Neuromuscular Junction Flashcards
Synapses
communication occurs via the release of chemical messengers (neurotransmitters) from presynaptic nerve terminals to act upon receptors on the postsynaptic membrane
Neuromuscular junction
synapse between neurone and a skeletal muscle fibre
Synaptic transmission at the neuromuscular junction
the release of transmitter onto receptors involves 5 steps, each of which can be affected by drugs and toxins resulting in either an increase or decrease in transmission
5 steps of synaptic transmission
- synthesis of the transmitter from a precursor and an enzyme
- storage of transmitter
- protect
- package (quanta) - transmitter released by vesicular exocytosis
- activation by binding of transmitter to receptors
- transmitter reaches inactivating enzyme and there is an uptake of transmitters
Drugs can enhance synaptic transmission by
Direct stimulation of post-synaptic receptors by - the natural transmitter - analogues Indirect action via - increased transmitter release - inhibition of transmitter removal
Drugs can inhibit synaptic transmission by
- blocking synthesis storage or release from the presynaptic neurone
- blocking postsynaptic receptors
Drugs acting directly on receptors (types)
agonists and antagonists
Agonists
- drugs, hormones or transmitters which bind to specific receptors and initiate a conformational change in the receptor resulting in a biological response
- affinity and efficacy
Affinity
the ability of agonists to bind to receptors
Efficacy
the ability of an agonist, once bound to a receptor, to initiate a biological response
What is the neurotranmitter at the NMJ?
acetylcholine
Activation of receptors by agonists
- an agonist binds with a receptor to produce an agonist/receptor complex
- the receptor is ligand-specific and so is like a ‘lock and key’
Binding step (agonists)
Agonist + receptor
- affinity
Activation step (agonists)
complex –> response
- efficacy
Antagonists
Antagonist + receptor -> complex
- affinity
- antagonists bind to receptors but do not activate them
- possess affinity but lack efficacy
- antagonists block receptor activation by agonists
Competitive nicotinic receptor antagonists
competes with the agonist for the agonist binding site on the receptor; block is reversed by the increase in agonist concentration
How are synapses classified?
- according to the transmitter released from the presynaptic neurone
- for synapses where the presynaptic neurone synthesises and released ACh transmission = cholinergic
- receptors which ACh acts on are called cholinoceptors
Types of cholinoceptors
Nicotinic
- activated by ACh or nicotine but not muscarine
Murcarinic
- activated by ACh or muscarine (fungal alkaloid( but not nicotine
What is the nicotinic ACh receptor?
a transmitter-gated ion channel
Transmitter-gated ion channels
- integral ion channel
- agonist binding to the receptor induces a rapid confrormational change to open the channel
- the channel is selective for certain ions
- signalling is extremely rapid (milliseconds)
- consist of seperate protein subunits that form a central, ion conducting, channel
- allow rapid changes in the permeability of the membrane to certain ions
- rapidly alter membrane potential
What does ACh released from a vesicle cause?
- a miniature endplate potential (MEPP)
- it activates many nicotinic ACh receptors
- upon activation the associated nicotinic cation channels open and Na ions flux into the muscle fibre to cause a local depolarisation at the endplate region
Synaptic transmission at the neuromuscular junction
- motor nerve stimulation causes synchronous release of many vesicles; summation of mepps to produce an epp
- membrane potential is negative
- MEPP molecules depolarise past the threshold
- this causes voltage gates Na channels to open
Quantal content
- number of vesicles/stimulus
- QC = (mean EPP amplitude (mV) / (mean MEPP amplitude (mV)
- each quanta gives rise to a miniature end plate potential (mepp) via activation of nicotinic ACh receptors
When do mepps occur?
spontaneously
When do epps occur?
in response to a motor nerve stimulation
What happens when mepps summate to give an epp?
Initiates an action potential which causes muscle contraction
Cholinergic transmission and the NMJ
- choline acetyl transferase (CAT) synthesises ACh from precursors choline and AcE A from mitochondria
- Re-uptake of choline is Na dependent and blocked competitively by hemicholinium 3
- there will be less ACh in each vessicle
- amplitude f the epp and mepp both decrease
Tetrodotoxin (TTX)
- blocks Na channels (no action potential, no release, no EPP)
Conotoxin
- blocks voltage-gated Ca channels
- a decrease in calcium influx decreases the release
- the epp amplitude decreases, no change in mepp amplitude
- decreased quantal content
Dendrotoxin
- blocks voltage-gated K channels
- prolonged action potential
- increased calcium influx causes an increase in release
- increased epp amplitude
- no change in mepp amplitude
- increased quantal content
Botulinum toxin
- blocks vesicle fusion
- no release
- EPP amplitude decreases
- MEPP amplitude doesn’t change
- quantal content decreases
Quantal content calculation
QC = EPP amplitude (mV)/MEPP amplitude (mV)
If tubocurarine has a decrease in EPP and decrease in MEPP, how will this effect the QC?
no change
If tubocurarine has an increase in EPP and decrease in MEPP, how will this effect the QC?
increase
If tubocurarine has a decrease in EPP and increase in MEPP, how will this effect the QC?
decrease
Non-depolarising neuromuscular blockers
- compete with ACh for binding to skeletal muscle nicotinic ACh receptors
- reduce the amplitude of the EPP to below the threshold for muscle fibre action potential generation
- block Na from leaving the cell
Tubocurarine
- used during surgery to produce skeletal muscle relaxation
- competitive non-depolarising neuromuscular blocking agent
- muscle block reversed by anticholinesterases e.g. neostigmine
- from the plant curare
Bungarotoxin
- snake venom from Taiwan Banded Krait
- not reversed by washout or by anticholinesterases
Suxamethonium
- a skeletal muscle relaxant
- rapid onset (30s) and short duration (5 min) of action
- used in ECT and for rapid tracheal intubation
- metabolised by PLASMA (butyryl cholinesterase)
- 1 in 3000 individuals express a genetic variant of the enzyme that does not degrade suxamethonium, causing a prolonged blockade
Skeletal NMJ depolarising blockers (suxamethonium)
Phase I block:
- persistent activation of endplate nicotinic receptors
- prolonged depolarisation of endplate
- inactivation of voltage-gated sodium channels
Phase II block
- desensitisation of endplate nicotinic receptors
- repolarisation of endplate
- receptor desensitisation maintains blockade
Pharmacology of ACh and nicotine
agonist
Pharmacology of suxamethonium
agonist
Tubocurarine
competitive antagonist
a-Bungarotoxin
irreversible antagonist
Cholinergic transmission and the NMJ (inactivation)
- ACh is terminated by acetylcholinesterase (AChE), which breaks down ACh to acetate and choline
- drugs which inhibit AChE anticholinesterases (e.g. nerve gases, neostigmine) increase the effects of ACh
True acetylcholinesterase (AChE)
- present at cholinergic synapses
- bound to the postsynaptic membrane in the synaptic cleft
Pseudo-cholinesterase (butyrylcholinesterase or plasma cholinesterase)
- widely distributed and found in plasma
- important in inactivating the depolarising neuromuscular blocker, suxamethonium
- both ‘true’ and pseudo cholinesterases are inhibited equally by most clinically-relevant anticholinesterases
Anticholinesterases (clinical uses)
- used to reverse non-depolarising skeletal muscle relaxants e.g. tubocurarine
Anticholinesterases and QC
- anticholinesterases such as neostigmine increase the amplitude of the EPP and MEPP
- no effect on QC
- neostigmine prolongs the duration of the mepp and epp due to the increased ‘life-time’ of ACh in the synaptic cleft which permits receptor rebinding
Anticholinesterase treatment of Myasthenia Gravis
- auto-immune disease
- loss of NMK nACh receptors
- muscular weakness, paralysis
- reversed by inhibitor of AChE e.g. treatment with neostigmine, diagnose with edrophonium
Anticholinesterases (organophosphates) as Nerve Gas Agents
- binds very stably
- recovery requires synthesis of new enzyme (weeks)
- dyflos: volatile, non-polar, lipid soluble, rapidly absorbed through mucous membranes and unbroken skin & cross the blood brain barrier
Reversal of organophosphate nerve gas
Atropine
- counteract effects of excessive muscarinic receptor stimulation by ACh
Oximes
- e.g. pralidoxime (2-PAM) antidote to Nerve Gas reactivates the AChase
Organophosphates as insecticides
Readily absorbed through the insect cuticle
