This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle, Subject name: Physiology
The coupling agent between electrical excitation and contraction of a muscle is said to be: 
Select and explain. 
A. Dihydropyridine receptor (DHPR), B. Ryanodine receptor, C. Ca++, D. Troponin C
C: Ca++
Answer: C: Ca++ 
Explanation: Coupling: electrical excitation travels into muscle, releases calcium & this calcium combines with troponin & causes muscle contraction. Events tills the release of calcium is electrical & after that is mechanical. T tubules contain DHPR & L tubules contain RYR.