What causes tetany? Explain at molecular level.
(I've read an article on Wikipedia about how low extracellular Ca2+ cause involuntary muscle contraction/tetany but I don't get it at all.)
Tetany is characterized by severe spasms of the hands and feet. It is an involuntary muscle contraction. The main trigger of tetany is low blood levels of calcium and magnesium, which increase the activity of nerve tissue nonspecifically, also can produce tetanic cramps. At molecular level tetany is caused by activation of nerve cells, which then stimulate the muscles. Low plasma calcium increases the permeability of neuronal membranes to sodium ions, causing a progressive depolarization. This increases the ease with which action potentials can be initiated. If the plasma Ca2+ decreases to less than 50% of the normal value action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscle.
In the other hand, calcium controls the contraction and relaxation of muscles. In the case of low blood calcium level muscles spasm and cannot relax. When muscle fibers contract, the head of the muscle proteins, also known as myofibrils, attach to strings known as actin filaments. Calcium is needed for the disassociation of myofibrils from actin in order for muscle relaxation. Because of muscle`s cell contains small stores of calcium they are released when a muscle needs to relax. But if calcium levels are too low muscles will be unable to relax, resulting in tetany.