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The access (conductance) through each kind of channel can be represented as a resistor in an equivalent circuit diagram, illustrated in Fig. 6.

The Na⁺ ions, carrying + charges inward, tend to make the inside positive to the outside. The K⁺ ions, moving outward, tend to make the outside positive to the inside. Imagine all the Na⁺ conductance channels lumped into one resistive (conductive) component, the K⁺ channels into another. The two components are arranged in parallel. Outside, the channels face the external medium; inside they face the cell interior.

 
 
Figure 6. The "double-battery" of the neuron membrane.

 
 
The Na⁺ distribution predicts inside +50 mV, and the K⁺ distribution predicts inside –100 mV (both with respect to outside considered as E_M = 0). In the rest state the inside is about –60 mV, and thus all the ion species contribute, but in unequal degrees, to the membrane potential.

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