  Noise induced excitability is studied in type I and II Morris-Lecar neurons
subject to constant sub threshold input, where fluctuations arise from sodium
and potassium ion channels. Ion channels open and close randomly, creating
current fluctuations that can induce spontaneous firing of action potentials.
Both noise sources are assumed to be weak so that spontaneous action potentials
occur on a longer timescale than ion channel fluctuations. Asymptotic
approximations of the stationary density function and most probable paths are
developed to understand the role of channel noise in spontaneous excitability.
Even though the deterministic dynamical behavior of type I and II action
potentials differ, results show that a single mechanism explains how ion
channel noise generates spontaneous action potentials.
