How the neonatal rat gets to the nipple: common motor modules and their involvement in the expression of early motor behavior.

One-day-old rat pups adopt a supine posture before attaching to the mother's nipple. Body rotations performed to reach the nipple occur in a typical kinematic structure. First, the pup rotates along the longitudinal axis of the trunk and lies on its side. Next, the pup arches the trunk to achieve a U-shaped posture and then rapidly relaxes the trunk. A second cephalocaudal rotation follows at the peak of trunk relaxation as the pup achieves a supine posture. After reaching a supine posture, the pup crawls to a nipple by performing "stepping" movements on the mother's ventral surface. The kinematic structure of these movements is reminiscent of the structure of righting as seen in the newborn rat. Both righting and achieving a supine posture under the mother involve the expression of common motor modules. During righting the modules are executed in the direction of gravity, and when achieving a supine posture the modules are executed against the force of gravity. Simple motor behaviors expressed by the rat pup during early postnatal development may have common origins and common control mechanisms.