Neuro: 8:00-9:00 Scribe: Ashley Holladay

Neuro: 8:00-9:00 Scribe: Ashley Holladay

Thursday, January 22, 2009 Proof: Teresa Kilborn

Dr. Lester Neuroscience Page 6 of 6

I.  Synaptic Transmission[S1]:

II.  Learning Objectives [S2]

a.  This looks like a lot of objectives, but if you can understand this much- then you are good.

b.  Just about all of the principles we know about have been worked out in the NMJ, and for the most part they all apply to central synapses. We will compare and contrast the properties of CNS and peripheral synapses.

c.  He approaches the synapse by using the prototypical junction- the motor neuron and neuromuscular junction. For the most part they all compare to nervous synapse.

III.  How is the signal transferred? [S3]

a.  Synaptic transmission- historically a big debate.

b.  There is a fundamental problem if you have 2 elements that you are trying to get a signal to go between. It comes back to plumbing analogy and the path of least resistance. If you have an electrical signal in one and want to get to the other, it will have to jump across the space. This space is the low resistance pathway. All of your current is just going to flow through the extracellular space and very little can get across to the next cell.

c.  Idea that we had to have some organized communication between 2 neurons. There were 2 main theories:

d.  First idea was that they just jumped across- not correct

e.  1. Electrical- not going to go into too much, but need to know that they exist

f. 2. Chemical

IV.  How do synapses work? [S4]

a.  Last century, Katz went to work with Eccles who were on opposite sides of the argument (electrical and chemical).

b.  Katz (more of a pharmacologist) was a proponent of chemical transmission, and Eccles (a physiologist) thought all synapses would work electrically. They had different backgrounds.

c.  It was a bid argument that went on to post WWII. During that time there were a lot of developments (like electron microscope) and people were able to identify synapses and vesicles, etc.

V.  Learning Objective #1 [S5]

a.  Electrical synapses- they do happen. They are sometimes more associated during early development. They are present in adult brain. More associated with communication between astrocytes. You can get huge networks of astrocytes that are linked by electrical junction.

b.  Electrical synapses cause continuity between one cell and the next. You get a hemi-channel formed by one cell and another formed by the next cell, and the membranes come very close.

c.  This is probably the closest that neuronal or glial membranes come in the CNS- they are right next to each other basically. The channels link up and you have a pathway. The channels can be regulated but are generally viewed as being open.

d.  They let ions through (Na, K, Ca and up to ~1000 MW- like cAMP). They are electrical but also metabolic to some extent.

e.  There were some questions on medical boards about this - must be reasonably important that you know it is not just chemical.

VI.  Learning Objective #2 [S6]

a.  Now talking about chemical

b.  Central principle: If you want to understand how the synapse is organized and what each individual part does, look at the overall physiological function of the synapse.

c.  If you want to move a muscle, you want to move a muscle and you want to be able to do it fast. Motor neurons to get the signal from the brain to the muscle fiber to contract quickly. You want to have the synapse work to release the transmitter quickly and react quickly with the postsynaptic cell so that you get a rapid muscle contraction.

d.  You want it to also be high fidelity. You cannot sacrifice everything to make it fast. It also needs to be very brief and ready to go again.

e.  We are pretty quick- a lot of rapid motor control.

f.  You can see how the synapse is organized to let that happen. You can extend that to fast excitatory transmission in the CNS. It uses a different transmitter, but it is basically the same.

VII.  NMJ Structure- Anatomy overview [S7]

a.  This is the NMJ. It is a motor neuron which sheds its myelin sheaths and forms buttons or terminals that synapse onto the muscle fiber itself. The organizing principle is to have synaptic vesicles across from the post-synaptic receptors (the Ach receptors).

b.  At the NMJ, if you pull off the nerve fiber and look down, you will see these little clusters. They are pentameric. Each one of the little rings is an Ach receptor. They are packed in very tightly. If transmitter is released and goes across the cleft, it will probably bump into a receptor, which is what you want.

VIII. NMJ- Physiology Overview [S8]

a.  Normally when you stimulate the motor neuron you get a muscle contract. This synapse has a high safety factor-there is always enough depolarization caused by the transmitter to get way beyond the threshold for AP initiation.

b.  Underlying synaptic event. If you block the AP, you would get the event due to Ach acting post-synaptically at the synapse.

c.  You can see that more clearly if you put on an antagonist (curare) to reduce the number of available nicotinic Ach receptors. You get a synaptic wave form that doesn’t reach threshold and you don’t get a synaptic response. The synaptic response has the same basic wave form of the synaptic response. It will be the same for this transmitter, glutamate (the principle transmitter in the CNS), GABA (the basic inhibitor).

d.  They will basically all the same shape. They will rise and last a msec to a few msec.

IX.  Origin of the EPP [S9]

a.  AP (like talked about from yesterday’s lecture) is generated at one point and regenerates itself to get propagation. It will always be the same when you start and when you end.

b.  These chemical signals will decay. They are only produced at the synapse and fade away as current leaks out and prevented from moving by the internal resistance. The current moves in just like it would through a Na channel at the synapse but then it just purely passive in its propagation. So, as you get further and further away, less of the current will move through the internal resistance and more will leak out through channels

c.  It will start of big at the synapse and by the time you get a long way away it will fade out. This is a principle that applies to CNS synapses more. At the NMJ, if an AP is generated, it will spread through the muscle fiber.

d.  In CNS neurons you have synapses that are hundreds of microns away from the soma.

e.  SQ: What does the EPP stand for?

f.  Answer: EPP is the specialized term for the endplate potential (NMJ sometimes called endplate). Other terms like EPSP (excitatory post-synaptic potential) is the more general term- anything that is a depolarizing excitation. IPSPs- inhibitory.

g.  This is passive and it contracts with AP which are active

X.  NMJ- Acetylcholine receptors [S10] (these were more than one slide in lecture)

a.  Nature provides us with tools to learn about nicotinic Ach receptors. The electric ray has electric organs on either side that are modified NMJs. Useful to biologists because it has thousands of NMJs, which means it has millions of nicotinic Ach receptors.

b.  As a biochemical source of protein it is very rich.

c.  The toxin from the snake (mostly highly characterized neurotoxin) binds with high affinity to the nicotinic Ach receptor and that is how it causes paralysis. People used this toxin to affinity purify the receptor.

d.  Basic structure is that you want a protein which is heteromeric (made of different subunits organized around a central pore. It has a gate like the Na channels and the K channels.

e.  This case is controlled by transmitter and not voltage. A binding site for the transmitter leads to activation of the gate. There has been very new data published claiming that they knew exactly where the gate was and what causes it to open.

XI.  Learning Objective #3 [S11]

a.  Ions passing in and out. Depolarize at the NMJ- depolarize towards threshold.

b.  Need to depolarize past the threshold to get an AP to fire.

XII.  Acetylcholine Receptor Channel [S12]

We need a net influx of Na ions into the cell. The only 2 ions that have a driving force that would drive them into the cell at rest are Na and K. Ca contributes a little bit through these channels.

a.  Equilibrium potentials or reversible potentials- potential at which the electrochemical gradients are balanced and there is no net current flow

b.  Think about what the membrane potential you would have to be at that would cause no current to move through. If you have a K channel, put the membrane at the K equilibrium and there won’t be any movement because there is no driving force.

c.  When done for the synaptic excitation channels, found the reversal potential was 0. There is not potential for any 1 ion that is 0, so there have to be multiple ions going through the channel. There is Na coming in and K going out.

d.  They have opposite equilibrium potentials and they will meet at roughly 0. At depolarized resting potentials, the main driving force is on Na to come in. As it depolarizes, there is more potential for K to move out.

e.  If you go past 0- K leaving the cell more than Na entering. They are mixed cation channels. At resting membrane potential, it will be mainly Na coming in- depolarizing.

f.  The same principle as the Na channel but not gated by voltage and it doesn’t regenerate.

XIII. Multi-ion Channels [S13]

a.  Can use the vector diagram to see that it is in between the Na and K equilibrium potential.

XIV.  Efficiency of EPP [S14]

a.  The post-synaptic receptor

b.  The channel was designed to work efficiently at the synapse to be very fast and transient.

c.  We need a receptor that is waiting to receive Ach and opens quickly and stay open only briefly and close and be ready to go again.

d.  We release massive amount of Ach molecules and by mass action- the rates of the forward reaction is proportional to the products and we have an excess of nicotinic receptors- We drive the binding action really fast. There is almost instantaneous binding when releasing Ach into the cleft.

e.  The channel opens and needs 2 molecules of Ach and opens in 10 usec (beyond the resolution of equipment so may be quicker). There is a high probability that the receptor is open. If a receptor has to molecules of Ach bound, it will open up. There would be huge amount of channel opening and huge amount of Na entering.

f.  Channel stays open for only ~1msec then it quickly closes and the molecule is gone. It has a reasonably low affinity so the Ach drops off very quickly.

g.  There are 2 mechanisms for clearance.

i.  Diffusion is very fast over short distances. It will be in microns that they are traveling. It will not take very long for the transmitter to move away.

ii.  The NMJ needs an additional method- uses enzyme hydrolysis- Ach-esterase that chews up and hydrolyzes Ach. It is so efficient that maybe 1/3 of the transmitter does not even make it across the synaptic cleft before it is chewed up. The system is geared to make sure that Ach only acts once, briefly and then it is gone- high fidelity.

h.  If we have an antagonist around for too long, might lead to desensitization. A drug could do this.

i.  All of our natural transmitters usually work on the timescale that is 10X what is needed to get to desensitization.

j.  The reaction is on the mSec time frame and recovered within a mSec. If we send signals down the axon at 100Hz, it is able to follow the signal. There are some CNS synapses that want to transmit in this fashion with 1 AP and 1 response. The post-synaptic response may summate with the previous one where you get build up or you have inhibitory signals. You want to blast past threshold, contract and recover.

k.  The muscle has to do this as well, but at the level of the synapse it is fast and efficient.

XV. Learning Objective #4 [S15]

XVI.  mEPPs quantal hypothesis [S16]

a.  Presynaptic side- The 2 critical organizing principles:

i.  1. Calcium is required

ii.  2. The transmitter is released in packets.

b.  The unitary event at the NMJ looked like a miniature full blown synaptic event- depolarization to threshold for AP.

c.  The way they did that was to lower Ca. You don’t release as much transmitter, it wasn’t graded- didn’t slowly reduce less and less. It was an all or none phenomenon. They were multiples of the unitary event. They are in quanta. At the NMJ, release about 200 and if each one depolarizes, a fraction of mV- you will get enough total depolarization-Quantal transmission.

d.  You might come across some diseases where there are some forms of NM diseases where you don’t release enough quanta and you don’t get to threshold in every case-clinical significance.

e.  They come from the hundreds of contacts/ release sites at to NMJ.

XVII.  Presynaptic mechanisms [S17]