Sarcomere — Structure

NCERT grounding

NCERT Class XI Biology, Chapter 17 (Locomotion and Movement), Section 17.2, defines the sarcomere word-for-word during its account of skeletal-muscle ultrastructure. The chapter first establishes that each myofibril carries alternating dark and light bands due to two proteins — actin in the light band and myosin in the dark band — and then locks the unit of contraction explicitly to the Z-line. NIOS Senior Secondary Biology, Chapter 16 (Locomotion and Movement), Section 16.3.2 supplements this by re-asserting the same Z-to-Z definition and adding that tropomyosin and troponin sit on the thin filament. Together these sources give NEET its non-negotiable anchor for every sarcomere question.

"The portion of the myofibril between two successive 'Z' lines is considered as the functional unit of contraction and is called a sarcomere."

NCERT Class XI Biology · Chapter 17 · Section 17.2

The same passage adds two further locked facts: the central A-band contains thick myosin and is called anisotropic; the I-band contains thin actin, is called isotropic, and is bisected by the Z-line. NCERT also designates the central region of the A-band where thick filaments lie alone — without thin-filament overlap — as the H-zone, with an M-line thin fibrous membrane holding the thick filaments together at the middle. The four words a NEET-bound student must own from this section are therefore Z-line, A-band, I-band, H-zone, plus the M-line as the centre of the A-band. Section 17.2.1 then breaks the filaments themselves into their contractile-protein composition, while Section 17.2.2 handles the mechanism in which these bands move.

Anatomy of one sarcomere

A skeletal muscle fibre is a single multinucleate cell bounded by the sarcolemma. Inside the sarcoplasm of that fibre lie hundreds of parallel rod-shaped myofibrils running the length of the cell. Each myofibril is not a uniform tube — it is a serial repeat of identical contractile blocks called sarcomeres. A sarcomere is defined geometrically: it is the segment of a myofibril lying between two successive Z-lines. Because Z-lines are shared between adjacent sarcomeres, a myofibril is essentially a string of sarcomeres glued end-to-end, and a single mammalian sarcomere at rest is roughly 2.5 micrometres long.

Looked at end-on along its long axis, one sarcomere has a strictly bilaterally symmetrical pattern around its centre. At each end stands a Z-line. Working inward from a Z-line, the sequence encountered is: half I-band → A-band → half I-band → next Z-line. The dark A-band sits in the middle of every sarcomere; the two light half I-bands lie on either side of it. Two adjacent half I-bands from neighbouring sarcomeres meet at a Z-line and visually merge into one full light I-band that is bisected by that Z-line — which is why microscope photographs and NCERT diagrams alike show the I-band as a single light stripe with a thin dark line down its middle.

The reason this banding exists is the spatial distribution of two contractile proteins, actin and myosin, both arranged as rod-like filaments oriented parallel to the long axis of the myofibril. Thick filaments are made of myosin and occupy the A-band. Thin filaments are made of actin (together with tropomyosin and troponin) and occupy the I-band, plus a portion at each lateral edge of the A-band where they overlap with the thick filaments. The thin filaments are firmly anchored to the Z-line; the thick filaments are anchored to one another at the M-line in the centre of the A-band.

Bands, zones and lines decoded

The A-band, I-band, H-zone, M-line and Z-line are not separate physical objects — they are regions of one continuous, repeating filament lattice. Names derive from how each region behaves under a polarising light microscope. The A-band is anisotropic: it rotates polarised light unequally along different axes because of the dense, ordered packing of thick filaments, and so it appears dark. The I-band is isotropic: it rotates polarised light equally in every direction and so appears light. The single letters A and I are deliberate mnemonic hooks for those optical properties. The H-zone derives its name from the German hell (bright), because in early light-microscope work the centre of the A-band looked slightly paler than its edges where actin overlaps were dense; the Z-line is so named from the German Zwischenscheibe, meaning "between-disc."

Why the A-band length never changes

A point students must internalise once and never confuse again: the lengths of the thick and thin filaments themselves are fixed. They neither stretch nor shrink. Because the A-band is defined as "the region occupied by the thick filament," its width is therefore a constant physical length equal to the length of one myosin filament. The I-band and H-zone, by contrast, are defined by the absence of overlap. When actin and myosin slide past each other, the geometry of overlap changes — and so the regions defined by that geometry change in width. The A-band is anchored by physics; the H-zone and I-band are anchored by overlap.

Filament architecture inside the sarcomere

Zooming in once more, the two filament classes that fill the sarcomere are themselves polymerised proteins built from defined sub-units. Each thin filament is made of two F-actin strands wound helically around each other; each F-actin strand is in turn a polymer of monomeric globular G-actin sub-units. Two long ribbons of tropomyosin lie in the grooves of the actin helix, and the regulatory complex troponin is distributed at regular intervals along the tropomyosin. In the resting sarcomere, a sub-unit of troponin masks the active sites on actin where myosin would otherwise bind. The thin filament is firmly attached at one end to the Z-line and extends inward toward the centre of the sarcomere.

Each thick filament is a bundle of meromyosin monomers. A meromyosin monomer has two functional halves: a long fibrous tail called light meromyosin (LMM) and a globular head with a short arm called heavy meromyosin (HMM). The HMM projects from the surface of the thick filament at regular angles as a cross-bridge. Each globular head carries an ATPase active site and a binding site for actin. Within the H-zone, the central portion of the A-band, thick filaments carry no cross-bridges (their tails meet here at the M-line), which is why no actin–myosin overlap can form in that zone even when actin slides inward — actin simply slides past a bare myosin tail.

Resting vs contracting sarcomere

At rest, the geometry of one sarcomere is fixed by the lengths of its filaments and by their degree of overlap. A typical mammalian skeletal-muscle sarcomere at rest is roughly 2.5 µm long. The A-band is about 1.6 µm; on either side, the half I-bands run for ~0.45 µm each; the central H-zone is roughly 0.2 µm wide. Thin filaments project from each Z-line, span the I-band, and overlap the lateral edges of the A-band, leaving the central H-zone as bare myosin. These resting values matter because every NEET question about "what shrinks during contraction" maps directly onto them.

During contraction the thin actin filaments slide inward, deeper into the A-band, toward the M-line. They drag the Z-lines inward with them. As a consequence: the A-band length is unchanged (the thick filament hasn't moved or shortened); the I-band shrinks (less of the actin now lies outside the A-band); the H-zone shrinks (actin now invades the centre of the A-band that was previously bare); and the sarcomere as a whole shortens because the two Z-lines are now closer together. NCERT codifies all four outcomes in Figure 17.5 of Chapter 17, and NEET 2021 Q.190 tests exactly this combination as a multi-statement composite.

Worked examples

Common confusion & NEET traps