The relationship of muscle electrical activity, tremor and heat production to shivering thermogenesis in Japanese quail.

We measured the electrical activity and the tremor of the pectoral muscle and total body heat production in control and cold-acclimated Japanese quail at +26 degrees C, +12 degrees C and +2 degrees C before and after 3 weeks of acclimation, using electromyography, accelerometer recordings, and indirect calorimetry. Japanese quail shiver in 0.2- to 3-s bursts that occur in groups. An increase in both the frequency and the duration of bursts and burst groups contributes to the increase in heat production by shivering at low temperatures. A compilation of shivering patterns in birds is given and its implications for the neural control and phylogeny of shivering are discussed. A rather non-specific increase in electromyographic (EMG) activity and heat production was observed after cold acclimation at all experimental temperatures, although many of the normal signs of cold acclimation (e.g. decrease in gonad mass, increase in heart mass and serum triiodothyronine) were seen. The increase in muscle electrical activity was greater than the increase in oxygen uptake, which resulted in a lower VO2/EMG ratio. The amplitude distribution of muscle electrical activity remained normal, but a shift towards higher frequencies occurred in the EMG spectra of cold-acclimated birds. Despite the increase in muscle electrical activity, power spectra of accelerometer recordings indicated that the amplitude of the muscle tremor was lower in cold-acclimated birds. The increase in the high-frequency components of the EMG indicates that decreased synchronization of motor unit firing may account for the lower tremor amplitude. We suggest that this change is adaptive because it reduces heat loss and/or because more fatigue-resistant motor units are recruited. These results show that temperature acclimation modifies the neural control of shivering in skeletal muscle.