The development of swimming rhythmicity in post-embryonic Xenopus laevis

The post-embryonic development of 'fictive' swimming in immobilized Xenopus laevis tadpoles has been examined during the first day of larval life. In Xenopus embryos (stage 37-38; Nieuwkoop and Faber 1956), the rhythmic ventral root activily underlying swimming occurs as single brief (ca. 7ms) compound impulses on each cycle. However, by stage 42 (about 24h after hatching), ventral root discharge consists of bursts lasting around 20ms per cycle. In addition to increased burst duration in each cycle of larval swimming, the range of cycle periods within an episode increases, although mean period values (ca. 70-80ms) remain similar to those of the younger animal. Consequently, motoneurons at developmental stage 42 are active during swimming for a greater percentage (ca. 25%) of cycle time than at stage 37-38 (ca. 10%) . Developmental stage 40 (ca. 12h post-hatching) is an intermediate stage in rhythm development. Ventral root discharge varies from bursts of 10-20ms at the start of an episode to embryonic (ca. 7ms) spikes at the end of an episode. Furthermore, discharge varies from bursts of activity in rostral segments of stage 40 larvae to 7ms spikes more caudally, as in embryos. The data thus suggest that Xenopus swimming rhythmicity develops relatively rapidly, along a rostrocaudal gradient, and may involve acquisition of multiple spiking in spinal neurons.