<P> The firing of the pacemaker cells is induced electrically by reaching the threshold potential of the cell membrane . The threshold potential is the potential an excitable cell membrane, such as a myocyte, must reach in order to induce an action potential . This depolarization is caused by very small net inward currents of calcium ions across the cell membrane, which gives rise to the action potential . </P> <P> Bio-pacemakers are the outcome of a rapidly emerging field of research into a replacement for the electronic pacemaker . The bio-pacemaker turns quiescent myocardial cells (e.g. atrial cells) into pacemaker cells . This is achieved by making the cells express a gene which creates a pacemaker current . </P>

Highest rate of automaticity in the normal heart