Neuromuscular junction Flashcards Preview

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Flashcards in Neuromuscular junction Deck (15)
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1
Q

What is the neuromuscular junction?

A

Synapse between muscle fibre and motor neurone

2
Q

How does a signal reach the NMJ?

A
  • It is a motor signal - nerve system sends information to muscles so they contract
  • Signals come from motor cortex
  • Reach muscle by going through neurons
  • First neuron is upper motor neuron, axon leaves motor cortex and goes to medulla
  • Travels through lateral corticospinal tract to reach synaptic contact with the lower motor neuron
  • Lower motor neurone has axons reaching muscles
  • 2 synaptic relays - one from brain to motor neuron and one from motor neuron to muscle fibers
  • Motor neuron divides into branches (motor units) when it reaches the muscle
3
Q

What is a motor unit?

A

One motor neurone and all the muscle fibres it innervates

4
Q

How many muscle fibres does one motor unit supply?

A

Several

5
Q

How many neurones innervate each muscle fibre?

A

One - the muscle fibre only receives one signal

6
Q

How does the NMJ work?

A
  • Signal comes from motor cortex through motor neuron and interacts with terminal
  • Action potential transmitted to pre-synaptic membrane
  • Opens voltage gated calcium channels
  • Allows calcium to move into the cell
  • Allows synaptic vesicles to fuse with pre-junctional membrane and release ACh into cleft
  • Passively diffuses into cleft
  • Binds to nAChR
  • 2 molecules of ACh needed for conformational change
  • Receptors open and allow ions to move through them according to concentration gradient - in this case sodium moves through, which creates mEPP
  • mEPP not big enough to produce AP at post-junctional membrane
  • Around 100 mEPPs needed for EPP
  • Continues through muscle fibres
  • Triggers release of calcium from SR to stimulate muscle contraction
7
Q

How is an electrical signal terminated?

A
  • If the signal isn’t removed, the muscle continues to contract and ACh stays in cleft
  • Acetylcholinesterase breaks down ACh into choline and acetate
  • Reuse choline to synthesise more ACh
  • Pre-junctional membrane takes up choline and makes more ACh in vesicles
8
Q

What do organophosphates do to the NMJ?

A

Normally, acetylcholinesterase catalyses the breakdown of ACh into acetate and choline. Organophosphates phosphorylate acetylcholine, reducing the ability of the enzyme to break it down. Concentration of ACh increases = cholinergic syndrome

9
Q

What is myasthenia gravis?

A

Antibodies block/destroy receptors for ACh at NMJ, preventing the muscle from contracting. Not enough nicotinic ACh receptors present means that mEPPs don’t accumulate to form an EPP for contraction

10
Q

What do neuromuscular blockers do to the NMJ?

A

Competitive antagonists that block transmission through the NMJ at nicotinic receptors by competing with ACh for the alpha sub units on the ACh receptors

11
Q

What does botulism toxin do to the NMJ?

A

Stops ACh being released at presynaptic membrane, which doesn’t allow muscles to contract and paralyses them, e.g. cardiac muscle in heart

12
Q

How many molecules of ACh are needed for conformational change?

A

2

13
Q

How many molecules of mEPP are needed for EPP?

A

Around 100

14
Q

What is ACh normally broken down into?

A

Acetate and choline

15
Q

Which enzyme breaks down ACh?

A

Acetylcholinesterase