Discharges of single hindlimb afferents in the freely moving cat.

1. Implanted semimicroelectrodes were used to record single afferent fiber discharges from L7 dorsal roots during unrestrained walking in the conscious cat. 2. A series of tests were used to identify an afferent during a short period of anesthesia following each recording session. The majority of afferents were from muscle spindle primary endings in hindlimb muscles. 3. Ankle extensor spindle primaries generally showed their highest firing rates during that phase of stepping in which they were passively stretched. During active muscle contraction there was evidence of fusimotor drive, although this was not usually sufficient to entirely overcome the unloading effect of rapid muscle shortening. The variability of firing rate from cycle to cycle was considerably greater for the phase of active muscle contraction. The EMG response to brisk stretches of the ankle extensor muscle indicated a rapid (disynaptic or trisynaptic) reflex arc in the conscious animal. 4. Knee flexor spindle primaries showed similarly higher firing rates during passive muscle stretching in the step cycle. The shorter periods of presumed alpha-gamma coactivation corresponded to the much more phasic role of these muscles in stepping. 5. Tendon organs in the physiological extensors of the toes were mainly active during stance, although some discharges were usually seen during the swing phase. It is suggested that previous experiments on mesencephalic preparations may have led to an exaggerated view of the degree of alpha-gamma coactivation during normal stepping movements.