Reduced extracellular calcium ion concentrations leads to tetanic convulsions in muscle.

Low ionized calcium levels in the extracellular fluid increase the permeability of neuronal membranes to Na+ ion, causing a progressive depolarization, which increases the possibility of action potentials. This occur because Ca2+ ions interact with the exterior surface of Na+ channels in the plasma membrane of nerve cells. When Ca2+ ions are absent the voltage level required to open voltage gated sodium channels is significantly altered (less excitation is required). If the plasma Ca2+ decreases to less than 50% of the normal value of 9.4 mg/dL, action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscles. Thus, hypocalcemia is a cause of tetany.
In the cardiac muscles, the amount of Ca2+ returned to the extracellular fluid and into the Sarcoplasmic reticulum exactly matches the amounts that entered the cytosol during excitations. And this property of cardiac Ca2+ channels underlines an important feature of cardiac muscle- that it cannot undergo tetanic contractions.