Sunday, April 1, 2012
Hypocalcemic tetany is the involuntary tetanic contraction of skeletal muscles that occurs when the extracellular calcium concentration falls to about 40 percent of its normal value. This may seem surprising, since calcium is required for excitation-contraction coupling. However, we must bear in mind, that this calcium is sarcoplasmic reticulum calcium, not extracellular calcium. The effect of changes in extracellular calcium is exerted not on the sarcoplasmic reticulum calcium, but directly on the plasma membrane. Low extracellular calcium (hypocalcemia) increases the opening of sodium channels in excitable membranes, leading to membrane depolarization and the spontaneous firing of action potentials. It is this that causes the increased muscle contractions.