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The sarcoplasmic reticulum of muscles is a membrane‐bound structure that is similar to the smooth endoplasmic reticulum in other cells (a network of membranous tubules in the cytoplasm of eukaryotic cells that is in essence a transportation system). The sarcoplasmic reticulum is a key structure specialized in calcium ion (Ca²⁺) storage (Figure 3.8). Depolarization of the sarcoplasmic reticulum membrane results in the release of Ca²⁺ ions. This release is initiated at the specialized neuromuscular junction (to be discussed shortly). For a uniform muscle contraction, the cell membrane of skeletal muscle fibers extends into and penetrates the center of the muscle fiber as a system of transverse (T) tubules that form a complex network encircling the A–I bands of each sarcomere. Adjacent to the opposite sides of each T‐tubule are the expanded terminal cisternae of the sarcoplasmic reticulum. Two small cisternae of the sarcoplasmic reticulum and the T‐tubule form a specialized complex known as a triad. It is this junction where a neurally mediated depolarization of the T‐tubules is transmitted to the sarcoplasmic reticulum, stimulating a quick and widespread release of Ca²⁺.

It is important to understand that muscle contraction depends on the availability of Ca²⁺ ions, while muscle relaxation occurs in the absence of Ca²⁺. The sarcoplasmic reticulum (Figure 3.8) regulates Ca²⁺ flow necessary for rapid contraction and relaxation cycles. After depolarization of the sarcoplasmic reticulum, Ca²⁺ ions concentrated within cisterna are passively released into the vicinity of the overlapping thick and thin filaments and bind to troponin. This allows bridging of the actin and myosin molecules. When the membrane depolarization ends, the sarcoplasmic reticulum actively transports the Ca²⁺ back into the cisternae, ending contractile activity.

Figure 3.8 Diagram of the sarcoplasmic reticulum and T‐tubule system of a mammalian skeletal muscle fiber. The sarcoplasmic reticulum enmeshes fibrils with transverse (T) tubules intersecting them. Invaginations of the T‐tubules occur at the transition point of A and I bands in every sarcomere. The T‐tubules are associated with terminal cisternae of the sarcoplasmic reticulum, forming triads. The cut surface of myofibrils also shows the thin and thick myofilaments. Satellite cells reside along the host muscle fiber, directly above the sarcolemma under the basal lamina of muscle and in proximity to the nuclei of muscle cells. Nerve endings and local capillaries extend along the length of the muscle fiber. The extracellular matrix, not shown here, successively encases each layer.

Mukund, K. & Subramaniam, S. (2019). Skeletal muscle: A review of molecular structure and function, in health and disease. Wiley Interdisciplinary Reviews: Systems Biology and Medicine, 12, e1462./John Wiley & Sons/CC BY‐4.0.