Biomedical Engineering Reference
In-Depth Information
wave of depolarization [Figure 3.8(a)]. The equilibrium values turn out to be a
slight excess of positive ions inside the cell, giving a Nernst potential inside the cell
of about
10 mV. This potential difference at the left side of the cell causes the ad-
jacent part of the cell membrane to similarly break down, which in turn causes its
adjacent part to break down. Thus, a wave of depolarization sweeps across the cell
from left to right. Now the individual dipole moments across the cell membrane
do not cancel each other out, and there is a net dipole moment of the cell pointing
in the direction of the wave of depolarization. As a wave of depolarization spreads
through the cell, it is electrically represented by a time varying electric dipole mo-
ment that goes to zero after the resting membrane potential is restored in the proc-
ess of repolarization.
In the depolarized state, there are individual dipole moments across the mem-
brane, this time pointing from the outside to the inside, unlike the muscle cell at
rest. As all the individual cells are aligned in the same direction in the cardiac
tissue, when the wave of depolarization reaches the right side of the cell, that po-
tential causes the adjacent cell to go through the wave of depolarization. Thus, a
+
Figure 3.8
Electrical activity in the heart and ECG: (a) muscle cell at rest and changes in charge dis-
tribution during membrane depolarization; (b) depolarization and repolarization of the heart during
a cardiac cycle (arrows indicate the direction of the resultant dipole); (c) electrical potentials during a
cardiac cycle; and (d) dipole moment and distribution of electrical fi eld during initial depolarization.
