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A Summary Ion Channels in the Nerve-Cell Membrane In this article Richard D Keynes details the workings of ion channels in nerve cell membranes Nerve impulses action potentials are the unit by which information travels in an organisms nervous system and the generation of this action potential is dependent on the nerve membrane being permeable to ions which in turn makes said membrane excitable Electrical activity of a nerve is triggered by a depolarization across the membrane and this also causes the sodium channels to open and allow sodium ions to flow inward due the electrochemical gradient Eventually the membrane potential falls to zero the sodium channels close and potassium channels open allowing potassium ions back in to the cell thus restoring the resting potential It is this exchange of ions that provided the immediate energy for the propagation of a nerve impulse The experimental technique that illustrated the different activities and timing of the opening and closing of sodium and potassium channels was the use of voltage-clamps Voltage-clamps allowed researchers to hold an axon at a predetermined membrane potential and observe the behavior of the ion channels at those levels Voltage-clamping has also been employed in the study of the selectivity of ion channels Bertil Hille collected evidence of four energy barriers in a sodium channel that prevent other ions from passing through and only allow one sodium ion through at a time The highest of these barriers results from the fact that sodium ions readily lose their stabilizing water molecules when they interact with the ionized carboxylic acid groups in the channel wall and are thus able to pass through the channel On the other hand the larger potassium ions do not interact correctly with the carboxylic acid group and therefore cannot surmount the energy barrier to pass through Hille also proposed that the electronegative and conformational properties of the molecules in the sodium channel also contribute to
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