Describe the role of t-tubules and terminal cisternae in skeletal muscle excitation-contraction coupling.

• A. T-tubules propagate action potentials into the muscle fiber and trigger Ca2+ release from the terminal cisternae via ryanodine receptors.
• B. T-tubules store calcium and release it passively from the sarcoplasmic reticulum.
• C. Terminal cisternae produce action potentials along the sarcolemma.
• D. Ca2+ is released directly from the cell membrane without involvement of the sarcoplasmic reticulum.

Answer: (A)

Excitation-contraction coupling in skeletal muscle hinges on electrical signals driving a chemical release of calcium from the storage sites inside the muscle fiber. When an action potential travels along the sarcolemma, it quickly invades the fiber through the transverse (T) tubules, bringing the depolarization deep into the interior. The terminal cisternae of the sarcoplasmic reticulum, arranged on either side of each T-tubule to form a triad, are ready to respond. The voltage sensors in the T-tubule membrane (the DHPRs) are mechanically linked to Ca2+ release channels (RyR1) on the terminal cisternae. The depolarization prompts RyR1 to open, releasing Ca2+ from the sarcoplasmic reticulum into the cytosol. The rise in intracellular Ca2+ binds to troponin C, causing a shift in tropomyosin that uncovers the myosin-binding sites on actin, allowing cross-bridge cycling and muscle contraction. After contraction, Ca2+ is pumped back into the SR by SERCA pumps to restore relaxation.

This explains why the other ideas don’t fit: Ca2+ release is not stored or released passively from the SR without triggering by the voltage signal, nor is the action potential generated by terminal cisternae itself. The action potentials originate at the sarcolemma, and Ca2+ is released from the sarcoplasmic reticulum rather than directly from the cell membrane.