Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
405
11
are immediately adjacent to each other. Continuous conduc-
tion is relatively slow.
Te presence of a myelin sheath dramatically increases the
rate of AP propagation. By acting as an insulator, myelin pre-
vents almost all charge from leaking from the axon and allows
the membrane voltage to change more rapidly. Current can pass
through the membrane of a myelinated axon
only
at the my-
elin sheath gaps, where there is no myelin sheath and the axon
is bare. Nearly all the voltage-gated Na
1
channels are concen-
trated in these gaps.
When an AP is generated in a myelinated fiber, the local
depolarizing current does not dissipate through the adjacent
membrane regions, which are nonexcitable. Instead, the current
is maintained and moves rapidly to the next myelin sheath gap,
a distance of approximately 1 mm, where it triggers another AP.
Consequently, APs are triggered only at the gaps, a type of con-
duction called
saltatory conduction
(
saltare
5
to leap) because
the electrical signal appears to jump from gap to gap along the
axon (Figure 11.15c). Saltatory conduction is about 30 times
faster than continuous conduction.
Homeostatic Imbalance
11.2
Te importance of myelin to nerve transmission is painfully
clear to people with demyelinating diseases such as
multiple
sclerosis (MS)
. Tis autoimmune disease affects mostly young
adults.
Multiple sclerosis gradually destroys myelin sheaths in the
CNS, reducing them to nonfunctional hardened lesions called
scleroses
. Te loss of myelin (a result of the immune system’s
attack on myelin proteins) shunts and short-circuits the cur-
rent so that successive gaps are excited more and more slowly,
and eventually impulse conduction ceases. However, the ax-
ons themselves are not damaged and growing numbers of Na
1
channels appear spontaneously in the demyelinated fibers. Tis
strong. Tis period, from the opening of the Na
1
channels until
the Na
1
channels begin to reset to their original resting state,
is called the
absolute refractory period
(Figure 11.14)
. It en-
sures that each AP is a separate,
all-or-none event
and enforces
one-way transmission of the AP.
Te
relative refractory period
is the interval following the
absolute refractory period. During the relative refractory pe-
riod, most Na
1
channels have returned to their resting state,
some K
1
channels are still open, and repolarization is occur-
ring. During this time, the axon’s threshold for AP generation is
substantially elevated. A stimulus that would normally generate
an AP is no longer sufficient, but an exceptionally strong stimu-
lus can reopen the Na
1
channels that have already returned to
their resting state and generate another AP. Strong stimuli trig-
ger more frequent APs by intruding into the relative refractory
period.
Conduction Velocity
How fast do APs travel? Conduction
velocities of neurons vary widely. Nerve fibers that transmit
impulses most rapidly (100 m/s or more) are found in neural
pathways where speed is essential, such as those that mediate
postural reflexes. Axons that conduct impulses more slowly
typically serve internal organs (the gut, glands, blood vessels),
where slower responses are not a handicap. Te rate of impulse
propagation depends largely on two factors:
Axon diameter.
As a rule, the larger the axon’s diameter, the
faster it conducts impulses. Larger axons conduct more rap-
idly because they offer less resistance to the flow of local cur-
rents, bringing adjacent areas of the membrane to threshold
more quickly.
Degree of myelination.
Action potentials propagate because
they are regenerated by voltage-gated channels in the mem-
brane
(Figure 11.15a, b)
. In
continuous conduction
, AP
propagation involving nonmyelinated axons, these channels
Threshold
Action
potentials
Stimulus
Time (ms)
Stimulus
voltage
Membrane potential (mV)
–70
0
+30
Figure 11.13
Relationship between stimulus strength and
action potential frequency.
APs are shown as vertical lines in
the upper trace. The lower trace shows the intensity of the applied
stimulus. A subthreshold stimulus does not generate an AP, but once
threshold voltage is reached, the stronger the stimulus, the more
frequently APs are generated.
Absolute refractory
period
Relative refractory
period
Membrane potential (mV)
Time (ms)
–70
0
+30
0
1
2
3
4
5
Depolarization
(Na
+
enters)
Repolarization
(K
+
leaves)
Hyperpolarization
Stimulus
Figure 11.14
Absolute and relative refractory periods in an AP.
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