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The Journal of Cell Biology, Vol 47, 107-119, Copyright © 1970 by Rockefeller University Press

ARTICLE

RELATIONS BETWEEN STRUCTURE AND FUNCTION IN RAT SKELETAL MUSCLE FIBERS

S. Schiaffino 1, Vecaronra Hanzlíková 1, and Sandra Pierobon 1

1 From the Institute of General Pathology and Center for the Study of Muscle Biology and Physiopathology, National Research Council, University of Padua, Italy.

Dr. Hanzlíková's permanent address is Institute of Physiology, Czechoslovak Academy of Sciences, Prague, Czechoslovakia

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.

Submitted on December 29, 1969
Revised on April 1, 1970


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