MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient presents with muscle weakness, fatigue, and difficulty coordinating movements. Genetic testing reveals a mutation in a gene encoding a protein crucial for the release of calcium from the sarcoplasmic reticulum into the sarcoplasm. Which of the following steps in excitation-contraction coupling would be directly impaired by this mutation?
- AConformational change in dihydropyridine receptors
- BCalcium binding to troponin
- CBinding of acetylcholine to receptors on the motor end plate
- DDepolarization of the T-tubules
Show answer & explanationAnswer & explanation
Correct answer: B. Calcium binding to troponin
The mutation affects the release of calcium from the sarcoplasmic reticulum. Reduced calcium in the sarcoplasm means less calcium available to bind to troponin, which is essential for initiating muscle contraction by moving tropomyosin.
Why the other options are wrong
- A. Dihydropyridine receptors are involved in sensing the T-tubule depolarization and triggering calcium release, but the mutation is in the SR calcium release protein itself, affecting what happens AFTER the dihydropyridine receptor's action.
- C. Acetylcholine binding initiates the action potential, which precedes calcium release.
- D. T-tubule depolarization occurs prior to calcium release and would not be directly affected by a defect in calcium release channels.
Excitation-Contraction Coupling
Excitation-contraction coupling is the physiological process of converting an electrical stimulus (action potential) to a mechanical response (muscle contraction).
- Involves the release of calcium from the sarcoplasmic reticulum.
- Calcium binds to troponin, causing a conformational change in tropomyosin.
- This exposes actin binding sites for myosin heads, initiating cross-bridge cycling.
Memory trick: A contraction is a 'CALCIUM' cascade.