Centrally generated rhythmic and non-rhythmic behavioural responses in Rana temporaria embryos

Embryos of the frog Rana temporaria up to and around the time of hatching show a range of rhythmic and non-rhythmic movements. These may occur spontaneously or in response to lightly touching the skin of the trunk or head. The first response to touching one side is usually on the opposite side. Non-rhythmic movements range from weak twitches centred on the mid trunk to strong flexions along much of one side of the body and part of the tail, which result in the animal becoming tightly coiled. Rhythmic movements range from slow, high-amplitude 'lashing' movements to faster, lower-amplitude 'swimming' movements. During rhythmic movements, a wave of bending passes along the animal from head to tail. The longitudinal phase delay in bending is constant for a range of cycle periods (88-193 ms) but is not uniform along the whole body. Bending is maximal along the body and rostral part of the tail, decreases towards the tip of the tail and is lowest at the head. Lateral displacement during rhythmic movements is lowest 0.2 body lengths from the snout, increases rostral and caudal to this level and is highest at the tip of the tail. In animals immobilised with curare, a range of patterns of motor discharge can be recorded in response to stimulation. Non-rhythmic responses range from single spikes to prolonged bursts, usually on the opposite side to the stimulus. Stronger bursts can alternate briefly between the two sides and are never synchronous on both. Episodes of sustained rhythmic activity can be evoked by touch, electrical stimulation of the skin or, rarely, dimming the lights. Cycle periods within each episode can vary considerably but often shorten as activity proceeds. Discharge on the two sides alternates (phase is approximately 0.5). Motor root burst duration correlates with cycle period, bursts being longer at longer cycle periods. Burst onset is delayed caudally, this delay being longer at longer cycle periods. Stimulating one side of the head evokes a large burst of discharge on the opposite side, often followed by sustained rhythmic discharge. These responses in immobilised animals are judged to constitute centrally generated correlates of the main behavioural responses of R. temporaria embryos.