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Which of the following best describes slow-twitch muscle fibers?
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4. Tissues & Histology
Introduction to Muscle Tissue
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The T tubules of a muscle fiber are extensions of the ______ and contain extracellular fluid.
A
sarcoplasmic reticulum
B
myofibril
C
sarcoplasm
D
sarcolemma
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Understand the structure of a muscle fiber: A muscle fiber is composed of several key components, including the sarcolemma (plasma membrane), sarcoplasm (cytoplasm), sarcoplasmic reticulum (specialized endoplasmic reticulum), and myofibrils (contractile units).
Learn the role of T tubules: T tubules, or transverse tubules, are invaginations of the sarcolemma that penetrate into the interior of the muscle fiber. They play a critical role in transmitting action potentials deep into the muscle fiber to ensure uniform contraction.
Recognize the relationship between T tubules and extracellular fluid: Since T tubules are extensions of the sarcolemma, they are continuous with the extracellular space and contain extracellular fluid.
Differentiate the options provided: The sarcoplasmic reticulum is an intracellular structure that stores calcium ions, the myofibrils are contractile units within the muscle fiber, and the sarcoplasm is the cytoplasm of the muscle cell. None of these are directly related to the extracellular fluid or the sarcolemma.
Conclude that the correct answer is 'sarcolemma': Based on the anatomical and physiological characteristics of T tubules, they are extensions of the sarcolemma and contain extracellular fluid.
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