It is known that curvature relation plays a key role in the propagation of two-dimensional waves in an excitable model. Such a relation is believed to obey the eikonal equation for typical excitable models (e.g., the FitzHugh-Nagumo (FHN) model), which states that the relation between the normal velocity and the local curvature is approximately linear. In this paper, we show that for a simplified model of intracellular calcium dynamics, although its temporal dynamics can be investigated by analogy with the FHN model, the curvature relation does not obey the eikonal equation. Further, the inconsistency with the eikonal equation for the calcium model is because of the dispersion relation between wave speed s and volume-ratio parameter γ in the closed-cell version of the model, not because of the separation of the fast and the slow variables as in the FHN model. Hence this simplified calcium model may be an unexpected excitable system, whose wave propagation properties cannot be always understood by analogy with the FHN model.
Since the celebrated works of of Hodgkin and Huxley [8], FitzHugh [5] andNagumo [15], wave propagation in excitable systems has been the subject of a vast number of applied mathematics studies. In particular, the understanding of waves in the FitzHugh-Nagumo (FHN) system has provided a great insight into waves propagation in a wide array of biological and chemical systems [4,10,20,18], ranging from action potentials in neurons to chemical waves in the Belousov-Zhabotinskii reaction.Although the FHN system is important in wave propagation theory of excitable systems, not all waves in biological systems can be well understood by this classic model. For example, earlier works on Goldbeter's model [7,16,17], which is derived from past theories on the mechanism underlying calcium waves and oscillations, has revealed that Goldbeter's model is an excitable system, but the description of waves in this model is different from that in the FHN system. We remark that most analytical works on Goldbeter's model are only for the reduced system which is a piecewise linear approximation to Goldbeter's model, and which is a phenomenological formulation, and so, does not involve many biological details.Recently, the analysis on a calcium model (CKKONS model [2,19]), which is based on current theory for waves of intracellular calcium concentration, indicates that the structure of one-dimensional waves in this model is quite different from that of the FHN system. Further, the stability analysis of waves in the CKKONS model is more subtle than those for the FHN system (see [19,6]). Motivated by these previous works, one may expect that the theory of two-dimensional waves in the CKKONS model is different from that in the FHN system.Previous studies [10,12,22,20] have demonstrated that the curvature relation of waves is crucial for the evolution of waves in two spatial dimensions. Specifically, previous theories [10,12,22,20] on the classical excitable models including the FHN system suggest that the propagation of t...