Electrophysiology

• Phase 0 corresponds to Na⁺ channel activation.

  • The maximum upstroke slope of phase 0 is proportional to the sodium current.

  • Phase 0 slope is related to the conduction velocity in that the more rapid the rate of depolarization the greater the rate of impulse propagation.

• Phase 1 corresponds to an early repolarizing K⁺ current.

  • rapidly inactivated.

• Phase 2 is the combination of an inward, depolarizing Ca²⁺ current balanced by an outward, repolarizing K⁺ current (delayed rectifier).

• Phase 3 is also the combination of Ca²⁺ and K⁺ currents.

  • Phase 3 is repolarizing because the outward (repolarizing) K⁺ current increases while the inward (depolarizing) Ca²⁺ current is decreasing.

• Phase 4 in normal His-Purkinje and ventricular muscle cells is characterized by a balance between outward Na⁺ current and inward K⁺ current.

  • As a result, the resting membrane potential would normally be flat.

  • In disease states or for other cell types (SA nodal cells) the membrane potential drifts towards threshold.

  • This phenomenon of spontaneous depolarization is termed automaticity and has an important role in arrhythmogenesis.

Category: Physiology Notes