text: Substantial decreases in extracellular Ca ion concentrations may result in a condition known as hypocalcemic tetany, which is marked by spontaneous motor neuron discharge. In addition, severe hypocalcaemia will begin to affect aspects of blood coagulation and signal transduction.
  Ca ions can damage cells if they enter in excessive numbers (for example, in the case of excitotoxicity, or over-excitation of neural circuits, which can occur in neurodegenerative diseases, or after insults such as brain trauma or stroke). Excessive entry of calcium into a cell may damage it or even cause it to undergo apoptosis, or death by necrosis. Calcium also acts as one of the primary regulators of osmotic stress (osmotic shock). Chronically elevated plasma calcium (hypercalcemia) is associated with cardiac arrhythmias and decreased neuromuscular excitability. One cause of hypercalcemia is a condition known as hyperparathyroidism.