Fusimotor modulation by spinal and skin temperature changes and its significance in cold shivering

Abstract

Single fusimotor fibers were isolated from the ventral root in lumbosacral segments of C1 spinal cats, and the firing activities were recorded during alteration of the spinal cord temperature (31 to 42°C) and the skin temperature of the ipsilateral hind limb foot pads (10 to 47°C). In 28 of the 31 fusimotor fibers isolated, the firing activity was significantly affected by the temperature changes of both the spinal cord and the skin. With constant skin temperature, the fusimotor fibers responded to cooling or warming of the spinal cord below 38°C by increasing or decreasing their firing rate, i.e., showing a negative temperature coefficient. Above 39°C they responded in the opposite manner, showing a positive temperature coefficient. Because the activities of the fusimotor fibers were highly dependent on spinal temperature while the skin temperature was constant, their steady-state firing rates (Δ impulses/s) were determined at various spinal temperatures between 31 and 42°C (mean ± sd: 35.44 ± 14.67 impulses/s at 33 to 36°C, 25.44 ± 13.09 impulses/s at 40 to 42°C). Maximum thermal sensitivity was calculated as −14.18 ± 7.72 impulses/s/°C below 38°C and 9.30 ± 4.57 impulses/s/°C above 39°C. In experiments in which temperatures of the spinal cord and the skin were independently modulated, an interaction between the effects of these two temperature changes on fusimotor activity was observed. It was suggested that modulation of the activity of the spinal fusimotor neurons elicited concurrently by temperature changes in the spinal cord and the skin probably plays an important role in the neuronal mechanisms for the genesis of cold shivering.