Interneurones in the Xenopus embryo spinal cord: sensory excitation and activity during swimming.

The dorsolateral spinal cord of embryonic Xenopus laevis has previously been shown to contain two anatomical classes of interneurones with dendrites in the dorsal tract where they could be contacted by the central axons of Rohon-Beard cells (Roberts & Clarke, 1982). The activity of these neurones within the dorsolateral spinal cord has been examined using intracellular micro-electrodes. Following electrical stimulation of Rohon-Beard neurites within the ipsilateral skin, dorsolateral neurones receive a short-latency, compound, excitatory post-synaptic potential (e.p.s.p.). The amplitude of the e.p.s.p. depends upon the number of Rohon-Beard cells stimulated. The e.p.s.p. consists of early and later components. The early components may result from monosynaptic connexions from Rohon-Beard cells, the later components from some unidentified interposed neurones. During episodes of fictive swimming the dorsolateral neurones are inhibited by rhythmic inhibitory post-synaptic potentials. Following Rohon-Beard neurite stimulation, neurones in the contralateral spinal cord receive e.p.s.p.s. These contralateral e.p.s.p.s are probably one of the post-synaptic effects of one of the dorsolateral neurone classes. The results suggest that the dorsolateral neurones are responsible for amplifying and distributing the primary afferent signals of Rohon-Beard cells, and may be involved in the initiation of swimming and reflex movements.