A number of substances affect nerve fiber membranes by influencing their permeability to ions.
For example, calcium ions seem to be needed for the closure of the sodium channels in the nerve fiber membrane during an action potential.
Consequently, if calcium is deficient, the sodium channels ma\ remain open, and sodium ions may diffuse through the membrane again and again so that impulses are transmitted repeatedly.
Such spontaneous impulses may travel along the nerve fibers to skeletal muscle fibers.
When this happens, the muscles may undergo continuous spasms (tetany). Such muscular spasms or cramps can sometimes be relieved by raising the level of calcium ions in the body fluids.
Certain drugs, such as procaine and cocaine, produce special effects by decreasing membrane permeability to sodium ions. When one of these drugs is present in the tissue fluids surrounding a nerve fiber, impulses are prevented from passing through the affected region.
Consequently, the drugs are useful as local anesthetics because they help keep impulses from reaching the brain and thus prevent the sensations of touch and pain.
“A decrease in the level of blood calcium, accompanied by tetany, sometimes occurs in women during pregnancy because of the increasing demand for calcium made by the developing fetus Tetany may also occur in persons who have an inadequate supply of calcium or vitamin D in their diets, or who suffer an increased loss of calcium due to prolonged diarrhea”.
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