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6-Action Potential Figs BW

Recf rmemb cmemb rmemb 1 surface area nav raxoplasm

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Unformatted text preview: b Rmemb ∝ 1 / Surface Area NaV Raxoplasm Raxoplasm ∝ 1 / r2 Spread of Graded Potentials the tip here is much smaller that you can poke through live cell membrane without killing it. the pipettes allow you to inject current into cell or measure membrane potential inside the cell at various point. if you inject I then at location 1, get dashed response. the amplitude of same event is smaller and the farther you go it get smaller and smaller. amplitude fall exponentially with distance. -the dash current where you inject half the current, then the amplitude is smaller than first injection, then get smaller in distance. -the line top is resistance of ECF (which is 0). at I we inject positive charge, if there's complete circuit, the current will move down the axon (it can't leak through lipid bilayer. the next place it can go out is next increment of length). at this point I, some current will go up across the membrane (get to ECF and go back to where it started) and some will continue( current divide). the current that continue is less the one at left. it will divide at the next branchpoint, the same fraction left and same fraction continue. this bleeding of current is why current fall off exponentially with distance because current continue to leak out of membrane with every increment of distance. -when current flows across membrane, it has a choice: it can go to the left through resistance (open ion channel) or to the right through capacitance of the membrane (capacitance of membrane is at left. the top conductor is ECF, bottom is ICF. the circle is charges carry by ions). at rest this capacitor is charged up. the outside of the capacitor bear an excess of positive charge, the inside bears an excess of negative charge. the potential difference (voltage) is resting membrane potential. the axon membrane is mostly lipid bilayer with a few ion channel. it's across the lipid bilayer that membrane potential exist, so it's the potential across the capacitance that is the actual membrane potential. this means that the charge of an ion can go through either path: going to resistance (ion go through open ion channel, easy but has to be the correct selective channel) or going to capacitance of membrane (if membrane is charged with excess of negative charge on bottom, here comes positive charge that wants to move up. remember we're...
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