p r o c e ss
diagram
Im pulse transm ission a t th e neuro m u scular junction • Figure 6.5
Axon of
motor neuron
Sarcolemma
Motor end
plate
Axon terminal
Synaptic
end bulb
Neuromuscular
junction (NMJ)
Axon terminal
Nerve impulse
Synaptic vesicle
containing
acetylcholine
(ACh)
Synaptic
end bulb
Synaptic cleft
(space)
Sarcolemma
Myofibril
Neuromuscular junction
Enlarged view of the
neuromuscular junction
ACh is released -----
from synaptic vesicle
Synaptic cleft
(space)
Q ACh binds to ACh
receptor
Junctional fold
Na
°
»
Z
v f ^
< *
*
t o »
^
.w
®
Vf
» J jU V .
.
-------- Synaptic end bulb
,j|j
ACh is broken down
Motor end plate
Muscle action
potential is produced
Binding of acetylcholine to ACh receptors in the motor end plate
Nerve Signals initiate the
Contraction of Skeletal Muscle
As mentioned earlier, skeletal muscle contraction can be
voluntary. If you want to pick up a pencil, your brain initi-
ates that voluntary muscle contraction using a nerve im-
pulse. So how is the nerve impulse
transmitted from the nerve cell to
the muscle cell? At a specialized
meeting place between the two
cells called a
neuromuscular junc-
tion
, the neuron sends a chemical
message (acethylcholine [ACh])
that starts an electrical impulse
in the muscle cell (
Figure 6.5
).
This impulse (muscle action potential) reverses the normal
electrical state of the membrane (
depolarization
) and ini-
tiates contraction along the length of the muscle fiber.
The muscle action potential passes along the cell
membrane and down into the T-tubules (
Figure 6.6
).
The action potential causes calcium ions to be released
through calcium channels from their storage site inside
the sarcoplasmic reticulum. The rise in calcium ion level
in the sarcoplasm (cytoplasm) initiates the contraction
cycle. The muscle continues to contract until enough cal-
cium ions get pumped back inside the sarcoplasmic reticu-
lum. Once the calcium levels in the cytoplasm return to
normal, the muscle relaxes. This process of returning to
the resting state is referred to as
repolarization
.
neuromuscular
junction
(noo-ro-
MUS-ku-lar) A synapse
(functional junction)
between the axon
terminals of a motor
neuron and the sarco-
lemma of a muscle fiber.
160 CHAPTER 6
The Muscular System
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