Effects of skin cooling and warming on stretch responses of the muscle spindle primary and secondary afferent fibers from the cat's tibialis anterior.

Fusimotor activation induced by thermal stimuli to the skin was investigated by recording the stretch responses of primary as well as secondary spindle afferent fibers from the tibialis anterior of the barbiturate-anesthetized, spinal cats. A standard ramp-and-hold stretch was repeatedly given during warming or cooling (20 to 40 degrees C) the skin of the ipsilateral foot pads. In each response of the spindle afferent fiber to stretch, the peak (P) and static firing rates (S) were measured and the dynamic index was calculated to assess the type of fusimotor fiber influence as dynamic or static. Of 47 primary spindle afferent fibers, 22 (46%) were affected by changes in skin temperature. Cooling of the skin below a certain temperature produced a marked increase in the dynamic sensitivity of their response to stretch, and this also occurred on warming above it. Such a "critical temperature" at which a directional relationship between skin temperature and muscle spindle activity was completely altered was determined (mean +/- SD: 35.10 +/- 1.66 degrees C, N = 7). However, a marked increase in static firing accompanied by a significant decrease in the dynamic index was observed at the dynamic phase of heating the skin beyond the critical temperature. None of the seven secondary afferent fibers studied showed a detectable temperature response. We conclude that the activation of primary spindle afferent fibers by thermal stimulation of the skin is to be attributed predominantly to activation of dynamic fusimotor neurons and is significant for the initiation of cold shivering.