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Sarcoplasmic Reticulum

Activation of skeletal muscle contraction is triggered through the release of calcium ions from a system of membranous sacs within the cell known as the sarcoplasmic reticulum (SR). Release of calcium from the SR is accomplished through rapid propagation of the muscle action potential throughout the muscle fiber via a complex system of membranes.

The muscle membrane system ensures that the muscle action potential initiated at the superficially located muscle end plate rapidly reaches the depths of the fiber and is then transmitted from one cell to the next with great velocity. The SR is made up of the longitudinal tubules and the transverse tubules (T tubules).

The action potential is carried the length of the fiber’s surface by the longitudinal tubule and lateral sac system and to the inner fiber by T tubules. The T tubules penetrate the muscle fibers near the Z line perpendicular to the depths of the muscle fiber.

Each T tubule is closely associated with two lateral sacs, which in turn are connected with the longitudinal tubules. The two lateral sacs and their related T tubule are collectively known as a triad.

The SR has a highly specialized membrane system for actively sequestering calcium and maintaining a low resting calcium concentration in the myoplasm. The maintenance of low myoplasmic calcium concentrations is accomplished by calcium adenosine triphosphatase (ATPase) pumps that transport calcium ions against a concentration gradient via cleavage of one ATP molecule.

When a muscle action potential is transmitted along the surface of the sarcolemma and into the T tubule and longitudinal tubule system, the SR becomes more permeable to Ca2+, and Ca2+ is released from the SR into the myoplasm.

Most of the free calcium binds to troponin to initiate contraction. Relaxation of the muscle occurs when the low myoplasmic Ca2+ concentration is restored via the Ca2+ ATPase pumps. This process is commonly referred to as excitation– contraction coupling.

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