370
UNIT THREE
the surface of the F ber and down into its transverse tubules.
See
table 10.3
for a summary of the events leading to the
conduction of a nerve impulse.
All-or-None Response
Nerve impulse conduction is an all-or-none response. In
other words, if a neuron responds at all, it responds com-
pletely. Thus, a nerve impulse is conducted whenever a
stimulus of threshold intensity or above is applied to an axon
and all impulses carried on that axon are the same strength.
A greater intensity of stimulation produces more impulses
per second, not a stronger impulse.
Refractory Period
±or a short time following passage of a nerve impulse, a thresh-
old stimulus will not trigger another impulse on an axon. This
brief period, called the
refractory period,
has two parts. During
the
absolute refractory period,
which lasts about 1/2,500 of a
second, the axon’s membrane is changing in sodium perme-
ability and cannot be stimulated. This is followed by a
relative
refractory period,
when the membrane reestablishes its rest-
ing potential. While the membrane is in the relative refractory
period, even though repolarization is incomplete, a threshold
stimulus of high intensity may trigger an impulse.
As time passes, the intensity of stimulation required to
trigger an impulse decreases until the axon’s original excit-
ability is restored. This return to the resting state usually
takes from 10 to 30 milliseconds.
The refractory period limits how many action potentials
may be generated in a neuron in a given period. Remembering
that the action potential takes about a millisecond, and add-
ing the time of the absolute refractory period, the maximum
theoretical frequency of impulses in a neuron is about 700 per
second. In the body, this limit is rarely achieved—frequencies
of about 100 impulses per second are common.
Milliseconds
1
0
80
60
40
20
0
+
20
+
40
2
345678
Membrane potential (millivolts)
Action potential
Resting potential
reestablished
Hyperpolarization
Resting
potential
(a)
Region of
action potential
Direction of nerve impulse
++
++
+
–––––––––
–––
––
––––
–––
––
––––
–––––––––
–––––––
––
–––––––
––
++++++++
+++++++++
(b)
++
++
+
++
++++++
+
++
++++++
(c)
++
++
+
++
++
++
++
+
++
++
++
++
FIGURE 10.17
An oscilloscope records an action potential.
FIGURE 10.18
Nerve impulse. (
a
) An action potential in one
region stimulates the adjacent region, and (
b
and
c
) a wave of action
potentials (a nerve impulse) moves along the axon.
TABLE
10.3
|
Events Leading to Nerve
Impulse Conduction
1. Nerve cell membrane maintains resting potential by diF
usion of
Na
+
and K
+
down their concentration gradients as the cell pumps
them up the gradients.
2. Neurons receive stimulation, causing local potentials, which may
sum to reach threshold.
3. Sodium channels in the trigger zone of the axon open.
4. Sodium ions diF
use inward, depolarizing the membrane.
5. Potassium channels in the membrane open.
6. Potassium ions diF
use outward, repolarizing the membrane.
7. The resulting action potential causes an electric current that
stimulates adjacent portions of the membrane.
8. Action potentials occur sequentially along the length of the axon as
a nerve impulse.
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