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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- Spring '09
- ECF, Positive charge