What is the biological role of the sarcoplasmic reticulum (SR) in the regulation of skeletal muscle contraction and relaxation?

Answer and explanation

Correct answer: D. Stores calcium ions and releases them into sarcoplasm upon T-tubule depolarization

  1. Step 1: The sarcoplasmic reticulum is a modified endoplasmic reticulum specialized for calcium handling in striated muscle.
  2. Step 2: In the relaxed state, it sequesters against a steep concentration gradient using ATP-powered SERCA pumps.
  3. Step 3: When an action potential depolarizes the adjacent transverse T-tubules, voltage-sensitive dihydropyridine receptors open ryanodine receptor channels on the SR, flooding the sarcoplasm with to trigger contraction.
  4. Step 4: Following excitation, SERCA pumps rapidly clear back into the SR lumen, restoring relaxation.
Answer
It stores calcium ions during relaxation and releases them into the sarcoplasm upon depolarization, then actively re-sequesters them via Ca2+-ATPase pumps to induce relaxation
Why not the others: 'It synthesizes and stores glycogen...' describes cytoplasmic glycogen rosettes, not SR function; 'It conducts action potentials...' is the function of the transverse T-tubule invaginations; 'It hydrolyzes myosin heads...' is catalyzed by the myosin ATPase active site.

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