Which eukaryotic organelle is described as a membranous channel extending inward from the muscle fiber membrane and plays a key role in transmitting action potentials?

• A. Sarcoplasmic reticulum
• B. Golgi apparatus
• C. Mitochondrion
• D. Transverse (T) tubule

Answer: (D)

Verified step by step guidance

1. Understand the problem: The question asks about a eukaryotic organelle that is a membranous channel extending inward from the muscle fiber membrane and is involved in transmitting action potentials. This is a structural and functional question related to muscle physiology.
2. Clarify the role of each option: The sarcoplasmic reticulum stores and releases calcium ions during muscle contraction. The Golgi apparatus is involved in packaging and transporting proteins, not muscle contraction. The mitochondrion is responsible for energy production (ATP synthesis) but does not transmit action potentials.
3. Focus on the correct answer: The transverse (T) tubule is a membranous channel that invaginates from the sarcolemma (muscle fiber membrane) into the muscle cell. It plays a critical role in transmitting action potentials deep into the muscle fiber to ensure synchronized contraction.
4. Explain the mechanism: When an action potential travels along the sarcolemma, it enters the T-tubules. This allows the signal to reach the sarcoplasmic reticulum, triggering calcium release and initiating muscle contraction.
5. Summarize the key concept: The transverse (T) tubule is essential for efficient communication between the muscle fiber membrane and the interior of the muscle cell, ensuring proper contraction during excitation-contraction coupling.