scholarly journals Myosin Cross-bridge Behaviour in Contracting Muscle – The T1 Curve of Huxley & Simmons (1971) Revisited

2019 ◽  
Author(s):  
Carlo Knupp ◽  
John M. Squire
Keyword(s):  
Actin Filaments ◽  
Striated Muscle ◽  
X Ray Diffraction ◽  
Step Size ◽  
Weakly Bound ◽  
X Ray ◽  
Cross Bridge ◽  
Key Factor ◽  
Length Changes ◽  
Cross Bridges

The stiffness of the myosin cross-bridges is a key factor in analysing possible scenarios to explain myosin head changes during force generation in active muscles.  The seminal study of Huxley and Simmons (1971: Nature 233: 533) suggested that most of the observed half-sarcomere instantaneous compliance (=1/stiffness) resides in the myosin heads.    They showed with a so-called T1 plot that, after a very fast release, the half-sarcomere tension reduced to zero after a step size of about 60Å (later with improved experiments reduced to 40Å).   However, later X-ray diffraction studies showed that myosin and actin filaments themselves stretch slightly under tension, which means that most (at least two-thirds) of the half sarcomere compliance comes from the filaments and not from cross-bridges.    Here we have used a different approach, namely to model the compliances in a virtual half sarcomere structure in silico.   We confirm that the T1 curve comes almost entirely from length changes in the myosin and actin filaments, because the calculated cross-bridge stiffness (probably greater than 0.4 pN/Å) is higher than previous studies have suggested.    In the light of this, we present a plausible modified scenario to describe aspects of the myosin cross-bridge cycle in active muscle.   In particular, we suggest that, apart from the filament compliances, most of the cross-bridge contribution to the instantaneous T1 response comes from weakly-bound myosin heads, not myosin heads in strongly attached states.   The strongly attached heads would still contribute to the T1 curve, but only in a very minor way, with a stiffness that we postulate could be around 0.1 pN/Å, a value which would generate a working stroke close to 100 Å from the hydrolysis of one ATP molecule.  The new program can serve as a tool to calculate sarcomere elastic properties for any vertebrate striated muscle once various parameters have been determined (e.g. tension, T1 intercept, temperature, X-ray diffraction spacing results).

scholarly journals Myosin Cross-Bridge Behaviour in Contracting Muscle—The T1 Curve of Huxley and Simmons (1971) Revisited

2019 ◽  
Vol 20 (19) ◽  
pp. 4892 ◽  
Author(s):  
Knupp ◽  
Squire
Keyword(s):  
Actin Filaments ◽  
Striated Muscle ◽  
Reliable Estimate ◽  
X Ray Diffraction ◽  
Step Size ◽  
X Ray ◽  
Cross Bridge ◽  
Length Changes ◽  
The Cross ◽  
Cross Bridges

The stiffness of the myosin cross-bridges is a key factor in analysing possible scenarios to explain myosin head changes during force generation in active muscles. The seminal study of Huxley and Simmons (1971: Nature 233: 533) suggested that most of the observed half-sarcomere instantaneous compliance (=1/stiffness) resides in the myosin heads. They showed with a so-called T1 plot that, after a very fast release, the half-sarcomere tension reduced to zero after a step size of about 60Å (later with improved experiments reduced to 40Å). However, later X-ray diffraction studies showed that myosin and actin filaments themselves stretch slightly under tension, which means that most (at least two-thirds) of the half sarcomere compliance comes from the filaments and not from cross-bridges. Here we have used a different approach, namely to model the compliances in a virtual half sarcomere structure in silico. We confirm that the T1 curve comes almost entirely from length changes in the myosin and actin filaments, because the calculated cross-bridge stiffness (probably greater than 0.4 pN/Å) is higher than previous studies have suggested. Our model demonstrates that the formulations produced by previous authors give very similar results to our model if the same starting parameters are used. However, we find that it is necessary to model the X-ray diffraction data as well as mechanics data to get a reliable estimate of the cross-bridge stiffness. In the light of the high cross-bridge stiffness found in the present study, we present a plausible modified scenario to describe aspects of the myosin cross-bridge cycle in active muscle. In particular, we suggest that, apart from the filament compliances, most of the cross-bridge contribution to the instantaneous T1 response may come from weakly-bound myosin heads, not myosin heads in strongly attached states. The strongly attached heads would still contribute to the T1 curve, but only in a very minor way, with a stiffness that we postulate could be around 0.1 pN/Å, a value which would generate a working stroke close to 100 Å from the hydrolysis of one ATP molecule. The new model can serve as a tool to calculate sarcomere elastic properties for any vertebrate striated muscle once various parameters have been determined (e.g., tension, T1 intercept, temperature, X-ray diffraction spacing results).

scholarly journals X-ray diffraction studies of cross-bridges weakly bound to actin in relaxed skinned fibers of rabbit psoas muscle

1997 ◽  
Vol 73 (5) ◽  
pp. 2292-2303 ◽  
Author(s):  
S. Xu ◽  
S. Malinchik ◽  
D. Gilroy ◽  
T. Kraft ◽  
B. Brenner ◽  
...  
Keyword(s):  
Psoas Muscle ◽  
Skinned Fibers ◽  
X Ray Diffraction ◽  
Weakly Bound ◽  
X Ray ◽  
Rabbit Psoas ◽  
Cross Bridges

X-ray diffraction studies on striated and smooth muscles

1964 ◽  
Vol 160 (981) ◽  
pp. 467-472 ◽  
Keyword(s):  
Actin Filaments ◽  
Sarcomere Length ◽  
Striated Muscle ◽  
Hexagonal Lattice ◽  
Smooth Muscles ◽  
Frog Muscle ◽  
X Ray Diffraction ◽  
X Ray ◽  
Molecular Forces ◽  
Resting Muscle

It is appropriate to start this contribution with a tribute to the pioneer small-angle X-ray studies of H. E. Huxley (1953) on living, resting, striated muscle, reported at another discussion meeting of this Society. These studies led to the prediction of many features of the sliding-filament model, and were ahead of their time in technique, as is evident, since it was nearly ten years before any other similar papers were published. 1. The filament lattice of striated muscle The work of Elliott, Lowy & Worthington (1963) on the filament lattice of striated muscle has shown that for both living, resting, muscle and glycerol- extracted muscle the relative intensity of the equatorial reflexions of the hexagonal lattice depends in the same way on sarcomere length. For long sarcomeres the intensity of the (1, 0) reflexion is greater than that of the (1, 1) reflexion, for short sarcomeres the reverse is true. The explanation offered was that the actin filaments contribute to the equatorial pattern only when stabilized by inter-molecular forces within the hexagonal myosin lattice and that they (the actin filaments) are comparatively disordered in the I -band. The actin contribution is in phase for the (1, 1) reflexion and out of phase for the (1, 0), so that, as the actin filaments slide in and the actin contribution increases, the (1, 0) becomes progressively weaker, and the (1, 1) progressively stronger. Calculations on this basis agreed well with the observed effect. It was found that the sarcomere length at which the two reflexions had equal intensity was greater for mammalian than for frog muscle. From this fact it was predicted that the actin filaments in mammalian muscle should be about 0.3 μm longer than in frog muscle; Miss Sally Page has now observed such a difference directly (Page & Huxley 1963).

scholarly journals Recent X-ray Diffraction and Electron Microscope Studies of Striated Muscle

1967 ◽  
Vol 50 (6) ◽  
pp. 71-83 ◽  
Author(s):  
H. E. Huxley
Keyword(s):  
Active Sites ◽  
Striated Muscle ◽  
Structural Features ◽  
Actin Binding ◽  
X Ray Diffraction ◽  
X Ray ◽  
Thick Filaments ◽  
Myosin Filaments ◽  
The Cross ◽  
Cross Bridges

The sliding filament model for muscular contraction supposes that an appropriately directed force is developed between the actin and myosin filaments by some process in which the cross-bridges are involved. The cross-bridges between the filaments are believed to represent the parts of the myosin molecules which possess the active sites for ATPase activity and actin-binding ability, and project out sidewise from the backbone of the thick filaments. The arrangement of the cross-bridges is now being studied by improved low-angle X-ray diffraction techniques, which show that in a resting muscle, they are arranged approximately but not exactly in a helical pattern, and that there are other structural features of the thick filaments which give rise to additional long periodicities shown up by the X-ray diagram. The actin filaments also contain helically arranged subunits, and both the subunit repeat and the helical repeat are different from those in the myosin filaments. Diffraction diagrams can be obtained from muscles in rigor (when permanent attachment of the cross-bridges to the actin subunits takes place) and now, taking advantage of the great increase in the speed of recording, from actively contracting muscles. These show that changes in the arrangement of the cross-bridges are produced under both these conditions and are no doubt associated in contraction with the development of force. Thus configurational changes of the myosin component in muscle have been demonstrated: these take place without any significant over-all change in the length of the filaments.

scholarly journals The Transient Mechanics of Muscle Require Only a Single Force-Producing Cross-Bridge State and a 100 Å Working Stroke

Biology ◽  
2020 ◽  
Vol 9 (12) ◽  
pp. 475
Author(s):  
Carlo Knupp ◽  
John M. Squire
Keyword(s):  
Simple Model ◽  
The Other ◽  
Active Muscle ◽  
Step Size ◽  
Constant Length ◽  
X Ray ◽  
Cross Bridge ◽  
Mechanical Explanation ◽  
Single Force ◽  
Cross Bridges

An informative probe of myosin cross-bridge behaviour in active muscle is a mechanical transient experiment where, for example, a fully active muscle initially held at constant length is suddenly shortened to a new fixed length, providing a force transient, or has its load suddenly reduced, providing a length transient. We describe the simplest cross-bridge mechanical cycle we could find to model these transients. We show using the statistical mechanics of 50,000 cross-bridges that a simple cycle with two actin-attached cross-bridge states, one producing no force and the other producing force, will explain much of what has been observed experimentally, and we discuss the implications of this modelling for our understanding of how muscle works. We show that this same simple model will explain, reasonably well, the isotonic mechanical and X-ray transients under different loads observed by Reconditi et al. (2004, Nature 428, 578) and that there is no need to invoke different cross-bridge step sizes under these different conditions; a step size of 100 Å works well for all loads. We do not claim that this model provides a total mechanical explanation of how muscle works. However, we do suggest that only if there are other observations that cannot be explained by this simple model should something more complicated be considered.

Step size, scanning speed and shape of X-ray diffraction peak

1994 ◽  
Vol 27 (5) ◽  
pp. 716-722 ◽  
Author(s):  
H. Wang
Keyword(s):  
Scanning Speed ◽  
Peak Position ◽  
Maximum Intensity ◽  
X Ray Diffraction ◽  
Step Size ◽  
X Ray ◽  
Scanning Time ◽  
Integral Width ◽  
Counting Statistics ◽  
Voigt Function

The influences of step size and scanning speed on the shape of a single X-ray diffraction (XRD) peak are analyzed quantitatively. For this purpose, it is assumed that XRD peak shapes are a mixture of Cauchy and Gauss curves. Six equations are established for the calculation of position, maximum intensity and full width at half-maximum (FWHM) errors caused by step size and two for the FWHM errors caused by counting statistics. The ratio of step size to FWHM is proposed as the shape-perfect coefficient of the XRD peak. From these equations and the relationship between the FWHM and the integral width of a peak based on the pseudo-Voigt function or Voigt function, three basic elements of a single symmetric XRD peak (peak position, maximum intensity and FWHM) can be refined. The optimum step size and scanning time can also be set from them.

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