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What opens first in response to a threshold stimulus?

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

What characterizes depolarization, 1st phase of action potential?

Membrane changes from a negative value to a positive value

What characterizes repolarization, 2nd phase of action potential?

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

What event triggers the generation of an action potential?

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.)

What is the first change to occur in response to a threshold stimulus?

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

Resting State

All gated Na+ and K+ channels are closed

Step 2

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.

Step 3

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

Step 4

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

How many gates/states do voltage gated Na+ channels have?

two gates and three states

Closed Na+

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

Opened Na+

opened by depolariztion, allowing Na+ to enter the cell

Inactivated

channels automatically blocked by inactivation gates soon after they open

How many gates/states do voltage gated K+ channels have?

one gate, two states

Closed K+

at the resting state, no K+ leaves

Opened K+

at depolarization, after delay, allowing K+ to leave

Why is an action potential self-generating?

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.

Why does regeneration of the action potential occur in one direction, rather than in two directions?

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.

What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization?

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.

What marks the end of the depolarization phase?

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 repolarization phase of the action potential, where voltage becomes more negative after the +30mV peak, is caused primarily by __________.

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.

During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to __________.

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.

During the hyperpolarization phase of the action potential, when the membrane potential is more negative than the resting membrane potential, what happens to voltage-gated ion channels?

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.

During the hyperpolarization phase of the action potential, voltage eventually returns to the resting membrane potential. What processes are primarily responsible for this return to the resting membrane potential?

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

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