7th February 2024

Generation of pacemaker action potential

The electrophysiology of action potential generation of cardiac pacemaker cells follows a cycle: 

Following an action potential, the pacemaker cell returns to a resting membrane potential of -60mV

• Phase 4: Increasing membrane permeability to sodium ions causes slow leakage of sodium ions via sodium channels into the cell, causing membrane potential to become less negative. This results in spontaneous depolarisation of the pacemaker cells. 
  • This is referred to as the funny current, If. The funny current is found in cardiac areas supporting spontaneous depolarisation, including the sinoatrial node, atrioventricular node and Purkinje fibres. 
• Phase 0: When the membrane potential reaches a threshold potential of -40mV, voltage-gated T-type calcium channels open, and rapid Ca2+ influx into the cell occurs, causing depolarisation to the membrane potential of +20mV (Phase 0)
• Phase 3: Following depolarisation, the voltage-gated T-type calcium channels close. Voltage-gated potassium channels open, causing efflux of K+ from the cell, returning membrane potential to resting potential of -60mV (Phase 3).

Image Source: Wikimedia Commons (2010). Pacemaker potential annotated, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/8/81/Pacemaker_potential_annotated.gif (Accessed: 4 January 2024).