In thehistologyofskeletal muscle,atriadis the structure formed by aT tubulewith asarcoplasmic reticulum(SR) known as theterminal cisternaon either side.[1]Eachskeletal muscle fiberhas many thousands of triads, visible in muscle fibers that have been sectioned longitudinally. (This property holds because T tubules run perpendicular to the longitudinal axis of the muscle fiber.) In mammals, triads are typically located at theA-I junction;[1]that is, the junction between theAandI bandsof thesarcomere,which is the smallest unit of a muscle fiber.
| Triad | |
|---|---|
 Skeletal muscle,showing Triad as well as T-tubule. | |
 Triad and T-tubule structure and relationship to thesarcoplasmic reticulumin skeletal muscle. | |
| Anatomical terminology |
Triads form the anatomical basis ofexcitation-contraction coupling,whereby a stimulus excites the muscle and causes it to contract. A stimulus, in the form of positively charged current, is transmitted from theneuromuscular junctiondown the length of theT tubules,activatingdihydropyridine receptors(DHPRs). Their activation causes 1) a negligible influx ofcalciumand 2) a mechanical interaction with calcium-conductingryanodine receptors(RyRs) on the adjacent SR membrane. Activation of RyRs causes the release of calcium from the SR, which subsequently initiates a cascade of events leading tomuscle contraction.These muscle contractions are caused by calcium's bonding to troponin and unmasking the binding sites covered by the troponin-tropomyosin complex on the actin myofilament and allowing the myosin cross-bridges to connect with the actin.
See also
edit- Diad,a homologous structure incardiac muscle
References
edit- ^abdi Fiore, Mariano SH; Eroschenko, Victor P (2008).Di Fiore's Atlas of histology: with functional correlations.Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. p. 124.ISBN978-0-7817-7057-6.