The parasympathetic nervous system: its role during torpor in the fat-tailed dunnart (Sminthopsis crassicaudata)

This study investigated the effect of parasympathetic inhibition on the cardio-ventilatory interaction during torpor in the fat-tailed dunnart (Sminthopsis crassicaudata). Studies on the influence of the autonomic nervous system on cardiac function during torpor have focused on deep hibernation in eutherians. S. crassicaudata was used as a representative of the Metatheria that exhibits shallow, daily torpor as a comparison for the patterns of cardiac function found in other mammalian heterotherms. During torpor, parasympathetic inhibition removed the cardio-ventilatory interaction, eliminated heart rate variability and increased the overall heart rate; these are responses that have been shown to be typical of eutherian hibernators under the same conditions. Similarly, there was evidence to suggest that as the bout of torpor progressed, the variation in instantaneous heart rate decreased as a result of the progressive removal of parasympathetic tone. It has been suggested that the ability to enter a "steady state" during torpor, which is characterised by a regular heart rate, is limited to deep hibernators. On the basis of this, and the results of previous physiological studies, it was proposed that there is little evidence to suggest that there is any physiological difference between shallow, daily torpor and deep hibernation.