Because ions are charged and water-soluble, they must move through small tunnels or channels (specialized proteins) that span the cell membrane's lipid bilayer. Each channel is specific for only one type of ion. There are specific channels for sodium ions, potassium ions, calcium ions and chloride ions. These channels make the cell membrane selectively permeable to various ions and other substances (like glucose). The selective permeability of the cell membrane allows the inside to have a different composition than the outside.
For the purposes of nerve signals, we are interested in the following characteristics:
- The outside fluid is rich in sodium, a concentration about 10 times higher than the inside fluid
- The inside fluid is rich in potassium, a concentration about 20 times higher inside the cell than outside.
- There are large negatively charged proteins inside the cell that are too big to move across the membrane. They give the inside of the cell a negative electrical charge compared to the outside. The charge is about 70 to 80 millivolts (mV) -- 1 mV is 1/1000th of a volt. For comparison, the charge in your house is about 120 V, about 1.2 million times more.
- The cell membrane is slightly "leaky" to sodium and potassium ions, so a sodium-potassium pump is located in the membrane. This pump uses energy (ATP) to pump sodium ions from the inside to the outside and potassium ions from the outside to the inside.
- Because sodium and potassium ions are positively charged, they carry tiny electrical currents when they move across the membrane. If sufficient numbers move across the membrane, you can measure the electrical currents.