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Abstract
Objective: This study investigated spinal cord mediation of rabbit localized twitch responses [R-LTRs] by transection of the spinal cord and later of the peripheral nerve. Rabbit localized twitch response [R-LTR] is a brisk contraction of a group of rabbit skeletal muscle fibers which is most responsive to mechanical stimulation of the pressure-sensitive site, the trigger spot [TrS]. An R-LTR is analogous to a local twitch response of a human taut band, and a TrS is analogous to a human myofascial trigger point [TrP]. Methods: The electromyographic [EMG] activity of R-LTRs was recorded for the biceps femoris muscle of 5 rabbits [ages: 2-3 months]. Each animal was anesthetized in a way that preserved most peripheral reflexes. R-LTRs were elicited by mechanical-tap stimuli [delivered by a solenoid-driven blunt metal probe] to the TrS. The percentage of occurence of R-LTRs and the changes in maximal amplitude and duration of EMG activity of R-LTRs [before the spinal cord transection at T4, T5, or T6, and then 5 minutes, 10 minutes, 30 minutes, 60 minutes, 120 minutes and 150 minutes after spinal cord transection] were analyzed. In addition, EMG activity was recorded immediately, 5 minutes, 30 minutes, and 60 minutes after further transection of the sciatic nerve. Results: Immediately after spinal cord transection, R-LTRs were unobtainable and the EMG activity had completely disappeared. There was evidence of partial recovery of the R-LTR at 5-10 minutes; there was nearly complete recovery by 2.5 hours after spinal cord transection. Additional transection of the muscle nerve completely abolished R-LTRs which lasted for the duration of the experiment. Conclusion: R-LTRs are mainly mediated through the spinal cord, and supraspinal structures are not essential.