MCAT® ExamBiological and Biochemical Foundations of Living SystemsEasy
A patient experiences symptoms of muscle weakness, fatigue, and difficulty coordinating movements. Genetic testing reveals a mutation affecting the sarcoplasmic reticulum's ability to release calcium ions. Which stage of excitation-contraction coupling is most directly impaired by this mutation?
- ATransmission of action potential along the T-tubules
- BAction potential generation at the neuromuscular junction
- CRelease of calcium from the sarcoplasmic reticulum
- DBinding of myosin to actin
Show answer & explanationAnswer & explanation
Correct answer: C. Release of calcium from the sarcoplasmic reticulum
Excitation-contraction coupling involves a sequence of events. The specific mutation described directly affects the sarcoplasmic reticulum's ability to release calcium ions, which is a critical step in this process, leading to the observed muscle weakness.
Why the other options are wrong
- A. Transmission along T-tubules also precedes calcium release; an impairment here would affect calcium release indirectly.
- B. Action potential generation at the neuromuscular junction precedes calcium release and would be a different impairment.
- D. Binding of myosin to actin occurs after calcium release and is dependent on it; this is a downstream effect, not the primary impairment described.
Excitation-Contraction Coupling
The physiological process that links electrical excitation (action potential) of the sarcolemma to mechanical contraction (muscle shortening) by releasing calcium ions from the sarcoplasmic reticulum.
- Action potential travels along sarcolemma and T-tubules.
- Dihydropyridine receptors (DHPR) on T-tubules change conformation.
- DHPR interacts with Ryanodine receptors (RyR) on SR.
- RyR opens, releasing Ca2+ from SR into sarcoplasm.
- Ca2+ binds to troponin, initiating cross-bridge cycling.
Memory trick: ECC: Electricity Calls Calcium for Contraction!