What is the role of the sarcoplasmic reticulum and transverse tubules in excitation-contraction coupling?

• A. The SR releases calcium independently of membrane depolarization.
• B. The transverse tubules conduct depolarization that triggers calcium release from the SR primarily through L-type calcium channels.
• C. Calcium release from SR is driven by extracellular calcium entering via voltage-gated channels.
• D. In skeletal muscle, depolarization triggers Ca2+ release via direct coupling between dihydropyridine receptors in the T-tubules and ryanodine receptors on the SR, with CICR being less prominent.

Answer: (D)

When a muscle fiber is stimulated, the electrical signal travels down into the transverse tubules (T-tubules), where the voltage-sensing dihydropyridine receptors (DHPRs) respond to depolarization. In skeletal muscle, these DHPRs are physically linked to the ryanodine receptors (RyR) on the adjacent sarcoplasmic reticulum. This direct mechanical interaction opens RyR channels, releasing Ca2+ from the SR into the cytosol. The sudden rise in intracellular Ca2+ allows troponin C to trigger actin-myosin cross-bridge cycling and contraction.

Extracellular Ca2+ entry through L-type channels is not the primary trigger here; the depolarization itself, via direct DHPR–RyR coupling, is sufficient. Calcium-induced calcium release (CICR), where Ca2+ entering from outside the cell triggers further Ca2+ release from the SR, is less prominent in skeletal muscle compared to cardiac muscle.

So, the key idea is direct coupling between DHPRs in the T-tubules and RyRs on the SR to initiate Ca2+ release, with CICR playing only a minor role.