Upon excitation, sodium ions penetrate into nerve cells which have to be pumped out after termination of excitations. There are several types of sodium pumps. The most important is the “sodium-potassium ATPase” which pumps sodium ions out of the neuron in exchange for potassium ions which are brought into the cell, to maintain the equilibrium of extracellular-intracellular concentration gradients for these two ions. The pump requires energy which is supplied by ATP in a stoichiometric fashion: one ATP for exchange of 3 sodium ions against 2 potassium ions. Since more positive charges are brought out of the neuron, the pump is “electrogenic,” i.e., it increases the hyperpolarization of the neuron.

 Nociceptors, Action Potentials, and Post-Firing Excitability Changes