----physio 1.3.97-----

Dr. Moore

CONCEPTS not understood from last time per the comment sheets:
Wheatstone bridge
Voltage clamp
Toothpaste experiment

these examples were discussed to show how we learned what ions are 
involved in generating potentials.

also not understood: why do we have to come back on Thursday and Friday.

suggestions to improve: follow the lecture notes, delete sound effects, 
speak louder and make sound effects lower.

well, Dr. Moore doesn't like to follow the lecture notes. we are supposed 
to READ the notes. He says he isn't organized well enough to follow the 
notes.

Wheatstone bridge: just understand that if resistance changes in the 
nerve segment, we can record it, and it indicates that conductance has 
gone up. 

Voltage clamp: just historically, this is what we did to learn what ions 
were involved in generating electrical activity.

Toothpaste experiment: we need to know that you can substitute what is 
inside and outside the membrane and see if the recorded voltage is as 
expected and it is.

Diffusion potential: lag potential- see previous notes.

[note: he's going over stuff from last time. i am not writing it down 
again!]

voltage gated na+ channels. when a propagated wavefront comes and excites 
the tissue, the channels open. the gates open, the selection filter is 
there, and the appropriate ion can go through. now, the closed state is 
the more stable state, where the channel wants to be. so they are opened 
by voltage and then they close again. every time you excite it, you get 
an action potential, membrane starts going from -80 mV to +35 and then 
the channels go back to inactive state from the voltage stage and also 
just because they tend to do that. so there is a voltage inactivation and 
a time inactivation process.

closed resting-->activated--->inactivated refractory--->open conducting
 ^^^^^^^
   |
 this is preferred.
****question: what is "open conducting" state?*****

recall that radiotagged TTX lets you count sodium channels because it 
sits on top of them.

So, channel proteins form hydrophilic pores and are rapid. carrier 
proteins which bind specific solutes to transport them are very slow.

also note that the inactivation of the channels is probably an active 
process using energy. TEA blocks voltage gated potassium channels. - he 
explained some reason how they proved it probably uses energy to close 
channels.

potassium leak channels - probably cause negative resting potential.

DO UNDERSTAND the squid nerve action potential diagram
at rest, K+ leak channels give rise to negative resting potential. enough 
Na+ channels open, and you get to threshold potential, and then all the 
Na+ channels rush to open state, and you get AP, followed by 
hyperpolarization. so, what happens is that the open and close of Na+ 
happens very quickly, but K+ voltage gated channels do not close nearlly 
as quickly as Na+ channels, rather remaining open, causing the 
hyperpolarization. this means that K+ is going very close to the 
theoretical equilibrium potential.

depolarization due to Na+ channels opening
repolarization due to Na+ channel shutting down and K+ channel opening. 
K+ leak channel always open.

since there are all different ions existing across this membrane, the 
relative permeabilities are important and are what give rise to the 
potentials.

research showed that in diseased tissue, APs were occuring due to calcium 
instead...not sure what that is supposed to mean.

effects of depolarization on conductance of Na+ and K+ (see handout)
increase in conductance Na+ causes depolarization which increases the 
conductance...

depolarization causes an increase in K+ conductance which...darn, he 
moved on.

Na+/K+ pump. Na+ enters the cell during an AP but it is a very MINIMAL 
amount. also very little K+ is lost. but over the LONG run, it adds up, 
so you need an active process to get the sodiumback out and the potassium 
back in. this rotating ATPase sits in the membrane and performs this 
function - maintains the concentration gradient. it brings in 2K+ and 
throws out 3Na+

ouabain binds to the K+ binding site of the pump...digitalis....digitalis 
most effective drug for congestive heart failure. it poisons the pump, 
allows calcium to accumulate, makes muscle pumping more effective.

about this pump...if you load a squid axon w/radiosodium, and put it in  
cyanide bath, and inject ATP into the axon, you can see how much Na+ is 
released with each injection

Electrogenic pump: toad bladder:

Membrane Channels: remember there are existing electrochemical gradients. 
we have K+ leak channels, we have pumps, voltage gated channels, carrier 
mediated transport, etc.

we can look at this with ion sensitive electrodes, or we can use patch 
clamp.
PATCH CLAMP is a technique people are starting to use instead of 
microelectrodes. it uses a largeer pipete. you attach it to the membrane 
and record the voltage, and if you depolarize the cell you can measure 
current. little blips occur that are sodium channels opening. so this way 
you can identify the opening of individual channels, not just quantify.  
also you can have it go into the cell and you can inject things into the 
cell or suck things out of the cell and youcan see what happens.

clinical correlates: 
vfib
anesthetic
DDT
digitalis
ca++ channel blocers
cottonmouth snake
kidney dz

Ca++ AP: most of Ca++ bound inside, high outside. first demonstrated ca++ 
channels in skeletal muscle and cardiac muscle. if you inject potassium 
into a cell it will diffuse freely, but if you inject calcium, it is 
bound and doesn't diffuse. it's a good trigger molecule.

can use aquorum, a luminescent marine molecule, to detect calcium ion 
movement.also quin2 and fura2 can be used, those are fluorescent dyes 
which fluoresce when exposed to light. they bind to the calcium and the 
wavelength will change so youcan see how much is bound and how much is 
unbound.

CABLE theory

nonmyelinated nerve: all or none APs. the AP will not look the same at 
the end of the nerve as at the beginning of the nerve without 
AMPLIFICATION. So, there IS amplification (due to the propagation of 
successive APs) to avoid a reduction of the AP due to resistance.

myelinated nerve: nodes between myelin patches
these fibers conduct VERY rapidly about 200 m/s, same as above re: AP 
same at the end. the myelin is an excellent insulator. the current is not 
LOST between nodes. so you only need fewer APs...it jumps from node to 
node to node, because the resistance and capacitance is lost in the 
myelinated segments. this is SALTATORY conduction.

telephone lines: no amplification of signal
due to resistance, signal is lost over distance, so they put in 
amplifiers.

resistance and capacitance would really wreak havoc on APs over any 
distance if we didn't have the amplification, which comes from the 
hodgson/huxley ideas.

we have capacitance across the cable, and resistance as well. if we 
stimulate the cable, the signal starts dropping in amplitude and changing 
its configuration due to ELECTROTONUS. but this doesn't occur in 
biological systems due to the renewal of the APs because of the ion 
channels.

REFRACTORY PERIOD: you can't generate an AP until the Na+ channels 
recover from previous excitation. there is an absolute refractory period 
first, when you just CAN NOT excite the fiber, and there is a relative 
refractory period, when you can cause an AP, but it will be a smaller AP.


understand that at rest the K+ leak causes the resting potential, that 
the sodium channels cause increased Na conductance, that depolarization 
itself opens the K+ voltage gated channels.

-----physio------