Action Potential Diagram Unlabeled

All 13 labels on the diagram

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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

Here is the action potential with no labels. The graph keeps its two axis captions and one voltage, +40 mV. The membrane strip keeps Outside the cell, Inside the cell and the ion symbols. Thirteen lines each take one name.

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Word bank for the action potential unlabeled diagram

On the graph, eight names: resting potential, stimulus, threshold, depolarization, peak, repolarization, hyperpolarization, refractory period. On the membrane, five: voltage-gated sodium channel, sodium ions moving in, voltage-gated potassium channel, potassium ions moving out, sodium-potassium pump. A class may write rising phase for depolarization, falling phase for repolarization and undershoot for hyperpolarization.

Shapes with no caption

The dashed line across the graph is the threshold, the block arrow under the curve the stimulus, and the bar under the time axis the refractory period. On the membrane the closed body with two arrows inside is the pump.

The curve shows the order of events, not measured values, so the time axis has no scale.
On paper the two bands under the curve print white. Their lines end under the curve, one on each side of the peak.

Questions and answers

What is the difference between the absolute and the relative refractory period?

In the absolute refractory period no second action potential can start, however strong the stimulus. In the relative refractory period one can start, but only with a stronger stimulus than the first needed. The diagram shows both as one bar.

Why does the voltage dip below the resting potential?

Potassium channels are slow to close. For a short time after the fall, potassium ions keep leaving the cell, so the inside becomes more negative than at rest.

How long does an action potential last?

In a neuron the whole rise and fall is over in one to a few thousandths of a second. In some other cells it lasts far longer: in a plant cell an action potential can take several seconds.

What does the sodium-potassium pump do during an action potential?

It makes neither the rise nor the fall; the two channels do. The pump works all the time, moving back the few ions that crossed, and so keeps the sodium and potassium concentrations on each side of the membrane steady.