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Brain in Health & Disease > L1 - Neuromuscular Junction > Flashcards

L1 - Neuromuscular Junction Flashcards

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The Brain flashcards
1
Q

What is ptosis?

A

Drooping of the upper or lower eyelid

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2
Q

What is a transmission disorder?

A

Faulty communication between nerves and muscles that leads to muscle weakness

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3
Q

What are 3 types of transmission disorder?

A

Axon fails to deliver strong signal
Fibre fails to respond to signal
Enhanced signal leads to uncontrolled activation

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4
Q

How does the neuromuscular junction work?

A
  1. action potential arrives to sarcoplasmic reticulum (SR)
  2. release of Ca
  3. muscle contracts
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5
Q

What three cells are involved in the NMJ?

A

Motor neuron
Schwann cell
Muscle cell

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6
Q

What neurotransmitter is present on the NMJ?

A

Acetylcholine

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7
Q

What is between the neuron and the muscle cell?

A

Basal lamina

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8
Q

What is the basal lamina?

A

Connective tissue crucial for maintain structure of NMJ and its function

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9
Q

What enzyme is in the basal lamina and what else is there?

A

Acetylcholinesterase associated with collagen

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10
Q

What structure is on the postsynaptic membrane and why?

A

Deep infolding of the sarcolemma (secondary synaptic folds) with anchored AChRs on the crests.
Accumulate as much receptor as possible and increase efficiency of transmission

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11
Q

What does acetylcholinesterase do?

A

Hydrolyses acetylcholine to transport it back into nerve terminal.
A serine in the active site does it

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12
Q

Why is acetylcholinesterase involved in disease?

A

Nerve gases and neurotxoins inhibit it by reacting with the same Serine - extending the period of membrane depolarisation

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13
Q

What are the two major events in development of NMJ?

A
  1. Receptors aggregating into clusters

2. Specialisation of nuclei sitting right underneath junction along muscle fibre

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14
Q

How many more AChRs are at the NMJ?

A

1000x more concentrated than at extra synaptic places

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15
Q

What experiment was used to show clustering of receptors?

A

Removed nerve and muscle fibre leaving only basal lamina

Signal sufficient for formation of synaptic folds rich in ACh

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16
Q

What fish was used to investigate thw NMJ and why?

A

Electric ray

Has 500 end places capable of releasing electric currents of 100V

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17
Q

What protein was eventually found to promote clustering?

A

Agrin

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18
Q

What are the features of agrin?

A

400 kDa proteoglycan
Secreted by motor neuron, muscle and schwann cell
Promotes clustering of AChR without enhancing AChR syn

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19
Q

What is the ratio of agrin to AChR at the NMJ and what does this show?

A

1:50 - 1:100

Clustering through signal rather than structural constraints

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20
Q

What was found to be the agrin receptor?

A

MuSK (muscle specific kinase)

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21
Q

How does agrin induce clustering?

A
  • Agrin interacts with MuSK, Masc complex
  • Musk is a receptor tyrosine kinase
  • MuSK activation leads to phosphorylation of rapsyn - a scaffolding protein and clusters AchRs
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22
Q

What is rapsyn?

A

A scaffolding protein that clusters AChRs

23
Q

What is MASC and what is its function?

A

myotube associated specificity component

Required to form agrin receptor

24
Q

What occurs in Lambert-Eaton syndrome?

A

Autoantibodies to P/Q-type voltage gated calcium channels

25
Q

What is hypothesised to be the origin of autoantibodies in Lambert-eaton?

A

Too much exposure to own protein

VGCCs known to be expressed by small cancer lung cells often present in this syndrome

26
Q

What is the function impaired in Lambert-Eaton syndrome>

A

Downregulation of presynaptic Ca channels causes myasthenia by preventing NT release - not enough fibres firing

27
Q

What occurs in Isaac’s syndrome?

A

Autoantibodies to Voltage gated potassium channels

28
Q

What are the internal symptoms of Isaac’s syndrome

A

Increased NT release so persistent depolarisation enhanced muscle contraction

29
Q

What are the external symptoms of Isaac’s syndrome?

A

Painful cramps and spasms

30
Q

What occurs in congenital myasthenic syndrome associated with episodic apnoea?

A

Mutations in CHAT impair catalytic efficiency of enzyme

31
Q

What are the features and function of CHAT?

A

Located in terminals of cholinergic neurons

Catalyses reversible syn of acetylcholine from acetyl CoA and choline

32
Q

Why causes acetylcholinesterase deficiency?

A

Mutations in collagen tail that anchors AChE

33
Q

What are the internal symptoms of acetylcholinesterase deficiency?

A

Persistent presence of Ach in cleft causing repetitive binding to AChRs causing muscle fatigue and endplate myopathy

34
Q

What do defects in repolarisation of the presynaptic membrane or elimination of ACh from cleft lead to?

A

Excessive transmission

35
Q

What causes Seronegative myasthenia gravis?

A

Antibodies against MuSK

36
Q

What causes congenital myasthenic syndrome?

A

Mutations in rapsyn

37
Q

What are the internal symptoms of Myasthenia gravis and myasthenic syndrome?

A

Inhibition of agrin-MuSK signal destabilises NMJ

38
Q

What causes postsynaptic congenital MG?

A

3 types of mutations in AChR genes

39
Q

What are the 3 types of mutations in AChRs?

A

Slow channel syndrome
Fast Channel Syndrome
ACh deficiency

40
Q

What is Slow channel syndrome?

A

channel takes longer to close and too much influx of NA takes place

41
Q

What is Fast channel syndrome?

A

channel closes too quickly and not enough depolarisation

42
Q

What is ACh deficiency?

A

Causes AChR to be unstable resulting in reduced receptors and deficient transmission

43
Q

What are the external symptoms of Myasthenia gravis?

A

Strong in the morning but progressively weaker through day because of depletin of ACh

44
Q

What is found in 80-90% of patients with MG?

A

Anti-AChR antibody

45
Q

What causes neonatal MG?

A

Placental transfer of maternal AChR antibodies

46
Q

What is arthrogryposis multiplex congenita?

A

MG associated in rare cases

Occurs when maternal AChR antibodies target fetal form

47
Q

What does arthrogyrposis multiplex congenita cause and how?

A

Bent hands and feet due to excessive tendon force

Maternal antibodies had attacked fetal AchRs so contractinos crucial for development were not there

48
Q

What does affecting the postsynaptic targets generally lead to?

A

Impaired transmsion and muscle weakness

49
Q

What are the postsynaptic targets?

A

Rapsyn
MuSK
AchRs

50
Q

What are the presynaptic and sypantic targets?

A

VGCC, VGKC, CHAT, ACHE

51
Q

What are treatments of MG?

A

Cholinesterase inhibitors
Thymectomy
Medication to alter immune sys
Immunoglobulin therapy

52
Q

In congenital myasthenia gravis what is the treatment of a reduced response?

A

Cholinesterase inhibitors

53
Q

In congenital myasthenia gravis what is the treatment of a increased response?

A

Open channel blockers of the AChR are used

Brain in Health & Disease (9 decks)

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