THE FINE STRUCTURE OF SHEEP MYOCARDIAL CELLS; SARCOLEMMAL INVAGINATIONS AND THE TRANSVERSE TUBULAR SYSTEM

An electron microscope study of sheep myocardial cells has demonstrated the presence of a transverse tubular system, apparently forming a network across the cell at each Z band level. The walls of these tubules resemble the sarcolemma in consisting of two dense layers—plasma membrane and basement menbrane; continuity of the tubule walls with the sarcolemma can be seen when longitudinal sections of a cell are obtained between two subsarcolemmal myofibrils and at the same time perpendicular to the cell surface. The demonstration of communication between the lumen of the transverse tubular system and the extracellular space appears to be more definite in this study than in any work hitherto published. It provides anatomical evidence of a possible direct pathway for transmission of the activating impulse from the sarcolemma to the myofibril Z bands.     

THE SARCOPLASMIC RETICULUM, THE T SYSTEM, AND THE MOTOR TERMINALS OF SLOW AND TWITCH MUSCLE FIBERS IN THE GARTER SNAKE

Twitch and slow muscle fibers, identified morphologically in the garter snake, have been examined in the electron microscope. The transverse tubular system and the sarcoplasmic reticulum are separate entities distinct from each other. In twitch fibers, the tubular system and the dilated sacs of the sarcoplasmic reticulum form triads at the level of junction of A and I bands. In the slow fibers, the sarcoplasmic reticulum is severely depleted in amount and the transverse tubular system is completely absent. The junctional folds of the postsynaptic membrane of the muscle fiber under an "en grappe" ending of a slow fiber are not so frequent or regular in occurrence or so wide or so long as under the "en plaque" ending of a twitch fiber. Some physiological implications of these differences in fine structure of twitch and slow fibers are discussed. The absence of the transverse tubular system and reduction in amount of sarcoplasmic reticulum, along with the consequent disposition of the fibrils, the occurrence of multiple nerve terminals, and the degree of complexity of the post junctional folds of the sarcolemma appear to be the morphological basis for the physiological reaction of slow muscle fibers.     

THE ORGANIZATION OF FLIGHT MUSCLE FIBERS IN THE ODONATA

The cytological organization of flight muscle fibers of Odonata has been investigated. These fibers, in representatives of the Zygoptera and Anisoptera, have been compared and found to be similar, except that, in the former, pairs of lamellar fibrils, rather than single fibrils, alternate with the mitochondria. In each instance, in these synchronous muscles, the actin filaments of the myofibrils are found to lie opposite to and midway between pairs of myosin filaments—a configuration previously reported in asynchronous flight muscle fibers. The disposition of the T system and sarcoplasmic reticulum membranes in glutaraldehyde-fixed anisopteran muscle is described in detail: the T system tubules are shown to be radially continuous across the fiber, and are derived as openmouthed invaginations from the surface cell-membrane. The detailed organization of the dyad junctions between these tubules and the adjoining cisternae of the sarcoplasmic reticulum is described. The accessibility of the T system interior to diffusion exchange with the general extracellular milieu has been investigated by studies on the penetration of ferritin into the fiber: molecules of this marker have been found to diffuse solely along the T system tubules, and their presence in the tubule extremities adjoining the centrally placed nuclei confirms the morphological evidence suggesting that these tubules provide open diffusion channels extending across the radius of the fiber. The possible physiological role of these membrane components and their distribution in synchronous muscles of insects and vertebrates and in asynchronous insect flight muscle are discussed.     

DIFFERENTIATION OF THE SARCOPLASMIC RETICULUM AND T SYSTEM IN DEVELOPING CHICK SKELETAL MUSCLE IN VITRO

The electron microscope was used to investigate the first 10 days of differentiation of the SR and the T system in skeletal muscle cultured from the breast muscle of 11-day chick embryos. The T-system tubules could be clearly distinguished from the SR in developing muscle cells fixed with glutaraldehyde and osmium tetroxide. Ferritin diffusion confirmed this finding: the ferritin particles were found only in the tubules identified as T system. The proliferation of both membranous systems seemed to start almost simultaneously at the earliest myotube stage. Observations suggested that the new SR membranes developed from the rough-surfaced ER as tubular projections. The SR tubules connected with one another to form a network around the myofibril. The T-system tubules were formed by invagination of the sarcolemma. The early extension of the T system by branching and budding was seen only in subsarcolemmal regions. Subsequently the T-system tubules could be seen deep within the muscle cells. Immediately after invaginating, the T-system tubule formed, along its course, specialized connections with the SR or ER: triadic structures showing various degrees of differentiation. The simultaneous occurrence of myofibril formation and membrane proliferation is considered to be important in understanding the coordinated events resulting in the differentiated myotube.     

THE STRUCTURE OF INTERSEGMENTAL MUSCLE FIBERS IN AN INSECT, PERIPLANETA AMERICANA L

The organization of intersegmental muscle fibers associated with the dorsal abdominal sclerites of the cockroach is described. These fibers correspond closely, in the disposition and derivation of the membranes of the transverse tubular system and sarcoplasmic reticulum cisternae, with insect synchronous flight muscle fibers, but differ markedly from these in their fibrillar architecture and mitochondrial content. The mitochondria are small and generally aligned alongside the prominent I bands of the sarcomere, and, in the best-oriented profiles of the A bands, thick filaments are associated with orbitals of twelve thin filaments, a configuration that has also been observed in striated fibers of insect visceral muscle. These structural features of insect muscles are compared and discussed in terms of possible variations in the control of contraction and relaxation, and in the nature of their mechanical role.     

CYTOCHEMICAL STUDIES OF ADENOSINE TRIPHOSPHATASES IN SKELETAL MUSCLE FIBERS

Mitochondrial ATPase and myosin ATPase have been localized in the muscle fibers of the rat diaphragm. The principal fiber type possesses a structure favorable for making this cytochemical separation with the light microscope. This small red fiber has numerous large, nearly spherical, mitochondria (ca. 1.5 µ) which are aggregated beneath the sarcolemma. In the interior of the fiber, smaller paired filamentous mitochondria (ca. 0.2 µ diameter) are aligned with the I band. Distribution of mitochondria was determined by sudanophilia, succinic dehydrogenase activity, and by direct examination with the electron microscope. ATPase activity at pH 7.2 is located in the large peripheral mitochondria and in the smaller mitochondria associated with the I band. The alignment of the small mitochondria results in a discrete cross-striated appearance in fibers stained for this enzymic activity. This mitochondrial ATPase does not cleave adenosine diphosphate or adenosine monophosphate; it is not sulfhydryl dependent and, in fact, is enhanced by the mercurial, p-hydroxymercuribenzoate. It requires magnesium ion and is stimulated by dinitrophenol. It is inhibited after formol-calcium fixation, but the residual activity is demonstrable by lengthening the incubation time. At pH 9.4 the ATPase is myofibrillar in origin and is located in the A bands. This myosin ATPase activity is sulfhydryl-dependent. Mercurial at this high pH has an interesting dual effect: it suppresses myosin ATPase but evokes mitochondrial ATPase activity. A third type of ATPase activity can be demonstrated, especially in the large white fibers. This activity occurs at pH 7.2 in the presence of cysteine. Its position is manifested cytochemically as a fine reticular pattern which surrounds individual myofibrils. The distribution suggests that it may originate in the sarcoplasmic reticulum.     

ULTRASTRUCTURE OF THE Z LINE OF SKELETAL MUSCLE FIBERS

A new model of Z-line structure in skeletal muscle is proposed. Unlike previous models it is capable of explaining the two apparently inconsistent lattice arrangements seen in thin sections, i.e., the "basket weave" lattice and the smaller lattice recently reported in the literature. The model is based on four looping helical strands derived from the I filaments within the Z line. Each of these four strands form hairpin-shaped loops within the Z line and then join with an adjacent I filament in the same sarcomere. The two apparently different lattices represent a common structure viewed at slightly different levels of section.     

RELATIONS BETWEEN STRUCTURE AND FUNCTION IN RAT SKELETAL MUSCLE FIBERS

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.     

ULTRASTRUCTURE AND ADENOSINE TRIPHOSPHATASE ACTIVITY OF RED AND WHITE MUSCLE FIBERS OF THE CAUDAL REGION OF A FISH, SALMO GAIRDNERI

Electron microscopy, together with quantitation using a tracing device linked to a digital computer, reveals that the red and white muscle fibers of Salmo gairdneri differ in diameter, organization of myofibrils, dimensions of myofilaments, volumes and surface areas of T system and sarcoplasmic reticulum, morphology of mitochondria, and content of mitochondria, lipid, and glycogen. Biochemical studies show that the ATPase activity of white fibers is three times that of the red fibers. Actomyosin content of red fibers is higher than that of the white fibers. The functional significance of these differences between two fiber types is discussed.     

电生理技术在鱼类尾部神经分泌系统研究中的应用

<正>200多年前,Galvani通过神经-肌肉兴奋性实验发现了神经系统与电活动在功能上具有本质的联系[1,2]。此后,众多研究者投入了大量的精力研发电生理设备,逐步开发出多种能够精确测量和控制神经元电活动的仪器,如:放大器、示波器、刺激器、数模/模数转换器、微电极等。现代电生理设备和技术的更新换代为研究单离子通道电     

地塞米松对大鼠坐骨神经损伤后各型肌纤维运动终板恢复作用的实验研究

本实验使用１２０只Ｗｉｓｔａｒ系大鼠,采用定位、定量、定时的方法压挫坐骨神经后,给予治疗剂量的地塞米松,动态地观察损伤后红、白、中间肌纤维运动终板ＡＣｈＥ的酶组织化学及各型肌纤维运动终板超微结构的变化。结果提示,早期适量应用地塞米松对周围神经损伤后运动终板的恢复有促进作用。