Part c - how an action potential affects ca2+ movement in a muscle cell 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.