Phasic Modulation of Reflexes during Rhythmic Activity
Ten years ago (Forssberg et al, 1975, 1976, 1977) experiments in the chronic spinal kitten were designed to evaluate how reactions to perturbations of a limb during walking were integrated into the locomotor pattern. It was observed that, when the dorsum of the hindfoot contacted a rod during swing, the whole leg elegantly hyperflexed over the obstacle. However, when the same region of the foot was touched by the rod during stance, mimicking for instance a twig flying back, there was no response in the flexor muscles but instead an increase in the activity of the extensor muscles. To ensure that the very same stimulation did indeed produce two opposite responses, the experiments were repeated with electrical stimulation of the dorsum of the hindfoot. This stimulation also induced an ipsilateral flexion response only during the swing phase of the step cycle and an ipsilateral extension response during the stance phase. This phenomenon, in keeping with the past terminology, was referred to as reflex reversal. Implied in this expression was the fact that the transmission of a given pathway was effective in one phase and ineffective in the opposite phase of the movement. This applied equally to pathways exciting flexors, mainly during the swing phase, as well as to pathways exciting extensors, mainly in the stance phase, although the latencies of the responses elicited through these pathways could be slightly different.
KeywordsStance Phase Swing Phase Step Cycle Excitatory Response Reflex Pathway
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