Figure14_5.

Principle underlying membrane-based heart models. Two model cells are depicted schematically, with their ion channels, pumps, and exchangers. Gap junctions connect the cells. In depolarizing cells, a large inward sodium current flows along its electrochemical gradient (1). This current passes through gap junctions to neighboring cells. There, part of it serves to charge the cell membrane (2) represented by a capacitor symbol, and another part leaks through potassium channels that are still open (3). The two parts form an outward current in this cell, and the current loop is closed in the interstitium and outside the heart (4), where it generates a potential field, schematically indicated here in red for positive potentials and blue for negative potentials. This potential field is picked up as an ECG. This figure is a courtesy of Dr. Mark Potse.[38]