Phasic cutaneous input facilitates locomotor recovery after incomplete spinal injury in the chick.

1. Walking and swimming of hatchling chicks was videotaped before hemisection of the left thoracic cord and thereafter at regular intervals, for up to 2 wk. With the use of kinematic techniques, movements of the left knee and ankle were quantified to assess recovery of the ipsilateral leg during walking and swimming trials. To study the effects of exteroceptive (cutaneous) feedback in the absence of limb loading, one group of animals was also provided with cutaneous stimulation during swimming in the form of neutrally buoyant tubes that only contacted the foot during the retraction (extension) phase of the swim cycle. 2. One day after hemisection, for both swimming and walking, the left knee failed to extend normally, and the ankle joint remained hyperextended. During walking, all chicks adopted an asymmetric gait, whereas during swimming the left leg remained retracted and motionless. 3. Over the next 2 wk, knee extension and ankle flexion during walking recovered to normal preoperative values, but neither measure returned to preoperative values during swimming trials. However, when chicks were provided with phasic cutaneous stimulation during swimming trials, they showed improvements in leg motion as soon as 5 days after hemisection. Temporary removal of the cutaneous stimulation during swimming (5 days after hemisection) resulted in reduced limb action. However, 14 days after hemisection, the improvement in limb motion was retained even when the cutaneous stimulation was not provided. 4. Improvement in leg motion after swim training with phasic cutaneous stimulation took the form of increased extension of the limb during retraction. Possible neurophysiological mechanisms for this behavior include reflex reinforcement of limb extensor activity in response to cutaneous stimulation of the foot. Repeated exposure to phasic stimulation during swimming trials results in a permanent alteration in limb action. Thus increased cutaneous afferent inputs, even in the absence of limb loading, can facilitate locomotor recovery after spinal cord injury.