Neuromuscular pathology Flashcards

1
Q

Neuromuscular anatomy:
what are muscles comprised of
where do motor neurones originate, and what do they innervate

A

Muscle composed of: fascicles, fibres and myofibrils
Primary motor neurons in ventral horn project to muscle targets where they form motor end plates on muscle fibres
One motor neuron innervates many fibres -together they are a motor unit

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2
Q

What are the main cytoskeletal proteins within muscle?

A

Actin
Filamin C
Dystrophin
Sarcoglycan complex
Dystroglycan complex
Laminin
Collagen ECM

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3
Q

Cytoskeletal proteins in muscle cells interaction

A

Sarcomere contains actin which is linked to the sarcoglycan complex in the cell membrane by filamin C and dystrophin.
Sarcoglycan complex is anchored in the collagen ECM as it is connected to dystroglycan complex in the cell membrane which laminin is binded to extracellularly

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4
Q

What are the different domains of dystrophin?

A

Actin-binding domain
Rod domain containing hinge regions
Cysteine rich domain
C-terminal domain

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5
Q

What tissues is dystrophin expressed in?

A

Muscle
Brain

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6
Q

What happens in MD?

A

Duchenne MD: nonsense or frameshift mutation causes production of non-functional protein
Becker MD: in-frame mutations lead to partially functional protein being produced, so less severe than DMD

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7
Q

What new treatments are being developed for MD?

A

Deliver functional dystrophin in a virus
Use anti-sense oligonucleotide to skip a mutated exon to restore the reading frame - produces protein slightly dysfunctional, but better than none (converts DMD > BMD)
Edit the genome - remove the mutated exon, or fix the mutation using CRISPR

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8
Q

What is required for motor neurons to stay alive in development?

A

Neurons need targets to stay alive - Innervation of muscle is required for neuronal survival
If limbs are removed, motor pool decreases if experimentally added, motor pool expands

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9
Q

How does innervation change throughout development?

A

Synaptic pruning
Motor neurons initially synapse with as many targets as possible, as development progresses, they are pruned back, synapses are lost with many targets
Allows for individual muscle fibres to be innervated by only one motor neuron
Occurs in muscle and ganglia and in brain - climbing fibres are pruned that innervate purkinje neurons in cerebellum

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10
Q

What is SMA?

A

Spinal muscular atrophy
LMN die during development which results in muscle atrophy which ultimately kills the patient

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11
Q

What causes SMA?

A

SMN1 mutation
Protein localised to nucleus and cytoplasm
Associated with RNA metabolism e.g. splicing, transport > and by extension cytoskeletal transport and dynamics
When SMN1 mutated, patients commonly only have 10% of SMN2 to fall back on

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12
Q

SMN genes in humans

A

Used for RNA metabolism, cytoskeletal transport
Humans have two SMN genes whose sequences nearly identical
SMN2 commonly mutated = 90% of the time exon spliced out

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13
Q

What determines the severity of SMA?

A

The copies of SMA2 genes
If they are mutated as well, each copy will only provide 10% production of the protein. If two copies are present = 20% etc.
The more copies of SMN2, the less severe the disease when SMN1 mutated

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14
Q

Where does most damage in SMA occur and why?

A

SMA genes are constitutively expressed, but RNA metabolism, processing and transport is particularly important in motor neurons due to their long axons
Motor neurons transmit mRNA for actin down to synapse to allow growth of neurone during development
Due to this increased need, it is LMN that die
Death of MN results in muscle atrophy which causes eventual death due to respiratory insufficiency

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15
Q

What markers can indicate SMA?

A

High levels from birth of neurofilament L in the blood are indicative of (but not specific to) SMA due to release of NfL when neurons are damaged or die
NfL is measurable in the blood in healthy adults due to Nf half lives, but in very low levels

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16
Q

What are the possible treatments for SMA?

A

> Anti-sense oligonucleotide that can protect the splice site in exon 7 of SMN2, so can produce more than 10% of the protein
Small molecules approach that inhibits splicing of exon 7 on SMN2 which will produce more functional SMN2 protein than 10% per copy
Gene therapy - onasemnogene abeparvovec - SMN cDNA (copy DNA), of SMN gene just containing exons beside a strong promotor inside a virus that will infect patient cells and provide the cells with a functional copy of SMN1 gene

17
Q

What is ALS?

A

Amyotrophic lateral sclerosis (MND - motor neurone disease - more of a catchall term)
Degenerative disorder occurring later in life
All neurons, LMN and UMN die progressively in degeneration > leads to atrophy (LMN death give the symptoms that lead to death)

18
Q

What are the symptoms of ALS?

A

Muscle cramps, spasticity, weakness
Respiratory insufficiency - symptom that kills the patient
Dysphagia
Dysarthria
Cognitive and behavioural impairment - due to all neurons dying, not just LMN

19
Q

What causes ALS?

A

Very complex - 30 genes implicated
Can run in families, but doesn’t have simple inheritance pattern - depends on environment
Genes involved in axon production, axon development, RNA metabolism, vesicle transport, glial cell dysfunction, cytoskeleton, protein homeostasis

20
Q

What causes CP?

A

Results from insult to brain during perinatal or prenatal or early postnatal periods
Associated with premature birth - but not causation but is a RF
Due to infection, trauma etc.

21
Q

What occurs in CP?

A

Hypoplasia - don’t get appropriate gyrification
Polymicrogyria - many small folds due to aberrant brain development
Periventricular leukomalacia - bleeding next to ventricle
Subcortical atrophy
Diagnosable: Neuromuscular developmental issues - deficit in MN, don’t innervate muscle - muscle atrophies

22
Q

What can be shown in a stain of a muscle biopsy in CP patients?

A

See more laminin - extracellular protein - due to smaller muscle fibres due to atrophy

23
Q

What clinical manifestations occur in CP, and what are they due to?

A

Result of the loss of UMN control of motor activity:
- Impaired motor control
- Weakness
Result of loss of LMN inhibition:
- Clonus
- Spasticity
Different patients will present differently - may present more with symptoms related to loss of UMN or more related to loss of LMN inhibition

24
Q

How may treatments vary in CP patients?

A

Patients with atrophy of the basal ganglia present similarly to parkinsons - can be given levadopa (parkinson medication) or
Anti-stimulant drugs can be given
Muscle weakness-presenting patients would not benefit from these, but orthopaedic surgery

25
Q

What are signs of UMN lesions?

A

Weakness/paresis
Paralysis
Hypertonia - long term often initial hypotonia
Hyperreflexia - long term, often initial hyporeflexia
Spasticity
Positive Babinski sign
Clonus

26
Q

What is hyperreflexia and how is it caused?

A

Loss of UMN descending inhibition modulation of motor pathways regulates how the stretch reflex is elicited - overactive reflex response

27
Q

What is spasticity and how is it caused?

A

Increase in muscle tone (tension) on passive movement in velocity dependent manner
Amount of resistance is proportional to speed of passive movement - rapid passive movement, the greater the resistance to stretch is felt

28
Q

What is positive Babinski’s sign, and why is it caused?

A

Firm stroke of lateral border of plantar surface of foot elicits large toe extension and abduction of other toes
Underdeveloped corticospinal tracts

29
Q

What is clonus and how is it caused?

A

Series of contractions that occur when muscle is suddenly stretched and held in that position
Arises via damaged descending inhibition that leads to exaggerated stretch reflex

30
Q

What are signs of LMN lesions? Why

A

Hyporeflexia/areflexia
Hypotonia/atonia
Flaccid muscle weakness or paralysis
Fasciculations
Muscle atrophy
- a and y motor neurones regulate voluntary movement, reflexes and tone LMN syndrome reflect a motor neurone loss

31
Q

What is hypo/areflexia and why is it caused?

A

Decreased or absent reflexes
Efferent aspect of reflex arc is damaged so decreases reflex when myotatic stretch reflex is elicited

32
Q

What is hypotonia/atonia and why is it caused?

A

Loss of a-motor neurones lead to reduced tone - tension
Reduces contraction of extrafusal fibres in response to stretch of muscle

33
Q

What is flaccid muscle weakness or paralysis caused by?

A

a motor neurone damage means muscles will receive reduced signal or no signal at all to contract when attempting to elicit voluntary movement

34
Q

What are fasciculations, and why are they caused?

A

Damage to a-motor neurone can cause firing of spontaneous APs
Causes contractions in the fibres of the motor unit, which can be seen as small involuntary muscle twitches

35
Q

How does LMN lesions lead to muscle atrophy?

A

Loss of neurotrophic factors from the a-motor neurone nerve terminal, which normally support the muscle causes atrophy
Different to disuse atrophy in UMN syndrome