MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A researcher is studying the effects of a new drug on muscle contraction. They observe that the drug inhibits the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm of muscle cells. Based on this observation, what stage of muscle contraction would be most directly affected?
- ABinding of acetylcholine to receptors on the sarcolemma
- BReuptake of calcium into the sarcoplasmic reticulum
- CExposure of myosin-binding sites on actin
- DATP hydrolysis for cross-bridge cycling
Show answer & explanationAnswer & explanation
Correct answer: C. Exposure of myosin-binding sites on actin
Calcium release from the sarcoplasmic reticulum is crucial because it binds to troponin, causing a conformational change that moves tropomyosin away from the actin binding sites, allowing myosin to bind and initiate contraction.
Why the other options are wrong
- A. Acetylcholine binding initiates the action potential on the sarcolemma, which then triggers calcium release; inhibition of release occurs after this step.
- B. Reuptake of calcium is for muscle relaxation, whereas the question describes an inhibition of *release* for contraction.
- D. ATP hydrolysis occurs after myosin has bound to actin; calcium release precedes this.
Excitation-Contraction Coupling
The physiological process of converting an electrical stimulus (action potential) to a mechanical response (muscle contraction), primarily involving the release and binding of calcium ions.
- Action potential travels along T-tubules.
- Triggers Ca2+ release from sarcoplasmic reticulum.
- Ca2+ binds to troponin, exposing actin active sites.
Memory trick: ACH (Acetylcholine) starts it, Ca (Calcium) uncovers it, Myosin pulls it, ATP fuels it.