Nature of the Excitatory Sarcoplasmic Reticular Junction.

The appositional regions between the surface membrane and sarcoplasmic reticulum in insects, decapod crustaceans, and barnacles are largely diadic and show a four-layered structure which is roughly circular in surface view. Each consists of the 56-angstrom double-membrane of the in-termediary (here called excitatory) element, and the 75-A double-membrane of the cisternal element of the reticulum, separated by a space of about 100 A. A sheet of electron-dense material is found between the two elements, giving the superficial appearance of an additional membrane. The orbits of thin filaments around the thick filaments adjacent to both excitatory and reticular elements are incomplete on the contact side. Regularly spaced bridges connect the thick filaments with both the excitatory elements and cisternal elements and hold the diads in place during stretch and contraction.