Reflex changes in discharge activities of gamma efferents to varying skin temperatures in cats

Summary

Changes in spontaneous discharges of single γ-efferents dissected from the lumbrosacral ventral root of spinal cats were observed during cooling of warming of a small area of the foot pad skin of the ipsilateral hind limb. In a certain range of skin temperature, varying from 34°C to 38°C from preparation to preparation, temperature changes resulted in either an increase or decrease in discharge frequency, whereas above this temperature range, heating produced an increase in discharge frequency. By contrast, all γ-efferents examined responded with a negative temperature coefficient to a change in skin temperature below the respective temperature. The thermal activation of these efferents elicited by a small temperature change of the skin was affected considerably by the velocity rather than by the degree of temperature changes but the effect of both variables differed according to the skin temperature at which the temperature change occurred. Ratios of increment or decrement in discharge frequency (±Δ imp./s) to degree (±Δ °C) and velocity of temperature change (±Δ °C/s) respectively were plotted as a function of skin temperature, and indicated a directional relationship between stimulus and response by attaching to each ratio either a positive or negative temperature coefficient. The maximum activity response in the range of 20–30°C was −19 imp./s per °C at 22°C and the average was −8.33±4.77 imp./s per °C. In the warmer range, the coefficients were positive and the average was 23.79±12.21 imp./s per °C in the range of 40–44°C. The steady state activity at a constant temperature showed no sign of dependency on skin temperature. On the basis of these results, the possible mechanisms for thermal inflow contributing to a thermally-induced reflex of the γ-motoneurons and the functional implications of these reflexes for the thermoregulatory system are discussed.