BiologyLocomotion and movementClass 11Class 12

Sliding filament theory & muscle contraction: interactive simulation

Contraction of a muscle fibre takes place by the sliding of thin filaments (Actin) over thick filaments (Myosin), proposed by Andrew Huxley and Hugh Huxley in the Sliding Filament Theory.

Useful for: NEET, AIIMS, CBSE Boards

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Muscle contraction is initiated by a signal from the central nervous system via a motor neuron. Acetylcholine release causes an action potential in the sarcolemma, releasing Ca²⁺ from the sarcoplasmic reticulum. Calcium binds to Troponin-C, pulling Tropomyosin away from active sites on actin. The energized myosin head forms a cross-bridge and pulls actin toward the M-line (Power Stroke).

  1. 1

    Drag the "Sarcomere Contraction Extent" slider from 0% to 100%.

    What you should see

    Watch the Z-lines move inward: the I-band narrows and the central H-zone shrinks to zero. But notice the purple A-band remains exactly 1.50 μm — the most tested question in NEET!

  2. 2

    Step through the 6-step Molecular Cross-Bridge Cycle.

    What you should see

    At Step 2, Ca²⁺ binds to Troponin-C, shifting the orange Tropomyosin ribbon away to reveal active binding sites. At Step 4, the myosin head rotates 45° in the Power Stroke.

  3. 3

    Observe Step 5 (ATP Binding & Detachment).

    What you should see

    Myosin head CANNOT release from actin until a new ATP molecule binds to it. This explains Rigor Mortis — after death, ATP production stops, so muscles remain permanently stiff.

  4. 4

    Switch display mode between "Sarcomere Bands" and "Cross-Bridge Cycle".

    What you should see

    See both the macroscopic sliding of protein bands and the microscopic biochemical interaction of actin and myosin heads.

Formulas behind it

Sarcomere Length (Resting)

Lsarcomere≈2.5–2.6 μm→Contraction1.8 μmL_{\text{sarcomere}} \approx 2.5\text{–}2.6\,\mu\text{m} \xrightarrow{\text{Contraction}} 1.8\,\mu\text{m}

A-Band Length (Constant)

LA-band=1.5 μm(Length of thick myosin filament, NEVER changes)L_{\text{A-band}} = 1.5\,\mu\text{m} \quad (\text{Length of thick myosin filament, NEVER changes})

H-Zone Behavior

H-zone (central non-overlap region)→0 μm(Completely disappears)\text{H-zone (central non-overlap region)} \to 0\,\mu\text{m} \quad (\text{Completely disappears})

ATP Hydrolysis Reaction

Myosin⋅ATP→ATPaseMyosin⋅ADP⋅Pi(High energy cocked state)\text{Myosin}\cdot\text{ATP} \xrightarrow{\text{ATPase}} \text{Myosin}\cdot\text{ADP}\cdot\text{P}_i \quad (\text{High energy cocked state})

Common mistakes

  • Thinking the A-band shortens during contraction. The A-band DOES NOT change length! Only the I-band and H-zone shorten.
  • Thinking filaments themselves shorten. Filaments (actin and myosin) do NOT contract or shorten; they merely SLIDE past one another.
  • Thinking ATP is required for the power stroke. ATP is required to DETACH the myosin head and re-cock it. The power stroke occurs upon release of ADP and Pi.
  • Confusing Troponin subunits: Troponin-C binds Calcium (Ca²⁺), Troponin-T binds Tropomyosin, and Troponin-I inhibits actin-myosin interaction.
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FAQ

Which band of the sarcomere does not change its length during muscle contraction?

The A-band (Anisotropic band) does not change its length because it represents the entire length of the thick myosin filament, which remains constant.

What is the role of Calcium (Ca²⁺) in muscle contraction?

Ca²⁺ released from the sarcoplasmic reticulum binds to Troponin-C on thin filaments. This induces a conformational shift in Tropomyosin, unmasking the myosin-binding active sites on actin.

Why does Rigor Mortis occur after death?

After death, cellular ATP synthesis halts. Because ATP is essential to detach the myosin cross-bridge from actin (Step 5), the actomyosin complex remains permanently locked in place, resulting in muscle rigidity.

What is the functional contractile unit of a muscle fibre?

The Sarcomere, which is the anatomical and functional portion of a myofibril between two successive Z-lines.

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