Isotropic bands

In physiology, isotropic bands (better known as I bands) are the lighter bands of skeletal muscle cells (a.k.a. muscle fibers). Isotropic bands contain only actin-containing thin filaments. The thin filaments are placed between 2 myosin filaments and contain only the actin filaments of neighboring sarcomeres. Bisecting the I band and serving as an anchoring point for the two adjacent actin filaments is the Z disc. During muscle contraction, the I band will shorten, while an A band will maintain its width.

The muscle is made up of several myofibrils packed into functional units surrounded by different layers of connective tissues (epimysium, perimysium, and endomysium). The main contractile unit is mainly composed of protein filaments (myofilaments), namely myosin (thick filaments) and actin (thin filaments).

Cellular anatomy

The darker bands are called anisotropic bands (A bands). A bands contain both thick and thin filaments and are the center of the sarcomere that spans the H-zone. Together the I-bands and A-bands contribute to the striated appearance of skeletal muscle.

Tropomyosin is a protein that covers myosin-binding sites to prevent actin from binding to myosin. Tropomyosin binds to troponin, which anchors the tropomyosin in place. In a relaxed muscle, the troponin-tropomyosin complex prevents the myosin heads from binding to the active sites on the actin microfilaments. Troponin also has a binding site for Calcium ions. These two regulatory proteins work together to respond to calcium and regulate sarcomere contraction. In muscle contraction, the position of tropomyosin is shifted to expose the myosin-binding site on an actin filament to allow the actin and myosin microfilaments to cross. The first step in the process of contraction is for Ca+ to bind to troponin causing an interaction that slides tropomyosin away from the binding sites on actin filaments. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges.

Further reading

• Menefee, W.; Jenks, J.; Mazzasette, C.; Nguyen, K.-L. (2021) Muscle fiber contraction and relaxation
• Standring, S. (2016). Gray's Anatomy (41st ed.). Edinburgh: Elsevier Churchill Livingstone
• Z disk - definition - glossary (2015) at PhysiologyWeb