AFFERENT DISCHARGE PRODUCED BY MUSCLE COOLING IN MICE MAINTAINED AT NORMAL AND LOW ENVIRONMENTAL TEMPERATURES

Abstract

Muscle receptors in the mouse were shown to discharge in response to cooling of the muscle. Afferent activity was recorded in vitro from the whole nerve of the soleus muscle of 20 male white mice. These animals were maintained at a constant temperature of 22 °C for at least 15 days. In muscles cooled from 37 °C, receptors first showed activity at 32.9 ± 0.89 °C. With further cooling, there was an increase in the number of receptors firing and in the frequency of their discharge. The temperature at which maximum afferent response to cooling (TMA) occurred was 26.6 ± 0.83 °C. Similar experiments were carried out on 61 mice which had been exposed to a constant temperature of 6 °C for periods ranging from 6 hours to 32 days. Sensitivity of the receptors to cooling was significantly decreased following 24 hours' cold exposure (TMA 22.5 ± 1.02 °C) and was lowest after 12 hours' exposure. In animals exposed to cold for 3 days, sensitivity returned to a norma! level (TMA 25.4 ± 1.02 °C), but there was again a decrease in the sensitivity of the receptors to cooling in mice exposed for 5 to 6 days (TMA 22.1 ± 1.13 °C) and in those exposed for 8 to 10 days (TMA 22.4 ± 0.94 °C). Following exposure for 14 to 20 and 30 to 32 days, the sensitivity of the receptors returned to normal levels (TMA 26.3 ± 1.48 °C and 24.6 ± 2.04 °C respectively). At present the mechanisms underlying these changes in sensitivity of muscle receptors to cooling are not known.