 The second most important ion is sodium (Na+) and this ion is most prominent outside of the cell.When there is a greater concentration of sodium ions outside of the cell and a greater concentration of potassium ions inside of the cell this will cause a slight negative charge to be present within the cell.This difference across the membrane is what the neuron uses to actually do the work of sending messages from the axon hillock of the neuron all the way down to the presynaptic terminal and then on to the postsynaptic terminal because of the release of neurotransmitter into the synaptic cleft.