Action Potential

Voltage Gated (activation gates) Na+ channels open and Na+ diffuses in the cytoplasm

Membrane changes from a negative value to a positive value

Once the membrane depolarizes to a peak value of 30+, it repolarizes to to its negative resting value of -70

The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.

( This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible.)

Voltage-gated Na+ channels change shape, and their activation gates open

All gated Na+ and K+ channels are closed

Depolarization; Na+ Channels Open

During the depolarization phase of the action potential, open Na+ channels allow Na+ ions to diffuse into the cell.

This inward movement of positive charge makes the membrane potential more positive (less negative). The depolarization phase is a positive feedback cycle where open Na+ channels cause depolarization, which in turn causes more voltage-gated Na+ channels to open.

Repolarization; Na+ channels are inactivating and K+ Channels Open

Hyperpolarization; Some K+ channels remain open and Na+ channels reset

two gates and three states

at the resting state, no Na+ enters the cell through them

opened by depolariztion, allowing Na+ to enter the cell

channels automatically blocked by inactivation gates soon after they open

one gate, two states

at the resting state, no K+ leaves

at depolarization, after delay, allowing K+ to leave

depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment

The Na+ diffusing into the axon during the first phase of the action potential creates a depolarizing current that brings the next segment, or node, of the axon to threshold.

The inactivation gates of voltage-gated Na+ channels close in the node, or segment, that has just fired an action potential

At the peak of the depolarization phase of the action potential, the inactivation gates close. Thus, the voltage-gated Na+ channels become absolutely refractory to another depolarizing stimulus.

Inactivation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open

Closing of voltage-gated channels is time dependent. Typically, the inactivation gates of voltage-gated Na+ channels close about a millisecond after the activation gates open. At the same time, the activation gates of voltage-gated K+ channels open.

As voltage-gated Na+ channels begin to inactivate, the membrane potential stops becoming more positive This marks the end of the depolarization phase of the action potential. Then, as voltage-gated K+ channels open, K+ ions rush out of the neuron, following their electrochemical gradient. This exit of positively-charged ions causes the interior of the cell to become more negative, repolarizing the membrane.

The opening of voltage-gated K+ channels allows K+ ions to exit the cell, repolarizing the membrane. In other words, the exit of K+ ions makes the membrane potential more negative. K+ also exits through leakage channels during this phase because leakage channels are always active. However, most of the membrane permeability to K+ during this phase is due to voltage-gated channels. Voltage-gated K+ channels make the action potential more brief than it would otherwise be if only leakage channels were available to repolarize the membrane.

The large number of voltage-gated K+ channels opening during the repolarization phase quickly makes the membrane potential more negative as positively-charged K+ ions leave the cell. K+ ions continue to leave through open channels as the membrane potential passes (becomes more negative than) the resting potential. This hyperpolarization phase of the action potential is therefore due to K+ ions diffusing through voltage-gated K+ channels. The membrane potential remains more negative than the resting potential until voltage-gated K+ channels close. This period of hyperpolarization is important in relieving voltage-gated Na+ channels from inactivation, readying them for another action potential.

Voltage-gated K+ channels are opened by depolarization. This means that as the membrane potential repolarizes and then hyperpolarizes, these K+ channels close. With the closing of voltage-gated K+ channels, the membrane potential returns to the resting membrane potential via leakage channel activity. Resetting voltage-gated Na+ channels to the closed (but not inactivated) state prepares them for the next action potential.

Voltage-gated K+ channels close. K+ and Na+ diffuse through leakage channels.