Repolarization: Function, Tasks, Role & Diseases

Repolarization is the excitation reversion of a cell that has previously established an action potential as a result of a stimulus. The cell’s resting membrane potential is restored.

What is repolarization?

The term repolarization describes the restored resting potential of a cell, specifically a nerve cell. The term repolarization describes the restored resting potential of a cell, specifically a nerve cell, after an action potential by redistribution of ions at the cell membrane. The sequence of an action potential can be divided as follows:

1) Resting potential,

2) Crossing of the threshold potential,

3) depolarization,

4) repolarization and

5) hyperpolarization. The membrane potential is approximately -70mV at the resting potential.

Function and task

For the action process to start, a defined threshold (- 50mV) must be exceeded at the axon hillock. If this value is not reached, no action potential occurs and the incoming stimuli are not transmitted. According to the “all or nothing principle”, either an action potential occurs when this threshold is exceeded across the axon or no response is triggered. With depolarization, the action potential travels across the axon. With the opening of the corresponding channels (Na+), Na+ ions flow from the outside into the cell interior of the axon. The so-called overshoot, a repolarization, occurs. The intracellular area is now positively charged. The depolarization is followed by repolarization. The opened K+ channels are the prerequisite for potassium to diffuse out of the positively charged cells. This process occurs within a short time due to the voltage difference. The voltage difference results from the positively charged cell interior and the negatively charged cell exterior. As a result of this repolarization process, the voltage of the cell interior decreases again. With hyperpolarization, the voltage drops below the original resting potential. After repolarization, the channels responsible for voltage reduction (Na+) have closed again, so that no renewed action potential is possible during this phase. This resting phase is called the refractory period. The sodiumpotassium pump regulates the voltage field back to the initial value of 70mV. The axon of the nerve cell is now ready for the next action potential. If the heart is affected by repolarization, this process causes considerable discomfort. The heart is an independent and autonomous organ with a fine system of triggered and distributed excitation waves. This vital organ has a large number of cardiac myocytes that are activated to contract based on a temporally and spatially optimized schedule. The sinus node in the right atrium sets the pace as the physiological and primary pacemaker of the heart, similar to a conductor. From this point, action potentials are conducted via the conduction system and the heart muscle. During repolarization, the interior of the cell is positively charged with respect to the external medium. The original ion distribution is now restored by the sodiumpotassium pump. The most common symptoms of this occur in the form of primary and early repolarization. This is a disturbed process in which the excitation states of the heart can no longer be discharged in a regular manner. In the case of right hypertrophy with repolarization disorders, incoming voltage states in the right part of the heart are no longer cleared regularly. The heart consists of a ventricle and an atrium on both the right and left sides. The deoxygenated and used blood first flows through the left atrium. From there, it makes its way to the right ventricle and from there is pumped to the lungs, where it is supplied with new oxygen. The left ventricle is enlarged with thickened heart walls, resulting in increased force. The right heart valve is the “gateway from the right ventricle to the lungs.” This no longer functions regularly and does not open to allow blood to pass through. Pulmonary valve stenosis is present. Due to the valve not opening regularly, the blood flows back into the ventricle and not into the pulmonary artery as intended. There, the irregular flow of blood causes congestion, causing the heart to use more pumping power and increase in volume.The heart is an electrical pump, as electrical excitations must constantly pass through the heart muscles to trigger contraction of the heart muscles and thus guarantee regular blood flow. After the state of excitation, however, the heart must be returned to a state of rest, the repolarization state, to relieve incoming stresses so that it is not overstressed. Only when a state of excitation has been regularly reduced again do the heart muscles begin to build up a new state of excitation. However, if this resting phase lasts too long, the regular repolarization state is disturbed and the heart no longer works regularly. This condition can cause a variety of symptoms, from mild arrhythmias to ventricular fibrillation and sudden cardiac death. Many patients are affected by early repolarization, and some are affected by idiopathic (baseless) ventricular fibrillation. Most ECG findings are unremarkable, and only in isolated cases are repolarization disorders responsible for life-threatening arrhythmias. The clinical findings of early repolarization do not yet allow a conclusive identification of high-risk groups at risk. The borderline between a non-life-threatening repolarization disorder and life-threatening ventricular fibrillation is thin. The most common causes are a genetic background and secondary factors such as age, lifestyle, the autonomic nervous system and, in individual cases, acute ischemia. Drugs can also be triggers of arrhythmias due to repolarization.

Diseases and conditions

Physicians follow the “all-or-nothing law” in diagnosing benign inferolateral signs of early repolarization. If special triggers are added to the usually benign ECG changes, extensive early repolarization changes are present, which can lead to an “electrical catastrophe” and sudden cardiac death because of voltage states that are not relieved in a regular fashion. Malignant disorders of the nervous system are significantly involved in repolarization abnormalities and the resulting cardiac arrhythmias. The extent to which the sympathetic nerve (stress nerve, sympathetic nervous system) and associated repolarization disorders affect sudden cardiac death is measured by an invasive procedure. A measuring electrode is inserted inside the nerve cell, while a second electrode is attached to the outside of the cell. Since identifying the at-risk groups affected by this secondary death is currently a medically unsolved problem, patients who show electrocardiological abnormalities can be given a defibrillator as a preventive measure. These unresolved medical problems also include sudden infant death syndrome, which physicians also attribute to repolarization disorders. Medical preventive measures are not yet known.