Body Temperature Regulation in the Rat by Muscle Tone

We have previously described an important role of skeletal muscle tone in body temperature regulation, muscle tone defined as the tonic motor unit activity recorded between movements. Here, we study muscle tone in an extensive sample of new muscles outside (external muscles) and inside (internal muscles) the body core.

Skeletal muscles of adult, male Wistar rats were chronically implanted with EMG electrodes, and EMG was recorded during exposures to changes in ambient temperatures between 30°C and 5°C. Every consecutive period between movements (M) was identified as one of rest (R), whether the rat was awake or asleep. The amount of tonic motor unit impulse activity, or muscle tone, within (R) was then measured by root mean square analysis of the raw EMG.

Muscle tone in external muscles depended strictly on ambient temperature, rising and falling with falling and rising ambient temperatures. In contrast, muscle tone in internal iliacus and psoas muscles showed no relation to ambient temperature but increased markedly during recovery from hypothermia caused by general anesthesia. Mean tonic motor unit firing rates varied between 10 and 75 Hz.

All examined muscles, except iliopsoas, participated in moment-to-moment regulation of body temperature by heat-producing muscle tone during rest. Iliopsoas appeared to have an emergency function coming into play when the body temperature fell below some critical value, as during anesthesia. The wide range of tonic firing rates indicated that not only slow but also fast, fatigue-resistant motor units contributed to heat-producing muscle tone during rest.