264
Chapter 9
Membrane Excitation:
The Neuromuscular Junction
We have just seen that an action potential in the plasma mem-
brane of a skeletal muscle fi
ber is the signal that triggers con-
traction. We will now back up one step and ask the question:
How are these action potentials initiated? Stimulation of the
nerve fi bers to a skeletal muscle is the only mechanism by
which action potentials are initiated in this type of muscle.
In subsequent sections you’ll see additional mechanisms for
activating cardiac and smooth muscle contraction.
The nerve cells whose axons innervate skeletal muscle
fi bers are known as
motor neurons
(or somatic efferent neu-
rons), and their cell bodies are located in either the brainstem
or the spinal cord. The axons of motor neurons are myelin-
ated and are the largest-diameter axons in the body. They are
therefore able to propagate action potentials at high velocities,
allowing signals from the central nervous system to travel to
skeletal muscle fi bers with minimal delay.
Upon reaching a muscle, the axon of a motor neuron
divides into many branches, each branch forming a single junc-
tion with a muscle fi ber. A single motor neuron innervates many
muscle fi bers, but each muscle fi ber is controlled by a branch
from only one motor neuron. A motor neuron plus the muscle
fi bers it innervates is called a
motor unit
(
Figure 9–13a
). The
muscle fi bers in a single motor unit are located in one muscle,
but they are scattered throughout the muscle and are not adja-
cent to each other (
Figure 9–13b
). When an action potential
occurs in a motor neuron, all the muscle fi bers in its motor
unit are stimulated to contract.
The myelin sheath surrounding the axon of each motor
neuron ends near the surface of a muscle fi ber, and the axon
divides into a number of short processes that lie embedded in
grooves on the muscle fi ber surface (
Figure 9–14a
). The axon
terminals of a motor neuron contain vesicles similar to the vesicles
found at synaptic junctions between two neurons. The vesicles
contain the neurotransmitter
acetylcholine (ACh).
The region
of the muscle fi
ber plasma membrane that lies directly under
the terminal portion of the axon is known as the
motor end
plate.
The junction of an axon terminal with the motor end
plate is known as a
neuromuscular junction
(
Figure 9–14b
).
Figure 9–15
shows the events occurring at the neu-
romuscular junction. When an action potential in a motor
neuron arrives at the axon terminal, it depolarizes the plasma
membrane, opening voltage-sensitive calcium channels and
allowing calcium ions to diffuse into the axon terminal from
the extracellular fl uid. This calcium binds to proteins that
enable the membranes of acetylcholine-containing vesicles to
fuse with the neuronal plasma membrane, thereby releasing
acetylcholine into the extracellular cleft separating the axon
terminal and the motor end plate.
ACh diffuses from the axon terminal to the motor end
plate where it binds to ionotropic receptors (of the nicotinic
type; see Chapter 6). The binding of ACh opens an ion channel
in each receptor protein; both sodium and potassium ions can
pass through these channels. Because of the differences in elec-
trochemical gradients across the plasma membrane (Chapter 6),
more sodium moves in than potassium out, producing a local
depolarization of the motor end plate known as an
end-plate
potential (EPP).
Thus, an EPP is analogous to an EPSP
(excitatory postsynaptic potential) at a neuron-neuron synapse
(Chapter 6).
The magnitude of a single EPP is, however, much larger
than that of an EPSP because neurotransmitter is released
over a larger surface area, binding to many more receptors and
Table 9–1
Functions of ATP in Skeletal Muscle
Contraction
1. Hydrolysis of ATP by myosin energizes the cross-bridges,
providing the energy for force generation.
2. Binding of ATP to myosin dissociates cross-bridges bound
to actin, allowing the bridges to repeat their cycle of activity.
3. Hydrolysis of ATP by the Ca
2+
-ATPase in the sarcoplasmic
reticulum provides the energy for the active transport of
calcium ions into the reticulum, lowering cytosolic calcium
to prerelease levels, ending the contraction, and allowing the
muscle fi ber to relax.
(a) Single motor unit
Neuromuscular junctions
Motor
neurons
(b) Two motor units
Motor
neuron
Figure 9–13
(a) Single motor unit consisting of one motor neuron and
the muscle fi bers it innervates. (b) Two motor units and their
intermingled fi bers in a muscle.
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