JOURNAL OF MORPHOLOGY 206:351-361 (1990)

Interfiber Tension Transmission in Series-Fibered Muscles of the Cat Hindlimb J.A. TRO'ITER Department of Anatomy, University of New Mexico, School of Medicine, Albuquerque,New Mexico 87131

ABSTRACT Several muscles of the cat hindlimb, including biceps femoris and tenuissimus, are composed of short, in-series muscle fibers with tapered intrafascicular terminations. Tension generation and transmission within such muscles requires that active fibers should be mechanically coupled in series via myomyous junctions, specialized connective tissue attachments, or the endomysium. This report establishes that the tapered fibers of the cat biceps femoris and tenuissimus muscles have insignificant numbers of either myomyous or specialized connectivetissue junctions. Tension appears to be transmitted in a distributed manner across the plasmalemma of the tapered (and probably the non-tapered) portions of the fibers to the connective tissue of the endomysium, which is therefore an essential series elastic element in these muscles. Subplasmalemmaldense plaques were identified and may play a role in transmembrane force transmission. In addition to the endomysium, passive muscle fibers may also serve to transmit tension between active fibers, and therefore should also be considered to be series elastic elements. Literature dating back to 1903documents the presence in many vertebrate muscles of fibers that are shorter than the fascicles they comprise and that terminate by tapering within the fascicle (Bardeen, '03; Huber, '16;Adrian, '25; Cooper, '25; Barrett, '62). The more recent studies have clearly shown that the strap muscles of the cat (Richmond et al., '85; Loeb et al., '87), goat (Ganset al., '89), and chicken (Gaunt and Gans, '90) are organized in this way. There is also evidence of tapered fibers in the tibialis anterior muscle of the cat (Ounjian et al., '87). In muscles composed of short tapered fibers, cholinesterase staining has revealed that the motor end-plates occur in many transverse bands arranged in rows parallel to the longitudinal axis of the fascicles (Englishand Weeks, '87; Loeb et al., '87; Gans et al., '89; Gaunt and Gans, '90). In at least some muscles the spacing of the end-plate bands is significantlyshorter than the mean length of the muscle's constituent fibers (Ganset al., '89; Gaunt and Gans, '90). Correspondingly, electromyographic (EMG) recordings made with a concatenated series of bipolar electrodes placed on the surfaces of fascicles have revealed an overlapping arrangement of short fibers that are activated when small divisions of the motor nerve are stimulated (Loeb et al., '87; Chanaud et al., '90). These observations together with those derived from glycogen depletion studies (Lev-tov et al., '88,Eldred et o 1990 WILEY-LISS, INC.

al., '89) support the developing concept that the arrangement of tapered muscle fibers into an overlapping array is a common pattern within vertebrate muscles (Gans, '89). Why some muscles have this architecture is not understood (Richmond et al., '85; Loeb et al., '87; Gans, '89), but the presence of this architectural arrangement raisessome important questionsabout neural control and muscle mechanics, including the mechanism(s) of force transmission. A muscle fascicle composed of fibers that attach at each end to connective tissue through muscle-tendonjunctions (MTJ)is relatively simple to describe in terms of force transmission. In contrast, a fascicle composed of short fibers that terminate by tapering intrafascicularly presents problems because the minimum contractile unit of the muscle must contain at least two muscle fibers that are mechanically coupled in series and presumably activated simultaneously by their motor axons (Gans, '89). Muscle fibers could be connected in seriesby direct mechanicaljunctions (myomyous junctions) between adjacent muscle fibers. Such junctions have been described in some vertebrate striated muscles (Bartels, '86; Torigoe and Nakamura, '871, and are the predominate mode of force transmission in Addreas reprint requests to Dr. John A. 'hotter, Department of Anatomy, University of New Mexico, School of Medicine, Albuquerqwa,NM87131.

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cardiac muscles (Fawcett, '86)and in the focally innervated fibers of cat extraocular muscles ( M a p et al., '75). Alternatively, series muscle fibers could be coupled through specialized connective tissue elements joining junctional regions of fibers in the same motor unit. This arrangement has been suggested as a formal possibility by Loeb et aL ('87) and by Gans ('89). Finally, force could be transmitted across the membrane of active muscle fibers to the overlying connective tissue of the endomysium and thence to other muscle fibers, as suggested by Richmond et al. ('85) and by Loeb et al. ('87). The studies described in this report establish that the latter mechanism is the major one in the biceps femoris and tenuissimus muscles of the cat. MATERIALS AND METHODS Female cats weighing between 1.5 and 2.5 kg were anesthetized with ketamine (Aveco, Fort Dodge, Iowa) given intramuscularly followed by Nembutal (Steris Laboratories, Phoenix, Arizona) given intravenously.The skin of the right hind leg was removed and the exposed biceps femoris muscle was kept moist with physiological saline during subsequent procedures. Longitudinal full thickness incisions were made in the anterior and posterior regions of the muscle to produce strips approximately 2 mm wide by 6 cm long. Sutures were tied to each end of a strip and the limb was positioned so as to place the muscle in short, long, and intermediate lengths, At each length the sutures of one strip were tied to a wooden stick so that the muscle strip would remain approximately a t that length as it was fixed. The strip was then excised from the muscle and placed into 2.5 % glutaraldehyde, 0.2 M sodium cacodylate buffer, pH 7.2 a t room temperature. This produced strips of anterior and posterior biceps femoris muscle fixed at sarcomere lengths representing the full physiological range. After the tissues had been excised, the cats were sacrificed by an overdoseof Nembutal. After approximately 30 minutes the strips were removed from the sticks and sliced with the aid of a dissecting microscope into strips about 1 mm square by 3 mm long for transmission electron microscopy (TEM) and about 1mm square by 2 cm long for scanning electron microscopy (SEM). Fixation was then continued overnight. Specimens for analysis by TEM were soaked for 1hour in fresh fixative of the same composition containing 0.2% tannic acid. They were then rinsed overnight in several changes of 0.2 M cacodylate buffer, postfixed in 1% osmium tetroxide, 0.1 M cacodylate buffer for 1 hour, and rinsed in water for one hr. After dehydration

in a graded ethanol series the specimens were infiltrated with Spurrk resin, which was polymerized for 24 hours at 65OC. Transverse ultrathin sections, approximately 100 nm thick, were produced using diamond knives and were imaged in a Hitachi H600 transmission electron microscope after staining with uranyl acetate and lead citrate. Tenuissimus muscles were prepared similarly, except that the thinness of the muscle made it unnecessary to cut strips. In order to prepare isolated muscle fibers for analysis in the scanningelectron microscope, the strips of biceps femoris that had been fixed overnight were rinsed in several changes of cacodylate buffer for several hours and were then placed in ice cold 50% glycerol,0.02% sodium azide, 0.1 M sodium phosphate buffer pH 7.0 (determined at room temperature). They were gently agitated for several days in the cold room and then stored in this solution at 4°C for up to 6 months. Single fiber segments were prepared from the strips by incubating them for 4 M 0 minutes at 6OoCin 9.6 M HC1. When individual fibers were seen to have begun to separate from the strips the acid was rapidly diluted with a large volume of water at room temperature. Fiber segments with tapered ends were selected with the aid of a dissecting microscope and were placed on freshly prepared, gelatin coated, 12 mm round cover slips. The gelatin was hardened by exposure to 2.5% glutaraldehyde in water, after which the cover slips were passed sequentiallythrough several changes of water and a graded ethanol series. After several changes of absolute ethanol, the fiber segments were dried from CO, by the critical point method. The cover slips were fixed onto aluminum stubs using silver paint, rendered conductive by a thin coating of gold/ palladium deposited in a cold cathode sputter coater, and imaged in a Hitachi S800 field emission electron microscope. RESULTS

Myomyous junctions Transmission electron micrographswere made from transverse sections of biceps femoris muscles containing small fiber profiles, equal to or less than 1.13 x lo-'* m2in area (equivalentto a circular diameter of 12 pm or less). A total of 220 profiles were analyzed from three different cats: 78 from cat 1,49 from cat 2, and 93 from cat 3. The average circular diameter for the analyzed fibers from the three cats were 8.2, 7.5, and 7.8 Nm. The plasmalemma was covered everywhere by basement membrane, and the basement membranes of adjacent fibers were separated from one another by collagen fibrils of the endomysium. No instances were encountered in which

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two adjacent fibers formed a junction comprised of complementary interdigitating folds, or in which the two adjacent basement membranes fused. Thus, no regions were encountered that could be interpreted to represent myomyous junctions. It can be estimated from these data that myomyous junctions constitute less than 0.5% (i.e., less than 1/220) of the length of the tapering region of muscle fibers, suggesting that there is no significantforce transmission through myomyous junctions. Similarly, no myomyous junctions were seen in 65 tapered ends from tenuissimus muscles.

Myotendinous or similar junctions The same biceps femoris fiber profiles analyzed for the presence of myomyous junctions were also analyzed for the presence of regions similar to myotendinousjunctions, in which the terminating portion of the muscle fiber forms deep folds and processes that interdigitate with the connective tissue. One such region was identified in the fibers from each of two cats, and two such regions were encountered in those from the third (Fig. la). From the lumped data it can be estimated that myotendinous-likejunctions represent an average of 1.8% (4/220) of the length of the tapered parts of fibers with circular diameters less than 12 pm. The 65 tapered ends of tenuissimus fibers that were examined showed no evidence of myotendinous-like specializations. Isolated biceps femoris fibers were also analyzed using SEM for the frequency of surface folds that would indicate specialized mechanical junctions. Two of the end tapers of 45 fibers from three different cats had surface projections that resembled those of the MTJ (Fig. lb), but the great majority of fibers tapered without any obvious surface features that could be interpreted as junctional specializations (Fig. 2). These data indicate that the low frequency of junctional specialization encountered in the TEM is due both to the absence of such specializations in most fibers and to the small part of the tapered end occupied by such specializations in those fibers that possess them. Morphology of tapered regions Both SEM and TEM analyses of the tapered portions of muscle fibersshowed that the surface was generally smooth or possessed of shallow longitudinalfolds (Figs. 2b, 3). The sarcoplasmic side of the plasmalemma of most (approximately 85 % ) of the tapered portions analyzed was associated with a discontinuouslayer of electron-dense material (Figs. 3, 4). This material was seen in all regions of the sarcomere, and was

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therefore not associated with any particular sarcomeric band or line. Both tapered and nontapered regions of the fiber surface contained this material (Figs. 3,4). Adjacent to the plasmalemma was a continuous basement membrane about 50 nm thick composed of an electron dense layer about 30 nm thick, the lamina densa, and an electron lucent layer about 20 nm thick, the lamina lucida (Figs. 3,4). The lamina lucida contained irregular filamentous structures that connected the membrane to the lamina densa (Fig. 4).These filaments were more numerous where the plasmalemma was associated with the subplasmalemmal dense bodies (Fig. 4). External to the lamina densa was a matrix comprised principally of small collagen fibrils (average diameter 41 nm, standard deviation 6 nm, N = 41), microfibrils (diameter approximately 10 nm), and fine filamentousmaterial (diameter 5 nm or less) associated with the basement membrane and with the collagen fibrils (Fig. 4). Amorphous elastin structures were infrequently seen (not shown). This matrix separated all muscle fibers throughout their lengths, although it was very thin in places (Fig. 3). The collagen fibrils did not appear to have a preferred orientation (Figs. 3,4). In the scanning electron microscope the tapered portion of isolated muscle fibers did not conform to any specific geometricpattern. Some fibers tapered over a length of several millimeters, within which there were several regions of taper separated by non-tapered intervals. Other fibers tapered over a shorter distance with a more or less constant taper. The cross-sectional shape of the tapered region was also highly variable. Some fibers were more or less circular in cross-section,but others were flattened or irregular. These characteristics make it impossible to measure taper angles with any accuracy. Nonetheless, by measuring the width (or caliper diameter) of the fibers at defined intervals along the length, it was possible to estimate that the fibers tapered with an angle on the order of.'1 DISCUSSION

Previous investigations with the light microscope have shown that muscle fibers of cat biceps femoristaper over about 50% of their length (25% at each end) (Chanaud et al., '90). The Sarcomere banding pattern of the tapered sections is the same as that in the body of the muscle fiber, and indeed, the sarcomere spacing in the tapered portion of a muscle fiber tends to be similar to that in a larger fiber to which the tapered end is closely applied (Richmond et al., '85; Loeb et al., '87). The present results are

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Fig.1. a: Transmiasion and b scanning electron micrographs of biceps femoria fibers that have a folded surface morphology simii to that of the MTJ. Figure la is a crosssection from a region of a fiber approximately equivalent to that marked in Figure lb, showing a process at the right of Figure l a that has separated from the body of the fiber. The

tapered end of the fiber contains a dense subplasmalemmal plaque (large arrowhead),and is completely enveloped by a basement membrane (small arrowhead) and endomysium containingcollagenfibrik (largearrow)andmicrofibrils(small arrow).Bar = 0.5pm in Figure la. and 10/~m in Figure lb.

TENSION TRANSMISSION IN SERIES-FJBEREDMUSCLES

Fig. 2. Scanning electron micrographs of isolated biceps femoris fibers. a: The low magnificationmicrographshows a fiber tapering from a diameter of 24 gm at the upper right of the Figure to its terminationat the lower left, over a distance

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of about 2 111111.b: Part of the middle of the taperedregion is shown. The fiber surface is generally smooth, with a shallow longitudinal groove, and a visible sarcomere pattern. Bar = 100pm in Figure 2%and 10pm in Figure 2b.

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Fig.3. Transmiesion electron micrograph of a t y p i d fiber end containingsubphmalemmd dense material (large arrowhead) and enveloped by a basement membrane (small arrowhead) and endomysium containing collagen fibrils (large arrow)and mimfibrila (small arrow). Bar = 1.0pm.

consistent with these observations, and support the notion that the tapered segments of muscle fibers have contractile properties identical to those of the non-tapered segments. Accordingly, if the force generated by a muscle fiber were not transmitted across the fiber surface of the tapered region, the myofilaments would be required to carry higher loads as the fiber decreased in diameter. The tapered portion of the fibers must therefore be involved in force transmission, which is why the studies described in this report were focused on this region. This is not to say that force transmission across the fiber surface occurs exclusively in the tapered region. Indeed, there are reasons to believe that non-tapered regions are also capable of lateral transmission of force (see below), as has been proposed previously (Street and Ramsey, '65; Street, '83). The present analysis revealed no myomyous junctions in transverse sections of 220 biceps

femoris fibers with cross-sectional areas corresponding to circular diameters of 12 pm or less. Because each fiber was randomly selected and each section was randomly taken through the tapered part of the fiber, it can be estimated that myomyous junctions, if they exist at all in these fibers, cover less than 0.5% of the total length of these 220 tapered ends (Zar, '84).Similarly they can be estimated to occur over less than 1.5%of the tapered ends of tenuissimus fibers. The rarity of such junctions means that they cannot represent a significant mechanism of force transmission. It is thus inferred that contractile force is transmitted across the tapered fiber surface directly to the endomysial connectivetissue. Given that force is transmitted to the connective tissue, it is important to know whether force transmission occurs in restricted regions of the fiber surface that are specialized for this function, as is the MTJ, or whether it occurs more generally over the plasmalemma of the tapered

TENSION TRANSMISSION IN SERIES-FIB-

Fig. 4. Transmission electron micrographs of the endomysium between adjacent biceps femoris fibers. a: Portions of a tapered end and nontapered part, respectively, of two fibers are shown at the bottom and top of the Figure. The subplasmalemmaldense plaques (large arrowhead) are associated with a concentration of transverse connecting threads in the lamiia lucida portion of the basement membrane (mediumarrowhead).Theendomyaiumcontainsfine filamen-

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tous material (small arrowheads) in addition t~ collagen fibrils and other amorphous or reticular material. b The space between two nontapered fiber regions is illustrated. Note the subplasmalemmal dense plaque (large arrowhead) with its associated connecting filamentsin the lamina lucida (medium arrowhead). In the endomysium are collagen fibrils (arrow) and fine filamentous material (4 arrowheads). Bar = 0.25 pm in bothmicrographs.

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region. Several studies have shown that the MTJ of vertebrate muscle fibers is characterized by three-dimensional folding of the surface. This surface folding significantly increases the area of the muscle-tendon interface and concomitantly reduces the stress to which the interface is subjected (Mackay et al., '69; Eisenberg and Milton, '84; Trotter et al., '85a,b; Tidball and Daniel, '86). It also produces a mechanical junction that is loaded almost exclusively in shear, which is inherently stronger than one loaded in tension (Mackay et al., '69; Tidball, '83; Trotter et al., '83a). Because this type of junction is always found at the interface between muscles and tendons of vertebrates, the question naturally arises whether the tapered intrafascicularends of muscle fibers have a similar morphology. The TEM data have shown that an average of 1.8%of the tapered length of biceps femoris fibers, and less than 1.5% of the equivalent region of tenuissimus fibers, are morphologically similar to the folded shape of the MTJ. The complementary SEM data have shown that most fibers lack any MTJ-like regions, and that such regions occupy only a small portion of the fiber surface on the tapered ends of those that possess them. Consequently, MTJ-like folding of the fiber surface cannot represent a significant mechanism for force transmission. The functional significance of the small quantity of MTJ-like folding that exists on a s m a l l fraction of biceps femoris fibers is unknown. Because there are insignificant quantities of myomyous or MTJ-like junctions, the transmission of force must occur generally across the cell surface of the tapered regions. Thus i t is interesting to note that tapering itself produces both of the major functional consequences of the folded morphology of the MTJ, but does so without folding. Because the taper angle is small (on the order of lo), the tapered ends have a large surfacebase ratio. For example, a right circular cone with a taper angle of lo has a surfacehase ratio of 57. Moreover, for any segment of such a cone, the ratio of the surface area of the segment to the difference in cross-sectional area of the two ends of the segment remains constant (i.e., 57 for a 1' taper). Since the difference in crosssectional area is proportional to the amount of force that must be transmitted across the surface area of the segment, this ratio is also the factor by which the stress at the interface is reduced. Further, a taper angle of about 1' produces an interface that is loaded almost completely in shear, since the shear load parallel to the plane of the membrane is proportional to the cosine of the angle whereas the tensile load nor-

mal to the plane of the membrane is proportional to its sine (Lubkin, '57). As discussed below, it seems likely that tension is transmitted not only at the tapered ends of the fibers, but also in their non-tapered regions. Assuming that this is the case, a principal function of fiber tapering may be to prevent the sort of stress concentrations that occur at the ends of the rigid members of glued lap joints (De Bruyne and Houwink, '51). In the absence of tapering, the extracellular matrix would need to carry higher loads near the ends of the fibers than elsewhere, and would thus need to be reinforced, as occurs in the MTJ. Because of the tapering of the fiber ends, however, the load borne by the matrix is proportionately reduced until, a t the end of the fiber, it is negligible. The morphology of the tapered and nontapered fiber surface of the cat fibers examined in this study is consistent with a distributed mechanism of force transmission. Beneath the plasmalemma of most fibers is a discontinuous layer of electron-densematerial. A similar entity associated with the membrane of the MTJ is thought to be important in the mechanical linkage between actin filaments and basement membrane, via transmembrane proteins (Trotter et al., '81, '83a,b; Bozyczko et al., '89; Swasdison and Mayne, '89). At both the MTJ (Hanak and Bock, '71; Korneliussen, '73; Ajiri et al., '78; Trotter et al., '83b) and the subsarcolemmaldensities described in the present report, the lamina lucida is enriched in fine filamentous connections between the plasmalemma and the lamina densa. Such connections are common components of basement membranes, including those of skeletal muscle (Fawcett, '81; Bonilla, '83), and they are probably composed of one or more of the molecules that make up the basement membrane (Inoue and Leblond, '88). The MTJ has been shown to contain a number of macromolecules thought to function in transmembrane mechanical connectionsbetween actin filaments and components of the extracellular matrix, including vinculin (Shear and Bloch, '85; Bozyczko et al., '89); talin ( T i d b d et al., '86; Rochlin et al., '89; Bozyczko et al., '89); integrin (Bozyczko et al., '89; Swasdison and Mayne, '89); and alpha actinin (Trotter et al., '83a) (see, however, Tidball, '87). The lateral surfaces of tonic muscle fibers in chickens also contain concentrations of connecting filaments in the lamina lucida associated with subsarcolemmal densities that stain positivelywith antibodies to vinculin (Shear and Bloch, '85; Bozyczko et al., '89) and integrin (Bozyczko et al., '89). Twitch fibers may contain similar elements (Pardo et al., '83; Bozyczko et

TENSION TRANSMISSION IN SERIES-FIB-

al., ’89), which may in some cases be localized into periodic belts that Pardo et al. (’83)called “costameres.” At least some of the muscles used in the above studies consist of in-series muscle fibers (Gaunt and Gans, ’90; Gaunt, personal communication). Mammalian skeletal muscle fibers have also been stained on their lateral surfaces with antibodies to alpha actinin (Endo and Masaki, ’84) and dystrophin (Hoffman et al., ’89).These studies, together with the expanding literature on the compositionof adhesion plaques in a variety of cell types (Burridgeet al., ’871, are consistent with the notion that the subplasmalemmal densities of tapered cat fibers are sites of tension transmission.All the evidence is correlative, however, and it will be important in the future to devise direct tests of this hypothesis, paying special attention to differences that may relate to the architectural form of the muscle. The hypothesis that force is transmitted t o the “sarcolemma” all along the fiber length has been proposed before (Street and Ramsey, ’65; Street, ’83; Pardo et al., ’83). Street (’83) has provided evidence for “lateral transmission of tension” in frog semitendinosus muscle fibers, and has discussed possible mechanisms for internal loading of the “sarcolemma” through cytoskeletal connections between myofilaments and the plasma membrane. A mechanical coupling between the myofilaments and pericellula matrix of frog muscle fibers is also suggested by the mechanical experiments of Tidball(’86) on isolated muscle fibers in rigor. Since the endomysium has been shown by the present study to be an essential elastic element in series-fibered muscles, its organization and compositionare clearly important. The endomysium of the cat muscles contains small collagen fibrils that appear to have no preferred orientation, associated with fine filamentous material, small, sparsely distributed elastin structures, and microfibrils. Structural studies on the interfiber material in other skeletal muscles have shown essentiallysimilarpictures, even in muscles where all fibers presumably end in regular MTJ (Schmalbruch, ’74; Swatland, ’75; Borg and Caulfield, ’80;Rowe, ’81;Bonilla, ’83).Both biochemicaland immunostainingstudies have identified collagen types I, 111, IV,V (Mayne and Sanderson, ’851, and VI (Linsenmayeret al., ’86) in the endomysium. Type I11collagen is likely to be the major collagen of the fibrils (Mayne and Sanderson, ,851, and type VI collagen may be an important component of the fibril-associatedfilaments. Recent studies suggest that type VI collagen may play an important role in the structural integration of fibrillar collagen matrices

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(Bonaldo et al., ’90); it may serve in the present instance to bind fibrils to the basement membrane and to one another. Fibronectin, which has also been localized in the endomysium (Mayneand Sanderson, ’85),may also help bind. The cohesive fibril-reinforced matrix formed in this way may constitute a continuous or interconnected network that effectively transfers load between the fibers which are embedded in it. Structural studies on cardiac muscle have also shown that finely filamentous material acts to intepate the bundles of collagen fibrils located between myocytes, and that the intermyocyte matrix plays an important role in cardiac mechanics (Borg et al., ’83; Robinson et al., ’83; Frank and Beydler, ’85; Yung and Frank, ’86; Ohayan and Chadwick, ’88). Type IV collagen, heparan sulfate proteoglycan, laminin, and merosin (Leivo and Engvall, ’88) occur between skeletal muscle fibers, principally in the basement membrane (Mayne and Sanderson, ’85). All these molecules, and probably others as well, may contribute to the elasticity in series with the tapered muscle fibers. A more detailed study of the intercellular matrix of these muscles is clearly warranted. The results of the present study establish that the mechanical properties of the muscle are determined both by the muscle fibers and by the connectivetissue associatedwith them. The presence of this extracellular series elasticity in the muscle would be expected to introduce a series compliancethat probably accounts for the small discrepancy between changes in fascicle length and sarcomere length that has been noted in the biceps femoris (Chanaud et al., ’90). Another important source of internal compliance in series-fibered muscles may reside in the inactive muscle fibers. It has been shown for cat tenuissimus (Lev-tov et al., ’881, and may be generally true of series-fiberedmuscles, that the muscle fibers that belong to a single motor unit are frequently adjacent mostly to fibers of other motor units,and thus often have no members of their own motor unit as nearest neighbors. This arrangement includes the tapered ends of the fibers as well as the non-tapered regions. Thus several fibers that belong to other motor units may intervene between the complementarily tapering portions of active fibers. Yet, in tenuissimus, stimulation of a single motor neuron produces tension with a fast rise-time at the ends of the muscle (Lev-tov et al., ’88). The non-adjacency of the tapered fibers of a motor unit thus requires that the tension generated by active muscle fibers must be transmitted not only through the endomysium, but also, at least later-

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ally, through passive fibers. Although specific mechanisms for such tension transmission have not yet been identified,it is possible that passive fibers could serve as series elastic elements. This would require all fibers in a muscle region to be coupled by the endomysium so as to maintain passive fibers a t the same sarcomere length as active fibers. It would also require passive fibers to possess an adequate shear stiffness, since the loads applied to them by active fibers would be shearing loads. Efforts are presently underway to test these hypotheses.

De Bruyne, N.A., and R. Houwink (1951) Adhesion and Adhesives. New York Elsevier Science Publishing Co. Eisenberg,B.R., and RL. Milton (1984) Muscle fiber tennination at the tendon in the frog's sartorius: a stereological study. Am. J. Anat. 171:27%284. Eldred,E., M. Ounjian, R.R Roy, and R.V. Edgerton (1989) Geometricpatterns of tapering in fast muscle fibers of cat tibialis anterior.Soc. Neurosci. Abs. 15:521a Endo, T., and T. Masaki (1984) Differential expression and distribution of chicken skeletal- and smooth-muscle-type alpha-adinins during myogenesis in culture. J. Cell Biol. 9923222332, English, A.W., and O.L. Weeks (1987) An anatomical and functionalanalysis of cat biceps femoris and semitendinosus muscles. J. Morphol. Z91:161-175. Fawcett, D.W. (1981) The CelL Philadelphia:W.B. Saunders ACKNOWLEDGMENTS Company. Fawcett, D.W. (1986) A Textbook of Histology. Philadelphia: The technical aspects of the studies described W.B. Saunders Company. in thii report were expertly carried out by Ray- Frank,J.S., and S. Beydler (1985)Intercellular connectionsin rabbit heart as revealed by quick-frozen,deep-etched,and anne Ozbaysal. The collaborative efforts of Gerrotary-replicated papillary muscle. J. Ultrastruct. Res. 90: ald Loeb and Carl Gans are gratefully acknowl18S193. edged, as are the helpful discussionswith Cheryl Gans, C. (1989) Consideringfunctionalcompartmentsin musChanaud and Frances Richmond. This work was cle. Behav. Brain Sci. 12:654-655. supported by NIH grant AR39922. Portions of Gans, C., G.E. Loeb, and F. De Vree (1989) Architecture and consequent physiological properties of the semitendinosus this publication have appeared in abstract form muscle in domestic goats.J. Morphol. 199287-297. (Trotter, '90;Trotter and McGuffee,'90). Gaunt, A.S., and C. Gans (1990) Architecture of chicken muscles: short fiber patterns and their ontogeny. Trans. R. LITERATURE CITED Soc. Lond. (B) 240:351-362. 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