Complex dynamics in the heart of the lobsterHomarus americanus

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Abstract
Nonlinear dynamics have been shown to be important in describing a large number of complex physiological systems. This work examined the dynamics of the relatively simple neurogenic heart of the lobster Homarus americanus. A non-invasive device was used to collect continuous data of spontaneous heart beats, and time series of consecutive beat-to-beat intervals were generated from these data. This study was concerned with dynamic changes in beat-to-beat intervals that were induced by external effects including changes in both the level of activity and body temperature. Two types of temperature changes, short term (acute) and long term (chronic), were examined. In both cases, decreasing the ambient temperature increased the mean interval length as well as the variation. The regression slope of the correlation between the mean and the variation was unique for each lobster. Variation around the mean included periodic components. Not only the magnitude of the variation but also its complexity were affected by temperature. Approximate entropy increased as temperature decreased, implying temperature dependence of the regularity of the beat-to-beat intervals. We suggest that future physiological studies focus on attempts to understand changes in the complexity of physiological processes.