Question

Depolarization of the T-tubule membrane activates the sarcoplasmic reticulum via the:

• A. Ryanodine receptor
• B. Dihydropyridine receptors
• C. Increased Na$^+$ and K$^+$ conductance in end-plate membrane
• D. IP$_3$ receptor

Hint:

In skeletal muscle: T-tubule depolarization $\rightarrow$ \textbf{DHPR activation} $\rightarrow$ Ryanodine receptor opening $\rightarrow$ Ca$^{2+}$ release.

Answer 1

The Correct Option is B

Step 1: In skeletal muscle fibers, T-tubules are invaginations of the sarcolemma that conduct action potentials deep into the muscle fiber.
Step 2: Depolarization of the T-tubule membrane activates dihydropyridine receptors (DHPR).
Step 3: Dihydropyridine receptors act as voltage sensors in the T-tubule membrane.
Step 4: These receptors are mechanically coupled to ryanodine receptors present on the sarcoplasmic reticulum (SR).
Step 5: Activation of DHPR causes opening of ryanodine receptors, leading to the release of Ca$^{2+$ ions} from the sarcoplasmic reticulum into the cytosol, initiating muscle contraction.