THE ORGANIZATION OF FLIGHT MUSCLE IN AN APHID, MEGOURA VICIAE (HOMOPTERA) : With a Discussion on the Structure of Synchronous and Asynchronous Striated Muscle Fibers

The organization of the indirect flight muscle of an aphid (Hemiptera-Homoptera) is described. The fibers of this muscle contain an extensive though irregularly disposed complement of T system tubules, derived as open invaginations from the cell surface and from the plasma membrane sheaths accompanying the tracheoles within the fiber. The sarcoplasmic reticulum is reduced to small vesicles applied to the T system surfaces, the intermembrane gap being traversed by blocks of electron-opaque material resembling that of septate desmosomes. The form and distribution of the T system and sarcoplasmic reticulum membranes in flight muscles of representatives of the major insect orders is described, and the extreme reduction of the reticulum cisternae in all asynchronous fibers (to which group the aphid flight muscle probably belongs), and the high degree of their development in synchronous fibers is documented and discussed in terms of the contraction physiology of these muscle cells.     

Disposition of membranes and the entry of haemolymph-borne ferritin in flight muscle fibers of the fly Phormia regina

The disposition of the surface plasma membrane and its inwardly directed derivatives, corresponding to the T-system of other fibers, and of the corresponding sarcoplasmic reticulum (SR) elements has been examined in a dipteran asynchronous flight muscle (Photmia regina). The presence of uniaxonal neuromuscular junctions within clefts approaching the center of the fiber is described. The most conspicuous SR component is present in the dyads adjoining plasma membrane derivatives, but it is also sparsely represented elsewhere. The accessibility of the plasma membrane-limited compartments within the fiber to the ambient haemolymph, in the living insect, has been investigated by tracing the distribution of ferritin by the circulatory system. The proportion of fiber volume occupied by the T-system and SR components in asynchronous and synchronous muscle is compared and the functional implications of these proportions is discussed.     

THE STRUCTURE OF INSECT FIBRILLAR FLIGHT MUSCLE : A Study Made with Special Reference to the Membrane Systems of the Fiber

The fine structure of fibrillar flight muscle of the mature adult beetle Tenebrio molitor is described. Although the very high frequency of contraction of fibrillar muscle has previously been in part accounted for as the result of mechanical specialization of the wing-bearing segment rather than of a correspondingly high rate of motor impulse supply, the problem of the nature of the pathway by which excitation is conducted into these large fibers remained. Therefore, particular attention has been given to the disposition and relationships of the plasma membrane and sarcoplasmic reticulum in this tissue. The invading tracheoles draw with them a sheath of plasma membrane from the surface to all depths in the fiber, and it is suggested that these sheaths, together with the extensive tubular arborisations arising from them, reduce the maximum plasma membrane-to-fibril distance from the radius of the fiber to a value of less than 2 µ. The evidence presented here confirms Veratti's contention that in fibrillar muscle the "reticulum" is associated with, though entirely distinct from the fibrils. Unlike other muscles so far examined, these flight muscle fibers contain a plasma membrane reticulum only, but it is possible that elsewhere the general "sarcoplasmic reticulum" includes a component derived from the plasma membrane, likewise acting as the pathway for inward conduction of excitation. Profiles of the internalised plasma membrane in Tenebrio showing the usual triple-layered 25-25-25 A organization are frequently seen, in sections, in close association with isolated vesicles (defined by "simple" 50 A membranes) which are here considered to represent, in vestigial form, the portion of the sarcoplasmic reticulum which in other types of muscle is complex and highly developed. Such associations, in Tenebrio, between these two dissimilar elements are here termed "dyads" and the possible morphological and functional homology between these and the "triads" of other types of fiber is considered.     

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.     

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 THE FLIGHT MUSCLE IN A DRAGONFLY, AESHNA SP. (ODONATA)

The structure of the flight muscle of a dragonfly (Aeshna sp.) has been studied with the light and electron microscopes, and the organization of this specialized tubular muscle is described. This tissue is characterized by the great development of the sarcosomes, which are slab-like and are arranged within the fiber opposite each sarcomere of the radially oriented lamellar myofibrils. A well developed and highly ordered sarcoplasmic reticulum is present, consisting of perforated curtain-like cisternae extending across the face of each fibril, together with tubular invaginations of the fiber plasma membrane situated within indentations in the sarcosomes and traversing the fibril surface midway between the Z and M levels. The structure of these fibers, and notably the organization of the reticulum, is compared with that of other types of muscle, and the possible role of the two components of the sarcoplasmic reticulum in the contraction physiology of the dragonfly muscle fiber is discussed.     

Shape and disposition of clefts,tubules, and sarcoplasmic reticulum in long and short sarcomere fibers of crab and crayfish

Summary The disposition of surface invaginations (clefts, Z and T tubules) and of the sarcoplasmic reticulum has been examined by electron microscopy at three accelerating voltages (100, 200 and 1000 kV) and by phase-contrast light microscopy in crustacean muscles infiltrated by the Golgi stain. In long-sarcomere, tonic type fibers, an extensive system of invaginating clefts has been observed, along with both Z and T tubules. Z and T tubules form interconnections with each other, but only T tubules form specific contacts with the sarcoplasmic reticulum, which in these fibers forms an extended and continuously fenestrated network. In short-sarcomere, phasic type fibers, a ladder-like disposition of an abundant T network is found. Z tubules are absent in these fibers. The sarcoplasmic reticulum forms more frequent junctions with flattened areas of T tubules and with clefts, but has less extensive free surfaces than in the long-sarcomere fibers.We wish to dedicate this paper to the late Graham Hoyle, whose lifetime of work and interest in the study of muscle from a comparative point of view has been an inspiration to us.     

A COMPARATIVE STUDY ON THE FINE STRUCTURE OF THE BASALAR MUSCLE OF THE WING AND THE TIBIAL EXTENSOR MUSCLE OF THE LEG OF THE LEPIDOPTERAN ACHALARUS LYCIADES

Basalar and tibial extensor muscle fibers of Achalarus lyciades were examined with light and electron microscopes. Basalar muscle fibers are 100–150 µ in diameter. T-system membranes and sarcoplasmic reticulum make triadic contacts midway between Z lines and the middle of each sarcomere. The sarcoplasmic reticulum is characterized by a transverse element situated among myofilaments halfway between Z lines in every sarcomere. The morphology of Z lines, hexagonal packing of thin and thick myofilaments, and thin/thick myofilament ratios are similar to those of fast-acting insect muscles. Tibial extensor muscle fibers are 50–100 µ in diameter. Except for a lack of the transverse element, the T system and sarcoplasmic reticulum are similar to those of basalar muscle. Wavy Z lines, lack of a hexagonal packing of myofilaments, and larger thin/thick myofilament ratios are similar to those of other postural muscles of insects. The morphology of basalar and tibial extensor muscle is compared to that of other insect muscle with known functions, and reference is made to the possible contribution of the transverse element of sarcoplasmic reticulum in basalar flight muscle to speed and synchrony in this muscle.     

THE ULTRASTRUCTURE OF THE CAT MYOCARDIUM : I. Ventricular Papillary Muscle

The ultrastructure of cat papillary muscle was studied with respect to the organization of the contractile material, the structure of the organelles, and the cell junctions. The morphological changes during prolonged work in vitro and some effects of fixation were assessed. The myofilaments are associated in a single coherent bundle extending throughout the fiber cross-section. The absence of discrete "myofibrils" in well preserved cardiac muscle is emphasized. The abundant mitochondria confined in clefts among the myofilaments often have slender prolongations, possibly related to changes in their number or their distribution as energy sources within the contractile mass. The large T tubules that penetrate ventricular cardiac muscle fibers at successive I bands are arranged in rows and are lined with a layer of protein-polysaccharide. Longitudinal connections between T tubules are common. The simple plexiform sarcoplasmic reticulum is continuous across the Z lines, and no circumferential "Z tubules" were identified. Specialized contacts between the reticulum and the sarcolemma are established on the T tubules and the cell periphery via subsarcolemmal saccules or cisterns. At cell junctions, a 20 A gap can be demonstrated between the apposed membranes in those areas commonly interpreted as sites of membrane fusion. In papillary muscles worked in vitro without added substrate, there is a marked depletion of both glycogen and lipid. No morphological evidence for preferential use of glycogen was found.     

SELECTIVE DISRUPTION OF THE SARCOTUBULAR SYSTEM IN FROG SARTORIUS MUSCLE : A Quantitative Study with Exogenous Peroxidase as a Marker

Skeletal muscles which have been soaked for 1 hr in a glycerol-Ringer solution and then returned to normal Ringer solution have a disrupted sarcotubular system. The effect is associated with the return to Ringer's since muscles have normal fine structure while still in glycerol-Ringer's. Karnovsky's peroxidase method was found to be a very reliable marker of extracellular space, filling 98.5% of the tubules in normal muscle. It was interesting to note that only 84.1% of the sarcomeres in normal muscle have transverse tubules. The sarcotubular system was essentially absent from glycerol-treated muscle fibers, only 2 % of the tubular system remaining connected to the extracellular space; the intact remnants were stumps extending only a few micra into the fiber. Thus, glycerol-treated muscle fibers provide a preparation of skeletal muscle with little sarcotubular system. Since the sarcoplasmic reticulum is not destroyed and the sarcolemma and myofilaments are intact in this preparation, of the properties of the sarcolemma may thus be separated from those of the tubular system.     

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.     

Regeneration of a tonic avian muscle: transformation to a twitch morphology.

The tonic anterior latissimus dorsi muscle of the pigeon was excised, minced into fine pieces, replaced into its original bed, and allowed to regenerate for periods up to 37 weeks. Although regeneration was asynchronous, regeneration patterns of the muscle fibers suggested the following sequence of fiber development: undifferentiated to tonic to twitch. Fiber types were identified on the basis of Z-line morphology and qualitative development and organization of the sarcotubular system as demonstrated by electron microscopy. Histochemical demonstration of myofibrillar adenosine triphosphatase and succinic dehydrogenase activities corroborated the morphological evidence, suggesting a transformation to a twitch morphology. In addition to the transformation to the twitch morphology, other alterations were observed in these regenerating fibers. Among these were large numbers of closely-packed 60-nm-diameter tubules, thought to be derived from the sarcoplasmic reticulum; mitochondria with intermembraneous dense material; and Z-line streaming. The transformation of the muscle fibers from tonic to twitch morphology is discussed in terms of alterations in nerve impulse activity to the regenerating muscle.     

A light and transmission electron microscope study of some cells and tissues associated with the tymbal muscle of a periodical cicada (Homoptera, Cicadidae)

Abstract. A light and transmission electron microscope study of sections of cells of—and of cells and tissues of—and of cells, associated with the previously undescribed tymbal muscle of a periodical cicada (Brood X of the 17-year cicada) was undertaken to (i) compare their features with similar features described for other cicada tymbal muscles, (ii) use that information to try to determine cytologically whether the muscle should be considered synchronous or asynchronous, and (iii) seek information about ultrastructural features not previously described for any cicada. In cross section the myofibrils are slightly angular and have an abundance of sarcoplasmic reticulum and T tubules. Longitudinal sections show a pair of T tubules, one of each pair located midway between the Z line and the center (H level) of each sarcomere. These cytological features are consistently found in the tymbal muscles of the majority of other cicada genera and species, which are designated synchronous muscles, and all of which are termed fast muscles. The amount of sarcoplasmic reticulum increases at the Z lines. The largest mitochondria occur in the largest axons, but the smallest axons have more neurotubules per cross-section area. Axon diameters range 0.14–20 μm. Multinucleate adipocytes, with vacuoles that appear either empty or content-containing, and tracheocytes, which could either be binucleate or have a lobate or U-shaped nucleus, are located at the periphery of the muscle. Large numbers of microtubules occur in the interface glia. The diameters of microtubules and neurotubules (∼27 nm) agree closely with the averages usually cited. This study indicates that the tymbal muscles of this cicada should be designated as synchronous, and it describes ultrastructural features that are typical and others that are unusual.     

Structural organization of two fast,rhythmically active crustacean muscles

Summary The organization of the flagellum abductor muscle and of a scaphognathite levator muscle of the green crab, Carcinus maenas, has been compared quantitatively using light and electron microscopy. These muscles are rhythmically active at relatively high frequencies and for long durations. Fibers of both muscles are interconnected to form fascicles of 50 or more fibers within which there is cytoplasmic continuity. A single muscle is made up of 8–12 fascicles. Individual fibers consist of a peripheral rind of densely packed mitochondria, a thick region of glycogen granules, and myofibrils arranged into scattered central islands. Less than half the volume-density of these muscles is contractile material, the balance being largely mitochondria and glycogen. The fibers within a muscle are structurally similar. They have short sarcomeres (about 2 m), thin to thick filament ratios of about 3:1, and junctions between the sarcoplasmic reticulum and the transverse tubules at the M line. Sarcoplasmic reticulum occupies about 10% of the myofibrillar volume-density. A well developed sarcoplasmic reticulum appears to underlie the capacities of these two muscles for high frequency contraction; extensive mitochondria and glycogen stores should confer fatigue resistance under both aerobic and anaerobic conditions.     

Discontinuity of sarcoplasmic reticulum in the mid-sarcomere region in flight muscle of dragonflies

The sarcoplasmic reticulum organization of dragonfly flight muscles is analyzed, with particular reference to the doubling existing at H-band level. This doubling could be explained as a consequence of a regular discontinuity in the sarcoplasmic reticulum covering myofibrils. In each sarcomere, two sleeves of the sarcoplasmic reticulum seem to overlap forming a telescopic system which can slide outside each other during the lengthening and shortening movements of the fiber.     

FINE STRUCTURE OF RAT INTRAFUSAL MUSCLE FIBERS : The Polar Region

An ultrastructural comparison of the two types of intrafusal muscle fibers in muscle spindles of the rat was undertaken. Discrete myofibrils with abundant interfibrillar sarcoplasm and organelles characterize the nuclear chain muscle fiber, while a continuous myofibril-like bundle with sparse interfibrillar sarcoplasm distinguishes the nuclear bag muscle fiber. Nuclear chain fibers possess well-defined and typical M bands in the center of each sarcomere, while nuclear bag fibers contain ill-defined M bands composed of two parallel thin densities in the center of the pseudo-H zone of each sarcomere. Mitochondria of nuclear chain fibers are larger and more numerous than they are in nuclear bag fibers. Mitochondria of chain fibers, in addition, often contain conspicuous dense granules, and they are frequently intimately related to elements of the sarcoplasmic reticulum (SR). Striking differences are noted in the organization and degree of development of the sarcotubular system. Nuclear bag fibers contain a poorly developed SR and T system with only occasional junctional couplings (dyads and triads). Nuclear chain fibers, in contrast, possess an unusually well-developed SR and T system and a variety of multiple junctional couplings (dyads, triads, quatrads, pentads, septads). Greatly dilated SR cisternae are common features of nuclear chain fibers, often forming intimate associations with T tubules, mitochondria, and the sarcolemma. Such dilatations of the SR were not encountered in nuclear bag fibers. The functional significance of these structural findings is discussed.     

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.     

Morphological studies of fiber types of striated muscle fibers of the cremaster in the guinea pig

THe fine structure of the striated muscle fibers of the cremaster of the guinea pig was studied using the cholinesterase technique and light and electron microscopy. Under light microscopy, isolated single muscle fibers showed two types of nerve endings: the first one presented elliptic or oval areas having digit-like structures inside, some of the borders of which were heavily stained. These fibers had only one end-plate. The second type presented elongated clear areas with most of the density located on the borders. Several nerve endings were apparent in these fibers. By electron microscopy, the former had large and numerous sarcolemmal foldings and these characteristics were also observed in unstained fibers. In the latter, the foldings were scanty or absent. At the ultrastructural level, the fibers having only one end-plate presented a regular array of fibrils with an abundant sarcoplasmic reticulum ('Fibrillenstruktur' type) in contrast to the multi-innervated fiber with an irregular distribution pattern of fibrils and a scarce sarcoplasmic reticulum ('Felderstruktur' type). The striated muscle fiber layer of the cremaster probably contains both fast and slow fibers. The possible functional role for the slow striated muscle fibers is discussed.     

STRUCTURE OF THE LONGITUDINAL BODY MUSCLES OF AMPHIOXUS

The structure of the longitudinal body muscles of Branchiostoma caribaeum has been studied by light and electron microscopy. These muscles are shown to be composed of fibers in the form of flat lamellae about 0.8µ in thickness, more than 100 µ wide, and reaching in length from one intermuscular septum to the next, a distance of about 0.6 mm. Each flat fiber is covered by a plasma membrane and contains a single myofibril consisting of myofilaments packed in the interdigitating hexagonal array characteristic of vertebrate striated muscle. Little or no sarcoplasmic reticulum is present. Mitochondria are found infrequently and have a tubular internal structure. These morphological observations are discussed in relation to a proposed hypothesis of excitation-contraction coupling. It is pointed out that the maximum distance from surface to myofilament in these muscles is about 0.5 µ and that diffusion of an "activating" substance over this distance would essentially be complete in less than 0.5 msec. after its release from the plasma membrane. It is concluded that the flat form of amphioxus muscle substitutes for the specialized mechanisms of excitation-contraction coupling thought possibly to involve the sarcoplasmic reticulum in higher vertebrate muscles.     

STUDIES ON THE ENDOPLASMIC RETICULUM : III. ITS FORM AND DISTRIBUTION IN STRIATED MUSCLE CELLS

Several types of striated muscle have been examined by the technics of electron microscopy and the findings in myotome fibers of Amblystoma larvae, the sartorius, and cardiac muscle of the rat are reported on in some detail. Particular attention has been given to structural components of the interfibrillar sarcoplasm and most especially to a finely divided, vacuolar system known as the sarcoplasmic reticulum. This consists of membrane-limited vesicles, tubules, and cisternae associated in a continuous reticular structure which forms lace-like sleeves around the myofibrils. It shows a definable organization which repeats with each sarcomere of the fiber so that the entire system is segmented in phase with the striations of the associated myofibrils. Details of these repetitive patterns are presented diagrammatically in Text-figs. 1, 2, and 3 on pages 279, 283, and 288 respectively. The system is continuous across the fiber at the H band level and largely discontinuous longitudinally because of interruptions in the structure at the I and Z band levels. The structure of the system relates it to the endoplasmic reticulum of other cell types. The precise morphological relation of the reticulum to the myofibrils, with specializations opposite the different bands, prompts the supposition that the system is functionally important in muscle contraction. In this regard it is proposed that the membrane limiting the system is polarized like the sarcolemma and that the corresponding potential difference is utilized in the intracellular distribution of the excitatory impulse.