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readerL5 - Action Potentials Communication between neurons...

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Action Potentials (3) The action potential starts with a rapid depolarization of the membrane potential. This rising phase reaches a peak, and is then followed by a repolarization ( falling phase ) back to resting membrane potential and then an after hyperpolarization (undershoot) below rest. Communication between neurons in the nervous system often requires that messages travel long distances down axons without loss of information. Neurons use action potentials to do this. (5) All of these events can be understood by examining the behavior of ion channels and the fl ow of ions across the cell membrane. (1) The action potential is a brief, transient change in membrane potential that sweeps down the membrane of an axon. (2) The action potential is triggered by a depolarization of the membrane above action potential threshold . If a depolarization is not large enough to reach threshold, the neuron will not fi re an action potential. (4) Following the fi ring of an action potential, there is a period called the absolute refractory period during which it is not possible to fi re an action potential. This absolute refractory period is followed by a relative refractory period during which an action potential can occur, but only if the membrane is depolarized to a much higher level than the normal threshold. Page 5-1
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What causes the rising phase of an action potential? (1) Let’s revisit the GHK equation that determines the membrane potential (at room temperature): V m = 58 log ( p K [K] o + p Na [Na] o + p Cl [Cl] i ) / ( p K [K] i + p Na [Na] i + p Cl [Cl] o ) What happens if the permeability to potassium ions increases dramatically? Intracellular Extracellular Na + K + Cl - A - 117 mM 3 mM 120 mM 0 mM 30 mM 90 mM 4 mM 116 mM What happens if the permeability to sodium ions increases dramatically? What happens if the permeability to chloride ions increases dramatically? (2) What’s the equilibrium potential for these three ions in neuron? Nernst equation: (3) When scientists fi rst thought about what ions might be responsible for the action potential, they immediately proposed that the rising phase might be due to sodium entry into the cell. Why was sodium an obvious choice? Page 5-2
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Experiments to test whether sodium really is driving the rising phase of the action potential Action potentials were recorded in a squid giant axon bathed in normal sea water (similar to the squid extracellular fl uid) and then in solutions containing lower concentrations of sodium. (1) What will lowering external sodium concentrations do to the driving force on sodium ions?
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