The heart beats continuously based on the unique features of its component cells. A cardiac cycle begins when spontaneousdepolarization of a pacemaker cell initiates an action potential;this electrical activity is transmitted to atrial muscle cells, which contract, and to the conduction system, which transmits the electrical activity to the ventricle. Activation depends on components of the cell membrane and cell that induce andmaintain the ion currents that maintain and promote electricalactivation.

Similar to most excitable cells in the body, the activity of cells in the heart is triggered by an action potential. An action potentialis a cyclic activation of a cell consisting of a rapid change in the membrane potential (the electrical gradient across the cell membrane) and subsequent return to a resting membrane potential. This process depends on a selectively permeable cell membrane and proteins that actively and passively direct ion passage across the cell membrane. . The myocyte action potential is characterized by a rapid initial depolarization mediated by fast channels (sodium channels) and then a plateau phase mediated by slow channels (calcium channels). Further details of this process are introduced as their components are described.

The Sarcolemma

The cardiac cell is surrounded by a membrane (plasmalemma, or more specific to a muscle cell, sarcolemma). The structural components of the sarcolemma allow for the origination and then conduction of an electrical signal through the heart with subsequent initiation of the excitation-contraction coupling process. This leads to depolarization of atrial myocytes and, with an appropriate delay, depolarization of ventricular myocytes. The sarcolemma also participates in the regulation of excitation, contraction, and intracellular metabolism in response to neuronal and chemical stimulation. Each of these functions will be considered, with emphasis on the features of the cardiac sarcolemma that differ from the plasmalemma of other cells.

The Phospholipid Bilayer

A phospholipid bilayer provides a barrier between the extracellularcompartment and the intracellular compartment, or cytosol. It is only two molecules thick, consists of phospholipids and cholesterol aligned so that the lipid, or hydrophobic, portion of the molecule is on the inside of the membrane, and the hydrophilic portion of the molecule is on the outside . The sarcolemma which is a phospholipid bilayer, provides a fluid barrier that is particularly impermeable to diffusion of ions. Small lipid-soluble molecules such as oxygen and carbon dioxide diffuse easily through the membrane. The water molecule, although insoluble in the membrane, is small enough that it diffuses easily through the membrane (or through pores in the membrane). Other, slightly larger molecules (e.g., sodium, chloride, potassium, and calcium) cannot diffuse easily through the lipid bilayer and require specialized channels for transport.