Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 108

Opening voltage-gated sodium channels, which increase the flow of sodium into the cell, further depolarizes the cell. This opens any other nearby voltage-dependent sodium channels. The situation is one of typical positive feedback in which the neuron is increasingly being driven toward the sodium equilibrium potential (approximately +55 mV, depending on the cell, from the Nernst equation). This amplification causes the neuron to remain stuck in this depolarized condition, unless something happens to get it back to its resting potential. Two things can work to do this:

 Sodium channels close themselves.

 Voltage-dependent potassium channels open.

In this chapter, I cover voltage-dependent sodium channels as though they only exist in two states — open or closed — but that isn’t quite right. A third state exists: When the neuron is in the resting state, the voltage-dependent sodium channels are in a state that is closed, but capable of opening (meaning they can be opened by voltage). When the membrane potential crosses the threshold, the channels transition to the open state. The third state occurs in less than one millisecond after the channels open. A mechanism within the channel causes the voltage-dependent channels to close themselves and transition to a state that is closed, but not immediately capable of reopening. That is, even though the membrane voltage may continue to remain above their opening threshold, they won’t open again. This process is called inactivation. The transition to this third, closed state causes the neuron to repolarize (meaning return to the resting potential), along with potassium channel activity, which we discuss in the next section. Milliseconds after the neuron repolarizes, the voltage-dependent sodium channels transition from the inactivated state back to the original closed state. Then the cycle can begin again.