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Questions and Answers
What triggers the release of calcium ions from the sarcoplasmic reticulum during muscle contraction?
What triggers the release of calcium ions from the sarcoplasmic reticulum during muscle contraction?
Which statement correctly describes the role of troponin in muscle contraction?
Which statement correctly describes the role of troponin in muscle contraction?
What is the primary purpose of the power stroke in muscle contraction?
What is the primary purpose of the power stroke in muscle contraction?
What initiates the detachment of myosin from the G actin active site during muscle contraction?
What initiates the detachment of myosin from the G actin active site during muscle contraction?
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How does ACh result in muscle action potential?
How does ACh result in muscle action potential?
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What position does the myosin head take when it is bound to G actin in the active contraction state?
What position does the myosin head take when it is bound to G actin in the active contraction state?
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What is the fate of myosin after completing its power stroke?
What is the fate of myosin after completing its power stroke?
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What is the primary function of tropomyosin in muscle contraction?
What is the primary function of tropomyosin in muscle contraction?
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Which event occurs when the myosin head releases the phosphate group during muscle contraction?
Which event occurs when the myosin head releases the phosphate group during muscle contraction?
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Which type of muscle fibers are primarily involved in maintaining posture?
Which type of muscle fibers are primarily involved in maintaining posture?
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What happens when calcium is transported back into the sarcoplasmic reticulum?
What happens when calcium is transported back into the sarcoplasmic reticulum?
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What triggers the increase in calcium ion concentration during muscle contraction?
What triggers the increase in calcium ion concentration during muscle contraction?
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Which event occurs first in the excitation-contraction coupling process?
Which event occurs first in the excitation-contraction coupling process?
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What is the role of the third subunit of the troponin molecule in muscle contraction?
What is the role of the third subunit of the troponin molecule in muscle contraction?
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How many myosin molecules approximately make up thick filaments in muscle?
How many myosin molecules approximately make up thick filaments in muscle?
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During which step does the myosin head change from a 90-degree angle to a position allowing the power stroke?
During which step does the myosin head change from a 90-degree angle to a position allowing the power stroke?
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What characteristic distinguishes slow-twitch muscle fibers from fast-twitch fibers?
What characteristic distinguishes slow-twitch muscle fibers from fast-twitch fibers?
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What happens during the latent period of muscle contraction?
What happens during the latent period of muscle contraction?
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Which of the following best describes the all-or-none response in muscle fibers?
Which of the following best describes the all-or-none response in muscle fibers?
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What is the term for a sustained contraction lacking relaxation?
What is the term for a sustained contraction lacking relaxation?
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How does a fast-twitch muscle fiber primarily generate ATP?
How does a fast-twitch muscle fiber primarily generate ATP?
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Which neurotransmitter is crucial for initiating muscle contraction at the neuromuscular junction?
Which neurotransmitter is crucial for initiating muscle contraction at the neuromuscular junction?
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What role does calcium play in muscle contraction?
What role does calcium play in muscle contraction?
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What is the primary reason fast-twitch fibers fatigue more easily than slow-twitch fibers?
What is the primary reason fast-twitch fibers fatigue more easily than slow-twitch fibers?
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Study Notes
Muscle Excitation and Contraction
- Action potential arrives at axon terminal of somatic motor neuron which causes voltage-gated calcium channels to open.
- Calcium entry triggers the exocytosis of ACh-containing synaptic vesicles.
- ACh diffuses across the synaptic cleft to bind with nicotinic receptors on the motor end plate of the muscle.
- This opens non-specific cation channels, allowing Na+ and K+ movement in response to electrochemical gradients.
- Net influx of positive charge depolarizes the muscle membrane, generating an end-plate potential.
- The end-plate potential always triggers a muscle action potential.
- The action potential spreads across the muscle fiber membrane and into the interior via the T-tubules.
- The action potential activates dihydropyridine receptors (DHP).
- DHP receptors open Ca2+ channels in the sarcoplasmic reticulum membrane.
- Ca2+ diffuses out of the sarcoplasmic reticulum and binds to troponin, moving tropomyosin away from the myosin-binding site.
- This allows myosin to complete its power stroke after releasing inorganic phosphate from ATP hydrolysis.
- At the end of the power stroke, the myosin crossbridge releases ADP and remains tightly bound to actin.
- Myosin must bind to ATP to release from this rigor state.
- Muscle relaxes when Ca2+ is removed from troponin, and tropomyosin blocks the myosin-binding site once again.
- Calcium is transported back into the sarcoplasmic reticulum via Ca2+-ATPase.
- Myosin ATPase hydrolyses ATP to ADP and inorganic phosphate, which both remain bound to the myosin head.
- The myosin swivels and binds to a new actin molecule, preparing for the next power stroke.
Molecular Basis of Muscle Contraction
- Myosin head is bound to G actin active site at a 45-degree angle during active contraction (rigor).
- Myosin detaches from the G actin active site when ATP binds to the nucleotide binding site of myosin.
- Myosin ATPase activity converts ATP to ADP and a phosphate group, providing energy to move the myosin head.
- The myosin head moves to a 90-degree angle, weakly binding to the next G actin active site on the thin filament.
- ADP and phosphate remain bound to the myosin head in the relaxed state.
- The myosin head releases the phosphate group and rotates towards the M line, moving the attached thin filament.
- This is known as the power stroke.
- At the end of the power stroke, the myosin head releases ADP and binds tightly to the G actin active site.
- This process repeats, returning to step 1.
The Regulatory Role of Tropomyosin and Troponin in Contraction
- Tropomyosin covers the active site of G actin in the relaxed state, preventing myosin from binding tightly to thin filaments.
- Ca2+ binds to the third subunit of the troponin molecule when the muscle is stimulated.
- The increased Ca2+ concentration allows troponin to pull tropomyosin away from the active site, enabling myosin binding.
Fast and Slow Muscles
- Slow-twitch muscles are involved in maintaining posture and contain high levels of myoglobin.
- They have a good blood supply and are often referred to as red muscles due to myoglobin.
- These muscles have many mitochondria and high respiratory capacity.
- They can contract for long periods without fatigue.
- Fast-twitch muscles have less myoglobin and are often called white muscles.
- They have a poorer blood supply compared to red muscles.
- They have fewer mitochondria and lower respiratory capacity.
- They have extensive sarcoplasmic reticulum, allowing for greater Ca2+ storage and reabsorption.
- They also have high ATPase activity which allows for rapid contraction, but they fatigue easily.
Skeletal Muscle Contraction
- The threshold stimulus is the minimum strength required to cause muscle contraction.
- Motor neuron impulses usually release enough ACh to bring muscle fibers in a motor unit to threshold.
- All-or-none response: If a muscle fiber is exposed to a stimulus at or above threshold, it contracts to its fullest extent.
- Increasing stimulus strength does not affect the degree of muscle fiber contraction.
- A muscle fiber will contract fully if stimulated, but it may not shorten fully.
- A myogram is the recording of an electrical stimulus of an isolated muscle.
- When a muscle is stimulated with a single stimulus of sufficient strength, it contracts and relaxes immediately.
- This single contraction is called a twitch, which lasts only a fraction of a second.
- The latency period refers to the delay between stimulation and response.
- Muscle has a refractory period where it is unresponsive for a short period after the first twitch.
- Summation: The force a muscle fiber can generate exceeds the maximum force of a single twitch.
- Multiple stimuli with increasing frequency prevent the muscle from relaxing completely before the next stimulation.
- The forces of individual twitches combine through summation.
- A tetanic contraction is a forceful, sustained contraction without any partial relaxation.
- The action potential propagates along axons from the CNS and reaches the terminal at the neuromuscular junction.
- The released Ach causes influx of Na+ and K+ into the muscle fiber, causing action potential in the muscle fiber.
- The action potential travels throughout the muscle fiber and causes the release of Ca2+ from the sarcoplasmic reticulum into the sarcoplasm.
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Description
This quiz explores the processes involved in muscle excitation and contraction. It covers topics such as action potential initiation, calcium signaling, and the role of neurotransmitters at the neuromuscular junction. Test your knowledge on how these mechanisms lead to muscle movement.
