If an intracellular microelectrode is inserted into a nerve or muscle cell, it is found that the inside of the cell is electrically negative to the outside by some tens of millivolts. This potential difference is known as the resting potential. If we slowly advance a microelectrode so that it penetrates the cell, the change in potential occurs suddenly and completely when the electrode tip is in the region of the cell membrane; thus the cell membrane is the site of the resting potential. In this chapter we shall consider some of the properties of the cell membrane that are associated with the production of the resting potential.

Membrane structure

Plasma membranes are usually composed of roughly equal amounts of protein and lipid, plus a small proportion of carbohydrate. Human red cell membranes, for example, contain about 49% protein, 44% lipid and 7% carbohydrate. Intracellular membranes tend to have a higher proportion of protein, whereas the protein content of myelin (p. 49) is only about 23%.

Figure 3.1 shows the chemical structure of some membrane lipids. Phospholipid molecules are esters of glycerol with two long-chain fatty acids, the glycerol moiety being attached via a phosphate group to various small molecules. The fatty acid chains thus form non-polar tails attached to polar heads. The fatty acid chains are usually fourteen to twenty carbon atoms long, and some of them are unsaturated, with one or more double bonds in the chain.