Neuromuscular Blocking Drugs Flashcards
What are alpha motor neurones?
Lower motor neurons whose cell bodies are found in the anterior horn of the spinal cord and whose axons travel in the body to innervate skeletal muscle

Describe how impulses are transmitted across synapses.
- Propagtion of action potential propagates along the presynaptic neurone until it reaches pre-synaptic terminal
- Depolarisation of presynaptic membrane
- Opening of coltage gated calcium channels
- Calcium influx
- Vesicle exocytosis
- Neurotransmitter release into synaptic cleft and diffuses across synapse to post-synaptic membrane
NOTE: Each step results in the subsequent step
What type of receptor is found at the neuromuscular junction?
Nicotinic acetylcholine receptors
- Ion-channel linked
REMEMBER: nAChR - different to ganglionic nAChR
- This allows drugs to be developed which are specifically targeted to skeletal muscle nAChRs
Where are nicotinic ACh receptors found on the muscle fibre?
On the motor end-plate
- The motor end plate is roughly in the middle of the skeletal muscle fibre
- NOTE: NMJ = junction between motor neurone and muscle fibre

What happens once ACh binds to the nicotinic receptors on the motor end-plate?
- ACh binding to the receptor causes opening of the ion channels and an influx of Na+
- This generates an end-plate potential
- This is a graded potential
- Therefore, the amount of depolarisation is dependent on:
- Amount of acetylcholine released from the presynaptic neurone
- Number of receptors stimulated on the motor end-plate
- Once the end plate potential reaches the threshold potential, it generates an action potential
- AP - all or nothing
- AP propagates in both directions along the muscle fibre
- You get excitation-contraction coupling → muscle contraction
NOTE: Ion channel doesn’t stay open for long - closes very quickly so whole process can occur again (i.e. can be stimulateed to open again)

Where is acetylcholinesterase found?
In the synaptic cleft
- ACh → choline + acetate
- Choline taken back up into pre-synaptic nerve terminal so it can be used in the re-synthesis of ACh
Describe the structure of nicotinic acetylcholine receptors.
- They span the whole width of the lipid bilayer
- They have both an intracellular and extracellular domain
- They consist of 5 subunits
- There are always 2 alpha subunits, which ACh bind to and activate the receptor
- So you need 2 molecules of ACh to activate each receptor - one for each subunit
- There are always 2 alpha subunits, which ACh bind to and activate the receptor
State different sites at which drugs can act to effect NM transmission
- Central process
- Sensory neurones stimulate APs in the spinal cord and therefore motor neurone activation
- Conduction of nerve AP in motor neurone
- ACh release
- Depolarisation of motor end-plate → AP initiation
- Propagation of AP along muscle fibre and muscle contraction
These all act to reduce NM transmission and relax the skeletal muscles

Name two drugs that are used as spasmolytics and describe their action.
- Diazepam
- Baclofen
They work to reduce the generation of APs in the spinal cord - centrally acting drugs
- They both facilitate GABA transmission
- GABA = chief inhibitory NT in CNS
Give some examples of conditions in which spasmolytics may be used.
- Multiple sclerosis
- Stroke
These patients may have spasticity - this could be helped by treatment with spasmolytics
-
Spasticity = increased, involuntary, velocity-dependent muscle tone that causes resistance to movement
- Muscle tone is increased - i.e. muscle is more contracted
- The faster you move the limb, the more the tone and the greater the resistance
- This makes the muscles more stiff and reduces their functionality
- Spasmolytics could help relax skeletal muscle to counteract the hypertonia
How do local anaethetics (LAs) work?
Normal action:
- They work by blocking VGSCs
- When an LA is injected around a wound, they would act on the sensory neurones in that area to reduce the generation and propagation of APs to the CNS
- This contributes the their analgesic (pain relief) action
Action on motor neurones:
- If you inject the LA too close to a motor neurone, it can also block VGSCs and reduce AP conduction down motor neurone
- This can cause some muscle weakness due to relaxation (reduced tone)
- This is an unwanted effect on LAs
State some drugs which inhibit ACh release.
- Hemicholinium
- Blocks reuptake of choline which is needed for further ACh synthesis and release
- Ca2+ entry blockers
- Block entry of Ca2+ into presynaptic terminal, therey preventing vesicle exocytosis
- Neurotoxins
- Have various mechanisms of action in general
- Botulinum toxin:
- Interacts with and damages proteins involved in vesicle fusion with pre-synaptic membrane (a key step in vesicle exocytosis)
- If this happens with respiratory muscles: respiratory muscle paralysis → respiratory failure and death
Name another spasmolytic that has a different action to the ones that act centrally.
Dantrolene
- It works in the skeletal muscle fibres themselves
- Reduced the release of Ca2+ from the sarcoplasmic reticulum
What do neuromuscular blocking drugs do?
Block NM transmission at the NMJ (i.e. the motor-end plate which is specific to the NMJ)
- They have a post-synaptic action
State the three main neuromuscular blocking drugs.
- Tubocurarine
- Atracurium
- Suxamethonium (otherwise known as succinylcholine)
What are the two types of neuromuscular blocking drugs? Give examples of drugs for each type
Depolarising e.g:
- Tubocurarine
- Atracurium
Non-depolarising e.g:
- Suxamethonium
State some characteristics of neuromuscular blocking drugs.
- Do not affect consciousness
- Do not affect pain sensation
- When giving these drugs you always assist respiration because of its effect on the respiratory muscles
- i.e. It would cause relaxation of the respiratory muscles which would mean you would struggle to self-ventilate
Describe the difference in structure between non-depolarising and depolarising NM blockers?
Tubocurarine (non-depolarising):
- Has quarternary ammonium groups, similar to ACh
- These similarities allow it to bind to nicotinic receptor (i.e. has affinity)
- BUT it has no efficacy
- Non-depolarising agents in general:
- Rigid, bulky molecules with limited movement around their bonds (free rotation)
- This makes them good receptor blockers
- Rigid, bulky molecules with limited movement around their bonds (free rotation)
Suxamethonium (depolarising):
- Made up of two acetylcholine molecules that are linked together
- As it is made up of two acetylcholine molecules it can binds to the two alpha subunits and activate the receptor
- Therefore it has a good efficacy - good stimulator of the nicotinic receptor
- Agonists in general:
- More free rotation

Describe the mechanism of action suxamethonium.
- Suxamethonium is a nicotinic receptor agonist so it can activate muscle fibres
- Initally you will see this as fasciculations (twitching of individual muscle fibres) as the suxamethonium begins to stimulate the nicotinic receptors
- However, it is not metabolised as rapidly as ACh
- So it remains in the synapse and can activate nicotinic receptors on the motor end-plate for longer
- Therefore, it causes an extended end plate depolarisation leading to a depolarisation block of the NMJ
- Also known as a phase 1 block
- Receptors are essentially being overstimulated, so the receptor response shuts down
- This results in flaccid paralysis
- Muscles relax due to depolarisation block so you have a lack of muscle tone → muscles can’t contract → prevents movement
- This results in flaccid paralysis
What is the route of administration of suxamethonium?
IV
- As it is highly charged it would not be very easily absorbed through the GI tract into the bloodstream
What is the duration of paralysis of suxamethonium?
5 mins - short-acting NM blocker
How is suxamethonium metabolised?
It is metabolised by pseudocholinesterase in the liver and plasma
NOTE: Pseudocholinesterase metabolises a variety of choline esters (non-specific)
What are some uses of suxamethonium?
Endotracheal intubation
- Relaxes the muscles of the airways which makes it easier to insert a tube into the trachea
Muscle relaxant for electroconvulsive therapy (ECT)
- ECT is a treatment for severe clinical depression
- Essentially with this, you are passing electrical currents through the brain to induce seizures
- Therefore, it is important to use a muscle relaxant to prevent motor symptoms that you would normally get from seizures such as jerking, in order to prevent musculoskeletal injury
State and explain four unwanted effects of suxamethonium.
Post-operative muscle pains
- This sometimes occurs due to the initial fasciculations
Bradycardia
- This is due to the direct muscarinic action on the heart
- Because suxamethonium is a ACh agonist, it can mimic ACh and can have parasympathetic effects on the heart by acting on muscarinic receptors
- But this effect tends to be prevented as atropine (competitive muscarinic antagonist) is included in thepre-medication
- Pre-medication given before GA
- Suxamethonium (a muscle relaxant) can be given with GA
Hyperkalaemia
- Soft tissue injury or burns can lead to ventricular arrhythmias or cardiac arrest
- If there is a burn or some soft tissue injury, you will lose some of the neurones innervating the muscle and so you get upregulation of the receptors in the skeletal muscle = deinnervation supersensitivity
- So, there will be more receptors on the muscle fibre for suxamethonium to act on and activate
- This results in bigger influx of sodium and bigger efflux of potassium through the nicotinic receptor
- Therefore, you get hyperkalaemia which leads to the arrythmia and cardiac arrest – disrupts normal electrical activity of the heart
- So in burns patients or patients with soft tissue damage, use a non-depolarising NM blocker
Raised Intraocular Pressure
- AVOID for eye injuries and glaucoma
- Due to increased contraction of the extraocular muscles
