Working Myocardial Action Potential

Working Myocardial Action Potential is the characteristic electrical signal generated by working myocardial cells (cardiomyocytes) during each heartbeat. It represents the sequence of voltage changes across the cell membrane as the myocyte undergoes excitation and recovery, ultimately leading to coordinated contraction and relaxation of the cardiac muscle. This action potential underpins the electrical activity that drives the synchronous pumping function of the heart.

Phases of the Working Myocardial Action Potential

The working myocardial action potential consists of several distinct phases, which reflect the dynamics of ion channel opening and closing, ion fluxes, and changes in membrane potential. These phases are traditionally divided into five stages: Phase 0 (Rapid Depolarization), Phase 1 (Early Repolarization), Phase 2 (Plateau), Phase 3 (Final Repolarization), and Phase 4 (Resting Membrane Potential). Each phase is critical for the electrical and mechanical function of the heart.

Phase 0: Rapid Depolarization

At rest, the cardiac myocyte membrane potential is approximately -85 to -90 mV. When a stimulus reaches the cell, voltage-gated fast sodium (Na⁺) channels open rapidly, causing a sudden influx of Na⁺ ions. This results in a swift rise in membrane potential from the resting negative value toward positive values, rapidly depolarizing the cell. The membrane potential can reach approximately +20 mV during this phase. The rapid depolarization ensures a fast conduction of the electrical signal through the myocardium.

Phase 1: Early Repolarization

Following the peak of depolarization, the fast sodium channels inactivate, and transient outward potassium (K⁺) channels (Ito) briefly open, allowing an outward K⁺ current. This results in a slight repolarization or "notch" in the action potential curve, bringing the membrane potential down from the peak level. This early repolarization shapes the initial decline and prepares the membrane for the plateau phase.

Phase 2: Plateau

The plateau phase is a defining feature of the working myocardial action potential, distinguishing it from action potentials in other excitable cells. During this phase, voltage-gated L-type calcium (Ca²⁺) channels open, permitting a sustained influx of Ca²⁺ into the cell, which balances the outward K⁺ current through delayed rectifier potassium channels. This balance between inward and outward currents maintains the membrane potential near 0 mV for an extended period (approximately 200-300 milliseconds). The plateau phase is critical for coupling electrical excitation to mechanical contraction, as the entry of Ca²⁺ triggers calcium-induced calcium release from the sarcoplasmic reticulum, initiating contraction.

Phase 3: Final Repolarization

As the L-type Ca²⁺ channels close, the delayed rectifier K⁺ channels remain open, allowing an increased outward flow of K⁺ ions. This results in a rapid repolarization of the membrane potential back toward the resting level. The membrane potential gradually returns to approximately -85 to -90 mV, restoring the negative internal environment of the cell. This phase ends the refractory period, making the cell ready for the next excitation.

Phase 4: Resting Membrane Potential

During phase 4, the cell remains at a stable resting membrane potential maintained primarily by the inward rectifier potassium current (IK1) and the Na⁺/K⁺ ATPase pump. This resting potential is essential for the cell's excitability and readiness to respond to the next depolarizing stimulus. The stable negative potential prevents spontaneous activation and ensures orderly propagation of electrical impulses.

Ionic Currents and Channels Involved

The working myocardial action potential is governed by the coordinated activity of multiple ion channels and transporters:

• Fast sodium channels (INa): Open rapidly during phase 0, causing rapid depolarization.
• Transient outward potassium channels (Ito): Open briefly during phase 1 to initiate early repolarization.
• L-type calcium channels (ICa,L): Open in phase 2, causing sustained Ca²⁺ influx and the plateau.
• Delayed rectifier potassium channels (IKr and IKs): Responsible for K⁺ efflux during phases 2 and 3 facilitating repolarization.
• Inward rectifier potassium channels (IK1): Maintain resting potential during phase 4.
• Na⁺/K⁺ ATPase pump: Maintains ionic gradients across the membrane.

Functional Significance

The unique shape and duration of the working myocardial action potential ensure proper cardiac function in several ways:

• The rapid depolarization phase allows for swift transmission of electrical impulses across the myocardium, promoting synchronized contraction.
• The prolonged plateau phase ensures a sustained contraction period, preventing premature relaxation and allowing efficient ejection of blood.
• The longer refractory period associated with the action potential prevents tetanic contractions, which could be fatal.
• The controlled repolarization and restoration of resting potential prepare the myocytes for subsequent heartbeats in a rhythmic and coordinated manner.

Summary of the Working Myocardial Action Potential Characteristics

This action potential profile is fundamental for the coordinated mechanical activity of the heart, enabling the myocardium to contract effectively and maintain the rhythmic pumping essential for life.