Indentations in the terminal cisternae of denervated rat EDL and soleus muscle fibers.

The effects of denervation on the structure of the triad and longitudinal sarcoplasmic reticulum have been investigated in freeze-fracture replicas of extensor digitorum longus (EDL) and soleus fibers that had been denervated for 2 to 70 days. In EDL fibers the density of indentations along 1 micron of terminal cisternae fell during the first 2 weeks after nerve section, from a normal value of 7.3 +/- 0.2 (mean +/- 1 SEM) to an average value of 2.0 +/- 0.5 in fibers denervated for 16 to 17 days. Denervation did not change the numbers of indentations in soleus fibers. There was a significant change in the orientation of the triads and organization of the longitudinal sarcoplasmic reticulum in denervated EDL and soleus fibers. The effect of denervation on the density of indentations was best correlated with the effect on asymmetric charge movement, when indentation density was compared with twitch contraction time, the voltage sensitivity of tension, and charge movement. The close relationship between indentation density and charge movement provides compelling evidence for a functional link between the two during excitation-contraction coupling.