6th March 2024

Nodal action potential

The pacemaker or nodal action potential consists of 3 phases: 4, 0 and 3. Phase 4 starts at a membrane potential of around -60mV with the opening of funny (If) channels. This allows an influx of Na⁺ ions into nodal cells and spontaneous depolarisation. This is propagated by the opening of Ca2+ channels (T-type at -50mV and L-type at -40mV) to allow an influx of Ca²⁺ ions. At the same time, there is reduced efflux of K+ due to closure of K+ ion channels.

Once a threshold (pacemaker potential) is reached, which is around -40 to -30 mV, there is an influx of Ca2+ ions through L-type channels leading to depolarisation, which constitutes phase 0.

The nodal cell then repolarises in phase 3 through the opening of K+ channels to allow K+ efflux and the closing of L-type Ca2+ channels, decreasing Ca²⁺ influx. This achieves a membrane potential of around -60mV and the cycle restarts again with no resting potential.