18
U N I T 1
Cell and Tissue Function
U N D E R S T A N D I N G
Membrane Potentials
Electrochemical potentials are present across the membranes of virtually all
cells in the body. Some cells, such as nerve and muscle cells, are capable
of generating rapidly changing electrical impulses and transmitting these
impulses along their membranes. Generation of membrane potentials relies
on (1) diffusion of current-carrying ions, (2) development of an electrochemical
equilibrium, and (3) establishment of a resting membrane potential and
triggering an action potential.
0
Outside cell
Inside cell
Volts
K + –permeable
membrane
Concentration gradient for K +
K
+
K
+
K
+
K
+
K
+
Diffusion potentials.
A diffusion
potential is a potential difference
generated across a membrane when
a current-carrying ion, such as the
potassium (K
+
) ion, diffuses down
its concentration gradient. Two
conditions are necessary for this to
occur: (1) the membrane must be
selectively permeable to a particular
ion, and (2) the concentration of the
diffusible ion must be greater on one
side of the membrane than the other.
The magnitude of the diffusion
potential, measured in millivolts
(mV), depends on the size of the con-
centration
gradient. The sign (+ or −)
or polarity of the potential depends
on the diffusing ion. It is negative on
the inside when a positively charged
ion such as K
+
diffuses from the
inside to the outside of the mem-
brane, carrying its charge with it.
1
Equilibrium potentials.
An equi-
librium potential is the membrane
potential that exactly balances and
opposes the net diffusion of an ion
down its concentration gradient. As a
cation diffuses down its concentration
gradient, it carries its positive charge
across the membrane, thereby gener-
ating an electrical force that will even-
tually retard and stop its diffusion.
An electrochemical equilibrium is one
in which the
chemical forces
driving
diffusion and the
repelling electrical
forces
are exactly balanced so that no
further diffusion occurs. The equilib-
rium potential (EMF, electromotive
force) can be calculated by inserting
the inside and outside ion concentra-
tions into the Nernst Equation.
2
Equilibrium
potential
Diffusion (chemical)
gradient
Nernst equation
EMF (mV) = –61 × log
10
(ion concentration inside/
ion concentration outside)
Electrical (ionic)
potential
