The cylindrical bundles of myofilaments (myofibrils) are separated from one another by a sac-like membrane system, the sarcoplasmic reticulum - SR (derived from the endoplasmic reticulum), which runs through each sarcomere. The sacs form a system of longitudinally running tubules (2) which meet and fuse at the level of the ends of the A-bands, to form a horizontally running terminal cistern (3). Also at the level of the ends of the A-band, the sarcolemma has a ring of fingerlike invaginations (transverse or T-tubules) which run around the myofibrils and fuse to form a system of tube-like girdles around each myofibril. The terminal cisterns from the SR on either side, closely abut, but do not fuse with, the membrane of the T-tubules. The membrane of the T-tubules (4) is continuous with the sarcolemma, and the lumen contains extracellular fluid. Like the sarcolemma, the T-tubular membrane can carry a propagated action potential.
Terminal cisternae and T-tubule loaded with ferritin by soaking the muscle fibre in a ferritin containing solution before fixing. The T-tubule is here seen aligned with the Z-line because this is a picture of frog (amphibian) muscle and not mammalian muscle which has the T-tubules at the A-I junction.
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The outer membrane of the SR and the sarcoplasm contain a soluble, calcium binding protein calbindin or parvalbumin. The SR membrane also has an integral Ca2+-Mg2+-ATPase-pump with a higher affinity for Ca2+ than parvalbumin. Ca2+ ion in the cytosol are slowly bound by the parvalbumin, and transferred to the pump which actively transports the Ca2+ into the lumen of the SR, where it is loosely and reversibly bound to calsequestrin. (Calsequestrin is to Ca2+ as haemoglobin is to oxygen.) This system, along with the mitochondria which also pick up Ca2+, helps to keep the Ca2+ concentration of the sarcoplasm (cytosol) of a resting muscle fibre at a very low level (<10-7 M). The T-tubules also have special Ca2+ channels termed 'dihydropyridine receptors'. These undergo configurational change when an action potential spreads along the sarcolemma and down the T-tubule. By an unknown mechanism, this change induces opening of other Ca2+ channels ('ryanodine receptors') in the membrane of the SR cisternae. The transient opening of these channels causes Ca2+ to be dumped from the cisternae into the cytosol, raising the concentration from <10-7 M to >10-4 M.
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