Sarcoplasmic reticulum

Sarcoplasmic Reticulum

The Sarcoplasmic Reticulum (SR) is a type of organelle found in muscle cells. It is a specialized form of endoplasmic reticulum (ER). The primary function of the sarcoplasmic reticulum is to store calcium ions (Ca2+), which it releases to trigger muscle contraction and absorbs to allow relaxation.

Structure

The sarcoplasmic reticulum is a network of tubules that extends throughout muscle cells, wrapping around each myofibril like a lace sleeve. It is composed of two major parts: the longitudinal tubules and the terminal cisternae. The longitudinal tubules are responsible for the rapid transmission of the action potential. The terminal cisternae store calcium ions and release them when an action potential is received.

Function

The primary function of the sarcoplasmic reticulum is to regulate the concentration of intracellular calcium ions. It does this by actively pumping calcium ions into the SR from the cytosol, which decreases the cytosolic calcium ion concentration. When the muscle cell is stimulated, the SR releases the stored calcium ions into the cytosol, which then bind to troponin, initiating muscle contraction.

Clinical Significance

Abnormalities in the function of the sarcoplasmic reticulum can lead to various muscle disorders. For example, malignant hyperthermia is a potentially fatal reaction to certain drugs used during general anesthesia. It is caused by a mutation in the ryanodine receptor, which causes the SR to release excessive amounts of calcium ions, leading to sustained muscle contraction and a dangerous increase in body temperature.

See Also

• Endoplasmic reticulum
• Muscle contraction
• Calcium in biology
• Malignant hyperthermia

References

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