T tubules

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].

References

1. ↑ A. Fullick, J.Locke, P.Bircher. A level biology for OCR. Oxford University Press, Great Clarendon Street, Oxford, OX2 6DP, United Kingdom. 2015.
2. ↑ Calcium regulation of muscle contraction. Biophys J. 1975;15(7):707-23.