---start physio 1.6.97----

dr moore: synapic transmission. 
handout: synaptic transmission

note: this guy's a DVM PhD. I hadn't realized that.

Concepts not understood last time:

axon voltage, gNa and gK interrelationships and timing:
this is important for the exam, need to know what causes the action 
potential etc. resting pot -80 mv. depolarizes once it reaches the 
threshold pot. sodium goes in rapidly giving rise to depolarization then 
stops. potassium channels lag. understand about the hyperpolarization. 
cardiac muscle is different.

refractory period
K+ voltage gated channels are still open, Na+ channels are in inactive 
state, not available for new AP, so can't generate another all or none 
AP.
this is absolute rp. then you go into relative refractory period where 
you can evoke only small APs.

cable theory, saltatory conduction, electrotonus: i believe i took good 
notes on this before. just consider if it weren't for hodgkin/huxley 
positive feedback cycle causing positive flow down membrane causing 
sequential equal APs due to amplification you would lose the signal going 
down the wire due to resisstance.

suggestions to improve: take us all sailing; find a comfortable lecture 
room.

know that electrotonus means you will lose the signal due to decreasing 
amplitude unless a new all or none AP is generated, because there are 
resistances and capacitances across the membranes.

COMPOUND NERVE:
has all different sizes of fibers in nerves. alpha fibers: motor control. 
beta, gamma fibers involved in sensory functions. recordings from these 
nerves show the different components. 

experimental setup:
stimulate the nerve (in oil) at one end and record at the other end, you 
will get a multiphasic recording, not just one AP, there are different 
components. this is an EXTRAcellular recording so you are recording the 
electrical events in all the cells in the fibers. the larger components 
look larger and show up first on the oscilloscope, because they get to 
the end first. the smaller fiber components show up later. the fibers 
have different conduction velocities. the larger fibers conduct REALLY 
fast. smaller fibers take longer to conduct. say you have an a-fiber and 
a g-fiber. stimulate them at the same time. impulse will reach other end 
of a faster than other end of g. clinically if you have fibers that are 
in compound nerves like sciatic nerve, and if you have an injury, the 
fibers most susceptible to crushing injury are the large motor fibers. 
animals can still maybe feel pain or something, so if you can elicit pain 
response that is indicative of good prognosis - nerve is jjust injured, 
not destroyed. the larger the fiber the faster the conduction, is the 
bottom line. it varies. note smaller fibers are sensitive to temperature.


motor end plate

the motor axon has an axon terminal which interacts with the muscle.
it wasn't til the availability of the microelectrode that they pinned 
down this chemical event. almost all neuromuscular conduction in mammals 
is a chemical event. it occurs only at the motor end plate, where the 
nerves synapse onto the sarcolemma of the muscle. when one looks at the 
anatomical make up of these motor axons, you see the nerve has a very 
tight connection to the muscle, synaptic cleft only a couple hundred 
angstroms. many receptors here - ACH receptors and lots of others, plus 
of course the ion channels in the membranes. there are ACH vesicles in 
the presynaptic axon terminal.

overall process of neuromuscular conduction...
nerve action potential --ca++ present----> ACH release--->diffuses across 
NM cleft---->combines with ACH ligand receptor protein--->motor end plate 
potential occurs (EPP)-->increase in conductance of Na, K, Ca, and 
Cl--------------->initiation of all or nothing action potentials (Na 
channels)----> EC coupling and muscle contraction.

the presynaptic axon terminal has the action potential coming down...once 
it arrives the depolarization opens up voltage gated calcium channels and 
calcium enters the cell and the ach vesicles exit and the ACH diffuses 
across the gap - about a millisecond is how long it takes for ACH to 
attach to receptor once AP reaches this area. when ACH binds receptor 
causes nonspecific changes in postsynaptic membrane - increases 
conductance of multiple ions. gives rise to EPP end plate potential of 
lower magnitude and longer duration than AP. not only are the ACH 
receptors present, but also sodium voltage gated channels and other 
stuff. if enough of the Na channels are opened to establish threshold 
potential, the membrane will depolarize and cause an action potential as 
seen in the nerve.

using patch clamp...the ACH channel is well understood. has been cloned. 
in the electric eel, there are tons of these receptors, and the membrane 
[he was interrupted here by electrical problems]
so with patch clamp you use the microelectrode which is very very tiny, 
below a micron. you can't see the tip.you get a very tight seal and what 
you do is you pull the pipette away and get a detached piece from the 
membrane and you can get just one ach channel, and you can manipulate or 
clone it or whatever. so you can get a great deal of information. 

bungarotoxin, snake venom, causes paralysis. similar to ttx which blocks 
Na channel...except blocks ACH channel/receptor. so you can tag it and 
use it to count channels. The K+ and Ca++ channels aren't so well 
understood because there isn't a good blocker that is so specific like 
this.

generation of EPP: in presence of ca++ ACH vesicles are exocytosed when 
triggered by AP. ACH diffuses across synaptic cleft, binds to ligand 
receptor channel, changes ACH channel, changes ion conductance of 
postsynaptic membrane. triggers EPP which persists, triggering action 
potentials and eventually tetany if ACH is not cleared from the receptor. 
so ACH is removed somehow.

ACH and voltage gated Na channels are in parallel, but ACH channels are 
only seen at motor end plate. ACH has no effect on membrane in other 
place.

if you cut the presynaptic axon, the ACH receptors will propagate 
themselves, increase in number and move to other spaces, ready to take 
advantage of any ACH that comes their way.

when the ACH channel is opened by the ACH, it opens a hole in the 
membrane. Na+ rushes in, K+ rushes out, ca++ rushes in...causing 
depolarization. stimulates EPP. this causes membrane to reach threshold 
potential, opening voltage gated Na channels....

eg, if enough ACH channels are opened, the EPP causes threshold potential 
to be reached initiating the action potential. (all or none response.)

voltage gated cation channels vs transmitter gated ion channels
eg Na channel							ACH channel
4 protein subunits					5 protein subunits
ion pore								ion pore
polypeptide								polypeptide

small alterations in AA sequencing account for various dzs. just 3 aa out 
of sequence causes that QT syndrome associated w/sudden death.also seen 
in dalmations and other breeds.

so we're learning more about the biochemical nature of these channels and 
hopefully how to manipulate them and treat, cure, and prevent dz.

electrotonic decay of EPP with distance....if you have a muscle and the 
EPP occurs at the motor end plate, due to resistance and capacitance of 
the membrane the EPP causes the whole muscle to depolarize, in different 
gradations, because the current is largest near the plate. current 
density - if sufficiently great to bring it above the threshold 
potential, will cause AP. actual physical properties of membrane are 
important, regulate how fast muscle will respond.

note that you only need to depolarize the membrane in one spot, to get 
the AP started. now if you don't have enough ACH receptors, you won't 
depolarize it enough, you won't have efficient control ofyour muscles, 
will have weakness. you need enough of the membrane depolarized to get 
the AP.
****this seems contradictory to me***

if you have just a few ACH receptors activated, you will get an EPP only. 
if you have a bunch of them activated, you'll reach threshold potential 
and you'll get an AP. can use TTX to block Na+ channels to just look at 
EPP without causing AP.

ligand gated channel
ligand binding--->channel opening-->ion flow (multiple ions)--> voltage 
change aka EPP

voltage gated channel
takes less time
channel opens--->ion flow--->voltage change--->|
	^                                          V
    |------------------------------------------											
when you record at the synapse, there are ligand channels open, so you 
don't see the normal +35mV you see only about -20 mV due to the multiple 
open channels. this is due to presence of ligand channels.

in presence of curare, curare will form competitive binding onto ACH 
channel. hops onto it and stays there and wont' let ACH on. the more 
curare you give, the more channels are tied up. EPP decreases as you add 
curare. lasts a long time. not used much clinically, succinylcholine used 
more often. unlike curare, when succ gets on receptor, it causes a 
depolarization. HUP has done anesthesia by giving a lot of morphine and 
then succinylcholine but then they don't notice that they morphine is 
wearing off and that's not good for the patient.

---break---

voltage gated Na and K channels   --    transmitter gated channels
both in the membrane

in the polarized state, the gates of Na and K channels close and ligand 
channel has no ligand attached.  once activation occurs ACH hooks up 
w/ligand channel, opens channel causes depolarization which, when reaches 
threshold, opens sodium channels causing further depolarization, opening 
the K+ channels, then recovery/repolarization occurs. but if the ligand 
channel stays open membrane stays depolarized. so we have an active 
process to remove the ligand.
an enzyme lyses the ACH-receptor complex and ACHesterase breaks down the 
ACH in to components. ACHesterase lies on external surface of the 
membrane as does the receptor protein. the ACHesterase story was worked 
out by a prof here using microelectrode. ACH is a + ion. by putting it 
INTO the cell there is no effect, but if you put it OUTSIDE the cell by 
the receptor, it gives rise to an EPP. the EPP doesn't last that long 
'cause the acetylcholinesterase breaks down the ACH. nerve gas will 
inhibit the ACHesterase...

if you have nerve gases, phosgene gases, they bind up the ACH esterase. 
so the EPP is prolonged, the channels stay open, it is very very very 
very unpleasant!
you keep getting APs due to persistent EPPs. you see tetanus followed by 
flaccid paralysis.  if APs are close enough together you get a 
contraction instead of a twitch. so the rapidity of APs is established by 
refractory period...can't go faster than that.

Neuromuscular conduction block:
TTX and local anesthetics block nerve AP.
botulinum toxin blocks ACH release - very very potent
low Ca = milk fever - membrane becomes unstable because ca++ needed for 
membrane stability, threshold potential changes, you get lots of APs due 
to the decreased TP, tetanus results
curare: competitive inhibitor
succinylcholine: depolarizing block
nerve gases: bind acetylcholinesterase.

myasthenia gravis:
new evidence may be specific antibodies that interfere with specific 
channels.
dogs: lack of ACH receptors - too few - causes muscle weakness.
Tx with ACHesterase blocker. you slow down the breakdown of ACH, which 
helps the EPP go up. you want to enhance the fx of ACH.


OVERVIEW:
we've talked about nerve conduction, neuromuscular conduction, and now we 
will go over how nerve conduction down to motor neuron gets integrated in 
CNS. 


how nerve APs cause muscle action...
motor neuron synapse: presynaptic terminals come up to large motor 
neurons. you have the soma dendritic part of the membrane recieving the 
presynaptic terminals, contains the ligand channels. the axon hillock is 
where the axon leaves the neuron - like optic disk, where nerve leaves 
eye - the axon contains nodes of ranvier and myelin.

NERVE SYNAPSE:
presynaptic AP--in presence of Ca++--->transmitter relase (various)-->200 
A synaptic cleft--> ligand receptor binding-->EPSP or IPSP--->AP.

the transmitter binds to the receptor and causes a potential. within the 
CNS are two kinds...EPSP is excitatory and IPSP is inhibitory (post 
synaptic potential). it's regulated by the type of neurotransmitter. so 
this is quite similar to neuromuscular conduction.

there are thousands of terminals on the dendritic membrane, and they have 
a predominant effect at the axon hillock.
the larger the nerve, the lower the excitability threshold (??***)

so you get an all or none response - -80 to +35...going from K battery to 
sodium and back to K. if you record at the axon hillock, there's a hump 
on the AP preceding it. this indicates the axon hillock has a lower 
threshold potential, which it does. if you stimulate the motor neurone 
via electrode or whatever, and record at axon hillock, you find the 
threshold is only about 10 mV wherease it's about 25 mV in the dendritic 
region. this is because the number of sodium channels is much lower in 
the dendritic region, but there is a high density of them at the axon 
hillock, so you get lots of +feedback very easily. actually, axon hillock 
depolarization propagates down axon and also back around dendritic 
portion as well, wiping everything clear, so to speak, and if a ligand is 
still bound, the effect will be maintained until it is removed. axon 
hillock also has lots of voltage activated K+ channels, so it's very 
snappy it depolarizes and repolarizes right again. very tight control 
with short refractory period. there are also Ca++ activated K+ channels 
that are much slower. the general concepts are what we are after here, 
not all the detail understand diff between ligand and voltage gated 
channels, know we are learning more and more, understand NM v motorneuron 
synapse. note repeated concepts eg electrotonus etc.

the thing is that if we want to look we've already said the axon hillock 
has lots of ion channels and is quite easily excited. if we look at an 
excitatory event, and EPSP, the ligand released by presynaptic terminal 
causes a depolarization by increasing conductance to a given ion. here we 
get ligands that are specific for opening particular ion channels, not 
opening channels for all membranes like the NM junction ACH situation. an 
EPSP depolarizing potential is due to Na+. so the threshold potential is 
much lower at axon hillock and higher in dendritic part. if 
depolarization occurs at dendritic part, the depolarization will 
propagate to hillock and possibly instigate an AP. electrotonus, again. 
if a presynaptic excitatory input comes in on dendritic membrane, it 
opens Na+ channels there, and they stay open til the ligand is lysed off. 
but you get a depolarization and it propagates toward axon hillock, with 
amplitude getting smaller and smaller. whether or not you get AP at axon 
hillock depends on whether there isenough depolarization remaining when 
impulse gets there. the larger the EPSP and the closer to the hillock, 
the more marked is its effect.

temporal summation: if you have multiple inputs into the motor neuron you 
get multiple EPSPs. if each causes the same change in conductance and 
they occur closely enough in time, they will sum to a larger potential.
rapid activity allowing summation over time. occuring at different times 
(**)

spatial summation: presynaptic inputs in various spots giving various 
potentials, if they are excited at the same time can also be summated 
over space

you can also have multiple synapses eg EPSP and IPSP occuring at same 
time. these are also integrated and may cancel each other out.

EPSP v IPSP
EPSP causes increased conductance of Na+, causes depolarization
IPSP causes increased conductance of K+ or Cl-, causes hyperpolarization. 
makes membrane more negative, so it is harder to reach threshold 
potential. this is obviously inhibitory.

again can have spatial or temporal summation of multiple synapses.

postsynaptic summation:

single EPSP doesn't reach TP. Two EPSP at same time in different spots: 
spatial summation: almost reaches TP. Two EPSP very rapidly in sequence 
in same spot: temporal summation

note: he's contradicted himself re: if these things occur at SAME OR AT 
DIFFERENT times. I'm curious and will hopefully remember to look it up.

strychnine eliminates inhibitory input, you get tetany. tetanus toxoid of 
course does the same thing. then you have rapid firing of motor neurons.

threshold potential and rate of firing/refractory period.

if you ahve an EPSP and it persists for x amt of time and is of y 
magnitude, you get an AP but it's a period of time before you can have 
another AP -there is an absolute refractory period with ion channels. so 
you can only have a certain number of action potentials. but there is no 
refractory period in the postsynaptic membrane. signal will persist until 
ligand is removed. 

amplitude of receptor potential governs how rapidly these will fire and 
if inhibitory or excitatory and how many of each: they integrate, 
temporally and spatially summate.

I'm sorry, he's still talking but I can no longer listen :( I am not 
feeling well. 

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