Can you identify the structure(s) identified by arrows in this muscle biopsy?

 

Tangential sections through three triads (pale T-tubules and darker adjacent sarcoplasmic reticulum ….. “Oreo cookie-like”) in association with sarcomeres (electron microscopy 15000x magnification).

T-tubules distribute the surface action potential into the interior of the muscle fiber, where it triggers release of calcium stored in the adjacent sarcoplasmic reticulum (i.e. excitation-contraction coupling).  Release and reuptake of calcium by the sarcoplasmic reticulum is critical to muscle contraction and relaxation.  Note the location of the triads adjacent to thick filaments where calcium is utilized for contraction.