Peripheral control of the cat's step cycle I. Phase dependent effects of ramp‐movements of the hip during “fictive locomotion”

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Abstract
Acute spinal and curarized cats can generate “fictive locomotor activity” after an i.v. injection of Nialamid followed by 4‐AP and L‐DOPA. The efferent burst activity to flexors and extensors can be recorded in peripheral nerve filaments. Ramp‐formed movements were applied in the hip at constant angular velocity in different phases of the spontaneous efferent burst activity. The cycle duration was markedly influenced. A flexion or an extension ramp applied in the early part of the “step‐cycle” (during flexor activity) will prolong the cycle duration, but in the later part of the cycle instead a marked shortening effect will occur. The transition from a prolongation to a shortening is very steep for the extension‐ramps, with a subsequent gradual increase from a shortening to a lengthening of the cycle. This type of phase response curve expresses a potent peripheral modulatory effect on the central pattern generator. A ramp movement (flexion or extension) applied in the beginning of the flexor burst will reinforce the flexor activity. In the end of the flexor burst instead there is a directional sensitivity with positive feedback, resulting in an excitation of the flexor activity for flexion ramps, but a depression of the flexor activity for extension‐ramps. Extension‐ramps also show a position dependent effect which enhances the response in the flexors for more extended hip positions.