What is the functional role of the T tubules?
Hold cross bridges in place in a resting muscle
Support the muscle fiber
Stabilize the G and F actin
Enhance spread of muscle action potential throughout muscle fiber
Synthesize glucose to provide energy for muscle contraction
The Correct Answer is D
Choice A reason: Cross bridges are formed between actin and myosin during contraction. T tubules do not hold them in place; their role is electrical, not structural.
Choice B reason: While T tubules are embedded in the muscle fiber, their function is not to provide structural support. That role is fulfilled by connective tissues like endomysium and cytoskeletal proteins.
Choice C reason: G and F actin are stabilized by proteins like tropomyosin and nebulin, not T tubules. T tubules are involved in electrical signaling.
Choice D reason: This is the correct answer. T tubules are invaginations of the sarcolemma that conduct action potentials deep into the muscle fiber. This ensures that the signal for contraction reaches all parts of the muscle simultaneously, triggering calcium release from the sarcoplasmic reticulum.
Choice E reason: Glucose synthesis occurs in the liver and other tissues via gluconeogenesis. Muscle cells use glucose for energy but do not synthesize it through T tubules.
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Related Questions
Correct Answer is A
Explanation
Choice A reason: Smooth and cardiac muscle are capable of contracting without direct nervous stimulation. Cardiac muscle has intrinsic pacemaker cells that generate rhythmic contractions, and smooth muscle can respond to chemical signals, stretch, and local factors independently of neural input.
Choice B reason: Skeletal muscle requires nervous stimulation to contract. It is under voluntary control and cannot initiate contraction on its own, making this choice incorrect.
Choice C reason: While cardiac muscle can contract independently, this choice omits smooth muscle, which also has autonomous contractile ability. Therefore, it is incomplete.
Choice D reason: Skeletal muscle is entirely dependent on neural input for contraction. It cannot contract without stimulation from motor neurons.
Choice E reason: Smooth muscle can contract without nervous stimulation, but this choice excludes cardiac muscle, which also shares this property. Thus, it is not the best answer.
Correct Answer is D
Explanation
Choice A reason: Actin is a structural protein that forms the thin filaments in muscle fibers. While it interacts with myosin during contraction, it does not bind calcium directly.
Choice B reason: Tropomyosin is a regulatory protein that blocks the myosin-binding sites on actin in a resting muscle. It shifts position when calcium binds to troponin but does not itself bind calcium.
Choice C reason: Titin is a large elastic protein that helps maintain the structural integrity of the sarcomere and contributes to passive elasticity. It does not function as a calcium receptor.
Choice D reason: Troponin is the correct answer. It is a regulatory protein complex associated with the thin filament. When calcium ions bind to troponin, it induces a conformational change that moves tropomyosin away from actin’s binding sites, allowing muscle contraction to occur.
Choice E reason: Dystrophin is a structural protein that connects the cytoskeleton of muscle fibers to the extracellular matrix. It is important for muscle integrity but does not bind calcium.
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