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 E
Explanation
Choice A reason: Osteoclasts are large, multinucleated cells responsible for bone resorption. They do not arise from osteoblasts and are not involved in matrix secretion.
Choice B reason: Osteogenic cells are stem cells that differentiate into osteoblasts. They are precursors, not the result of matrix entrapment.
Choice C reason: Chondrocytes are cartilage cells, not bone cells. They originate from chondroblasts and are unrelated to osteoblast transformation.
Choice D reason: Osteoid refers to the unmineralized bone matrix secreted by osteoblasts. "Osteoid cell" is not a recognized term for a mature bone cell.
Choice E reason: Osteocyte is the correct answer. Once an osteoblast becomes encased in the bone matrix it secretes, it differentiates into an osteocyte. Osteocytes reside in lacunae and maintain bone tissue.
Correct Answer is A
Explanation
Choice A reason: The sarcomere is the smallest contractile unit of a skeletal muscle. It is composed of actin and myosin filaments arranged in a precise pattern that allows for muscle contraction. Sarcomeres are repeated along the length of a myofibril and are responsible for the striated appearance of skeletal muscle.
Choice B reason: The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum in muscle cells that stores and releases calcium ions, which are essential for muscle contraction. However, it is not the contractile unit itself.
Choice C reason: The sarcolemma is the plasma membrane of a muscle fiber. It plays a role in conducting electrical signals but is not the structural unit responsible for contraction.
Choice D reason: A myofibril is a long, cylindrical organelle found within muscle cells that contains many sarcomeres. While it contributes to muscle contraction, it is composed of multiple sarcomeres and is not the fundamental unit.
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