<prompt> In tetany the increased membrane excitability is caused by:Decreased release of inhibitory neurotransmitter from nerve terminalsDepolarization of the nerve and muscle membranesSpontaneous release of calcium from the sarcoplasmic reticulumActivation of sodium channels at more negative membrane potentials  <response>: 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. 