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Tetany is the point at which signals from nerves (action potentials) are arriving to skeletal muscle rapidly enough in succession to cause a steady contraction, and not just a series of individual twitches.
Tetany can be demonstrated by applying quick electrical charges to the skin close to where a nerve lies (such as by the elbow near the ulnar nerve). If these charges are coming once a second, the hand muscles (which the ulnar nerve supplies) will flex once a second. If the frequency is increased, the hand will close up, and individual twitches will not be seen.
To move a muscle, the brain sends an action potential to the muscle at a very fast rate, and the muscle contracts smoothly.
Low serum levels of calcium can produce a condition called Tetany. In tetany, the skeletal muscle fibers are persistently contracted because they are 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 is simply low Calcium levels and the cell thinks that it is depolarized and the channels will open.