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Department of Medicine, Baylor College of Medicine, Houston, Texas 77030
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ABSTRACT |
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Abstract
Introduction
Methods
Results
Discussion
References
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Boriek, Aladin M., Charles C. Miller III, and Joseph R. Rodarte. Muscle fiber architecture of the dog diaphragm.
J. Appl. Physiol. 84(1): 318-326, 1998.
Previous measurements of muscle thickness and length ratio of
costal diaphragm insertions in the dog (A. M. Boriek and J. R. Rodarte.
J. Appl. Physiol. 77: 2065-2070,
1994) suggested, but did not prove, discontinuous muscle fiber
architecture. We examined diaphragmatic muscle fiber architecture using
morphological and histochemical methods. In 15 mongrel dogs, transverse
sections along the length of the muscle fibers were analyzed
morphometrically at ×20, by using the BioQuant System IV
software. We measured fiber diameters, cross-sectional fiber shapes,
and cross-sectional area distributions of fibers. We also determined
numbers of muscle fibers per cross-sectional area and ratio of
connective tissue to muscle fibers along a course of the muscle from
near the chest wall (CW) to near the central tendon (CT) for midcostal
left and right hemidiaphragms, as well as ventral, middle, and dorsal
regions of the left costal hemidiaphragm. In six other mongrel dogs,
the macroscopic distribution of neuromuscular junctions (NMJ) on
thoracic and abdominal diaphragm surfaces was determined by staining
the intact diaphragmatic muscle for acetylcholinesterase activity. The
average major diameter of muscle fibers was significantly smaller, and
the number of fibers was significantly larger midspan between CT and CW
than near the insertions. The ratio of connective tissues to muscle
fibers was largest at CW compared with other regions along the length
of the muscle. The diaphragm is transversely crossed by multiple
scattered NMJ bands with fairly regular intervals offset in adjacent
strips. Muscle fascicles traverse two to five NMJ, consistent with
fibers that do not span the entire fascicle from CT to CW. These
results suggest that the diaphragm has a discontinuous fiber
architecture in which contractile forces may be transmitted among the
muscle fibers through the connective tissue adjacent to the fibers.
connective tissue; neuromuscular junction; tension transmission
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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THE MECHANICAL PROPERTIES of the diaphragm are determined both by muscle fiber architecture and the connective tissue surrounding the muscle fibers. It is now well recognized that fiber architecture has a significant effect on the length-tension relationship, shortening velocities, and force-developing capabilities of the muscle (12). Our previous anatomic study of the dog diaphragm (2) has demonstrated that muscle thickness is essentially constant along the length of the relaxed midcostal region. Furthermore, the lengths of costal muscle insertion on the chest wall (CW) and its origin on the central tendon (CT) were significantly different. These data are consistent with discontinuous fiber architecture. In such an architecture, either not all fibers span the entire length of the muscle from CW to CT or the number of fibers per cross-sectional area must increase and/or the diameter of muscle fibers must decrease as fascicles run from the broader CW length of insertion to the narrower CT length of origin.
To further explore the fiber architecture of the dog diaphragm, we performed morphological and histochemical studies. These studies address issues of fiber architecture and distribution of neuromuscular junctions (NMJ) on the surfaces of the diaphragm. From these, we inferred the mechanism of tension transmission between muscle fibers and connective tissue. Using morphological techniques, we examined the fiber architecture, including determining the number of fibers per cross-sectional unit area and ratio of connective tissue to muscle fibers, as well as measuring major fiber diameter, fiber cross-sectional shape, and fiber cross-sectional area along the course of muscle from CT to CW. Using histochemical methods, we examined the NMJ distribution on the thoracic and abdominal surfaces of the costal and crural regions of the diaphragm. Both morphological and histochemical studies supported the hypothesis of a discontinuous fiber architecture in which tension generated by active muscle fibers in the diaphragm is transmitted not only along fibers spanning from CT to CW but also among nonspanning muscle fibers through the connective tissue surrounding those fibers.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Muscle morphology. Fifteen adult mongrel dogs, weighing from 13-17 kg each, were killed by intravenous pentobarbital sodium. The diaphragm and ribs of insertion were removed together and placed in a physiological saline solution. In eight dogs, we excised portions of the midcostal region of the left and right hemidiaphragms. In seven other dogs, we excised portions of the ventral, middle, and dorsal regions of the left costal diaphragm. Strips, ~2 in. wide, were cut along the muscle fascicles from the CW to the CT at their relaxed (unstressed) length. These strips were pinned along their edges to a sheet of cork and placed overnight in a 5% formaldehyde solution. Sections were cut for analysis from midspan between CT and CW and from ~4 mm from CT origin or CW insertion. Schematics of excised diaphragms, viewed from the abdominal surface, show locations of 2-in.-wide strips in the midcostal regions of the left and right hemidiaphragms (Fig. 1A) as well as the ventral, middle, and dorsal regions of the left hemidiaphragm (Fig. 1B). The pinned edges of these strips and the location of the analyzed samples from near insertion on CW, midway between origin and insertion (M) and origin on CT are also shown. Care was taken to cut the samples, ~1 cm wide, 1.5 cm from the area where the muscles were pinned. Samples taken were placed in histological cassettes, cut into transverse sections, and stained by using a trichrome staining technique. This technique allowed intracellular structures of different architecture and composition to be visibly varied: the muscle fibers and connective tissue of our samples stained red and blue, respectively (1).
Stained sections were analyzed using light microscopy (×20 magnification). The BioQuant System IV software (R & M Biometrics, MEGm, vol. 305, version 2.10.92) was used to create a grid of 42 points defined as a field of vision. For each section, 20 fields were analyzed according to the following protocol. Our measurements were started at the top right-hand corner of each section and moved horizontally to analyze every fourth field. We skipped every other row and stayed at least one field of vision away from the boundaries of the section. Within each field, the number of points falling on muscle fibers, connective tissue, and other (nonfiber, nonconnective tissue) material was counted. From these data, we were able to compute an estimate of the fractional content of connective tissue vs. muscle fiber (Pct-f), the percentage of nonfiber and nonconnective tissue components (blood vessels, nerves, fat cells, space, and so on), and fiber numbers per unit cross-sectional area. The Pct-f was computed by dividing the number of grid points lying on connective tissue by the number of grid points lying on muscle fibers. Number of fibers per unit cross-sectional area was computed simply by dividing the number of fibers for each field by the area of the field and then averaging these values for all analyzed fields. Within each field, we chose the five fibers nearest to five preselected points of the grid. Along the course of the costal diaphragm muscle, we determined fiber cross-sectional areas, major fiber diameter, and shape of the cross-sectional profile of the fiber (Sfibers). The major fiber diameter is defined as the greatest length across a transverse cross-section of the fiber. Sfibers is the ratio of the measured perimeter of a transversely sectioned fiber to the circumference of a circle having the same area as that of the fiber cross-sectional area. We use the term "muscle tissue" to include muscle fibers, connective tissue, and nonfiber and nonconnective tissue components. We computed the total fiber areas per unit area of muscle tissue at CT, M, and CW as follows. We measured the cross-sectional areas of the fibers at the specified grid points and averaged them for all fields for each slide. To compute a measure of total fiber area per unit area of muscle tissue for each position, we multiplied the average area of fibers per slide by number of fibers per slide and then divided the resulting value by the total area of all analyzed fields.
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i 
X/s,
where
i is
the ith observation on variable X (area, perimeter, major diameter,
shape factor), is the overall
mean for variable X, and
s is the SD of
X. Statistical analyses were then
performed on the normalized data by four-way analysis of variance
(ANOVA) with repeated measures. Using BioQuant software, as described
above, we analyzed 100 fibers on each slide for each position along a
muscle bundle. The measurements of each of the variables analyzed
(perimeter, major diameter, fiber cross-sectional area, shape factor)
for each of the 100 fibers were recorded as separate variables (sample)
and were handled as an adjustment effect in the ANOVA model. The dog
identifier was handled as a subject effect, and side and position on
the muscle were handled as main and crossed fixed effects. SAS Proc GLM
was used to perform the analysis. Pairwise comparisons of significant
fixed effect levels were made by linear contrast.
Fiber number and Pct-f were
measured in the same places as the morphological variables, but each
location was sampled once for each field of vision. Data were analyzed
by two-way ANOVA with repeated measures. Because only one measurement
was made at each location, no effect for sample was necessary. The dog identifier was handled as a subject effect as in the analysis described
above.
Because highly significant differences were not found between right and
left midcostal regions in the first set of studies described above, we
limited further studies of regional effects to ventral-middle-dorsal
muscular regions in the left costal diaphragm. These studies of
regional effects were undertaken in the second group of seven dogs. The
same model was used, except that the term for side was eliminated, and
a three-level term (ventral, middle, dorsal) for location was added.
Fiber number and connective tissue-fiber ratios were evaluated for this
study as described above.
NMJ histochemistry. An established technique was used for histochemically testing the intact muscle for acetylcholinesterase activity (10). Six adult mongrel dogs, weighing 13-17 kg each, were killed by pentobarbital sodium overdose. Within 5 min of death, we removed the diaphragm with care to preserve the integrity of the muscle close to its attachments on the rib cage and vertebral column. The entire diaphragm was sutured onto a rigid polyester screen to limit shrinkage and distortion during subsequent processing. Muscles were incubated overnight in 0.5 mg/ml acetylthiocholine iodine in 0.1 M phosphate buffer (pH 6.4) containing 0.1 M sodium citrate, 30 mM cupric sulfate, and 5 mM potassium ferricyanide at room temperature. The samples were then rinsed in distilled water and counterstained in 1% ammonium sulfide until groups of end plates appeared as brown spots. Care was taken to avoid too long a reaction time in the acetylcholinesterase stain because the reaction product tends to diffuse and migrate to the connective tissues adjacent to the fibers (6). The reaction was blocked by rinsing the muscle in distilled water and immersing it in 5% formaldehyde solution.
To obtain the distribution of NMJ of the diaphragm on the thoracic and abdominal surfaces, we put a transparent plastic sheet on the flat excised diaphragm, and the locations of the NMJ were traced along 480 fascicles in the costal and crural regions. Those fascicles were arranged as follows: 32 fascicles on the costal regions and 8 on the crural regions of each of the surfaces of the diaphragm (abdominal and thoracic) for each of the six dogs. The relative distribution of these muscle fascicles was kept essentially the same across all dogs. The lengths of the fascicles were measured by a flexible scale from the tracings in the same positions from which NMJ was traced. Measuring the distance from each muscle insertion to the nearest row of NMJ assessed NMJ spacing in the CT and CW locations along the muscle bundle. In the M position, NMJ spacing was computed by measuring NMJ-to-NMJ distances and then taking the average distance as the measure. In a previous work (2), we considered the potential for bias due to placing the diaphragm flat on a flat surface. Briefly, we made radial cuts along muscle bundles in 13 regions throughout the diaphragm and found no opened gaps between the sectors, probably because there was enough friction to prevent any sliding. The cuts made a negligible difference in the perimeter measurements of the origin on CT and of the insertion on the CW. We have measured the lengths of the muscle bundles at the locations where the cuts were made and found them to be the same as the corresponding lengths of the sections of the cuts. This suggested that the surface area of the intact diaphragm which was laid flat is the same as the sum of the surface areas of the 13 wedge-shaped segments. To illustrate the methods in simple form, we show in Table 1 the lengths of four well-identified muscle fascicles and the lengths of the multiple NMJ bands from the midcostal region of the left hemidiaphragm from one dog. The longest fascicles of the midcostal region were crossed by a series of four or five short NMJ bands spaced ~0.7-0.8 cm apart.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Fiber architecture.
The first set of data presented are solely for the first group of eight
dogs in which we studied only the midcostal regions of the right and
left hemidiaphragms. Main effects of position along the muscle bundle
(CT, M, CW) were highly significant for all four standardized
morphometric variables. The side of the diaphragm (left vs. right
hemidiaphragm) was not significant for shape factor and major diameter
and was marginally significant for perimeter and fiber area. Total
fraction of muscle fiber area was not significantly different along the
midcostal muscle fascicles at CW, M, and CT for the left or right
hemidiaphragms. These were 0.65 ± 0.11, 0.68 ± 0.15, and 0.70 ± 0.11 along the CW, M, and CT for the right hemidiaphragm and 0.68 ± 0.14, 0.67 ± 0.14, and 0.72 ± 0.13 at the same positions
on the left hemidiaphragm. Normal SD for variables representing area,
perimeter, major diameter, and shape factor are listed in Table
2. Because observations are normalized to
the global mean for the diaphragmatic muscle, negative values represent
areas that are smaller than the overall mean for the variable. For
example, in Table 2, the value 0.365 for area in the M of the
right costal diaphragm indicates that fiber cross-sectional area at
this position is 0.365 SD less than that of the average midcostal
(right and left hemidiaphragms) fiber area. In general, all positions
differ from one another, with CT being the largest, CW being the next
largest, and the midspan region between CT and CW being the smallest,
except for shape factor in the right costal regions, where the factor
is smallest at the CW.
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Distribution of NMJ. The surface distribution of acetylcholinesterase-stained end-plate zones was determined to reveal NMJ. Photographs of the abdominal and thoracic surfaces of the midcostal region of the diaphragm of a mongrel dog (17 kg) are shown in Fig. 4, a and b, respectively. Magnified sections of the midcostal region of the diaphragm are shown in Fig. 4, c and d, for the abdominal and thoracic surfaces, respectively. The darkly stained bands are acetylcholinesterase-positive zones associated with the NMJ of innervating motoneurons. The photomicrographs illustrate that NMJ are numerous and may number as many as six across the muscle from origin on the CT to its insertion on the CW. The multiple end-plate bands that cross the muscle run in an irregular pattern. These bands are scattered across most of the muscle surface, as would be required to innervate the short muscle fibers distributed at various positions along the course of the muscle. Within each muscle strip, the longitudinal interval between NMJ bands is fairly regular but offset from the adjacent strips. A line parallel to the fascicles traverses two to six NMJs distributed along the muscle from the CW to CT. In the costal and crural regions of the diaphragm, the number of NMJ appeared to be directly proportional to muscle length.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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It has been generally accepted that, in mammalian strap muscles, the myofibers run the full length of the muscle (5, 9). With this muscle fiber architecture, the fiber would have uniform tension transmitted through a myotendinous junction at each end. The muscle would be innervated by a single neural junction near the central region of the fiber. Initial results of experiments using acid dissection in the cat and dog showed that the fiber ran full length of the diaphragmatic muscle (13). In these experiments, the investigators used 30% concentration of nitric acid, and they did not use the gold chloride technique; therefore, distinguishing the short single fibers from the long spanning ones may have been difficult.
Our previous study on the anatomy of the diaphragm of the dog (2) demonstrated that muscle thickness of the costal region along muscle fascicles was essentially constant but that the length of the origin on the CT was only ~60% of that of the insertion on the CW. This macroscopic structure would be inconsistent with the diaphragm being composed of cylindrical spanning myofibrils, unless nearly 40% more of the tissue near the CW insertion were not muscle. The data from this study was consistent with many of the muscle fibers of the costal diaphragm being shorter than the length of the muscle from CW to CT. A previous study of the cat diaphragm (8) showed that only ~40% of the muscle fibers spanned from CW to CT and that multiple bands of NMJ were strewn throughout the muscle, as would be expected if the fibers of various lengths and origins were innervated near midspan between CT and CW.
There was a slight increase in connective tissue near the CW of the costal diaphragm, perhaps related to the increase in the shear strains in that region (3); the total percentage of muscle along the fascicle is essentially constant. Because muscle thickness is constant from CW to CT, the volume of the muscle per unit length of the diaphragm should also be constant from CW to CT. However, as illustrated in Fig. 3, A-C, the number of fibers per unit length of diaphragm cross-sectional area is systematically greatest in the midspan between CT and CW, and cross-sectional area and major diameter of fibers are systematically smallest in midspan region of the muscle. These data suggest that the dog diaphragm may not be composed of spanning fibers that taper from the CW to the CT. The number of fibers per millimeter squared near the CW and CT were not significantly different. Because muscle thickness is constant along the length of the fascicle, these morphometric data can only be explained by fibers originating from CT or fibers inserting on CW and tapering in midspan region, with or without additional nonspanning fibers that may reach neither CT nor CW. Therefore, there is a region along the length of the muscle, presumably the midspan region, where there is overlapping between those nonspanning fibers that originate on the CT and those nonspanning fibers inserting on the CW. Because the proportion of connective tissue to muscle fibers is smallest at M and fiber number per unit cross-sectional area of muscle tissue is greatest at M, the thickness of endomysial component of connective tissue surrounding the single fibers may therefore be smaller at M than near CT or CW. Contractile forces generated by active muscle fibers could be transmitted through the delicate connective tissue, especially in the overlapping regions of the muscle fibers.
From the assumptions that a single myofibril has only one myoneural junction and that myoneural junctions are located near the middle of the myofibril, then the large number of NMJ for each grossly visible muscle bundle as shown in Fig. 4 suggests that a muscle bundle is composed of fibers in a variety of lengths. The separation between origins of the muscle and the first NMJ and between NMJ on the same fascicle is on the order of 1 cm, with no consistent differences between the abdominal and thoracic surface of the diaphragm or in NMJ spacing along the fascicle. From ventral to dorsal, muscle fiber bundles increase in length, and it is therefore possible that the proportion of spanning fibers to nonspanning fibers decreases from ventral to dorsal. This would be consistent with our observation of a tendency of an increased number of NMJ as well as an increased separation between them as one moves from ventral to dorsal. It is clear that the number of NMJ is directly proportional to the length of the muscle as shown in Fig. 5. From Fig. 5, we are able to extrapolate an average muscle length value for a hypothetical muscle bundle with a single NMJ. This number would represent the average length of a muscle bundle composed of fibers that span the entire length of the bundle. If the diaphragm were composed of fibers that spanned from the CW to the CT, the length of the diaphragm muscle would be ~2.7 cm in the costal region and 3.2 cm in the crural region. These data also suggest that the longer the muscle bundles, the greater is the likelihood that the bundles are composed of nonspanning fibers. Therefore, the ventral region of the diaphragm, where the muscle is shortest, is expected to have a greater percentage of spanning fibers than the midcostal region, where the muscle is longest. This is consistent with the data in Fig. 6A, which show that the NMJ spacing in the ventral region is significantly less than that in the middle or dorsal regions of the costal diaphragm.
The data in Fig. 6, B and C, show that NMJ spacing is greater near the CW insertion than near either the CT or the M of the costal or the crural muscle bundle. This may suggest that muscle fibers inserting on CW are longer than those originating from CT. However, the difference in NMJ spacing between the CW and other regions along the bundle appears to be greater in the crural muscle than in the costal muscle. Because muscle bundles is longer in the crural than in the costal region (2), it is likely that the length of single fibers inserting on CW in the crural region is greater than those inserting on CW in the costal region. It is possible also that the fraction of connective tissue to muscle in the crural region may be greater than that of the costal region near the CW insertion. There are two other possibilities: 1) the percentage of nonspanning crural muscle fibers that insert on the CW may be less than the percentage of those that originate from the CT, or 2) crural muscle fibers that insert on CW could be longer than those that originate from CT.
The observation that NMJ spacing was greater in the right than in the left costal hemidiaphragm may suggest that in general the length of nonspanning fibers may be longer in the right costal hemidiaphragm than in the left costal hemidiaphragm. An average separation of ~1 cm between NMJ does not mean that the average length of myofibrils is in that range. Preliminary results (4) from dissection of single fibers confirm that fibers are ~3 cm long. Any single motor unit may innervate on the order of 100 motor end plates, but we have no data on whether the NMJ of a motor unit would be located longitudinally along the fascicle, in a row of NMJ in the same region, or some combination thereof.
It is possible that there is a correlation between NMJ distribution and fiber arrangement between the CW and CT. However, it is difficult to understand the significance of the arrangement of NMJ without more a detailed knowledge of muscle fiber architecture . In the costal and crural regions of the diaphragm, the band of NMJ closest to the CT may serve nonspanning fibers that originate from the CT. The most peripheral band of NMJ near the CW may serve nonspanning muscle fibers that insert on the CW. The midspan band of NMJ may serve either fibers that span the entire length of the muscle from CW to CT or possibly nonspanning fibers that may not insert on CW or originate from CT. There were four to six bands running transversely across muscle fibers. It is therefore possible that muscle fibers may not be necessarily innervated at their middle portion. The data of Trotter and Purslow (16) on quail pectoralis muscle, however, supported the general model of the arrangement of short muscle fibers: each fiber, innervated in its middle one third, passes through the next two NMJ bands in both directions without being innervated.
The original descriptions of series architecture in skeletal muscle were of the long limb muscles of a medium-size animal such as a goat (6). A teleological explanation for discontinuous fiber architecture is that myoelectrical transmission along a fiber is slow, ~4.5 m/s. If the myoneural junction were located in the middle of a muscle 0.45 m long, it would take 50 ms for depolarization to reach the two ends of the muscle, and by that time the central region would be mostly relaxed. The quiescent part of the muscle would constitute series elastance, and twitch tension would be greatly impaired. However, the diaphragmatic muscle fibers of the dog are all <0.09 m long, and transmission time should not be a problem. There are geometric considerations, however, that would make nonspanning fibers mechanically advantageous. Assume that the diaphragm were flat and the muscle were modeled as an isotropic material between two concentric circles, with the inner circle representing the CT and the outer circle representing the length of CW. In this model, if the material between the two insertions were under tension, force applied at each insertion would be of equal magnitude, and tension, or force per unit length, would be inversely proportional to the fiber radius. For stress in the material to be constant, thickness would also have to increase as 1/radius to balance the gradient of tension. This would occur if the diaphragm were made of spanning cylindrical fibers. If, on the other hand, the diaphragm were modeled as a section of a spherical membrane with one insertion at the equator and another halfway to the pole, even though the two circles are unequal, a load of constant pressure would result in uniform tension because of the force supported by the curvature of the membrane. In this hypothetical model, tension would be uniform, and for stress to be uniform, thickness should be constant. The discontinuous fiber architecture supported by the results of this study is consistent with constant stress in a curved membrane.
Tapering fibers ending intramuscularly are inconsistent with force transmission occurring only at the ends of a muscle. If this occurred, force would be constant along the body of a muscle, and stress at the sarcomere level would be inversely proportional to the diameter of the myofibril. Also, if there were myofibrils of different lengths, maximal contraction of the longest myofibers could exceed the shortening capabilities of shorter fibers and unload them completely. These considerations suggest that the force of individual myofibers is transmitted only through myotendinous junctions at each end. This discontinuous architecture implies that, during submaximal contraction, adjacent muscle cells may contribute to the parallel elastance. The nature of the connective tissue network is not completely known.
An interesting point raised by Loeb et al. (12) is that if the individual fibers insert diffusely into an epimysial extracellular matrix, then it is possible that the extracellular series elasticity in the muscle can change the way in which the stretch is distributed within the muscle. This would permit the sliding of discontinuous muscle fibers with respect to each other. Therefore, the overall changes in muscle length may not necessarily be reflected in proportional changes in sarcomere length. Passive muscle fibers may be considered another important source of internal compliance (15). Lev-tov et al. (11) demonstrated that fibers of the cat tenuissimus muscle which belong to a single motor unit are frequently adjacent mostly to fibers belonging to other motor units. Thus the fibers often have no members of their own motor unit as their nearest neighbors. Therefore, tension generated by active muscle fibers must be transmitted not only through the endomysium but also, at least laterally, through passive fibers (15). When a cell contracts, the tension generated is dispersed to and through both collagen fibers and adjacent noncontracting cells (7).
The diaphragm differs from limb muscles and may differ from other trunk muscles in that it carries a load transverse to its muscle fibers in the plane of the diaphragm. Previous results demonstrated very little in-plane strain in the direction transverse to the muscle fibers (3). This, again, has a teleological advantage. If the diaphragm were elastically extensible in the plane of the diaphragm, transverse to the direction of the muscle fibers, then as the muscle contracted and tension in the diaphragm increased, extension in the direction transverse to the fibers would reduce the volume displacement of the diaphragm, and part of the work of muscle contraction would be expended on elastic extension in the transverse fiber direction. No strain in the direction transverse to the fibers means that muscle thickness is inversely proportional to fiber length and that the cross-sectional shape of the muscle fibers must change during active shortening. The length of excised unstressed diaphragm is very similar to that at passive total lung capacity (TLC) in vivo (14) and ~25% longer than that occurring during maximal activation at high lung volume. Specimens for the morphometric analysis were fixed at the unstressed length (length at TLC). If the major fiber diameter tended to be perpendicular to the plane of the diaphragm, when the diaphragm is passively stretched from the length at TLC to spontaneous functional residual capacity length, dimensional changes would preferentially decrease thickness, and the muscle fiber would become more circular. During maximal active shortening, however, the myofibrils would be less circular at end of inspiratory effort, and the surface area would be greater than for circular cross section. Presumably, this would create an increased tension on the cell membrane and possibly influence the transmission of forces from the sarcomere through the connective tissue framework of the muscle.
In summary, our morphological and histochemical analyses demonstrated that the diaphragmatic muscle has a greater number of fibers at M than near CW or CT and cross-sectional area at M is smaller than near CW or CT. Furthermore, the muscle fascicles are crossed by two to six NMJ bands. The results of our study are consistent with the diaphragm having a discontinuous fiber architecture with greater number of fibers midway between the insertions than near the CW or CT. Muscle fibers in such architecture may not span the entire length of the muscle and may end by tapering before reaching either CT or CW. Therefore, forces are expected to be transmitted not only from CW to CT along the length of spanning fibers but also laterally among the adjacent nonspanning fibers through the connective tissue.
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ACKNOWLEDGEMENTS |
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The authors thank Drs. Clair Langston and Michael Reid for discussions about the data. The authors are grateful to Q. Lin, D. Zhu, O. Siddiqi, and A. M. Doneski for technical assistance.
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FOOTNOTES |
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This work was supported by National Heart, Lung, and Blood Institute Grants HL-54198 and HL-46230.
Address for reprint requests: A. M. Boriek, Pulmonary Section, Suite 520B, Dept. of Medicine, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030.
Received 24 April 1997; accepted in final form 9 September 1997.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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a new trichrome staining technique.
Z. Mikrosk. Anat. Forsch.
96:
138-144,
1994.
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