7th February 2024

Conduction of electrical signals of cardiac myocytes

• Phase 0: Action potentials from pacemaker cells or nearby myocytes induce transient opening of Na+ channels, resulting in an influx of Na+ into myocytes. This produces rapid depolarisation to the membrane potential of +20mV
• Phase 1: Immediately following phase 0, Na+ channels close in response to depolarisation, voltage-gated K+ channel transiently opens, and electrochemical gradient causes transient efflux of K+ ions.
• Phase 2: Membrane potential is stable at this phase. Inward and outward currents are balanced. K+ channels remain open. L-type Ca2+ channels open. This results in the influx of Ca2+ ions, balanced by the efflux of K+ ions. The influx of Ca2+ ions initiates the release of intracellular Ca2+ stores in excitation-contraction coupling (see later).
• Phase 3: Repolarisation begins due to the closing of L-type Ca2+ channels. K+ channels remain open, resulting in the outward flow of K+.
• Phase 4: Membrane potential is maintained at resting membrane potential at -85mV. This phase also allows an interval between action potentials to return Ca2+ into intracellular stores.

Image Source: Wikimedia Commons (2019). Heart rhythm ionic flow, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/b/bc/Heart_rhythm_ionic_flow.svg  (Accessed: 4 January 2024).