Muscle: structure and mechanism of contraction

Muscle

In plain words: a skeletal muscle is a bundle of bundles. Its long cells contain myofibrils made of repeating sarcomeres, where myosin pulls on actin to produce force. When the muscle shortens, these units shorten together. Smooth muscle shows no striations (NCERT page 218); the detailed mechanism below is for skeletal muscle.

Skeletal, visceral and cardiac muscles

• Muscles contribute about 40–50 per cent of the body weight of an adult human. They have the special properties of excitability, contractility, extensibility and elasticity (NCERT page 218).
• They are classified by location, appearance and nature of regulation. By location there are three types (page 218; see muscle tissue):
  1. Skeletal muscles: closely associated with the skeletal components; striped under the microscope, so striated; under voluntary control of the nervous system, so voluntary. They are mainly for locomotion and changes of body posture (page 218).
  2. Visceral muscles: in the inner walls of hollow visceral organs such as the alimentary canal and reproductive tract; no striations, so smooth (non-striated); not under voluntary control, so involuntary. They move food through the digestive tract and gametes through the genital tract (page 218).
  3. Cardiac muscles: the muscles of the heart; many cells assemble in a branching pattern; striated but involuntary. Cardiac pacemaker cells initiate the rhythm; autonomic nerves modify rate and force rather than voluntarily triggering each beat (NCERT page 219; Chapter 15, regulation of cardiac activity).

Structure of a skeletal muscle fibre

• Each skeletal muscle is made of muscle bundles (fascicles) held together by a collagenous connective tissue layer, the fascia. Each bundle contains many muscle fibres (NCERT page 219).
• Each muscle fibre is lined by the plasma membrane, the sarcolemma, enclosing the sarcoplasm. A muscle fibre is a syncytium, because its sarcoplasm contains many nuclei (page 219; NCERT prints “syncitium”).
• The endoplasmic reticulum of the fibre, the sarcoplasmic reticulum, is the store house of calcium ions (page 219).
• The sarcoplasm has a large number of parallel threads, the myofibrils, each with alternate dark and light bands (page 219).
• NCERT’s Summary calls the muscle fibre the anatomical unit of muscle; the sarcomere is the functional unit (page 228).
• NCERT calls these threads “myofilaments or myofibrils” (page 219), but the two are not the same thing. Beyond the NCERT text, each myofibril is made of many thin and thick myofilaments, of actin and myosin, which repeat as sarcomeres along its length (Zhao and colleagues, “Architecture and molecular machinery of skeletal myofibers”, Frontiers in Cell and Developmental Biology, 2025). NCERT’s own Summary keeps them apart: each muscle fibre has many myofibrils, and the thin and thick filaments make up the bands of each sarcomere (page 228).

Sarcomere

• The striations come from the arrangement of two proteins, actin and myosin (NCERT page 219):
  • the light band, the I band (isotropic), contains actin;
  • the dark band, the A band (anisotropic), contains myosin.
• Actin filaments are thinner than myosin filaments, so they are called thin and thick filaments. Both run parallel to each other and to the long axis of the myofibril (page 220).
• A Z line bisects each I band, and the thin filaments are firmly attached to it. An M line holds the thick filaments together in the middle of the A band (page 220).
• The part of a myofibril between two successive Z lines is the sarcomere, the functional unit of contraction (page 220).
• At rest, the edges of the thin filaments partly overlap the free ends of the thick filaments. The central part of the thick filaments, not overlapped, is the H zone (page 220).
• So the A band, the whole length of the thick filaments, also holds thin filaments near each end, where the two overlap. Only the H zone has thick filaments alone, and only the I band has thin filaments alone (NCERT page 220, Figure 17.2). Beyond the NCERT text, a 2025 review puts it the same way: “the myosins are assembled into thick myofilaments in the A-band that overlap with thin myofilaments” (Zhao and colleagues, Frontiers in Cell and Developmental Biology, 2025, CC BY 4.0).

NCERT Exercise 1 asks you to draw a sarcomere and show its regions. Draw the relaxed panel above and label a Z line at each end, a half I band (thin filaments only) beside each Z line, the A band (the whole length of the thick filaments) in the middle, the H zone at the centre of the A band, and the M line through the H zone (NCERT page 220, Figure 17.2).

Structure of contractile proteins

NCERT Section 17.2.1 (page 221):

• Actin (thin) filament: two F (filamentous) actins wound helically round each other; each F actin is a polymer of G (globular) actins.
  • Two filaments of tropomyosin run close to the F actins along their length.
  • A complex protein, troponin, sits at regular intervals on the tropomyosin; at rest, NCERT says a subunit of troponin masks the active binding sites for myosin on actin. Beyond the NCERT text (Wikipedia, “Muscle contraction”, facts in our words): tropomyosin itself covers part of each binding site, and when Ca^²⁺ binds troponin C, the troponin–calcium complex moves the tropomyosin aside.
• Myosin (thick) filament: an assembly of myosin molecules. NCERT calls the monomers meromyosins and describes each using these regions:
  • a globular head with a short arm, the heavy meromyosin (HMM);
  • a tail, the light meromyosin (LMM).
• The HMM parts project outwards at regular distances and angles from the thick filament as cross arms. The globular head is an ATPase enzyme, with binding sites for ATP and active sites for actin.

Beyond the NCERT text: a myosin II molecule has two heads and a long tail in which the two heavy chains coil round each other, and in muscle the tails of many such molecules join to form the thick filament (Wikipedia, “Myosin”, facts in our words). HMM and LMM are the two parts obtained when myosin is cut with the enzyme trypsin (Wikipedia, “Meromyosin”, facts in our words), so they are not separate units from which a thick filament is built (our reasoning). Use NCERT’s labelled regions for its diagram questions.

Mechanism of muscle contraction

NCERT Section 17.2.2 (pages 222–223):

1. A signal from the central nervous system arrives by a motor neuron. A motor neuron with the muscle fibres it connects to is a motor unit; its junction with the sarcolemma is the neuromuscular junction, or motor-end plate.
2. The signal releases a neurotransmitter, acetylcholine, which generates an action potential in the sarcolemma. This spreads through the fibre and releases calcium ions into the sarcoplasm.
3. Ca^²⁺ binds a subunit of troponin, removing the mask from the active sites for myosin on actin.
4. Using energy from ATP hydrolysis, the myosin head binds the exposed sites on actin, forming a cross bridge.
5. This pulls the actin filaments towards the centre of the A band. The Z lines attached to them are pulled inwards too, so the sarcomere shortens: contraction. During contraction, the I bands shorten while the A bands keep their length.
6. Releasing ADP and P_ᵢ, the myosin returns to its relaxed state; a new ATP binds and the cross bridge breaks.
7. The ATP is hydrolysed again and the cycle repeats, causing further sliding, until the Ca^²⁺ is pumped back to the sarcoplasmic cisternae. Actin is masked again, the Z lines return, and the muscle relaxes.

NCERT slip (page 222): NCERT prints “ADP and P_₁”, with a subscript 1; it means P_ᵢ, inorganic phosphate, the partner of ADP when ATP is split (compare NCERT Chapter 12, page 160: ATP is made “from ADP and inorganic phosphate”). Write ADP and P_ᵢ in an answer.

Beyond the NCERT text (Wikipedia, “Muscle contraction”, facts in our words): in the modern account of the cycle, the head sheds its phosphate as the power stroke begins and its ADP when the stroke is over, then stays locked to actin (the rigor state) until a fresh ATP frees it; relaxation follows as a calcium pump of the sarcoplasmic reticulum (SERCA) takes Ca^²⁺ back. Force can also be produced with no change in length, an isometric contraction, for example when a heavy bag is held steady; NCERT’s sliding diagrams show the shortening case.

Sliding filament theory

NCERT states the theory (page 221):

Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.

NCERT Biology (Class 11), Section 17.2.2, page 221

• During shortening the thin filaments slide inwards, so the I bands and the H zone become shorter, while the A band keeps its length, and the thin and thick filaments keep theirs (NCERT pages 222–223, Figure 17.5).

Red and white muscle fibres

• The reaction time of fibres varies between muscles. NCERT links repeated activation, anaerobic glycogen breakdown and lactic-acid accumulation with fatigue (page 223; see lactic acid fermentation). Beyond the NCERT text: muscle fatigue was once blamed on a build-up of lactic acid, and whether lactic acid worsens fatigue or eases it is not settled; a build-up of other metabolites can bring on fatigue within the muscle fibres by cutting the release of calcium from the sarcoplasmic reticulum or by making actin and myosin less sensitive to calcium (Wikipedia, “Muscle fatigue”, facts in our words). In an NCERT-based answer, give NCERT’s lactic-acid account.
• Muscle contains a red, oxygen-storing pigment, myoglobin (page 223).
• Red fibres: high myoglobin, so reddish; plenty of mitochondria that use the stored oxygen for ATP; they can be called aerobic muscles (page 223).
• White fibres: very little myoglobin, so pale; few mitochondria, but a lot of sarcoplasmic reticulum; they depend on anaerobic processes for energy (page 223). Red and white differ in which pathway they lean on more; neither fibre type uses only one energy pathway, since white fibres still have a few mitochondria and NCERT puts anaerobic glycogen breakdown in muscles generally (reasoned from NCERT page 223).

Solved examples

Common mistakes to avoid

Quick revision summary

• Muscle is mesodermal; it is 40–50 per cent of adult body weight. Skeletal muscle is striated and voluntary, visceral muscle smooth and involuntary, cardiac muscle striated, branched and involuntary.
• Muscle → fascicles (in fascia) → muscle fibres (sarcolemma, many nuclei, sarcoplasmic reticulum storing Ca^²⁺) → myofibrils of alternating I and A bands.
• Sarcomere (Z line to Z line) is the functional unit: thin actin filaments from the Z lines, thick myosin filaments of the A band held at the M line; the H zone is the unoverlapped middle of the thick filaments.
• Thin filament: two F actins (of G actin), tropomyosin, troponin. Thick filament: myosin molecules with ATPase heads; NCERT describes head/short arm (HMM) and tail (LMM). Beyond the NCERT text: HMM and LMM are the parts obtained when myosin is cut with trypsin (Wikipedia, “Meromyosin”).
• Shortening contraction: nerve signal → acetylcholine → Ca^²⁺ → troponin → cross bridges → thin filaments slide in; I bands and H zone shorten, A band unchanged; Ca^²⁺ pumped back → relaxation.
• Red fibres: myoglobin, many mitochondria, aerobic. White fibres: little myoglobin, much sarcoplasmic reticulum, anaerobic.

Questions students ask

Why is calcium needed for muscle contraction?

Calcium is needed for muscle contraction because it is the switch that lets myosin reach actin: in a resting fibre a subunit of troponin masks the active binding sites for myosin on the actin filaments, and Ca^²⁺ binding to that subunit removes the mask, so that myosin heads can form cross bridges (NCERT pages 221–222). The Ca^²⁺ comes from the fibre’s own store, the sarcoplasmic reticulum, released into the sarcoplasm as the action potential spreads (pages 219 and 222). When it is pumped back into the sarcoplasmic cisternae, actin is masked again and the muscle relaxes (page 223). Beyond the NCERT text, the calcium-binding part of the troponin complex is troponin C (MeSH, “Troponin C”).

Why do the I band and H zone shorten during contraction but the A band does not?

The A band keeps its length because it spans the whole length of the thick myosin filaments, which do not change, while the I band and the H zone are the parts where thin and thick filaments do not overlap, and these shrink as the thin filaments slide further in (NCERT pages 220–223). The I band, bisected by a Z line, holds only thin filaments, and the H zone is the middle of the A band that the thin filaments have not reached (page 220). As cross bridges pull the thin filaments towards the centre of the A band, the Z lines come closer: in NCERT’s words the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length, and its Figure 17.5 shows the H zone narrowing too (pages 222–223).

What is the difference between troponin and tropomyosin?

Troponin and tropomyosin are both thin-filament proteins: tropomyosin runs as two filaments close to the F actins along their length, while troponin is a complex protein placed at regular intervals on the tropomyosin (NCERT page 221). At rest, in NCERT’s account, a troponin subunit masks the myosin-binding sites on actin (page 221). Beyond the NCERT text, troponin is a complex of three proteins, troponin C, I and T, and heart muscle has its own forms of troponin I and T (MeSH, “Troponin”, “Troponin I”, “Troponin T”). These leak into the blood from damaged heart muscle, so a troponin test helps assess heart injury, though other conditions can also raise it (MedlinePlus, Troponin Test).

What is the difference between a sarcomere, a myofibril and the sarcolemma?

A sarcomere, a myofibril and the sarcolemma are three different parts of a skeletal muscle fibre: the sarcolemma is the plasma membrane lining each fibre, a myofibril is one of the many parallel threads in the fibre’s sarcoplasm, and a sarcomere is one repeating stretch of a myofibril between two successive Z lines (NCERT pages 219–220). So one muscle fibre, the anatomical unit of muscle, holds many myofibrils, and each myofibril is a chain of sarcomeres, the functional units of contraction (NCERT Summary, page 228). The sarcolemma is also where contraction starts, since acetylcholine released at the neuromuscular junction generates an action potential in it (page 222).

Who proposed the sliding filament theory of muscle contraction?

The sliding filament theory was proposed in 1954 by two pairs of researchers working separately, Andrew Huxley with Rolf Niedergerke and Hugh Huxley with Jean Hanson, whose papers appeared back to back in Nature (beyond the NCERT text, which states the theory without naming its authors; Huxley and Niedergerke, Nature, 1954; Huxley and Hanson, Nature, 1954). Both teams found that the A band keeps its length while a muscle shortens or is stretched, and the I band changes instead, which they explained by the filaments sliding past each other. NCERT states the theory as contraction of a muscle fibre by the sliding of the thin filaments over the thick filaments (page 221).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What is a motor unit?

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   A motor neuron together with the muscle fibres connected to it (NCERT page 222).

2. Q2. In NCERT’s wording, what masks the active sites for myosin on actin at rest?

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   A subunit of troponin (NCERT page 221). Beyond the NCERT text, tropomyosin covers part of each binding site and the troponin complex moves it aside when Ca^²⁺ binds, as explained above (Wikipedia, “Muscle contraction”).

3. Q3. How does NCERT explain muscle fatigue, and what qualification is needed?

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   Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue (NCERT page 223); give this in an NCERT-based answer. Beyond the NCERT text: fatigue was once blamed on lactic acid, and whether lactic acid worsens fatigue or eases it is not settled; a build-up of other metabolites can bring on fatigue by reducing calcium release or the response of actin and myosin to calcium (Wikipedia, “Muscle fatigue”, facts in our words; see Red and white muscle fibres above).