Sodium conductance does not increase until the cell is depolarized to the threshold potential for the sodium inward current, -60mV, by either the electrotonic effect of depolarization of adjacent cells or by an external source, such as a pacemaker. At this membrane potential (-60mV), sodium conductance increases, sodium ions flow rapidly across the membrane into the cell because of the large driving force and the upstroke of the action potential (phase 0) is generated.