In tetany hyperexcitability is due to

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.