Myocardial action potential

A recording of membrane potential during an action potential in a myocardial (force-producing) fibre shows a different profile from that in the pacemaker cells. The resting membrane potential in myocardial fibres is constant and considerably more negative. Once the cells have been depolarized to threshold by an external stimulus (normally spread of activity from a neighbouring fibre) the overshoot of the action potential is much steeper than that seen in the pacemaker cells. It is followed by a partial repolarization and a long shoulder (with the inside of the cell positive) before the membrane repolarizes to its resting level.