MEDICAL PHYSIOLOGY

Detailed explanation-1: -SA node pacemaker cells depolarize at a rate of 60 to 100 per minute, while the AV node at 40 to 60 per minute. The pacemaker with the highest rate of depolarization takes over as the primary pacemaker.

Detailed explanation-2: -Depolarization spreads from the AV node to the bundle of His in the interventricular septum. The bundle splits into right and left bundle branches, supplying the respective ventricles. The left bundle itself divides into anterior and posterior divisions.

Detailed explanation-3: -Cells within the sinus node have the fastest rate of spontaneous depolarization, and, therefore, the sinus node is the main pacemaker region of the heart. The AV node has the second fastest rate of spontaneous depolarization, which allows it to create an escape rhythm if the sinus node is diseased.

Detailed explanation-4: -In nerve and muscle cells, the depolarization phase of the action potential is caused by an opening of fast sodium channels. This also occurs in non-pacemaker cardiac cells; however, in cardiac pacemaker cells, calcium ions are involved in the initial depolarization phase of the action potential.