Plateau in Some Action Potentials
In some instances, the excited membrane does not repolarize immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential for many milliseconds, and only then does repolarization begin. Such a plateau is shown in Figure 5–13; one can readily see that the plateau greatly prolongs the period of depolarization. This type of action potential occurs in heart muscle fibers, where the plateau lasts for as long as 0.2 to 0.3 second and causes contraction of heart muscle to last for this same long period.
The cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels enter into the depolarization process: (1) the usual voltage-activated sodium channels, called fast chan-nels, and (2) voltage-activated calcium-sodium chan-nels, which are slow to open and therefore are called slow channels. Opening of fast channels causes thespike portion of the action potential, whereas the slow, prolonged opening of the slow calcium-sodium chan-nels mainly allows calcium ions to enter the fiber, which is largely responsible for the plateau portion of the action potential as well.
A second factor that may be partly responsible for the plateau is that the voltage-gated potassium chan-nels are slower than usual to open, often not opening very much until the end of the plateau. This delays the return of the membrane potential toward its normal negative value of –80 to –90 millivolts.