The density attached to the inside surface of the apposed sarcoplasmic reticular membrane in vertebrate cardiac and skeletal muscle fibres.
D000818:Animals; D001001:Anura; D004285:Dogs; D004721:Endoplasmic Reticulum; D066298:In Vitro Techniques; D008566:Membranes; D008854:Microscopy, Electron; D009132:Muscles / Q000166:cytology*; D009206:Myocardium / Q000166:cytology*; D009210:Myofibrils / Q000166:cytology; D051381:Rats; D013194:Staining and Labeling