A Summary Ion Channels In The NerveCell

A Summary: Ion Channels In The Nerve-Cell Membrane Essay, Research Paper 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 organism?s 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 its selectivity. Two important tools in the study of sodium channels and their voltage-sensitive gating mechanism have been the nerve poisons, tetrodotoxin and saxitoxin. These two toxins bind specifically to sodium channels and effectively block them. Because

only one molecule of each toxin binds to each sodium channel, these toxins were used in a bioassay to count the number of sodium channels on the membrane of an axon. Results proved that small axons have the fewer sodium channels per square micrometer than large axons. This result also satisfied previous calculations of how many channels would be necessary to obtain the maximum conduction velocity in a 500 micrometer axon. Tetrodotoxin has also been enormously valuable in the study of ionic gating. The mechanism that controls the opening and closing of ion channels involves the movement of charged particles that result in a small charge displacement. However since this gating Ayele, 2 current is so much smaller that the ionic current through the channel, it was nearly impossible

to measure. Tetrodotoxin enables this measurement by blocking the sodium channels (potassium ions are also blocked in this experiment) and stopping the ionic current, while still allowing the opening and closing of the sodium channels. Using this technique, the gating current was found to rise and then fall to zero when the (approximately) three or four charged particles reached their new configuration. The mechanism that closes the sodium channels was found to be electrically silent. Sodium channels appear to have three operational states. They are either at rest, conducting, or inactivated. The molecular model of a sodium channel has not yet been described however. This is due the many complexities of the channel including its complex kinetics, and hydrophobic and hydrophilic