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There are three phases of the pacemaker potential (phase 0, phase 3 and phase 4) in pacemaker cells of the heart.During phase 4 the membrane potential is depolarizing because sodium ions are entering the cell through funny channels, creating an inward positive current (if). This is often called "phase 4 depolarization" of "diastolic depolarization" (because it is during diastole, the resting part of the cardiac cycle).
During phase 0, calcium-selective ion channels allow calcium ions to enter the cell, initiating the pacemaker cell action potential. The threshold potential for opening voltage-gated calcium channels is shown (diagram to the right). These calcium channels open when phase 4 depolarization reached the threshold potential.
During phase 3, potassium channels allow potassium ions to leave the cell. Cells hyperpolarize during phase 3.
The "funny" channels are "hyperpolarization-activated"; they open at the very negative membrane potentials that are reached at the end of phase 3.
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