Question

Conduction of depolarization from the surface of a muscle fiber to its inner aspects is accomplished by the

• A. Neuromuscular junction
• B. Actin filaments
• C. Endomysium
• D. Sarcotubular system

Hint:

The triad structure (one central T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum) is the structural basis of excitation-contraction coupling.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Muscle contraction requires the rapid transmission of an action potential from the cell membrane (sarcolemma) deep into the muscle fiber to trigger calcium release.
This process is known as excitation-contraction coupling.

Step 2: Detailed Explanation:
The skeletal muscle fiber is a large, cylindrical cell.
To ensure that all myofibrils contract in unison, the depolarization signal must travel rapidly from the surface of the sarcolemma to the deep interior of the fiber.
This rapid transmission is carried out by the sarcotubular system, which consists of two main membrane networks:
Transverse Tubules (T-tubules): These are deep, finger-like invaginations of the sarcolemma that run perpendicular to the muscle fiber axis.
They carry the action potential from the cell surface deep into the cell interior.
Sarcoplasmic Reticulum (SR): A specialized network of endoplasmic reticulum that stores calcium ions.
Its terminal cisternae sit adjacent to the T-tubules, forming junctions called "triads."
When the action potential travels down the T-tubules, it activates voltage-gated dihydropyridine receptors (DHPR), which mechanically open ryanodine receptors (RyR) on the SR membrane.
This opens calcium channels, releasing calcium into the sarcoplasm to initiate contraction.
Therefore, the sarcotubular system is responsible for conducting this depolarization signal.

Step 3: Final Answer:
Conduction of depolarization from the surface to the inner aspects of a muscle fiber is accomplished by the sarcotubular system.