Why does lower blood calcium levels (or lower calcium levels in ECF) cause nervo

Question

Why does lower blood calcium levels (or lower calcium levels in ECF) cause nervous hyperexcitaton? Why does it cause over stimulation of nerves and muscles and spasmic contractions of muscles?
This is why undersecretion of parathormone causes parathyroid tetany.
I am aware of the role of calcium in opening the synaptic vesicles for transmission of impulses and the role of calcium in muscle contraction but fail to understand how that might hep me understand the overexcitation of nerves and spasmic contraction of muscles. It actually seems that higher calcium in ECF might cause over stimulation and spasms of muscles due to sustained contraction.

Explanation / Answer

It is a question of transmembrane potential. Ca++ being a cation, means that if you decrease the amount of ionized calcium in the extra cellular fluid, it conceptually is nearly equivalent to having a more positively charged intracellular fluid. This in turn means that the cell will be closer to its threshold potential for depolarization, therefore accounting for its hyper excitability.

What you should keep in mind, is that when we say something as: "this cell has a transmembrane potential of -70 mV", we always define it relatively to the extra cellular fluid.

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