45 Cardiac pacemaker cells

Action potentials are considerably different between cardiac pacemaker cells and cardiac muscle cells. While Na⁺ and K⁺ play essential roles, Ca²⁺ is also critical for both types of cells. Unlike skeletal muscles and neurons, cardiac pacemaker cells do not have a stable resting potential. Pacemaker cells contain a series of Na⁺ channels that allow a normal and slow influx of Na⁺ ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. This is called drift. The resulting movement of Na⁺ ions creates a spontaneous depolarization and brings the cell to threshold. At this point voltage-gated Ca²⁺ channels open and Ca²⁺ influx rapidly causes depolarization until it reaches a value of approximately +5 mV. At this voltage, the Ca²⁺ channels close and voltage-gated K⁺ channels open, allowing K⁺ efflux which causes repolarization. When the membrane potential reaches approximately −60 mV, the K⁺ channels close and voltage-gated slow Na⁺ channels open, and the drift phase begins again. This phenomenon explains the autorhythmicity properties of cardiac muscle (Figure 45.1).

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 19