Actin filaments in muscle: Pattern of myosin and tropomyosin/troponin attachments

X-ray patterns from lobster and crayfish muscles show very clear layer lines from the thin filaments, well separated from the myosin layer lines. The intensities in patterns from relaxed muscles include an important contribution from the regulatory proteins, and allow the arrangement of the troponin complexes to be deduced. Moreover, the troponin diffraction indirectly provides an accurate value for the pitch of the actin helix in relaxed muscle.In rigor, the attachment of cross-bridges modifies the intensities. These X-ray patterns support Reedy's (1968) concept that cross-bridges in rigor attach only to certain azimuths on the actin filaments (“target areas”); the 145 Å repeat of their origins on the thick filaments is not reflected in the pattern of attachment. Our calculations show that the observed intensities agree quantitatively with those expected for models based on such attachment, but depend significantly on the locations of the troponin complexes. The arrangement of the filament components is discussed in terms of design requirements. Our conclusions may be applicable to many other muscles, especially insect flight muscle and other invertebrate muscles.     

Drosophila muscle regulation characterized by electron microscopy and three-dimensional reconstruction of thin filament mutants

Wild-type and mutant thin filaments were isolated directly from "myosinless" Drosophila indirect flight muscles to study the structural basis of muscle regulation genetically. Negatively stained filaments showed tropomyosin with periodically arranged troponin complexes in electron micrographs. Three-dimensional helical reconstruction of wild-type filaments indicated that the positions of tropomyosin on actin in the presence and absence of Ca(2+) were indistinguishable from those in vertebrate striated muscle and consistent with a steric mechanism of regulation by troponin-tropomyosin in Drosophila muscles. Thus, the Drosophila model can be used to study steric regulation. Thin filaments from the Drosophila mutant heldup(2), which possesses a single amino acid conversion in troponin I, were similarly analyzed to assess the Drosophila model genetically. The positions of tropomyosin in the mutant filaments, in both the Ca(2+)-free and the Ca(2+)-induced states, were the same, and identical to that of wild-type filaments in the presence of Ca(2+). Thus, cross-bridge cycling would be expected to proceed uninhibited in these fibers, even in relaxing conditions, and this would account for the dramatic hypercontraction characteristic of these mutant muscles. The interaction of mutant troponin I with Drosophila troponin C is discussed, along with functional differences between troponin C from Drosophila and vertebrates.     

Fine structure of muscles in the tickHyalomma (Hyalomma) dromedarii (Ixodoidea: Ixodidae)

Skeletal and visceral muscles are distinguished in the unfed nymphHyalomma (Hyalomma) dromedarii according to position, structure and function. The skeletal muscles include the capitulum, dorsoventral and leg oblique muscles. Their muscle fibres have the striated pattern of successive sarcomeres whose thick myosin filaments are surrounded by orbitals of up to 12 thin actin filaments. The cell membrane invaginates into tubular system (T) extending deeply into the sarcoplasm and closely associated to cisternae of sarcoplasmic reticulum (SR). The T and SR forming two-membered dyads are considered to be the main route of calcium ions whose movements are synchronized with the motor impulse to control contraction and relaxation in most muscles. Two types of skeletal muscle fibres are recognized, and are suggested as representing different physiological phases.In the visceral-muscle fibres investing tick internal organs, the actin and myosin filaments are slightly interrupted, and the T and SR are well demonstrated. Both skeletal and visceral muscles are invaginated by tracheoles and innervated by nerve-axons containing synaptic vesicles.     

"Twitchin-actin linkage hypothesis" for the catch mechanism in molluscan muscles: evidence that twitchin interacts with myosin, myorod, and paramyosin core and affects properties of actomyosin

"Twitchin-actin linkage hypothesis" for the catch mechanism in molluscan smooth muscles postulates in vivo existence of twitchin links between thin and thick filaments that arise in a phosphorylation-dependent manner [N.S. Shelud'ko, G.G. Matusovskaya, T.V. Permyakova, O.S. Matusovsky, Arch. Biochem. Biophys. 432 (2004) 269-277]. In this paper, we proposed a scheme for a possible catch mechanism involving twitchin links and regulated thin filaments. The experimental evidence in support of the scheme is provided. It was found that twitchin can interact not only with mussel myosin and rabbit F-actin but also with the paramyosin core of thick filaments, myorod, mussel thin filaments, "natural" F-actin from mussel, and skeletal myosin from rabbit. No difference was revealed in binding of twitchin with mussel and rabbit myosin. The capability of twitchin to interact with all thick filament proteins suggests that putative twitchin links can be attached to any site of thick filaments. Addition of twitchin to a mixture of actin and paramyosin filaments, or to a mixture of Ca(2+)-regulated actin and myosin filaments under relaxing conditions caused in both cases similar changes in the optical properties of suspensions, indicating an interaction and aggregation of the filaments. The interaction of actin and myosin filaments in the presence of twitchin under relaxing conditions was not accompanied by an appreciable increase in the MgATPase activity. We suggest that in both cases aggregation of filaments was caused by formation of twitchin links between the filaments. We also demonstrate that native thin filaments from the catch muscle of the mussel Crenomytilus grayanus are Ca(2+)-regulated. Twitchin inhibits the ability of thin filaments to activate myosin MgATPase in the presence of Ca(2+). We suggest that twitchin inhibition of the actin-myosin interaction is due to twitchin-induced switching of the thin filaments to the inactive state.     

The Regulation of Catch in Molluscan Muscle

Molluscan catch muscles are smooth muscles. As with mammalian smooth muscles, there is no transverse ordering of filaments or dense bodies. In contrast to mammalian smooth muscles, two size ranges of filaments are present. The thick filaments are long as well as large in diameter and contain paramyosin. The thin filaments contain actin and appear to run into and join the dense bodies. Vesicles are present which may be part of a sarcoplasmic reticulum. Neural activation of contraction in Mytilus muscle is similar to that observed in mammalian smooth muscles, and in some respects to frog striated muscle. The relaxing nerves, which reduce catch, are unique to catch muscles. 5-Hydroxytryptamine, which appears to mediate relaxation, specifically blocks catch tension but increases the ability of the muscle to fire spikes. It is speculated that Mytilus muscle actomyosin is activated by a Ca⁺⁺-releasing mechanism, and that 5-hydroxytryptamine may reduce catch and increase excitability by influencing the rate of removal of intracellular free Ca⁺⁺.     

Contribution of myosin rod protein to the structural organization of adult and embryonic muscles in Drosophila

Myosin rod protein (MRP) is a naturally occurring 155 kDa protein in Drosophila that includes the myosin heavy chain (MHC) rod domain, but contains a unique 77 amino acid residue N-terminal region that replaces the motor and light chain-binding domains of S1. MRP is a major component of myofilaments in certain direct flight muscles (DFMs) and it is present in other somatic, cardiac and visceral muscles in adults, larvae and embryos, where it is coexpressed and polymerized into thick filaments along with MHC. DFM49 has a relatively high content of MRP, and is characterized by an unusually disordered myofibrillar ultrastructure, which has been attributed to lack of cross-bridges in the filament regions containing MRP. Here, we characterize in detail the structural organization of myofibrils in adult and embryonic Drosophila muscles containing various MRP/MHC ratios and in embryos carrying a null mutation for the single MHC gene. We examined MRP in embryonic body wall and intestinal muscles as well as in DFMs with consistent findings. In DFMs numbers 49, 53 and 55, MRP is expressed at a high level relative to MHC and is associated with disorder in the positioning of thin filaments relative to thick filaments in the areas of overlap. Embryos that express MRP in the absence of MHC form thick filaments that participate in the assembly of sarcomeres, suggesting that myofibrillogenesis does not depend on strong myosin-actin interactions. Further, although thick filaments are not well ordered, the relative positioning of thin filaments is fairly regular in MRP-only containing sarcomeres, confirming the hypothesis that the observed disorder in MRP/MHC containing wild-type muscles is due to the combined action between the functional behavior of MRP and MHC myosin heads. Our findings support the conclusion that MRP has an active function to modulate the contractile activity of muscles in which it is expressed.     

Immunocytochemical electron microscopic study and Western blot analysis of paramyosin in different invertebrate muscle cell types of the fruit flyDrosophila melanogaster, the earthwormEisenia foetida, and the snailHelix aspersa

Summary The presence and distribution pattern of paramyosin have been examined in different invertebrate muscle cell types by means of Western blot analysis and electron microscopy immunogold labelling. the muscles studied were: transversely striated muscle with continuous Z lines (flight muscle fromDrosophila melanogaster), transversely striated muscle with discontinuous Z lines (heart muscle from the snailHelix aspersa), obliquely striated body wall muscle from the earthwormEisenia foetida, and smooth muscles (retractor muscle from the snail and pseudoheart outer muscular layer from the earthworm). Paramyosin-like immunoreactivity was localized in thick filaments of all muscles studied. Immunogold particle density was similar along the whole thick filament length in insect flight muscle but it predominated in filament tips of fusiform thick filaments in both snail heart and earthworm body wall musculature when these filaments were observed in longitudinal sections. In obliquely sectioned thick filaments, immunolabelling was more abundant at the sites where filaments disappeared from the section. These results agree with the notion that paramyosin extended along the whole filament length, but that it can only be immunolabelled when it is not covered by myosin. In all muscles examined, immunolabelling density was lower in cross-sectioned myofilaments than in longitudinally sectioned myofilaments. This suggests that paramyosin does not form a continuous filament. The results of a semiquantitative analysis of paramyosin-like immunoreactivity indicated that it was more abundant in striated than in smooth muscles, and that, within striated muscles, transversely striated muscles contain more paramyosin than obliquely striated muscles.     

X-ray diffraction patterns from mammalian heart muscle

We have obtained light and X-ray diffraction patterns from trabecular and papillary muscles of various mammalian hearts in the living resting state and in rigor. Equatorial X-ray diffraction patterns from living muscles show the 1,0 and 1,1 reflections from a hexagonal lattice of filaments. The lattice spacing varies with sarcomere length over the observable range (2·0 to 2·5 μm) in such a manner that the lattice volume remains constant. In the living resting state the 1,0 reflection is stronger than the 1,1 reflection, whereas in rigor the 1,1 reflection is almost as strong as the 1,0 reflection. These intensity changes are similar to those found in vertebrate skeletal muscle, suggesting that the mechanism of cross-bridge attachment to actin is similar in both muscles.Two types of meridional X-ray diffraction pattern were observed in muscles in different conditions. One type, obtained from dead or glycerol-extracted muscles or from muscles treated with iodoacetate, showed a strong actin-related pattern but only a weak pattern associated with myosin. This type of pattern was similar to that from vertebrate skeletal muscle in rigor. The other type, obtained from living, resting muscle, showed a weaker actin pattern but a stronger myosin pattern. The myosin pattern included layer-line reflections associated with projections from the thick filaments. This second type of pattern was similar to that from resting vertebrate skeletal muscle, but the layer lines were weaker. The weakness of the myosin layer lines may indicate that part of the high resting tension found in heart muscle arises from a small amount of actin-myosin interaction in the resting state. Such interaction could provide a mechanism for varying the diastolic length of heart muscle and thereby the diastolic volume of the heart.     

Calcium ions modulate regulation of smooth muscle contraction mediated by phosphorylation of myosin regulatory light chains

The effect of calcium ions on conformational changes of F-actin initiated by decoration of thin filaments with phosphorylated and dephosphorylated heavy meromyosin from smooth muscles was studied by fluorescence polarization spectroscopy. It is shown that heavy meromyosin with phosphorylated regulatory light chains (pHMM) promotes structural changes of F-actin which are typical for the "strong" binding of actin to the myosin heads. Heavy meromyosin with dephosphorylated regulatory light chains (dpHMM) causes conformational changes of F-actin which are typical for the "weak" binding of actin to the myosin heads. The presence of calcium enhances the pHMM effect and attenuates the dpHMM effect. We propose that a Ca2+-dependent mechanism exists in smooth muscles which modulates the regulation of actin--myosin interaction occurring via phosphorylation of myosin regulatory light chains.     

General model of myosin filament structure. 3. Molecular packing arrangements in myosin filaments

Following the original proposals about myosin filament structure put forward as part of a general myosin filament model (Squire, 1971, 1972) it is here shown what the most likely molecular packing arrangements within the backbones of certain myosin filaments would be assuming that the model is correct. That this is so is already indicated by recently published experimental results which have confirmed several predictions of the model (Bullard and Reedy, 1972; Reedy et al., 1972; Tregear and Squire, 1973).The starting point in the analysis of the myosin packing arrangements is the model for the myosin ribbons in vertebrate smooth muscle proposed by Small &; Squire (1972). It is shown that there is only one reasonable type of packing arrangement for the rod portions of the myosin molecules which will account for the known structure of the ribbons and which is consistent with the known properties of myosin molecules. The dominant interactions in this packing scheme are between parallel myosin molecules which are related by axial shifts of 430 Å and 720 Å. In this analysis the myosin rods are treated as uniform rods of electron density and only the general features of two-strand coiled-coil molecules are considered.Since the general myosin filament model is based on the assumption that the structures of different types of myosin filament must be closely related, the packing scheme derived for the myosin ribbons is used to deduce the structures of the main parts (excluding the bare zones) of the myosin filaments in a variety of muscles. It is shown in each case that there is only one packing scheme consistent with all the available data on these filaments and that in each filament type exactly the same interactions between myosin rods are involved. In other words the myosin-myosin interactions involved in filament formation are specific, they involve molecular shifts of either 430 Å or 720 Å, and are virtually identical in all the different myosin filaments which have been considered. Apart from the myosin ribbons, these are the filaments in vertebrate skeletal muscle, insect flight muscle and certain molluscan muscles.In the case of the thick filaments in vertebrate skeletal muscle the form of the myosin packing arrangement in the bare zone is considered and a packing scheme proposed which involves antiparallel overlaps between myosin rods of 1300 Å and 430 Å. It is shown that this scheme readily explains the triangular profiles of the myosin filaments in the bare zone (Pepe, 1967, 1971) and many other observations on the form of these myosin filaments.Finally it is shown that the cores of several different myosin filaments, assuming they contain protein, may consist of different arrangements of one or other of two types of core subfilament.     

THE FILAMENT LATTICE OF COCKROACH THORACIC MUSCLE

The fine structure of the tergo-coxal muscle of the cockroach, Leucophaea maderae, has been studied with the electron microscope. This muscle differs from some other types of insect flight muscles inasmuch as the ratio of thin to thick filaments is 4 instead of the characteristic 3. The cockroach flight muscle also differs from the cockroach femoral muscle in thin to thick filament ratios and diameters and in lengths of thick filaments. A comparison of these latter three parameters in a number of vertebrate and invertebrate muscles suggests in general that the diameters and lengths of the thick filaments and thin to thick filament ratios are related.     

The in vitro motility assay to study the calcium-mechanical relationship in skeletal and cardiac muscles

In a series of experiments on regulated contractile systems (i.e., in vitro mobile systems with reconstructed thin filaments), the velocities of the movement of a thin filament on the surface covered by either rabbit skeletal or rat cardiac myosin at various concentrations of calcium ions in solution (in the pCa range from 4 to 8) were assessed. The corresponding "pCa-velocity" relationships were plotted, which proved to be of the sigmoid form. It was found that, at a saturating calcium concentration (pCa 4), the velocity of regulated thin filaments was 65% higher than for unregulated ones in the case of skeletal myosin and 87% higher than for unregulated thin filaments in the case of cardiac myosin. It was also found that the Hill coefficient was 1.95 and 2.5 for skeletal and cardiac myosins, respectively. The difference in the Hill coefficients for skeletal and cardiac myosins is discussed in terms of the difference in contribution of cooperativity mechanisms of contractile and regulatory proteins in the regulation of contraction in these types of muscles.     

Forces measured with micro-fabricated cantilevers during actomyosin interactions produced by filaments containing different myosin isoforms and loop 1 structures

Background

There is evidence that the actin-activated ATP kinetics and the mechanical work produced by muscle myosin molecules are regulated by two surface loops, located near the ATP binding pocket (loop 1), and in a region that interfaces with actin (loop 2). These loops regulate force and velocity of contraction, and have been investigated mostly in single molecules. There is a lack of information of the work produced by myosin molecules ordered in filaments and working cooperatively, which is the actual muscle environment.

Methods

We use micro-fabricated cantilevers to measure forces produced by myosin filaments isolated from mollusk muscles, skeletal muscles, and smooth muscles containing variations in the structure of loop 1 (tonic and phasic myosins). We complemented the experiments with in-vitro assays to measure the velocity of actin motility.

Results

Smooth muscle myosin filaments produced more force than skeletal and mollusk myosin filaments when normalized per filament overlap. Skeletal muscle myosin propelled actin filaments in a higher sliding velocity than smooth muscle myosin. The values for force and velocity were consistent with previous studies using myosin molecules, and suggest a close correlation with the myosin isoform and structure of surface loop 1.

General significance

The technique using micro-fabricated cantilevers to measure force of filaments allows for the investigation of the relation between myosin structure and contractility, allowing experiments to be conducted with an array of different myosin isoforms. Using the technique we observed that the work produced by myosin molecules is regulated by amino-acid sequences aligned in specific loops.     

E-Tmod capping of actin filaments at the slow-growing end is required to establish mouse embryonic circulation

Tropomodulins are a family of proteins that cap the slow-growing end of actin filaments. Erythrocyte tropomodulin (E-Tmod) stabilizes short actin protofilaments in erythrocytes and caps longer sarcomeric actin filaments in striated muscles. We report the knockin of the beta-galactosidase gene (LacZ) under the control of the endogenous E-Tmod promoter and the knockout of E-Tmod in mouse embryonic stem cells. E-Tmod(-/-) embryos die around embryonic day 10 and exhibit a noncontractile heart tube with disorganized myofibrils and underdevelopment of the right ventricle, accumulation of mechanically weakened primitive erythroid cells in the yolk sac, and failure of primary capillary plexuses to remodel into vitelline vessels, all required to establish blood circulation between the yolk sac and the embryo proper. We propose a hemodynamic "plexus channel selection" mechanism as the basis for vitelline vascular remodeling. The defects in cardiac contractility, vitelline circulation, and hematopoiesis reflect an essential role for E-Tmod capping of the actin filaments in both assembly of cardiac sarcomeres and of the membrane skeleton in erythroid cells that is not compensated for by other proteins.     

The contractile apparatus of striated muscle in the course of atrophy and regeneration

Summary The ultrastructure of the contractile apparatus of the rat soleus muscle during the course of denervation atrophy was investigated. It was found that the ratio of thin to thick filaments increased in myofibrils of atrophying muscle fibers. Elevation of the ratio was observed as early as the second day after denervation, and became more pronounced with the progress of atrophy. Parallel measurements of the amounts of actin and myosin in the myofibrils and in the muscle protein extracts revealed a lower proportion of myosin heavy chains to actin in the fractions from denervated muscles, compared with the control values. Both the electron-microscopic observations and the biochemical evaluation of the actin content of the muscle, suggests that the elevated ratio of thin to thick filaments seen in the course of the muscle atrophy appears as the result of an earlier and more intensive disappearance of thick filaments. Thin filaments disappeared more slowly, in parallel to the decrease in muscle weight.On the basis of the results presented a mechanism of progress of simple atrophy of muscle in suggested.     

Thick Filaments in Unstretched Mammalian Smooth Muscle

THE controversy concerning the organization of myosin in mammalian smooth muscle was reviewed (Nature New Biology, 231, 225; 1971) at a time when the studies of Rice's laboratory and our own demonstrated a regular, quasi-rectangular array of thick filaments in guinea-pig taenia coli (TC) and rabbit portal-anterior mesenteric vein (MV), and, further, that, by excessive stretch and by the use of hypertonic incubation solutions, the thick filaments in this lattice could be aggregated into ribbon-like structures^1,2. These observations were made on muscles stretched to approximately 1.5 times their excised length. Both the TC³ and the rabbit MV^2,4 are spontaneously active smooth muscles, which shorten to less than their in vivo length when excised from the body: stretching by approximately 1.5 times brings these muscles close to their in vivo length. Nevertheless, recent reports^5,6, indicating that thick filaments were more readily visualized (but see Figs. 2 and 3 in ref. 5) in stretched smooth muscles, prompted the editorial writer of Nature (231, 423; 1971) to consider it a debatable question whether thick filaments are present in unstretched muscle. Thick filaments have been observed in relaxed muscles^1,5,6 and we now show that an array of thick filaments can also be observed in completely unstretched guinea-pig and rabbit MV smooth muscle (excised and dropped into the fixative) and that such arrays are present after two different modes of fixation.     

The Effect of Experimental Hyperthyroidism on Characteristics of Actin–Myosin Interaction in Fast and Slow Skeletal Muscles

The molecular mechanism of the failure of contractile function of skeletal muscles caused by oxidative damage to myosin in hyperthyroidism is not fully understood. Using an in vitro motility assay, we studied the effect of myosin damage caused by oxidative stress in experimental hyperthyroidism on the actin–myosin interaction and its regulation by calcium. We found that hyperthyroidism-induced oxidation of myosin is accompanied by a decrease in the sliding velocity of the regulated thin filaments in the in vitro motility assay, and this effect is increased with the duration of the pathological process.     

Immunohistochemistry of chaetognath body wall muscles

Abstract. A light and electron immunohistochemical study was carried out on the body wall muscles of the chaetognath Sagitta friderici for the presence of a variety of contractile proteins (myosin, paramyosin, actin), regulatory proteins (tropomyosin, troponin), and structural proteins (α‐actinin, desmin, vimentin). The primary muscle (～80% of body wall volume) showed the characteristic structure of transversely striated muscles, and was comparable to that of insect asynchronous flight muscles. In addition, the body wall had a secondary muscle with a peculiar structure, displaying two sarcomere types (S1 and S2), which alternated along the myofibrils. S1 sarcomeres were similar to those in the slow striated fibers of many invertebrates. In contrast, S2 sarcomeres did not show a regular sarcomeric pattern, but instead exhibited parallel arrays of 2 filament types. The thickest filaments (～10–15 nm) were arranged to form lamellar structures, surrounded by the thinnest filaments (～6 nm). Immunoreactions to desmin and vimentin were negative in both muscle types. The primary muscle exhibited the classical distribution of muscle proteins: actin, tropomyosin, and troponin were detected along the thin filaments, whereas myosin and paramyosin were localized along the thick filaments; immunolabeling of α‐actinin was found at Z‐bands. Immunoreactions in the S1 sarcomeres of the secondary muscle were very similar to those found in the primary muscle. Interestingly, the S2 sarcomeres of this muscle were labeled with actin and tropomyosin antibodies, and presented no immunore‐actions to both myosin and paramyosin. α‐Actinin in the secondary muscle was only detected at the Z‐lines that separate S1 from S2. These findings suggest that S2 are not true sarcomeres. Although they contain actin and tropomyosin in their thinnest filaments, their thickest filaments do not show myosin or paramyosin, as the striated muscle thick myofilaments do. These peculiar S2 thick filaments might be an uncommon type of intermediate filament, which were labeled neither with desmin or vimentin antibodies.     

Effect of denervation, reinnervation and hypertrophy on the state of actin filaments in skeletal muscle fibres

The conformational state of actin filaments was studied in the rat soleus muscle atrophying after denervation, recovering following reinnervation, hypertrophying following tenotomy of synergists and in intact muscle. Intrinsic (tryptophan residues of F-actin) and extrinsic (rhodamine-phalloidin or 1,5-IAEDANS attached to F-actin) polarized fluorescence was measured. In parallel, the influence of ATP or NEM on the state of F-actin was studied. The results show that the conformational state of F-actin is changed in all experimental muscles. These changes of the denervated muscle differ from those of the reinnervated and hypertrophying muscles. In the reinnervated muscle, beginning with the first days of recovery, the structure of F-actin seems to "recover" to the state in intact muscle. In the later stage of muscle recovery, the state of F-actin is similar to that in hypertrophying muscle. Differences between the mentioned muscles in the conformational state of actin monomers, in the orientation of monomers and in the flexibility of thin filaments are discussed.     

OBLIQUELY STRIATED MUSCLE : III. Contraction Mechanism of Ascaris Body Muscle

Segments of the obliquely striated body muscle of Ascaris were fixed at minimum body length after treatment with acetylcholine and at maximum body length after treatment with piperazine citrate and then studied by light and electron microscopy. Evidence was found for two mechanisms of length change: sliding of thin filaments with respect to thick filaments such as occurs in cross-striated muscle, and shearing of thick filaments with respect to each other such that the degree of their stagger increases with extension and decreases with shortening. The shearing mechanism could account for great extensibility in this muscle and in nonstriated muscles in general and could underlie other manifestations of "plasticity" as well. In addition, it is suggested that the contractile apparatus is attached to the endomysium in such a way that the sarcomeres can act either in series, as in cross-striated muscle, or individually. Since the sarcomeres are virtually longitudinal in orientation and are almost coextensive with the muscle fiber, it would, therefore, be possible for a single sarcomere contracting independently to develop tension effectively between widely separated points on the fiber surface, thus permitting very efficient maintenance of isometric tension.