This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Contractility of cardiac muscle depends on 
Select and explain. 
A. On T tubules, B. On T tubules and extracellular Ca2+, C. On extracellular Ca2+, D. On intracellular Ca2+
B: On T tubules and extracellular Ca2+
Answer: B: On T tubules and extracellular Ca2+ 
Explanation: Excitation and contraction coupling is there in cardiac muscle. When an action potential reaches the T tubules there is activation of Dihydropyridine channels. This causes influx of extracellular calcium into sarcoplasm. Ref: Textbook of medical physiology by N Geetha, 2nd edition, page no. 53