Which structure conducts the action potential from the surface into the interior of the muscle fiber?

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

Answer: (A)

The main idea is how the electrical signal that travels along the muscle cell surface actually reaches the inner parts of the fiber to trigger contraction. Transverse tubules are deep invaginations of the sarcolemma that dive into the cytoplasm and run between the myofibrils. This network brings the action potential right into the interior of the muscle cell, placing it near the near-by stores of calcium in the sarcoplasmic reticulum. When the AP reaches these tubules, voltage sensors in their membrane activate calcium release channels on the SR, flooding the cytoplasm with Ca2+ and allowing contraction to begin.

Other cellular structures don’t perform this conduction role. Mitochondria supply ATP, nuclei house genetic material, and the Golgi apparatus handles protein processing and trafficking, but none of them act as the conduits that carry the electrical signal from the surface into the fiber interior.