ABSTRACT
Excitability of a myocyte is dictated by the make-up of its active and passive membrane properties. The passive electrical behavior of a cell membrane is similar to a fixed resistor and capacitor in parallel, and determines the behavior of flow of the subthreshold currents of the excitatory impulse. The membrane length constant is the distance at which potential has decayed to 1/e (37%) of the value at the source of the current application. Because atrial cells are smaller in diameter than ventricular cells,1 ri (internal resistance of the fiber per unit length) is higher in atrial cells assuming that the cytosol of atrial and ventricular myocytes has a similar conductivity. This effect makes the membrane length constant shorter in atrial versus ventricular cells.1
