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IonChannelsFinal

The clever experiment involves tuning the

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The clever experiment involves tuning the extracellular concentrations of the different ion species in turn and then examining the resulting membrane potential. (Ralph Greenspan, An Introduction to Nervous Systems)
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The action potential Action potentials are a stereotyped response once a critical threshold has been crossed. The shape and duration of the action potential are key variables. NOTE: Helmholtz measured the velocity of propagation of nerve impulses.
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The avoidance response: details These single-celled eukaryotes manage a sophisticated response to their environment. One of the things I am most excited to tell you is that this response is mediated by similar tricks to those we are used to in the context of nervous systems. (Ralph Greenspan, An Introduction to Nervous Systems)
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Ion transport rates in ion channels Another way in which cells manipulate transport rates: selectively and transiently altering the permeability of cell membranes through protein channels and pumps A typical ion channel, which fluctuates btw closed and open conformation Estimate the flux of ions trough the channel, assuming purely diffusive motion Ion channel = D small ions 2000 μ m 2 /s (e.g., Na + ) 5 nm (width of lipid bilayer) 0.5 nm (size of hydrated ion) Fick’s law (details - later): where Δ c = difference in ion c across the cell membrane, l = distance btw the two “reservoirs” mammal.cell: c 100 mM ( 6 × 10 –2 molecules/nm 3 ); l 5 nm A channel = d 2 π /4 0.2 nm 2 => N of ions traversing the membrane per second: J ion D Δ c l J ion 2 × 10 9 nm 2 s × 6 × 10 2 molecules / nm 3 5 nm 2 × 10 7 nm 2 s 1 dN ion dt J ion A channel 2 × 10 7 nm 2 s 1 × 0.2 nm 2 = 4 × 10 6 s 1
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Two-state systems There are two models that a statistical mechanician is interested in, the one they are working on and the Ising model. A Statistical Mechanician A paving stone at UBC: Ising model of spins on a regular lattice In many bio problems, the state of the system is described by one or several simple two-state variables ( σ ), each of which can only take the values of 0 or 1 Examples of the internal state variable description of macromolecules:
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The clever experiment involves tuning the extracellular...

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