Mutable and immutable features of paw-shake responses after hindlimb deafferentation in the cat

Hindlimb paw-shake responses were assessed before and after unilateral deafferentation (L3-S1) in chronic-spinal cats (n = 5), spinalized at the T12 level 1 yr earlier. Selected ankle flexor tibialis anterior (TA) and extensor lateral gastrocnemius (LG) and knee extensor vastus lateralis (VL) muscles were surgically implanted with chronic electromyographic (EMG) electrodes to determine mutable features of cycle characteristics and muscle synergies that are modulated by motion-dependent feedback as opposed to immutable features that are centrally programmed and not modulated by limb afference. 2. Paw-shake responses were difficult to elicit in the extensively deafferented hindlimb; this was true particularly during the first recovery weeks after deafferentation. By the end of the first month, however, brief responses of 1 or 2 cycles were commonly elicited in four of five cats, and responses of 3-7 cycles were common by the end of the second month in three of five cats. Initially, responses in the deafferented limb were elicited by stimuli applied to the dorsolateral thigh, an oval patch of skin innervated by intact S2 afferents. Over the 4-mo recovery period, however, the receptive field of the largely denervated skin expanded, and responses were also elicited by stimuli applied to the lateral aspect of the knee and shank, but usually not the paw. 3. In addition to fewer average cycles per response (5 vs. 10 cycles), paw shaking evoked in the deafferented hindlimb was characterized by longer-than-average cycle periods (124 vs. 98 ms), but the average cycle period varied widely among responses, ranging from 99 to 239 ms. Before deafferentation, the temporal organization of consecutive cycles was stereotypic; cycle periods increased linearly throughout a response. After deafferentation, however, there was no systematic relationship between cycle period and cycle number, and approximately 14% of the records with greater than or equal to 3 cycles were characterized by arhythmical sequences of EMG bursts. 4. At the ankle, LG burst duration was not altered by deafferentation, but TA onset and burst duration were affected. Before deafferentation, TA onset was invariant with respect to the beginning of the cycle, and burst duration increased linearly with cycle period. After deafferentation, however, TA onset was delayed, and the delay increased linearly with cycle period. Consequently, the TA burst duration was brief and unrelated to cycle period.(ABSTRACT TRUNCATED AT 400 WORDS)