T-Tubules

A system of chambers of the endoplasmic reticulum of (striated) muscle cells (sarcoplasmic reticulum), which can cause muscle contraction by myosin filaments and actin filaments by expulsion of calcium.

https://de.wikipedia.org/wiki/Endoplasmatisches_Retikulum#Sarkoplasmatisches_Retikulum_.28Glattes_Retikulum.2C_SR.29 _{(Wikipedia CC-by-sa-3.0)}