Figure 1.

The experimental setup. A, Photographs of embryos (A1; stage 37/38) and larvae (A2; stage 42) of the South African clawed frog Xenopus laevis, staged according to Nieuwkoop and Faber (1956). B, The intermyotomal clefts (as numbered from the otic capsule) and spinal cord were exposed to facilitate ventral root recordings with ipsilaterally placed electrodes (postotic clefts v7, v12) and recordings from motor neurons with an intracellular microelectrode (mn). Fictive swimming from immobilized tadpoles was elicited by a stimulating electrode (se). See Materials and Methods for more details. C, The fictive swimming rhythm at slow (C1) and fast (C2) time scales illustrates that on-cycle excitation (▿) just precedes ipsilateral ventral root activity and midcycle inhibition (▾), both of which are superimposed on a gradually declining tonic depolarization. KCl-filled electrodes render chloride-dependent inhibition depolarizing (see Materials and Methods). RMP, Resting membrane potential; BD, burst duration; CP, cycle period; LD, longitudinal delay; ED, episode duration. Note the stimulation artifact (se) arrowed in C1.