Neurones have a constant resting membrane potential (RMP), set at about -70 mV. The inside is negative relative to the exterior.

The RMP is created by the differences in ionic concentrations across the membrane which are generated by the Na⁺-K⁺ pump, moving K⁺ ions in and Na⁺ ions out of the cell against their concentration gradients (Fig 1).

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Because three Na⁺ ions are lost from the cell in exchange for two K⁺ ions entering the cell, the system is said to have a coupling ratio of 3/2, which itself accounts for part of the RMP.

In addition the membrane is slightly permeable to K⁺ ions (but impermeable to Na⁺). The resulting slow outward leak of K⁺ ions contributes the major share of the RMP.

The active transport of Na⁺ and K⁺ is a major energy-utilizing process, and accounts for almost three quarters of the energy used by nerve cells.