# Isotropic bands

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{{Short description|Lighter bands of skeletal muscle}}
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In [physiology](/source/physiology), '''isotropic bands''' (better known as [I bands](/source/I_bands)) are the lighter bands of skeletal muscle cells (a.k.a. [muscle fibers](/source/muscle_fibers)). Isotropic bands contain only [actin](/source/actin)-containing [thin filaments](/source/thin_filaments).<ref>{{cite book | vauthors = Goldblum JR | chapter = Section 9: Bone and Soft Tissue Pathology, Chapter 41: Soft Tissues | veditors = McKenney JK, Goldblum JR, Lamps LW, Myers JL |title=Rosai and Ackerman's Surgical Pathology |date=2018 |publisher=Elsevier |pages=1810–1914 |edition=11}}</ref> The thin filaments are placed between 2 myosin filaments and contain only the [actin filament](/source/actin_filament)s of neighboring [sarcomere](/source/sarcomere)s. 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.<ref>{{cite web | url =https://www.physiologyweb.com/glossary/z/z_disk.html | title =Z disk - definition - glossary | date =2015 | work = PhysiologyWeb }}</ref>

[[File:ಸಾರ್ಕೊಮೆರ್.png|thumb|The muscle is made up of several [myofibril](/source/myofibril)s packed into functional units surrounded by different layers of connective tissues ([epimysium](/source/epimysium), [perimysium](/source/perimysium), and [endomysium](/source/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 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.<ref>{{cite book | vauthors = Standring S | date = 2016 | title = Gray's Anatomy | edition = 41st | location = Edinburgh | publisher= Elsevier Churchill Livingstone }}</ref> Tropomyosin, a protein, shields the myosin-binding sites, hindering actin from binding to myosin. It attaches to [troponin](/source/troponin), which secures it in place. During muscle relaxation, the troponin-tropomyosin complex inhibits [myosin](/source/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](/source/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.<ref>{{cite web |url= https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Human_Anatomy_(OERI)/09%3A_Skeletal_Muscle_Tissue/9.04%3A_Muscle_Fiber_Contraction_and_Relaxation| vauthors = Menefee W, Jenks J, Mazzasette C, Nguyen KL |year=2021|title=Muscle fiber contraction and relaxation | work = LibreTexts }}</ref>

== References ==
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Category:Wikipedia Student Program
Category:Muscular system
Category:Medicine

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