Lecture 3 - Very few ions actually cross the membrane to...

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Very few ions actually cross the membrane to set up a potential, they are electrostatically attracted to each other at the membrane. Thus: 1) Ion concentrations remain virtually the same on either side of the membrane. 2) Virtual electroneutrality. The bulk of the ions in solution are electroneutral.
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Properties of action potentials (AP) More current produces more APs Analog Digital
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The action potential Threshold • overshoot • peak of AP • repolarization • after hyperpolarization • refractory period •All-or-none (I) Refractory period 1 msec.
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Inactivation of Na + channels
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The bottom line Resting potential is due to a large “leak” conductance to K + (E K ~ -70 mV) and sometimes a smaller “leak” conductance to Na + (E Na = ~ +60 mV). These conductances are NOT voltage dependent (i.e.—these channels are always open). They are molecularly different channels than the voltage-dependent channels. Rising phase of AP: is due to the rapid (< 1msec) opening (activation ) of voltage-dependent Na + channels (bringing membrane potential to E na ). Falling phase of the AP is due to the slightly slower (1-2 msec) closing (inactivation ) of voltage-dependent Na + channels and the even slower (~ 2-3 msecs) opening of voltage-dependent K + channels (these are different K + channels than the ones responsible for RP).
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The bottom line Refractory period is due to the fact that Na + channels remain inactivated (2-4 msecs), and that voltage- dependent K + channels remain open (they are slower to open, slower to close) which holds the membrane potential at E K . Recovery from refractory period is due to the recovery of Na + channels from inactivation (they must be at a hyperpolarized potential for a few msecs in order for this to occur) and the closing of the voltage-dependent K + channels. Then all voltage dependent channels are closed and the membrane returns to its RP determined by the two “leak” channels.
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Schematic diagram of which channels are open during each phase of the AP Leak K+ Leak Na+ V-dep Na+ V-dep K+ R P 10 1 0 0 AP rise 10 1 50 0 AP peak 10 1 100 50 AP fall 10 1 50 100 refractory 10 1 0 100
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Great moments in the study of the action potential: the pre-classical age.
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This note was uploaded on 09/17/2009 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas at Austin.

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Lecture 3 - Very few ions actually cross the membrane to...

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