The absolute and relative refractory periods refer to the intervals during which the cardiac cells are either entirely or partially unresponsive to further stimulation, respectively. The refractory periods are crucial in regulating the timing and rhythm of the heart’s contractions.
Suppose a stimulus causes the transmembrane potential (TMP) to reach a threshold (-55 mV). In that case, an “all-or-nothing” action potential (AP) is fired with the influx of sodium through open voltage-gated Na+ channels. These channels can be in three states: open, closed, or inactivated. After a brief opening period, these channels become inactivated, rendering the myocardial cells unresponsive to further stimulation, no matter how strong. This is the absolute refractory period (ARP).
After the peak AP (+30-40 mV) is reached and repolarization begins with potassium efflux from the voltage-gated K+ channels, the voltage-gated Na+ channels begin resetting to their closed state. These are partially responsive to further stimulation, although the stimulus required for excitation is much higher than usual since K+ channels remain open during repolarization. This is called the relative refractory period (RRP) and is completed when the Na+ channels have all been reset to their closed state. Remember that the TMP overshoots and hyperpolarizes during repolarization until the Na+/K+ ATPase restores the TMP back to the resting potential.
On an EKG, the ARP coincides with the beginning of the QRS complex till the first third of the T-wave, whereas the RRP is the last 2/3 of the T-wave.
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