***********************yCole, who was another great influence on the Cambridge pair, and unluckynot to share their Nobel laurels.ClampedAn axon is a long tubular outgrowth from a cell, wrapped in a cellmembrane. One of the differences between the outside and the inside of thecell is the concentration of various types of ion--atoms carrying electriccharge. To take one example, cells contain a high concentration ofpositively charged potassium ions.If the membrane becomes permeable to potassium ions, they will leak out ofthe cell into the fluid outside. Force of numbers drives them from placeswhere they are concentrated to places where they are scarce. If themembrane stops negatively charged ions joining the exodus, an electricalpotential, or voltage, quickly builds up across the membrane as positivecharge leaves the cell. Eventually that voltage becomes strong enough tostop the flow of potassium. The electrical force encouraging the ions tostay in the cell becomes as strong as the force driving them out.The cell can quickly overturn this balance, though, by making its membraneporous to other ions. These charged particles will flow to where they areless common, just as potassium did, until a new balance between electricityand concentration is struck. To the outside world, the movement of chargeshows up as a sudden change in the voltage across the membrane--an actionpotential.Dr Hodgkin and Dr Huxley realised that they could watch this process as ithappened by looking at ions flow across the membrane of a single nervefibre. They called the moving charge the "membrane current", and set out tomeasure it using Cole's fancy electronic apparatus. They inserted two tinyelectrodes down the middle of the nerve. Since the electrodes could not beallowed to touch, the wide-bore squid nerve-fibre was a godsend. Eachelectrode was connected, through the membrane, to another in the fluidoutside the nerve. Currents in one of these pairs of electrodes were ...
