5.4: The Sarcomere — Functional Unit of Muscle

The sarcomere is the functional unit of skeletal muscle, where actin and myosin filaments interact to produce contraction. 

Master this section and you'll be able to

• Identify areas of the skeletal muscle fibers
• Describe excitation-contraction coupling

The Sarcomere

As you have learned already, skeletal muscle fibers look striped (striated) under the microscope. This striped pattern comes from the precise arrangement of two types of protein filaments: actin (thin filaments) and myosin (thick filaments). These filaments, along with regulatory proteins like troponin and tropomyosin, are organized into repeating units called sarcomeres.

Functional Unit of Muscle

• The sarcomere is the functional unit of a muscle fiber.

• Sarcomeres are linked end to end along the length of structures called myofibrils. Myofibrils are rod-like organelles inside the long, tubular muscle cells, also known as myofibers.

• Each myofibril runs the entire length of the muscle cell and is anchored to the cell membrane (sarcolemma) at its ends.

• When the sarcomeres in a myofibril contract, the entire myofibril shortens — causing the whole muscle fiber to contract.

Size and Arrangement

• Myofibrils are very thin, only about 1.2 micrometers in diameter, but each fiber contains hundreds to thousands of myofibrils, and each myofibril contains thousands of sarcomeres.

• A typical sarcomere is about 2 micrometers long and has a cylindrical, three-dimensional arrangement.

• Each sarcomere is bordered by Z-discs (or Z-lines), which anchor the thin filaments (actin).

Figure \(\PageIndex{1}\): Sarcomere Structure. Cross-section of a muscle fiber showing sarcomere, sarcolemma, sarcoplasm, myofibrils, and striations.

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Thin vs. Thick Filaments

• Thin filaments: made mostly of actin, with troponin and tropomyosin as regulatory proteins. These filaments extend from the Z-discs toward the center of the sarcomere.

• Thick filaments: made of myosin, with many projecting “heads.” These extend from the center of the sarcomere outward, but they do not reach all the way to the Z-discs.

• The overlap of thin and thick filaments gives muscle its striated appearance and is key to contraction.

Figure \(\PageIndex{2}\): The sarcomere. The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber.

 

The Sarcomere

The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other. Each packet of these microfilaments and their regulatory proteins, troponin and tropomyosin (along with other proteins) is called a sarcomere.

The sarcomere is the functional unit of the muscle fiber. The sarcomere itself is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end. As myofibrils contract, the entire muscle cell contracts. Because myofibrils are only approximately 1.2 μm in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle fiber. Each sarcomere is approximately 2 μm in length with a three-dimensional cylinder-like arrangement and is bordered by structures called Z-discs (also called Z-lines, because pictures are two-dimensional), to which the actin myofilaments are anchored. Because the actin and its troponin-tropomyosin complex (projecting from the Z-discs toward the center of the sarcomere) form strands that are thinner than the myosin, it is called the thin filament of the sarcomere. Likewise, because the myosin strands and their multiple heads (projecting from the center of the sarcomere, toward but not all to way to, the Z-discs) have more mass and are thicker, they are called the thick filament of the sarcomere.