lecture 4

lecture 4 - Equilibrium with more than one permeant ion...

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Equilibrium with more than one permeant ion Solution 1 contains 10mM KCl and 1mM NaCl, solution 2 contains 1mM KCl and 10mM NaCl. Membrane is permeable to both K+ and Na+ but not necessarily equally permeable. On one extreme if only permeable to K+, than potential is -58 mV. If only permeable to Na+, than potential is +58. Intermediate values in between. Nernst cannot do this calculation. Requires Goldman equation V=58 log (P k (K)o/(K)i + P na (Na)o/(Na)i + P cl (Cl)i/(Cl)o) The relevant ions in neurons are K+ , Na+, and Cl-
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Resting and Action Potentials Entail Permeabilities to Different Ions
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Rising phase Overshoot phase Falling phase Undershoot phase The action potential
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Voltage dependent membrane permeability • Hodgkin Huxley, hypothesis-action potential explained if there are voltage-gated ion channels. • Would like to measure ion permeability at varying membrane potentials. • Problem-hard to systematically vary the cell potential and also measure ion permeability. • Solution- voltage clamping. Fix membrane potential in a cell without triggering an action potential while measuring ion permeability.
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The Voltage Clamp Method
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Voltage clamp technique Can depolarize or Hyperpolarize cell Holds potential constant and measures ion conductances
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Hodgkin and Huxley 1952 • Do neuronal membranes have voltage-dependent permeabilities? • If so, what ions are changing permeability? • Experiment- Change potential, make it more negative, (hyperpolarize) cell. No currents need to be injected into cell to maintain that potential. • Change potential, depolarize cell, now see both inward and outward currents.
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Electric Current Flow Across a Squid Axon Membrane nothing inward and outward currents
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Current Produced by Membrane Depolarizations to Several Different Potentials no inward current outward current
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Relationship between Current Amplitude and Membrane Potential early fow shiFts From inward to outward at 55 mV external Na 440mM internal Na 50mM Nernst says E Na = 55mV
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current is sodium? • Prediction- If you could change the Na+ concentrations in the system, for example make outside less sodium than inside (instead of the normal high outside low inside), Nernst would predict an early outward current instead of an early inward current. • Experiment, change the Na+ concentration in the
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lecture 4 - Equilibrium with more than one permeant ion...

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