Cardiac Resting Membrane Potential

Cardiac Resting Membrane Potential is the stable transmembrane voltage difference maintained by non-pacemaker cardiac muscle cells between action potentials, established primarily by the selective permeability of the cell membrane to potassium ions and serving as the baseline from which the cardiac action potential is initiated.

Ionic Basis of the Resting Potential

Potassium Permeability

At rest, the cardiac cell membrane is highly permeable to potassium ions relative to other ions, due to the activity of inward rectifier potassium channels, causing the resting membrane potential to lie close to the equilibrium potential for potassium as predicted by the Nernst equation.

Ionic Gradients

The concentration gradients that establish this potential are maintained by the sodium-potassium ATPase pump, which actively transports sodium out of and potassium into the cell against their respective concentration gradients, consuming metabolic energy to preserve the ionic asymmetry across the membrane.

Typical Resting Potential Values

Working Myocardial Cells

In atrial and ventricular working myocardial cells, the resting membrane potential is stable and strongly negative, typically close to negative ninety millivolts, remaining essentially constant between successive action potentials due to the dominant, steady potassium conductance.

Contrast with Pacemaker Tissue

Unlike working myocardial cells, cells of the sinoatrial and atrioventricular nodes lack a true stable resting potential; instead, their membrane potential drifts continuously and spontaneously toward threshold following each action potential, a property termed the pacemaker potential, distinguishing them functionally from cells that possess a genuine resting state.

Functional Role of the Resting Potential

Foundation for Excitability

The stable, strongly negative resting potential of working myocardial cells ensures that voltage-gated sodium channels remain available in their closed, activatable state, allowing a rapid and reliable upstroke when the cell is subsequently depolarized by an approaching wave of excitation.

Determinant of Excitation Threshold

The magnitude of the resting membrane potential relative to the threshold potential determines how much depolarizing stimulus is required to trigger an action potential, so that abnormal shifts in resting potential, whether toward or away from threshold, alter the excitability of the affected cardiac tissue.

Maintenance and Stability

Continuous Ion Pump Activity

The resting potential is not a static, passive state but an actively maintained condition requiring continuous operation of the sodium-potassium ATPase and other transport mechanisms to counteract the slow ionic leak that would otherwise dissipate the concentration gradients over time.

Sensitivity to Extracellular Ion Concentration

Because the resting potential depends closely on the ratio of intracellular to extracellular potassium concentration, changes in extracellular potassium level produce significant and predictable shifts in the cardiac resting membrane potential, with consequences for cardiac excitability and conduction.

Physiological Significance

The cardiac resting membrane potential provides the essential electrical baseline from which the rapid depolarization, plateau, and repolarization phases of the cardiac action potential proceed, and its stability in working myocardial tissue is fundamental to reliable, coordinated excitation of the heart during each cardiac cycle.