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.