Action potentials, pacemaking propagation

The electrical impulse for contraction (propagated action potential p. 136) originates in pacemaker cells of the sinoatrial node and spreads through the atria, atrioventricular (AV) node, and adjoining parts of the His-Purkinje fiber system to the ventricles (A). Irregularities of heart rhythm can interfere dangerously with cardiac pumping func-tioa... 

In healthy cardiac cells, the recovery time from inactivation of sodium channels (back to the resting state) is quite rapid, so that the maximum number of channels is available for activation. In contrast, in sick cells, the recovery time is quite slow. In these sick cells, the action potential develops from the opening of fewer sodium channels, so the action potential is a slow sluggish upstroke as opposed to the fast upstroke in a healthy cell. A slow sluggish upstroke results in poor and perhaps no propagation of the action potential. Chronically depolarized or ischemic cells may (1) fail to conduct an action potential and therefore fail to contract or to transmit the action potential to neighboring cells or (2) become an ectopic pacemaker (due to a... 

As with the normal cardiac conducting system and its propagation of impulses through the His Purkinje network, electrostimulation by an artificial cardiac pacemaker depends on the depolarization of a single or a group of myocyte cell membranes which can then act as pacemaker cells. In order for these cells to depolarize, the electric field of the applied artificial pacemaker stimulus must exceed a threshold voltage. This initiates a complex cascade of ionic currents both in and out of the cell membrane referred to as the action potential. The impulse or wave of depolarization then propagates away from the site of stimulation from cell to cell across gap junctions or intercalated disks, which with normal cells provide very low resistance to depolarization. 

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