Muscle and Peripheral Nerves

Question 1

NT used at the NMJ

Answer 1

Acetylcholine

Question 2

Type 1 fibers

Answer 2

White
Slow sustained contractions
Resist fatigue
Rich in oxidative enzymes
Ex: postural/endurance
Use oxidative phosphorylation pathway

Question 3

Type 2 fibers

Answer 3

Red
Rapid short contractions
Easily fatigued
Rich in glycogen
Ex: sprinting
Use glycogen storage which runs out soon

Question 4

Definitions

1. Myotonus
2. Fibrillation
3. Myalgia

Answer 4

1. Repetitive contraction of muscles
2. Spontaneous contraction
3. Muscle pain

Question 5

Function of muscles cells depends on (2 things)

Answer 5

1. Proteins in the muscle membrane (ex: dystrophin, tropomyosin)
2. Generation of ATP from lipid and glycogen in the mitochondria (hence metabolic myopathies)

Question 6

Upper motor neuron

Answer 6

Provides inhibitory stimulation
When removed the muscle will become more excitable
The reflex arcs are more able to cause contraction
Still innervated by lower motor neuron

Question 7

Lower motor neuron

Answer 7

Cut this and you will get neurogenic atrophy of the muscle

Question 8

Guillain-Barre Syndrome

Answer 8

Autoimmune neuritis - autoimmune attack on motor nerve axons
2-4 weeks after viral or bacterial infection
Antibodies directed against nerve sheaths
Leads to limb weakness and total paralysis
Treat with immunosuppressors and plasmapheresis

Question 9

Fiber type grouping

Answer 9

Once a muscle is reinnervated, it is only controlled by a single axon
Since axon controls fiber type, they will all be the same

Question 10

Myasthenia Gravis

Answer 10

Autoimmune disease involving NMJ
Antibodies to ACh receptors in most cases
In younger group, associated with thymoma
No full denervation, there are just reduced receptors
Easy fatigability, muscle weakness
Ptosis, inability to chew, diplopia, bland expression
Death from resp paralysis

Question 11

1. Ptosis
2. Diplopia
3. Myotonia

Answer 11

1. Dropping eyelids
2. Double vision
3. Sustained muscle contration

Question 12

How to diagnose and treat MG?

Answer 12

Diag: block cholinesterase, will see transient increase in ACh levels (reverse symptoms), EMG, serologic testing for antibodies
Treat: thymectomy, anticholinesterases, plasmapheresis

Question 13

Becker’s Dystrophy

Answer 13

Some loss of dystrophin, but milder

Question 14

Duchenne’s muscular dystrophy

Answer 14

Most common MD
Caused by complete deficiency of dystrophin
Gene on X chromosome - X linked recessive so mostly in boys
Weakness of pelvic girdle muscles
Hypertrophy to compensate, fiber splitting (trying to increase ability to work), replacement by scar tissue (fibrosis and fat)
Respiratory insufficiency eventually and premature death

Question 15

Myotonic Dystrophy

Answer 15

2nd most common genetic muscle disease
Autosomal dominant, CTG trinucleotide repeat expansion in DMPK gene on chromosome 19
Myotonia, hatchet face, weakness of eye muscles, systemic symptoms (diabetes, heart disease, gonadal dystrophy), ptosis, frontal balding

Question 16

Repeat in Huntington’s vs Myotonic Dystrophy

Answer 16

H: CAG
MD: CTG

Question 17

Congenital myopathies

Answer 17

Rare, muscle weakness in infancy (floppy infant syndrome), often lethal at young age
Many different disorders

Question 18

Immune vs neurogenic diseases in regards to the muscles they effect

Answer 18

Immune: proximal rather than distal muscles
Neurogenic: distal > proximal

Question 19

Dermatomyositis

Answer 19

Lilac blue rash on upper eyelids
Inflammation is not limited to muscles and may involve other organs
Skin changes are prominant
Cells invade blood vessels in the perimysium
Perifascicular atrophy

Question 20

Polymyositis

Answer 20

T cells and macrophages
Invade the endomysium
Necrotic and regenerating fibers
Chronic inflammation of the muscles