The Cardiac Action Potential Explained

The cardiac action potential describes the electrical activity of a cardiac myocyte (muscle cell), and its relationship to the activity of ions across the cell’s membrane.

Four Properties of Cardiac Myocytes

Cardiac myocytes all have four basic properties which differentiate them from skeletal myocytes and other smooth muscle cells. These are:

• Automaticity: A myocyte’s ability to initiate its own pace (chronotropism)
• Conductivity: A myocyte’s ability to conduct/carry an electrical impulse (dromotropism)
• Contractility: A myocyte’s ability to contract on stimulation (inotropism)
• Excitability: A myocyte’s ability to generate an action potential due from an inward depolarisation (bathmotropism)

These four properties allow for the following phases of the Cardiac Action Potential to occur. A myocyte’s resting membrane potential is -90mV. An action potential is propagated and contraction generated when the membrane potential reaches 20mV.

The Four Phases of the Cardiac Action Potential

Phase 4: Depolarisation of a neighbouring cell causes Na⁺ and Ca²⁺ to be released. These ions flow through the gap junctions of the resting cell. This raises the resting membrane potential from -90mV to -70mV.

Phase 0: At -70mV voltage-gated fast Na⁺ channels opening causing a rapid influx of Na⁺, causing the membrane potential to spike at 20mV.

Phase 1: At 20mV voltage-gated K⁺ open, causing an efflux of K⁺, causing the membrane potential to drop.

Phase 2: Ca⁺ channels open allowing Ca⁺ to travel into the cell, as K⁺ is flowing out, resulting in an equilibrium. Thus membrane potential is maintained at ¬5mV for a period of time.

Phase 3: Ca⁺ channels close, K⁺ remain open, closing at -90mV and bringing the membrane potential back to its resting point. Na⁺ and K⁺ are then actively transported to their respective intra/extra cellular spaces, causing a refractory period in which further depolarisation cannot occur.