In tetany the increased membrane excitability is caused by:: (A) Decreased release of inhibitory neurotransmitter from nerve terminals, (B) Depolarization of the nerve and muscle membranes, (C) Spontaneous release of calcium from the sarcoplasmic reticulum, (D) Activation of sodium channels at more negative membrane potentials
Answer is B. 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.