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Questions and Answers
ACh is produced in the nerve terminal and stored in ______.
synaptic vesicles
After release, ACh binds to receptors on ______.
Muscle
Type I muscle fibers are also known as ______ twitch fibers.
slow
Type II muscle fibers are also known as ______ twitch fibers.
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The NMJ is also known as a __________ synapse.
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The contractile unit of skeletal muscle is called a ______.
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The neurotransmitter released at the NMJ is __________.
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Actin and Myosin give muscle a striated ______.
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The sarcoplasmic reticulum serves as a ______ storehouse.
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The __________ motor neuron is also referred to as the alpha motor neuron.
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The ______ cross-bridge cycle explains the mechanisms of muscle contraction.
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There is a synaptic cleft of about __________ micrometers at the NMJ.
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The muscle action potential is initiated when ACh binds to __________ on the muscle.
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Skeletal muscle fibers are organized into ______ that consist of actin and myosin.
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Mitochondria in muscle fibers are responsible for producing ______.
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The sequence of events that occurs when a skeletal muscle contracts is known as __________ formation.
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The area of the neuron that releases neurotransmitters is called the __________ terminal.
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__________ are numerous in both the presynaptic and postsynaptic terminals at the NMJ.
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The region between the motor nerve and the muscle fiber is known as the __________ cleft.
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An __________ motor neuron is responsible for transmitting signals to skeletal muscle.
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Study Notes
Neuromuscular Junction (NMJ)
- The NMJ is the synapse between a motor neuron and a muscle fiber.
- Alpha motor neurons are also known as lower motor neurons and are responsible for stimulating muscle contraction.
- The synaptic cleft is the space between the motor neuron and the muscle fiber.
- Acetylcholine (ACh) is the neurotransmitter released at the NMJ.
Structure of the NMJ
- The motor neuron terminal has a presynaptic terminal and the muscle fiber has a postsynaptic terminal.
- The synaptic cleft is about 20 µm wide, ensuring no physical contact between the nerve and the muscle.
- The presynaptic terminal has many mitochondria for ATP production needed for neurotransmitter release.
- The postsynaptic terminal contains receptors for the neurotransmitter.
Neurotransmitter Release at the NMJ
- ACh is stored in synaptic vesicles in the presynaptic terminal.
- The arrival of an action potential at the presynaptic terminal triggers the release of ACh via exocytosis.
- ACh diffuses across the synaptic cleft and binds to receptors on the muscle fiber membrane.
Events after Neurotransmitter Release
- Binding of ACh to its receptors on the muscle fiber membrane opens ion channels, leading to depolarization of the muscle fiber membrane.
- This depolarization generates an end-plate potential (EPP) which triggers an action potential in the muscle fiber.
- The muscle fiber action potential travels along the sarcolemma, leading to muscle contraction.
- ACh is rapidly broken down by acetylcholinesterase (AChE) in the synaptic cleft, ending the signal and preventing continuous muscle contraction.
Types of Skeletal Muscle Fibers
-
Type I (Slow Twitch) Muscle Fibers (Red):
- Used for postural muscles
- High myoglobin content
- Many mitochondria for oxidative metabolism
- Fatigue-resistant
-
Type II (Fast Twitch) Muscle Fibers (White):
- Found in extraocular muscles
- Lower myoglobin content
- Fewer mitochondria
- Use glycolytic metabolism for energy production
- More fatigue-prone
Gross Structure of Muscle
- Muscle fibers are composed of myofibrils, which are in turn made up of myofilaments of actin and myosin.
- The arrangement of actin and myosin filaments gives skeletal muscle a striated appearance.
Organization of Skeletal Muscle Fibers
- Myofibrils are made up of repeating contractile units called sarcomeres.
- The sarcomere is the basic unit of muscle contraction.
- T-tubules allow the propagation of the muscle action potential throughout the muscle fiber.
- Sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum in muscle fibers that stores and releases calcium ions for muscle contraction.
- The SR is connected to the T-tubules.
Histology of Skeletal Muscle Fibers
- The thin filaments are made of the protein actin.
- The thick filaments are made of the protein myosin.
- Titin is a large protein that connects myosin to the Z-disc.
- Troponin and tropomyosin are proteins that regulate actin-myosin interaction during muscle contraction.
The Contraction-Relaxation Cycle (Cross-Bridge Formation)
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Muscle contraction:
- Calcium ions released from the SR bind to troponin.
- This causes a conformational change in tropomyosin, exposing the myosin-binding site on actin.
- Myosin heads attach to the actin filament (cross-bridge formation).
- The myosin heads swivel, pulling the actin filament towards the center of the sarcomere.
- ATP is hydrolyzed, providing energy for the power stroke .
- The myosin heads detach from the actin filament, and the cycle repeats.
-
Muscle relaxation:
- Calcium ions are pumped back into the SR by active transport.
- Troponin returns to its original conformation.
- Tropomyosin covers the myosin-binding sites on actin.
- Myosin can no longer bind to actin, and the muscle relaxes.
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Description
Explore the structure and function of the neuromuscular junction (NMJ) in this quiz. Learn about the roles of alpha motor neurons, acetylcholine release, and the components of the NMJ. Test your knowledge on how these elements work together to stimulate muscle contractions.
