Stumbling corrective reaction: a phase-dependent compensatory reaction during locomotion.

1. Tactile stimuli to the paw consisting of a stick making contact or an air puff aimed at the dorsum were used to study the phasic influence of locomotor activity on the reflex pattern elicited in extensor and flexor muscles and on the induced compensatory movements in intact cats. The resulting movements and reflex pattern are called "stumbling corrective reactions." 2. The basic reflex pattern and movements of the stumbling corrective reaction are: a) if the stimulus occurs during the swing phase, a short-latency activation of the flexor muscles, which induces an additional flexion of the limb lifting the paw over the obstacle; b) if the stimulus occurs during the support phase, an inhibition followed by an excitation of the extensor muscles, which neither increase nor decrease the extension. However, the stimulus evokes an increased flexor activity in the succeeding swing phase, which induces a brisker flexion. 3. Tactile stimuli to the proximal part of the limb or to the belly in front of the knee evoked the same type of phase-dependent compensatory reactions. Such reactions would presumably be beneficial for the animal to avoid high obstacles that impede movement. 4. A nonnoxious electrical stimulus (typically 2 mA; 1 ms) applied to the dorsum of the paw was used to study systematically the reflex pattern of the stumbling corrective reaction. Two pathways were defined to the knee flexor (semitendinosus). One early burst was evoked at about 10 ms and one later at about 25 ms after the stimulus. Short inhibitory pathways and longer excitatory pathways (20-50 ms) projected to the extensor nuclei. A short-latency (10 ms) excitatory pathway to the ankle extensor (lateral gastrocnemius) was also activated. 5. A painful electrical stimulus applied to the dorsum of the paw evoked large flexor responses during the whole step cycle. During the support phase the locomotion was disrupted as the supporting limb was withdrawn. 6. The results demonstrate that intact cats are able to compensate rapidly for unpredicted perturbations and that the reflex pattern and the induced corrective movements are adapted to the locomotor activity so that functionally meaningful movements are evoked in each phase of the step cycle. 7. The evoked reflexes and their modulation are consistent with those previously found in chronic spinal cats during walking and in paralyzed spinal cats performing "fictive locomotion." It is suggested that the same spinal pathways are used, and that they are controlled by the spinal "locomotor generator."