GROSS ANATOMY

Detailed explanation-1: -The spontaneous opening of the pacemaker Na+ channels causes a progressive increase in the cell’s Na+ permeability (label b in Fig. 19.7). The increase in Na+ permeability allows Na+ to enter the cell from the extracellular fluid, which contributes to the depolarization of the cell toward threshold.

Detailed explanation-2: -Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles. Since the cells re connected by gap junctions, the action potential spreads form cell to cell by ion flow through the gap junctions.

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: -Repolarization in the ventricles of the heart is a process allowing the muscle cells of the ventricles to regain their ability to depolarize again. Repolarization entails movement of the ions, which entered the cell during the depolarization phase of the cycle, to flow out of the cell.