Cardiac Arrhythmias: Difference between revisions

In summary during the depolarization, sodium ions stream into the cell followed by a influx of Ca²⁺ ions (both from the inside (sarcoplasmatic reticulum) and outside of the cell). These Ca²⁺ ions cause the actual muscular contraction. Shortly thereafter K+ ions stream out of the cell. During repolarization the ion concentration returns to its precontraction state. The action potential can be divided in five phases:  

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===Phase 0: Rapid Depolarization===

Rapid depolarization is started once the membrane potential reaches a certain threshold (about -70 to -60 mV), independent of the size of the depolarizing stimulus. This produces a rapid influx of Na⁺ and a rapid upstroke of the action potential. At higher potentials (-40 to -30) Ca²⁺ influx participates in the upstroke. In sinus node and AV node a slower upstroke can be observed (Figure 1). This caused because the upstroke in these cells are mainly mediated by the slower acting Ca²⁺ ion channels. The slow activation and inactivation produce a slower upstroke.