A number of substances alter nerve fiber membrane permeability to ions. For example, calcium ions are needed to close sodium channels in nerve fiber membranes during an action potential. Consequently, if calcium is deficient, sodium channels remain open, and sodium ions diffuse through the membrane again and again so that impulses are transmitted repeatedly. If these spontaneous impulses travel along nerve fibers to skeletal muscle fibers, the muscles continuously spasm (tetanus or tetany). This can occur in women during pregnancy as the developing fetus uses maternal calcium. Tetanic contraction may also occur when the diet lacks calcium or vitamin D, or when prolonged diarrhea depletes the body of calcium.
A small increase in the concentration of extracellular potassium ions causes the resting potential of nerve fibers to be less negative (partially depolarized). As a result, the threshold potential is reached with a less intense stimulus than usual. The affected fibers are very excitable, and the person may experience convulsions.
If the extracellular potassium ion concentration is greatly decreased, the resting potentials of the nerve fibers may remain so negative that action potentials cannot occur. In this case, impulses are not triggered, and muscles become paralyzed.
Certain anesthetic drugs, such as procaine, produce special effects by decreasing membrane permeability to sodium ions. These drugs present in the tissue fluids surrounding a nerve fiber prevent impulses from passing through the affected region. Consequently, these drugs keep impulses from reaching the brain, preventing the perception of touch and pain.