Cardiomyocytes maintain a coherent mechanical rhythm, but the sarcomeres connected inside them do not necessarily return to exactly the same internal configuration on every cycle. This preprint starts from high-speed recordings of five consecutive sarcomeres in each of seven living neonatal rat cardiomyocytes during hyperthermal sarcomeric oscillations (HSOs). These recordings were used to construct a five-unit mesoscale chain model that separates shared fast timing from local amplitude–phase organization.
In the experimental recordings, local amplitude variability exceeded fast-period variability in all seven cells, and neighboring sarcomeres displayed both co-directed and opposed motion. In the model, a common phase represents the stable fast temporal scaffold, while five interacting local amplitude–phase states represent changing internal redistribution.
Complete ten-dimensional Lyapunov spectra from 360 numerical runs identified a reproducibly positive largest Lyapunov exponent at intermediate coupling. Thus, the model’s fast timing can remain stable while its internal amplitude–phase configuration remains sensitive to small differences in initial conditions and evolves nonrepetitively.
The same intermediate-coupling regime also broadened the repertoire of internal responses. An identical perturbation accessed more independent response directions, nearby initial states produced less similar later responses, and a simple linear predictor transferred less successfully between trajectories. Small second- and third-harmonic terms improved the HSO-like waveform shape in held-out cells 5–7 without changing the internal dynamical equations.
The model also reproduced the experimentally observed predominance of one-link phase updates: 95.4% in the model compared with 93.9% in the experiment. Its length redistribution, however, remained more local than the experimental redistribution, suggesting that explicit serial force balance or weak longer-range elastic load sharing may be required in future models.
The central conclusion is that a stable beat-like rhythm does not require a connected sarcomere ensemble to repeat the same internal configuration on every cycle. Temporal order can coexist with sensitive, nonrepeating internal organization. In this restricted mesoscale-model sense, the study gives a quantitative dynamical meaning to chaordic homeodynamics.
This manuscript is a bioRxiv preprint and has not yet undergone peer review.
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Five consecutive sarcomeres were recorded simultaneously in seven cells, and local amplitude variability consistently exceeded fast-period variability. The model separates a stable shared phase from five changing local amplitude–phase states. Positive Lyapunov exponents, diversified perturbation responses, and reduced cross-trajectory predictor transfer emerge together at intermediate coupling. The model reproduces the predominance of experimental one-link phase updates, although its redistribution reach remains more local than in the recordings.
Seine A. Shintani. Chaotic internal dynamics coexist with a stable temporal scaffold in a mesoscale sarcomere model informed by high-resolution recordings. bioRxiv (Preprint, Version 1, 2026).
Posted: 20 July 2026
DOI: 10.64898/2026.07.20.739477
Keywords: cardiomyocyte; sarcomere; oscillation; nonlinear dynamics; Lyapunov exponent; mesoscale