pharmacology - skeletal neuromuscular junction

outline the presynaptic events of cholinergic transmission

• ACh synthesised from choline plus acetylcoenzyme A by the action of enzyme Chline Acetyl Transferase (CAT)

• choline is taken into the nerve terminal from the extracellular space by a transporter protein

• ACh is stored in synaptic vesicles that are released by the process of exocytosis

• exocytosis is triggered by a rise in the intracellular calcium concentration within the nerve terminal

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outline the post synaptic events of cholinergic transmission

• after ACh is released, it diffuses across the synaptic gap and activates receptors on the post-synaptic membrane

• there are two types of ACh receptors, nicotinic receptors (nAChRs) and muscarinic receptors (mAChRs)

• ACh diffuses across the synapse and is hydrolysed by acetylcholinesterase found in the synaptic gap or in the muscle

• neutralises actions of released ACh very quickly preventing spontaneous or continued activation

• choline part of the molecule is recycled back into the new terminal to make new ACh

outline the process for the synthesis, release and metabolism of ACh

• acetylcholine is made from choline and acetyl CoA

• packaged into vesicles

• acetylcholine is broken down by acetylcholinesterase in synaptic cleft

• choline is transported back into the axon to make more ACh

what is neurotransmitter release stimulated by

entry of calcium ions into the presynaptic axon membrane through voltage-gated calcium channels. this produces vesicle fusion and expulsion of their contents

what is vesicle docking

when ACh vesicles fuse to the presynaptic membrane mediated by the SNARE complex

what is a SNARE complex

• complex multifactorial arrangement

• made up of 4 main proteins

calcium trigger: synaptotagmin

vesicle protein: synaptobrevin

membrane proteins: syntaxins/ SNAP 25

• synaptobrevin and syntaxins/ SNAP 25 bring the vesicles into contact with the membrane

• synaptotagmin senses the valcium and switches on the complex

• vesicle fusion

outline the function of the nicotinic acetylcholine receptor

• binds to activate nicotinic acetylcholine receptors

• nicotine is the endogenous agonist

• formed by 5 protein subunites (α₁,α, β₁γ, δ) arranged around a non-selective cation channel permeable to Na+, K+

• γ subunit in foetal muscle is replaced by an epsilon (ε) in adult muscle

• channel opens when 2 molecules of ACh bind to 1 protein complex, each to an alpha subunit

• opening of the channel leads to depolarisation of the end-plate region of the muscle membrane which acts as a trigger for a muscle action potential

how does muscle contraction occur through ACh binding

• ACh binds to nicotinic receptors

• opens the channel to allow sodium ions through

• stimulated action potential in muscle cell - causes Ca2+ release from terminal cisternae

• allows the change in morphology of troponin and the dissociation of tropomyosin from action

• muscle contraction

which drugs can act pre-synaptically to inhibit cholinegeric transmission

1. synthesis: hemicholinium blocks the uptake of choline into the nerve terminal

2. storage: vesamicol blocks the uptake of ACh into synaptic vesicles

3. release: conotoxines (from fish-eating marine snails) blocks voltage-gated calcium channels

4. release: botulinium toxins blocks the process of exocytosis

what is omega conotoxin

• found in sea snails

• used to block N type calcium channels

• pain therapeutic ziconitide (inhected into spine)

what is botulism

botulinium toxin - from clostridium botulinium, found in uncooked foods giving rise to botulism

• extremely potent

• death occurs due to paralysis of respiratory muscles

• symptoms: double vision (diplopia), blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, muscle weakness

what is the botulinium toxin

• produced by clostridium botulinium

• 7 neurotoxins (A-G), human versions: A,B,E,F

• work on SNARE complex

• A and E cleaves SNAP 25

• B, D, F cleave synaptovrevin

• C cleaves syntaxin

• toxin prevents ACh release

therapeutic uses of BTX

• focal dystonia

involuntary, sustained or spasmordic patterened muscle activity

cervical dystonia (torticollis)

• spasticity

increased muscle tone

stroke, cerebral palsy

• headaches

• sweating, salivary disorders

• cosmetic use

what is tubocurarine

nicotinic antagonists

• has affinity but zero efficacy, binds to the receptor but doesnt open the associated ion channel

• prevents ACh binding and activating the receptor

what is atropine

muscarinic antagonist

• has affinity but zero efficacy, binds to the receptor but doesnt open the associated ion channel

• prevents ACh binding and activating the receptor

what are neuromuscular blockers and give examples

• curare extract from south american woody vines

d-tubocuraine

non-depolarising muscle relaxant

blocks nicotinic ACh receptor (competitive antagonist)

rocuronium and pancuronium

• used as a muscle relaxant in surgery

• antidote: acetylcholinesterase inhibitor (physostigmine and neostimine)

clinical uses of reversible, competitive antagonists

• used as muscle relaxants during surgery to stop reflex muscle contractions and allow lower doses of general anaesthetics to be administered

• effects last 15-40 minutes depending on drug

• effects can be reversed by the acetylcholinesterase inhibitor neostigmine

• because the diapgragm is a skeletal muscle, patients administered muscle relaxants cannot breath for themselves and must be artifically ventilated

outline acetylcholinesterase inhibitor drugs acting post-synaptically to alter cholinegeric transmission

these drugs enhance the effects of ACh by preventing its destruction and include:

• edrophonium: short duration of action, binds reversibly and briefly to the active site of the enzyme

• neostigmine - medium duration of action, is slowly hydrolysed by the enzyme occupying the active aire and so prevening to breaking down ACh

• irreversible organophosphates e.g. Novichok nerve agent and parathion (incesticide). irreversibly phosphorylate the active site of the enzyme and recovery only occurs once new enzyme has been synthesised

what is myasthenia gravis

• autoimmune disease characterised by neuromuscular weakness

• antibodies bind to a-subunits of the nicotinic ACh receptors leading to their destruction

• edrophonium has a short duration of action (5mins) and can be used to diagonise myasthenia gravis. neostigmine lasts 4 hours and can treat the disease

• the increased ACh concentration enhances neuromuscular transmission increasing the liklihood that ACh is able to activate fewer remaining nAChRs

outline examples and the action of depolarising muscle relaxants

• suxamethonium drug

• acts as an agonist at nicotinic ACh receptors

• not broken down by acetylcholinesterase so it lingers at the NMJ producing a prologed depolarisation

• allows the action potential to repoliase fully and the membrane potential to return to its resting value so that the voltage dependent sodium channels can recover from inactivation

• with suxamethonium not broken down by acetylcholinesterase, the depolarisation is prolonges, sodium channels are trapped in an inactive state and cannot support another action potential.

• initially it causes an action potential leading to muscle twitches and then relaxation