The transverse tubules in a skeletal-muscle fiber

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Transverse tubules, also known as T-tubules, are invaginations of the sarcolemma (plasma membrane) that penetrate deep into the muscle fiber, running perpendicular to the myofibrils. They provide a means for transmitting an action potential, which depolarizes the T-tubules and triggers the release of calcium ions from the sarcoplasmic reticulum, leading to muscle contraction. T-tubules do not store calcium ions, form Z lines, store ATP, or run in parallel with the myofibrils.

The Transverse Tubules in a Skeletal-Muscle Fiber