Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
401
11
changing shape to reopen their inactivation gates and close
their activation gates.
Repolarization restores resting electrical conditions, but it
does
not
restore resting ionic conditions. Afer repolarization,
the
sodium-potassium pump
redistributes the ions. While it
might appear that tremendous numbers oF Na
1
and K
1
ions
change places during an action potential, this is not the case.
Only small amounts oF sodium and potassium cross the mem-
brane. (Te Na
1
influx required to reach threshold produces
only a 0.012% change in intracellular Na
1
concentration.) Tese
small ionic changes are quickly corrected because an axon
membrane has thousands oF Na
1
-K
1
pumps.
Threshold and the All-or-None Phenomenon
Not all local
depolarization events produce APs. Te depolarization must
reach threshold values iF an axon is to “fire.” What determines
the
threshold point
?
One explanation is that threshold is the membrane poten-
tial at which the outward current created by K
1
movement is
exactly equal to the inward current created by Na
1
movement.
Treshold is typically reached when the membrane has been
depolarized by 15 to 20 mV From the resting value. Tis depo-
larization status represents an unstable equilibrium state. IF one
more Na
1
enters, Further depolarization occurs, opening more
Na
1
channels and allowing more Na
1
to enter. IF, on the other
hand, one more K
1
leaves, the membrane potential is driven
away From threshold, Na
1
channels close, and K
1
continues to
diffuse outward until the potential returns to its resting value.
Recall that local depolarizations are graded potentials and
their magnitude increases when stimuli become more intense.
BrieF weak stimuli (
subthreshold stimuli
) produce subthreshold
depolarizations that are not translated into nerve impulses. On
the other hand, stronger
threshold stimuli
produce depolarizing
currents that push the membrane potential toward and beyond
the threshold voltage. As a result, Na
1
permeability rises to such
an extent that entering sodium ions “swamp” (exceed) the out-
ward movement oF K
1
, establishing the positive Feedback cycle
and generating an AP.
Te critical Factor here is the total amount oF current that
flows through the membrane during a stimulus (electrical charge
3
time). Strong stimuli depolarize the membrane to threshold
quickly. Weaker stimuli must be applied For longer periods to pro-
vide the crucial amount oF current flow. Very weak stimuli do not
trigger an AP because the local current flows they produce are so
slight that they dissipate long beFore threshold is reached.
An AP is an
all-or-none phenomenon
: It either happens com-
pletely or doesn’t happen at all. We can compare the generation
oF an AP to lighting a match under a small dry twig. Te changes
occurring where the twig is heated are analogous to the change in
membrane permeability that initially allows more Na
1
to enter
the cell. When that part oF the twig becomes hot enough (when
enough Na
1
enters the cell), it reaches the flash point (threshold)
and the flame consumes the entire twig, even iF you blow out the
match. Similarly, the AP is generated and propagated whether or
not the stimulus continues. But iF you blow out the match beFore
the twig reaches the threshold temperature, ignition will not take
Both gates must be open in order For Na
1
to enter, but
the closing oF
either
gate effectively closes the channel. By
contrast, each active potassium channel has a single volt-
age-sensitive gate that is closed in the resting state and
opens slowly in response to depolarization.
2
Depolarization: Na
1
channels open.
As local currents de-
polarize the axon membrane, the voltage-gated sodium
channels open and Na
1
rushes into the cell. Tis influx oF
positive charge depolarizes that local patch oF membrane
Further, opening more Na
1
channels so the cell interior be-
comes progressively less negative.
When depolarization at the stimulation site reaches a
certain critical level called
threshold
(ofen between
2
55
and
2
50 mV), depolarization becomes selF-generating,
urged on by positive Feedback. Tat is, afer being initi-
ated by the stimulus, depolarization is driven by the ionic
currents created by Na
1
influx. As more Na
1
enters, the
membrane depolarizes Further and opens still more chan-
nels until all Na
1
channels are open. At this point, Na
1
permeability is about 1000 times greater than in a resting
neuron. As a result, the membrane potential becomes less
and less negative and then overshoots to about
1
30 mV as
Na
1
rushes in along its electrochemical gradient. Tis rapid
depolarization and polarity reversal produces the sharp up-
ward
spike
oF the action potential (±igure 11.11).
Earlier, we stated that membrane potential depends on
membrane permeability, but here we say that membrane
permeability depends on membrane potential. Can both
statements be true? Yes, because these two relationships
establish a
positive feedback cycle
: Increasing Na
1
perme-
ability due to increased channel openings leads to greater
depolarization, which increases Na
1
permeability, and so
on. Tis explosive positive Feedback cycle is responsible For
the rising (depolarizing) phase oF an action potential—it
puts the “action” in the action potential.
3
Repolarization: Na
1
channels are inactivating, and K
1
channels open.
Te explosively rising phase oF the action
potential persists For only about 1 ms. It is selF-limiting be-
cause the slow inactivation gates oF the Na
1
channels begin
to close at this point. As a result, the membrane perme-
ability to Na
1
declines to resting levels, and the net influx
oF Na
1
stops completely. Consequently, the AP spike stops
rising.
As Na
1
entry declines, the slow voltage-gated K
1
chan-
nels open and K
1
rushes out oF the cell, Following its elec-
trochemical gradient. Tis restores the internal negativity
oF the resting neuron, an event called
repolarization
. Both
the abrupt decline in Na
1
permeability and the increased
permeability to K
1
contribute to repolarization.
4
Hyperpolarization: Some K
1
channels remain open, and
Na
1
channels reset.
Te period oF increased K
1
permeabil-
ity typically lasts longer than needed to restore the resting
state. As a result oF the excessive K
1
efflux beFore the potas-
sium channels close, a hyperpolarization is seen on the AP
curve as a slight dip Following the spike. Also at this point,
the Na
1
channels begin to reset to their original position by
(Text continues on p. 404.)
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