The NMJ Flashcards
what type of motor neurones are skeletal motor neurones
- alpha motor neurones
describe where motor neurones exit the spinal cord
- Cell bodies in ventral (anterior) horn of spinal cord
* Exit via ventral (anterior) roots
what is a motor unit
• The motor neurone plus the group of muscle fibres
describe what happens if more or less muscle fibres are innervated
- More muscle fibres innervated more power but less precisions
- Less muscle fibres innervated less power but more precision
what do the motor axons divide into as they reach the neuromuscular junction
- Each motor axon as it approaches the muscle fibre divides into a number of fine terminal branches – motor end plate
- Within the end plate individual branches form a bulbous presynaptic axon terminal – terminal boutons
what is the terminal bouton filled with
synaptic vesciles
describe the structure of the neuromuscular junction
- Muscle fibres goes up and down into ridges, at the top at the neck of the ridges is the nicotinic acetylcholine receptor
- The single neurotransmitter is filled with acetylcholine
- The terminal boutons contain many mitochondria – metabolically active
- Lots of synaptic vesicles – about 50nm in diameter, contain acetylcholine
- Muscle membrane under each bouton is extensively folded
- Nicotinic – ligand gated sodium ion channel that allows the sodium in in order to conduct waves of depolarisation,
- Synaptic vesicles are all near the base of the synapse where they are needed
how large are synaptic vesicles
50 nm in diameter
how is acetylcholine produced
- making acetylcholine in the pre-synaptic bouton
- Choline and acetyl-CoA is combined by enzyme choline acetyltransferase which acetylates the choline
How is acetylcholine broken-down
- Acetylcholine is made from choline and acetyl CoA, in the synapse ACh is rapidly broken down by the enzyme acetylcholinesterase, choline is then transported back into the axon terminal and used to make more ACh
how is acetylcholine released
- Action potential in motor neurone axon (1) gets to terminal bouton
- Voltage gated calcium channels (2) open and calcium enters, these help with the vesicles dock to the presynaptic membrane
- Docked vesicles fuse with pre-synaptic membrane (3)
- Release contents into synaptic cleft (4)
what are the two main types of calcium channels
- L and N types
describe type L calcium channels
- Heart and vascular smooth muscle
- L-type calcium channel blockers = reduce blood pressure or heart rate
- Calcium channel blocker – used to reduce blood pressure or heart rate as they cause the vascular smooth muscle to relax
describe type N calcium channels
- In skeletal muscle and the autonomic nervous system as presynaptic terminals
- Mediate transmitter release
where are calcium channels located in the neuromuscular junction
• Calcium channels are actually positioned close to pre-synaptic membrane and vesicles, they are located on the baso membrane of the pre-synaptic membrane
How do calcium cause the vesicle to fuse with the presynaptic membrane
- calcium causes membrane depolarisation when it enters
- calcium binds to the binding protein synaptotagmin
– calcium binding site where calcium can bind to C2B and C2A this synapotgamin is the linker between the SNARE proteins, - once this has been bound it causes a conformational change in syntaxin and synaptobrevin to interact, this is associated with SNAP 25
• Can wind the vesicle in, syntaxoin and synaptobrevin attach to each toerh and are closer together
what does synaptotagmin do?
- synaptoagmin links syaptobrevin and syntax
what are the proteins on the vesicle
• Synaptobrevins
what are the proteins on the presynaptic membrane
- Syntaxins
* SNAP-25 associates with syntaxin and interacts with synaptobrevin
how are the vesicles recycled
- After ACh released into synaptic cleft empty vesicles are coated with clathrin
- Clathrin coated vesicles move back into synaptic terminal cytoplasm
- Refilled with ACh
- The presynaptic membrane would continue to increase in size, this gets the plasma membrane that forms the vesicle back, need to refill it with the newly made acetylcholine
how many proteins is clathrin made up of
3 linked protein
what does the acetylcholine bind to in a NMJ
• ACh in synaptic cleft binds nicotinic cholinergic receptor embedded in muscle membrane
describe the nicotinic cholinergic receptor
- nACh receptor = ligand gated sodium channel
- 5 sub-units
- 2 identical alpha sub-units (α1)
- Needs 2 Ach to bind to activate
- 1 beta, 1 delta and (1 epsilon or 1 gamma) sub-units
what happens when acetylcholine binds to an acetylcholine receptor
– causes a relative change in the membrane potential and cause an action potential
- Need two acetylcholine to bind and they bidn to the alpha units of the acetylcholine receptor
- opens sodium channels
what does the inward movement of sodium ions into the muscle produce
- Inward movement of Na+ ions into the muscle produces a depolarisation in the muscle called
- End-plate potential (EPP)
what does end-plate potential depend on
• EPP is a graded depolarisation that depends on the amount of acetylcholine released
when is the action potential triggered in muscle
- An action potential is triggered in the muscle when the depolarisation reaches a threshold (typically about -40 mv)
- Muscle is higly excitatble and it only needs about 20mv of change in order to get it to react
what is one to one transmission
: Each action potential in the nerve produces an action potential in the muscle. We get ONE-TO-ONE transmission from nerve to muscle. This is very important for the reliable conversion of a signal from the brain to a specific force of contraction of a muscle.
what is myasthenia gravis
- Autoimmune condition
- This is where you make antibodies that attach the acetylcholine receptors
- Antibodies stick to the acetylcholine receptors and block them therefore the acetylcholine can no longer bind to the receptors
- So the EPP is not often a large enough to trigger an action potential
what are the symptoms of Myasthenia gravis
- This causes muscle weakness and fatigue
- Can often see it in the face – cant keep eyelids open cant smile so they look expressionless
where is acetylcholinesterase located
- Acetylcholinesterase is embedded in muscle membrane not lose int eh cytoplasm, it is close to the receptor
- As soone as the acetylcholine is released or excess acetylcholine, it goes into acetylchcolinesterase and tis broken down quickly
what does acetylcholinesterase break acetylcholine into
- Choline
* Acetic acid
what happens to choline when it is broken down from acetylcholine
• Choline is taken back up into axon by the choline transporter, it is combined with acetyl CoA and put back into the empty vesicles
describe the characteristics acetylcholinesterase
• It is a very quick acting enzyme, acts in milliseconds, so neutralises actions of released ACh very quickly preventing spontaneous or continued activation
how many vesciles are released with each stimulus release
- Each stimulus releases ~10% of docked vesicles,
* Which are ~1% of the total vesicles that we have in the terminal
how long would it take for acetylcholine to run out without recycling
- So each stimulus releases ACh from about 1 in 1000 of the vesicles in the synapse
- So without recycling transmission would fail after approximately 1000 action potentials
what drugs prevent the uptake of choline
, can use hemicholinius to block uptake of choline
how can you prevent the release of acetylcholine
- Have drugs to prevent uptake of chloine, can use hemicholinius to block uptake of choline
- Vesamicol blocks refilling of empty vesicles not used clinically
- Can block calcium channels
- Can use botulinum toxin to prevent vesicle release
what does vesamicol block
• Vesamicol blocks refilling of empty vesicles not used clinically
what does the botulinum toxin do
- Shuts down the SNARE and SNAP proteins and prevent the release of acetylcholine
what is the botulin toxin made from
o Clostridium botulinum – a gram positive anaeorbic bacterium
what are the neurotoxins in the botulin toxin
o 7 neurotoxins (A, B, C, D, E, F and G)
what neurotoxins from the botulin toxin poison humans
o Human botulism mainly types A, B, E and rarely F
what neurotoxins from the botulin toxin poison animals
o Types C and D cause toxicity in animals
what do type A and E do from botulin toxin
Cleaves synaptosome associated protein (SNAP-25) – presynaptic
what do type B, D and F do from botulin toxin
Cleave synaptobrevin – the one that is embedded in the acetylcholine vesicle
what do type C do from botulin toxin
Cleaves syntaxin
what is the botulin toxin used for
- Focal dystonia’s
- Involuntary, sustained or spasmodic patterned muscle activity
- Cervical dystonia (torticollis)
- Spasticity
- Increased muscle tone
- Stroke
- Cerebral palsy
- MS
- Headaches
- Sweating, salivary disorders
- Cosmetic use
how is the botulin toxin reversed
• The neuromuscular junction reforms with new snares and new snaps with a new position next to it, therefore after about 3 months you get the wrinkles back
what are NMJ blockers
conotoxins
- no acetylcholine is released
what are the therapeutic use of conotoxins
- N-type voltage calcium channels related to algesia (sensitivity to pain) – could put in conotide which is an artificial version of conotoxins and this blcoks the calcium entery – not used outside of the CNS as it has side effects such as blocking breathing muscles
- Ziconotide has to be used intrathecally because of profound side effects
- Blocks Ca2+ entry = analgesic actions
what are non depolarising NMJ blockers
- D-tubocuraine
- Non-depolarising muscle relaxant
- Blocks nicotinic ACh receptor (competitive antagonist)
- Rocuronium and pancuronium
- Antidote acetylcholinesterase inhibitor
- Physostigmine and neostigmine
how do non depolarising NMJ blockers work
- Binds to the nticitonic receptor and blcoks acetylcholine from being able to access its bdingin site
- Can be use for incubation
- If you put enough acetylcholine in then it overcomes this blockage
- Can also give acetycholine antidote so you get more acetychylien that can compelte with the tubucuraine
what is an example of a depolaring blocker
- E.g. Succinylcholine
how do depolarising blockers work
o Stimulate ACh receptors like ACh and activate muscle
o But they do not detach
o Muscle unable to initiate depolarisation after the 1st one or two even if membrane has repolarised
o Desensitized to ACh
o Widely used for tracheal intubation
Succinylcholine
what is the difference between non depolarising and depolarising NMJ blockers
- Non-depolarising blocker
- Competitive blockers (they have no intrinsic activity)
- Anatagonism overcome by increasing ACh
- Depolarising blocker
- Non-competitive or competitive
- Act like Ach but persist at the synapse and constantly stimulate receptor
- 1st opening of Na+ channel leads to transient twitching of muscle but continued binding makes receptor incapable to transmit impulses
how do acetylcholinesterase inhibitors work
– they prevent the acetylcholine from breaking into choline and acetate
• Makes NMJ hyperactive muscle goes into convulsions and then paralysis
what are examples of acetylcholinesterase inhibitors
• Tabun, sarin and Novichok
what are acetycholineusterases inhibitors used for
- Neostigmine and physostogmine for a treatment
* Echothiophate – “irreversible” used for glaucoma
what are examples of
- agonists
- antagonists
- anticholinesterase
- depolarising blocker
- release inhibitor
- synthesis inhibitor
- acetylcholine, nicotine, carbachol
- tubucurarine , pancuronium
- neostigmine, physostigmine
- succinylcholine
- botulinus toxin, conotoxin
- hemicholinium
