Chaordic homeodynamics: The periodic chaos phenomenon observed at the sarcomere level and its physiological significance

Cardiomyocytes repeatedly contract and relax based on electrical stimulation and fluctuations in intracellular calcium. Recently, self-sustained oscillations at the sarcomere level, termed Hyperthermal Sarcomeric Oscillations (HSOs), have been observed under high-temperature stimulation and reported to exhibit chaotic characteristics. In this study, we analyzed HSO waveforms from multiple sarcomeres in cultured neonatal rat cardiomyocytes, obtained via fluorescence microscopy and SL-nanometry. From the presence of positive Lyapunov exponents, recurrence plot analysis (RQA), and surrogate data analysis, we confirmed clear evidence of chaos in HSOs. At the same time, the oscillation period remained constant, revealing a dual property of being both "periodic and chaotic." These findings advance the concept of "homeochaos," which posits that biological homeostasis is dynamically maintained and regulated by chaotic fluctuations, and reposition it as Chaordic Homeodynamics-where periodic order (Order) and chaos (Chaos) coexist, and homeostasis is regulated in a dynamic rather than static manner. This study suggests a novel nonlinear control mechanism at the microscopic level within cardiomyocytes, contributing to a deeper understanding of dynamic homeostasis in cardiac physiology and biophysics.