REVERSAL OF THE INFLUENCE OF GROUP IB AFFERENTS FROM PLANTARIS ON ACTIVITY IN MEDIAL GASTROCNEMIUS-MUSCLE DURING LOCOMOTOR-ACTIVITY

1. Rhythmic locomotor activity was evoked in clonidine-treated acute a nd chronic spinal cats, and the effect of stimulating group I afferent s from the plantaris muscle on the timing and magnitude of bursts in m edial gastrocnemius (MG) motoneurons was examined. 2. The locomotor rh ythm was entrained when group I afferents in the plantaris nerve were electrically stimulated with trains of stimuli presented at rates abov e and below the intrinsic frequency of the rhythmic activity. During e ntrainment at rates higher than the intrinsic frequency, a burst of ac tivity in ipsilateral MG motoneurons was initiated approximately 40 ms after the onset of each stimulus train. At lower rates of entrainment the onset of MG bursts preceded the onset of the stimulus trains, and each stimulus train had an excitatory effect on the MG burst with a l atency in the range of 30-50 ms. A similar excitatory effect was obser ved when the stimulus trains were triggered at a preset delay after th e endogenous generation of the MG bursts. 3. The excitatory action of plantaris group I afferents on the MG motoneurons was only seen during periods of locomotor activity. In the absence of rhythmic activity, t he same stimulus trains reduced any ongoing tonic activity in MG moton eurons. 4. Vibration of the plantaris muscle to preferentially activat e group la afferents neither entrained the locomotor rhythm nor increa sed the magnitude of the MG bursts. 5. We conclude that during locomot or activity, input from group Ib afferents of the plantaris muscle has an excitatory action on the system of interneurons generating the ext ensor bursts, i.e., on the extensor half-center of the central rhythm generator. One functional implication of this finding is that feedback from the force-sensitive Ib afferents in the plantaris muscle functio ns to reinforce the generation of activity in the ankle extensor muscl es during the stance phase of walking.