Sarcomere

Sarcomere is the basic unit of muscle tissue. It is the smallest functional unit of a myofibril, which is a long, cylindrical organelle found in muscle cells. Sarcomeres are responsible for the contraction of muscles, and their structure and function are essential to understanding the mechanics of muscle movement.

Structure

A sarcomere is composed of long, fibrous proteins that slide past each other when the muscles contract and relax. Two types of protein filaments make up the structure of the sarcomere: actin and myosin. Actin filaments are thinner and occur at the ends of the sarcomere, while myosin filaments are thicker and located in the center of the sarcomere.

The sarcomere is divided into several zones and bands. The A band is the region of the sarcomere where the actin and myosin filaments overlap. The I band is the region containing only actin filaments. The H zone is the region containing only myosin filaments. The Z line or Z disc forms the borders of the sarcomere, and the M line runs down the center of the sarcomere, anchoring the myosin filaments.

Function

The primary function of the sarcomere is muscle contraction. This occurs through a process known as the sliding filament theory. According to this theory, when a muscle contracts, the Z lines move closer together and the I band and H zone become smaller, while the A band remains the same size. This is due to the actin filaments sliding over the myosin filaments, causing the sarcomere to shorten and the muscle to contract.

Clinical significance

Abnormalities in the structure or function of the sarcomere can lead to various muscular disorders. For example, mutations in the genes encoding sarcomeric proteins can cause cardiomyopathy, a disease of the heart muscle. Other conditions, such as muscular dystrophy, can also affect the sarcomere.

See also

• Muscle contraction
• Myofibril
• Actin
• Myosin

References

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  Molecular model of the Sarcomere in the A-band

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  Cardiac sarcomere structure