MUSCLE PHYSIOLOGY

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: -Myocardial contraction begins when sodium channels open and positively charged sodium ions flow into the cell and cause membrane depolarization (phase 0).

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: -During phase 0, membrane permeability to potassium decreases and fast sodium channels open, producing rapid depolarization from −90 mV to +10 mV. During phase 1, there is partial repolarization, because of a decrease in sodium permeability. Phase 2 is the plateau phase of the cardiac action potential.