What Is It and When Does It Occur?
The absolute refractory period (ARP) is a state in which tissue excitability drops to 0%, and the threshold of stimulation approaches infinity. The tissue is completely incapable of responding to any stimulus.
This process begins instantly upon reaching the threshold potential (critical level of depolarization). On an action potential (AP) trace, this period corresponds to the spike—the peak of the action potential.
In general terms, complete unexcitability coincides with the phase of depolarization and early repolarization. In Purkinje fibers, the absolute refractory period encompasses phases 0, 1, and 2.
Mechanism of the Absolute Refractory Period
The state of cell membrane ion channels forms the fundamental mechanism behind complete unexcitability.
During the depolarization phase, all voltage-gated $Na^+$ channels open, causing a rapid, avalanche-like influx of sodium into the cell. At this moment, no external stimulus can further augment this process because available sodium channel capacity is completely exhausted.
Subsequently, the sodium channels undergo inactivation. If a second stimulus is applied during this period, the muscle will not respond: a second action potential and second muscle contraction cannot occur because impulse generation is physiologically impossible.
Physiological Significance in the Heart
A prolonged absolute refractory period plays a critical role in cardiomyocyte function. Unlike skeletal muscle, cardiac muscle is incapable of tetanic contraction (summation of individual twitches). This occurs because the refractory period in the myocardium overlaps the duration of contraction.
Action potential and contraction relationships across different cardiac regions:
- Atria: AP duration is ~180 ms, while contraction lasts 100–110 ms. The AP lasts longer, so a new excitation can occur only after relaxation begins.
- Ventricles: Myocyte AP duration roughly equals systolic duration (~330 ms).
- Conduction system cells (atypical cardiomyocytes): AP duration is up to 450 ms, with an absolute refractory period around 400 ms. On standard traces, ARP duration is approximately 270 ms.
Such prolonged unexcitability prevents tetanus and ensures continuous rhythmic blood ejection while allowing adequate time for ventricular relaxation (diastole) to fill the heart with blood. By comparison, in skeletal muscle, the refractory period is far shorter than the twitch duration, enabling wave summation and tetanus.
Relationship to Other Excitability Phases
The dynamics of excitability reflect changes in tissue responsiveness to stimuli (expressed in %). The absolute refractory period is only one stage of this cycle:
- Supernormal excitability (prespike) — precedes the spike; the stimulus threshold is decreased.
- Absolute refractory period (depolarization and plateau) — complete unexcitability. In cardiomyocytes, it encompasses depolarization, early rapid repolarization, the plateau, and the beginning of final rapid repolarization.
- Relative refractory period (end of final rapid repolarization) — excitability begins to recover from 0% toward 100%. The cell can generate an impulse, but the threshold is elevated, requiring a suprathreshold stimulus.
- Supernormal period / Exaltation (negative afterpotential) — phase of increased excitability with a lowered threshold.
- Subnormal period (positive afterpotential) — phase of decreased excitability with an elevated threshold.
Effective Refractory Period (ERP)
The concept of ERP is closely linked to absolute unexcitability and is important when evaluating antiarrhythmic drugs.
- Effective refractory period — the time window during which a cell cannot conduct a full, propagating action potential.
- It spans approximately two-thirds of final repolarization time.
- Clinical significance: The longer the repolarization process, the longer the ERP. Lengthening this period reduces the risk of premature action potentials (extrasystoles) arising in response to aberrant stimulation.