To explore the role of tonic motor unit activity in body temperature control.
Motor unit activity in soleus and several other skeletal muscles was recorded electromyographically from adult rats placed in a climate chamber on a load sensitive floor, which, together with video monitoring, allowed detection of every successive period of movement and no movement.
In the absence of movements during rest or sleep, motor unit activity was exclusively tonic and therefore equivalent to muscle tone as defined here. The amount of tonic activity increased linearly in the soleus as the ambient temperature decreased from 32°C to below 7°C, owing to progressive recruitment and increased firing rate of individual units. Brief movements occurred randomly and frequently during rest or sleep in association with brief facilitation or inhibition of motor neurons that turned tonic motor unit activity on or off, partitioning the tonic activity among the available motor units. Shivering first appeared when a falling ambient temperature reached ≤7°C in several muscles except soleus, which was as active between shivering bursts as during them.
Muscle tone and overt shivering are strikingly different phenomena. Tonic motor unit activity in the absence of movements evokes isometric contractions and, therefore, generates heat. Accordingly, when the amount of tonic activity increases with falling ambient temperature, so must heat production. Consequently, graded muscle tone appears as an important and independent mechanism for thermogenesis during rest or sleep at ambient temperatures ranging from <7°C to at least 32°C.
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