Activation and inactivation kinetics of voltage-gated channels.

ρ represents the postulated number of gates, m∞ is the steady-state activation variable, τm is the activation time constant, h∞ is the steady-state inactivation variable, and τh is the inactivation time constant. The seven voltage-gated ion channels are: a fast sodium channel (INa), a fast (ICaT) and slow (ICaS) calcium channel, a fast and transient potassium channel (A-type channel, IA), a calcium-activated potassium channel (IKCa), a delayed-rectifier potassium channel (IKd), and a hyperpolarization-activated inward channel (h-type channel, Ih).