FIGURE 17-33 The sarcoplasmic reticulum regulates the level of free Ca²⁺ in myofibrils. (a) When a nerve impulse stimulates a muscle cell, the action potential is transmitted down a transverse tubule (yellow), which is continuous with the plasma membrane (sarcolemma), leading to release of Ca²⁺ from the adjacent sarcoplasmic reticulum (blue) into the myofibrils. (b) Thin-section electron micrograph of skeletal muscle, showing the intimate relationship of the sarcoplasmic reticulum to the muscle fibers.

[Part (b) courtesy of Keith R. Porter and Clara Franzini-Armstrong.]