Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
397
11
more easily than sodium ions can enter the cell along theirs. K
1
flowing out of the cell causes the cell to become more negative
inside. Na
1
trickling into the cell makes the cell just slightly
more positive than it would be if only K
1
flowed. Terefore,
at resting membrane potential, the negative interior of the cell
is due to a much greater ability for K
1
to diffuse out of the cell
than for Na
1
to diffuse into the cell.
Because some K
1
is always leaking out of the cell and some
Na
1
is always leaking in, you might think that the concentration
gradients would eventually “run down,” resulting in equal concen-
trations of Na
1
and K
1
inside and outside the cell. Tis does not
happen because the A±P-driven sodium-potassium pump first
ejects three Na
1
from the cell and then transports two K
1
back
into the cell. In other words, the
sodium-potassium pump (Na
1
-
K
1
ATPase)
stabilizes the resting membrane potential by main-
taining the concentration gradients for sodium and potassium.
Check Your Understanding
9.
For an open channel, what factors determine in which
direction ions will move through that channel?
10.
For which cation is there the greatest amount of leakage
(through leakage channels) across the plasma membrane?
For answers, see Appendix H.
Membrane Potentials That Act as Signals
Compare and contrast graded potentials and action
potentials.
Explain how action potentials are generated and
propagated along neurons.
Define absolute and relative refractory periods.
Define saltatory conduction and contrast it to continuous
conduction.
electrical currents and voltage changes across the membrane
according to the rearranged Ohm’s law equation:
Voltage (
V
)
5
current (
I
)
3
resistance (
R
)
Ions move along chemical
concentration gradients
when they
diffuse passively from an area of their higher concentration
to an area of lower concentration. Tey move along
electrical
gradients
when they move toward an area of opposite electrical
charge. ±ogether, electrical and concentration gradients consti-
tute the
electrochemical gradient
. Ions flowing along electro-
chemical gradients underlie all electrical events in neurons.
The Resting Membrane Potential
Define resting membrane potential and describe its
electrochemical basis.
A voltmeter is used to measure the potential difference between
two points. When one microelectrode of the voltmeter is in-
serted into a neuron and the other is in the extracellular fluid, it
records a voltage across the membrane of approximately
2
70 mV
(Figure 11.7)
. Te minus sign indicates that the cytoplasmic
side (inside) of the membrane is negatively charged relative to
the outside. Tis potential difference in a resting neuron (
V
r
)
is called the
resting membrane potential
, and the membrane
is said to be
polarized
. Te value of the resting membrane po-
tential varies (from
2
40 mV to
2
90 mV) in different types of
neurons.
Te resting potential exists only across the membrane. In
other words, the bulk solutions inside and outside the cell are
electrically neutral. ±wo factors generate the resting membrane
potential: differences in the ionic composition of the intracel-
lular and extracellular fluids, and differences in the permeability
of the plasma membrane to those ions.
Differences in Ionic Composition
First, let’s compare the ionic makeup of the intracellular and
extracellular fluids, as shown in
Focus on Resting Membrane
Potential
(Figure 11.8)
. Te cell cytosol contains a lower con-
centration of Na
1
and a higher concentration of K
1
than the
extracellular fluid. Negatively charged (anionic) proteins (not
shown) help to balance the positive charges of intracellular
cations (primarily K
1
). In the extracellular fluid, the positive
charges of Na
1
and other cations are balanced chiefly by chlo-
ride ions (Cl
2
). Although there are many other solutes (glucose,
urea, and other ions) in both fluids, potassium (K
1
) plays the
most important role in generating the membrane potential.
Differences in Plasma Membrane Permeability
Next, let’s consider the differential permeability of the mem-
brane to various ions (Figure 11.8, bottom). At rest the mem-
brane is impermeable to the large anionic cytoplasmic proteins,
very slightly permeable to sodium, approximately 25 times more
permeable to potassium than to sodium, and quite permeable to
chloride ions. Tese resting permeabilities reflect the properties
of the leakage ion channels in the membrane. Potassium ions
diffuse out of the cell along their
concentration gradient
much
Voltmeter
Microelectrode
inside cell
Plasma
membrane
Ground electrode
outside cell
Neuron
Axon
+
+
+
+
+
+
+
+
+
+
Figure 11.7
Measuring membrane potential in neurons.
The
potential difference between an electrode inside a neuron and the
ground electrode in the extracellular fluid is approximately
2
70 mV
(inside negative).
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