Tetany is a case of involuntary muscle convulsion. It is a muscular physical state at which action potentials from nerves arrive to the skeletal muscle motor end plate rapidly enough in succession to cause a steady contraction. This differs from individual twitches in that the twitches are summed. A single twitch may elicit from one action potential at the motor end plate.
Tetany can be demonstrated by applying quick electrical pulses to the skin close to where a nerve lies (such as by the elbow near the ulnar nerve), or the sciatic nerve. If the frequency of charge is once per second, the hand muscles (which the ulnar nerve supplies) will flex once per second. If the frequency is increased, the muscle contractions will sum and appear as one smooth contraction. The hand will smoothly close.
Low serum levels of calcium can produce a medical sign called tetany. In this case, the skeletal muscle fibers are persistently contracted because they are not persistently depolarized, and this is occurring because the serum calcium is very low - without replacement. If serum calcium was low with replacement with another divalent cation, such as magnesium, then there would not be persistent contraction because magnesium would competitively inhibit the voltage gated calcium channels, thereby inhibiting calcium's entry. However, in tetany, there are simply low calcium levels and the cell thinks that it is depolarized.
