Control of cardiac and ventilation frequencies during hibernation in ground squirrels

Entrance into hibernation is accompanied by a massive increase in parasympa thetic tone which dramatically decreases heart late. 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 As body temperature does fall, parasymapathetic and sympathetic tones are slowly withdrawn and heart rate appears to become determined directly by c omplex interactions between metabolism, temperature and "state" The dramati c increase in heart rate during arousal is due to a massive increase in sym pathetic tone which also parallels the rise in metabolism and this tone is progressively withdrawn as body temperature subsequently rises.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 metaboli sm. As body temperature fails, breathing frequency slows, depending on spec ies, 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 breath ing frequency giving rise to an episodic breathing pattern. In the latter c ase, the waxing and waning, and resulting episodic pattern, appear to be du e 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 hibernatio n. 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 nun-stochastic changes in hear t rate and breathing pattern in hibernating animals, and the mechanisms und erlying their immediate control. The central neural mechanisms underlying t he interactions between metabolism, body temperature and "state" and the ma nner in which these affect autonomic control processes, however, remain eni gmatic.