FUSIMOTOR NEURON RESPONSES TO MEDIAL PLANTAR NERVE-STIMULATION IN THEDECEREBRATE CAT

1. The effect of single shock electrical stimulation, up to 20 x thres hold (T), of the medial plantar nerve on the discharges of single medi al gastrocnemius static and dynamic gamma-efferents has been investiga ted 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 affecte d by stimulation of the medial plantar nerve. Dynamic units showed net inhibition while facilitation dominated the responses of static neuro nes. 4. The responses of dynamic units consisted of powerful short lat ency (15 +/- 1.2 ms, mean +/- S.D.) spinal inhibition followed by weak er facilitation that was difficult to characterize due to concomitant rephasing of neuronal discharge. 5. Static neurones showed two pattern s 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 strong er 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 afferen ts contributed to the effects without changing their qualitative natur e. 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 involve d. 8. The results are discussed in relation to the control and functio n of fusimotor neurones and the possible existence of subdivisions wit hin the static system.