In tetany the increased membrane excitability is caused by:

Membrane excitability is related to the ease with which depolarization opens Na+ channels. The opening of the Na+ channel in response to depolarization is, in pa, related to the extracellular Ca2+ concentration; the lower the extracellular Ca2+ concentration, the easier it is for Na + channels to open when the membrane depolarizes. Hyperventilation (lowering aerial CO2 tension) decreases extracellular Ca+ concentration by increasing aerial pH. When pH rises, H+ is released from plasma proteins in exchange for Ca and ionized Ca+ concentration decreases.