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In this paper, we use a one-dimensional model of the rabbit sinoatrial node (SAN), and we investigate the response of the model to hyperpolarizing and depolarizing stimulus. Depending on the stimulus timing, either a delay or an advance in the occurrence of next action potential is produced. This resetting behavior of the model is quantified in terms of phase transition curves (PTCs) for short electrical current pulses of varying amplitude which span the whole period. The main focus of this paper is to compare the dynamic properties of the spatially extended system and the single cell model. The detailed analysis of the results provides new insights in the understanding of the transition from the theoretical single cell models to the spatially extended systems. Index Terms--Cardiac models, phase resetting, phase transition curves, sinoatrial node.

The ability of the Preisach formalism to model hysteresis processes in antiferromagnetically coupled (AFC) recording media is investigated in this paper. Two modeling approaches are proposed, each one using a weighed mixture of two normal bivariate probability function densities as the characteristic density and either classical or appropriately modified one-dimensional or two-dimensional hysteresis operators. The models are identified for three samples of different lower layer thickness, [t.sub.L.] In all cases, the resulting weighing factor [w.sub.L] correlates with [t.sub.L] as do some of the density parameters with material parameters. The calculated major loops compare well with the experimental data while the discrepancy of the calculated minor loops at fields lower than the coercive field indicates the limitations of the proposed models. Index Terms--Antiferromagnetically coupled media, hysteresis, vector Preisach modeling.