Sarcomeres are contractile units of skeletal muscle that divide into “I” and “A” bands, “M” and “Z” lines, and the “H” zone.

The sarcomere is the basic contractile unit of skeletal muscle. It is made of thick and thin filaments. Thick filaments are organized bundles of myosin, while thin filaments are made of actin along with the two other regulatory proteins (troponin and tropomyosin).

Each sarcomere divides into different lines, bands, and zone: “I” and “A” bands, “M” and “Z” lines, and the “H” zone.

– Z-lines define the boundaries of each sarcomere.
– The M-line runs down the center of the sarcomere, through the middle of the myosin filaments.
– The I-band is the region containing only thin filaments.
– The H-zone contains only thick filaments.
– The A-band contains both thick and thin filaments and is the center of the sarcomere that spans the H zone.

During contraction, the H-zone, I-band, the distance between Z-lines, and the distance between M-lines all become smaller. However, the A band’s size remains constant during contraction.

The Sarcomere. A sarcomere is a contractile unit of skeletal muscle that is divided into I and A bands, M and Z lines, and H zone.

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Key Points

• A sarcomere is the basic contractile unit of skeletal muscle that is made of thick and thin filaments.

• Thick filaments are organized bundles of myosin, while thin filaments are made of actin along with the two other regulatory proteins-troponin and tropomyosin.

• Z-lines define the boundaries of each sarcomere.

• The M-line runs down the center of the sarcomere, through the middle of the myosin filaments.

• The I-band is the region containing only thin filaments.

• The H-zone contains only thick filaments.

• The A-band contains both thick and thin filaments and is the center of the sarcomere that spans the H zone.

• When the muscle contracts, the H-zone, I-band, the distance between Z-lines all become smaller, whereas the A band remains the same.

Key Terms

Sarcomere: The functional unit of the skeletal muscle.

Thin filament/actin: A protein that forms (together with myosin) the contractile filaments of muscle cells, and is also involved in motion in other types of cells.

Thick filament/myosin: A fibrous protein that forms (together with actin) the contractile filaments of muscle cells and is also involved in motion in other types of cells.

Troponin: A complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle, or any member of this complex.

Tropomyosin: A protein involved in skeletal muscle contraction and that wraps around actin and prevents myosin from grabbing it.

I-band: The area adjacent to the Z-line, where actin is not superimposed by myosin.

A-band: The length of a myosin within a sarcomere.

M-line: The line at the center of a sarcomere to which myosin bind.

Z-line: Neighboring, parallel lines that define a sarcomere.

H-band: the area adjacent to the M-line, where myosin is not superimposed by actin.