M04.05.02 Ultrastructure of the Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a specialized form of smooth endoplasmic reticulum found in muscle cells, playing a key role in calcium storage and release during muscle contraction. This structure is critical for regulating calcium ions (Ca²⁺), which are necessary for muscle contraction in both skeletal and cardiac muscle.

Structure of a Skeletal Muscle Cell

The ultrastructure of a skeletal muscle cell includes various components that work together to facilitate muscle contraction. These include:

1. Myofibrils
   Myofibrils are the contractile elements of muscle cells. They are composed of repeating units called sarcomeres, which contain actin and myosin filaments.
2. Sarcolemma
   The sarcolemma is the plasma membrane of the muscle cell. It envelops the muscle fibers and helps in conducting electrical impulses for contraction.
3. Terminal Cisternae
   These are enlarged areas of the sarcoplasmic reticulum adjacent to the T-tubules. They are the primary storage site for calcium ions.
4. T-Tubule (Transverse Tubule)
   T-tubules are invaginations of the sarcolemma that penetrate the interior of the muscle cell. These extensions allow electrical signals to be transmitted rapidly across the muscle fiber, coordinating contraction. The interior of the T-tubules is part of the extracellular environment.

Table of Skeletal Muscle Cell Components

Role of the Sarcoplasmic Reticulum in Calcium Storage

The sarcoplasmic reticulum forms a network around the myofibrils and is an integral part of the internal membrane system. Its main function is to regulate intracellular calcium levels, which is crucial for muscle contraction.

• Terminal Cisternae: In skeletal muscle, the majority of calcium is stored in the terminal cisternae of the sarcoplasmic reticulum. These cisternae are located adjacent to the T-tubule system, ensuring that calcium release is synchronized with the electrical signals transmitted through the T-tubules.

Points to Remember

• The T-tubule membranes are extensions of the sarcolemma and are part of the extracellular compartment.
• The sarcoplasmic reticulum stores calcium, which is essential for muscle contraction.
• Terminal cisternae are closely associated with T-tubules and play a key role in releasing calcium when an action potential is propagated.

Figure

• Displays the relationship between T-tubules, sarcolemma, myofibrils, and terminal cisternae.

Bibliography

1. Alberts, B. et al. (2014). Molecular Biology of the Cell (6th ed.). Garland Science.
2. Friel, D. D., & Tsien, R. W. (1992). A caffeine- and ryanodine-sensitive Ca²⁺ stored in bullfrog sympathetic neurons. The Journal of Neuroscience, 12(10), 4007-4021.
3. Rios, E., & Brum, G. (1987). Involvement of dihydropyridine receptors in excitation-contraction coupling in skeletal muscle. Nature, 325(6106), 717-720.