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The myocardial action potential occurs in all cardiac myocytes except those in the sinoatrial (SA) and atrioventricular (AV) nodes. Unlike skeletal muscle, cardiac muscle has unique characteristics, including a plateau phase due to calcium (Ca²⁺) influx and potassium (K⁺) efflux, and calcium-induced calcium release for contraction.
The cardiac action potential consists of five distinct phases, each governed by the movement of specific ions across the cell membrane.
Phase | Description | Key Ionic Movements |
---|---|---|
Phase 0 (Rapid Upstroke & Depolarization) | Voltage-gated sodium (Na⁺) channels open, leading to a rapid influx of Na⁺ and depolarization. | Na⁺ influx (I_Na) |
Phase 1 (Initial Repolarization) | Inactivation of Na⁺ channels and opening of voltage-gated K⁺ channels, causing a slight repolarization. | K⁺ efflux |
Phase 2 (Plateau) | Ca²⁺ influx through voltage-gated Ca²⁺ channels balances K⁺ efflux, maintaining a plateau. Ca²⁺ influx triggers calcium release from the sarcoplasmic reticulum, leading to myocyte contraction. | Ca²⁺ influx, K⁺ efflux (I_Ca & I_K) |
Phase 3 (Rapid Repolarization) | Opening of slow delayed-rectifier K⁺ channels results in massive K⁺ efflux, while Ca²⁺ channels close. | K⁺ efflux (I_K) |
Phase 4 (Resting Potential) | High K⁺ permeability maintains the resting membrane potential. | K⁺ permeability (I_K) |