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

Detailed explanation-4: -Phase 0 is the depolarization phase of the action potential. This is followed by phase 3 repolarization. Once the cell is completely repolarized at about-60 mV, the cycle is spontaneously repeated.

Detailed explanation-5: -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.