Relations Between Structure and Function in Rat Skeletal Muscle Fibers

Abstract

The fast-twitch extensor digitorum longus (EDL) and the slow-twitch soleus muscle of the rat consist of heterogeneous fiber populations . EDL muscle fibers differ in size, mitochondrial content, myoglobin concentration, and thickness of the Z line . The sarcoplasmic reticulum, on the other hand, is richly developed in all fibers, with only small variation . Myofibrils are clearly circumscribed at both the A and I band level . The soleus muscle is composed primarily of fibers with moderate mitochondrial content and myoglobin concentration . In most fibers the sarcoplasmic reticulum is poorly developed, with the exception of the portion of reticulum in phase with the Z line . As a consequence the myofibrillar fields are amply fused together. Contacts between sarcoplasmic reticulum and T system are discontinuous and may occur in the form of "dyads" instead of the typical triad structure . In a small proportion of soleus muscle fibers the organization and development of the sarcoplasmic reticulum is similar to that of EDL muscle fibers, with prominent fenestrated collars at the H band level . In these fibers mitochondria are larger and more abundant . The results are correlated with physiological studies on motor units in the same and in similar rat muscles . It is suggested that the variable structural pattern of rat muscle fibers is related to two distinct physiological parameters, speed of contraction and resistance to fatigue .

Peripheral Couplings in Adult Vertebrate Skeletal Muscle

“…In their location and morphology, the peripheral couplings were thus similar in all avian fibers studied. In the avian leg muscles and both skeletal muscles from the mouse, numerous fenestrations were observed in the SR retes over both the M line and Z line in accordance with earlier observations on the disl;ribution of SR fenestrations in rat soleus muscle (14), in certain other avian muscle (15), and in the mouse diaphragm (16).…”

Changes in the Sarcoplasmic Reticulum and Transverse Tubular System of Fast and Slow Skeletal Muscles of the Mouse During Postnatal Development

“…However, there was a clear-cut difference between most SOL and EDL fibers in the extent of the SR and TTS. Such differences have been shown in rat SOL and EDL fibers (32) . In 'Abbreviations used in this article : EDL, extensor digitorum longus ; SOL, soleus ; SR, sarcoplasmic reticulum ; TTS, transverse tubular system .…”

“…Such strap-like mitochondria have been reported by several authors (2,5,28,34) . Variation in mitochondrial content occurred in fibers of mouse EDL muscles, as in rat EDL (32) . There was less variation among SOL fibers.…”

“…Their primary categories are "red," "white," and "intermediate," with a secondary grouping into "slow" and "fast ." Ultrastructural differences are also known to exist in the different fiber types (16,23,32) .…”

“…Previous work on postnatal development of mam- 40), and several studies have been done on the malian fast and slow muscles has been concerned fine structure of various fibers in adult muscles mainly with biochemical and physiological fea- (16,23,27,30,32,34) . No quantitative comparitures.…”