What is the Sliding Filament Theory?
The sliding filament theory explains how muscles contract at the molecular level: thin actin filaments slide past thick myosin filaments, shortening the sarcomere without either filament changing length. It is the accepted model for how chemical energy (ATP) becomes mechanical force in skeletal and cardiac muscle.
According to the sliding filament theory, myosin heads repeatedly bind actin, pull it toward the sarcomere's center (power stroke), release, and re-bind further along — sliding actin over myosin and shortening the muscle.
- 1.ATP binds myosin — ATP attaches to the myosin head, causing it to release from actin.
- 2.ATP hydrolysis — ATP splits into ADP + Pi, re-cocking the myosin head into a high-energy position.
- 3.Cross-bridge forms — With Ca2+ bound to troponin exposing binding sites, the myosin head attaches to a new actin site.
- 4.Power stroke — Pi is released, the myosin head pivots and pulls the actin filament toward the sarcomere center; ADP is released.
- 5.Cycle repeats — A new ATP binds the myosin head, detaching it again — the cycle continues as long as Ca2+ and ATP are present.
Step-by-step worked examples
During a bicep curl, calcium ions flood the sarcoplasm. What happens to the troponin-tropomyosin complex, and why does it matter for contraction?
Ca2+ released from the sarcoplasmic reticulum binds to troponin Troponin changes shape and pulls tropomyosin away from the myosin-binding sites on actin Myosin heads can now attach to actin, beginning the cross-bridge cycle and contraction
A muscle cell completely runs out of ATP (as happens after death, causing rigor mortis). What happens to its cross-bridges?
Without ATP, myosin heads cannot release from actin (ATP binding is required for detachment) All cross-bridges lock in the bound position The muscle becomes stiff and cannot relax — this is rigor mortis
A sarcomere shortens from 2.2 µm to 2.0 µm during contraction. What happens to the A band versus the I band?
The A band (spans the full length of the myosin filament) stays the same width — myosin doesn't shrink The I band (actin only, no myosin overlap) narrows, because actin slides further into the A band The H zone (myosin only, no actin overlap) also narrows or disappears as overlap increases
Flashcards
Quick quiz
Q1.According to the sliding filament theory, muscle contraction occurs because:
Q2.What triggers the exposure of myosin-binding sites on actin?
Q3.What happens to the myosin head immediately after the power stroke?
Q4.During muscle contraction, which sarcomere zone narrows?
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Common mistakes
Myosin and actin filaments themselves shrink during contraction. — Correct: The filaments stay the same length — they slide past each other rather than shrinking.
Contraction needs calcium but not ATP. — Correct: Both are essential: Ca2+ exposes the actin binding sites, while ATP powers cross-bridge cycling and detachment.
The power stroke happens before ATP is hydrolyzed. — Correct: ATP hydrolysis (into ADP + Pi) cocks the myosin head first; the power stroke happens after the cross-bridge forms and Pi is released.
Rigor mortis happens because muscles run out of calcium. — Correct: Rigor mortis occurs because ATP runs out, so myosin heads can't detach from actin — it's an ATP problem, not a calcium one.
FAQ
What is the sliding filament theory?
It's the model of muscle contraction stating that thin actin filaments slide over thick myosin filaments, shortening the sarcomere, while the filaments themselves stay the same length.
What triggers muscle contraction according to the sliding filament theory?
A nerve impulse releases calcium into the muscle cell; Ca2+ binds troponin, exposing actin's binding sites so myosin cross-bridges can form and pull the filaments together.
Why don't actin and myosin filaments change length during contraction?
Contraction is caused by filaments sliding past each other via the cross-bridge cycle, not by the filaments themselves stretching or shrinking.
What role does ATP play in the sliding filament theory?
ATP hydrolysis cocks the myosin head for the power stroke, and a new ATP binding is what allows the myosin head to detach from actin and repeat the cycle.