The action potential causes current to flow deep within the skeletal muscle fiber by which mechanism?

The action potential causes current to flow deep within the skeletal muscle fiber by which mechanism?

• A. Transmission along T tubules
• B. Electrotonic spread of potential from one myofibril to the next
• C. Electrogenic Na-K pump activity at the sarcoplasmic reticulum
• D. Release of sodium from the sarcoplasmic reticulum

Answer: (A)

The action potential reaches deep into the muscle fiber by traveling along transverse tubules. These are invaginations of the sarcolemma that dive into the cell, carrying the depolarizing current from the surface deep into the interior. This rapid spread into the interior allows the voltage change to activate the signaling machinery (DHPRs) in the T-tubule membranes, which in turn triggers calcium release from the sarcoplasmic reticulum to initiate contraction. The other ideas—electrotonic spread through cytoplasm, Na-K pump activity, or sodium release from the SR—don’t provide a pathway for propagating the depolarization into the fiber interior.

The action potential reaches deep into the muscle fiber by traveling along transverse tubules. These are invaginations of the sarcolemma that dive into the cell, carrying the depolarizing current from the surface deep into the interior. This rapid spread into the interior allows the voltage change to activate the signaling machinery (DHPRs) in the T-tubule membranes, which in turn triggers calcium release from the sarcoplasmic reticulum to initiate contraction. The other ideas—electrotonic spread through cytoplasm, Na-K pump activity, or sodium release from the SR—don’t provide a pathway for propagating the depolarization into the fiber interior.