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The cellular matrix and the metabolic machinery of the cell maintain large differences in concentrations of various chemical entities between the inside and the outside of the cell. For the nerve impulse, the distributions of Na⁺ and of K⁺ are of special importance. The cell maintains across its membrane a concentration ratio for K⁺ of about 1:10 outside to inside and of about 20:1 outside to inside for Na⁺. Thus a profound gradient favors diffusion of K⁺ outward and of Na⁺ inward through the membrane. The large diffusion gradients of K⁺ outward and of Na⁺ inward are responsible for the membrane potential of the cell. These ions provide the electric charges that move as current through the conductance channels. In the resting membrane, the conductance ("ease of passage") through the channels is 25 times greater for K⁺ than for Na⁺. This difference is reversed during the action potential.

 
 
Figure 4. Na⁺ and K⁺ gradients across the membrane.

 
 
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