Disorders of the Motor Unit

Question 1

What is a motor unit?

Answer 1

• An alpha motoneurone
• All of the extrafusal muscle fibres it supplies
• The NMJ.

Question 2

What is the functional difference between an alpha motoneurone and a gamma motoneurone?

Answer 2

• An alpha motoneurone innervates extrafusal muscle fibres.

- A gamma motoneurone innervates intrafusal muscle fibres.

Question 3

What is the difference between intrafusal muscle fibres and extrafusal muscle fibres?

Answer 3

• Intrafusal muscle fibres are sensory organs that are stimulated in response to stretch.
• Extrafusal muscle fibres contract to displace the limbs.
• Both are skeletal muscle.

Question 4

What is polio myelitis?

Answer 4

• A communicable infection.
• Targets and destroys the soma of the lower motoneurones in the ventral horn.
• Caused by polio virus.
• AKA infantile paralysis.

Question 5

List 2 symptoms of polio myelitis.

Answer 5

1 - Muscle weakness

2 - Paralysis of affected muscles.

Question 6

What are the two types of motoneurone disease?

Answer 6

• The disease destroys both upper and lower motoneurones.
• Diseases of this type are known progressive supranuclear palsy.

Or

-The disease only destroys the lower motoneurone.

Question 7

What is the pathophysiology of motoneurone disease?

Answer 7

Motoneurones undergo apoptosis.

Question 8

List 2 motoneurone diseases.

Answer 8

• Lou Gherig’s Disease

- Amyotrophic lateral sclerosis

Question 9

Which motoneurones of the body do not undergo apoptosis under motoneurone disease?

Answer 9

• Motoneurones supplying extraocular muscles.

- Motoneurones supplying the anal sphincter (S2-S4 roots).

Question 10

When is Guillan-Barré syndrome acquired?

Answer 10

Following a viral infection such as a common cold.

Question 11

How can diabetic neuropathies affect the nervous system?

Answer 11

Demyelination of both sensory and motor axons.

Question 12

What do Guillan-Barré syndrome and diabetic neuropathies share in common?

Answer 12

They are both demyelinating diseases.

Question 13

What do polio myelitis and motoneurone disease share in common?

Answer 13

Both result in the death of cell bodies of motoneurones.

Question 14

What can be affected in motoneurone disease that cannot be affected in polio myelitis? Is this always the case?

Answer 14

• Upper motoneurones.

- Only in the progressive supranuclear palsy variant of motoneurone disease.

Question 15

What is a common cause of botulinum toxin culturing?

Answer 15

Poorly preserved (tinned) foods.

Question 16

How does the botulinum toxin affect the nervous system?

Answer 16

Causes the presynaptic terminal of the NMJ to be unable to release acetylcholine into the synaptic cleft.

Question 17

Give an example of a substance that acts as a nerve agent.

Answer 17

Organophosphates.

Question 18

How do nerve agents affect the nervous system?

Answer 18

They block acetylcholinesterase in the NMJ, preventing the breakdown of acetylcoline.

Question 19

Briefly describe the aetiology and pathophysiology of Duchenne Muscular Dystrophy.

Answer 19

• Genetic aetiology.

- Characterised by defects in muscle proteins that lead to death of muscle tissue.

Question 20

List 6 lower motoneurone signs.

Answer 20

• Flaccid muscle weakness
• Hypotonia or atonia
• Hyporeflexia or areflexia
• Denervation muscle atrophy
• Fasciculations (acute phase)
• Muscle wasting

Question 21

What is ptosis?

Answer 21

Drooping of the upper eyelid.