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: -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-4: -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-5: -The threshold potential is the potential an excitable cell membrane, such as a myocyte, must reach in order to induce an action potential. This depolarization is caused by very small net inward currents of calcium ions across the cell membrane, which gives rise to the action potential.