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Sliding Filament Theory Muscle Contraction Class 11

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BiologyCBSE Class 11 · Locomotion and Movement

Skeletal Muscle Structure and the Sliding Filament Theory

Updated 17 October 2026 · Fact-checked

A skeletal muscle fibre contains myofibrils made of sarcomeres, the units between two Z lines, with thin actin and thick myosin filaments. In the sliding filament theory, a nerve signal releases acetylcholine, Ca2+ is released from the sarcoplasmic reticulum, myosin heads bind actin and pull it inward using ATP, shortening the sarcomere: the I band and H zone narrow while the A band stays the same.

Understand Muscle Structure and Sliding Filament Theory

Types of muscle. Skeletal (striated, voluntary, attached to bones), visceral (smooth, involuntary, in walls of hollow organs) and cardiac (striated, involuntary, branched, with intercalated discs).

Structure of a skeletal muscle fibre. Each fibre is a multinucleate cell bounded by the sarcolemma, with sarcoplasm and sarcoplasmic reticulum (store of Ca²⁺). Each fibre contains parallel myofibrils, which show alternate dark A (anisotropic) bands and light I (isotropic) bands. The I band is bisected by a Z line; the A band is bisected by an M line. The portion between two successive Z lines is the sarcomere, the functional unit of contraction. The central part of the A band, with only thick filaments, is the H zone.

Contractile proteins.

  • Actin (thin filaments): two 'F' (filamentous) actin strands twisted helically, each a polymer of 'G' actin monomers; two filaments of tropomyosin run close to F actin; troponin (complex) at regular intervals on tropomyosin. At rest, tropomyosin masks the myosin-binding sites on actin.
  • Myosin (thick filaments): polymer of many monomers, each meromyosin with a globular head (heavy meromyosin, with ATPase and actin-binding sites) and a tail (light meromyosin). The head and short arm project as cross arms.

Mechanism (sliding filament theory).
1. Signal from the motor neuron reaches the neuromuscular junction (motor end plate); acetylcholine is released, producing an action potential in the sarcolemma.
2. The action potential releases Ca²⁺ from the sarcoplasmic reticulum into the sarcoplasm.
3. Ca²⁺ binds troponin, causing a shift in tropomyosin that exposes the myosin-binding sites on actin.
4. Myosin head (with ATP hydrolysed to ADP + Pi) binds actin, forming a cross bridge, then pulls the actin filament toward the centre of the sarcomere (A band), releasing ADP and Pi.
5. A new ATP binds, the cross bridge breaks, and the cycle repeats.
6. Z lines are pulled inward: the sarcomere shortens. The I band and H zone get narrower, A band remains the same (the filaments slide, not shorten).
7. Relaxation: Ca²⁺ is pumped back into the sarcoplasmic reticulum, the troponin-tropomyosin masks the sites again, and the filaments return.

Fatigue and fibre types. Anaerobic breakdown of glycogen produces lactic acid, causing fatigue. Red fibres have a high content of myoglobin (oxygen store), many mitochondria, aerobic, slow, sustained contraction (posture); white fibres have less myoglobin, fewer mitochondria, anaerobic, fast, fatigue quickly.

Table: A band, I band, H zone on contraction: A unchanged; I narrower; H narrower.

Draw and label: a sarcomere with Z line, A band, I band, H zone and M line.

Muscle Structure and Sliding Filament Theory: key facts

Sarcomere
Z line to Z line
Functional unit
On contraction
  • A band unchanged
  • I band shorter
  • H zone shorter
Sliding
Trigger
Acetylcholine -> action potential -> Ca2+ -> troponin -> cross bridge
Sequence
Fibre types
Red: myoglobin, aerobic, slow | White: less myoglobin, anaerobic, fast
Two types

How to answer Muscle Structure and Sliding Filament Theory questions

Questions ask for bands changes or the order of events.

  1. 1List events from nerve signal to relaxation in order.
  2. 2State which bands change and which do not.
  3. 3Name the protein with its role (troponin binds Ca2+; tropomyosin blocks).
  4. 4Mention ATP at two points: cross-bridge cycling and breaking.
  5. 5For fibre types, link myoglobin to aerobic capacity.

Muscle Structure and Sliding Filament Theory in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Sprinter versus marathoner

Suppose a sprinter's leg muscles tire in a minute but a marathoner's keep going for hours.

Relate to fibre types.

Try it first, then show the solutionSolution
  1. Sprinters rely on white fibres, anaerobic, fast, fatigue quickly with lactic acid.
  2. Marathoners rely on red fibres with myoglobin and mitochondria, using oxygen.
  3. So red fibres sustain longer.

Answer: White fibres for bursts; red fibres for endurance.

Memory hook: White quick, red slow and steady.

Tricks to remember Muscle Structure and Sliding Filament Theory

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • A stays, I and H shrink

    Contraction: A stays; I and H narrow.

    Use it when: Bands

  • Ca2+ unlocks

    Calcium moves tropomyosin and exposes sites.

    Use it when: Mechanism

  • Red = Rich in myoglobin

    Red fibres rich in myoglobin.

    Use it when: Fibres

Muscle Structure and Sliding Filament Theory: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Sarcomere bands · 3 marks · Short answerBoard level

State what happens to the A band, I band and H zone when a muscle contracts, and why.

Try it first, then show the solutionSolution
  1. A band length stays the same (1 mark).
  2. I band and H zone become narrower (1 mark).
  3. Actin filaments slide over myosin toward the centre; filaments do not shorten (1 mark).

Answer: A unchanged; I and H narrow; sliding.

Example 2: Role of Ca2+ · 2 marks · Very short answerBoard level

What is the role of calcium ions in muscle contraction?

Try it first, then show the solutionSolution
  1. Ca2+ is released from the sarcoplasmic reticulum and binds troponin (1 mark).
  2. This moves tropomyosin and exposes myosin-binding sites on actin (1 mark).

Answer: Ca2+ unmasks binding sites.

Example 3: Sequence · 3 marks · Short answerHarder board level

Arrange in order: cross bridge formation, action potential, Ca2+ release, ACh release, ATP binding and detachment, shortening of the sarcomere.

Try it first, then show the solutionSolution
  1. ACh release then action potential (1 mark).
  2. Ca2+ release, cross bridge formation (1 mark).
  3. Sarcomere shortening, ATP binding and detachment to repeat (1 mark).

Answer: ACh, action potential, Ca2+, cross bridge, shortening, ATP detachment.

Example 4: Fibre comparison · 4 marks · Short answerHarder board level

Compare red and white muscle fibres on myoglobin, mitochondria, metabolism and speed.

Try it first, then show the solutionSolution
  1. Myoglobin: high versus low (1 mark).
  2. Mitochondria: many versus few (1 mark).
  3. Metabolism: aerobic versus anaerobic (1 mark).
  4. Speed: slow, sustained versus fast, fatigue quickly (1 mark).

Answer: Four differences.

Example 5: Rigor mortis · 5 marks · Long answerHOTS / competency

After death muscles stiffen because ATP is no longer made. Explain using the cross bridge cycle, and why ATP is needed for relaxation as well as contraction.

Try it first, then show the solutionSolution
  1. Myosin heads bind actin to form cross bridges (1 mark).
  2. A new ATP is needed to detach the head from actin (1 mark).
  3. Without ATP, heads stay attached and the filaments are locked (1 mark).
  4. Ca2+ pumping back to the SR also needs ATP (1 mark).
  5. So both contraction cycling and relaxation need ATP (1 mark).

Answer: No ATP: heads cannot detach.

Where students lose marks in Muscle Structure and Sliding Filament Theory

The slips examiners see most often, and what to write instead.

  • Mistake: A band shortens

    Misconception.

    Wrong working

    A band becomes shorter.

    Right working

    A band stays the same.

    Do this: A is thick filaments.

    Usually costs: 1 mark.

  • Mistake: Troponin blocks sites

    Role mixed.

    Wrong working

    Troponin masks the sites.

    Right working

    Tropomyosin masks them; troponin binds Ca2+.

    Do this: Tropomyosin masks.

    Usually costs: 1 mark.

  • Mistake: Ca2+ from outside

    Source mixed.

    Wrong working

    Ca2+ enters from blood.

    Right working

    Ca2+ is released from the sarcoplasmic reticulum.

    Do this: SR store.

    Usually costs: 1 mark.

Exam tips

  • Draw the sarcomere and mark the bands.
  • Write the events in a numbered order.
  • Name proteins with roles.
  • Mention ATP twice.

Practice questions on Muscle Structure and Sliding Filament Theory

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    During contraction of a sarcomere the band whose length does not change is the
    (a) I band (b) H zone (c) A band (d) none of these

    Hint

    Thick filaments.

    Answer

    (c).

  2. Q2 · 1 mark · Assertion-reasonHarder board level

    Assertion (A): The H zone narrows during contraction.
    Reason (R): Thin filaments from both sides move inward over the central region of the thick filaments.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Sliding.

    Answer

    (a).

  3. Q3 · 2 marks · Very short answerBoard level

    Name the protein that blocks the myosin-binding sites at rest and the protein that binds calcium.

    Hint

    Two proteins.

    Answer

    Tropomyosin blocks the binding sites; troponin binds Ca2+.

  4. Q4 · 4 marks · Case-basedHarder board level

    Read and answer.
    Suppose an electron micrograph of a muscle shows an A band of 1.6 um unchanged while the I band shrinks.
    (a) Name the process. (1)
    (b) Which filaments shorten? (1)
    (c) Name the trigger ion. (2)

    Hint

    Sliding filaments.

    Answer

    (a) Contraction by sliding filaments. (b) None; the filaments do not shorten. (c) Ca2+ binding to troponin.

  5. Q5 · 3 marks · Short answerHOTS / competency

    Suppose a toxin blocks acetylcholine receptors at the neuromuscular junction. Predict the effect on Ca2+ release and on muscle contraction, and name a disorder caused by loss of such receptors.

    Hint

    No signal.

    Answer

    Without ACh action there is no action potential, so Ca2+ is not released and the muscle does not contract (paralysis). Myasthenia gravis, an autoimmune disorder affecting the neuromuscular junction, shows similar weakness.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.Functional unit of muscle?Show answer

    Sarcomere.

  2. 2.Store of Ca2+?Show answer

    Sarcoplasmic reticulum.

  3. 3.Which band is dark?Show answer

    A band.

  4. 4.Neurotransmitter at NMJ?Show answer

    Acetylcholine.

  5. 5.Cause of fatigue?Show answer

    Lactic acid.

  6. 6.Myoglobin does what?Show answer

    Stores oxygen.

Muscle Structure and Sliding Filament Theory: frequently asked questions

What is the sliding filament theory?

Contraction by actin sliding over myosin.

What is a motor unit?

A motor neuron with the fibres it supplies.

What are red fibres?

Myoglobin-rich aerobic fibres.

What is the H zone?

Central part of the A band with only myosin.

Why is ATP needed?

For cross-bridge detachment and cycling.