signal transduction at the neuromuscular junction - post-junctional events Flashcards
what is a miniature end plate potential (MEPP)
spontaneous miniature potentials are recorded by intracellular microelectrode inserted into a muscle cell
the size of deflection is quantal and frequency of occurrence is random
acetylcholine released from a vesicle causes a MEPP
Ach released from a single vesicle activates many nicotinic Ach receptors
upon activation the associated nicotinic receptor, cation channels open, and Na+ ions flux into the muscle fibre to cause a local depolarisation at the endplate region
simplified model of ‘fast’ synaptic transmission
1) presynaptic action potential
2) a synchronus Ca2+ influx via voltage-gated Ca2+ channels
3) many vesicles undergo exocytosis releasing a large cloud of acetylcholine
4) activation of many nicotinic Ach receptors
5) this causes a large depolarisation of the end plate region of the muscle cell (an EPP)
6) if the depolarisation is large enough it activates postsynaptic voltage-gated sodium channels to initiate an action potential
what is the involvement of calcium at the NMJ
essential for neurally-evoked neurotransmitter release
localised calcium entry via voltage-gated calcium channels
↑ in calcium concentration = ↑ in release
calcium triggering of vesicle fusion is very fast ≈ 0.1ms
magnesium blocks the voltage-gated calcium ion channel
what is the issue with studying the neuromuscular junction
nerve stimulation causes muscle contraction resultin gin the glass microelectrode breaking
therefore a hihgh Mg2+/low Ca2+ buffer solution is used to reduce the EPP to below the threshold for firing an action potential
what is quantal release?
the amplitude of the EPP is a multiple of the amplitude of the MEPP, with the smallest EPP amplitude equal to that of the MEPP amplitude
what is the quantal content (QC)
number of vesicles / stimulus
QC = mean EPP amplitude (mV) / mean MEPP amplitude (mV)
release of a vesicle gives a ‘quanta’ of transmitter
each quanta gives rise to a miniature end plate potential via activation of nicotinic acetylcholine receptors
MEPPs occur spontaneously (without nerve stimulation)
EPPs occur in response to motor nerve stimulation
upon nerve stimulation MEPPs summate to give an EPP which initiates an action potential > muscle contraction
synaptic transmission at the NMJ - synthesis / hemicholonium 3
choline acetyl transferase (CAT) synthesises Ach from precursors choline and acetyl CoA from the mitochondria
re-uptake of choline is Na+ dependent and is blocked competitively by hemicholonium 3 so there will be less Ach in each vesicle
amplitude of the EPP and the MEPP are both decreased to a similair extent = no change in the quantal content
synaptic transmission at the NMJ - storage / vesamicol
transport of Ach into vesicles blocked by inhibition of the Ach vesicular transporter by vesamicol
less Ach in each vesicle so the amplitude of the EPP and the MEPP are decreased to a similair extent = no change in the quantal content
⍺-latrotoxin
influences spontaneous transmitter release
- massive Ach release
- muscle spasms
- depletion of vesicles
- inhibition of endocytosis
- distended terminal
- paralysis
endocytosis
recovery of vesicular membrane after fusion
tetrodotoxin
extremely toxic
- paralysis of the diaphragm
- respiratory failure
- blocks Na+ channels = no activation of voltage-gated Ca2+ channels = no action potential = no release = no EPP
conotoxins
block voltage-gated Ca2+ channels = ↓ Ca2+ influx = ↓ release
EPP amplitude decreases
MEPP amplitude remains the same
= decreased quantal content
dendrotoxin
block voltage-gated K+ channels = prolonged action potential = ↑ Ca2+ influx = ↑ release
EPP amplitude increases
MEPP amplitude remains the same
= increased quantal content
botulinum toxin
respiratory paralysis
blocks vesicle fusion by cleaving a vesicular protein required for exocytosis = ↓ release
EPP amplitude decreases
MEPP amplitude remains the same
= decreased quantal content
