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

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 extensive-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.