Postinhibitory rebound facilitation of the extension reflex in the high spinal cat

Neural mechanisms of the reflex rebound of the hindlimb were studied in the high spinalized cat. After the animal had recovered from spinal shock, a light touch-pressure on the foot pad elicited a rebound extension reflex of the ipsilateral hindlimb. Excitability changes in hindlimb motor nuclei during the rebound phenomenon were examined by recording test monosynaptic reflexes (MSR). During touch-pressure stimulation, flexors such as the biceps femoris and tibialis anterior were facilitated, while extensors such as the gastrocnemius and quadriceps femoris were inhibited. After cessation of a stimulus, extensor MSRs showed a rebound facilitation which lasted for a few seconds. The greater the preceding suppression of MSR, the greater the rebound facilitation. Membrane potential changes induced in extensor motoneurons by touch-pressure were investigated by intracellular recordings. In most of the motoneurons tested (21/24), a clear membrane hyperpolarization was observed during stimulation and was followed by a rebound firing on cesstion of stimulation. Possible mechanisms underlying the rebound extension reflex were discussed.