pacemaker cell

In the heart, pacemaker cells in the sinoatrial node are specialized cardiac muscle cells that spontaneously generate action potentials. This pacemaker activity controls the heart rate.Cardiac pacemaker cells have high levels of expression of the HCN4 gene and they have many funny channels in their surface membrane. Entry of sodium ions into the pacemaker cells through the funny channels causes a spontaneous depolarization of the membrane potential that initiates the action potentials.

This "phase 4" depolarization is characteristic of pacemaker cells.