MICROANATOMY

Detailed explanation-1: -Phase zero is the phase of depolarization. This phase starts when the membrane potential reaches-40 mV, the threshold potential for pacemaker cells. There is the opening of voltage-gated Ca2+ channels on reaching the threshold, causing the influx of Ca2+ ions.

Detailed explanation-2: -Phase 0 is the phase of rapid depolarization. The membrane potential shifts into positive voltage range. This phase is central to rapid propagation of the cardiac impulse (conduction velocity, =1 m/s).

Detailed explanation-3: -Phase 0. This phase consists of a rapid, positive change in voltage across the cell membrane (depolarization) lasting less than 2 ms in ventricular cells and 10–20 ms in SAN cells. This occurs due to a net flow of positive charge into the cell.

Detailed explanation-4: -Sodium-channel blockers comprise the Class I antiarrhythmic compounds according to the Vaughan-Williams classification scheme. These drugs bind to and block fast sodium channels that are responsible for rapid depolarization (phase 0) of fast-response cardiac action potentials.

Detailed explanation-5: -The cardiac transmembrane action potential consists of five phases:phase 0, upstroke or rapid depolarization; phase 1, early rapid repolarization; phase 2, plateau; phase 3, final rapid repolarization; andphase 4, resting membrane potential and diastolic depolarization (Fig.