1. When a hind leg of a walking cat fails to contact the ground at the end of swing, the limb is rapidly lifted and replaced in an attempt to seek support. In this investigation we tested the hypothesis that one factor in the initiation of this corrective response is the absence of signals from the group I afferents of extensor muscles. 2. Experiments were performed on decerebrate cats walking on a treadmill. The corrective response to loss of ground support occurred when one hind leg stepped into a hole cut into the treadmill belt. Group I afferents in various extensor nerves were stimulated via implanted cuff electrodes when the foot entered the hole. 3. Stimulation of extensor group I afferents suppressed the corrective flexion response. Instead of a flexor burst being generated soon after the foot entered the hole, extensor activity was maintained for a period exceeding the duration of the stimulus trains. The onset of the corrective response occurred at a relatively fixed latency of 70 ms after the end of the short stimulus trains (200-400 ms) provided that the contralateral limb was still in stance phase. For longer stimulus trains (500-1,000 ms) that terminated during or just prior to the swing phase of the contralateral leg, the onset of ipsilateral flexor activity occurred only after the contralateral leg had regained support. 4. We conclude that the absence of signals from group I afferents when a foot fails to contact the ground allows the locomotor rhythm generator to re-initiate a flexor burst to produce the corrective response.(ABSTRACT TRUNCATED AT 250 WORDS)
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