Current concepts on the electrophysiology of the myocardium

Each heart beat is triggered by a characteristic change in the electrical potential that exists across the membranes of the cells of the heart. This 'cardiac action potential' is shown in figure 1 A and it is the result of currents of ions (figure 1 B) which flow across the cell membrane via so-called ion channels. During diastole the cell is at 'rest' and the cell interior is negative; this resting potential is the result of positive K ions flowing out of the cell via one type of K channel. This particular K current is known as iK, 1. During the upstroke (phase 0) of the action potential the cell is depolarized and the cell interior becomes positive because of a large influx of positive Na ions into the cell. This Na current (iNa) is short lived but despite this the cell remains depolarized for several hundred milliseconds during the characteristic plateau (phase 2) of the action potential because positive Ca ions now flow into the cell. Although this Ca current (iCa) is an important factor underlying the plateau phase, it is helped because the efflux of K ions via iK, 1 is curtailed as a result of a special property known as anomalous rectification. Finally the action potential is brought to an end as the Ca current gradually declines (inactivates) whereas the movement of K out of the cell (via a second set of K channels) gradually increases. This K current is known as iK.(ABSTRACT TRUNCATED AT 250 WORDS)