Ion pumps are membrane proteins that serve both as an enzyme and a channel. When concentration differences exist, there is a net molecular movement from a region of high conc. All ions in the body exist in different concentrations inside vs. outside cells i.e., ions have concentration gradients across cells 4. Because ions are charged, they can also be made to move by an electrical field. There are electrically charged ions in solution on either side of the neuronal membrane.
Ions can cross the membrane only by way of protein channels.
The protein channels can be highly selective for specific ions.