Membrane Potential Purves Chapter 2 Kandel Chapter 7

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Membrane Potential Purves Chapter 2Kandel
Chapter 7
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Bases of neuronal excitability
Ion movement is the basis of neuronal excitability
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Ions involved in neuronal excitablity

• Sodium Na
• Chloride Cl-
• Potassium K
• Calcium Ca2
• Magnesium Mg2
• Bicarbonate HCO3-

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Cell Membrane
Ions cannot cross the lipid bilayer
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Ion Channel
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Channels are made up of proteins(folded amino
acid chains)
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Hydrophobic
Hydrophilic
Other
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Several proteins (subunits) make up a complete
channel
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Channels are selectively permeable to specific
ion species
In a resting situation, by definition, there
are no ion currents across the membraneIonic
Equilibrium
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Resting Potential

• Differences in ionic concentration
• ion pumps
• Selective permeability
• ion channels

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Ion pumps transform chemical energy (ATP) in
ionic gradient energy
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Sodium-Potassium Pump
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Exchange pumps differ from ATPases as they
exploit a ion gradient to move a different ion
against its gradient (no ATP involved)
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Ion Channel
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Main voltage- gated channels
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Main ligand-gated channels
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Both types of channels can be modulated by
phosphorylation
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• Once the channel is open the flow of current in
  the channels is passive
• V RI (Ohms law holds)
• But opening of a channel is active
• the presence of a voltage change or of a ligand
  induce a changing in the opening state (gating)
  of the channel
• Except for leakage channels which are by
  definition always open

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Electrophysiological techniques can reveal the
active properties of the membrane
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No ion gradient no voltage difference
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Concentration difference creates membrane
potential
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Nernst EquationEx RT ln Xout zF
XinXin intracellular ion concentration
Xout extracellular ion concentration
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Nernst Equation - SimplifiedEx 58 log X out
z X in
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Goldman Equation for the resting potentialEr
58 log PKKout PNaNaoutPClClin
PKKin PNaNain PClClout
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K determines resting potential