The R15 neuron model with the intracellular calcium concentrations coupled to the transmembrane voltage was studied by the symbolic dynamics. The symbolic expressions of periodic and chaotic firing patterns were constructed through the first-return maps of ISIs (interspike interval series). And also the intermittent chaos close correlative to the fold bifurcation and the generating mechanism of periodic windows embedded in the chaos region were discussed. The corresponding relation between firing rhythms with different topological structures and continuously varying parameter values was revealed on the basis of ordering rules of symbolic sequences. The corresponding parameter value of some firing pattern measured from the electrophysiological experiment or simulated by the mathematical model could be approximately fixed according to the ordering relation. Results show that symbolic approach is a powerful tool to study neural firing activities. In particular, such a coarse-grained way can be generalized in neural electrophysiological experiments to extract more valuable information from complicated experiment data.
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