1. We examined the ability of muscular and joint afferents from the hip region to entrain fictive locomotion evoked by stimulation of the mesencephalic locomotor region in the decerebrate cat by mechanically imposed, sinusoidal hip flexion and extension movements. 2. A method is presented for qualitative and quantitative analysis of entrainment. 3. Hip joint capsular afferents were shown by denervation experiments to be unnecessary for mediating locomotor entrainment. 4. As the population of muscular afferents was progressively decreased by selective denervation, the strength of entrainment concomitantly decreased, even though as few as two small intrinsic hip muscles were still effective in producing entrainment. The ability to entrain locomotion was abolished with complete ipsilateral denervation. 5. Entrainment was observed with low amplitude hip angular displacement of 5-20°, which would be expected to activate low-threshold, stretch sensitive muscle afferents. 6. The extensor burst activity occurred during the period of imposed hip flexion, which corresponded to passive stretching and loading of the extensor muscles, while the flexor burst activity occurred during the latter portion of the imposed hip extension, which corresponded to passive stretching of the flexor muscles (when attached) and release of the extensors. During harmonic entrainment, the match of hip cycle duration and step cycle duration was accomplished by a variation in extensor electroneurogram (ENG) burst duration. These results are consistent with a positive feedback mechanism where low-threshold afferent activity from the extensor musculature is used by the rhythm generator to prolong the extension phase of locomotion. 7. A hip cycle frequency-dependent phase shift of ENG activity was observed. This may indicate that the locomotor rhythm generator is dependent on more than just static positional or threshold load information for modulation of the step cycle frequency and switching between flexion and extension phases. 8. Subharmonic forms of entrainment were observed when the number of innervated muscles was markedly reduced. The occurrence of subharmonic entrainment characterizes the locomotor rhythm generator as a nonlinear oscillator. 9. To modulate the stepping frequency, the afferent pathways responsible for entrainment must be directly connected to the neural circuitry responsible for rhythm generation. The rhythm generating interneurons must receive a high degree of convergence from afferents arising from a variety of muscles spanning the hip joint.
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