Unit 6.3 - AP for Contractile Myocytes & ECG

Ionic Basis of AP for Contractile Myocyte

There are 5 phases which characterized depolarization and repolarization:

Phase 0 - rapid depolarization phase

Phase 1 - early repolarization phase

Phase 2 - plateau phase

Phase 3 - repolarization phase

Phase 4 - restoration phase

Phase 0

Rapid Depolarization Phase

Occurs when pacemaker potential spreads to the contractile myocytes and reaches its threshold (-65mV)

Triggers influx of Na⁺ through fast Na⁺ channels.

Phase 1

Early Repolarization Phase

Rapid inactivation of the fast Na⁺ channels.

Activation of transient outward K⁺ current.

Brief efflux of K⁺.

Phase 2

Plateau Phase

The balance between K⁺ efflux and Ca²⁺ influx.

K⁺ efflux through delayed rectifier channels.

Ca²⁺ influx through L-type (long-lasting) channels.

Phase 3

Repolarization Phase

Inactivation of L-type Ca²⁺ channels.

Continuation on the efflux of K⁺.

Cells refractory (unexcitable) during phases 0, 1, 2, and part of 3.

Phase 4

Restoration Phase

Concentration of Na⁺ and K⁺ return to their resting state.

Electrocardiogram (ECG)

Graphic recording of the biopotentials generated by the myocardium during the cardiac cycle.

Reflects the rhythmic electrical events of depolarization and repolarization wave, followed by mechanical events of contraction and relaxation.

ECG - Depolarization

The ECG shows and upward deflection when the depolarization wave (positive) flows toward the positive recording electrode.

The larger the tissue mass, the higher the deflection.

ECG - Repolarization

The ECG shows a downward deflection when the repolarization wave (negative) flows toward the positive recording electrode.

The larger the tissue mass, the higher the deflection.

ECG Waves

P Wave: depolarization of atria

QRS complex: depolarization of ventricles (repolarization of atria not visible)

T wave: repolarization of ventricles

U wave: unknown

PR Interval

Time from the beginning of atrial depolarization to the start of ventricular depolarization.

Time for the impulse to travel from SA node to the ventricular septum.

PR Segment

Time from the end of atrial depolarization to the start of ventricular depolarization.

Time related to the time of delay for the AP passing throught the AV node.

QRS Duration

The duration of ventricular depolarization.

Associated with the functional integrity of the ventricular conduction system and contractile myocardium.

ST Segment (Isoelectric Line)

Time between the end of ventricular depolarization and the start of ventricular repolarization.

Should be no electrical activity for atria and ventricles.

QT Interval

Time between the beginning of ventricular depolarization and the end of ventricular repolarization.

Directly correlated with heart rate.