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Biological Sciences
Peter A.Bowler

e109 Ch. 8 Continued Neurons Graded Potentials -Graded: means not the same everywhere -transient/ weaker as you get farther from the stimulus (distance-dependent) -associated with input signal Membrane potential and ion movement -May assume that ion concentrations do not change appreciably as ions migrate across membrane changing membrane potential -“Asignificant change in membrane potential requires the movement of very few ions…To change the membrane potential by 100mV, only one out of every 100,000 K+ must enter or leave the cell. This is such a tiny fraction of the total number of K+ ions in the cell that the concentration gradient for K+ remains essentially unchanged” pg. 166 Electrical neutrality For purposes of calculating ion concentrations, you may always assume that any fluid compartment is electrically neutral (the number of positive charges=number of negative charges) “electrical neutrality” -For example, if [Na+] is 0.1 mM, you should assume that negative charges are also 0.1mM Asolution contains 0.1 moles of Ca2 . The only cation is Ca++ and the only anion is Cl-. How many moles of Cl- are present? For every Ca++, we need 2Cl- : 0.2 moles of Cl- are needed *always assume electrical neutrality Membrane Potential vs. Time (msec) -monitoring changes in a single location -voltage-gated Na+ and K+ channels will open and close at different times What is a voltage gated channel? -channel in membrane that opens or closes depending on local potential (voltage) example: voltage-gated Na+ channel ECF is +++ and ICF is ---- At a single point on the axon, 1) resting membrane potential (-70 mV) Ion permeability: the permeability of the membrane to Na+ or K+ or percent of channels open -At -70 mV (t=0), the ion permeability is very low (although the permeability is a little higher to K+ than to Na+ in a resting cell) 2) Depolarizing stimulus (graded potential) --heading in more positive direction—curve going up 3) Membrane depolarizes to threshold (-55mV). Na+ channels open quickly, K+ channels open slowly. Na+ moves in. Therefore, ion permeability to Na+ opens much more rapidly. 4) Na+ channels open quickly, Na+ moves in. Example of positive feedback loop (drives process to extreme) ====the channels all open gradually 5) Na+ channels close. Why? Na+ channels have two gates (one opens faster and one closes slower in response to depolarization). K+ channels continue to open. 6) K+ moves out of cell because it is now positive inside the cell more, which brings the membrane potential back
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