ECG Basics · Lesson 1 · From cell to trace

How a heart muscle cell fires

Every QRS and T wave starts as millions of small voltage changes in single muscle cells. Learn the shape of one cell's electrical cycle first; the surface ECG is its sum.

Resting, then a fast upstroke

A resting ventricular cell sits at about −85 to −90 mV, held there mainly by an inward-rectifier potassium current (I_K1). When a neighbouring cell depolarises it past threshold, fast sodium channels open and the membrane jumps positive within a millisecond or two. That upstroke is what spreads cell to cell as the depolarisation wave.

Phase	What happens	Main currents
4 · rest	Stable resting potential between beats	I_K1 (potassium out)
0 · upstroke	Rapid depolarisation; drives conduction velocity	I_Na (sodium in)
1 · early repolarisation	Brief notch, larger in epicardium than endocardium	I_to (potassium out)
2 · plateau	Calcium entry balances potassium exit; triggers contraction	I_Ca,L in; I_Kr, I_Ks out
3 · repolarisation	Potassium currents win and return the cell to rest	I_Kr, I_Ks, I_K1

Refractory periods

While most sodium channels are inactivated the cell cannot fire again. In the absolute refractory period no stimulus produces an action potential; the slightly longer effective refractory period, from the upstroke to roughly mid phase 3, is the time in which no stimulus produces one that propagates. A stronger-than-usual stimulus can fire the cell during the relative refractory period later in phase 3, but the response is smaller and conducts more slowly. Refractoriness is why a premature impulse can fail to conduct, conduct with aberration, or set up re-entry.

The shaded bands in the figure use simple voltage thresholds for teaching. Real refractoriness depends on channel recovery, not voltage alone.

Not every cell has the same shape

Cells across the ventricular wall differ. Epicardial cells have a prominent phase-1 notch (spike-and-dome) and a shorter action potential; mid-myocardial (M) cells have the longest. That gradient matters for the T wave, which the third lesson explains. Use the buttons above the figure to compare the three model cells.

Key points

Phase 0 sodium entry makes the fast upstroke; phase 2 calcium entry makes the plateau.
Potassium currents end the action potential (phase 3) and hold rest (phase 4).
During the effective refractory period no stimulus, however strong, produces an action potential that propagates.

References

Grant AO. Cardiac ion channels. Circ Arrhythm Electrophysiol 2009;2:185–194.
Bueno-Orovio A, Cherry EM, Fenton FH. Minimal model for human ventricular action potentials in tissue. J Theor Biol 2008;253:544–560 (model used for the cell figures).