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The mechanism of reciprocal inhibition between antagonistic motor centres during swimming in the paralysed Xenopus embryo has been investigated further. Paired intracellular recordings have been made from interneurones and motoneurones in an attempt to identify neurones which make direct inhibitory synapses onto motoneurones on the opposite side of the spinal cord. A physiological class of inhibitory interneurones is described which, when stimulated by intracellular current passage, evoke short-latency, probably monosynaptic, strychnine-sensitive inhibitory potentials in contralateral motoneurones. These inhibitory interneurones 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 motoneurones of the antagonistic motor centre at an appropriate part of the swimming cycle. The intracellular injection of horseradish peroxidase (HRP) has allowed the morphology of these inhibitory interneurones 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 interneurones' first described by Roberts & Clarke (1982). There are also some inhibitory interneurones which can inhibit motoneurones on the same side of the spinal cord. At least some of these interneurones may be commissural interneurones with ipsilateral axons and they may play a role in the generation of the swimming rhythm.

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