Cardiac Muscle, Dr. D. Penney
The 5 Phases of Myocardial Cell Electrophysiology (follower cells only):
- Phase 4 -
Polarized Cell; (-) inside, (+) outside; due mainly to Na+ & K+ ion positioning and higher permeability of membrane to K+, allowing loss of intracellular (+) charge.
- Phase 0 -
Cell Depolarization; greatly increased membrane permeabilty to Na+ ions, which rush in through fast channels, down conc. gradient, reversing cell polarity (fast current).
- Phase 1 -
Partial Repolarization; loss of Na+ conductance, & transient influx of Cl- ions and outflow K+ ions.
- Phase 2 -
Plateau; due to the slow inward flow of Ca++ ions through slow channels (i.e. increased Ca++ conductance) (also some inward movement of Na+ through slow channels and outward movement of K+). Phase 2 includes most of the absolute refractory period.
- Phase 3 -
Rapid Repolarization; decreased Ca++ conductance and increased K+ conductance, thus K+ moves out; inside of cell again becomes (-) relative to outside; Na+/K+ pump re-establishes distribution of ions. Supranormal excitabilty present early in phase 3, thus greatest chance of ectopic beat.
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