Title: The Neuromuscular Junction, The Synapse Between a Motor Neuron and a
Skeletal Muscle Fiber, is Where a Muscle Action Potential is initiated.
1- A muscle fiber contracts in response to muscle action potentials propagating
along its sarcolemma and through its system of T tubules.
a-motor neurons stimulate nerves
b- Muscle action potentials arise at the neuromuscular junction (NMJ), the
structural point of synaptic contact and functional site of communication
between a motor neuron and a skeletal muscle fiber
c- A synapse is a region where communication occurs between a neuron and
another cell—in this case, between a motor neuron and a skeletal muscle
fiber.
d- A small gap, called the synaptic cleft, separates the two cells. Because the
cells do not physically touch, the action potential from the motor neuron
cannot “jump the gap” to directly excite the muscle fiber. Instead, the motor
neuron communicates with the muscle fiber indirectly, by releasing a
chemical called a neurotransmitter.
e- The principle neurotransmitter is acetylcholine – There are 30-40 million
AcH receptors on each muscle fiber
f- A nerve impulse excites a skeletal muscle fiber in the following way
1- Release of Acetylcholine
2- Activation of AcH receptors
3- Generation of muscle action potential
4- Termination of AcH receptors
g- Each nerve impulse normally elicits one muscle action potential. If another
nerve impulse releases more acetylcholine, then steps 2 and 3 are repeated.
h- Because skeletal muscle fibers often are very long cells, the NMJ usually is
located near the midpoint of a skeletal muscle fiber. Muscle action potentials
arise at the NMJ and then propagate toward both ends of the fiber. This
arrangement permits nearly simultaneous activation (and thus contraction)
of all parts of the muscle fiber.
2- Excitation–Contraction Coupling
a- Excitation–contraction coupling refers to the steps that connect excitation
(a muscle action potential propagating along the sarcolemma and into the T
tubules) to contraction (sliding of the filaments).
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3b- An Action Potential Releases Calcium Ions that Allow the Thick Filaments
to Bind to and Pull the Thin Filaments Toward the Center of the Sarcomere.
1- Once an action potential is propagated along the sarcolemma and
T tubules, muscle contraction begins. The model describing the
contraction of muscle is known as the sliding filament mechanism.
2- Muscle contraction occurs because myosin heads attach to and
“walk” along the thin filaments at both ends of a sarcomere,
progressively pulling the thin filaments toward the M line
3- the thin filaments slide inward and meet at the center of a
sarcomere. They may even move so far inward that their ends
overlap
4- As the thin filaments slide inward, the Z discs come closer
together, and the sarcomere shortens. Note that the lengths of the
individual thick and thin filaments do not change; it is the amount
of overlap between them that changes. Shortening of the
individual sarcomeres causes shortening of the whole muscle
fiber, and ultimately shortening of the entire muscle.
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