Muscle and the neuromuscular junction

Question 1

What are motor end plates?

Answer 1

The synapses that form between motor neurones and muscle

Question 2

Neurotransmitter release to the motor end plate of muscle

Answer 2

Action potential causes voltage calcium channels to open–> Influx and release of ACh

Question 3

What happens when ACh binds on the target muscle?

Answer 3

The ACh receptors open and render the membrane permeable to Na/K ions

Question 4

Which enzyme is responsible for cessation of neurotransmission?

Answer 4

Acetylcholinesterase

Question 5

How does botulinum toxin cuase weakness?

Answer 5

Cleaves presynaptic proteins involved in vesicle formation and block vesicle docking with presynaptic membrane

Question 6

How does Lambert Eaton myasthenic syndrome cause weakness?

Answer 6

Antibodies to presynaptic calcium channels leads to less vesicle release

Associated with small cell carcinoma

Question 7

Why does myasthenia gravis cause weakness?

Answer 7

Antibodies to ACh receptors–> Reduced number of functioning receptors leading to weakness

ie it is a post synaptic condition

Question 8

When do symptoms of myasthenia gravis begin?

Answer 8

When ACh receptor number are reduced to 30% of normal

Question 9

How is the thymus linked to myasthenia gravis?

Answer 9

75% of patients have hyperplasia or thymoma

Question 10

Clinical features of MG?

Answer 10

Typically fluctuating weakness
Most common extraoccular, facial and bulbar weakness

Limb weakness typically proximal

Question 11

Treatment of MG?

Answer 11

Acetylcholinesterase inhibitor- Pyridostigmine

Thymectomy

immunomodulation

Steroids/Azathioprine

Question 12

features of skeletal muscle?

Answer 12

Long, cylindrical multinucleate with mitochondria

Surrounded by thin layer of endomysium

Question 13

Features of smooth muscle?

Answer 13

Single central nucleus

Gap junctions between cells

Question 14

What are fasciculations?

Answer 14

Visible, fast fine spontaneous twitches

Question 15

When do fasciculations happen?

Answer 15

Can happen in healthy muscle

Occurs in denervated muscle which becomes hyperexcitable

Question 16

What is myotonia?

Answer 16

Failure of muscle to relax after use

Question 17

Symptoms/signs of muscle disease

Answer 17

Myalgia
Muscle weakness
Wasting
Hyporeflexia

Question 18

Inflammatory muscle conditions

Answer 18

Dermato and polymyositis

Question 19

Inherited muscle conditions?

Answer 19

Muscular dystrophies

Question 20

Congenital muscle conditions?

Answer 20

Congenital myasthenic syndromes

Congenital myopathies

Question 21

Clinical features of polymyositis?

Answer 21

Symmetrical, progressive proximal weakness over weeks to months

Raised CK- responds to steroids

Question 22

Clinical features of dermatomyositis?

Answer 22

Same as polymyositis +skin lesions

Heliotrope rash

Up to 50% have underlying malignancy

Question 23

Features of inclusion body myositis?

Answer 23

Slow progressive weakness in 6th decade with characteristic thumb sparing

Question 24

What is myotonic dystrophy

Answer 24

Commonest musclular dystrophy

Autosomal dominant

Multisystem involvement

Question 25

Features of myotonic dystrophy?

Answer 25

Myotonia
Weakness
Cataracts
Ptosis
Frontal balding
Cardiac defects

Question 26

What is rhabdomyolysis?

Answer 26

Damage to skeletal muscle casuing leakage of large quantities of toxic intracellular contents into plasma

Myalgia, weakness and myoglobinuria