1.6.2 Cardiac Membrane Potential Definition

Cardiac Membrane Potential Definition is the precise characterization of the cardiac membrane potential as the voltage difference across the cell membrane of a cardiomyocyte, arising from the unequal distribution of ions, particularly sodium, potassium, and calcium, and from the selective permeability of the membrane to these ions at any given moment. This definition establishes the membrane potential as the electrical variable that changes during depolarization and repolarization, forming the basis of the cardiac action potential.

Elements of the Definition

Voltage Difference Across the Membrane

The cardiac membrane potential is defined as the difference in electrical voltage between the interior and exterior of a cardiomyocyte, measured in millivolts and varying continuously throughout the phases of the action potential.

Dependence on Ionic Distribution

Central to the definition is the unequal distribution of ions across the membrane, with intracellular and extracellular concentrations of sodium, potassium, and calcium differing substantially and being maintained by active transport mechanisms.

Dependence on Selective Membrane Permeability

The definition specifies that the membrane potential at any moment depends not only on ionic concentration gradients but also on the relative permeability of the membrane to each ion, which changes dynamically as ion channels open and close.

Distinguishing Features

Contrast Between Resting and Active States

The cardiac membrane potential is distinguished between its resting state, a stable negative value maintained between action potentials, and its actively changing state during depolarization and repolarization phases of excitation.

Distinctive Plateau Phase

A defining feature of the cardiac membrane potential, setting it apart from membrane potentials in other excitable tissues, is the prolonged plateau phase sustained by balanced calcium influx and potassium efflux during the action potential.

Variability Across Cardiac Tissue Types

The definition accounts for variation in membrane potential behavior across different cardiac tissues, with pacemaker cells exhibiting spontaneous diastolic depolarization not present in ordinary working cardiomyocytes.

Purpose of the Definition

Establishing the Electrical Variable Underlying Cardiac Excitability

A precise definition of the cardiac membrane potential establishes the specific electrical quantity whose changes constitute the action potential and, ultimately, cardiac excitation.

Foundation for Understanding the Cardiac Action Potential

The definition provides the basis for understanding the distinct phases of the cardiac action potential, each corresponding to specific changes in membrane potential driven by particular ionic currents.

Clarifying the Membrane Potential's Role Within Cardiac Electrical Activity

By specifying that the membrane potential reflects ionic distribution and permeability, the definition delineates its role as the physical substrate upon which the broader processes of cardiac electrical activity depend.