The core cylinder in the junctional SR of single intact skeletal muscle fibers of the frog after quick-freezing: Its topography

Abstract

The "core cylinder" (CC) represents confluent electron lucencies seen in the junctional SR (JSR) when skeletal muscle is fixed in the presence of cations such as ruthenium red. CCs are a consistent feature in quick-frozen muscle fibers (Fig.l), both in the resting state and after various intervals between stimulation and freezing, incl. tetanus (5 sec, 50Hz). The CCs are excentrically located inside the JSR and are separated from the junctional membrane toward the transverse tubule by the coextensive line(CL) and from the rest of the membranous envelope of the JSR by granular material (junctional granules, JG). JGs are also found elsewhere in the free SR. Whereas the JGs fill the JSR completely in many cases, often they are separated from the JSR envelope by a peripheral clear halo except in the region of the CL; in favourable sections they form regular rosettes about the CC (Fig.3). Very frequently, the CC loose their more circular configuration (in one dimension)(Fig.4), with flares connecting the CCs with the peripheral halo at either end of the CL (Fig.3). The junction between the electron-lucent flares and the ends of the CL subtends, approximately, the location of the staggered rows of pits on the E face of the freeze-fractured JSR(Fig.2).