Sarcomere Structure

The sarcomere is the basic functional unit of muscle contraction. It is located between two Z-lines and consists of thin and thick myofilaments. The arrangement of these filaments within the sarcomere is responsible for the characteristic striated appearance of skeletal muscle fiber. In a Sarcomere, each thin myofilament is surrounded by 3 thick myofilaments and each thick myofilament is surrounded by 6 thin myofilaments. Sarcomere is divided into certain bands and zones based on the presence or absence of thick or thin filaments:

• A-band: A-Band is also known as an anisotropic or dark band because light does not pass through it. It is the region where the thick filament is present. 
• H-Zone: H-Zone is also known as Henson’s Zone. It is the non-overlapping region between the A-Band. It is located in the middle of the sarcomere where only thick filament is present. It can also be defined as the central gap between actin filaments extending through myosin filaments in A-Band.
• O-Zone: O-Zone is the region where both actin and myosin are present. It is also present within A-Band.
• I-Band: I-Band is also known as an isotropic band or light band. It is the region where only thin filament is present. It is part of two sarcomeres. It is divided into two halves by the Z- line.

In a Sarcomere, each thin myofilament is surrounded by 3 thick myofilaments and each thick myofilament is surrounded by 6 thin myofilaments.

The mechanism of muscle contraction refers to the process by which muscles generate force and produce movement. Muscle contraction involves a complex interplay of biochemical and physiological events within the muscle fibers. When a nerve signal reaches a muscle, it triggers the release of calcium ions, which allows actin and myosin proteins to interact and causes these filaments to slide past each other. This sliding filament mechanism shortens the muscle fibers and results in contraction and the generation of force.