1. What are the contractile elements of a muscle fiber?
2. What is the difference between Type I and Type II muscle fibers in terms of recruitment, fatigue, effort required, firing frequency, movements, and oxygen capacity?
3. How do muscle fibers contract?
4. How does relaxation occur?
5. Why does rigor mortis occur?
Answers:
1. Actin and myosin
2. Type I (slow twitch) fibers are recruited early, highly resistant to fatigue, require mild effort (4-8 Hz), fire slowly, are used for fine movements, and are aerobic. Type II-A and II-B fibers are recruited late, more sensitive to fatigue, require high to intermediate effort (20-30 Hz), fire fast, are used for gross movements, and are anaerobic.
3. The stimulus penetrates the T-tubule system and causes Ca to be released from the sarcoplasmic reticulum (SR). This binds to the troponin-tropomyosin complex and exposes actin's active sites, which bind with myosin heads. The actin and myosin filaments slide over each other to shorten the muscle.
4. For relaxation, Ca is actively pumped back into the SR powered by ATP so that tropomyosin can block actin's active sites.
5. After death, there is an absence of ATP so the Ca ions cannot be pumped back into the SR, so actin and myosin remain permanently joined.
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1 comment:
Type IIA fibers are actually a hybrid of Type I and Type II muscle fibers in that they are oxidative (aerobic) and glycolytic (anerobic). They can change based on consistent level of activity or lack thereof. With constant use, be it resistive or endurance training, they become more aerobic and "Type I-like" as the demands placed on them induce greater myoglobin, mitochondria, & capillary growth/upregulation.
Likewise, with disuse & atrophy, they have no need for the extra aerobic players / mechanisms and are better served with a system designed for a sedentary lifestyle so they convert more toward Type IIB glycolytic design.
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