BiologyISC Class 11 · Locomotion and Movement

Muscle Structure and the Sliding Filament Theory of Contraction

Updated 17 October 2026 · Fact-checked

Muscles are of three types: skeletal, smooth (visceral) and cardiac. A skeletal muscle fibre contains myofibrils made of sarcomeres with thin actin and thick myosin filaments. In the sliding filament theory, Ca²⁺ exposes the binding sites on actin, myosin heads form cross bridges using ATP and pull the actin filaments toward the centre of the sarcomere, so the sarcomere shortens while the filaments keep their length.

Understand Muscle Structure and Sliding Filament Theory

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

Structure of a skeletal muscle. The muscle is made of fascicles, each with many muscle fibres (syncytial cells) covered by the sarcolemma, with sarcoplasm containing the sarcoplasmic reticulum (storing Ca²⁺) and many myofibrils. A myofibril shows alternate dark (A or anisotropic) and light (I or isotropic) bands. The A band contains thick myosin filaments; the I band has the thin actin filaments and is bisected by the Z line; the M line lies in the middle of the A band, and the H zone is the region of the A band without actin. The sarcomere is the functional unit, from one Z line to the next.

Contractile proteins. *Actin* (thin filament): two F-actin helical strands, with tropomyosin lying along them and troponin at intervals; troponin masks the myosin-binding sites on actin in the resting state. *Myosin* (thick filament): a polymer of meromyosins; the globular head (heavy meromyosin) has ATPase activity and actin-binding sites, and projects as a cross arm.

Sliding filament theory (Huxley and Niedergerke; Hanson and Huxley). (1) A motor neuron releases acetylcholine at the neuromuscular junction; the sarcolemma is depolarised and Ca²⁺ is released from the sarcoplasmic reticulum. (2) Ca²⁺ binds troponin, which moves tropomyosin and exposes the active sites on actin. (3) The myosin head, energised by ATP hydrolysis (ADP + Pi), binds actin forming a cross-bridge. (4) The head tilts, pulling the actin filament towards the M line (the power stroke), releasing ADP and Pi. (5) A new ATP binds the head, breaking the cross-bridge; the cycle repeats. During contraction the A band stays the same, the I band and H zone become narrower, and the sarcomere shortens. On relaxation Ca²⁺ is pumped back into the sarcoplasmic reticulum.

Fatigue and fibre types. Repeated stimulation depletes ATP and accumulates lactic acid from anaerobic breakdown of glycogen, causing fatigue. Red fibres contain myoglobin, many mitochondria and use aerobic respiration (slow, sustained); white fibres have little myoglobin and few mitochondria and rely on anaerobic glycolysis (fast, brief). Rigor mortis is the stiffening after death caused by the lack of ATP, which leaves the cross-bridges attached.

Muscle Structure and Sliding Filament Theory: key facts

Sarcomere bands: A band myosin
I band actin
H zone myosin only
Z line ends of the sarcomere; Between two Z lines.
Thin filament: $F - a c t in + t r o p o m y os in + t r o p o nin$; Troponin binds Ca²⁺.
Contraction: 1.Ca²⁺
2.troponin
3.exposed actin sites
4.cross bridge (ATP)
5.power stroke
6.detach (ATP); The A band stays constant.
Fibre types: Red: myoglobin, mitochondria, aerobic | White: few mitochondria, anaerobic; Slow and sustained versus fast and brief.

A band	myosin
I band	actin
H zone	myosin only
Z line	ends of the sarcomere

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.

A cramp in the calf

Suppose a runner suddenly gets a painful cramp in the calf muscle after a long sprint.

Link it to events in the muscle.

Try it first, then show the solutionSolution

In prolonged exercise the oxygen supply falls and the fibres use anaerobic glycolysis.
Lactic acid accumulates, and ATP runs low, causing fatigue.
Without ATP, myosin heads cannot detach from actin, which can give a stiff, cramped muscle until oxygen and ATP are restored.

Answer: Lactic acid and ATP depletion cause fatigue and cramps.

Memory hook: ATP is needed to detach the heads as well as to power the stroke.

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 constant, I and H shorten
During contraction only the I band and H zone narrow.
Use it when: Sarcomere changes.
Ca²⁺ unlocks, ATP drives
Ca²⁺ exposes the sites; ATP provides energy and detaches heads.
Use it when: Roles in the cross-bridge cycle.

Muscle Structure and Sliding Filament Theory: solved examples

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

Example 1: Calcium and contraction · 1 mark · MCQBoard level

In muscle contraction, Ca²⁺ ions bind to:
(a) actin
(b) myosin
(c) troponin
(d) tropomyosin

Try it first, then show the solutionSolution

Ca²⁺ binds troponin, moving tropomyosin and exposing the actin sites.

Answer: (c) troponin

Example 2: Structure of a sarcomere · 3 marks · Short answerBoard level

Draw (describe) a sarcomere and label the A band, I band, H zone, M line and Z line.

Try it first, then show the solutionSolution

The sarcomere lies between two Z lines. (0.75 mark)
The A band (dark) has thick myosin filaments; the I band (light) has thin actin filaments and is bisected by the Z line. (1.25 marks)
The H zone is the central, lighter region of the A band with only myosin; the M line is in the middle of the H zone. (1 mark)

Answer: A: myosin; I: actin; H: only myosin; Z at the ends; M in the middle.

Example 3: Changes in the sarcomere · 4 marks · Short answerHarder board level

A relaxed sarcomere has an A band of 1.6 µm and an I band (total per sarcomere) of 0.6 µm. On contraction the I band is 0.2 µm. Find the length of the sarcomere before and after contraction and the percentage shortening. What happens to the H zone?

Try it first, then show the solutionSolution

Relaxed: 1.6 + 0.6 = 2.2 µm. (1 mark)
Contracted: 1.6 + 0.2 = 1.8 µm (the A band stays constant). (1 mark)
Shortening = 0.4/2.2 × 100 = 18.2%. (1 mark)
The H zone becomes narrower or disappears as actin filaments slide in. (1 mark)

Answer: 2.2 µm; 1.8 µm; 18.2%; the H zone narrows.

Example 4: Sliding filament mechanism · 4 marks · Short answerHarder board level

Describe the steps of the sliding filament theory beginning with the nerve impulse and including the role of ATP.

Try it first, then show the solutionSolution

A motor neuron releases acetylcholine at the junction, depolarising the sarcolemma and releasing Ca²⁺ from the sarcoplasmic reticulum. (1 mark)
Ca²⁺ binds troponin, moves tropomyosin and exposes the myosin-binding sites on actin. (1 mark)
The myosin head with ADP + Pi (from ATP hydrolysis) attaches to actin, forms a cross-bridge and pulls the actin inward. (1 mark)
A new ATP binds and detaches the head, and the cycle repeats; on relaxation Ca²⁺ is pumped back. (1 mark)

Answer: Impulse, Ca²⁺ release, cross-bridge, power stroke, ATP detachment.

Example 5: Muscle length and fibre types · 5 marks · Long answerHOTS / competency

A muscle of 20 cm contains sarcomeres of 2.2 µm in series. (i) How many sarcomeres lie along one myofibril? (ii) By how much does the muscle shorten if every sarcomere shortens to 1.8 µm? (iii) Distinguish red and white fibres. (iv) Why does rigor mortis occur?

Try it first, then show the solutionSolution

(i) 0.20 m/2.2 × 10⁻⁶ m = 90 909 ≈ 9.1 × 10⁴ sarcomeres. (1.5 marks)
(ii) New length = 90 909 × 1.8 µm = 16.4 cm; shortening = 3.6 cm (18%). (1.5 marks)
(iii) Red fibres: myoglobin and many mitochondria, aerobic and slow-fatiguing; white fibres: little myoglobin, anaerobic and fatigue quickly. (1 mark)
(iv) After death no ATP is made, so the myosin heads stay attached to actin and the muscle stiffens. (1 mark)

Answer: About 9.1 × 10⁴ sarcomeres; 3.6 cm shortening; red aerobic vs white anaerobic; rigor mortis due to lack of ATP.

Where students lose marks in Muscle Structure and Sliding Filament Theory

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

Mistake: Saying filaments shorten
Writing that actin and myosin become shorter.
Do this: The filaments slide; their lengths do not change.
Usually costs: 2 marks.
Mistake: Mixing the A and I bands
Giving actin for the A band.
Do this: A band: myosin (and overlapping actin); I band: actin only.
Usually costs: 1 mark.
Mistake: Missing Ca²⁺ and troponin
Skipping the trigger.
Do this: Mention Ca²⁺ binding troponin and exposure of actin sites.
Usually costs: 1 to 2 marks.

Exam tips

Draw the sarcomere in relaxed and contracted states.
Write the sequence of steps with numbers.
State the role of ATP twice: energy and detachment.
Compare red and white fibres in a table.

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.

Q1 · 1 mark · MCQBoard level
Which of the following muscles shows branching and intercalated discs?
(a) smooth
(b) skeletal
(c) cardiac
(d) visceral
Hint
The heart muscle.
Answer
(c) cardiac
Q2 · 2 marks · Short answerBoard level
What is the neuromuscular junction?
Hint
The meeting of nerve and muscle.
Answer
The junction between a motor neuron and the sarcolemma of a muscle fibre where acetylcholine is released to start contraction.
Q3 · 3 marks · Short answerHarder board level
Differentiate between actin and myosin filaments.
Hint
Thin and thick.
Answer
Actin is the thin filament (F-actin, tropomyosin, troponin) in the I band; myosin is the thick filament in the A band, whose head has ATPase activity and binds actin.
Q4 · 3 marks · Short answerHarder board level
What is the role of the sarcoplasmic reticulum in contraction?
Hint
It stores Ca²⁺.
Answer
It stores Ca²⁺ and releases it into the sarcoplasm on stimulation, triggering contraction; it takes the Ca²⁺ back during relaxation.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): In a contracting sarcomere the length of the A band remains constant. Reason (R): The thick myosin filaments do not shorten; the actin filaments slide over them. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Think of the sliding filament theory.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a patient shows progressive muscle weakness because antibodies block receptors at the neuromuscular junction.
(i) Name the disorder. (1 mark)
(ii) Name the neurotransmitter involved. (1 mark)
(iii) Name one other muscular disorder that is genetic. (1 mark)
(iv) Name the disorder from low Ca²⁺. (1 mark)
Hint
Autoimmune muscle disorder.
Answer
(i) Myasthenia gravis. (ii) Acetylcholine. (iii) Muscular dystrophy. (iv) Tetany.

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.Contractile proteins?Show answer
Actin and myosin.
2.Unit of contraction?Show answer
Sarcomere.
3.Source of Ca²⁺?Show answer
Sarcoplasmic reticulum.
4.Fatigue causes?Show answer
Lactic acid and ATP depletion.

Muscle Structure and Sliding Filament Theory: frequently asked questions

What is the difference between the I band and the A band?

The I band is light and has only actin; the A band is dark and contains myosin.

Why do muscles need mitochondria?

For ATP, which is needed for contraction and relaxation.

What is tetanus in muscle physiology?

A sustained contraction from the summation of repeated stimuli, unrelated to the infectious disease.