Abstract
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.
Highlights
In cold environments, birds and mammals increase their heat production through non‐shivering and shivering thermogenesis
This work shows (a) that muscle tone likely plays an essential and substantial, but so far generally unrecognized, role in body temperature regulation during rest or sleep, (b) that muscle tone and shivering are strikingly different phenomena and, should not both be described as shivering with the implication that they are “different degrees of the same phenomenon”, and (c) that regulated muscle tone occurs in the thermoneutral zone (TNZ) as described in the literature, questioning the present description of this zone in rats
We propose that these effects arise because falling ambient temperatures increase the serotonergic inputs to the relevant motor neurons, thereby promoting and enhancing the generation of plateau potentials in them (Figure 16D,E)
Summary
Birds and mammals increase their heat production through non‐shivering and shivering thermogenesis. Non‐shivering thermogenesis (NST) occurs in brown adipose tissue (BAT).[1] The extent to which it occurs in electrically inactive skeletal muscles is debated, as evidence both for and against such thermogenesis exists.[2,3,4,5] Studies on birds, beginning with Steen and Enger,[6] have provided important information on the physiology of shivering.
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