Neuronal Signaling and the Structure of the Nervous System
145
of positive and negative charges that have to be separated
across a membrane to account for the potential is actually an
infi nitesimal fraction of the total number of charges in the
two compartments.
Table 6–2
lists the concentrations of sodium, potas-
sium, and chloride ions in the extracellular fl uid and in the
intracellular fl uid of a representative nerve cell. Each of these
ions has a 10- to 30-fold difference in concentration between
the inside and the outside of the cell. Although this table
appears to contradict our earlier assertion that the bulk of the
intra- and extracellular fl
uids are electrically neutral, there are
many other ions, including Mg
2+
, Ca
2+
, H
+
, HCO
3
, HPO
4
2–
,
SO
4
2–
, amino acids, and proteins, in both fl
uid compartments.
Of the diffusable ions, sodium, potassium, and chloride ions
are present in the highest concentrations, and the membrane
permeability to each is independently determined. Sodium
and potassium generally play the most important roles in
generating the resting membrane potential, but in some cells
chloride is also a factor. Note that the sodium and chloride
concentrations are lower inside the cell than outside, and that
the potassium concentration is greater inside the cell. The
concentration differences for sodium and potassium are estab-
lished by the action of the sodium-potassium pump (Na
+
/K
+
-
ATPase, Chapter 4) that pumps sodium out of the cell and
potassium into it. The reason for the chloride distribution var-
ies among cell types, as will be described later.
The magnitude of the resting membrane potential
depends mainly on two factors: (1) differences in specifi c ion
concentrations in the intracellular and extracellular fl
uids, and
(2) differences in membrane permeabilities to the different
ions, which refl ect the number of open channels for the differ-
ent ions in the plasma membrane.
To understand how concentration differences for sodium
and potassium create membrane potentials, fi rst consider what
happens when the membrane is permeable (has open channels)
Table 6–2
Distribution of Major Mobile Ions
Across the Plasma Membrane of a
Typical Nerve Cell
Concentration, mmol/L
Ion
Extracellular
Intracellular
Na
+
145
15
Cl
100
7*
K
+
5
1
5
0
A more accurate measure of electrical driving force can be obtained using mEq/L, which factors
in ion valence. Since all the ions in this table have a valence of 1, the mEq/L is the same as the
mmol/L concentration.
*Intracellular chloride concentration varies signifi cantly between neurons due to differences in
expression of membrane transporters and channels.
Figure 6–8
(a) Apparatus for measuring membrane potentials. The voltmeter
records the difference between the intracellular and extracellular
electrodes. (b) The potential difference across a plasma membrane as
measured by an intracellular microelectrode. The asterisk indicates
the moment the electrode entered the cell.
+
Intracellular
(recording)
microelectrode
Extracellular
(reference)
electrode
Voltmeter
+
+
+
+
+
+
+
+
+
+
+
+
+
+
R
ecorded potential
(
mV
)
Extracellular fluid
Time
(a)
(b)
0
70
l
l
el
e
Ce
Ce
Ce
Ce
+
– –
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Cell
Extracellular
fluid
Figure 6–9
The excess positive charges outside the cell and the excess negative
charges inside collect in a tight shell against the plasma membrane.
In reality, these excess charges are only an extremely small fraction
of the total number of ions inside and outside the cell.
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