One of several membrane pumps—consisting of a specialized MOLECULE embedded in a MEMBRANE—that actively pumps ions across membranes against the CONCENTRATION GRADIENT. An ION Can exist in different concentrations on either side of neuronal membranes, and even when neurons are at rest (see MEMBRANE POTENTIAL) there is a flow of these ions across membranes. However, ACTION POTENTIAL generation causes very significant changes in the concentrations of ions on either side of neuronal membranes: principally, sodium ions flow in and potassium ions out.
In order to restore the appropriate balance and prevent accumulation of sodium ions within neurons, the sodium-potassium pump is activated. The relative concentrations of sodium (Na+) and potassium I (K+) ions activates SODIUM-POTASSIUM ATPASE, an ENZYME that degrades ADENOSINE TRIPHOSPHATE (ATP) which in turn leads to the production of energy to fuel the pump. The sodium-potassium pump does not simply exchange the one ion for the other: approximately three sodium ions are pumped out of the neuron for every potassium ion that is pumped in. Since the action potential sweeps along an axonal membrane, sodium-potassium pumps exist in considerable density: approximately 750 sites per square micrometer.
