Ia afferent activity during a variety of voluntary movements in the cat

Abstract

Implanted dorsal root electrodes were used to record discharge trains of single spindle primary afferents (Ia) of the cat''s hind limb during different types of movement. The length of the ipsilateral ankle extensors was continuously monitored by an implanted length gauge. Length changes occurring during active stepping were subsequently passively reproduced during brief anesthesia. A comparison of the Ia responses in active and simulated step cycles revealed that moderate fusimotor drive to ankle extensor spindles probably occurred mainly, if not exclusively, during the E1, E2 and E3 phases of active stepping. A temporal advance in the Ia response to passive stretching in the F-phase was attributed to the after-effects of fusimotor activity in the extension phases. Light thrust applied to the animal''s back evoked a potent fusimotor response. This load compensation effect may provide an explanation for the apparently higher degree of .alpha.-.gamma. co-activation seen in the mesencephalic locomotor preparation. Ankle extensor Ia discharge decreased during falls, despite an increase in extensor electromyogram. This is a clear example of independent .alpha. and .gamma. control. Placing reactions during walking were consistent with the notion that cutaneous inputs dominate over proprioceptive inputs in these movements. The .alpha. and Ia discharge during paw-shaking showed many of the characteristics of that in decerebrate and spastic clonus. Movements resembling clonus may be part of the animal''s normal repertoire. Isometric co-contraction of agonists and antagonists involved .alpha.-.gamma. co-activation. Hamstring Ia discharge behavior during stepping further highlighted the increases in firing rate which normally occur during passive muscle stretching in pre-programmed movements.