Numerous myocytes make up muscle tissue and the controlled production of tension in these cells can generate significant force. Muscle tissue can be classified functionally as voluntary or involuntary and morphologically as striated or non-striated. Voluntary refers to whether the muscle is under conscious control, while striation refers to the presence of visible banding within myocytes caused by the organization of myofibrils to produce constant tension.
The above classifications describe three forms of muscle tissue that perform a wide range of diverse functions. Skeletal muscle mainly attaches to the skeletal system via tendons to maintain posture and control movement. For example, contraction of the biceps muscle, attached to the scapula and radius, will raise the forearm. Some skeletal muscle can attach directly to other muscles or to the skin, as seen in the face where numerous muscles control facial expression.
Skeletal muscle is under voluntary control, although this can be subconscious when maintaining posture or balance. Morphologically skeletal myocytes are elongated and tubular and appear striated with multiple peripheral nuclei. Cardiac muscle tissue is found only in the heart, where cardiac contractions pump blood throughout the body and maintain blood pressure. As with skeletal muscle, cardiac muscle is striated; however it is not consciously controlled and so is classified as involuntary.
Cardiac muscle can be further differentiated from skeletal muscle by the presence of intercalated discs that control the synchronized contraction of cardiac tissues. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two centrally located nuclei.
Smooth muscle tissue is associated with numerous organs and tissue systems, such as the digestive system and respiratory system. It plays an important role in the regulation of flow in such systems, such as aiding the movement of food through the digestive system via peristalsis. Smooth muscle is non-striated and involuntary. Smooth muscle myocytes are spindle shaped with a single centrally located nucleus.
Types of muscle : The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle, visualized here using light microscopy. Visible striations in skeletal and cardiac muscle are visible, differentiating them from the more randomised appearance of smooth muscle. Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin. Myocytes, sometimes called muscle fibers, form the bulk of muscle tissue.
They are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Myocytes contain numerous specialized cellular structures which facilitate their contraction and therefore that of the muscle as a whole.
The highly specialized structure of myocytes has led to the creation of terminology which differentiates them from generic animal cells. Myocytes can be incredibly large, with diameters of up to micrometers and lengths of up to 30 centimeters. The sarcoplasm is rich with glycogen and myoglobin, which store the glucose and oxygen required for energy generation, and is almost completely filled with myofibrils, the long fibers composed of myofilaments that facilitate muscle contraction.
The sarcolemma of myocytes contains numerous invaginations pits called transverse tubules which are usually perpendicular to the length of the myocyte.
Each myocyte contains multiple nuclei due to their derivation from multiple myoblasts, progenitor cells that give rise to myocytes. These myoblasts asre located to the periphery of the myocyte and flattened so as not to impact myocyte contraction. Myocyte: Skeletal muscle cell : A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm called the sarcoplasm. A muscle fiber is composed of many myofibrils, packaged into orderly units.
Each myocyte can contain many thousands of myofibrils. Myofibrils run parallel to the myocyte and typically run for its entire length, attaching to the sarcolemma at either end.
Each myofibril is surrounded by the sarcoplasmic reticulum, which is closely associated with the transverse tubules. Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins. These proteins are organized into regions termed sarcomeres, the functional contractile region of the myocyte. Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction.
The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance. Sarcomere : The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments.
Myofibrils are composed of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments. Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments.
Thin filaments are composed of actin, tropomyosin, and troponin. The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle. In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere. Movement often requires the contraction of a skeletal muscle, as can be observed when the bicep muscle in the arm contracts, drawing the forearm up towards the trunk.
The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle. To understand the sliding filament model requires an understanding of sarcomere structure. A sarcomere is defined as the segment between two neighbouring, parallel Z-lines. Z lines are composed of a mixture of actin myofilaments and molecules of the highly elastic protein titin crosslinked by alpha-actinin.
Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules. Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments. Cell Metabolism. The Origin of Mitochondria. Mitochondrial Fusion and Division. The Origin of Plastids.
The Origins of Viruses. Discovery of the Giant Mimivirus. Volvox, Chlamydomonas, and the Evolution of Multicellularity. Yeast Fermentation and the Making of Beer and Wine. Dynamic Adaptation of Nutrient Utilization in Humans. Nutrient Utilization in Humans: Metabolism Pathways.
An Evolutionary Perspective on Amino Acids. Mitochondria and the Immune Response. Stem Cells in Plants and Animals. Promising Biofuel Resources: Lignocellulose and Algae. The Discovery of Lysosomes and Autophagy.
The Mystery of Vitamin C. Krans, Ph. Citation: Krans, J. Nature Education 3 9 How do muscles contract? What molecules are necessary for a tissue to change its shape?
Aa Aa Aa. Muscle is a specialized contractile tissue that is a distinguishing characteristic of animals. Changes in muscle length support an exquisite array of animal movements, from the dexterity of octopus tentacles and peristaltic waves of Aplysia feet to the precise coordination of linebackers and ballerinas.
What molecular mechanisms give rise to muscle contraction? The process of contraction has several key steps, which have been conserved during evolution across the majority of animals. What Is a Sarcomere? Figure 1: A gastrocnemius muscle calf with striped pattern of sarcomeres. The view of a mouse gastrocnemius calf muscle under a microscope. The Sliding Filament Theory. Figure 2: Comparison of a relaxed and contracted sarcomere.
A The basic organization of a sarcomere subregion, showing the centralized location of myosin A band. Figure 3: The power stroke of the swinging cross-bridge model, via myosin-actin cycling. Actin red interacts with myosin, shown in globular form pink and a filament form black line. Figure 4: Illustration of the cycle of changes in myosin shape during cross-bridge cycling 1, 2, 3, and 4.
ATP hydrolysis releases the energy required for myosin to do its job. What Regulates Sarcomere Shortening? Figure 5: Troponin and tropomyosin regulate contraction via calcium binding.
Simplified schematic of actin backbones, shown as gray chains of actin molecules balls , covered with smooth tropomyosin filaments. Unresolved Questions. Muscle contraction provides animals with great flexibility, allowing them to move in exquisite ways.
The molecular changes that result in muscle contraction have been conserved across evolution in the majority of animals. By studying sarcomeres, the basic unit controlling changes in muscle length, scientists proposed the sliding filament theory to explain the molecular mechanisms behind muscle contraction. Within the sarcomere, myosin slides along actin to contract the muscle fiber in a process that requires ATP.
Scientists have also identified many of the molecules involved in regulating muscle contractions and motor behaviors, including calcium, troponin, and tropomyosin. This research helped us learn how muscles can change their shapes to produce movements. References and Recommended Reading Clark, M. Article History Close. Share Cancel. Revoke Cancel.
Recently new methods of light microscopy have been developed to extend the resolution limit. This is very informative, but it is still far from what X-rays or electrons can do [ 92 ].
This whole story has changed dramatically in the last few years using new electron microscopes, better electron detectors, cryo-protected specimens and powerful computer algorithms [ 94 ].
Now, instead of using 2D crystals to get multiple images of the same object, isolated single protein molecules or molecular assemblies for example can be spread onto a grid in a thin layer of water and then the grid can be rapidly frozen to get amorphous ice. The frozen grid can be kept very cold by liquid nitrogen cooling of the speciman stage of the electron microscope, and the specimen viewed in a very high vacuum.
The electron image can be recorded, with minimal electron dose on the specimen, using a highly efficient electron detector [ 95 ]. It is noteworthy that the Nobel Prize for Chemistry was awarded to Drs. In the case of muscle, several of the molecular assemblies of interest are not single molecules, but are extended filaments e.
They can then be averaged together in different ways to give good structures for the filaments [ 98 , 99 ]. This is what has been done in references [ 20 , 39 , 51 , 52 , 53 , 54 ] and is the technique used to generate Figure 4 and Figure 9 of this Review. Later we will see that some of this can be done by X-ray diffraction, but here we consider the unique contribution that spectroscopy can make.
We start with fluorescence. It happens that amino acids like tryptophan are intrinsically fluorescent [ , , ]. It was shown by Aronson and Morales [ ] that P is sensitive to the physiological state of the muscle, relaxed, rigor or active, and, although there are tryptophans in other molecules than myosin, Nihei et al. Inset B , the microwave absorption spectrum top from the spin flip in A and its first derivative below. To be more specific, and to label other things than the myosin head, extrinsic fluorophores can be added to skinned fibres fibres with their outer membrane sarcolemma removed either physically or chemically using detergents in which some of the known molecules, such as troponin or the myosin light chains or MyBP-C, can be exchanged for equivalent purified proteins carrying extrinsic fluorescent tags, such as IAEDANS [ 71 ].
One of the problems of such extrinsic fluorophores is that they can have significant mobility on their parent molecule, even if the molecule itself is fairly static. This mobility can be reduced substantially by the use of bi-functional probes, where the probe is covalently linked to two sites on the parent molecule rather than one e.
This makes use of what is known as Forster resonance energy transfer FRET, sometimes called fluorescence resonance energy transfer. This can either use intrinsic fluorophores like tryptophan or added fluorescent labels with mono- or bi-functional binding [ , ]. A possible problem with extrinsic labels is that they may modify the way that a protein normally functions. It is important in the case of muscle to test whether force generation and movement are affected by the labelling.
Exciting developments in spectroscopic methods include luminescence resonance energy transfer LRET and total internal reflection fluorescence detection TIRF [ ]. A complementary method to using fluorescence is to use spin probes [ ]. The technique, known as electron paramagnetic resonance EPR or electron spin resonance ESR , makes use of the fact that all electrons possess a magnetic moment and will align in a magnetic field. If there is an unpaired electron, this will align either parallel to or antiparallel to the field.
A common arrangement is to have the sample, which may be a solution or a skinned or intact fibre, in a strong magnetic field. With the microwave frequency scanned past the appropriate excitation frequency, the absorbtion spectrum can be detected, recorded Figure 14 b; Panel B and its first derivative calculated.
Alternatively, this can also be done the other way round with the microwave source of fixed frequency and the magnetic field scanned through the optimal field strength.
As with fluorescence, the spin label can be an extrinsic probe such as a nitroxide radical site directed spin labelling , or it can also be a bifunctional probe to reduce probe mobility, but once again there is a need to check that the probe is not altering the normal function of the host protein. The precise application of EPR depends both on the field strength, the excitation frequency and which signal is recorded. However, muscle proteins have interesting motions in the microsecond to millisecond time window.
EPR can be used to measure both motions and probe orientations. In addition, measurements of distance can be determined by double electron-electron resonance DEER and ab initio high resolution structure determined in muscle fibres by a combination of two-probe BEER with single probe EPR [ , , , ].
Bifunctional probes attached to the lever arm of the myosin head can be used to determine the lever arm orientation [ ]. The biochemical properties of enzymes like myosin, with its substrate of ATP, have been characterised in solution by measuring rate constants, for example between various steps in the acto-myosin ATPase cycle [ 55 , , ].
All of the transitions in the Lymn-Taylor scheme of Figure 8 are reversible steps, so the relative abundances of the different states around the cross-bridge cycle are determined by forward and backward rate constants. Many of these rates were originally measured in solution using stopped flow or quenched flow methods [ 55 , ] Figure However, the fact that the acto-myosin II interaction normally occurs in muscle fibres, where the filament geometry will have an effect, means that results from solution may not be directly applicable to what goes on in muscle.
After that there are additives sensitive to, for example, Pi concentration. Such a Pi sensor, a phosphate binding protein, will bind Pi when it is released from myosin and as a result will fluoresce [ , ]. The recorded fluorescence intensity is a measure of Pi concentration. An experiment of this kind is illustrated in Figure 16 , combining mechanical measurements, release of caged ATP and the use of a phosphate binding protein [ ]. Apparatus used for rapid reaction kinetics: a stopped flow apparatus, b quenched flow apparatus.
In a the reactants are held in syringes A and B and are forced into the mixing chamber C within a few msec. The mixing chamber can be monitored optically to follow the progress of the reaction. In b the reactants A and B, as before, are mixed in a small chamber mixer 1. The flow is continuous, and the reaction proceeds in the tube between mixer 1 and mixer 2, The reaction is quenched in the mixer 2 chamber by the addition of acid from syringe C.
Adapted from [ 3 ] after White and Thorson [ ]. Simultaneous measurement of length a , force b and phosphate release c in a single skinned muscle fibre, illustrating the acceleration of Pi release rate during shortening. A permeabilized muscle fibre was mounted between two hooks, one attached to a length-adjusting motor and the other to a force transducer. The Figure shows two consecutive measurements on a single fibre.
The fibre was initially at rest length in a rigor solution no ATP. At zero time a contraction was initiated by the release into a muscle fibre of around 1. At either 0. The trace shows clearly that the rate of Pi liberation was increased during shortening steps adapted from Figure 5 of He et al.
Apart from the application of X-rays in protein crystallography, so-called low-angle X-ray cameras can be used to record X-ray diffraction patterns directly from whole muscles or isolated muscle fibres, or at moderate angles even preparations of isolated actin filaments [ 3 , 7 , 18 , 19 , 22 , 38 , 57 , 58 , 59 , 62 , 63 , 64 , 80 , 81 , 82 , , , , , , ]. For reviews on low-angle X-ray diffraction theory and results see references [ 7 , , ].
The basic idea is similar to Figure 13 c, except that now the specimen is a whole muscle or a fibre or an oriented gel of actin filaments.
Because of the reciprocal nature of diffraction i. Long X-ray cameras i. An advantage of low-angle diffraction is that electronic detectors can be used to record the diffraction patterns [ ] and these can often be read out within a ms or less, permitting fast time-resolved X-ray diffraction studies of contracting muscles and fibres [ 57 , , , , , , , , ].
The spacings of the observed diffraction peaks can be determined, often with high accuracy, and this is very informative, especially about the size and symmetry of the diffracting objects. The main problem with such diffraction techniques is that, unlike protein crystallography, it is not easy to solve the phase problem. For this reason a common approach is to use what knowledge there is about the proteins involved and then to set up structural models in the computer, using this information, with the molecular organisation in 3D defined by adjustable parameters, for example to define the positions and orientations of particular proteins or protein domains.
The parameters are then adjusted in the computer and the predicted diffraction pattern calculated. The goodness of fit between the observed and calculated diffraction patterns can then be assessed, usually using a so-called R-factor see [ ], to find which combination of parameters gives the best fit to the observations. This approach can be very powerful [ , ]. Using time-resolved X-ray diffraction with patterns recorded on fast readout detectors [ ], the changing diffraction pattern from an active muscle can be recorded on a millisecond or shorter time scale as force is being generated.
The steps of the crossbridge cycle can be followed during changes in intensity of various parts of the X-ray pattern and these changes can be modelled to yield time courses of changing populations of cross-bridge states in the contractile cycle. In the right hands, this technique is immensely powerful. But caution is required, since misinterpretation can lead to false conclusions see discussion in [ 7 , ].
In particular, I believe that those modelling X-ray diffraction data should show clearly that the number of parameters required to fit their model to the data, is significantly smaller than the number of truly independent observations that are available. If this is not done, then the analysis can be assumed to be under-determined and there is no reason why anyone should believe what is being claimed. The second point included in the Knupp et al.
On its own it simply contains the information that a lump of material of unknown shape is situated at There is no more information than this other than possibly the extent of the diffracting array. If the peak is sampled by an interference function, then there is additional information about the interference function, but no more information about the shape of the diffracting object. Only by full modelling of all the myosin meridional peaks out to a reasonable resolution say 2 nm would one get useful information about the axially-projected shape of the diffracting object.
This might then reveal lever arm movements. Apart from studying muscle proteins either biochemically in solution or in the intact fibre, methods have been developed to study the motion or behaviours of isolated filaments or molecules in the light microscope, sometimes with the proteins labelled with fluorescent tags which can be detected, even though not imaged at high resolution.
These are called motility assays. For example, isolated myosin heads or heavy meromyosin two heads and about one third of the myosin rod , can be laid down on a microscope cover slip in an appropriate solution with ATP and calcium and fluorescently labelled actin filaments can be watched as the myosin heads propel the actin filaments across the cover slip Figure 17 a,b. Or use can be made of the photon power in focused laser beams to manipulate small beads in the light microscope; so-called optical trap methods.
If these beads are attached to either end of an actin filament Figure 17 c , then this filament can be lowered onto an isolated myosin molecule on a pedestal or on another bead and the force generated by the interaction can be determined by the observed displacement of the actin-attached beads.
These are very useful and powerful techniques, which can separate out the effects of different steps between strong states in the cross-bridge cycle, as discussed fully in Special Issue Review [ ], where Mansson et al. The reader is referred to that Review for further discussion of these techniques and appropriate references. In vitro motility assay and optical tweezers set-up for single-molecule mechanics. The curved red arrow indicates a possible filament sliding path; c schematic diagram of the three-bead optical tweezers assay where an actin filament is suspended between two beads held in optical traps.
The filament is then lowered down to allow the actin filament to interact with single myosin motor fragments adsorbed to a third bead or another type of pedestal as indicated here. Reproduced from Special Issue Review [ ] with permission. An interesting recent development in electron microscopy has been the application of an environmental chamber to visualise domain movements in the actin-myosin system while the proteins are in a hydrated state; the proteins are almost in a physiological environment within the electron microscope Figure Sugi and his colleagues have been studying the movements of various parts of myosin heads labelled with specific antibodies carrying gold particles, which are visible in the microscope.
We will discuss some of their results later, but full details are given in the Special Issue Review [ ] by Sugi et al. Diagram of the environmental chamber in the electron microscope. The chamber is sealed top and bottom by carbon films through which the electrons can penetrate. Reproduced from Sugi et al. Some of the major insights into the contractile mechanism have come from protein crystallographic studies of different myosin heads with various substrates bound. Following the initial ground-breaking study of Rayment and his colleagues [ 12 ], the more recent work, reviewed by Houdusse and Sweeney [ ], has been particularly notable.
Figure 19 summarises some of the conclusions of these studies from [ ]. The myosin head motor domain see Figure 3 has a cleft in it, which, depending on the substrate bound or whether actin is attached, can be closed or open, or partially open.
This is called the actin-binding cleft. If no nucleotide is bound, the sheet is curved and relaxed. There is an intermediate state of the transducer when the motor domain binds actin and MgADP with high affinity, as in the major force-generating part of the cross-bridge cycle.
Various stages of the actin-myosin ATPase cycle cf. Figure 8 as expanded by Houdusse and Sweeney [ ]. Reproduced from [ ] with permission. When ATP binds to the myosin head, the products ADP and Pi are rapidly produced, but these products are not readily released until the head binds to actin.
In Figure 19 , the left side shows head states off actin or weak-binding to actin, whereas the right hand figures show strong head states on actin. In each head image the actin-binding cleft is on the left and the converter and lever arm are on the right.
The numbered double arrow shows the state of the transducer. The progress of Pi release is not straightforward in that its exit via the ATP binding pocket is blocked by the MgADP that is still bound, so there must be another exit a back door in the motor domain for the Pi to be released from.
Such a back door has been seen in crystals of myosin VI [ ], but not in heads bound to actin. In the non-attached states this Pi exit is blocked. A recent disagreement has been about whether force-production occurs before or after Pi release [ , , ]. However, protein crystallography studies suggest that the Pi released from the ATP-binding pocket and exiting through the back door may get trapped there and may only be fully released and detectable at a later time.
In this case, Pi release from the ATP binding pocket might be an early event that triggers the power stroke, but Pi release into the sarcoplasm would only be detectable after a delay. ATP state which rapidly detaches from actin to give the M. ATP post-rigor state 1 in which the lever arm is still down. In this scheme, resetting of the lever arm occurs before ATP hydrolysis when there is a change of state of the transducer to give conformation 2. Hydrolysis then occurs 2 to 3 , and the resulting M.
ADP Pi state can then interact with actin in a rapid equilibrium, weak-binding, state during which the attached head may explore the actin surface to find a suitable, strong binding site. When it finds this, there is stereospecific labelling of actin by the head, the actin-binding cleft partially closes, and Pi release is triggered to give the AM. ADP state 6 with the actin-binding cleft almost closed and the lever arm tending to swing down.
Further closure of this cleft may be associated with a change of state of the transducer from 6 to 7. ADP release then fully closes the cleft and further lever arm rotation at least in some myosins can occur from 7 to 8 , after which the cycle can start again. One of the main driving forces in muscle studies, for obvious medical reasons, is to understand in detail how heart muscle works.
One of the main aspects of heart muscle that enables it to function as it does is that it pulls harder when it is stretched. This is the well-known Frank-Starling law [ ]. So, what might change as the sarcomere length increases? An obvious possibility is that perhaps the actin filament becomes more switched on with stretch because the tropomyosin shift is enhanced [ ].
But an unexpected finding in the Special Issue paper by Eakins et al. One was not just a scaled down version of the other, as might be expected because of the reduced filament overlap at 2. Could this kind of effect, if it occurs in heart muscle, be associated with the Frank-Starling Law? See also [ ]. Recent time-resolved X-ray diffraction studies of contracting muscle [ ] have attempted to model the varying time-courses of several X-ray reflections on the equator of the diffraction pattern in terms of the occupancy of various states and their likely diffraction contributions.
The cross-bridge cycle used for the modelling was less complicated than that in Figure 19 , but it included groups of states with contributions to the equator which were likely to be similar. So there was Group a , heads off actin, Group b , heads in the weak-binding and transiently-attached pre-powerstroke states, Group c , strong and rigor-like heads i.
Most notable here is that of state b which has a significant number of heads in the weak-binding states. Some of the advantages of using insect flight muscle as a specimen for both static and time-resolved X-ray diffraction studies are described in the Special Issue Review by Iwamoto [ ].
Even diffraction from single sarcomeres and from muscle cross-sections can now be achieved. Some remarkable time-resolved experiments include those from a whole Drosophila fruit fly actually in the synchrotron X-ray beam and using its flight muscles to do oscillatory work [ ]. Above we discussed the Huxley-Simmons [ 76 ] experiments on the tension transients resulting from rapid step changes of length of frog muscle fibres, and their improved results with better experimental set-ups [ 77 , 78 ].
These experiments have formed the backbone of the thinking of researchers on how muscle works ever since. However, in a recent paper in this Special Issue, the interpretation of the T 1 curve has been questioned [ 79 ]. What was done was to model in the computer all the known compliances in the sarcomere as Hookean springs with different stiffnesses depending on whether it was the myosin filament backbone, the actin filament, the cross-bridges and even titin.
What was found was that, instead of the myosin cross-bridges accounting for about one-third of the half-sarcomere compliance, the observed X-ray spacing changes seen by Huxley et al. In this case the apparent cross-bridge stiffness was much higher than had been thought previously. Instead of being around 0. But it was also realised that, as well as the strong-binding heads, the weak-binding bridges must also be contributing to the instantaneous stiffness seen in the T 1 curve.
In fact, it is under the very fast shortening conditions of the T 1 curve measurements that the weak-binding heads would show stiffnesses as great as or even more than the stiffnesses of rigor bridges [ 56 ]. What can be going on? Happily, there is a way out of this dilemma. The suggestion in [ 79 ] is that by far the biggest cross-bridge effect on the T 1 curve is from the weak binding bridges. So the high stiffness based on the T 1 curve would in this case not be primarily reporting the behaviour of the strong bridges.
It was suggested that the effect of the first strong-binding state associated with phosphate release could be to release the lever arm so that it can now act as a softer spring with a stiffness of only 0.
With this scheme, the Huxley-Simmons observations would be entirely consistent with conclusions from other experimental approaches about the lever arm movement that can result from a single ATP turnover. Note that, with this scenario, during the T 1 step, both the weak-binding and strong-binding heads would be contributing to what is seen and the tension change will still be reporting what the strong heads, the weak heads and the myosin and actin filaments do.
But the stiffness slope of the T 1 curve will be dominated by the compliance of the filament backbones and the stiffness of the weak-binding heads and their effect on any changes in muscle length, because the weak-binding heads in this model are much stiffer than the strong-binding heads. Note that, as yet, the idea that the strong binding states might be less stiff than the weak binding states is only a conjecture; it is yet to be tested properly.
The T 2 curve of Huxley and Simmons [ 76 ] was originally thought to be reporting the behaviour of the strong bridges as they rebuilt the tension in the filaments and redistributed themselves amongst the available strong states. There is no reason, as yet, to doubt that this is still the case, but, once again, interpretation of various results on the T 2 tension recovery processes also needs to be clarified. The review by Ranatunga [ ] discusses how the mechanics of the crossbridge cycle are affected by changing the temperature, sometimes using rapid T-jumps.
Temperature has a marked effect on the level of tension produced by most muscle types. The sigmoidal dependence of steady force on temperature is due to this endothermic nature of crossbridge force generation. During shortening, the force-generating step and the ATPase cycle are accelerated, whereas during lengthening, they are inhibited.
The endothermic force generation is seen in different muscle types fast, slow, and cardiac. Two reviews in the Special Issue discuss the application of quick-freeze electron microscopy to capture transient sarcomere structures and visualise them in the electron microscope.
In one [ 91 ], by Taylor and his colleagues, the quick freezing of intact fibres in this case insect flight muscle was followed by low-temperature embedding freeze-substitution , sectioning and electron tomography to reveal the 3D distribution of density within the sections.
Because of the beautiful 3D regularity of insect flight muscles [ 83 ], this technique is particularly effective. In this way, a variety of images of myosin heads attached to actin have been visualized and attempts made to fit the known crystal structure of the myosin head into the density. In another approach [ ], Katayama and his colleagues have combined quick freezing with motility assays on a mica surface and then produced deep-etch, shadowed, replicas using something like the Heuser technique [ ].
They have then isolated individual electron microscope images of myosin heads on actin filaments and have tried to correlate these with the known myosin head structures in different states as they would appear after heavy metal shadowing. They claim to recognize different known head shapes and even some previously unknown ones. In the totally different and heroic approach by Sugi and his colleagues [ , ], discussed earlier Figure 18 , some results from their studies of hydrated actin-myosin samples in the environmental chamber of an electron microscope are at the limit of what can be achieved and are tantalizingly provocative Figure They used gold particle-labelling to identify myosin head positions and structures through labels attached via antibodies to different parts of the myosin head.
In this way, they believe they have seen evidence of reversible myosin head movements in the presence of ATP, but in the absence of actin, away from the myosin filament bare zone and in opposite directions on opposite sides of the bare zone. They take this to be the resetting stage or recovery stroke 1 to 2 in Figure Results from use of the electron microscope environmental chamber to study myosin head conformations under different conditions.
These are based on histograms such as a — c and similar histograms for the movement in the presence of actin. In e the heads start in rigor and then ATP is added. The heads are thought to come off actin, hydrolyse ATP, rebind to actin and then go through a power stroke. Adapted from ref [ ] with permission. In the presence of actin and with three different antibodies labelling the heads on either the motor domain or the light chains they believe they can actually see a change in myosin head shape consistent with the kind of lever arm movements that are illustrated in the conventional cross-bridge cycle Figure With results from a variety of techniques available it is possible to put together features in the crossbridge cycle other than those depicted in Figure 19 which was purely based on results from protein crystallography.
These results are illustrated in Figure Schematic diagram showing the different states that have been identified from electron microscopy and low-angle X-ray diffraction studies: a the fully relaxed super-relaxed state; b the normal activated relaxed state in intact muscle; c the attached part of the weak-binding state where heads are in a rapid detached-attached equilibrium; d the strong states during which the lever arm is supposed to swing axially towards the rigor state e ; f the rapid detachment caused by binding of ATP to the rigor-like heads in e ; g the resetting state where ATP is hydrolysed and the lever arm returns towards its configuration in resting muscle.
All transitions are reversible, but some are more likely in the forward direction around the cycle. For discussion see text. Firstly, we can think about resting muscle. We have seen that many electron microscopy studies of isolated myosin filaments using single particle analysis [ 50 ] have come up with structures such as that in Figure 4 which show heads in the interacting head motif conformation Figure 6. However, X-ray diffraction patterns from normal resting muscles show that the heads are in a different conformation on myosin filaments in the intact A-band filament lattice of bony fish muscle [ 22 ], insect flight muscle [ 22 ] and mouse Soleus and EDL muscles [ ].
To quote from the Special Issue paper by Ma et al. The quasi-helically ordered configuration is taken to be the super-relaxed state and the normal activated relaxed state for bony fish muscle is modelled to be as in Hudson et al. It remains to be seen what controls the transitions between the super-relaxed and normal activated relaxed states. The first attached state of the myosin heads is thought to be the weak-binding state where the heads are in a very rapid equilibrium between attachment and detachment and the head stiffness in muscle mechanics experiments then depends on how fast the muscle is pulled.
During normal contractions the weak heads will be on and off so fast that they contribute little to the muscle stiffness [ 79 ]. What do we know about the structure of this state? The study by Eakins et al.
What is seen is a different distribution of intensity along the layer lines, consistent with heads still being myosin-centred, but perhaps with variable azimuthal shifts to make the heads point towards actin where they can take part in the weak-binding attachment to actin.
This energy is expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head is in a low-energy position.
After the power stroke, ADP is released; however, the cross-bridge formed is still in place, and actin and myosin are bound together. ATP can then attach to myosin, which allows the cross-bridge cycle to start again and further muscle contraction can occur Figure 1.
The movement of the myosin head back to its original position is called the recovery stroke. Resting muscles store energy from ATP in the myosin heads while they wait for another contraction. Figure 1. With each contraction cycle, actin moves relative to myosin. When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation and preventing contraction in a muscle without nervous input.
0コメント