Autonomic events associated with entrance into hibernation are primarily mediated by the parasympathetic nervous system while those associated with arousal from hibernation are primarily sympathetically mediated. During deep hibernation, both sympathetic and parasympathetic tone are greatly reduced or absent. Within this context, the pattern of autonomic influence on cardiorespiratory control is intriguing. The dramatic fall in heart rate exhibited by mammals entering hibernation begins before there is any noticeable fall in body temperature and is due to a cyclic vagal activation that induces skipped beats and regular asystoles, and also slows the even heart beat. As body temperature subsequently falls, the vagal influence is progressively withdrawn and periods of vagal and sympathetic dominance alternate and give rise to regular periods of arrhythmia (tachycardia followed by bradycardia). Superimposed on this is a vagally mediated, respiratory sinus arrhythmia. As metabolic rate and body temperature fall, breathing frequency slows, depending on species, either by a prolongation of the pause between breaths, or by a waxing and waning of breathing frequency giving rise to discrete periods of apnoea interspersed by periods of continuous breathing. In such cases, the waxing and waning, and resulting episodic pattern, appear to be due to alternating positive and negative influences, possibly arising from vagal-pontine interactions acting on the medullary respiratory centres. The respiratory sinus arrhythmia is accentuated in these species. While advances have been made in describing the overt changes in autonomic control of cardiorespiratory processes during hibernation, the mechanisms behind, and the biological significance of cardiac and respiratory changes mediated through orchestrated arrhythmias and apnoeas, remain enigmatic.

Experimental Physiology (2001) 86.6, 791-796.