The role of cutaneous afferents in the control of gamma-motoneurones during locomotion in the decerebrate cat.

1. The effect of electrical stimulation, up to 20x threshold (T), of the sural nerve on the discharges of single medial gastrocnemius static and dynamic gamma-motoneurones has been investigated at rest and during locomotion in the decerebrate cat. 2. A total of twenty-three gamma-motoneurones were recorded. The neurones were identified as static (15) or dynamic (8) on the basis of their discharge characteristics (Murphy, Stein & Taylor, 1984). 3. Low intensity stimulation (less than or equal to 1.5T) had no effect on the discharges of most (22 of 23) gamma-efferents at rest or during locomotion. Hence the largest afferents in the sural nerve had little influence on the discharges of static or dynamic gamma-motoneurones in either condition. 4. Higher intensity stimulation (greater than 1.5T) excited both types of gamma-efferent in the resting state and response size was graded with stimulus intensity. For most neurones (20 of 23) excitatory effects appeared in the range 1.5-2T. 5. Stimulation at intensities greater than 1.5T also excited dynamic and some static gamma-motoneurones during locomotion. The responses of dynamic gamma-motoneurones were unchanged during locomotion compared to the resting state. In contrast, the responses of static neurones were significantly reduced, or even abolished, during locomotion and stimuli less than or equal to 3T generally (12 of 13) had no effect. Thus the responses of static, but not dynamic, gamma-efferents were task dependent. Further, the thresholds of responses indicate that activation of low threshold mechanoreceptors in the sural receptive field excites both types of gamma-efferent at rest, and dynamic neurones during locomotion. In contrast, it is proposed that the same peripheral input does not affect static gamma-efferents during locomotion. 6. The responses of static and dynamic gamma-motoneurones during locomotion were not obviously related to step cycle phase, or gamma rate, and responses occurring during or between homonymous electromyogram (EMG) bursts were not significantly different. Thus gamma responses during locomotion were not phase dependent. 7. Stimulation at intensities greater than 3T excited dynamic and some static gamma-motoneurones during locomotion but simultaneously inhibited on-going EMG activity. Peripheral inputs are therefore capable of influencing alpha- and gamma-motoneurones independently during locomotion. 8. The significance of the results is discussed in relation to the control and function of gamma-motoneurones.