1.6.5 Repolarization Definition

Repolarization Definition is the precise characterization of repolarization as the process by which a cardiomyocyte's membrane potential returns from its depolarized state back toward its negative resting value, driven primarily by the outward flow of potassium ions through voltage-gated channels as sodium and calcium currents subside. This definition establishes repolarization as the concluding phase of the cardiac action potential, restoring the cell's electrical state in preparation for subsequent excitation.

Elements of the Definition

Directional Return Toward Resting Potential

Repolarization is defined by the specific direction of membrane potential change, moving from a depolarized or plateau state back toward the negative resting membrane potential characteristic of a quiescent cardiomyocyte.

Dependence on Potassium Channel Activity

Central to the definition is the role of voltage-gated potassium channels, whose opening allows an outward flow of positive charge that drives the membrane potential back toward negative values.

Decline of Depolarizing Currents

The definition specifies that repolarization depends not only on increasing potassium efflux but also on the progressive inactivation of the sodium and calcium currents that had sustained the depolarized and plateau phases.

Distinguishing Features

Contrast with Depolarization

Repolarization is distinguished from depolarization by both the direction of membrane potential change and its predominant ionic basis, with repolarization driven by potassium efflux rather than the sodium influx characteristic of depolarization.

Gradual Kinetics Relative to Depolarization

A defining feature of repolarization is its comparatively gradual time course, particularly during the plateau-associated phase, contrasting with the rapid kinetics of the initial depolarizing upstroke.

Association with the Refractory Period

The definition accounts for the close relationship between repolarization and the refractory period, since the cell remains largely unresponsive to new stimuli until repolarization has proceeded sufficiently to restore excitability.

Purpose of the Definition

Establishing the Concluding Phase of the Cardiac Action Potential

A precise definition of repolarization establishes the specific phase responsible for restoring the resting membrane potential, completing the cycle of electrical activity initiated by depolarization.

Foundation for Understanding Cardiac Refractoriness

The definition provides the basis for understanding how the timing of repolarization determines the duration of the refractory period and, consequently, the minimum interval between successive cardiac excitations.

Clarifying Repolarization's Role Within the Cardiac Action Potential

By specifying that repolarization restores the resting membrane potential through potassium efflux, the definition delineates its role relative to the depolarization and plateau phases that precede it within the complete cardiac action potential.