Recovery and perturbation of paw-shake responses in spinal cats

Paw-shake responses (PSRs) were evoked by wrapping masking tape around the hind-paw in nine cats spinalized at the T12 level either at 2 or 12 wk of age or as young adults (10-12 mo). Electromyographic responses of ankle extensors (LG and SOL) and one ankle flexor (TA) were recorded through the 6th mo after cord transection. Activity of the LG was used to determine the cycle characteristics. Cycle characteristics did not differ among cats spinalized at different ages. The average PSR, consisting of 11 cycles with a cycle time of 85 ms, was similar to the PSR of normal adult cats (16). Activity in the TA and LG muscles alternated with the onset of the flexor burst occurring at 52% of the extensor cycle. Burst durations averaged 39 and 57 ms for LG and TA muscles, respectively. Relatively normal PSRs were evoked within 48 h following cordotomy of the young-adult cats; differences being that the responses were elicited less frequently with fewer and slightly longer cycle times than normal. Within 2 wk following cord transection, PSR parameters returned to normal values. In the spinal cats, the SOL was active during PSR, showing either tonic low-level activity or discrete bursts that were coactive with the LG. In normal adult cats, the slow extensor (SOL) is usually inactive (16). In spinal cats, participation of the SOL may depend on a conversion of muscle units from slow to fast contracting (8, 13) or on the absence of inhibition of slow motor units from descending tracts (11). Both mechanisms are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)