Hyperexcitability of nerve and muscle when the concentration of extracellular ionized calcium is decreased.

Tetany or tetanic seizure is a medical sign consisting of the involuntary contraction of muscles, which may be caused by disorders that increase the action potential frequency of muscle cells or of the nerves that innervate them. Muscle cramps caused by the disease tetanus are not classified as tetany; rather, they are due to a lack of inhibition to the neurons that supply muscles. Tetanic contractions (physiologic tetanus) have a broad range of muscle contraction types, of which tetany is only one..