Human Physiology, 14th edition (2016)

152 Chapter 6

and closing of Na^1 and K^1 channels that are involved, but

gated channels for Ca^2 1 and Cl^2 are also very important in

physiology.

The resting membrane potential of most cells in the body

ranges from 2 65 mV to 2 85 mV (in neurons it averages 2 70 mV).

This value is close to the E (^) K because the resting plasma membrane
is more permeable to K^1 than to other ions. During nerve and
muscle impulses, however, the permeability properties change, as
gradient thus promotes the movement of Na^1 into the cell,
and, in order to oppose this diffusion, the membrane poten-
tial would have to have a positive polarity on the inside of
the cell. This is indeed what the Nernst equation would
provide. Thus, using an intracellular Na^1 concentration of
12 mEq/L,
ENa 5 61 mV log
145 mEq/L

12 mEq/L
5 1 66 mV
This means that a membrane potential of 66 mV, with
the inside of the cell positive, would be required to prevent
the diffusion of Na^1 into the cell. This is why the equilibrium
potential for Na^1  ( E (^) Na ) was given as 1 66 mV in the earlier
discussion of equilibrium potentials.

Resting Membrane Potential

The membrane potential of a real cell that is not producing
impulses is known as the resting membrane potential. If the
plasma membrane were only permeable to Na^1 , its resting

membrane potential would equal the E (^) Na of 1 66 mV; if it were
only permeable to K^1 , its resting membrane potential would
equal the E (^) K of 2 90 mV. A real resting cell is more permeable
to K^1 than to Na^1 , but it is not completely impermeable to
Na^1. As a result, its resting membrane potential is close to the
E (^) K but somewhat less negative due to the slight inward diffu-
sion of Na^1. Since the resting membrane potential is less nega-
tive than the E (^) K , there will also be a slight outward diffusion
of K^1. These leakages are countered by the constant activity of
the Na^1 /K^1 pumps.
The actual value of the resting membrane potential depends
on two factors:

1. The ratio of the concentrations ( X (^) o / X (^) i ) of each ion on the
   two sides of the plasma membrane.


2. The specific permeability of the membrane to each differ-
   ent ion.
   Many ions—including K^1 , Na^1 , Ca^2 1 , and Cl^2 —contribute
   to the resting membrane potential. Their individual contributions
   are determined by the differences in their concentrations across
   the membrane ( fig. 6.27 ), and by their membrane permeabilities.
   This has two important implications:


3. For any given ion, a change in its concentration in the
   extracellular fluid will change the resting membrane
   potential—but only to the extent that the membrane is
   permeable to that ion. Because the resting membrane is
   most permeable to K 1 , a change in the extracellular con-
   centration of K^1 has the greatest effect on the resting
   membrane potential.


4. A change in the membrane permeability to any given ion
   will change the membrane potential. This fact is central
   to the production of nerve and muscle impulses, as will
   be described in chapter 7. Most often, it is the opening
   Figure 6.27 The resting membrane potential.
   Because some Na^1 leaks into the cell by diffusion, the actual
   resting membrane potential is not as negative as the K^1
   equilibrium potential. As a result, some K^1 diffuses out of the cell,
   as indicated by the dashed lines.

+

K+

Na+

K

+

Na

+

Fixed anions

–70 mV

Voltmeter

CLINICAL APPLICATION

The resting membrane potential, which depends upon the

extracellular K^1 concentration, affects the electrical activity

of the heart. Thus, the plasma K^1 concentration is main-

tained within a very narrow range (3.5 to 5.0 mEq/L), primar-

ily by the kidneys. An abnormal increase in plasma K^1 , called

hyperkalemia, causes the intracellular concentration of K^1

to increase and the [K o 1 ]/[K i 1 ] ratio of the Nernst equation

to decrease. This reduces the membrane potential (brings

it closer to zero), interfering with the normal functioning of

the heart and causing electrocardiogram (ECG) abnormali-

ties. The heartbeat is stopped at plasma K^1 concentrations

above 8 mEq/L. Indeed, lethal injections for executions con-

tain KCl to accomplish this. These dangers require careful

monitoring of the blood electrolyte concentrations in patients

with heart or kidney disease. Hypokalemia (abnormally low

plasma K^1 ), which can result from taking diuretics (drugs

that promote urine production), may also cause ECG

abnormalities.