Neuromuscular Junction Flashcards
What is the anatomy of NMJ?
Motor neurone nerve terminal and Motor end plate of myocyte.
With Schwann cells surrounding synaptic cleft.
How is ACh produced and degraded?
Acetyl-CoA and Choline are combined to ACh by Choline Acetyl Transferase.
ACh degraded by Acetylcholinesterase to form CHoline and acetic acid.
Choline reuptaken by Choline transporter.
What is curare?
Nicotinic ACh receptor antagonist.
= Reducing EPSP of Motor EPP
What is alpha-bungarotoxin?
An antagonist of nicotinic ACh receptors.
What are antagonists to NAChRs?
Alpha-bungarotoxin
Curare.
What are neonicotinoids?
Neurotoxic Insecticides.
Agonists of NAChRs but with greater affinity to insect nAChRs than vertebrates.
What are agonists of nAChRs?
Nicotine
Anatoxin-A - produced by cyanobacteria.
(Neonicotinoids but greater affinity to insect receptors).
What is an EPP?
nicotinic ACh receptors are permeable to both NA+ and K+.
K+ leaves, Na+ entry - due to respective electrochemical gradient, there is more Na+ current than K+ current.
= Depolarisation of End Plate.
EPPs are large ampltitude EPSPs, and always trigger an AP.
(Around 60/70mV EPP).
There is a local response at motor end plate, which experiences DECREMENTAL PROPAGATION along muscle fibre sarcolemma….
= but will trigger an AP.
0.4ms synaptic delay.
Possible temporal summation.
What can you measure with a voltage clamp or a current clamp of an EPP?
Voltage clamp - fixing EM using injected currents.
= Record compensatory current.
= Measure IACh = current responsible for EPP.
Current clamp - using an injected current to record changes to EM.
= Measure EPP as a function of EM.
(Inject current to set a certain EM, then see how Em changes with ACh).
Why is EPP an EPSP and not IPSP?
nAChRs are permeable, equally, to K+ and Na+
Modifying conc. of either ion changes EPP.
And EPP is different based on EM during current-clamp (where current injected to set EM at specific value when ACh added).
If equally permeable to both ions, then gNa/gK should be 1.
= Therefore, you consider electrochemical gradients.
When Er is -70mV, then Na+ electrochemical gradient is much greater than K+.
Reversal potential of EPP is 0mV.
What are electroplates?
Electrocytes/Electroplates are small, disc-like cells arranged into columns in the electric organ of Torpedo Rays.
What is the structure of nAChRs?
Pentameric
With 2 alpha subunits, 1 beta, 1 gamma and 1 delta.
ACh binds to the alpha subunits = requires 2 ACh bound to open.
Each of the 5 subunits has 4 TM segments = M1, M2, M3, M4.
M2 segment of each of 5 subunits forms central pore for ion channel.
M2 contains L251 = leucine-251.
When ACh not bound, L-251 blocks channel port.
When ACh bound, L-251 is displaced and replaced with Serine.
.
Ion channel lned by serine and threonine residues - with cationic selectivity.
What is site directed mutagenesis?
Replacing residues with other residues in ACh.
If you replace L-251 of M2 segment, you can increase the frequency of channel opening.
The more subunits mutated, the greater the ffect.
What are examples of AChE inhibitors?
When are they used?
Reversible:
Neostigmine, Eserin
Irreversible:
Sarin, Novichock, Other organophosphorous compounds.,
Reversible AChE inhibitors can be used:
Alzheimer’s:
Neostigmine as cholinergic neurones dying.
Myasthenia Gravis:
Autoimmune destruction of NMJ
