Calcium affects nerve and muscle excitability, neurotrans-mitter release from axon terminals, and excitation-contraction coupling in muscle cells. It serves as a second or third messenger in several intracellular signal transduction pathways. Some enzymes use calcium as a cofactor, including some in the blood-clotting cascade. Finally, calcium is a major constituent of bone.
Of all these roles, the one that demands the most careful regulation of plasma calcium is the effect of calcium on nerve excitability. Calcium affects the sodium permeability of nerve membranes, which influences the ease with which action potentials are triggered. Low plasma calcium can lead to the generation of spontaneous action potentials in nerves. When motor neurons are affected, tetany of the muscles of the motor unit may occur,- this condition is called hypocalcemic tetany. Latent tetany may be revealed in certain diagnostically important signs. Trousseau's sign is a characteristic spasm of the muscles of the forearm that causes flexion of the wrist and thumb and extension of the fingers. It may occur spontaneously or be elicited by inflation of a blood pressure cuff placed on the upper arm. Chvostek's sign is a unilateral spasm of the facial muscles that can be elicited by tapping the facial nerve at the point where it crosses the angle of the jaw.
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