Action Potential Diagram – Phases and Ion Channels Labeled

All 13 labels on the diagram

Show the list

The graph 8

Resting potential The steady voltage across the membrane between impulses. The cell is negative inside compared with outside.
Stimulus Whatever pushes the voltage up toward the threshold. One too weak to reach the threshold starts no impulse.
Threshold The voltage that must be reached to start an action potential.
Depolarization (rising phase) The rising phase: the voltage climbs steeply. Sodium channels open and sodium ions flow into the cell.
Peak The highest point of the curve, about +40 millivolts. Here the rise stops and the fall begins.
Repolarization (falling phase) The falling phase: the voltage drops back toward rest. Sodium channels shut and potassium ions flow out.
Hyperpolarization (undershoot) A short dip below the resting level. It is also called the undershoot.
Refractory period The time when a second impulse cannot start, or starts less easily.

The membrane 5

Voltage-gated sodium channel A channel that opens when the membrane is depolarized. It lets sodium ions into the cell, then shuts itself.
Sodium ions moving in Sodium ions flow into the cell through the open channel. This makes the inside more positive.
Voltage-gated potassium channel A channel that also opens with depolarization, but more slowly. It lets potassium ions out of the cell.
Potassium ions moving out Potassium ions flow out of the cell. This brings the voltage back down toward rest.
Sodium-potassium pump A pump that works all the time. It keeps sodium low and potassium high inside the cell.

How this list was compiled: Every name checked on 2026-10-01 in both of these. (1) Wikipedia, English edition, Action potential, https://en.wikipedia.org/wiki/Action_potential (the phases, the threshold, the channels and the pump). (2) Neuroscience Online, McGovern Medical School at UTHealth Houston, Section 1, Chapters 1 and 2, https://nba.uth.tmc.edu/neuroscience/m/s1/chapter01.html and https://nba.uth.tmc.edu/neuroscience/m/s1/chapter02.html (resting potentials and action potentials; their ionic mechanisms). Detail lines are in our own words. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

The graph follows the voltage across a nerve cell membrane through one impulse, phase by phase. The second drawing shows the action potential at the cell membrane: two channels, the ions that cross, and the pump.

Read the full guide

Read the graph from left to right

Number 1 is the resting potential, the flat line. The stimulus, 2, pushes the voltage up to the threshold, 3, the dashed line. Then the curve rises, 4, depolarization; reaches its peak, 5; and falls, 6, repolarization. It dips below rest, 7, hyperpolarization. The bar under the axis, 8, is the refractory period. On the membrane, sodium ions, 10, come in through their channel, 9. Potassium ions, 12, go out through theirs, 11. The pump is 13.

Sodium in makes the rise and potassium out makes the fall, so match each channel to its half of the peak.
Depolarization and repolarization are the two bands under the peak; hyperpolarization is the dip after them.

Questions and answers

What are the phases of an action potential?

The rising phase (depolarization), the peak, the falling phase (repolarization), the undershoot (hyperpolarization) and the refractory period, which overlaps the others. Before and after them the membrane sits at its resting potential.

What happens at the threshold?

If a stimulus lifts the voltage to the threshold, sodium channels open and an action potential starts. A stronger stimulus does not make the impulse bigger, and one that stays under the threshold starts none.

Which ions move during an action potential?

Sodium ions flow into the cell through voltage-gated sodium channels and make the rise. Potassium ions flow out through voltage-gated potassium channels and make the fall. The sodium-potassium pump keeps sodium low and potassium high inside the cell.

What voltage is the resting potential?

It differs from cell to cell. One of this page's sources uses -70 millivolts and the other -60. Both put the peak at about +40, so that is the one figure printed on the graph.

Is this the answer key to "label the diagram modeling the formation of a membrane potential"?

No. That sentence is the title of a publisher's homework activity. This is our own drawing of the same subject: the membrane view shows the channels, the ions and the pump.