Chapter 27: Skeletal Muscle Part I

Question 1

Most primary myopathies feature damage to these kinds of fibers, which are responsible for movement

Answer 1

• extrafusal!

intrafusal fibers are within stretch receptors and are usually not affected by primary myopathies

Question 2

What is the name of the absent or altered protein that is mutated in Duchenne or Becker muscular dystrophies and where is its gene located?

Answer 2

• dystrophin, on the short arm of the X chromosome

Question 3

A 40 yo man presents with progressive weakness and stiffness in his distal limbs for the past few months. He also notices that his face is not as expressive and he sometimes has trouble holding his neck up. You do a muscle biopsy, and find type I fiber atrophy with type II fiber hypertrophy and central nuclei. What disease does he likely have and what is the molecular pathogenesis?

Answer 3

• myotonic dystrophy (DM1), due to expansion of CTG repeat

- extramuscular clues can also be cataracts, testicular atrophy, and personality changes

Question 4

Dr. Goljan has been working on his arm wrestling skills, and completes a vigorous arm routine 4x’s/week. What changes in muscle fiber would be expected in his bulging biceps as compared to average folk?

Answer 4

• hypertrophy of type II fibers: fast, short, more powerful contraction
• As opposed to type I which are slow, sustained contraction for endurance

Question 5

Widespread acute necrosis of skeletal muscle fibers that releases myoglobin into the circulation, which can clog up those kidneys and lead to acute renal failure

Answer 5

rhabdomyolysis

Question 6

What are the three most common forms of congenital myopathies? What are their common clinical symptoms?

Answer 6

central core disease, nemaline (rod) myopathy, and central nuclear myopathy.

• all show hypotonia, decreased deep tendon reflexes, decreased muscle bulk, and delayed motor milestones