Skeletal Muscle Pathology

Question 1

What are the biochemical markers for muscle injury?

Answer 1

• Injury leads to leakage of cell contents
• Lose cell integrity but not function
• Creatine kinase (CK) - specific to muscles, elevation indicates skeletal muscle damage
• Aspartate aminotransferase - less specific
• Myoglobin - stores oxygen in muscles, nephrotoxic

Question 2

What clinical signs could indicate muscle damage?

Answer 2

• Weakness, lethargy
• Gait examination - limp
• Muscle size and tone
• Red urine - severe muscle breakdown releases myoglobin which gives urine a red colour

Question 3

What 2 fibres are found in skeletal muscle?

Answer 3

1. Type 1 fibres

• Slow response
• Slow to fatigue
• Oxidative metabolism
• Postural muscles
• ‘Red’ muscle
  2. Type 2 fibres
• Fast response
• Fast to fatigue
• Glycolytic metabolism
• Propulsive muscles
• ‘White’ muscle
  *

Question 4

What is the response of muscle to injury?

Answer 4

Necrosis followed by regeneration

Question 5

What appearance do necrotic muscle cells have?

Answer 5

• Segmental
• Cells hypercontract
• Have a homogenous, eosinophilic appearance
• Lose striations
• Myofibre size and shape changes from angular to rounded

Question 6

What happens if the basal lamina is left intact after necrosis?

Answer 6

• Macrophages enter and remove debris
• Satellite cells become myoblasts
• Myoblasts move to the centre and fuse to form myotubes
• Essential structures reform, primary myofibrils
• Nuclei move to a peripheral position

REGENERATION

Question 7

What happens if the basal lamina is destroyed? e.g. burn, ischaemia

Answer 7

• If satellite cells survive, these develop into multinucleated muscle giant cells
• There is an attempt at regeneration from these cells

FIBROSIS

Question 8

How can necrosis and regeneration be classified?

Answer 8

• Focal monophasia - one site at one time e.g. IM injection
• Multi-focal monophasia - at one time in numerous muscles e.g. exposure to toxin
• Focal polyphasic - repeated trauma at the same site
• Multi-focal polyphasic - repeated trauma in numerous sites e.g. Se deficiency

Question 9

How does atrophy affect muscle cells?

Answer 9

• Reduces myofibre diameter and muscle volume
• Physiological - disuse, starvation/emaciation, cachexia associated with disease
• Slowly progressive
• Typically affects type 2 fibres
• Denervation - myofibres are reliant on intact innervation for size and structure. Denervation leads to rapid disintergration of myofibrils. Cells take on flattened, angular appearance

Question 10

How does hypertrophy affect muscle cells?

Answer 10

• Increase in size by adding myofibrils
• Physiological - increased work/exercise
• Pathological - compensation of remaining myofibres in a partially denervated muscle

Question 11

Give examples of skeletal muscle diseases

Answer 11

1. Degenerative - ischaemia, nutritional, toxic, exertional, traumatic
2. Inflammatory - bacterial, parasitic, immune mediated
3. Inherited - muscular dystrophy
4. Neoplasia - primary, secondary, metastatic