Tetany - Sodium Permeability

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So I was doing a passage about PTH and Calcitonin. There is a question that asks what effect would a person with hypoparathyroidism would get. The answer is neuronal excitability. The reason is that hypoparathyroidism would lead to low PTH which in turns lead to low extracellular Ca concentration. Low Ca increases sodium permeability of neurons, leading to self depolarization and tetany. I understand all this but I do not understand why low extracellular Ca concentration would lead to increased sodium permeability 😕

 

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dont try to understand this, just remember that low EC Ca causes hyper excitability of neurons, as Ca normally has a dampening effect on the activity of sodium channels