Repetitive activity of a molluscan neurone driven by maintained currents: a supercritical bifurcation.

The amplitude and rate of transient and maintained responses of an identified giant neurone of the pond-snail, Lymnaea stagnalis, to a maintained injected current were investigated. The responses of the isopotential soma to a maintained current density of less than 10 microamperemeter cm -2 includes repetitive discharges of action potentials, sawtooth oscillations and amplitude-modulated oscillations. The frequency of the maintained response increases as the injected current increases, while the amplitude decreases until the maintained response is a stable steady state at a membrane potential of about -20 mV. This bifurcation from small amplitude periodic solutions to a depolarized stable steady state has the characteristics of a supercritical Hopf bifurcation.