A 45-year-old man presents with perioral numbness, carpopedal spasm, and positive Chvostek sign after thyroid surgery. Serum calcium is 7.2 mg/dL. Which physiological mechanism explains why a reduction in ionised calcium causes increased neuronal excitability?
- A Hypocalcemia increases the resting membrane potential (makes it more positive), lowering the threshold for action potential firing
- B Hypocalcemia directly depolarises nerve membranes by reducing the K+ equilibrium potential
- C Low calcium inhibits the Na+/K+-ATPase, raising intracellular Na+ and depolarizing neurons
- D Calcium normally stabilises voltage-gated Na+ channels in the inactivated state; low Ca2+ increases surface charge, lowers the activation threshold, and facilitates Na+ channel opening ✓
Explanation
Extracellular Ca2+ binds to negative surface charges on the external face of voltage-gated Na+ channels, effectively shifting the voltage-activation curve rightward (requiring more depolarization to activate). When Ca2+ falls, this stabilizing effect is lost: the Na+ channel activation threshold shifts toward the resting membrane potential, making the neuron more excitable even at normal resting potential. This is the physicochemical basis of tetany and paresthesias in hypocalcemia — the membrane is not depolarized, but it is hair-trigger sensitive to minor stimuli.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.