Different classifications of UPOs in the parametrically different chaotic ISI series of a neural pacemaker

Various interspike intervals (ISIs) in a series of spontaneous discharges o f experimental neural pacemakers were identified as chaotic and as lying be tween stable period 2 and stable period 3. The method of So et al. for dete ction of unstable periodic orbits (UPOs) was applied to analyze two chaotic ISI series generated under the action of [Ca2+](o) in different concentrat ions. In the chaotic ISI series near the stable period 2, an unstable perio d 2 orbit was detected. In the chaotic ISI series near the stable period 3, both an unstable period 2 orbit and an unstable period 3 orbit were detect ed. The location of the unstable period 2 orbit was close to that of the st able period 2 in the return map. while the location of the unstable period 3 orbit was close to that of the stable period 3. The results not only reve aled the structures of various chaotic ISI series, but also indicated that the classification of UPOs could reflect the experimental control parameter s at which the chaotic ISI series were generated. The previously discovered period adding bifurcation in the ISI series generated by experimental neur al pacemakers [12] was further described in terms of the evolution of the p eriod orbits. NeuroReport 12:2121-2124 (C) 2001 Lippincott Williams & Wilki ns.