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Old 04-19-2010
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Question Physiology of Disuse Muscular Atrophy

A healthy 35-year-old woman has a cast removed from her leg after 6 weeks of immobilization. Which of the following best characterizes her gastrocnemius muscle at this time?

(A) Conversion to fast fibers
(B) Decrease in number of fibers
(C) Decrease in number of myofibrils
(D) Increase in mitochondrial content
(E) Increase in number of satellite cells
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Old 04-20-2010
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Default answer is A

Muscle fibers do not decrease in number, that would be called hypoplasia and not atrophy or hypotrophy. So options B and C are certainly wrong.
Increased mitochondrial content occurs in the context of exercise not immobility. So D is incorrect.
It is the decrease not the increase of satellite cells that's seen in atrophy. So E is also wrong.

So the correct choice is A. That makes sense in immobility to switch to white Type II muscle fibers that are less endurable.

Please reply if you know the correct answer
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Old 04-20-2010
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Default You corrected my understanding

Seriously I thought the decrease in muscle fibers
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Old 04-21-2010
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Default

that's a real good explanation and argument, however,

the answer is actually C. decrease in myofibrils. not mucle fibres but myofibrils.

this attached review on the molecular basis of muscle atrophy may help a better understanding.
Attached Files
File Type: pdf muscle disuse atrophy.pdf (394.0 KB)
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Old 04-21-2010
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Default I insist on the smae answer

Quote:
Originally Posted by Seetal View Post
that's a real good explanation and argument, however,

the answer is actually C. decrease in myofibrils. not mucle fibres but myofibrils.

this attached review on the molecular basis of muscle atrophy may help a better understanding.
Did u read the article that u attached! They are saying there's a decrease in the diameter of the myofibrils not in their number.
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Old 04-22-2010
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Default

Quote:
Originally Posted by RRMadukha View Post
Did u read the article that u attached! They are saying there's a decrease in the diameter of the myofibrils not in their number.
im pretty sure they said number. ill read it again and copy and paste a few lines that explain it. but the answer is C.
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Old 04-24-2010
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Quote:
Originally Posted by RRMadukha View Post
Did u read the article that u attached! They are saying there's a decrease in the diameter of the myofibrils not in their number.
sorry it took so long to reply. i forgot.

to answer yr question, YES i did read it. and if U had READ past the first page u would see how clearly and beautifully they had explained it with figures too!

the entire article explains how there are many things involved in the signalling of muscle atrophy. basically muscle atrophy occurs due to 2 things which are decreased in protein synthesis and increase in protein degradation. and there are many signals involved in initiating the process (which is explained in the article i attached).

but to summarize it, protein degradation in muscle atrophy occurs mostly due to degradation of myofibrillar proteins that make up most of the muscle cell proteins. i think the confusion here is that some of u think decrease in myofibrils means decrease in muscle cells. HOWEVER, this is not the case. muscle cells do not decrease in number (this is something everyone should know by now), but rather decrease in protein number which causes the muscle cells to flatten (atrophy). here's a quote from the article:

"At least half of total muscle protein is myofibrillar protein, and this
fraction is lost at a faster rate than other muscle proteins during
atrophy"


and with this quote (page C838) u will see this beautiful picture that shows a myofibrillar protein being degraded.

hope that was helpful

ps: the article also explains that in people with TGF-Beta deficiency, they cannot stop the over synthesis of myofibrillar protein and hence tend to have a muscle hypertrophy disorder. thats another interesting piece of information.
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