What are tubular infoldings of the sarcolemma that penetrate through the cell and emerge on the other side and contain voltage-gated channels to rapidly conduct action potentials into the cell interior?

• A. T tubules
• B. Mitochondria
• C. Nuclei
• D. Golgi apparatus

Answer: (A)

T-tubules are tubular infoldings of the sarcolemma that extend into the muscle fiber. They carry voltage-gated channels so an action potential spreads quickly from the surface into the interior, ensuring the electrical signal reaches deep parts of the cell. This depolarization triggers calcium release from the sarcoplasmic reticulum through coupling between the voltage-sensing receptors in the T-tubule membrane and calcium-release channels in the SR, enabling a coordinated contraction. They are typically found forming triads with terminal cisternae at the A–I band junction, which optimizes the timing of calcium release. Mitochondria, nuclei, and the Golgi apparatus serve other roles and do not mediate this rapid conduction into the cell interior.