T tubules
From The School of Biomedical Sciences Wiki
The action potential generated travels deep into the cell by the aid of t-tubules, which are invaginations of the sarcolemma. When the muscle is stimulated by Action Potential, the t tubules are triggered to release Ca2+ ions from the Terminal cristae of the Sarcoplasmic reticulum, which causes muscle contraction. They help spread the impulse so that the cell is able to contract at the same time[1][2].