Comparative Cytophysiology of Striated Muscle With Special Reference to the Role of the Endoplasmic Reticulum

651

364

The sarcoplasmic reticulum of the frog's sartorius muscle was examined by electron microscopy following sequential fixation in glutaraldehyde and osmium tetroxide and embedding in Epon. The earlier results of Porter and Palade on Ambystoma muscle were confirmed in the sartorius. In addition, the transverse tubules were observed to be continuous across the width of the fiber, a set of flat intermediate cisternae was seen to connect the terminal cisternae to the longitudinal tubules in the A band, and the continuous reticulum collar at the center of the A band was found to be perforated by circular and elongated pores (the fenestrated collar). The transverse tubules have a volume about 0.3 per cent of the fiber volume, and a surface area about 7 times the outer cylindrical surface area for a fiber 100 µ in diameter. The terminal cisternae, the intermediate cisternae, and the longitudinal tubules together with the fenestrated collar each have a volume of 4 to 5 per cent of the fiber volume and a surface area 40 to 50 times the outer surface area of a fiber 100 µ in diameter. Some evidence for continuity of the transverse tubules with the fiber surface is presented, but this is thought to be not so convincing as evidence presented by others. The results are discussed in terms of a possible mechanism for a role of the transverse tubules and sarcoplasmic reticulum in excitation-contraction coupling, as suggested by their morphology and a variety of physiological studies. In this scheme, the transverse tubules are thought to be electrically coupled to the terminal cisternae, so that depolarization of the fiber surface spreads inward along the transverse tubules and to the terminal cisternae, initiating the release of a contraction-activating substance.

Section: Discussionmentioning

confidence: 80%

Section: The Triadsmentioning

confidence: 90%

The Sarcoplasmic Reticulum and Transverse Tubules of the Frog's Sartorius

Peachey

1965

651

364

“…This is true especially where the inner membrane of the tubular profile is least well defined (Figs. 1,6,9). It is recognized that images such as T and SI in Fig.…”

Section: A Nelson and E S Benson Rabbit And Human Myocardial Cellsmentioning

confidence: 99%

“…A simple mechanism might be provided by physical continuity of the lumen of this transverse tubular system with the extracellular space, a relationship not yet demonstrated in vertebrate skeletal muscle (2,6,7).…”

Section: Figurementioning

confidence: 99%

On the Structural Continuities of the Transverse Tubular System of Rabbit and Human Myocardial Cells

Nelson

Benson

1963

151

An electron microscopic study of rabbit and human myocardium provides further evidence of the existence of two distinct components of the sarcoplasmic reticulum. A thin-walled tubular system (termed longitudinal system) is arranged in anastomosing channels sursurrounding each sarcomere and has transverse and possibly also longitudinal connections with the tubules of adjacent sarcomeres. A thick-walled tubular system traverses the myofiber transversely at the level of the Z lines of the myofibrils. The structure of these tubules very closely resembles that of deep sarcolemmal invaginations. Indeed, the membranes of the tubules appear to be continuous with the sarcolemma in favorable sections so that there seems to be an extension of the cell membrane and extracellular fluid to all depths of the myocardial fiber. Certain physiologic data which support this concept are discussed. The calculations of A. V. Hill comparing the kinetics of diffusion and the time-distance relationships between excitation and activation in frog sartorius muscle are reconsidered for cardiac muscle.After Bennett and Porter's first electron microscopic observations of the sarcoplasmic reticulum in the breast muscle of the domestic fowl (I), Porter and Palade presented detailed descriptions of this subcellular structure in different types of striated muscle (2). These findings provided a structural basis for consideration of the mechanisms of excitation-contraction coupling in muscle.Using a micropipette electrode to achieve local depolarization of the cell membrane, Huxley and Taylor (3) demonstrated transverse, two-dimensional conduction of excitation toward the central axis of an isolated intact fiber of frog striated muscle. The geometry of excitation and spread indicated very strongly that conduction inward took place along some structure in the plane of the Z lines of the myofibrils lying in register with one another. This conclusion conformed with an earlier prediction by A. V. Hill (4), based on consideration of the kinetics of diffusion and the time-distance relationships between excitation and activation in frog sartorius muscle. Hill proposed that, while excitation undoubtedly occurred at the cell surface, diffusion was far too slow a means of bearing the impulse into the interior of the fiber. He concluded that a process, not a substance, must carry the excitation inward.The electron microscopic observations of Porter and Palade (2) suggested the possible role of transversely oriented elements of the sarcoplasmic reticulum in excitation-contraction coupling.

“…5). For more details of normal muscle structure the reader is referred to the literature (19)(20)(21)(22)(23)(24)(25)(26)(27)32). …”

Section: Observationsmentioning

confidence: 99%

Electron Microscopic and Histochemical Observations of Muscle Degeneration After Tourniquet

Moore

Rüska

Copenhaver

1956

Self Cite

Muscle degeneration and a shock syndrome, which frequently results in death, follow the release of limb tourniquets. Albumin and other plasma components are transferred into the affected muscle tissue (1-4) while toxic products of autolysis and hemolytic substances appear in the circulating blood (5-7). It is possible that this abnormal transfer results in part from damage of capillaries and muscle fiber membranes. The application of cold delays autolysis and fluid exchange, modifies the shock symptoms, and reduces mortality (8). Chemical changes, such as the large decrease in phosphorus (9) and morphologic alterations, as revealed with the light microscope (9, 10), have been carefully studied. Wide interfibrillar spaces are observed in tissues taken immediately after release of the tourniquet, and some fibers show discoid decay of their fibrils. The primary alterations are followed several hours later by advancing granular and hyaline degeneration.The present communication extends the light microscopic observations of the succinic dehydrogenase distribution in normal fibers (11) to the degenerated fibers. With the electron microscope an attempt is made to visualize the finer details in the development of discoid, hyaline, and granular degeneration seen after release of the tourniquet. The alterations thus produced are common to an extremely wide variety of muscle lesions. Further experience will determine whether degenerations of different etiology, such as those caused by bacterial toxins, vitamin E deficiency, or Coxsackie virus infection will expose specific details characteristic of each lesion. MethodsAs in antecedent electrophoretic studies of blood and tissue extracts after shock-producing injuries (2-4), the technique of Rosenthal (12) was used for traumatizafion of hind legs of C57