Talk about the mechanism of Ca sequestering in SER.

Regulated calcium delivery into cytosol which triggers many functions (muscle contraction, nerve impulses, fusion of secretory vesicles.) AP in muscle fibers reach SER via transverse tubules (extensions of plasma membrane touching SER membrane) which opens SER Ca channels, and enables Ca to interact with proteins (actin, myosin, tropomyosin, troponin...) which triggers contraction, when AP goes out, Ca are pumped actively into the SER lumen so that cytosol Ca concentration becomes less than lumen, which causes muscle relaxation.