NE102 Lecture Notes 3

E aps regenerate as they move down an axon signal

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i.e. APs regenerate as they move down an axon (signal strength doesn’t degrade) –  they are all or none. Myelination increase membrane resistance, allowing APs to propagate faster (salutatory  conduction) Current can propagate farther w/o dissipating b/c of myelin insulation (lipid bilayer does  have high resistance but still leaks a bit – myelin reduces this) i.e. APs regenerate at the unmyelinated nodes of Ranvier Na-K-ATPase activity does not cause repolarization Inhibiting the activity of the Na-K-ATPase with drugs (e.g., dinitrophenoal) does not alter  membrane potential or ability of neuron to fire APs in the short term.  ACTION POTENTIALS – PROPERTIES OF V-G SODIUM CHANNELS APs fire when v-g sodium channels open up.  Pore is composed of S5 & S6 helices of alpha subunit.  Specific AA residues between S5 and S6 are responsible for ion selectivity. 
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Neuronal Biophysics II 18:04 Site-directed mutagenesis of only lys-1422 or ala-1714 to glu (which is present in similar  regions of calcium ion channels) results in mutant channels that admit calcium ions. Voltage gating is due to S4 helices Contain positive residues, move outward into membrane when it depolarizes (“helical  screw” model), changing protein conformation Inactivation gate found on intracellular loop between domains III and IV Inactivation loop also changes conformation when membrane depolarizes, but slower  than the pore opens.  Hydrophobic phe-1489 is critical AA in this process: binds to pore opening, closing it off.  Mutating this site to hydrophilic AAs reduces ability of channel to inactivate.  Why are these APs so different from one another? Different relative numbers of ion channels. Different ion channel subtypes with different kinetics. Different speeds of opening/closing/inactivation Different voltage thresholds More or less selective for only a single ion
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Neuronal Biophysics III/Synaptic Plasticity 18:04 GRADED POTENTIALS AND SUMMATION How/why does the membrane depolarize to the threshold of v-g sodium channels in the  first place?  GRADED POTENTIALS Graded potentials  are responsible for membrane depolarization.  Two main classes: Excitatory postsynaptic potentials (EPSPs):  depolarize membrane (i.e., result from  opening of l-g sodium channels) Inhibitory postsynaptic potentials (IPSPs):  What are they? Local changes in postsynaptic cell membrane potential due to opening of ligand-gated  ion channels.
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