Sarcoplasmic reticulum

In striated muscle, sarcoplasmic reticulum is a type of smooth endoplasmic reticulum specially adapted to surround the myofibrils, forming dyads in cardiac muscle and triads in skeletal muscle with invaginations of the plasma membrane called T-tubules.

The sarcoplasmic reticulum contains large stores of calcium, which it sequesters and then releases when the cell become depolarised. This has the effect of triggering muscle contraction.