A COMPARATIVE-STUDY OF THE TRANSVERSE TUBULAR SYSTEM OF THE RAT EXTENSOR DIGITORUM LONGUS AND SOLEUS MUSCLES

Abstract

An ultrastructural comparison was made between the transverse tubular systems of the extensor digitorum longus (fast twitch) muscles in the rat. Quantitative measurements were made of the volume fraction, surface to volume ratio, surface density and foot density of the T system membrane. The values for the volume fraction and surface density were .apprx. 50% higher in the fast twitch (extensor digitorum longus) than the slow twitch (soleus) muscle. The dimensions of the tubules and hence the surface to volume ratio was the same in the 2 muscles. There was no difference in the spacing of the feet, so the reported 3- to 5-fold difference in charge movement cannot be associated with a difference in foot density. Material lying within the lumen of the T tubules was observed in the majority of tubule profiles. This material was laminar in form.