156
Chapter 6
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Resting membrane
Resting membrane
depolarized toward
threshold by
local current
Initial site
of action
potential
3
2
1
Resting membrane
depolarized toward
threshold by local
current
Present site of
action potential
Membrane is
refractory; local
current cannot
stimulate a
second action
potential
Membrane is
refractory; local
current cannot
stimulate a
second action
potential
3
2
1
+
Present site of
action potential
Resting membrane
3
2
1
+
0
1
2
3
4
Time (ms)
im
e (
70 mV
0
m
0
1
2
3
4
70 mV
0
1
2
3
4
–70 mV
0
1
234
Time (ms)
im
e (
–70 mV
0
m
0
1
2
3
4
70 mV
0
1
2
3
4
70 mV
0
1
2
3
4
Time (ms)
im
e (
70 mV
0
m
0
1
2
3
4
70 mV
0
1
2
3
4
–70 mV
(a)
(b)
(c)
Figure 6–22
One-way propagation of an action potential.
For simplicity, potentials are shown only
on the upper membrane, local currents are
shown only on the inside of the membrane,
and repolarizing currents are not shown. (a)
Action potential is initiated in region 1, and
local currents depolarize region 2. (b) Action
potential in region 2 generates local currents;
region 3 is depolarized toward threshold, but
region 1 is refractory. (c) Action potential in
region 3 generates local currents, but region 2 is
refractory.
Figure 6–22
physiological
inquiry
Striking the ulnar nerve in your elbow against
a hard surface (sometimes called “hitting
your funny bone”) initiates action potentials
in the middle of sensory and motor axons
traveling in that nerve. In which direction
will those action potentials propagate?
Answer can be found at end of chapter.
potential. How is the threshold potential attained, and how do
various types of neurons actually generate action potentials?
In afferent neurons, the initial depolarization to thresh-
old is achieved by a graded potential—here called a
receptor
potential,
which is generated in the sensory receptors at the
peripheral ends of the neurons. These are the ends farthest
from the central nervous system where the nervous system
receives information. In all other neurons, the depolarization
to threshold is due either to a graded potential generated by
synaptic input to the neuron, known as a
synaptic potential,
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