W1: Neuromuscular pathologies

Question 1

Describe the structure of skeletal muscle

Answer 1

A sarcomere consists of myofilaments (actin and myosin).
These are arranged together in a myofibril.
Myofibrils are grouped together into a muscle fibre (muscle cell), withintin the cell there will be multiple peripheral nuclei, sarcolemma and a sarcooplasm.
Each muscle fibre is surrounded by endomysium.
Group together to form muscle fascicles which are surrounded by periymysium
Group together the form muscle surrounded by epimysium.

Question 2

What is the process of action potential generation at a neuromuscular junction?

Answer 2

Action potential at nerve terminal causes release of ACh from nerve terminal
Binds to nicotinic receptors on the motor end plate, these are ligand gated Na+ channels that allow the influx of Na+ causing depolarisation of the membrane, when depolarisation is above threshold voltage-gated Na+ channels also open furthering depolarisation so action potential can be reached.
Depolarisation spread along the sarcolemma and transverse tubules.
T tubules contain dihydropyridine receptors (voltage-gated Ca2+ channels), the activation of these receptors causes a conformational change in themselves and ryanodine receptors in the close proximity sarcoplasmic reticulum, this allows Ca2+ to be released from the SR into the sarcoplasm and cause muscle contraction.
Hence Ca2+ provides the link in excitation-contraction coupling.

Question 3

What is a motor unit?

Answer 3

All the muscle fibres innervated by the same motor neuron

Question 4

What is a motor end plate?

Answer 4

Also referred to as a neuromuscular junction
Is a site of specialised chemical synpase between the terminal branches of an axon and a muscle fibre.

Question 5

What protein is involed in limb girldel muscular dystrophy?

Answer 5

The sarcoglycan complex in the sarcolemma.

Question 6

What are the clinical presentations of DMD?

Answer 6

Positive Gowers sign
Hypertrophy of the calf
Loss of motor development (regression or failure)
Loss of or late milestones
Waddling gait
Characterised by Proximal muscle weakness
Heel cord shortening
Blunted intellect
TYpically tend to be a male patient aged 3-6yrs
Progressive condition often wheelchari bound to adolescence and cardiac arrhythmias and respiratory problems in later stages.

Question 7

What is the difference between Becker and Duchenne’s muscular dystrophy on a pathogenesis level?

Answer 7

Both are mutations in the same gene - dystrophen protein
Beckers - in frame mutation, typically mis-sense mutation. Some function of dystrophen protein remains, as the crucial ABD and CR,CT domains remain
Duchennes - out of frame mutation tends to be frameshift or non-sense mutation. Loss of function of protein. As the cystene binding domain tends to be absent.

Question 8

What are the differences in the clinical presentations of Duchennes and Beckers Muscular Dystrophy?

Answer 8

Beckers - diagnosed from age 5 upwards, typically up to 15 years old but can be after this. life expectancy of 40-50yrs, often able to walk until teens and early adult life, often from cardiac effects before skeletal problems

Duchennes - diagnosed between 3-6 years, life expectancy around 20-30yrs, wheelchair-bound by early teenage years, muscular skeletal symptoms are obvious from early on.

Question 9

Describe the role of dystrophin protein.

Answer 9

In skeletal muscle is part of a large complex of sarcolemma associated proteins that provides stability to the sarcolemma.
Protein consist of an N-terminal actin binding domain (ABD1) a central rod domain containing 4 hinge regions and 24 spectrin-like repeats, cystein rich domain for binding a beta dystroglycan and a C-terminus domain.
These carboxyl-terminal domains are important for anchoring to the sarcolemma via the dystrophin-glycoprotein complex, which then links to the endomysium by laminin.
This maintains the structural integrity os muscle fibre.

Has total of 79 exons

Question 10

Explain the consequences of a malfunctional dystrophin protein that is seen in DMD.

Answer 10

Non functional protein leads to inability to anchor actin to sarcolemma then endomysium
Therefore intracellular contractile forces are not spread from the sarcolemma to the ECM
Can lead to small membrane tears and calcium ion influx, accumulates in nucleus (alter gene expression) and the mitochondria (holes in membrane release of cytochrome C triggering apoptosis pathway) that cause myofibre degeneration.

Question 11

What are the morphological changes shown in muscle fibres in both DMD and BMD?

Answer 11

Variation in myofibre diameter
Increased numbers of internalized nuclei
Degeneration necrosis and phagocytosis of muscle fibres
Proliferative of endomysial connective tissue
late stage - muscle replaced by adipose tissue and water

Question 12

What is the common presentation of limb-girdle muscular dystrophy?

Answer 12

Common onset in later childhood or early adulthood
Muscle weakness in hips and things - struggle standing up, back pain, difficulty climbing stairs
Muscle weakness in arms - struggle lifting heavy objects, struggle to lift arms above the head.
Loss of muscle mass in affected areas

Question 13

What is the pathogenesis of limb-girdle muscular dystrophy?

Answer 13

Autosomal recessive condition fot type 2 or autosomal dominant for type 1.
Specific to LGMD 2F involves mutations in the δ-sarcoglycan gene, role in anchoring actin filaments to the sarcolemma
Weakness in the pelvic or shoulder girdle
Variable progression
Muscle fibre destruction and regeneration (milder than DMD)

Question 14

Where is the dystrophin gene expressed?
How?

Answer 14

Dystrophin is expressed predominantly in smooth, skeletal and cardiac muscle as well as some brain neurons.

Has eight different promoters hence allows different functional proteins to be made from the same gene. These different proteins are all dystrophin isoforms.

Question 15

What is meant by a hinge region in DNA?*

Answer 15

Region that in a functional protein will link to other functional units together

Question 16

What is a spectrin repeat region in DNA?*

Answer 16

Codes for a a specifc protein shape - three alpha helical coiled coils

Question 17

What promotor acts on the dystrophin gene to form the dystrophin gene relevant in skeletal muscle in DMD?

Answer 17

Dp427m.

Question 18

What are some treatments currently being invested to treat DMD?

Answer 18

1) gene therapy - delivering a functional dystropgen gene using a viral vector
2) using an anti-sense oligonucleotide to skip the mutated exon so is excluded from final expressed mRNA in splicing (hence does not influence protein structure) - note still some change from normal gene typically changes from DMD to BMD
3) Use CRISPR to edit the genome

Question 19

What influence neural survival in development?

Answer 19

Neurons are overproduced during development
Motor neurons that do not find a muscle fibre to innervate die,
Those that find a muscle to innervate survive.
This is clearly demonstrate by the changes in surviving motor neurons when limbs have been removed or added during embryological development.

Question 20

Describe how innervation of muscle develops from birth onwards?

Answer 20

Neurons that have no found a muscle fibre to innervate have already died
However a neuron may have multiple axons innverating a muscle fibre and a muscle fibre may be innervated by many motor neurons.
Synaptic pruning occurs, axons that innervate multiple targets die back -ensures that individual target fibres are innervated by single neurons (one neuron can still innervate multiple muscle fibres)
This process starts rapidly from birth, one axon terminal will outcompete the other and the loser will retract and die.

Question 21

Describe the pattern of synaptic pruning across the brain.

Answer 21

Occurs thorughout the brain
Happens faster between age 2 to 10 years.
Number of synapses tends to peak 4 months after birth then gradually increases as we age
Process occurs faster in girls (hence reach maturity faster)
Rate and degree of loss depends on the location in the brain
Key regions in the brain that undergone pruning include the limbic system, the visual system, the motor system and the prefrontal system.

Question 22

Why do we undergo the process of synaptic pruning?

Answer 22

Increase the efficiency of synaptic connections - faster and more efficient neurological activity
Often causes solidification of behavioural and thought patterns.

Question 23

Give an overview of what spinal muscular atrophy is.

Answer 23

A range of hereditary diseases
Results in death of lower motor neurons typically due to mutated SMN1.
Lack on innervation to muscle results in muscle atrophy

Question 24

What are the four different types of Spinal Muscular Atrophy and how are they different?

Answer 24

Types 0-4, higer numbers are less seveere
0 - presents in foetus - die soon after birth as no communication, feeding, motor control and very little pulmonary control
1 - presents at 3 months - no sitting to rolling, requires support feeding, communicating and pulmonary function - survives around 10 months
2 - present at 12 months - sits but no walking, som variable support needed to feeding, communication and plumanory function - live 20yrs
3 - present at 3 years - limited walking, normal feeding and communication, mild pulmonary impairment but no symptoms - normal survival
4 - present older than 18yrs, can walk, feed, communicate and good pulmonary function - normal lifeexpectancy

Question 25

What is the cause of spinal muscular atrophy?

Answer 25

SMN1 mutation (autosomal recessive) - leads to decreased expression of SMN1 protein Survival of Motor neurons 1 is a protein located in the centre of the SMN complex - found mainly in the cytoplasm but also the nucleus Loss of ability to regulate RNA metabolism, translation and cytoskeleton function Results in neuron death, particularly alpha motor neurons, results in muscle atrophy from underuse.

Question 26

Why does the genetic evolution of humans make us more vulnerable to Spinal Muscular Atrophy?

Answer 26

Undergone human specific gene duplication to produce an SMN1 and an SMN2 gene. These sequences are almost identical but SMN2 commonly has a mutation that causes splicing problems, splicing out exon 7 90% of the time, meaning only 10% of SMN2 protein is functional. This means patients with a mutated SMN1 so are reliant on SMN2 are more vulnerable to SMA is they have less copy numbers of SMN2 (as sequence can repeat).

Question 27

How does the number of SMN2 repeats relate to the severity of Spinal Muscular Atrophy disease?

Answer 27

1 copy tends to be incompitable with life or type 1 spinal atrophy, with 2 copies being most common in type 1. Type 2 is predominantly 3 copes with some 2 copies Type 3 - is mainly3 or 4 copies Hence more copies of SMN2 means less severe spinal muscular atrophy.

Question 28

What test can be done to indicate spinal muscular atrophy?

Answer 28

Neurofilament L (light chain) levels in the blood Elevated can indicate spinal muscular atrophy - as released from damaged neurons particularly myelinated axons

Question 29

What is the pathogenesis of spinal muscular atrophy?

Answer 29

Mutated SMN1 and low copies of SMN2 increase vulnerability Leads to poor functioning SMN comples leads to poor RNA metabolism, translation and cytoskeleton Results in neuronal death. Effect at ventral horn: synaptic dysfunction, loss of synapses, motorneuron hyperexcitability, motorneuron death Effect on motor neuron: reduced radial growth, impaired schwann cell ensheathment and myelination, degeneration of neuron Ejject at neurmuscular junction: reduced NMJ maturity, reduced quanta content (vesicle release), denervation Effect on muscle: reduced myofibre size, impaired development, myofibre loss.

Question 30

What are the treatments for spinal muscular atrophy?**

Answer 30

a) exon skipping approach using anti-sense oligonucleotide - nursinersin - nursinersin, inhibits splicing factors to include exon 7 in final transcripy bind to preMRNA b) risdiplam - small molecule - premRNA splicing modifier to insure the inclusion of exon 7, increase functional protein expressed from SMN2. Binds to preMRNA c) gene therapy approach to insert functional, SMN gene using an adeno-associated virus taken up by endocytosis, gene in nucleus (not inserted into host genome) expressed alongside host gene, replaces protein functionally.(onasemnogene adaparvovec)

Question 31

What is amyotrophiv lateral sclerosis?

Answer 31

A type of motor neuron disease Loss of motor neurons - die, loss of motor neurons in the ventral horn is clrealy identified on histological images Complex polygenic disease

Question 32

What are the clincal presentations of a person with amyotrophic lateral sclerosis?

Answer 32

Cognitive and behavioural impairement Dysphagia and dysarthria respiratory insufficiency Muscle cramps Spasticity Muscle weakness Muscle atrophy

Question 33

What is thought the be the cause of amytrophic lateral sclerosis?

Answer 33

30 genes implicated but is polygenic and with high influence from the environment No clear inheritance pattern but is thought to be a familial link Mutated genes tend to be involved in cellular metabolism, such as mitochondrial dysfunction, dysregulated vesciel transport, abnormal RNA metabolism, impaired DNA repait, oxidative stress and impaired homeostasis. Effects muscle most due to higher metabolic rate. Alternativly some are more specific to neurons - glial dysfunction, hyperexcitability and axonopathy. Characterised by protein aggregates accumulating in the neuron.

Question 34

How do the symptoms of amyotrophic lateral sclerosis typically progress?

Answer 34

Starts as lower motor neurons symptoms Then develops into upper motor neuron symptoms Both tend to be affected in typical onset

Question 35

What is thought to cause cerebral palsy?

Answer 35

Non-progressive damage to the brain in antenatal, perinatal or early postnatal period. Includes: preamture birth, hypoxia, infection, head injury, drowning

Question 36

What are the common effects of cerebral palsy on the brain?

Answer 36

Hypoplasia - reduced growth Polymicrogyria - increased number of irregular formed gyri Subcortical atrophy - loss of basal ganglia or fibres in the subcortical white matter. And periventricular leukomalacia

Question 37

How does the prevelance of cerebral palsy vary bt the gestational week?

Answer 37

The risk of develpoing cerebral palsy is significantly higher at earlier weeks of gestation However, the majority of children with cerebral palsy are born at normal gestation

Question 38

What muscle pathology is often observed in cerebral palsy?

Answer 38

Loss of satellite cells Decrease in muscle fibre diameter Increased ECM deposition, particularly laminin. Decrease growth of muscle, often atrophy, Highly stretched sarcomeres. Results in restricted movement and force of contraction

Question 39

What are the clincal manifestation of cerebral palsy?

Answer 39

Due to loss of UMN control of motor activity (negative features of UMN syndrome): - impaired selective motor control - weakness of muscles - poor coordination and sensory deficits Can present as non neural symptoms such as contractures, bony torsion and hip displacement Due to loss of LMN inhibition (causes positive features of UMNL): - clonus - spasticity - hyperreflexia -co-contraction This can present as neural problems like contractures, scoliosis and changes in posture or hip displacement

Question 40

What is meant by diversity in cerebral palsy?

Answer 40

Highly heterogeneous disease with a range of causes hence the manifestation of symptoms Meaning requires a range of interventions

Question 41

What are the different types of disorders in which cerebral palsy may present?*

Answer 41

1) Movement disorders - can be from abnormal muscle activation or abnormal reflexes - both of which can cause a dystonic response (involuntary contraction) which can present as a movement disorder to a postural disorder. 2)Stretch reflex disorders - which are due to supraspinal inhibition and present as a spastic response 3) muscle disorder due to trophic changes in the muscle - resulting in muscle fibre-type transformation which causes reduced muscle extensibility and joint range.

Question 42

How do the different types of cerebral palsy disorders present?*

Answer 42

Movement and posture disorders (dystonic response) and stretch reflex disorder (spastic response) - show as EMG positive and dynamic muscle resistance. (movement) Muscle disorders, mainly muscle fibre-type transformation present as EMG negative and passive muscle resistance, can cause contracture and deformity. (note the above can also lead to this)

Question 43

What are the different types of treatment intervention that may be considered in cerebral palsy?

Answer 43

Levodopa - treat movement disorders, improves control over movement Deep brain stimulation - interrupts signals that cause tremors and movement symptoms Medication to relax skeletal muscles such as - Botox, baclofen or ITB, benzodiazepines, anticholinergics, gabapentin Selective dorsal rhizotomy to reduce spasticity - irreversibly remove some sensory innervation from the limbs Orthopaedic surgery - on muscles, tendons, bones and nerves aims to improves anatomical alignment.