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.
Decreased release of inhibitory neurotransmitter from nerve terminals
Depolarization of the nerve and muscle membranes
Spontaneous release of calcium from the sarcoplasmic reticulum
Activation of sodium channels at more negative membrane potentials