns: swimming and struggling. Both can be generated by spinal cord circuitry and evoked by stimulation of a single skin sensory pathway (Soffe, 1991b). This presents a valuable opportunity to explore mechanisms for vertebrate motor pattern switching. Swimming and struggling have been compared using intracellular recording from spinal neurons in immobilized embryos. Underlying synaptic drive was similar; motoneurons and premotor interneurons were excited in phase with ipsilateral motor root discharge and inhibited in phase with contralateral motor root discharge. Excitation was stronger during struggling and associated with short bursts of impulses, contrasting with single spikes per cycle during swimming. Excitation was reduced in both patterns by local application of 1 mM kynurenic acid, indicating excitatory amino acid mediation. Inhibition was antagonized by 1 microM strychnine, indicating glycine mediation. Many motoneurons (76%) and premotor interneurons (68%) fired during both swimming and struggling, including examples of all three spinal premotor interneuron classes. Most of the remaining motoneurons (20%) and premotor interneurons (24%) fired only during struggling, providing roughly 30% more active neurons than during swimming. To investigate whether new neuronal classes become active during struggling, recordings were made from sensory neurons and sensory interneurons. Rohon-Beard sensory neurons did not fire during either swimming or struggling. Dorsolateral commissural sensory interneurons received rhythmic, strychnine-sensitive inhibition during both swimming and struggling and also did not fire. Neither of these neuronal classes is therefore recruited to the circuitry for struggling. Although behaviorally distinct, Xenopus embryo swimming and struggling motor patterns appear to employ similar synaptic drive. I propose that this reflects the common nature of much of the premotor circuitry that drives them. Extra neurons are recruited to this circuitry during struggling, but only from within classes that also participate in swimming.