Sliding filament theory explains how skeletal muscles contract at the microscopic level. It shows how thin actin filaments slide past thick myosin filaments inside sarcomeres to shorten muscle fibers. This cheat sheet helps students connect muscle structure, ATP use, calcium signaling, and force production.
It is useful for reviewing anatomy, physiology, and cell biology concepts in one clear reference.
Key Facts
- A sarcomere is the basic contractile unit of a muscle fiber and runs from one Z line to the next Z line.
- During contraction, actin filaments slide toward the center of the sarcomere while myosin filaments stay the same length.
- Calcium ions bind to troponin, causing tropomyosin to move away from the myosin-binding sites on actin.
- The cross-bridge cycle is attachment, power stroke, detachment, and reactivation of the myosin head.
- ATP binds to myosin to release it from actin, and ATP hydrolysis re-cocks the myosin head for the next stroke.
- The I band and H zone become smaller during contraction, but the A band stays the same length.
- A motor neuron triggers contraction by releasing acetylcholine at the neuromuscular junction, starting an action potential in the muscle fiber.
- Relaxation occurs when calcium ions are pumped back into the sarcoplasmic reticulum and tropomyosin blocks actin’s binding sites again.
Vocabulary
- Sarcomere
- The repeating unit of a myofibril that shortens during muscle contraction.
- Actin
- The thin filament that contains binding sites for myosin during contraction.
- Myosin
- The thick filament with heads that pull on actin to produce movement.
- Troponin
- A regulatory protein that binds calcium and helps move tropomyosin off actin’s binding sites.
- Tropomyosin
- A regulatory protein that blocks myosin-binding sites on actin when the muscle is relaxed.
- Sarcoplasmic Reticulum
- A specialized membrane network in muscle cells that stores and releases calcium ions.
Common Mistakes to Avoid
- Saying the filaments shrink is wrong because actin and myosin keep the same length while they slide past each other.
- Forgetting ATP is needed for detachment is wrong because myosin cannot release from actin without ATP binding.
- Thinking calcium directly pulls the filaments is wrong because calcium changes the position of troponin and tropomyosin to expose binding sites.
- Mixing up band changes is wrong because the I band and H zone shorten during contraction, while the A band stays constant.
- Leaving out the sarcoplasmic reticulum is wrong because calcium release and reuptake from this structure control contraction and relaxation.
Practice Questions
- 1 A sarcomere shortens from 2.4 micrometers to 2.0 micrometers during contraction. By how many micrometers did it shorten?
- 2 If one myosin head uses 1 ATP per cross-bridge cycle, how many ATP molecules are used by 200 myosin heads completing 3 cycles each?
- 3 During contraction, the H zone changes from 0.6 micrometers to 0.2 micrometers. What is the decrease in H zone length?
- 4 Explain why a muscle cannot relax properly if calcium ions are not pumped back into the sarcoplasmic reticulum.