MECHANICAL ENTRAINMENT OF FICTIVE LOCOMOTION IN THE DECEREBRATE CAT

1. We examined the ability of muscular and joint afferents from the hi p region to entrain fictive locomotion evoked by stimulation of the me sencephalic locomotor region in the decerebrate cat by mechanically im posed, 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 populat ion of muscular afferents was progressively decreased by selective den ervation, the strength of entrainment concomitantly decreased, even th ough as few as two small intrinsic hip muscles were still effective in producing entrainment. The ability to entrain locomotion was abolishe d with complete ipsilateral denervation. 5. Entrainment was observed w ith low amplitude hip angular displacement of 5-20 degrees, which woul d be expected to activate low-threshold, stretch-sensitive muscle affe rents. 6. The extensor burst activity occurred during the period of im posed hip flexion, which corresponded to passive stretching and loadin g of the extensor muscles, while the flexor burst activity occurred du ring the latter portion of the imposed hip extension, which correspond ed to passive stretching of the flexor muscles (when attached) and rel ease of the extensors. During harmonic entrainment, the match of hip c ycle duration and step cycle duration was accomplished by a variation in extensor electroneurogram (ENG) burst duration. These results are c onsistent with a positive feedback mechanism where low-threshold affer ent activity from the extensor musculature is used by the rhythm gener ator to prolong the extension phase of locomotion. 7. A hip cycle freq uency-dependent phase shift of ENG activity was observed. This may ind icate that the locomotor rhythm generator is dependent on more than ju st static positional or threshold load information for modulation of t he step cycle frequency and switching between flexion and extension ph ases. 8. Subharmonic forms of entrainment were observed when the numbe r of innervated muscles was markedly reduced. The occurrence of subhar monic entrainment characterizes the locomotor rhythm generator as a no nlinear oscillator. 9. To modulate the stepping frequency, the afferen t pathways responsible for entrainment must be directly connected to t he neural circuitry responsible for rhythm generation. The rhythm gene rating interneurons must receive a high degree of convergence from aff erents arising from a variety of muscles spanning the hip joint.