Heart Action Potential Diagram – Muscle and Pacemaker Cells

All 10 labels on the diagram

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Muscle cell 6

Phase 4 (resting potential) The resting potential between beats. The inside of the cell stays steadily negative.
Phase 0 (rapid depolarization) The fast rise at the start. Fast sodium channels open and sodium rushes into the cell.
Phase 1 (early repolarization) A small, brief fall right after the peak. It is called early repolarization.
Phase 2 (plateau) The plateau, where the voltage stays high. Calcium enters the cell while potassium leaves.
Phase 3 (repolarization) The final fall back to rest. Calcium channels close and potassium keeps leaving the cell.
Refractory period The time when a new beat cannot start. It runs from the rise until late in phase 3.

Pacemaker cell 4

Pacemaker potential The slow upward drift between beats. A pacemaker cell has no steady resting potential.
Threshold The voltage at which the cell fires. The drift reaches it with no outside signal.
Depolarization (calcium in) The rise once threshold is reached. Calcium, not sodium, carries it, so it is less steep.
Repolarization (potassium out) The fall after the peak. Potassium leaves the cell, and then the drift starts again.

How this list was compiled: Every name read on 2026-10-03 and kept only where two independent sources give it. (1) Wikipedia, text only: Cardiac action potential, Pacemaker potential and Sinoatrial node, https://en.wikipedia.org/wiki/Cardiac_action_potential : phases 0 to 4 of a ventricular cell with their ions, the two refractory periods, and the pacemaker cell that is never at rest, drifts up to a threshold and rises on calcium. (2) Three books read by full-text search at archive.org: Berne and Levy, Cardiovascular Physiology (2001), https://archive.org/details/cardiovascularph00robe (an upstroke, phase 0; an early repolarization, phase 1; a plateau, phase 2, during which calcium enters; a final repolarization, phase 3; the slow response whose upstroke is less steep; the effective refractory period); Basic Cardiac Electrophysiology for the Clinician (1999), https://archive.org/details/basiccardiacelec0000unse_x0x0 (phase 4 as rest in muscle cells; the plateau held while calcium channels stay open; repolarization by potassium currents; in nodal cells an upstroke carried by the calcium current); W. C. De Mello, Electrical Phenomena in the Heart (1972), https://archive.org/details/electricalphenom0000deme (pacemaker cells with a slow diastolic depolarization that reaches threshold). The chart is our own. Licence: own work

The drawing: Drawn for this site on 2026-10-03 from plain geometry; nothing traced. Licence: own work

How it works

A heart cell fires an action potential with every beat. The first graph is a ventricular muscle cell, with its numbered phases and long plateau. The second is a pacemaker cell action potential diagram: a sinoatrial node cell.

Read the full guide

Muscle cell

Read the curve from the left. Phase 4 is the flat resting line. Phase 0 is the steep rise, phase 1 a small early fall, phase 2 the long plateau and phase 3 the fall back to rest. The bar under the axis is the refractory period.

Pacemaker cell

There is no flat resting line. The voltage drifts up (the pacemaker potential) until it reaches the threshold, the dashed line. Then it rises and falls, and the drift starts again.

The plateau is what sets heart muscle apart: a nerve impulse has none.

Questions and answers

What are the phases of the cardiac action potential?

In a heart muscle cell: phase 0, the fast rise as sodium enters; phase 1, a small early fall; phase 2, the plateau, as calcium enters; phase 3, the fall as potassium leaves; phase 4, the resting potential.

What causes the plateau in a heart action potential?

Calcium channels stay open and calcium flows into the cell while potassium flows out. The two nearly balance, so the voltage stays high for a while before it falls.

How is a pacemaker cell action potential different?

A pacemaker cell has no steady resting potential. Its voltage drifts up to the threshold by itself, the rise is carried by calcium and is less steep, and there is no plateau.

What is the pacemaker potential?

The slow rise in voltage between two beats of a pacemaker cell, such as a cell of the sinoatrial node. When it reaches the threshold, the next action potential starts.