Isotropic bands
In
Cellular anatomy
The darker bands within skeletal muscle, known as anisotropic bands (A bands), encompass both thick and thin filaments and constitute the central region of the sarcomere, extending across the H-zone. Collectively, the A bands and the I bands create the distinctive striped appearance of skeletal muscle tissue. [3] Tropomyosin, a protein, shields the myosin-binding sites, hindering actin from binding to myosin. It attaches to troponin, which secures it in place. During muscle relaxation, the troponin-tropomyosin complex inhibits myosin heads from binding to the active sites on actin microfilaments. Troponin also possesses a calcium ion binding site. These two regulatory proteins cooperate in response to calcium levels, overseeing sarcomere contraction. During muscle contraction, tropomyosin shifts to expose the myosin-binding site on an actin filament, allowing the interaction between actin and myosin microfilaments to occur. The initiation of contraction involves calcium ions binding to troponin, prompting a reaction that displaces tropomyosin from the actin filament binding sites. Consequently, myosin heads can attach to these exposed sites, forming cross-bridges and initiating muscle contraction.[4]
References
- ^ Goldblum, John (2018). Rosai and Ackerman's Surgical Pathology (11 ed.). Elsevier. pp. 1810–1914.
- ^ Z disk - definition - glossary (2015) at PhysiologyWeb
- ^ Standring, S. (2016). Gray's Anatomy (41st ed.). Edinburgh: Elsevier Churchill Livingstone
- ^ Menefee, W.; Jenks, J.; Mazzasette, C.; Nguyen, K.-L. (2021). "Muscle fiber contraction and relaxation" – via libretexts.org.