Anatomy of the NMJ
NMJ is called the end-plate, and is the site of contact between a motor neuron and a skeletal muscle fiber
Synaptic boutons at end of axons of motor fiber contain synaptic vesicles containing ACh
ACh released from active zones on presynaptic terminal (sites of NT release)
Postsynaptic muscle fiber has regularly spaced invaginations called junctional folds, which contain acetylcholine receptors (nicotinic) on the crests of the junctional folds
Synaptic basal lamina (basement membrane) within synaptic cleft which contains acetylcholinesterase (technically on postsynaptic membrane)
Synaptic transmission at the NMJ
1) Depolarization of presynaptic terminal membrane by action potential activates voltage-sensitive Ca2+ channels at presynaptic terminal
2) Ca2+ flows into presynaptic terminal and triggers fusion of synaptic vesicles with presynaptic membrane
3) ACh in vesicles released into synaptic cleft and diffuses to postsynaptic membrane
4) ACh receptors in postsynaptic membrane are ligand-gated ion channels that undergo conformational change to open ion channel and let Na+ in (also permeable to K+) to depolarize (depolarization produced by activation of nicotinic receptors called end-plate potential, or EPP)
5) ACh cleared from synaptic cleft by being broken down by acetylcholinesterase
Is an EPP an action potential?
EPP is just the depolarization that occurs when ACh binds nicotinic receptors at NMJ
AP is created if EPP strong enough (so, usually but not always). Then, voltage gated Na+ channels open to cause AP
What does it mean that NT release is quantal?
NTs are released in small packets corresponding to individual synaptic vesicles
1 quanta = 1 synaptic vesicle
Miniature end plate potentials (MEPPs)
Postsynaptic responses to ACh spontaneously released from presynaptic nerve terminal
Small, randomly occurring spontaneous potentials in postsynaptic cell occur about 1-5 per second
Identical to normal EPPs but much smaller
What happens when extracellular [Ca2+] is decreased?
Normal EPP becomes smaller and smaller
What are EPPs?
Near simultaneous release of multiple quanta (synaptic vesicles)
Size of EPP depends on number of quanta released (quantal content)
Quantal content (m)
Number of quanta that make the EPP
How does Ca2+ trigger exocytosis of ACh synaptic vesicles?
1) v-SNAREs on vesicle (synaptobrevin) bind t-SNAREs on presynaptic terminal membrane (syntaxin and SNAP-25) and vesicle "docks" on membrane
2) Complexin prevents vesicle from fusing and releasing ACh
3) Ca2+ comes into presynaptic terminal from outside and binds vesicle protein called synaptotagmin causing it to undergo a conformational change
4) Synaptotagmin completes fusion of vesicle to presynaptic membrane and ACh is released into synaptic cleft
Note: synaptobrevin is v-SNARE that binds SNAP-25 and syntaxin; synaptotagmin is the Ca2+ sensor
How does Botulinum Toxin (BoTox) block synaptic transmission?
Cleaves synaptobrevin (v-SNARE) so that it can't bind t-SNAREs to dock the synaptic vesicle to presynaptic membrane
Other components of BoTox also cleave syntaxin and SNAP-25
Basically, disrupt SNAREs so no synaptic transmission at NMJ
Note: produced by anaerobic spore-forming bacteria and most often caused by eating improperly prepared/stored food
For one presynaptic action potential, you will trigger a postsynaptic action potential
NMJ has high safety factor: only need 100 quanta to pass threshold and get postsynaptic AP, but instead release about 200 just to make sure you'll get an AP
"synapses always work"
For one presynaptic action potential, do we always get the same strength of postsynaptic EPP?
No, it is very variable!
For one presynaptic AP could get release of 200 quanta, but the next will be 170 quanta and the next 210 quanta, etc etc
When multiple APs occur in rapid succession and EPPs evoked by each AP progressively increase in amplitude
This happens because transmitter release is sensitive to Ca2+ concentration and higher level of Ca2+ that builds up evokes release of larger number of synaptic vesicles which result in larger EPP
Usually, once AP over Ca2+ is removed from release sites, but if rapid succession of APs, there is residual Ca2+ that stays around which summates with new Ca2+ influx resulting in higher levels of Ca2+ in presynaptic terminal
During prolonged burst of presynaptic action potentials, size of EPPs evoked by each AP eventually begins to decline
This happens because number of vesicles filled with NT and available for release declines (all used up!)
Usually this is not a problem, but people with myasthenia gravis (lower safety factor) could have decreased synaptic transmission because of depression
Reversible, competitive inhibitor of ACh derived from bark of tropical tree
Used to partially block postsynaptic ACh receptors to reduce EPP to below threshold if you want to see just the EPP alone
(alpha-bungarotoxin does the same thing)
Nicotinic ACh receptor
Ligand-gated ion channel
ACh binding causes conformational change that opens ion channel permeable to both Na+ and K+ (lets a little Ca2+ in also, but not much)
At normal resting potential, you'll get influx of Na+ into cell
Pentameric protein with ACh binding site on alpha subunits; membrane-spanning
When does the end plate current (EPC)/synaptic current terminate?
When there's no more ACh around (because it's been hydrolyzed by AChE)
Now the nicotinic ACh receptor is no longer open and membrane no longer permeable to Na+ and K+, so back to resting membrane potential
Equation for current (I) through channel
I = g(Vm - Erev)
g = membrane conductance
Vm - Erev = driving force (moves ions through the channel)
Equation for current through ACh receptor channels at any given membrane potential
Iepc = gNa x (Vm - ENa) + gK x (Vm - EK)
Sum of current carried by Na and current carried by K; end plate current
Reversal, or equilibruim potential for nicotinic ACh ion channels
Between ENa (+40) and EK (-90)
From black widow spiders
Induces massive spontaneous release from nerve terminals via 2 mechanisms:
1) Activates presynaptic neurexin to directly cause NT release (even without Ca2+)
2) Forms Ca2+ permeable pores in plasma membrane and influx of Ca2+ triggers NT release