Which structure is formed by enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules?

• A. Terminal cisternae
• B. T tubules
• C. Myofibrils
• D. Z discs

Answer: (A)

It focuses on how Ca2+ release sites are organized for excitation-contraction coupling in skeletal muscle. Terminal cisternae are enlarged regions of the sarcoplasmic reticulum that surround the transverse tubules. These enlarged SR sacs, together with the T-tubule, form the triad that enables rapid, coordinated release of Ca2+ into the cytosol when an action potential travels along the T-tubule, triggering contraction. The transverse tubules themselves are invaginations of the muscle cell membrane that propagate electrical signals inward; myofibrils are the contractile elements within the fiber, and Z discs are the boundaries between sarcomeres. Since the description specifically refers to enlarged SR areas around the T-tubules, the structure described is terminal cisternae.