Transmembrane Potential

Figure 16.1 shows the phases of the cardiac action potential recorded with an intracellular microelectrode. The characteristic action potential is the result of activation and inactivation of multiple ion channels, which allows the flow of charged ions across the sarcolemmal membrane. The ion channels are transmembrane proteins possessing two important features: an ion selective pore that allows the passage of a specific cation or anion and regulatory components that respond to chemical stimulation or changes in the transmembrane potential by opening or closing. The ions flow through open channels according to the electrochemical driving forces at any given moment.

Like all other electrically active cells, the interior of the cardiac muscle cell is electrically negative with respect to the surrounding medium. This difference between the exterior and interior of a myocardial cell results from the action of several energy-requiring pumps, such as the Na+-K+-ATPase, which pumps Na+ out of and K+ into the cell in a ratio of 3Na+ to 2K+, and the presence of large negatively charged intracellular proteins that do not diffuse freely across the sarcolemmal membrane. The normal resting [K+]i is 140 mM, whereas the extracellular K+ concentration, [K+]0, is 4 mM. The resting myocardial cell tends to be highly permeable to K+ and less so to Na+ and Ca++; therefore, a net diffusion of K+ flows out of the cell, leaving behind negatively charged proteins. As a result, the interior of the cell becomes electronegative, and two opposing forces are established: a chemical force due to a concentration gradient and a counteracting electrostatic force established by the negatively charged ions within the cell.

At equilibrium, the chemical and electrostatic forces are equal, and there is no net flow of ions across the

Resting State

Inactivated State hh Activated State

h I 1 h h T h hh Activated State

200 Time (msec)

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