Neuron ionic gradients.
<P>
   The  concentrations  of  Na^+ and Cl^- are higher in the extracellular
   fluid than in the cytosol of a neuron.
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   The reverse is true for K^+.
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   These  gradients are maintained by the sodium-potassium pump and cause
   the  ions  to  diffuse  back  across the membrane and create a resting
   potential.