What is it and when does it occur?
Absolute refractoriness is a state in which the level of tissue excitability drops to 0%, and the stimulation threshold tends to infinity. The tissue is unable to respond to a stimulus.
The process begins instantly upon reaching the level of critical depolarization (threshold potential). On the action potential (AP) graph, this period corresponds to the spike — the peak of the action potential.
Broadly speaking, absolute inexcitability coincides with the phase of depolarization and initial repolarization. In Purkinje fibers, absolute refractoriness spans phases 0, 1, and 2.
What is the mechanism of absolute refractoriness?
Complete inexcitability is based on the state of the cell membrane ion channels.
During the depolarization phase, all $Na^+$ ion pores are open, and an avalanche-like influx of sodium into the cell occurs. At this point, no external stimulation strength can further enhance the process, as the sodium channel resources are completely exhausted.
Subsequently, sodium channels are inactivated. If a repeated stimulus is applied during this period, the muscle will not respond: a second action potential and a second muscle contraction will not occur, as impulse generation is physiologically impossible.
What is its physiological significance for the heart?
A prolonged absolute refractory period (ARP) plays a critical role in cardiomyocyte function. Unlike skeletal muscle, the heart muscle is not capable of tetanic contraction (summation of single contractions). This is because the refractory period in the myocardium overlaps the contraction period.
Ratio of AP and contraction in different parts of the heart:
- Atria: AP duration is 180 ms, and contraction is 100–110 ms. The AP lasts longer, so a new excitation is only possible after relaxation.
- Ventricles: The AP duration of myocytes is approximately equal to the duration of systole (330 ms).
- Atypical cardiomyocytes (conduction system): AP duration is 450 ms, absolute refractory period is 400 ms. On the graph, the duration of the absolute refractory period is 270 ms.
Such prolonged inexcitability prevents tetanus and ensures uninterrupted rhythmic blood ejection, as well as the opportunity for relaxation (diastole) to fill the heart with blood. For comparison: in skeletal muscle, the refractory period is shorter than the contraction duration, which makes impulse summation and tetanus possible.
Relationship with other excitability phases
Excitability dynamics reflect changes in the tissue's ability to respond to a stimulus (in %). Absolute refractoriness is just one stage of this cycle:
- Supernormal excitability (prespike) — precedes the spike, stimulation threshold is lowered.
- Absolute refractoriness (depolarization and plateau) — complete inexcitability. In cardiomyocytes, it encompasses depolarization, rapid initial repolarization, plateau, and the onset of rapid final repolarization.
- Relative refractoriness (end of rapid final repolarization) — excitability begins to recover from 0 to 100%. The cell is capable of generating an impulse, but the threshold is elevated, and a strong (suprathreshold) stimulus is required.
- Exaltation (negative after-potential) — phase of increased excitability, threshold is lowered.
- Subnormal excitability (positive after-potential) — excitability is reduced, threshold is elevated.
Effective refractory period (ERP)
Closely related to the concept of absolute inexcitability is the ERP, which is of great importance in the study of antiarrhythmic drugs.
- Effective refractory period is the time during which a cell cannot conduct another full-fledged impulse.
- It occupies approximately 2/3 of the time of final repolarization.
- Clinical significance: The longer the repolarization process, the longer the ERP. Lengthening this period reduces the risk of an ectopic action potential (extrasystole) in response to accidental stimulation.
Class and exam questions
Can a strong stimulus cause excitation during the absolute refractory period?
No. During this period, the cell is completely inexcitable (excitability is 0%). Generating a new impulse is impossible regardless of stimulus strength, as the stimulation threshold tends to infinity.
Why is the heart muscle incapable of tetanus?
Due to the prolonged absolute refractory period, which overlaps the contraction period. New excitation is only possible after myocardial relaxation, which ensures the alternation of systole and diastole.
How does absolute refractoriness differ from relative refractoriness?
During absolute refractoriness, excitability is zero, and a response to a stimulus is impossible. During relative refractoriness, excitability begins to recover, and the cell can generate an impulse, but only in response to a strong (suprathreshold) stimulus.
What else to study on this topic
- Action potential
- Relative refractory period
- Properties of the heart muscle
- Extrasystole
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