RECIPROCAL INHIBITORY INTERNEURONES IN THE XENOPUS EMBRYO SPINAL-CORD

The mechanism of reciprocal inhibition between antagonistic motor centers during swimming in the paralyzed Xenopus embryo was investigated. Paired intracellular recording were made from interneurons and motoneurons in an attempt to identify neurons which make direct inhibitory synapses onto motoneurons on the opposite side of the spinal cord. A physiological class of inhibitory interneurons is described which, when stimulated by intracellular current passage, evoke short-latency, probably monosynaptic, strychnine-sensitive inhibitory pontentials in contralateral motoneurons. These inhibitory interneurons fire once per swimming cycle in phase with the ipsilateral motor root discharge. They therefore have a pattern of activity which would cause them to inhibit motoneurons of the antagonistic motor center at an appropriate part of the swimming cycle. The intracellular injection of horseradish peroxidase (HRP) allowed the morphology of these inhibitory interneurons to be characterized. They have unipolar cell bodies with a thick proximal process with short dendrites which crosses the spinal cord ventrally and then bifurcates with one axonal branch ascending into the hind brain and the other descending the spinal cord. These anatomical features are typical of the commissural interneurons first described by Roberts and Clarke (1982). There are also some inhibitory interneurons which can inhibit motoneurons on the same side of the spinal cord. At least some of these interneurons may be commissural interneurons with ipsilateral axons and they may play a role in the generation of the swimming rhythm.