The sarcoplasmic reticulum of striated muscle myocytes is the equivalent to the smooth endoplasmic reticulum of ordinary cells. It consists of a separate cytoplasmic compartment containing flat cisternae (sacs) and anastomosing tubules (sarcotubules) which are interconnected as collar-like complexes around each myofibril. Fast, calcium-release channels are present in the membrane of the sarcoplasmic reticulum. Each pair of terminal cisternae lies at an A band - I band junction of the sarcomere. Each sarcomere is encircled by two T tubules, and each T tubule in turn is flanked by two terminal cisternae.
A second part of the sarcoplasmic reticulum is the network of narrow, anastomosing tubules, which are known as sarcotubules. These tubules interconnect cisternae which belongs to adjacent pairs of terminal cisternae, but not the neighboring cisternae of each pair. Sarcotubules extend somewhat longitudinally. However, in the midregion of the sarcomere, they anastomose extensively. The resulting network gives the sarcoplasmic reticulum the overall appearance of a lacy sleeve.
Function
The main function of the sarcoplasmic reticulum is to regulate the calcium ion concentration within the myofibrils, which in turn determine whether thin filaments interact with thick ones to bring about a contraction. When the muscle fiber is relaxed, calcium ions are stored in the lumen of its sarcoplasmic reticulum. The moment the sarcolemmal lining of the T tubules depolarizes, the fast calcium-release channels in the sarcoplasmic reticulum membrane open and release membrane-segragated calcium ions. When they enter the myofibrils, the liberated calcium ions enable the thick filaments to interact with the thin ones.
Below, a picture showing the sarcoplasmic reticulum as it really is: a network of tubules and cisternae surrounding the myofibril.
