<prompt> In tetany hyperexcitability is due toLow ca causes T permeability to NaPrevent K. releasePrevent Na IC- releaseI.Ca produce generation of AP  <response>: A i.e. Low Ca CausesT Na permeabilityA decrease in extra cellular Ca Concentration increases the excitability of nerve and muscle cells by decreasing the amount of depolarization necessary to initiate the change in Na IC conductance that produce the action potentialQ. In fact Ca needs to fall only 50 below the normal before spontaneous discharge occur from peripheral nerves, often causing tetany.The voltage gated calcium channels are slightly permeable to Na as well as Ca;when they are activated both Na and Ca flow to interior of the fiber. When there is deficit of Ca2 in ECF, Na channels become activated opened by very little change of membrane potential from its very negative resting level to a less negative level. Therefore nerve fibers become highly excitable. Sometime discharging repetitively without provocation rather than remaining in resting stage.Conversely, an increase in ECF Ca stabilizes the membrane by decreasing excitability. 