Question at position 7 The major function of the sarcoplasmic reticulum in skeletal muscle contraction is to: provide a source of myosin for the contraction processstore ATPregulate intracellular calcium concentrationsynthesize actin and myosin myofilaments

Question

Question at position 7 The major function of the sarcoplasmic reticulum in skeletal muscle contraction is to: provide a source of myosin for the contraction processstore ATPregulate intracellular calcium concentrationsynthesize actin and myosin myofilaments

BlackTom AI · Accepted Answer

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum in muscle fibers whose primary role is to control intracellular calcium (Ca2+) concentration, which directly regulates contraction and relaxation.

1. Relevant knowledge points and concepts
- The SR stores Ca2+ bound in its lumen and releases it rapidly into the cytosol when the muscle fiber is excited.
- Excitation–contraction coupling: action potentials travel along the sarcolemma and into T-tubules, triggering SR Ca2+ release.
- Ca2+ binds troponin C, causing tropomyosin to move off actin’s myosin-binding sites so cross-bridge cycling can occur.
- Relaxation requires active reuptake of Ca2+ into the SR by SERCA (an ATP-driven Ca2+ pump).

2. Step-by-step reasoning (mechanistic sequence)
a) Motor neuron releases acetylcholine → muscle action potential is generated on the sarcolemma.
b) Action potential propagates into T-tubules and is sensed by voltage-sensitive dihydropyridine receptors.
c) These trigger ryanodine receptors on the SR membrane to open, releasing stored Ca2+ into the cytosol.
d) Cytosolic Ca2+ binds troponin C → tropomyosin shifts → myosin binds actin → cross-bridge cycling produces contraction.
e) To relax, SERCA pumps (using ATP) resequester Ca2+ into the SR, lowering cytosolic Ca2+ and ending contraction.

3. Intermediate steps shown above are the detailed molecular events linking SR function to muscle contraction and relaxation.

4. Why the correct answer is correct
Because the SR’s defining structural and biochemical specialization is Ca2+ storage and regulated release/reuptake, its major function in skeletal muscle contraction is to regulate intracellular calcium concentration, making contraction possible and controlling its termination.

5. Why the other options are incorrect
- “Provide a source of myosin”: Myosin is a contractile protein synthesized on ribosomes and assembled into filaments; the SR does not supply myosin.
- “Store ATP”: ATP is produced mainly by mitochondria and glycolytic pathways; the SR does not function as an ATP store.
- “Synthesize actin and myosin myofilaments”: Protein synthesis occurs on ribosomes (rough ER/cytosol); the SR is not the site of filament synthesis.

6. Conclusion
The SR’s major function is to regulate intracellular calcium concentration — this matches the provided correct answer, “regulate intracellular calcium concentration.”

Question at position 7 The major function of the sarcoplasmic reticulum in skeletal muscle contraction is to: provide a source of myosin for the contraction processstore ATPregulate intracellular calcium concentrationsynthesize actin and myosin myofilamentsSingle choice

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