The effect of single shock electrical stimulation, up to 20 x threshold (T), of the medial plantar nerve on the discharges of single medial gastrocnemius static and dynamic gamma-efferents has been investigated in the decerebrate cat. 2. The neurones were classified as static (15) or dynamic (8) indirectly on the basis of their locomotor and/or resting discharge characteristics. 3. All gamma-efferents were affected by stimulation of the medial plantar nerve. Dynamic units showed net inhibition while facilitation dominated the responses of static neurones. 4. The responses of dynamic units consisted of powerful short latency (15 +/- 1.2 ms, mean +/- S.D.) spinal inhibition followed by weaker facilitation that was difficult to characterize due to concomitant rephasing of neuronal discharge. 5. Static neurones showed two patterns of response. Some units (7 of 15) were facilitated at medium latency (39.9 +/- 12.2 ms) while the remainder showed mixed effects in which short latency (18 +/- 3.6 ms) spinal inhibition was followed by stronger facilitation (latency, 38.1 +/- 5.3 ms). 6. Fusimotor facilitation and inhibition were generally present at 2T. The inhibition of dynamic and static gamma-efferents, and the facilitation of the latter type, increased with stimulus intensity. Thus low and high threshold afferents contributed to the effects without changing their qualitative nature. 7. We conclude that low threshold cutaneous mechanoreceptors in the plantar surface of the foot are capable of influencing the discharges of medial gastrocnemius static and dynamic gamma-efferents. Further, the cutaneous responses of fusimotor neurones appear to vary according to both the source of the afferent input and the type of unit involved. 8. The results are discussed in relation to the control and function of fusimotor neurones and the possible existence of subdivisions within the static system.
