Action potential in cardiac muscle

The resting membrane potential is expressed in millivolts (mV) and intracellular potential is expressed relative to extracellular one. Myocardial fibers have a resting membrane potential of approximately -90 mV. Changes in membrane potential are caused by flow of ions into or out of cells. Depolarization causes the membrane potential to become less negative, it occurs when there is a movement of a positive charge into the cell. Repolarization causes the membrane potential to become more negative and it occurs when there is a movement of a positive charge out of the cell.
The trans-membrane action potential of single cardiac muscle cells is characterized by rapid depolarization (phase 0), an initial rapid repolarization (phase 1), a plateau (phase 2), and a slow repolarization process (phase 3) that allows return to the resting membrane potential (phase 4).
Phases of cardiac action potential
a. Phase 0 (depolarization): When the cardiac cell is stimulated and depolarizes, the membrane potential becomes more positive. Voltage-gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it. The membrane potential reaches about +20 millivolts before the sodium channels close.
b. Phase 1 (initial repolarization): The sodium channels close, the cell begins to repolarize, and potassium ions leave the cell through open potassium channels.
c. Phase 2 (plateau): A brief initial repolarization occurs and the action potential then plateaus as a result of increased calcium ion permeability and decreased potassium ion permeability. The voltage-gated calcium ion channels open slowly during phases 1 and 0, and calcium enters the cell. Potassium channels then close, and the combination of decreased potassium ion efflux and increased calcium ion influx causes the action potential to plateau.
Benefits of plateau to cardiac cells:
1. Prolongation of the duration of excitation (refractory period).
2. Prolongation of the exposure to high cytoplasmic Ca2+ concentration, thereby increasing the force of cardiac muscle contraction.
3. Giving more time that all cardiac cells can be activated together and contract as one unit in atria and ventricles.
d. Phase 3 (rapid repolarization): The closure of calcium ion channels and increased potassium ion permeability, permitting potassium ions to rapidly exit the cell, ends the plateau and returns the cell membrane potential to its resting level.
e. Phase 4 (resting membrane potential): Averages about ?90 millivolts.
Figure (1) Action potential of cardiac muscle and associated ionic currents
Excitation-contraction coupling
It is the mechanism by which the action potential causes the myofibrils of muscle to contract.
When an action potential passes over the cardiac muscle membrane, it