Transitions between two different motor patterns in Xenopus embryos.

The Xenopus embryo is a valuable system in which to study the poorly understood mechanisms underlying vertebrate motor pattern switching. Here, we present a combined kinematic and electrophysiological description of the changes that occur during a switch between two rhythmic behaviours: struggling and swimming. Stable struggling movements evoked by sensory stimulation were followed by a transitionary period of variable duration leading to swimming. During the transition, cycle period and bending strength (local bending angle) progressively decreased and longitudinal delay progressively reversed. These changes were paralleled by similar changes in cycle period, burst duration and longitudinal delay of the motor pattern in immobilised embryos. The three movement parameters and their motor pattern correlates all scaled together during struggling and transitionary patterns. Our results indicate that transitions can be gradual (consistent with an earlier conclusion that a single set of pattern generating circuitry is involved); that the transitional movements are centrally programmed; and that they form a continuum with struggling movements. The correlated change of motor pattern parameters suggests that either a single mechanism underlies the components of the switch from struggling to swimming, or that separate mechanisms are closely linked.