What happens during the plateau phase?
During the action potential plateau, cardiomyocyte membranes maintain a constant charge, regardless of its magnitude. At this moment, the heart structure is completely encompassed by excitation.
Since excitation simultaneously encompasses a significant portion of the muscle fibers, there is no electric field around the heart. There is no potential difference between different areas of the myocardium. Unlike the depolarization and repolarization processes, which form the ECG waves, the plateau phase forms the isoelectric line—a straight line between the waves.
Contraction mechanism and "calcium surge"
The general mechanism of myocardial contraction, like that of skeletal muscle, is based on the interaction of the proteins actin and myosin under the influence of $Ca^{2+}$ ions. However, the sources of this calcium differ:
- In skeletal muscle, $Ca^{2+}$ ions are supplied to the contractile proteins primarily from the endoplasmic reticulum (sarcoplasmic reticulum).
- In the myocardium, calcium comes partially from the endoplasmic reticulum and partially from outside the cardiomyocytes—from the extracellular environment via the T-system.
The entry of $Ca^{2+}$ ions into cardiomyocytes from the outside is called the calcium surge. It occurs during the action potential plateau phase and determines the specific features of myocardial contraction.
ECG manifestation: atria and ventricles
On the electrocardiogram, the plateau phase is recorded as segments—portions of the isoelectric line.
- PQ segment (in the atria): Reflects slow repolarization (the plateau phase) in the atria. The duration of the segment is 60 ms. During this period, the atria are excited, and atrioventricular delay occurs—a slowing of excitation conduction from the atria to the ventricles. Together with the P wave, this segment forms the PQ interval of 100 ms duration, corresponding to the time of atrial systole.
- ST segment (in the ventricles): Reflects the plateau phase of ventricular cardiomyocyte action potentials. The segment duration is 120 ms. The entire ventricular myocardium is encompassed by excitation, and there is no potential difference. The segment is located between the QRS complex and the T wave.
Clinical significance
The ST segment, formed during the ventricular plateau, is an important diagnostic criterion for myocardial ischemia.
Selective potassium channel blockers, such as ibutilide and nibentan, selectively prolong the effective refractory period of the bundle of His fibers. This blocks the conduction of premature impulses from the atria to the ventricles in supraventricular tachyarrhythmias. Normally, phase 3 of the action potential in these fibers is short, which ensures a short refractory period and high conductivity.
Class and exam questions
Why is a straight line recorded on the ECG during the plateau?
During the plateau phase, the charge on the cardiomyocyte membranes remains unchanged, and the heart structure is completely encompassed by excitation. Because of this, there is no potential difference between myocardial regions, the electric field around the heart disappears, and an isoelectric line is recorded on the ECG.
Where does the calcium for myocardial contraction come from during this phase?
Unlike skeletal muscles, where calcium is supplied to the contractile proteins primarily from the endoplasmic reticulum, a "calcium surge" occurs in the myocardium. During the plateau, $Ca^{2+}$ ions enter cardiomyocytes from the outside—from the extracellular environment via the T-system.
Which ECG segments correspond to the plateau phase?
In the atria, the plateau phase, corresponding to slow repolarization, is reflected by the PQ segment with a duration of 60 ms. In the ventricles, the plateau corresponds to the ST segment with a duration of 120 ms.
What else to study on this topic
- Cardiomyocyte action potential
- Slow diastolic depolarization (SDD)
- Electrocardiography (ECG): waves, segments, and intervals
- Mechanism of muscle contraction
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