Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 110

The voltage-dependent potassium channels also close on their own after opening, but much more slowly than the sodium channels. A lingering potassium current remains after each action potential that makes firing another action potential more difficult, because a subsequent depolarizing input must fight this hyperpolarizing potassium current.

The period immediately following an action potential when it’s more difficult or impossible to elicit a second action potential is called the refractory period. It has two phases:

 Absolute refractory period: The absolute refractory period is when the sodium channels are in the inactivated state. In this state no additional action potential can be elicited, no matter how strong the depolarizing input.

 Relative refractory period: The relative refractory period is when the potassium channels are still open, but the sodium channels have transition from closed, inactivatable, to closed, activatable. Another spike is hard — but not impossible — to produce because of the relative refractory period after the sodium channel transitions (from the inactivated to the closed state) and potassium currents are lingering.

See how these events play out in Figure 3-2.

Figure 3-2: Event sequence underlying action potential, including voltage across the membrane (V), sodium channel conductance (g_Na), and potassium channel conductance (g_K).

The trace in Figure 3-2 labeled “V” is the membrane potential, and the scale for this trace is on the left. At the start of the plot, the membrane potential is at the resting level, or about –65 mV. As sodium channels open, the potential moves toward the sodium equilibrium potential (although it may not actually reach this 55 mV level). The membrane potential then declines to a level below the resting potential, and then finally returns to the resting potential.

The other two traces in Figure 3-2 show the sodium and potassium permeabilities that control this membrane potential. The action potential (voltage, membrane potential trace) is caused by any input that depolarizes the cell enough to open the voltage-dependent sodium channels. The permeability (a biophysical term corresponding to electrical conductance) of the sodium channels is given by the g_Na trace, whose values (conductance) correspond to the axis on the right. This conductance is nearly zero at the resting potential, rises to a maximum near the membrane potential peak, and then returns to zero through the transition from the open to the inactivated state (refer to “Getting back to resting potential” earlier in this chapter).

The opening and closing of the voltage-dependent potassium channels, whose conductance is g_K, happens more slowly than the sodium channels. They tend to drive the membrane potential toward the potassium reversal potential, usually near –75 mV. The depolarizing phase of the action potential tends to be stopped by the closing of the sodium channels (which the voltage-dependent sodium channels do on their own) and the opening of potassium channels.

The absolute refractory period is the time during which the sodium channels are in the inactivated state, before they transition to the closed state. The relative refractory period occurs after the action potential when potassium channels remain open and sodium channels have partially recovered from inactivation. The lingering potassium current drives the membrane potential below the resting potential, and makes it harder to elicit a spike than was the case in Figure 3-2 when the cell was at the resting potential at the beginning of the plot.

The sodium-potassium transporter pump runs all the time, causing the imbalance in ionic concentrations between the inside and outside of the neuron. A single action potential creates a temporary current through the membrane, which changes the voltage across the membrane, but it has little effect on the total ion concentrations within the entire cell. Even if all the sodium-transporter pumps shut down (refer to the “Sodium-potassium pump” section, earlier in this chapter), up to millions of action potentials can occur before the cell starts to lose its concentration difference with respect to the cell exterior.