288
UNIT 2
Covering, Support, and Movement of the Body
9
membrane voltage, referred to as
threshold,
is reached, an
action potential is generated (initiated).
Te action potential
propagates
(moves along the
length of the sarcolemma) in all directions from the neu-
romuscular junction, just as ripples move away from a
pebble dropped into a stream. As it propagates, the local
depolarization wave of the AP spreads to adjacent areas of
the sarcolemma and opens voltage-gated sodium channels
there. Again, Na
1
, normally restricted from entering, dif-
fuses into the cell following its electrochemical gradient.
3
Repolarization: Restoring the sarcolemma to its initial
polarized state.
Te repolarization wave, like the de-
polarization wave, is a consequence of changes in mem-
brane permeability. In this case, Na
1
channels close and
voltage-gated K
1
channels open. Since the potassium ion
concentration is substantially higher inside the cell than in
the extracellular fluid, K
1
diffuses rapidly out of the muscle
the entire surface of the sarcolemma. Essentially three steps are
involved
(Figure 9.9)
:
1
Generation of an end plate potential.
Binding of ACh
molecules to ACh receptors at the neuromuscular junction
opens
chemically
(
ligand
)
gated ion channels
that allow Na
1
and K
1
to pass (also see Figure 9.8). Because the driving
force for Na
1
is greater than that for K
1
, more Na
1
diffuses
in than K
1
diffuses out. A transient change in membrane
potential occurs as the interior of the sarcolemma becomes
less negative, an event called depolarization. Initially, depo-
larization is a local event called an
end plate potential
.
2
Depolarization: Generation and propagation of an action
potential.
Te end plate potential (local depolarization at
the neuromuscular junction) ignites an action potential
by spreading to adjacent membrane areas and opening
voltage-gated sodium channels there. Na
1
enters, fol-
lowing its electrochemical gradient, and once a certain
Ca
2+
Ca
2+
ACh-containing
synaptic vesicle
Axon terminal of
neuromuscular
junction
Synaptic
cleft
Wave of
depolarization
Na
+
Closed Na
+
channel
Open K
+
channel
K
+
Na
+
Open Na
+
channel
Closed K
+
channel
Action potential
K
+
1
2
3
An end plate potential is generated at the neuromuscular
junction (see Figure 9.8).
Depolarization: Generating and propagating an action potential
(AP).
The local depolarization current spreads to adjacent areas of the
sarcolemma. This opens voltage-gated sodium channels there, so Na
+
enters following its electrochemical gradient and initiates the AP. The
AP is propagated as its local depolarization wave spreads to adjacent
areas of the sarcolemma, opening voltage-gated channels there.
Again Na
+
diffuses into the cell following its electrochemical gradient.
Repolarization: Restoring the sarcolemma to its initial polarized
state (negative inside, positive outside).
Repolarization occurs as
Na
+
channels close (inactivate) and voltage-gated K
+
channels open.
Because K
+
concentration is substantially higher inside the cell than
in the extracellular fluid, K
+
diffuses rapidly out of the muscle fiber.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Figure 9.9
Summary of events in the generation and propa-
gation of an action potential in a skeletal muscle fiber.
previous page 322 Human Anatomy and Physiology (9th ed ) 2012 read online next page 324 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off