Sodium Channel Activation and Inactivation.

In the center of this picture is shown the recorded current flowing through a single sodium ion channel in response to a step change in transmembrane potential. Surrounding the current-time trace are states of the of the sodium ion channel. Prior to the step change in transmembrane potential, the ion channel is in the inactive-activateable state. When the membrane potential decreases, the channel will begin conduction, followed by channel blockage by the ball and chain representation of the III-IV linker. Immediately after the membrane potential is returned to the resting state, the channel remains blocked for a finite period of time. Eventually the channel returns to the inactive-activateable state. The magnitude of the current change is constant while the channel is open and the magnitude is dependent on the driving force (Vm - Vna). Keep in mind the finite inactivateable period after the channel has been activated as this state may be employed to manipulate axon excitability. The inactivatable period corresponds to the refractory period, the time that an axon cannot be reactivated following stimulation induced depolarization.