Muscle
265
Figure 9–15
Events at the neuromuscular junction that lead to an action potential in the muscle fi ber plasma membrane. Although potassium also exits the
muscle cell when ACh receptors are open, sodium entry and depolarization dominates, as shown here.
(a)
(b)
Myelin
Motor nerve fiber
Axon terminal
Schwann cell
Synaptic vesicles
(containing ACh)
Active zone
Sarcolemma
Region of
sarcolemma
with ACh receptors
Junctional
folds
Nucleus of muscle fiber
Synaptic
cleft
Figure 9–14
The neuromuscular junction. (a) Scanning electron micrograph showing branching of motor axons, with terminals embedded in grooves in the
muscle fi ber’s surface. (b) Structure of a neuromuscular junction.
3
9
2
5
4
6
7
8
1
+
+
Acetylcholine
release
Motor neuron
action potential
Muscle fiber
action potential
initiation
Local current between
depolarized end plate and
adjacent muscle plasma
membrane
Acetylcholine receptor
Acetylcholine
degradation
Acetylcholinesterase
Motor end plate
Acetylcholine
vesicle
Voltage-gated
Na
+
channels
++
+
–––
+
+
+
+
+
––
+
+
+
+
+
+
+
+
+
+
+
Na
+
entry
Acetylcholine binding
opens ion channels
Ca
2+
enters
voltage-gated
channels
Propagated action
potential in muscle
plasma membrane
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