In: Anatomy and Physiology
Troponin (Tn) is the sarcomeric Ca2+ regulator for striated
(skeletal and cardiac) muscle contraction. On binding Ca2+ Tn
transmits information via structural changes throughout the
actin-tropomyosin filaments, activating myosin ATPase activity and
muscle contraction.
The sarcoplasmic reticulum stores calcium ions, which it releases
when a muscle cell is stimulated; the calcium ions then enable the
cross-bridge muscle contraction cycle.
The neuromuscular symptoms of hypercalcaemia are caused by a
negative bathmotropic effect due to the increased interaction of
calcium with sodium channels. Since calcium blocks sodium channels
and inhibits depolarization of nerve and muscle fibers, increased
calcium raises the threshold for depolarizationThis results in
diminished deep tendon reflexes (hyporeflexia), and skeletal muscle
weakness.
thus decreasing excitability, which leads to hypotonicity of smooth
and striated muscle. This explains the fatigue, muscle weakness,
low tone and sluggish reflexes in muscle groups.