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BIO 140 Human Biology I - Questions and Solutions

Muscle Fiber Contraction and Relaxation - Solutions

Review Questions

1. In relaxed muscle, the myosin-binding site on actin is blocked by ________.

  1. titin
  2. troponin
  3. myoglobin
  4. tropomyosin

2. According to the sliding filament model, binding sites on actin open when ________.

  1. creatine phosphate levels rise
  2. ATP levels rise
  3. acetylcholine levels rise
  4. calcium ion levels rise

3. The cell membrane of a muscle fiber is called ________.

  1. myofibril
  2. sarcolemma
  3. sarcoplasm
  4. myofilament

4. Muscle relaxation occurs when ________.

  1. calcium ions are actively transported out of the sarcoplasmic reticulum
  2. calcium ions diffuse out of the sarcoplasmic reticulum
  3. calcium ions are actively transported into the sarcoplasmic reticulum
  4. calcium ions diffuse into the sarcoplasmic reticulum

5. During muscle contraction, the cross-bridge detaches when ________.

  1. the myosin head binds to an ADP molecule
  2. the myosin head binds to an ATP molecule
  3. calcium ions bind to troponin
  4. calcium ions bind to actin

6. Thin and thick filaments are organized into functional units called ________.

  1. myofibrils
  2. myofilaments
  3. T-tubules
  4. sarcomeres


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Critical Thinking Questions

1. How would muscle contractions be affected if skeletal muscle fibers did not have T-tubules?

Solution: Without T-tubules, action potential conduction into the interior of the cell would happen much more slowly, causing delays between neural stimulation and muscle contraction, resulting in slower, weaker contractions.

2. What causes the striated appearance of skeletal muscle tissue?

Solution: Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell cause the entire cell to appear striated.

3. How would muscle contractions be affected if ATP was completely depleted in a muscle fiber?

Solution: Without ATP, the myosin heads cannot detach from the actin-binding sites. All of the “stuck” cross-bridges result in muscle stiffness. In a live person, this can cause a condition like “writer’s cramps.” In a recently dead person, it results in rigor mortis.


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OpenStax, Muscle Fiber Contraction and Relaxation. OpenStax CNX. Dec 15, 2016 http://cnx.org/contents/12d59670-933e-45cf-b4b6-c29f2dde2847@5. © Dec 15, 2016 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License 4.0 license.