NMJ Flashcards
What is the NMJ?
A specialised synapse for the transmission of a signal from the motor nerve terminal to a postsynaptic region on a muscle fibre.
Why is the NMJ the primary model for studying synaptic structure and function?
Large, efferent excited by electrical stimulation.
What does conformational change in synaptotagmin enable it to do?
Stabilise SNARE complex at the vesicles, where v-SNAREs and t-SNAREs in a state of hemi-fusion (outer leaflets merged only). Stabilisation via conformational change allows to progress to full fusion, resulting in formation of a fusion pore.
What does fusion pore provide?
Direct route for ACh to diffuse across cleft.
What toxin cleaves SNARE proteins, preventing exocytosis? (Flaccid paralysis).
Botulinum toxin.
Mutuations in what gene encoding what channels reduce ion entry and thus impair NT release. Results in Human Episodic Ataxia Type II.
CANCA1A, P/Q type v-g Ca2+ channels.
What study used freeze-fracture electron micrographs to show that there was a gap between motor axon and muscle fibre, and vesicles in active zones?
Heuser, 1979.
How does the presynaptic neuron facilitate efficient exocytosis? (Shown by FF electron micrographs).
Vesicles concentrated in active zones where there are high concentrations of v-g Ca2+ channels.
How does the motor endplate facilitate efficient transmission?
Junctional folds, increase SA and thus enhance the likelihood of ACh binding, promoting stronger depolarisation. Lots of nAChRs there, lots of v-g Na+ there too (low threshold).
How does the observation of mEPPs (less than 1mV) provide evidence for quantal release?
Fixed amplitude, corresponds to individual quanta.
How does binding lead to conformational change in nAChR?
Energy of binding translated into change.
Why is Na+ flux through v-g channels regenerative?
Increasing polarisation opens more channels. Makes it all or none.
What toxin evidences role of nAChRs?
Curare, competitive antagonist of them, blocks ACh binding.
What is evidence for role of sodium ions?
TTX application, AP not fired even if EPP.
How are postsynaptic ionic gradients restored?
Calcium ATPase pumps. Actively transport Ca2+ out of cytoplasm. Also sodium-potassium pump, re-establihses RMP.
Why is ACh rapidly degrated in the cleft?
To prevent continuous stimulation of the muscle fibre (and facilitate temporal precision in muscle movement).
What nerve gas blocks acetylcholinerase, leading to overactivation of muscle cells and desensitisation of receptors -> convulsive spasms.
Sarin.
What happens in kiss and stay?
Vesicle remains docked at active zone, VAChT uses a proton gradient as energy source to actively transport ACh into vesicle.
What does kiss and stay allow? When is this useful?
Prolonged NT release, useful in periods of lower synaptic activity.
What happens in kiss and run?
Fusion pore closes immediately after exocytosis, retaining some contents.
What does kiss and run allow for? When is this useful?
Quick re-use of the vesicle in subsequent rounds of neurotransmission, critical during high-frequency stimulation.
How do vesicles endocytyse?
Via clathrin-coated pits, reacidifying, filling with ACh either directly or after passing through an endosomal intermediate.
Why is enodocytysing useful?
Slow, but means damaged or dysfunctional vesicles are removed from circulation, preserving the quality of the vesicle pool, which is itself integral.
Who showed that, at the frog NMJ, increases in Ca2+ concentraton increases the number of quanta released from the nerve?
Del Castillo and Katz (1954).
What do animal models show to highlight the role of Ca2+?
Mice with synatptotagmin-1 “knocked out” experience severely reduced synchronous NT release.
How many molecules of ACh does each vesicle contain?
5,000-10,000
Example of vSNARE and tSNARE:
Synaptobrevin, syntaxin.
What does physical gap between preS and postS suggest?
That transmission may depend on diffusion of substance across cleft.
What is myasthenia gravis? What is it caused by?
Autoimmune condition which leads to varying degrees of muscle weakness; difficulties speaking, swallowing etc.
Caused by destruction of nACHRs at NMJ, preventing nerve impulses from triggering muscle contractions.
If ACh were released in varying amounts (not quanta) what would we expect mEPP amplitudes to do?
Vary more widely.
