{{short description|Structure of muscle tissue}} {{Infobox anatomy | Name = Triad | Image = Blausen_0801_SkeletalMuscle.png | Caption = Skeletal muscle, showing Triad as well as T-tubule. | Image2 = 1023 T-tubule.jpg | Caption2 = Triad and T-tubule structure and relationship to the sarcoplasmic reticulum in skeletal muscle. | Precursor = | System = | Artery = | Vein = | Nerve = | Lymph = }} In the histology of skeletal muscle, a '''triad''' is the structure formed by a T tubule with a sarcoplasmic reticulum (SR) known as the terminal cisterna on either side.<ref name="atlas">{{cite book |author1=di Fiore, Mariano SH |author2=Eroschenko, Victor P |title=Di Fiore's Atlas of histology: with functional correlations |publisher=Wolters Kluwer Health/Lippincott Williams & Wilkins |location=Philadelphia |year=2008 |pages=124 |isbn=978-0-7817-7057-6 }}</ref> Each skeletal muscle fiber has 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 the A-I junction;<ref name="atlas"/> that is, the junction between the A and I bands of the sarcomere, which is the smallest unit of a muscle fiber.
Triads form the anatomical basis of excitation-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 the neuromuscular junction down the length of the T tubules, activating dihydropyridine receptors (DHPRs). Their activation causes 1) a negligible influx of calcium and 2) a mechanical interaction with calcium-conducting ryanodine 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 to muscle 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== * Diad, a homologous structure in cardiac muscle
==References== {{reflist}}
{{DEFAULTSORT:Triad (Anatomy)}} Category:Histology Category:Muscular system