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Medicine Year 1 - Semester 2 - Part 1 > Pharmacology of the Neuromuscular Junction > Flashcards

Pharmacology of the Neuromuscular Junction Flashcards

1
Q

Briefly describe what happens at the NMJ (refer to ACh)

A

ACh is created and packaged into vesicles, so that when we have excitation of that motor neuron, that stimulates the release of the vesicle.

ACh is released and acts on the post-synaptic nicotinic receptor.

This action is degraded rapidly by the presence of acetylcholine esterase enzymes.

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2
Q

State 3 ways to block the neuromuscular junction

A

Presynaptically by INHIBITING ACh synthesis

Presynaptically by INHIBITING ACh release

Postsynaptically by interfering with the actions of ACh on the receptor

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3
Q

State ways of achieving presynaptic blockade

A

By inhibiting ACh release :

  • Local anaesthetics
  • General inhalational anaesthetics
  • Inhibitors/competitors of calcium
  • Neurotoxins
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4
Q

How do local anaesthetics work ?

A

They bind and block the voltage gated sodium channels.

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5
Q

State some inhibitors of calcium release

A

Antibiotics :

  • Aminoglycosides (e.g. gentamicin)
  • Tetracycline
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6
Q

State a competitor of calcium

A

Magnesium ions

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7
Q

Name some neurotoxins

A

Boutilin toxin
Beta-Bungarotoxin

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8
Q

Boutilin toxin cause

A

clostridium botulinum

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9
Q

Uses of botox

A

Prevents muscle spasticity
Cosmetic reasons
Prevents hyperhydrosis (sweating)

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10
Q

How does botox work ?

A

Prevents the release of ACh, but also influences other neurons, not specific to inhibit the control of muscle.

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11
Q

State ways of achieving postsynaptic blockade

A

Endotracheal intubation
During surgical procedures
Infrequently in intensive care
During electroconvulsive therapy

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12
Q

Why are neuromuscular blocking drugs used during surgical procedures ?

A

To allow access to abdominal cavity

To ensure immobility

To allow relaxation to reduce displaced fracture or dislocation

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13
Q

Advantage of using neuromuscular blocking drugs during surgical procedures

A

Lowers the concentration of general anaesthetic needed

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14
Q

Describe the structure of the nicotinic acetylcholine receptor

A

Alpha2Beta1Gamma1Delta1
Has 2 ACh gates

The ACh gates require the binding of 2 molecules of ACh to the receptor to cause a conformational change that allows it to open.

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15
Q

What are agonists ?

A

Things that open the channel and act as the endogenous ligand would on that system.

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16
Q

What are antagonists ?

A

They block the action of ACh on the receptor, so they prevent ACh from opening the channel and having its effect.

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17
Q

Name some nicotinic ACh receptor agonists

A

ACh
Nicotine
Suxamethonium

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18
Q

Name some nicotinic ACh receptor antagonists

A

Tubocurarine
Atracurium

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19
Q

What are non-depolarising blockers ?

A

They act as competitive antagonists of nicotinic ACh receptors at the NMJ.

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20
Q

Name some non-depolarising blockers

A

Tubocurarine
Atracurium

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21
Q

Describe the action of non-depolarising blockers

A

Prevents ACh binding to the receptor by occupying site

Decreases the motor end plate potential (EPP)

Decreases depolarisation of the motor end plate region

No activation of the muscle action potential

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22
Q

What happens at the end-plate ?

A

The end plate is where you get the synapse between your motor neuron and the muscle fibre that’s controlling.

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23
Q

What are depolarising blockers ?

A

They are agonists of nicotinic ACh receptors at the NMJ.

24
Q

Name a depolarising blocker

A

Suxamethonium

-> opens nicotinic ACh receptors

25
Q

Describe the action of suxamethonium

A

Suxamethonium will activate nicotinic acetylcholine receptors and open them.

This allows sodium to come into the cell, which would cause an action potential.

This results in contraction of the muscle.

Initial contraction, relaxation and no further ability to cause contraction. (overstimulation of cell)

Suxamethonium is not degraded as rapidly as ACh. It is only degraded by the acetylcholine esterase found in the patients plasma.

26
Q

Describe the action of depolarising blockers

A

Persistent depolarisation of the motor end plate
Prolonged EPP
Prolonged depolarisation of the muscle membrane

Membrane potential above the threshold for resetting of voltage gated sodium channels.

Sodium channels remain refractory

No more muscle action potentials generated

27
Q

Describe phase 1 of the depolarising block

A

Muscle fasciculations observed, then blocked

Repolarisation inhibited
- K+ leaks from cells (hyperkalaemia)

Voltage gated Na+ channels kept inactivated

28
Q

Describe phase 2 of the depolarising block

A

Occurs when prolonged/ increased exposure to drug

“Desensitisation blockade”
- Depolarisation cannot occur, even in absence of drug

29
Q

List the drugs that affect NMJ neurotransmission

A

Pancuronium
Vecuronium
Rocuronium
Atacurium
Mivacurium
Suxamethonium

30
Q

Describe the drug : Pancuronium

A

Onset : Medium
Duration : Long
Main side effect(s) : Tachycardia

31
Q

Describe the drug : Vecuronium

A

Onset : Medium
Duration : Medium
Main side effect(s) : Few side effects

32
Q

Describe the drug : Rocuronium

A

Onset : Fast
Duration : Medium
Main side effect(s) :Tachycardia

33
Q

Describe the drug : Atacurium

A

Onset : Medium
Duration : Medium
Main side effect(s) : Hypotension / Bronchospasm

34
Q

Describe the drug : Mivacurium

A

Onset : Fast
Duration : Short
Main side effect(s) : Hypotension / Bronchospasm

35
Q

Describe the drug : Suxamethonium

A

Onset : Fast
Duration : Short

Main side effect(s) :

  • Bradycardia
  • Cardiac dysrhythmias
  • Malignant hypothermia

etc.

36
Q

Describe the action of atracurium

A

Atracurium undergoes ester hydrolysis and Hofmann elimination

37
Q

Describe the action of mivacurium and suxamethonium

A

These drugs are metabolised by plasma cholinesterases

This breaks down ACh which is present in plasma.

38
Q

Describe the action of pancuronium and vecuronium

A

These drugs undergo hepatic metabolism and are dependent on liver function.

39
Q

Describe the action of rocuronium

A

Unchanged in bile/urine

40
Q

State ways to reverse neuromuscular blockade

A

Cholinesterases : the duration of action of ACh is regulated by hydrolysis

41
Q

Describe acetylcholinesterase

A

True cholinesterase, specific for hydrolysis of ACh

Present in conducting tissue and red blood cells

Bound to the basement membrane in the synaptic cleft

42
Q

Describe plasma cholinesterase

A

Pseudocholinesterase, broad spectrum of substrates

Widespread distribution

Soluble in plasma

43
Q

Describe how cholinesterases act

A

If you inhibit the activity of acetylcholinesterase, in the synaptic region, the concentration of ACh present in the synaptic region increases.

As the concentration of ACh released increases, this increases the amount able to bind to the nicotinic receptors.

Restoration of transmission

44
Q

What are anti-cholinesterase drugs ?

A

They are all inhibitors of cholinesterase enzymes.

45
Q

How do anti cholinesterase drugs act ?

A

They increase the availability of ACh @ the NMJ by decreasing degradation.

This increases the duration of activity of ACh @ NMJ

More ACh to compete with non-depolarising blockers

46
Q

Name some anti cholinesterase drugs

A

Neostigimine
Pyridostigmine

47
Q

Function of carbamylation

A

Slows the rate of hydrolysis

48
Q

Describe some effects of anticholinesterases on the CNS

A

Initial excitation with convulsions
Unconsciousness and respiratory failure

49
Q

Describe some effects of anticholinesterases on the ANS

A

Salivation
Lacrimation
Urination
Defecation
Gastrointestinal upset
Emesis

Bradycardia
Hypotension
Bronchoconstriction
Pupillary constriction

50
Q

State some clinical uses of anticholinesterases

A

In anaesthesia

Myasthenia Gravis

Glaucoma

Alzheimer’s disease

51
Q

How do anticholinesterases work in anaesthesia ?

A

Reverse non-depolarising muscle blockade

Given with atropine or glycopyrrolate to counteract parasympathetic effects

52
Q

How do anticholinesterases work in myasthenia gravis ?

A

Increase neuromuscular transmission

53
Q

How do anticholinesterases work in glaucoma ?

A

Decrease intraocular pressure

54
Q

How do anticholinesterases work in Alzheimer’s disease ?

A

Enhances the cholinergic transmission in the CNS

55
Q

Myasthenia Gravis

A

Autoantibodies may be produced against the acetylcholine receptor, blocking the interaction of the acetylcholine receptor with its ligand.

This leads to increased muscle stiffness and weakness.

Medicine Year 1 - Semester 2 - Part 1 (51 decks)

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