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Flashcards in Musculoskeletal 1 Deck (58)
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What are the two types of muscle fibers?

1 (slow, red) - depends on oxidative metabolism (fat)

2 (fast, white) - more powerful, depends on anaerobic glycolysis


How does each muscle fiber type stain for myosin ATPase at high pH?

Type 1 (slow, red) fibers don't stain, Type 2 (fast, white) fibers stain darkly.


Is each human muscle composed of either type 1 or type 2 muscles, or are they mixed?

They are mixed


What is the determinant for the development of a type 1 or 2 muscle fiber?

The nerve that supplies it (fiber type can switch if the nerve type is changed)


What is a motor unit?

A nerve and all the muscle fibers that it innervates.


Muscular atrophy is characterized by abnormally _______ myofibers.



Can denervation atrophy affect both fiber types? What do fibers that have undergone denervation atrophy look like microscopically?

It can affect both fiber types. Fibers become small and angular on cross section.


What is fiber-type grouping? What is group atrophy and what is it a hallmark of?

When adjacent nerves make new connections to muscles that have been affected by dernervation atrophy, the fibers switch to the fiber type of the new nerve (checkerboard pattern). If this new nerve dies, then large groups of fibers atrophy - a hallmark of recurrent neurogenic atrophy.


What does disuse atrophy look like microscopically?

Small, angular fibers, primarily type 2, random distribution.


What muscles are primarily affected by steroid-induced (exogenous or endogenous glucocorticoid) atrophy?

Proximal muscles, type 2 fibers.


In advanced cases of muscular dystophy, muscle fibers are replaced by ______ tissue, and labs will show elevated serum ______ ______.

fibrofatty tissue replaces muscle

elevated serum creatine kinase from degenerating muscle


Name two X-linked muscular dystrophy diseases. Which one is more severe? Which one is more common?

Duchenne (more severe, more common) and Becker


Name the disease: begins with weakness in the proximal muscles of extremities at about 1 year of age. Clinically evident by age 5, progressing to immobilization and wheelchair dependence by ages 10-12, wasting, contracture and death by the early 20s.

Duchenne muscular dystrophy


What is the etiology (genetic and protein product) for both Duchenne and Becker muscular dystrophy?

Mutation on short arm of X chromosome --> decreased (Becker) or absence (Duchenne) of dystrophin, a protein that connects the contractile elements to the plasma membrane of the muscle cell.


Duchenne and Becker muscular dystrophy are characterized by relentless _______ of muscle fibers, followed by regeneration and progressive ________.

relentless necrosis --> repair, regeneration --> fibrosis


What type of inflammatory cells are often found in muscles of patients with Duchenne/Becker?

Macrophages - scavenging


Does Duchenne/Becker affect cardiac muscle?



How is Duchenne/Becker muscular dystrophy diagnosed (other than clinical features)?

Abnormal staining for dystrophin or western blot.


What is the main difference between the immunohistological staining of muscle of a patient with Duchenne vs. a patient with Becker?

In Duchenne, there is NO dystrophin staining. In Becker, there is diminished dystrophin staining (reflects the relative severities of the diseases)


What are seven clinical features of the x-linked muscular dystrophies (Duchenne, Becker)?

1. Boys with DMD are normal at birth.
2. Elevated serum creatine kinase.
3. Walking is delayed.
4. Weakness begins in pelvic girdle, then goes to the shoulder girdle.
5. PSEUDOHYPERTROPHY of the calves.
6. Immobilization by age 10-12, bedridden by age 15.
7. Death from resp. insufficiency or cardiac arrhythmias.


Aside from staining for dystrophin, what other microscopic changes can be seen in DMD/BMD (5)?

1. Over-contracted segments of sarcoplasm between degenerated segments.
2. Macrophages.
3. Some fibers are small with a granular sarcoplasm.
4. Enlarged, vesicular nuclei with prominent nucleoli (degenerating fibers).
5. Collagen deposition.


What is the pattern of inheritance of myotonic dystrophy?

Autosomal dominant


At what age do symptoms of myotonic dystrophy usually manifest?

Age of onset and severity are variable, but onset usually occurs in adult years


What is the incidence of myotonic dystrophy?

14 per 100,000


What is the genetic mutation involved in myotonic dystrophy? What does the gene encode?

CTG trinucleotide repeat expansion of the gene for dystrophia myotonica protein kinase (DMPK), located on the long arm of chromosome 19. It encodes a novel serine-threonine protein kinase.


Myotonic dystrophy is characterized by impaired muscle _______, along with weakness associated with ________ and muscle wasting.

impaired muscle relaxation; weakness associated with myotonia (delayed muscle relaxation)


Describe early symptoms of myotonic dystrophy (2).

1. Early childhood - gait abnormalities due to weak foot dorsiflexors.
2. Progression to weakness of intrinsic hand muscles and wrist extensors, followed by facial muscle atrophy and ptosis (eyelid drop).


Name three seemingly random other clinical features that myotonic dystrophy is associated with.

1. Cataracts
2. Testicular atrophy
3. Baldness


Myotonic dystrophy worsens with each passing generation due to trinucleotide repeat expansion. What is this characteristic called?



What microscopic changes are seen in a muscle of a person affected with myotonic dystrophy (3)?

1. Central nuclei
2. Variation in fiber size
3. Fibrosis