dd up to make curve A.The overall effect is of a wave of current washing in and out of the cell.The initial balance between the electric potential and the force drivingthe ions across the membrane is disturbed. It swings first one way assodium pushes into the cell, then the other way as potassium rushes out. Ifthere was no clamp around, the current surge would make the voltage swingwildly: that swing in voltage is the action potential. And it would changethe porosity of the membrane nearby.Imagine the action potential running along an axon like a bead along athread. At the front edge of the bead, sodium is moving into the cell;behind it, potassium is flowing out. In front of the bead, the oncomingsodium current is increasing the voltage across the membrane; once thevoltage passes a certain level, the membrane becomes porous to sodium ions.The action potential has arrived. Thus the ring of activity movesforward--a pulse running along a nerve.Dr Hodgkin and Dr Huxley had little time for generalisations, so they wentto remarkable lengths to develop their story.They calculated the number of ions that crossed the membrane in an actionpotential and showed that it agreed with Dr Keynes's radioactive results.They showed how the size of the action potential depends on theconcentration of sodium outside the nerve; the less sodium, the less theforce pushing sodium into the cell when the membrane becomes porous.With data from a whole range of voltages, they used standard physicscalculations to work out what shape the action potential should have; theiranswer matched measurements from living nerves almost exactly.The finishing touch, ten years later, was similar in style: a physicalapproach to the nerve. Peter Baker, who worked under Dr Hodgkin, found thathe could extrude a nerve-fibre's innards, as one would squeeze toothpastefrom its tube. As long as the nerve-fibre is refilled with a mixture thatis rich in potassium but poor in sodium, it will go...
