THE CARDIAC ACTION POTENTIAL

The ECG is best understood by first considering some electrical and chemical features of the myocardial cell. Resting cells have a potassium concentration (about 140mmol/litre) which is high in comparison with that in the extracellular tissues (about 4mmol/litre), whereas extracellular sodium concentration is much greater than intracellular. This ionic disequilibrium between the cell and its environment is maintained by a sodium-potassium ions into the cell and sodium ions out of the cell.

During diastole there is a relative negative potential within the cell of the order of 90mV the Trans membrane resting potential. This is primarily due to two factors, the high concentration of potassium ions intracellularly and the high permeability of the cell membrane to potassium ions. As a result, potassium ions tend to diffuse out of the cell down their concentration gradient, creating a negative charge in the interior of the cell which offsets and almost balances the concentration gradient for potassium.

The cardiac action potential arises due to a sequence of changes in permeability to sodium,calcium and potassium ions. At rest,cell membrane is relatively impermeable to sodium ions. The rapid upstroke of AP is due to a sudden increase in sodium permeability, causing a rapid influx of sodium ions. This sodium current is short-lived,because the ion channels which open to cause the increase in sodium permeability rapidly close once again. The first inward sodium current is succeeded by a slower inward current,comprised predominantly of calcium ions and to a lesser extent sodium ions. This slow inward current is responsible for the plateau phase, preventing the cell from re-polarizing rapidly like a nerve. During this phase potassium permeability is reduced. Re polarization is achieved by a gradual increase in potassium permeability once again, accompanied by a gradual decrease in the slow inward current.

CURRENT                                 ION                                    FUNCTION

1) Fast inward current                 Na+                                    Rapid depolarization

2) Slow inward current              Ca2+                                  Plateau maintenance
                                                 (mainly)                           Excitation-contraction coupling

3) Outward curves                      K+                                      Re-polarization
                                                                                         Resting membrane potential