Chaotic nature of bursting discharges in the Onchidium pacemaker neuron

Neurons in the brain or ganglia spontaneously fire due to non-linear characteristics. Spontaneous discharges fluctuate in many cases. In this paper, the dynamical properties of discharges of the Onchidium pacemaker neuron are investigated using an intracellular recording method. The d.c. current through the cell membrane was used as a control parameter. The neuron shows regular beating, regular bursting and irregular bursting discharges depending on the d.c. current. The three-dimensional phase space reconstruction of the trajectory from the irregular discharges resembles that of a strange attractor. The one-dimensional Poincare map in the case of irregular discharges shows features of chaotic dynamics. The steady-state current-voltage curve of the pacemaker neuron indicates that the neuron has a negative slope conductance in the membrane potential range of −70 to −40 mV. The dependence of the negative slope conductance on Na+ and Ca2+ ion concentrations suggests the presence of a slow inward Na+ current, and the bifurcation from the beating state to the bursting state with increasing Co2+ ion concentration suggests that the slow outward current is Ca2+-insensitive. Therefore, a Hodgkin-Huxley type neuron model which essentially has two fast channels and two slow channels is presented. The model replicates the experimental results very well.