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 been seen.
If we want to move a muscle in our body, our brain sends action potentials to the muscle at a very fast rate, and the muscle contracts smoothly.
Diseases and other conditions that increase action potential frequency cause unwanted contraction of muscles. The disease tetanus blocks inhibition to the neurons that supply muscles, causing them to contract.
When the membrane potential is upset, for instance by low levels of ions (such as calcium) in the blood (hypocalcemia), neurons will depolarise too easily. As a result, too many action potentials are sent to muscles causing spasm.
