The postnatal ontogeny of interlimb co-ordination was studied in rats from day 1 after birth to day 22. Swimming rather than walking was studied in order to minimize any effect of weak limb muscles in very young rats. Videotape records of swimming were analysed on a frame-by-frame basis to determine stroke cycle duration for individual limbs and interlimb latency and phase relationships. Interlimb co-ordination typical of swimming (or trotting) in adult quadrupedal vertebrates was already present on postnatal day 1, and so apparently the neural pattern generating circuitry for this behaviour is already established by this stage. In young rats it was possible to see 1 limb, or almost any combination of 2, 3 or 4 limbs, moving during swimming, and usually with proper phase relationships. This suggests that each limb has its own separate neural pattern generator and that there are effective interlimb co-ordinating mechanisms controlling moving limbs during swimming, even at very early stages. There was a gradual decrease in stroke cycle duration (increase in frequency) during the first two postnatal weeks. This may be due to maturing sensory input.

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