Synaptic Transmission and Neuromuscular Junction

Hello All,

Here are your questions regarding synaptic transmission and the neuromuscular junction.  There are a lot, so please look through these if you sent me a question at all regarding this subject.  I have grouped like-questions together but included each question so that you can find your own.  As always, if you have further questions regarding this material please feel free to follow-up with questions below.

A heads-up that I will be off-island for 10 days beginning this Saturday.  I will not be available, therefore, for office hours during this time.  I will be available electronically during that time, however, and will be posting skeletal muscle and smooth muscle blog posts in addition to a few videos that help explain a few commonly-difficult points about skeletal muscles.  Please stay tuned, therefore, for those postings!

1. Please clarify the synaptic transmission relationships within the body. Here is what I understand:
In Peripherial Nervous system:
Autonomic nervous system:
1) sympathetic division: the result of communication between preganglionic-postganglionic neurons (in the paravertebral or prevertebral ganglions) would ALWAYS lead to excitatory response (depolarization) in the postganglionic neuron (since the only neurotransmitter is acetylcholine and the only receptor for it on the postganglionic cell is nicotinic acetylcholine receptor)
2) parasympathetic division: Likewise, the result of communication between preganglionic-postganglionic neurons (in their respective ganglions) would ALWAYS lead to excitatory response in the postganglionic neuron (since the only neurotransmitter is acetylcholine and the only receptor for it on the postganglionic cell is nicotinic acetylcholine receptor)

Indeed your understanding of those relationships are correct.  The level with which the action potential is fired in the secondary neuron, however, is dependent upon the frequency of the action potentials arriving from the presynaptic neuron.  In both of these places, however, it is an excitatory neurotransmitter and an excitatory nicotinic ACh Receptor on the postsynaptic membrane.

3) Enteric division: What is happening in here along the same lines?

The details of the physiology of the enteric nervous system is quite complex and is covered, therefore, within the GI module.  In essence, this division of the nervous system is called the 'brain' of the gut because it utilizes over 30 neurotransmitters and more postsynaptic receptors (just like the actual brain in the CNS).  Therefore I will not explain the number of details of that system here, but encourage you to stay tuned for this information next semester.

In somatic nervous system (At the neuromuscular junction): Also that, the result of communication between presynaptic neuron and skeletal muscle cell would ALWAYS lead to excitatory response (depolarization) in the skeletal muscle cell (since the only neurotransmitter is acetylcholine and the only receptor for it on the skeletal muscle cell is nicotinic acetylcholine receptor) 

Yes.  We call this excitatory postsynaptic response an end-plate potential (EPP).  It is similar to what is called an excitatory post-synaptic potential (EPSP) in the central nervous system, but has an individual name special to the neuromuscular junction.

In the central nervous system: the result of communication between presynaptic-postsynaptic neurons (in the nuclei of CNS) would depend on the type of neurotransmitter(s) released, type of receptor(s) it (they) is (are) acting on and, the overall summation of graded potentials (received across dendrites) at the axon hillock of postsynaptic neuron.

Yes.  Also remember that within this system there are LOTS of inputs.  Between 1000-10,000 neurons are synapsing on any one neuron at any given moment, so all of those inputs and the frequency with which they are sending information to the one neuron all summate to determine if an action potential will be propagated or not.

2.  Does an stimulus from the external environment or internal environment cause the release of only one specific neurotransmitter at a given time across synapses in the CNS or multiple ones? how about a combination of stimuli?

For years in research it was thought that one neuron released only one type of neurotransmitter.  This has been re-researched however and now it is viewed that each neuron can synthesize and release a few neurotansmitters.  However, any action potential arriving at the end of the axon of a neuron will cause the release of all of the neurotransmitters within that neuron that are primed (the synaptic vesicles are ready to fuse with the membrane).  Therefore it is not that just one type would be released if multiple types were being synthesized, but all types would be released.  Some neurons, therefore, only release a single type of neurotransmitter and others release many.

3.  Given that 1 action potential generated by a neuron causes 1 action potential in the muscle, does that mean a neuromuscular junction is not associated with EPSPs and IPSPs?

The last learning objective you have for us is to distinguish between an endplate potential and an action potential in skeletal muscles. Can you give me some clarification with this? I think  I must have missed this whole concept during lecture! I understand how an AP is propagated from the neuron to the skeletal muscle, but to what are you referring when you say end plate potential?  

I indeed did forget to mention the difference between endplate potential and EPSPs.  I addressed it a bit today in class.  However, the change in membrane potential that occurs at the motor endplate and leads to the action potential within the skeletal muscle membrane is called an endplate potential.  An endplate potential is essentially similar to EPSPs, but they lead to action potential initiation with a greater degree of efficacy.  IPSPs, in contrast, are changes in membrane potential to bring the membrane potential away from threshold and do not occur at the neuromuscular junction, but only within the CNS.

4.  I'm confused as to why the Sodium flux is greater that the K flux? In lecture I figured it had something to do with the membrane permeability but when referring to my notes I saw that the cell is more permeable to K? Is it because the in/outside concentration from Na vs. K is larger? 

1) If a ligand-gated channel permeable to both sodium and potassium was briefly opened at a specific location on the membrane of a typical resting neuron, what would be the result?

A: Local currents will flow from inside to the outside of the membrane and away from that region

I get the later half (moving away from that region), and I get that Potassium will want to move out of the cell. My confusion come from the fact that Sodium will also want to enter the cell, so how is it included in the 'current'?

This question is simply stating that you are making the membrane permeable equally to Na+ and K+ by opening up a channel that is equally permeable to both of those ions.  Yes, at rest you are more permeable to K+, but that is because you have channels allowing that.  Now, say you open a different-type of channel, one that is mixed-cationic and permeable to both K+ and Na+.  A channel that is equally permeable to both K+ and Na+ would have the equivalent of an 'equilibrium potential' that is halfway between their equilibrium potentials.  Remember the equilibrium potential is for a single ion, but if you are equally permeable to two ions the membrane potential would go to a membrane potential directly between their two equilibrium potentials so for this mixed-cationic channel it will have the equivalent of an equilibrium potential.  If the actual equilibrium potential for Na+ is quite positive (~+60mV) and the actual equilibrium potential for K+ is quite negative (~-90mV), then the 'equilibrium potential' for this mixed cationic channel would be halfway between those two values, or approximately -15mV.  In order for the membrane to  move from its resting membrane potential (between -50mV and -70mV) to this 'equilibrium potential', then in order to bring the membrane potential towards this 'equilibrium potential', positively charged ions need to enter the cell.  Because Na+ concentration is much greater outside of the cell than K+ concentrations, then the majority of the positively charged ions entering the cell would be Na+ ions.

5.  It does not matter then what type of neurotransmitters bind to the receptor because it's the receptor that determine the response of the postsynaptic membrane? 

Yes and no.  Postsynaptic receptors only having binding sites for specific neurotransmitters.  For example, nAchR only have binding sites for ACh (and nicotine which it is named after).  Therefore, other neurotransmitters will not activate that ligand-gated ion channel.  The point is not that the receptors can bind to anything, but that the neurotransmitters that can bind to multiple different receptors will only cause the postsynaptic response that the receptor that it the neurotransmitter binds to facilitates.

6.  Is the depolarization/hyperpolarization (EPSP/IPSP) caused by the different ions (K+, Na+, etc.) going throught he ligand-gated ion channels not the voltage-gated channels?

Yes.  As stated on slide 15, the influx of Na+ (and sometimes Ca2+) causes a depolarization of the postsynaptic membrane (also called an EPSP) while the efflux of K+ or the influx of Cl- causes a hyperpolarization of the postsynaptic membrane (IPSP).

7.  Almost like #2, in the lecture, when the depolarizing charges came in, it spread to the k+/Na+voltage-gated channels and cause AP.  I was under the understanding that these ions (K+/Na+) come in through ion channels cause a threshold to be reached and then Na+ influx via volage-gated channels to elicit AP.  Please clarify this for me.

I believe what you are asking is, do depolarizing charges through ligand-gated ion channels cause depolarizing graded potentials that summate to allow the voltage-gated Na+ channels to open causing the activation of an action potential?  Yes.  In the CNS, this happens, but so does the influx of Cl- which can prevent the summation of the EPSPs (the movement of Na+ into the cell) from bring the membrane potential to threshold.  At the neuromuscular junction (NMJ), the equivalent to the EPSPs which we call EPPs do occur.  If that is not what you are asking, please clarify.

8.  Can you please provide clarification on my questions?  I understand that ions can flow into or out of the cell via ionotropic receptors, thus allowing for depolarization or hyperpolarization  depending on the ion. What about metabotropic receptors? After the GCPR alpha subunit initiates the ion channel to open, can ions move in and out of the cell?  Or is this a one way direction only into the cell?   

If the secondary cascade of events causes the G-protein alpha subunit to initiate an ion channel to open, then indeed ions would move down their electrochemical gradients either into or out of the cell.  The cellular signaling cascades that occur from activation of a GPCR do not always cause the opening of channels, but can also change properties within the cell.

This leads me to my second question regarding the metabotropic receptor on slide 17.  After Ach binds the the GCPR, the beta/gamma subunit activate the ion channel to open.  According to the diagram it says that there is an "activation of inward rectifier K+ channel" thus leading to hyperpolarization.  However wouldn't an influx of K+ depolarization the cell? Wouldn't the cell have to have an eflux of K+ to hyperpolarize the cell and decrease heart rate?  

Indeed that seems confusing, but in fact the title of a 'inward rectifying K+ channel' is just a title of the channel itself.  When the channel is open it will still allow for the movement of K+ down its electrochemical gradient allowing for K+ to move out of the cell.  If more K+ moves out of the cell it would hyperpolarize the membrane potential.

Is the K+ channel that is activated by the By subunit of the G protein in the cardiac muscle a ligand- gated or voltage gated channel?

It is technically a voltage-gated K+ channel that is modulated by the binding of a ligand.  Similar to how phosphorylation can make some enzymes more ore less active, the binding of the beta-gamma subunit of the G protein causes the K+ channel to become more active. 

9. 8.  Periodic hyperkalemic paralysis is characterized by high potassium concentration and muscle weakness.  Which of the following is likely to cause muscle weakness as a result of increased extracellular potassium concentration?

  A.  Hyperpolarization of muscle cells

  B.  Inactivation of sodium channels in muscle cells

  C.  Increased release of neurotransmitters from alpha motorneurons

  D.  Decreased potassium conductance in muscle cells

  E.  Increased duration of action potentials produced by alpha motorneurons

Is the answer B, because an excess of extracellular K+ would prevent the efflux of intracellular K+ thus preventing hyperpolarization and the Na+ would stay inactivated?

 
Essentially yes.  It is not quite a prevention of the efflux of K+, but it is a reduction in the drive for the efflux of intracellular K+ which definitely reduces repolarization of the action potential causing the Na+ channels to stay inactivated.

10.  While watching the Synaptic transmission lecture today I became confused when you started talking about the Acetyl choline receptors.  One slide said that these receptors are excitatory receptors which depolarize the membrane (slide 15).  Then, on slide 17, where you were explaining the muscarinic ACh receptor, it said that the hyperpolarization of the membrane would occur.  Do ACh receptors depolarize and hyperpolarize the membrane?

Remember on slide 13 two different types of postsynaptic receptors are described.  nicotinic ACh receptors are ionotropic and themselves are mixed-cationic ion channels.  As described above in question #4, the movement through these channels will depolarize the membrane.  Muscarinic ACh receptors, in contrast, are metabotropic postsynaptic receptors which stimulate a series of secondary cellular signaling events which leads to the opening of a K+ channel thereby causing the hyperpolarization of the membrane.  The quick answer is yes, but the details of how both of those things occur is what is the physiology behind why ACh can cause both depolarization and hyperpolarization of the postsynaptic cell.

 
11. (from the comment on the Membrane Potentials lecture):  I might be over thinking this one, but figured I'd ask..

The "Biogenic Amine" neurotransmitters include Epi/NE. These two are types of adrenergic neurotransmitters because they are released from post-ganglionic sympathetic ANS axons.

Dopamine, serotonin, and histamine are also a types of biogenic amine neurotransmitters. Are ALL biogenic amine neurotransmitters considered adrenergic neurotransmitters too? Or are the classes of neurotransmitters from today's lecture simply a description of what they are derived from?  

They are a description of where they are derived from, therefore Epi and NE are adrenergic, but in the class of biogenic amine neurotransmitters with dopamine, serotonin, and histamine.


12.  From what I understand, temporal summation is the summation of postsynaptic potentials from same location at different times while spatial summation is the summation of postsynaptic potentials at different times from the same location. How would you define a situation where you have both spatial and temporal summation?

Your understanding of temporal summation is true: multiple action potentials coming close together in time from the same location.  Spatial summation, on the other hand, is multiple action potentials coming from different locations that are close together but at the same time.  Indeed, sometimes action potentials come from different locations at different times.  Scientifically we call that temporal and spatial summation of graded potentials.  Indeed, this occurs all the time.  Summation is simply the adding together in changes in membrane potential and that can occur both spatially and temporally.

13. From my understanding, summation of postsynaptic potentials would only be associated with electrical synapses and not chemical synapses. Is this correct?

On 6^th slide of today’s lecture, I understand there is electrical synapse between presynaptic neuron and postsynaptic neuron. If direct transmission is happening here, would summation of postsynaptic potentials cause it? 

No.  Summation is simply the addition (or summing) of graded potentials which typically occur at chemical synapses due to the influx or eflux of ions across the postsynaptic membrane.  Electrical synapses allow for the direct flux of ions from one cell into another and therefore can be an entire change in membrane potential equivalent to initiate an action potential in the next cell (as we will see in smooth and cardiac muscle cells).  Direct transmission of the electrical activity from one cell to the next would therefore simply allow for whatever changes in membrane potential are happening in one cell (ex. an action potential) would pass through the open pore into the next cell and initiate the same change in membrane potentials within the next cell.  Postsynaptic potentials that occur from chemical synapses from previous cells could cause the initiation of the action potential within that cell, but there is no need to summate through electrical synapses as the whole changes themselves could simply be passed from one cell to the next.

14.  I was wondering if you could clear up the electric and chemical synapses for me. Is there a definitive start at stop point for each, or are they both happening at the same time? I know you said that it goes from electrical (ions depolarizing) to chemical (neurotransmitters) and then back to electrical again, but I am confused as to when these happen. 

I think that your confusing electrical synapses and chemical synapses with what I said happens at chemical synapses.  Electrical synapses simply allow for the movement of electrical charges from one cell to the other through a pore (as explained in #13) allowing for electrical changes that are occurring in one cell to be directly communicated to a neighboring cell.  In a chemical synapse, however, you are taking an action potential (which is an electrical event) and changing it into neurotransmitters moving across a synapse (a chemical event) that activates graded potentials and the possible initiation of an action potential (an electrical event).  Therefore, the series of events takes electrical communication and turns it into chemical communication and then re-turns it back into electrical communication.  Furthermore, once an action potential is initiated in a skeletal muscle cell, that event then converts the release of calcium as a chemical event from the sarcoplasmic reticulum and then that activates a mechanical event of the cross-bridge cycling to increase muscle tension.

15.  I was wondering why turning point question number #3 has the answer 2 and NOT 4? Please let me know as soon as possible.  

For future reference, it would be helpful to provide a bit of thoughts on why you picked one answer over another.  However, Turningpoint quesiton #3 asks you to identify why the muscle cell would be refractory to ACh.  Now, blocking the break down of acetylcholinesterase would cause an additional concentration of ACh within the junctional cleft.  This itself cause s increased activity of the ACh receptors and continuous depolarization of the motor endplate.  Subsequently the voltage-gated Na+ channels surrounding the motor endplate cannot recover from inactivation not allowing further action potentials of the skeletal muscles.  During this time the ACh become desensitized to further opening as well, thereby not allowing follow-up depolarization of the membrane.  Eventually the ACh within the vesicles in the presynaptic membrane will begin to deplete, but it will take a much longer time for that to occur than it would for the initial set of events.

This is a turningpoint question that is used for discussion and learning about the whole system.  It is more difficult than the questions you will see on the exam, but due to time we could not cover that discussion within the lecture.