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Structure of Vertebrate Striated Muscle as Determined by X‐ray‐Diffraction Studies

John C. Haselgrove

10.1002/cphy.cp100105

Source: Supplement 27: Handbook of Physiology, Skeletal Muscle

Originally published: 1983

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Muscle Structure: An Overview
1.1 Description of Low‐Angle Diffraction Patterns
2 Interpretation of X‐Ray Patterns
2.1 Theoretical Aspects
2.2 Experimental Approaches
3 Filament Lattice
3.1 Lattice Spacing
3.2 Filament Orientations
3.3 Radial Electron‐Density Distribution
4 Actin‐Containing Filaments
4.1 Helix Parameters
4.2 Thin‐Filament Regulation of Contraction
5 Myosin‐Filament Backbone
5.1 Axial Packing of Myosin
5.2 Helical Packing of Myosin
5.3 Axial Repeat of C Protein
6 Cross Bridges
6.1 Relaxed Muscle
6.2 Rigor Muscle
6.3 Contracting Muscle
7 Analogues of ATP
Figure 1.

Electron micrographs of the filament lattice in vertebrate striated muscle. A: longitudinal section parallel to the 1.1 plane (see also Fig. 5). Thick myosin‐containing filaments lie in longitudinal register and thinner actin‐containing filaments extend on either side of the Z line and interdigitate with myosin filaments. Cross bridges project from the thick filaments. Different zones are marked. B: transverse section through A bands of several myofibrils. Thick filaments are located at lattice points of a hexagonal lattice with actin filaments at trigonal positions, equidistant from 3 myosin filaments (see Fig. 5). [From H. E. Huxley, unpublished observations.]
Figure 2.

A 4‐step, 4‐state model of cross‐bridge action showing mechanical states with corresponding predominant biochemical species. Cross‐bridge orientations in states C and D are often depicted as 45° angles; both A and B are shown as 90° angles. Although states A and D can be distinguished biochemically, differences in physical orientation are still unclear. The hinge region between head and tail of the myosin molecule is thought to be very flexible. A: myosin cross bridge, with cleaved ATP‐hydrolysis products still bound to it, is not yet attached to actin. B: cross bridge attaches to actin monomer at approximate 90° angle. C: cross bridge‐actin angle changes to 45°, pulling the filaments past each other while the cleaved nucleotide products dissociate from myosin. Resultant state is the rigor cross‐link. D: ATP binds to rigor cross‐link causing myosin cross bridge to dissociate from actin filament. Subsequent hydrolysis of ATP leaves the cross bridge in original state (A), ready to bind to the next available actin monomer.
Figure 3.

Low‐angle X‐ray‐diffraction patterns of frog sartorius muscles at rest length, displayed as if taken with muscle axis vertical. The vertical axis of symmetry is called the meridian; the horizontal axis is the equator. Right (in each picture), axial position of myosin meridional reflections (14.3 nm); left, axial position of actin layer line (5.9 nm). A: relaxed muscle. Pattern is dominated by layer lines arising from helical arrangement of cross bridges around myosin filament. B: relaxed muscle, meridional region (very low angle pattern). T, doublet near 38.5 nm arising from troponin repeat; C, doublet near 43.0 nm arising from C protein. C: rigor muscle. Relaxed myosin layer lines have disappeared, replaced by a new series of layer lines arising from cross bridges attached to actin. D: contracting muscle. The layer lines arising from myosin cross bridges are extremely weak and the pattern is dominated by layer lines from actin filament. [With permission from Haselgrove 46. Copyright by Academic Press Inc. (London) Ltd.]
Figure 4.

Schematic representation of principal X‐ray reflections from rest‐length VSM. Right, spacings of myosin reflections; left, spacing of actin reflections. [From Haselgrove and Rodger 51.]
Figure 5.

Hexagonal filament lattice with myosin filaments at lattice positions, actin filaments at trigonal positions. (See also Fig. LA.) A and B: different sets of planes have different crystallographic notations and their spacing determines the spacing of corresponding equatorial reflections. When the sarcomere's electron‐density distribution is projected along the lattice planes, the resulting one‐dimensional distribution can be described in terms of sine waves. the amplitude (F) of the fundamental sine wave (with a wavelength equal to the separation of the planes) controls and is proportional to the equatorial reflection amplitude for those planes. Transferring cross‐bridge mass from the vicinity of thick filaments to that of thin filaments decreases intensity F2 of the 1.0 reflection and increases intensity of the 1.1 reflection. Relaxed muscle (A): myosin cross bridges (hatched area) lie close to myosin filaments. Rigor muscle (B): myosin cross bridges lie close to actin filaments. C, D, E: radial electron‐density maps for rest‐length muscle constructed by Fourier summation from the amplitudes of the 1.0 and 1.1 equatorial reflections with phases of 0° for both reflections. Amplitudes taken from patterns of relaxed (C), contracting (D), and rigor (E) muscle. F: radial electron‐density distribution of relaxed muscle calculated by Fourier summation; amplitudes calculated from intensities in Table 3. The 1.0, 1.1, and 3.0 reflections had phases of 0°; the 2.0 and 2.1 reflections had phases of 180°. Calculations assuming 6‐fold symmetry gave rise to 6 projections, but no fine details can be usefully interpreted in terms of cross‐bridge shape and postion. [C‐E with permission from Haselgrove and Huxley 49. Copyright by Academic Press Inc. (London) Ltd.; F from Haselgrove et al. 52.]
Figure 6.

A: equatorial diffraction pattern of relaxed frog sartorius muscle at rest length, showing the first 5 equatorial reflections arising from the filament lattice. From right to left from the origin, they are 1.0, 1.1, 2.0, 2.1, 3.0. A weak reflection lying between 1.0 and 1.1 is attributed to the Z line 183. Relative reflection intensities are shown in Table 3. Center, meridional reflection at 14.3 nm is indicated. To right of backstop, higher‐order reflections obscured by part of the camera. B‐G: densitometer tracings from equator of relaxed, contracting, and rigor muscles at different sarcomere lengths. Thick line, 1.0 reflection; thin line, 1.1 reflection. Tracings are not all to the same vertical scale.

A from Haselgrove et al. 48, © 1977 by The Institute of Physics. B‐G with permission from Haselgrove and Huxley 49. Copyright by Academic Press Inc. (London) Ltd
Figure 7.

A: relation between myosin‐filament spacing (d) and sarcomere length (s) in intact living muscle fiber. Data fit a line of the form d2 ∝ 1/s. Thus lattice volume is independent of sarcomere length. B: comparison of filament lattice behavior of intact and skinned fibers. The lattice is constrained by the membrane in intact living fiber. Filled circles, relative unit cell areas of living intact fibers plotted against the inverse of the osmolarity of the bathing medium; filled triangles, area of skinned fibers plotted against the inverse of the corresponding ionic strength. Open symbols, mean values for muscles in normal physiological conditions. [A from April 3; B from April 2.]
Figure 8.

A: X‐ray pattern showing myosin layer lines in relaxed muscle sampled by Bragg reflections, arrows, from the myosin‐filament lattice. Sampling on 1st and 2nd layer lines occurs at different radial positions from the equator and 3rd layer line, indicating that the lattice is not a simple one with all myosin filaments having the same orientation. B: superlattice proposed by H.E. Huxley and Brown to explain the X‐ray sampling. (This illustration is for a 2‐stranded filament, but the logic may be applied to filaments of any symmetry.) Numbers on the cross bridges represent the level (in steps of 14.3 nm) along the filament at which the cross bridges project in the direction shown. Nearest neighbors are rotated through 1/3 or 2/3 revolution with respect to each other. Superlattice, broken line, links adjacent filaments with the same orientation. In cross section, myosin filaments have triangular profiles and on such a superlattice all triangles would point in the same direction. C: Squire's superlattice in which filaments face in only 2 directions. [A from Haselgrove et al. 48; B with permission from Huxley and Brown 70. Copyright by Academic Press Inc. (London) Ltd.; C adapted from Luther and Squire 83.]
Figure 9.

Plot of equatorial intensity ratio I1.0/I1.1 for different muscle states. All data are for frog striated muscle except the open circles for live relaxed rabbit psoas muscle. [From Huxley 66.] A, relaxed living muscle; B and C, isometrically contracting muscle; D, rigor muscle. Vertical line, ratios depend on tension for muscles that, although contracting isometrically, generate only a fraction of the maximum isometric tension. Tensions of 0%, 25%, 50%, and 100% are shown 182. Open triangle, isometric contraction 105. Two reflections arise from lattice planes across the muscle (see Fig. 5). Ratio I1.0/I1.1 falls when electron density at actin position relative to myosin position rises. This occurs 1) if actin filaments are more ordered at short as opposed to long sarcomere lengths or 2) if cross bridges move from the myosin‐filament region to that of actin when the muscle contracts or passes into rigor. [Data for A, B, D from Haselgrove and Huxley 49; data for C from Podolsky et al. 129.]
Figure 10.

A: the actin helix may be considered either a two‐stranded structure with a half‐pitch of about 37.5 nm (top part of helix) or a single‐stranded genetic helix with a pitch of about 5.9 nm (lower part). Because the pitches of the actin and myosin helices are different in VSM, actin‐myosin interaction does not repeat exactly; note the difference in attachment geometry between the 1st and 4th cross bridges. Here only the helical structure of the actin filament is emphasized, although attachment positions of myosin cross bridges to actin are determined by the helical structures of both filaments. B: structure of the actin filament with tropomyosin and troponin. [A adapted from Haselgrove and Reedy 50.]
Figure 11.

A and B: diagrams of two actin filaments with different axial repeats. A repeats after 2 × 35.5 nm whereas B repeats after 2 × 40.9 nm. Superficially the filaments appear very similar, but differences are clear if one holds the page at eye level and sights from left to right. C and D: low‐angle diffraction patterns anticipated from the structures of A and B, respectively. By sighting across the page one can see that changes in the long helix pitch make only very slight changes in layer‐line positions. Since layer lines tend to be broad parallel to the meridian it is impossible to measure their axial spacing with any accuracy or, therefore, to obtain a reliable measure of the long helix pitch to an accuracy of better than ∼3% (2 × 1 nm in 2 × 38.5 nm).
Figure 12.

X‐ray patterns printed by enhancement technique [see 166] to show actin‐filament layer lines (cf. Fig. 11). Positions of first 3 layer lines, influenced by tropomyosin, are marked between the 2 patterns. A: relaxed muscle; 3rd layer line is stronger than 2nd because tropomyosin molecule is positioned near the edge of the actin groove (see also Fig. 13). B: rigor muscle; 2nd layer line is stronger than 3rd because tropomyosin has moved nearer to center of groove. [From Vibert et al. 166. Copyright by Academic Press Inc. (London) Ltd.]
Figure 13.

Schematic diagrams illustrating movement of tropomyosin when thin filaments are activated, and possible correlation to myosin S1‐ac‐tin binding. All diagrams drawn as if looking along the filament from M line to Z line. A and B: the 3‐D reconstruction data of Moore et al. indicate that myosin S1 binds on each filament strand approximately as shown. X‐ray data show that tropomyosin moves toward the center of the helix groove when the relaxed filament (solid circle position) is activated (broken circle position); X‐ray data, however, cannot be used to determine on which side of the filament tropomyosin sits. One is, therefore, unable to distinguish between 2 possible geometries, A and B. A, original steric‐blocking mechanism; B, tropomyosin position as deduced by Seymour and O'Brien. In the latter model, tropomyosin position cannot control S1. C: recent data indicating that S1 probably extends to a position where tropomyosin can physically prevent attachment. TMa, position of tropomyosin in activated muscle; TMb, position in relaxed muscle. [C with permission from Taylor and Amos 157. Copyright by Academic Press Inc. (London) Ltd.]
Figure 14.

A: schematic representation of layer lines from myosin cross‐bridge array. Layer lines are indexed on a helix with a subunit pitch of 14.3 nm and a helix pitch of N × 14.3 where N = rotational symmetry of the filament. B, C, D: nets of surface lattices that can give rise to layer lines. The N values of myosin VSM filament are thought to be 2, 3, or 4. Note that each lattice has an axial repeat subunit of 14.3 nm and repeats exactly after 42.9 nm, although the basic helical strand, thin line, has a pitch of N × 42.9 nm. Perfect lattices like those in B, C, and D would give meridional reflections at orders of 14.3 nm only; the presence of forbidden reflections (see Tables 1 and 2; Fig. 3) indicates that the lattice is not perfect. A possible perturbation might be for the axial subunit repeat to be nonuniform. E: a 2‐stranded filament with regular repeat of 14.3 nm between cross bridges. F: sequence of 16, 11, and 16 nm. Note that since the filament's rotational symmetry is not known, any suitable symmetry may be used for illustrating general structural features. [B‐D with permission from Luther and Squire 83. Copyright by Academic Press Inc. (London) Ltd.; E and F from Yagi et al. 180.]
Figure 15.

Possible cross‐bridge orientations on backbone of relaxed muscle. Cross‐bridge shape is described by Elliott and Offer 26. A: view perpendicular to filament axis. Two parts of the cross bridges, perhaps representing 2 S1 heads, are tilted slightly along the axis. B: view looking down on one crown of a 3‐stranded model. Heads lie close to filament surface, and one head of each molecule lies behind the other in this view.

Adapted from Haselgrove 47
Figure 16.

Electron micrographs of insect flight muscle, IFM, in rigor (cf. Fig. 1.). A: longitudinal section, 2 cross bridges attach to actin every 38.5 nm along all filaments throughout the fibril. B: insert, optical‐diffraction pattern of longitudinal section. C: transverse section, 4 cross bridges connect each myosin filament to 4 of the adjacent actin filaments. Because this section is cut at a slight angle, the axial level of sectioning changes across the picture: flared X made by the cross bridges rotates clockwise by about 60° from left to right. Note that IFM actin filaments are in different lattice positions than those in VSM. [From Haselgrove and Reedy 50.]
Figure 17.

X‐ray pattern of semitendinosus muscle in rigor at non‐overlap length (cf. Fig. 3C). Myosin layer lines have disappeared, but because the actin filaments have been withdrawn from the A band, cross bridges do not attach to actin and no rigor layer lines are seen. [With permission from Haselgrove 46. Copyright by Academic Press Inc. (London) Ltd.]
Figure 18.

Kinetic data following time course of changing X‐ray reflections during tension development and relaxation. Open squares, intensity; filled circles, tension. A and B: curves plotted as percentage of maximal change. A: 42.9‐nm layer line during a single twitch. Three hundred separate twitches were summed for a total integrated time of 3 s for each 10‐ms time interval. Note that intensity decreased by 90%. B: 14.3‐nm meridional reflection and tension during a twitch at 10°C. Total exposure is 1.5 s in each 10‐ms time slot. C: time course of change in intensity of the 14.3‐nm reflection during a quick release in a tetanus. Release was 1.3% of initial length. [A and B from Huxley et al. 72; C from Huxley et al. 75.]


Figure 1.

Electron micrographs of the filament lattice in vertebrate striated muscle. A: longitudinal section parallel to the 1.1 plane (see also Fig. 5). Thick myosin‐containing filaments lie in longitudinal register and thinner actin‐containing filaments extend on either side of the Z line and interdigitate with myosin filaments. Cross bridges project from the thick filaments. Different zones are marked. B: transverse section through A bands of several myofibrils. Thick filaments are located at lattice points of a hexagonal lattice with actin filaments at trigonal positions, equidistant from 3 myosin filaments (see Fig. 5). [From H. E. Huxley, unpublished observations.]



Figure 2.

A 4‐step, 4‐state model of cross‐bridge action showing mechanical states with corresponding predominant biochemical species. Cross‐bridge orientations in states C and D are often depicted as 45° angles; both A and B are shown as 90° angles. Although states A and D can be distinguished biochemically, differences in physical orientation are still unclear. The hinge region between head and tail of the myosin molecule is thought to be very flexible. A: myosin cross bridge, with cleaved ATP‐hydrolysis products still bound to it, is not yet attached to actin. B: cross bridge attaches to actin monomer at approximate 90° angle. C: cross bridge‐actin angle changes to 45°, pulling the filaments past each other while the cleaved nucleotide products dissociate from myosin. Resultant state is the rigor cross‐link. D: ATP binds to rigor cross‐link causing myosin cross bridge to dissociate from actin filament. Subsequent hydrolysis of ATP leaves the cross bridge in original state (A), ready to bind to the next available actin monomer.



Figure 3.

Low‐angle X‐ray‐diffraction patterns of frog sartorius muscles at rest length, displayed as if taken with muscle axis vertical. The vertical axis of symmetry is called the meridian; the horizontal axis is the equator. Right (in each picture), axial position of myosin meridional reflections (14.3 nm); left, axial position of actin layer line (5.9 nm). A: relaxed muscle. Pattern is dominated by layer lines arising from helical arrangement of cross bridges around myosin filament. B: relaxed muscle, meridional region (very low angle pattern). T, doublet near 38.5 nm arising from troponin repeat; C, doublet near 43.0 nm arising from C protein. C: rigor muscle. Relaxed myosin layer lines have disappeared, replaced by a new series of layer lines arising from cross bridges attached to actin. D: contracting muscle. The layer lines arising from myosin cross bridges are extremely weak and the pattern is dominated by layer lines from actin filament. [With permission from Haselgrove 46. Copyright by Academic Press Inc. (London) Ltd.]



Figure 4.

Schematic representation of principal X‐ray reflections from rest‐length VSM. Right, spacings of myosin reflections; left, spacing of actin reflections. [From Haselgrove and Rodger 51.]



Figure 5.

Hexagonal filament lattice with myosin filaments at lattice positions, actin filaments at trigonal positions. (See also Fig. LA.) A and B: different sets of planes have different crystallographic notations and their spacing determines the spacing of corresponding equatorial reflections. When the sarcomere's electron‐density distribution is projected along the lattice planes, the resulting one‐dimensional distribution can be described in terms of sine waves. the amplitude (F) of the fundamental sine wave (with a wavelength equal to the separation of the planes) controls and is proportional to the equatorial reflection amplitude for those planes. Transferring cross‐bridge mass from the vicinity of thick filaments to that of thin filaments decreases intensity F2 of the 1.0 reflection and increases intensity of the 1.1 reflection. Relaxed muscle (A): myosin cross bridges (hatched area) lie close to myosin filaments. Rigor muscle (B): myosin cross bridges lie close to actin filaments. C, D, E: radial electron‐density maps for rest‐length muscle constructed by Fourier summation from the amplitudes of the 1.0 and 1.1 equatorial reflections with phases of 0° for both reflections. Amplitudes taken from patterns of relaxed (C), contracting (D), and rigor (E) muscle. F: radial electron‐density distribution of relaxed muscle calculated by Fourier summation; amplitudes calculated from intensities in Table 3. The 1.0, 1.1, and 3.0 reflections had phases of 0°; the 2.0 and 2.1 reflections had phases of 180°. Calculations assuming 6‐fold symmetry gave rise to 6 projections, but no fine details can be usefully interpreted in terms of cross‐bridge shape and postion. [C‐E with permission from Haselgrove and Huxley 49. Copyright by Academic Press Inc. (London) Ltd.; F from Haselgrove et al. 52.]



Figure 6.

A: equatorial diffraction pattern of relaxed frog sartorius muscle at rest length, showing the first 5 equatorial reflections arising from the filament lattice. From right to left from the origin, they are 1.0, 1.1, 2.0, 2.1, 3.0. A weak reflection lying between 1.0 and 1.1 is attributed to the Z line 183. Relative reflection intensities are shown in Table 3. Center, meridional reflection at 14.3 nm is indicated. To right of backstop, higher‐order reflections obscured by part of the camera. B‐G: densitometer tracings from equator of relaxed, contracting, and rigor muscles at different sarcomere lengths. Thick line, 1.0 reflection; thin line, 1.1 reflection. Tracings are not all to the same vertical scale.

A from Haselgrove et al. 48, © 1977 by The Institute of Physics. B‐G with permission from Haselgrove and Huxley 49. Copyright by Academic Press Inc. (London) Ltd


Figure 7.

A: relation between myosin‐filament spacing (d) and sarcomere length (s) in intact living muscle fiber. Data fit a line of the form d2 ∝ 1/s. Thus lattice volume is independent of sarcomere length. B: comparison of filament lattice behavior of intact and skinned fibers. The lattice is constrained by the membrane in intact living fiber. Filled circles, relative unit cell areas of living intact fibers plotted against the inverse of the osmolarity of the bathing medium; filled triangles, area of skinned fibers plotted against the inverse of the corresponding ionic strength. Open symbols, mean values for muscles in normal physiological conditions. [A from April 3; B from April 2.]



Figure 8.

A: X‐ray pattern showing myosin layer lines in relaxed muscle sampled by Bragg reflections, arrows, from the myosin‐filament lattice. Sampling on 1st and 2nd layer lines occurs at different radial positions from the equator and 3rd layer line, indicating that the lattice is not a simple one with all myosin filaments having the same orientation. B: superlattice proposed by H.E. Huxley and Brown to explain the X‐ray sampling. (This illustration is for a 2‐stranded filament, but the logic may be applied to filaments of any symmetry.) Numbers on the cross bridges represent the level (in steps of 14.3 nm) along the filament at which the cross bridges project in the direction shown. Nearest neighbors are rotated through 1/3 or 2/3 revolution with respect to each other. Superlattice, broken line, links adjacent filaments with the same orientation. In cross section, myosin filaments have triangular profiles and on such a superlattice all triangles would point in the same direction. C: Squire's superlattice in which filaments face in only 2 directions. [A from Haselgrove et al. 48; B with permission from Huxley and Brown 70. Copyright by Academic Press Inc. (London) Ltd.; C adapted from Luther and Squire 83.]



Figure 9.

Plot of equatorial intensity ratio I1.0/I1.1 for different muscle states. All data are for frog striated muscle except the open circles for live relaxed rabbit psoas muscle. [From Huxley 66.] A, relaxed living muscle; B and C, isometrically contracting muscle; D, rigor muscle. Vertical line, ratios depend on tension for muscles that, although contracting isometrically, generate only a fraction of the maximum isometric tension. Tensions of 0%, 25%, 50%, and 100% are shown 182. Open triangle, isometric contraction 105. Two reflections arise from lattice planes across the muscle (see Fig. 5). Ratio I1.0/I1.1 falls when electron density at actin position relative to myosin position rises. This occurs 1) if actin filaments are more ordered at short as opposed to long sarcomere lengths or 2) if cross bridges move from the myosin‐filament region to that of actin when the muscle contracts or passes into rigor. [Data for A, B, D from Haselgrove and Huxley 49; data for C from Podolsky et al. 129.]



Figure 10.

A: the actin helix may be considered either a two‐stranded structure with a half‐pitch of about 37.5 nm (top part of helix) or a single‐stranded genetic helix with a pitch of about 5.9 nm (lower part). Because the pitches of the actin and myosin helices are different in VSM, actin‐myosin interaction does not repeat exactly; note the difference in attachment geometry between the 1st and 4th cross bridges. Here only the helical structure of the actin filament is emphasized, although attachment positions of myosin cross bridges to actin are determined by the helical structures of both filaments. B: structure of the actin filament with tropomyosin and troponin. [A adapted from Haselgrove and Reedy 50.]



Figure 11.

A and B: diagrams of two actin filaments with different axial repeats. A repeats after 2 × 35.5 nm whereas B repeats after 2 × 40.9 nm. Superficially the filaments appear very similar, but differences are clear if one holds the page at eye level and sights from left to right. C and D: low‐angle diffraction patterns anticipated from the structures of A and B, respectively. By sighting across the page one can see that changes in the long helix pitch make only very slight changes in layer‐line positions. Since layer lines tend to be broad parallel to the meridian it is impossible to measure their axial spacing with any accuracy or, therefore, to obtain a reliable measure of the long helix pitch to an accuracy of better than ∼3% (2 × 1 nm in 2 × 38.5 nm).



Figure 12.

X‐ray patterns printed by enhancement technique [see 166] to show actin‐filament layer lines (cf. Fig. 11). Positions of first 3 layer lines, influenced by tropomyosin, are marked between the 2 patterns. A: relaxed muscle; 3rd layer line is stronger than 2nd because tropomyosin molecule is positioned near the edge of the actin groove (see also Fig. 13). B: rigor muscle; 2nd layer line is stronger than 3rd because tropomyosin has moved nearer to center of groove. [From Vibert et al. 166. Copyright by Academic Press Inc. (London) Ltd.]



Figure 13.

Schematic diagrams illustrating movement of tropomyosin when thin filaments are activated, and possible correlation to myosin S1‐ac‐tin binding. All diagrams drawn as if looking along the filament from M line to Z line. A and B: the 3‐D reconstruction data of Moore et al. indicate that myosin S1 binds on each filament strand approximately as shown. X‐ray data show that tropomyosin moves toward the center of the helix groove when the relaxed filament (solid circle position) is activated (broken circle position); X‐ray data, however, cannot be used to determine on which side of the filament tropomyosin sits. One is, therefore, unable to distinguish between 2 possible geometries, A and B. A, original steric‐blocking mechanism; B, tropomyosin position as deduced by Seymour and O'Brien. In the latter model, tropomyosin position cannot control S1. C: recent data indicating that S1 probably extends to a position where tropomyosin can physically prevent attachment. TMa, position of tropomyosin in activated muscle; TMb, position in relaxed muscle. [C with permission from Taylor and Amos 157. Copyright by Academic Press Inc. (London) Ltd.]



Figure 14.

A: schematic representation of layer lines from myosin cross‐bridge array. Layer lines are indexed on a helix with a subunit pitch of 14.3 nm and a helix pitch of N × 14.3 where N = rotational symmetry of the filament. B, C, D: nets of surface lattices that can give rise to layer lines. The N values of myosin VSM filament are thought to be 2, 3, or 4. Note that each lattice has an axial repeat subunit of 14.3 nm and repeats exactly after 42.9 nm, although the basic helical strand, thin line, has a pitch of N × 42.9 nm. Perfect lattices like those in B, C, and D would give meridional reflections at orders of 14.3 nm only; the presence of forbidden reflections (see Tables 1 and 2; Fig. 3) indicates that the lattice is not perfect. A possible perturbation might be for the axial subunit repeat to be nonuniform. E: a 2‐stranded filament with regular repeat of 14.3 nm between cross bridges. F: sequence of 16, 11, and 16 nm. Note that since the filament's rotational symmetry is not known, any suitable symmetry may be used for illustrating general structural features. [B‐D with permission from Luther and Squire 83. Copyright by Academic Press Inc. (London) Ltd.; E and F from Yagi et al. 180.]



Figure 15.

Possible cross‐bridge orientations on backbone of relaxed muscle. Cross‐bridge shape is described by Elliott and Offer 26. A: view perpendicular to filament axis. Two parts of the cross bridges, perhaps representing 2 S1 heads, are tilted slightly along the axis. B: view looking down on one crown of a 3‐stranded model. Heads lie close to filament surface, and one head of each molecule lies behind the other in this view.

Adapted from Haselgrove 47


Figure 16.

Electron micrographs of insect flight muscle, IFM, in rigor (cf. Fig. 1.). A: longitudinal section, 2 cross bridges attach to actin every 38.5 nm along all filaments throughout the fibril. B: insert, optical‐diffraction pattern of longitudinal section. C: transverse section, 4 cross bridges connect each myosin filament to 4 of the adjacent actin filaments. Because this section is cut at a slight angle, the axial level of sectioning changes across the picture: flared X made by the cross bridges rotates clockwise by about 60° from left to right. Note that IFM actin filaments are in different lattice positions than those in VSM. [From Haselgrove and Reedy 50.]



Figure 17.

X‐ray pattern of semitendinosus muscle in rigor at non‐overlap length (cf. Fig. 3C). Myosin layer lines have disappeared, but because the actin filaments have been withdrawn from the A band, cross bridges do not attach to actin and no rigor layer lines are seen. [With permission from Haselgrove 46. Copyright by Academic Press Inc. (London) Ltd.]



Figure 18.

Kinetic data following time course of changing X‐ray reflections during tension development and relaxation. Open squares, intensity; filled circles, tension. A and B: curves plotted as percentage of maximal change. A: 42.9‐nm layer line during a single twitch. Three hundred separate twitches were summed for a total integrated time of 3 s for each 10‐ms time interval. Note that intensity decreased by 90%. B: 14.3‐nm meridional reflection and tension during a twitch at 10°C. Total exposure is 1.5 s in each 10‐ms time slot. C: time course of change in intensity of the 14.3‐nm reflection during a quick release in a tetanus. Release was 1.3% of initial length. [A and B from Huxley et al. 72; C from Huxley et al. 75.]

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Article Title: Structure of Vertebrate Striated Muscle as Determined by X‐ray‐Diffraction Studies
 

How to Cite

John C. Haselgrove. Structure of Vertebrate Striated Muscle as Determined by X‐ray‐Diffraction Studies. Compr Physiol 2011, Supplement 27: Handbook of Physiology, Skeletal Muscle: 143-171. First published in print 1983. doi: 10.1002/cphy.cp100105