Control of Cardiac and Ventilation Frequencies During Hibernation in Ground Squirrels

Abstract

Entrance into hibernation is accompanied by a massive increase in parasympathetic tone which dramatically decreases heart rate. These changes parallel the fall in metabolism rather than the subsequent fall in body temperature. As body temperature does fall, parasymapathetic and sympathetic tones are slowly withdrawn and heart rate appears to become determined directly by complex interactions between metabolism, temperature and “state.” The dramatic increase in heart rate during arousal is due to a massive increase in sympathetic tone which also parallels the rise in metabolism and this tone is progressively withdrawn as body temperature subsequently rises. Ventilation rate, too, decreases during entrance and parallels the changes in metabolism. As body temperature falls, breathing frequency slows, depending on species, either by a prolongation of the pause between breaths giving rise to a pattern of slow evenly spaced breaths, or by a waxing and waning of breathing frequency giving rise to an episodic breathing pattern. In the latter case, the waxing and waning, and resulting episodic pattern, appear to be due to alternating descending positive and negative influences acting on the medullary breathing centres. In deep hibernation, “state” itself appears to have a direct role in regulating these episodes in addition to an indirect influence, related to the low body temperatures associated with hibernation. During arousal, a massive excitation of breathing frequency occurs which, again, is correlated to metabolism rather than temperature. The field has made substantial progress in describing the non-stochastic changes in heart rate and breathing pattern in hibernating animals, and the mechanisms underlying their immediate control. The central neural mechanisms underlying the interactions between metabolism, body temperature and “state”, and the manner in which these affect autonomic control processes, however, remain enigmatic.