usedto "clamp" the membrane at a particular voltage. With the membranepotential fixed by this first pair of electrodes, the second pair could beused to measure the resulting membrane current.Dr Hodgkin and Dr Huxley had found a way around the problems ofall-or-nothing action potentials. Like the good physicists the war had madethem, they had succeeded in controlling one variable--the potential--andhad thus won the freedom to explore how the other variable--the membranecurrent--depended upon it.The diagram summarises one set of results. It shows the currents that flowat a spot on the membrane if the membrane potential is suddenly clamped ata new value, higher than its resting value. Curve A is taken from a nervebathed in a fluid that is rich in sodium ions, as it would be in the body.At first, charge flows into the cell; within a millisecond, it begins toflow out again.Richard Keynes, one of Dr Hodgkin's students, had used radioactive isotopesof sodium and potassium to show that the two elements moved in and out ofthe nerve cell when it was stimulated. Armed with this information, DrHodgkin and Dr Huxley could explain what was happening. Having realisedthat changes in porosity lead to changes in voltage, they now argued thatchanges in voltage lead to changes in porosity, as well.Clamping the voltage at above its resting value makes the membrane porousto positively charged sodium ions. They flood into the cell from outside,where their concentration is high, bringing their positive charge withthem. That influx provides a sudden and transient inward current, seen incurve B.This leakiness to sodium is only transitory: the sodium current soon diesaway to nothing. Instead the membrane becomes porous to potassium. The flowof potassium was isolated and measured by looking at a cell bathed in afluid containing no sodium ions: the result is shown in curve C. Potassiumflows out of the cell, carrying positive charge with it. Curves B and Ctogether a...
