Explain how the dystrophin-glycoprotein complex contributes to muscle integrity.

• A. Dystrophin links the cytoskeleton to the extracellular matrix, stabilizing the sarcolemma; loss leads to muscle degeneration.
• B. Dystrophin promotes calcium release from the sarcoplasmic reticulum.
• C. Dystrophin increases sarcolemma stiffness independent of contraction.
• D. Dystrophin binds actin to myosin directly during contraction.

Answer: (A)

The main idea is that the dystrophin-glycoprotein complex provides a mechanical link between the internal cytoskeleton and the external matrix, stabilizing the muscle fiber membrane during contraction. Dystrophin binds actin inside the cell and connects to a transmembrane complex of dystroglycans and sarcoglycans, which in turn anchors to laminin in the extracellular matrix. This linkage distributes the forces generated during muscle contraction across the membrane, preventing tears in the sarcolemma. When this complex is disrupted, the membrane becomes fragile, leading to muscle fiber damage and degeneration, as seen in conditions like Duchenne muscular dystrophy.

Calcium release from the sarcoplasmic reticulum is controlled by calcium channels and signaling pathways, not directly by dystrophin. Increasing sarcolemma stiffness independent of contraction isn’t the primary function of the complex; its main role is membrane stabilization under mechanical stress. Dystrophin does not bind actin to myosin directly during contraction—the contractile interaction is between actin and myosin filaments, while dystrophin serves as the membrane-linking scaffold.