Model the sliding-filament mechanism
Explain how myosin pulls actin so a sarcomere shortens: without the filaments themselves getting shorter.
- Muscle fiber structure: A muscle fiber is built from repeating units; you need this layout before you can track what slides.
- Proteins have shapes that do jobs: Myosin and actin are proteins whose shapes let one grab and pull the other.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
During contraction, myosin heads grab actin and pull it toward the sarcomere's center. The filaments slide past each other and overlap more, so the sarcomere shortens: the filaments themselves do not get shorter.
A muscle contracts. Which statement correctly describes what happens to the sarcomere and its filaments?
Approved- A.The actin and myosin filaments each get shorter, so the sarcomere shortens.
- B.Myosin pulls actin toward the center; the filaments slide and overlap more, so the sarcomere shortens while the filaments stay the same length.
- C.The actin filaments stretch out, making the sarcomere longer.
- D.The myosin breaks apart and the pieces fill the gap.
Show the worked solution ▾
Answer: B. Myosin pulls actin toward the center; the filaments slide and overlap more, so the sarcomere shortens while the filaments stay the same length.
- Step 1: Recall who pulls: Myosin heads attach to actin and pull it inward toward the sarcomere center.
- Step 2: Check the filament length: Sliding means the filaments slide past each other and overlap more: they do not change length.
- Step 3: Check the unit length: Because actin is pulled inward from both ends, the sarcomere as a whole gets shorter.
Why it's right: In the sliding-filament model, myosin pulls actin so the filaments overlap more and the sarcomere shortens, while each filament keeps its own length.
- A: The filaments do not shorten: that is the most common misconception this skill targets.
- C: Contraction shortens the sarcomere; it does not lengthen it.
- D: Myosin does not break apart during contraction.
Aligned to HBS 1.2: sliding-filament mechanism · reading level ~grade 9
A relaxed sarcomere measures 2.0 micrometers across. After it contracts it measures 1.5 micrometers. If each actin filament was 1.0 micrometer long before, how long is each actin filament after contraction?
Reviewed- A.0.5 micrometers
- B.0.75 micrometers
- C.1.0 micrometers
- D.1.5 micrometers
Show the worked solution ▾
Answer: C. 1.0 micrometers
- Step 1: Recall the rule: In sliding-filament, filaments slide; they do not change length.
- Step 2: Apply it: If the actin was 1.0 micrometer before, it is still 1.0 micrometer after: only the sarcomere's overall length changed.
Why it's right: Filament length is conserved during sliding, so the actin stays 1.0 micrometer even though the sarcomere shortened from 2.0 to 1.5 micrometers.
- A: This wrongly assumes the filament shrank with the sarcomere.
- B: This wrongly scales the filament down by the same fraction as the sarcomere.
- D: The actin did not lengthen; 1.5 is the sarcomere length, not the filament length.
Aligned to HBS 1.2: sliding-filament mechanism · reading level ~grade 9
- Charting a strength assessment: a clinician describes contraction as filaments sliding, which explains why a muscle can shorten fast but cannot pull beyond full overlap.
Fill these in as you work through the lesson.
- Sarcomere (the repeating unit):
- Actin (the thin filament):
- Myosin (the thick filament with heads):
- Filament (a long protein strand):
When a muscle contracts, the myosin heads pull the filaments toward the center, so the gets shorter even though the filaments do not.
- Two diagrams show the same sarcomere. In the second one the ends are closer together. What moved, and what did NOT change length?
- Why is it wrong to say 'the filaments shrink' when a muscle contracts?
- Which protein does the pulling, and which protein gets pulled?
A sarcomere is 2.0 micrometers long when relaxed and 1.4 micrometers long when contracted. Explain what slid, and confirm that neither the actin nor the myosin changed length.