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T tubule (transverse tubule)

(Science: cell biology) invagination of the plasma membrane (sarcolemma) of striated muscle that lies between two tubular portions of the endoplasmic (sarcoplasmic) reticulum to form a triad of membrane profiles adjacent to the A band/I band junction in some cases, in other cases to the z disc, of the resting sarcomere. Depolarisation of the t-tubule membrane triggers the release of calcium (Ca2+) from the sarcoplasmic reticulum and eventually muscle contraction.