404
UNIT 3
Regulation and Integration of the Body
11
Te propagation process we have just described occurs on
nonmyelinated axons. On p. 405, we will describe propagation
along myelinated axons.
Although the phrase
conduction of a nerve impulse
is com-
monly used, nerve impulses are not really conducted in the
same way that an insulated wire conducts current. In fact,
neurons are fairly poor conductors, and as noted earlier, local
current flows decline with distance because the charges leak
through the membrane. Te expression
propagation of a nerve
impulse
is more accurate, because the AP is
regenerated anew
at
each membrane patch, and every subsequent AP is identical to
the one that was generated initially.
Coding for Stimulus Intensity
Once generated, all APs are in-
dependent of stimulus strength, and all APs are alike. So how
can the CNS determine whether a particular stimulus is in-
tense or weak—information it needs to initiate an appropriate
response?
Te answer is really quite simple: Strong stimuli generate
nerve impulses more
oFen
in a given time interval than do weak
stimuli. Stimulus intensity is coded for by the number of impulses
per second—that is, by the
frequency of action potentials
—rather
than by increases in the strength (amplitude) of the individual
APs
(Figure 11.13)
.
Refractory Periods
When a patch of neuron membrane is gen-
erating an AP and its voltage-gated sodium channels are open,
the neuron cannot respond to another stimulus, no matter how
place. Likewise, if the number of Na
1
ions entering the cell is too
low to achieve threshold, no AP will occur.
Propagation of an Action Potential
If it is to serve as the
neuron’s signaling device, an AP must be
propagated
along the
axon’s entire length. As we have seen, the AP is generated by
the influx of Na
1
through a given area of the membrane. Tis
influx establishes local currents that depolarize adjacent mem-
brane areas in the forward direction (away from the origin of
the nerve impulse), which opens voltage-gated channels and
triggers an action potential there
(Figure 11.12)
.
Because the area where the AP originated has just generated
an AP, the sodium channels in that area are inactivated and no
new AP is generated there. For this reason, the AP propagates
away from its point of origin. (If an
isolated
axon is stimulated
by an electrode, the nerve impulse will move away from the
point of stimulus in all directions along the membrane.) In the
body, APs are initiated at one end of the axon and conducted
away from that point toward the axon’s terminals. Once initi-
ated, an AP is
self-propagating
and continues along the axon at a
constant velocity—something like a domino effect.
Following depolarization, each segment of axon membrane
repolarizes, which restores the resting membrane potential in
that region. Because these electrical changes also set up local
currents, the repolarization wave chases the depolarization
wave down the length of the axon.
Voltage
at 2 ms
Voltage
at 4 ms
–70
+30
Membrane potential (mV)
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Voltage
at 0 ms
Recording
electrode
+
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(a) Time = 0 ms.
Action potential has
not yet reached the recording
electrode.
(b) Time = 2 ms.
Action potential
peak reaches the recording
electrode.
(c) Time = 4 ms.
Action potential
peak has passed the recording
electrode. Membrane at the
recording electrode is still
hyperpolarized.
Resting potential
Peak of action potential
Hyperpolarization
Figure 11.12
Propagation of an action potential (AP).
Recordings at three successive
times as an AP propagates along an axon (from left to right). The arrows show the direction of
local current flow generated by the movement of positive ions. This current brings the resting
membrane at the leading edge of the AP to threshold, propagating the AP forward.
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