T-tubules

T-tubules (also transverse tubules) are invaginations of the muscle membrane (sarcolemma) which penetrate deep into the muscle fibre. Action potentials inside t-tubules triggers Ca²⁺ release from terminal cisternae of the sarcoplasmic reiticulum, which in turn will produce a contraction. The arrangement of T-tubules and their sizes are vary depending on the muscle type.