I-band
Summary
The I-band is the light region of a striated muscle sarcomere that contains only thin filaments (actin) and is bisected by the Z-line. During contraction the I-band gets shorter, while the A-band stays the same length. That contrast is the most testable point.
Detail
Meaning: The I-band is the lighter-staining stretch of a sarcomere where thin filaments (actin with troponin and tropomyosin) are present and thick myosin filaments are absent. The Z-line runs through its middle, so each I-band is shared by two neighboring sarcomeres. In the sliding filament model, myosin heads pull thin filaments toward the sarcomere center. More actin then overlaps with myosin, and the I-band narrows. The name comes from "isotropic," describing how it looks under polarized light.
On the MCAT: A passage shows a diagram or micrograph of a sarcomere at rest and during contraction, then asks which regions change. When a muscle contracts, the I-band, the H-zone and the whole sarcomere shorten. The A-band does not change. When the muscle is stretched or relaxes, the I-band lengthens. Questions may also ask you to locate the I-band on a labeled figure or to say which band contains only thin filaments.
Common confusion: Students often say the A-band shrinks because "the filaments move." The A-band spans the length of the thick filaments, and that length stays constant. It includes the zone where thin and thick filaments overlap. Neither set of filaments shortens. The I-band shortens because the filaments overlap more. Do not mix up the I-band with the H-zone. The H-zone lies inside the A-band and has only thick filaments.
Sources
- Biology (Campbell)
- Human Physiology: An Integrated Approach (Silverthorn)
- Principles of Anatomy and Physiology (Tortora)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.