This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Size of action potential is decreased as a result of: March 2010 
Select and explain. 
A. Lower extracellular sodium, B. Raised extracellular calcium, C. Lower extracellular calcium, D. Raised extracellular sodium
A: Lower extracellular sodium
Answer: A: Lower extracellular sodium 
Explanation: Ans. A: Lower extracellular sodium Decreasing the external Na+ concentration decreases the size of the action potential but has little effect on the resting membrane potential. The lack of much effect on the resting membrane potential would be predicted, since the permeability of the membrane to Na+ at rest is relatively low. Conversely, increasing the external K+ concentration decreases the resting membrane potential. A decrease in extracellular Ca2+ concentration increases the excitability of nerve and muscle cells by decreasing the amount of depolarization necessary to initiate the changes in the Na+ and K+ conductance that produce the action potential. Conversely, an increase in extracellular Ca2+ concentration "stabilizes the membrane" by decreasing excitability