The sarcoplasmic reticulum is a specialized form of ER that surrounds each myofibril. The sarcoplasmic reticulum functions to control cytosolic Ca2+ levels in the muscle cell. Changes in cytosolic Ca2+ concentrations couple action potentials to muscle contraction.
The concentration of Ca2+ ions in the sarcoplasmic reticulum is typically much higher than the Ca2+ concentration in the cytosol. This concentration gradient is key to the movement of Ca2+ in the muscle cell in response to an action potential.
The cycle diagram below shows the sequence of events that affect Ca2+ levels in a muscle cell, beginning with the propagation of an action potential down a T tubule (top of the diagram).
Drag the labels to their appropriate locations on the cycle diagram below. Note that SR stands for sarcoplasmic reticulum.