The primary function of cardiac myocytes
is to contract. Electrical changes within the
myocytes initiate this contraction. This chap-
ter examines (1) the electrical activity of indi-
vidual myocytes, including resting membrane
potentials and action potentials; (2) the way
action potentials are conducted throughout
the heart to initiate coordinated contraction
of the entire heart; and (3) the way electri-
cal activity of the heart is measured using the
electrocardiogram (ECG).
Resting Membrane Potentials
Cardiac cells, like all living cells in the body,
have an electrical potential across the cell mem-
brane. This potential can be measured by insert-
ing a microelectrode into the cell and measuring
the electrical potential in millivolts (mV) inside
the cell relative to the outside of the cell. By
convention, the outside of the cell is considered
0 mV If measurements are taken with a rest-
ing ventricular myocyte, a membrane potential
of about -90 mV will be recorded. This rest-
ing membrane potential (Em) is determined by
the concentrations of positively and negatively
charged ions across the cell membrane, the rela-
tive permeability of the cell membrane to these
ions, and the ionic pumps that transport ions
across the cell membrane.
Of the many different ions present inside and
outside of cells, the concentrations of Na+,
K+, and Ca++ are most important in determin-
ing the membrane potential across the cell
membrane. Although chloride ions are found
inside and outside the cell, they contrib-
ute relatively little to the resting membrane
concentrations of Na+, K+, and Ca++ inside
and outside the cell. Of the three ions, K+ is
the most important in determining the rest-
ing membrane potential. In a cardiac cell, the
concentration of K+ is high inside and low out-
side the cell. Therefore, a chemical gradient
K +
(4 mM)
(145 mM)
(2.5 mM)
■ FIGURE 2.1 Concentrations of K+, Na+, and Ca++
inside and outside a cardiac m yocyte at a resting
membrane potential of -9 0 mV.
P r,
charged proteins.
(concentration difference) exists for K+ to dif-
fuse out of the cell. The opposite situation is
found for Na+ and Ca++; their chemical gradi-
ents favor an inward diffusion. The concen-
tration differences across the cell membrane
for these and other ions are determined by
the activity of energy-dependent ionic pumps
and the presence of impermeable, negatively
charged proteins within the cell that affect the
passive distribution of cations and anions.
understand how concentration gra-
dients of ions across a cell membrane affect
membrane potential, consider a cell in which
K+ is the only ion across the membrane other
charged proteins on the inside of the cell. In
this cell, K+ diffuses down its chemical gra-
dient and out of the cell because its concen-
tration is much higher inside than outside
the cell (see Fig. 2.1). As K+ diffuses out of
the cell, it leaves behind negatively charged
proteins, thereby creating a separation of
charge and a potential difference across the
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