||Figure 5. Schematic depiction of normal cardiac
action potential, as described in text.
The fast movement of positive Na+ ions into the myocyte depolarizes the
resting membrane potential and initiates an action potential. This is
followed by outwardly directed potassium currents: Ito, the initial outward
current and Ik, the delayed rectifier current which move positive K+ ions
out of the cells repolarizing the myocytes back to their resting membrane
potential. An inward calcium current, ICa,L temporarily delays the
repolarization of the membrane potential by bringing positive Ca2+ into the
myocytes producing the plateau of the action potential; this is called the
L-type Ca2+ current. The orderly occurrence of these currents creates the
action potentials, which couple the electrical and mechanical functions of
the heart resulting in its rhythmic contractions. Fatal arrhythmias occur
when the electrical signals become chaotic and the heart can no longer
function as a pump.