A-band

Region of a sarcomere in muscle tissue

Anatomy

The A-band is a region within the sarcomere of striated muscle fibers. It is characterized by its dark appearance under a microscope and is primarily composed of thick myosin filaments. The A-band plays a crucial role in the muscle contraction process.

Structure

The A-band is one of the key structural components of the sarcomere, the basic unit of a muscle's cross-striated myofibril. It spans the entire length of the thick myosin filaments and includes regions of overlap with thin actin filaments. The A-band remains constant in length during muscle contraction, distinguishing it from the I-band, which shortens.

Function

The primary function of the A-band is to facilitate muscle contraction through the sliding filament mechanism. During contraction, the thin actin filaments slide past the thick myosin filaments, causing the sarcomere to shorten while the A-band itself remains unchanged in length. This interaction is essential for the generation of force and movement in muscle tissues.

Components

The A-band contains several important sub-regions:

• The H-zone: A lighter region in the center of the A-band where there is no overlap between actin and myosin filaments.
• The M-line: The central part of the H-zone, which holds the thick filaments together.

Related Structures

• I-band: The region of the sarcomere that contains only thin actin filaments and shortens during muscle contraction.
• Z-line: The boundary of the sarcomere, anchoring the thin filaments.
• Sarcomere: The repeating unit of a myofibril in striated muscle.

See also

• Muscle contraction
• Myofibril
• Sliding filament theory
• Striated muscle