Physiological and developmental aspects of intersegmental coordination in xenopus embryos and tadpoles

The rostro-caudal delay in the activation of the segmentally organised trunk muscles is a consistent feature of forward swimming in the tadpoles of Xenopus laevis. Our evidence suggests that such a delay may be achieved because rostral neurons are associated with a higher level of excitability than caudal neurons and so fire first on cycles of swimming. In addition we have found developmentally related changes in the properties of rostro-caudal delays in Xenopus, whereby the duration of motor nerve activity increases, a constant phaselag becomes established and there is increased sensitivity to 5-HT. These changes may be directly related to the incorporation of descending serotonergic neurones into the spinal cord network, so providing a means of determining the role played by identifiable neural elements in intersegmental coordination in the Xenopus preparation.