Sarcoplasmic reticulum
= Sarcoplasmic reticulum
The <sarcoplasmic reticulum> is the specialized <endoplasmic reticulum> of muscle cells that stores and releases <calcium>. Pump uptake, channel-mediated release and passive leak exchange <calcium> with the <cytoplasm>; these internal transfers cancel in the appropriately volume-weighted total-calcium balance. They can nevertheless change cytoplasmic <calcium> rapidly and control contraction.