Lecture 20: Synaptic Transmission Flashcards
What are the 2 types of synapses?
- Electrical = gap junctions 2. Chemical
How do the synapses in the brain compare to those in the gut?
Those in the brain communicate more rapidly
What is the neuromuscular junction (NMJ)?
The neuroanatomical contact point between nerve and muscle
What is another name for the neuromuscular junction?
The End Plate
What is the depolarization of muscle fibers due to?
Excitatory postsynaptic potentials (EPSPs)
What are 2 another names for EPSPs?
- End Plate potentials (EPPs)
- Excitatory Junction Potential (EJPs)
Which one is the presynaptic neuron?
The one releasing the NTs
Which one is the postsynaptic cell?
The one receiving the NTs: muscle cell
What happens once the AP reaches the the axon terminal?
Ca2+ voltage-gated channels open and permit an influx of ions
What is excitation-secretion coupling?
Process by which depolarization increases free cytosolic calcium which enters the axon terminal and induces synaptic vesicle fusion on the presynaptic membrane and exocytosis of NTs
How do the NTs affect the postsynaptic cell?
They directly or indirectly alter its conductance
What is the active zone of the presynaptic cell?
The region that is highly concentrated with vesicles fused along the inner leaflet of the membrane in parallel rows containing a total of ~100mM of acetylcholine + proteins to facilitate NT release
How many vesicles can be released upon nerve stimulation?
20-30
What is another name for the NT containing vesicle?
Quanta
How many ACh receptor-ion channels can each NT containing vesicle activate?
1,000
What type of channels are ACh receptor-ion channels?
Ligand-gated channels
What is the reversible competitive antagonist to Ach? What is it used for? Pre or postsynaptic mechanism?
Curare, preventing ACh from binding REVERSIBLY
Used during surgery (d-tubocurarine) on men: to paralyze skeletal muscle nerves so that the patient does not twitch
POSTSYNAPTIC MECHANISM
What is the irreversible competitive antagonist to Ach? Pre or postsynaptic mechanism?
Alpha bungarotoxin (Cobra poison)
POSTSYNAPTIC MECHANISM
What does it mean for an antagonist to be competitive?
It means that it can be overcome by increasing concentrations of the agonist
What does it mean for an antagonist to be noncompetitive?
It means it permanently closes the channel, regardless of how much agonist is added
What does the synaptic cleft contain?
The basal lamina, which contains acetylcholinesterase (AChE) which instantly cleaves Ach into acetyl CoA + choline after it’s released by the receptor
What is myastenia gravis? How is it treated?
Autoimmune disease where antibodies block ACh receptors blocking the NMJ. Can be treated with inhibitors of acetylcholinesterase (eg: tensilon) to increase time of ACh in the synaptic cleft and then increase the effect of ACh on the receptors
What are 3 specializations of the postsynaptic membrane?
- Junctional folds 2. NT receptors 3. Scaffolding proteins
What are junctional folds?
Located on the postsynaptic membrane to: (1) increase surface area to allow for more receptors and (2) decrease the distance between the pre and post synaptic cells
What is the structure/location of a motor neuron when it enters a muscle?
It splits into many unmyelinated branches, runs along a muscle fiber, and ends at the NMJ
How many muscle fibers per axon terminal in adults?
Multiple!
How many axon terminals per muscle fiber in adults?
1
How many axon terminals per muscle fiber in children?
Multiple because we are born with more NMJ than we need and the neurons compete for survival and innervation
Where are AChRs located in early life?
All along the length of the muscle cell, not only at the motor end plate
What is the motor end plate?
Location on muscle fiber which contains AChRs
What happens to muscle fibers with development in early life?
AChR cluster at the NMJ, extra End Plates are pruned off, and polyneuronal innervation ends
What is the part of the brain that exerts ultimate control over skeletal muscles?
Motor cortex
How is the motor cortex connected to skeletal muscles?
One long axon extends from the motor cortex to the spinal cord and synapses in the spinal cord on motor neurons
What are the 2 scaffolding proteins? What is their on the post synaptic muscle cell?
Agrin and dystrophin: they help AChRs cluster at the motor end plate
What is the exact role of dystrophin?
It links the muscle membrane to the actin cytoskeleton giving structure to the motor end plate
What is Duchenne muscular dystrophy caused by?
Gene defects in dystrophin, which causes the motor end plate to lack structure
What is the synaptic delay? What is it due to?
The time between AP arrival at axon terminal and production of post synaptic response Due to the time it takes V-gated Ca channels to open, Ca to induce vesicle fusion, and NTs to travel to post synaptic cell
What is the input-output relation?
A measure of the post synaptic response as a function of presynaptic potential
When does calcium enter the axon terminal? Start and end?
Starts when the axon starts getting repolarized and ends when it is back to RMP
When does the post synaptic AP start with regards to the pre synaptic AP?
It starts when the pre synaptic cell is completely repolarized (before the undershoot though)
How do the AChR cause cell depolarization?
The receptors are permeable to Na+/K+, so when ACh binds, Na+ rushes in
What is the safety factor? What is it due to?
The excess voltage produced by the presynaptic nerve (well over threshold) caused by excess vesicle release:
Quantal content >>>>>> Threshold
What is the main difference between an EPP/EPSP and an action potential? What are the 5 mechanical differences?
The purpose of the EPSP is to depolarize the muscle cell enough to cause an AP in it (vs AP between 2 neurons) and also it is a graded potential meaning its size will depend on the number of vesicles released
Does the number of vesicles released at each NMJ the same?
It fluctuates around an average value
What are Miniature End Plate Potentials (MEPPs)?
Small spontaneous depolarizations of postsynaptic muscle cells due to spontaneous release of ACh
What is the Quantal Theory of NT release?
MEPPs result from spontaneous irreducible units of NTs (quanta) which added up together form an EPP (the more MEPPs, the stronger the EPP)
What are the 2 types of ligand-gated channels? Different names for each?
- Ionotropic = primary 2. Metabotropic = secondary
Describe ionotropic ligand-gated channels.
Directly activated by ligand biding: when 2 ACh bind both Na+ and K+ channels open and both enter the cell
What is an example of an ionotropic ligand-gated channel? Explain how it works.
Nicotinic acetylcholine receptor: stimulated by ACh
Describe metabotropic ligand-gated receptors
Indirectly activated by ligand binding: ligand activates either a G protein or second messengers (to activate enzymatic pathways) that in turn activate an ion channel (the metabotropic receptor is not an ion channel!)
What are 2 examples of metabotropic ligand-gated channels? Explain how each works. Where are they found?
- M2 muscarinic AChR: G-protein coupled receptor
- M1 muscarinic AChR: second messenger cascade receptor
Both found in smooth muscle, heart, and neurons
Can an NT activate multiple types of receptors?
Yes
Describe the permeability of Na+ and K+ in the muscle fiber?
The same!
What is the RMP of a muscle cell?
-90mV
What is the Na+ Erev of a muscle cell?
+90mV
What is the K+ Erev of a muscle cell?
-90mV
What happens in the muscle cell once the ACh bind to the motor end plate?
Na+ rushes in: depolarization, which causes K+ to rush out until they balance each other out at 0 mV: equal flux of Na+ and K+ through the ACh channel Vm = Erev
What is the concentration of calcium in cerebrospinal fluid?
1-2 mM
What is the concentration of calcium in cytosol compared to in cerebrospinal fluid?
Much lower
What is the concentration of calcium in inside the cell? What does this mean?
10-50 nanomolars, which means the concentration gradient with the cytosol is HUGE and requires a lot of energy to maintain
How much Ca remains free when it enters the presynaptic axon terminal?
For 1,000 Ca in only one remains free
What 3 processes result in calcium homeostasis?
- Ca binding 2. Ca sequestration 3. Ca extrusion
What 5 molecules allow for calcium binding in the presynaptic axon terminal?
4 proteins: 1. Calmodulin 2. Calcineurin 3. Calreticulin 4. Synaptotagmin + head groups of phospholipids (eg: PLA2)
What are the 2 roles of synaptotagmin?
- Ca binding 2. Vesicle fusion
What sequesters calcium in the presynaptic axon terminal?
Sequestered by mitochondria and SER by Ca-ATPase pumps
Describe mitochondrial calcium sequestration by the Ca-ATPase
Na+/Ca2+ antiport: low affinity and high capacity
Describe SER calcium sequestration by the Ca-ATPase
High affinity, low capacity
Which Ca-ATPase pump plays a bigger role: mitochondrial or SER?
SER pump under normal conditions, and mitochondrial if the cell is highly active or injured
What are the 2 ways for the SER of the presynaptic neuron to release free calcium?
- IP3 (results from cleavage of PLA2 into IP3 and DAG) binds receptors that cause calcium release for increase in cytosolic concentration
- Ryanodine binds receptors and release calcium for muscle contraction
What is a ryanodine receptor stimulator?
Caffeine
What are the 2 mechanisms to pump out calcium out of the presynaptic axon terminal?
- Ca-ATPase 2. Na/Ca antiport (3 Na for 1 Ca)
What are the 4 types of calcium channels on the membrane of the presynaptic axon terminal? Does each bring calcium in or out?
- Ca-ATPase - OUT
- Na/Ca antiport (3 Na:1 Ca) - OUT
- Voltage-gated Ca2+ - IN
- NMDA ligand-gated channel - IN
What are the 2 MAIN types of voltage-gated calcium channels? Describe each.
- L-type (long-lasting): less sensitive, open slower, and remain open longer
- T-type (transient): more sensitive, open faster, and remain open for shorter periods of time
What are L-type calcium channels good for?
Sustaining an AP
What are T-type calcium channels good for?
Initiating an AP
What do T-type calcium channels play an important role in?
Rhythmic processes: heart beat, breathing, sleep, walking
Draw the table of all the different kinds of V-gated calcium channels
What are dihydropyridines (DHPs)? What do we use them to treat?
Pharmacological agents that block L-type calcium channels (but do not modify them) used to treat angina and hypertension
What is w-conotoxin?
Venom produced by fish-eating snail (conus geographus) which block N-type calcium channels
What calcium channels does the funnel web spider’s venom block?
P-type calcium channels at the NMJ
How many ACh receptors on the post synaptic membrane?
10,000/micron2
What allows the vesicles to be very concentrated with ACh?
Proton pumps that make the milieu of the vesicle very acidic. Without it acetylcholine would precipitate and would not be able to be released
What is the concentration of the ACh in the vesicle?
150 mM
How big is the vesicle?
15-16 nm in diameter
What transporter protein is responsible for bringing the ACh within the vesicle?
Cholineacetyl transferase (CAT)
What are the 3 main electrical events happening at the NMJ?
- Presynaptic nerve action potential
- Postsynaptic end plate potential
- Postsynaptic muscle cell action potential
What are decreasing if you decreasing number of AChRs: a quantal or the quantal size/content?
Quantal size
How does botox work?
PRESYNAPTIC MECHANISM
Inhibiting proteins that are necessary for the fusion of the vesicles at the presynaptic neuron
What is the voltage/current relationship in an EPP?
Linear
What is the voltage/current relationship in an AP?
Non-linear because of voltage-gated channels
How long does it take for the ACh to be released, to diffuse and bind to the receptor, and to be broken down?
1 ms at room temperature
What are decreasing if you decreasing number of AChRs on the postsynaptic neuron: a quantal content or the quantal size?
Quantal size
What is the concentration of ACh in the synaptic cleft?
1 mM
How many ACh molecules per receptor?
2
What are decreasing if you decreasing number of calcium channels on the presynaptic neuron: a quantal content or the quantal size?
Quantal content
What is the quantal size vs content?
Quantal size: the synaptic response to the release of neurotransmitter from a single vesicle
Quantal content: number of effective vesicles released in response to a nerve impulse
What is the exact role of agrin?
It tells the muscle membrane to form a new NMJ
What happens when a nerve is injured on a skeletal muscle?
It spreads all over the muscle, not just at NMJ
The EPP propagation on the muscle is electrotonic - what does this mean? Why is this?
Means it decreases with distance (vs the AP which actively propagates) because AChRs are only found at the end plate
Which one produces a contraction: EPP or AP?
AP in muscle!
Do ion channels open for different or same amounts of time?
Different each time! Overall there is an average time though
What determines the speed of the synaptic current?
The average time the AChR ion channels are open
What is the reversal potential of the end plate? Explain why.
0 mV because Na+ and K+ are equally permeable to the channel and ENa = - EK so they balance each other out until INa = - IK
What is a synapse?
Junction between 2 nerve cells
Does an EPP include an overshoot?
YES
What is nicotine an agonist to?
ACh receptors
What does nicotine enhance the release of?
Dopamine
How is ACh put into the vesicles?
ACh-H exchanger
How were L and T type calcium channels discovered?
Voltage clamp experiments on cardiac myocytes
How are EPPs terminated?
By the removal of acetylcholine from the synaptic junction
Can the release of NT vesicles be modified by second messenger-induced processes?
YES
Where does the recycling of vesicles occur?
At coated pits
Do all noncompetitive antagonists permanently close the ion channel?
Not necessarily
Why is EPP repolarization passive?
Because the AChRs simply close and there is not a specific repolarizion channel, just the leakage ones.
Why would a channel specifically permeable to Ca2+ cause a larger depolarization than one permeable to Na+?
Because of the larger concentration gradient of calcium and the added charge on calcium
What happens if you depolarize the post in NMJ really slowly? Why?
The AP will not fire, quantal content not big enough
