366
UNIT THREE
Resting Potential
A resting nerve cell is not being stimulated to send a nerve
impulse. Under resting conditions, nongated (always open)
channels determine the membrane permeability to sodium
and potassium ions.
Sodium and potassium ions follow the laws of diffusion
described in chapter 3 (pp. 90 and 92) and show a net move-
ment from areas of high concentration to areas of low concen-
tration across a membrane as their permeabilities permit. The
resting cell membrane is only slightly permeable to these ions,
but the membrane is more permeable to potassium ions than
to sodium ions
(f g. 10.14
a
)
. Also, the cytoplasm of these cells
has many negatively charged ions (anions) which include phos-
phate (PO
4
–2
), sulfate (SO
4
–2
), and proteins, that are synthesized
inside the cell and cannot diffuse through cell membranes.
If we consider a hypothetical neuron, before a membrane
potential has been established, we would expect potassium
to diffuse out of the cell more rapidly than sodium could dif-
fuse in. This means that every millisecond (as the membrane
potential is being established in our hypothetical cell), a few
more positive ions leave the cell than enter it (F
g. 10.14
a
).
As a result, the outside of the membrane gains a slight sur-
plus of positive charges, and the inside refl ects a surplus of
the impermeant negatively charged ions. This creates a sep-
aration of positive and negative electrical charges between
the inside and outside surfaces of the cell membrane (F g.
10.14
b
). All this time, the cell continues to expend metabolic
energy in the form of ATP to actively transport sodium and
potassium ions in opposite directions, thus maintaining the
concentration gradients for those ions responsible for their
diffusion in the F
rst place.
The difference in electrical charge between two points is
measured in units called volts. It is called a potential differ-
ence because it represents stored electrical energy that can be
used to do work at some future time. The potential difference
across the cell membrane is called the
membrane potential
(transmembrane potential) and is measured in millivolts.
In the case of a resting neuron, one that is not send-
ing impulses or responding to other neurons, the membrane
potential is termed the
resting potential
(resting membrane
potential) and has a value of –70 millivolts (F g. 10.14
b
). The
negative sign is relative to the inside of the cell and is due to the
excess negative charges on the inside of the cell membrane. To
understand how the resting potential provides the energy for
sending a nerve impulse down the axon, we must F rst under-
stand how neurons respond to signals called stimuli.
With the resting membrane potential established, a few
sodium ions and potassium ions continue to diffuse across
the cell membrane. The negative membrane potential helps
sodium ions enter the cell despite sodium’s low permea-
bility, but it hinders potassium ions from leaving the cell
despite potassium’s higher permeability. The net effect is
that three sodium ions “leak” into the cell for every two
potassium ions that “leak” out. The Na
+
/K
+
pump exactly
balances these leaks by pumping three sodium ions out for
every two potassium ions it pumps in (F
g. 10.14
c
).
Some channels are always open, whereas others may be
either open or closed, somewhat like a gate. Both chemical
and electrical factors can affect the opening and closing of
these
gated channels
(f
g. 10.13)
.
Direction of
nerve impulse
Synaptic
vesicles
Synaptic
vesicle
Mitochondrion
Synaptic knob
(a)
Synaptic cleft
+ + + +
+
+
+
+
+
+
+
+
+
+
+
––
––
Neurotransmitter
Axon
Ca
+
2
Ca
+
2
Ca
+
2
Cell body or dendrite
of postsynaptic neuron
Presynaptic neuron
Depolarized
membrane
Polarized
membrane
Axon
membrane
Vesicle releasing
neurotransmitter
Synaptic
vesicle
Synaptic
cleft
Postsynaptic
membrane
(b)
Mitochondrion
FIGURE 10.12
The synapse. (
a
) When a nerve impulse
reaches the synaptic knob at the end of an axon, synaptic
vesicles release a neurotransmitter that diF
uses across the
synaptic cleft. In this case the neurotransmitter is excitatory.
(
b
) A transmission electron micrograph of a synaptic knob ±
lled
with synaptic vesicles (37,500×).
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