Short-range Elastic Component Research Articles

David Keynes Hill.

David Hill followed his father, A.V. Hill FRS, into the study of muscular contraction. Using a wide range of experimental techniques, he made several important advances of which the most important was the discovery of the 'short-range elastic component', a phenomenon which implied that even in the resting state there was an interaction between the thick (myosin) and thin (actin) filaments. He also studied physical changes in nerve when stimulated.

A cross-bridge mechanism can explain the thixotropic short-range elastic component of relaxed frog skeletal muscle.

1. The passive tension and sarcomere length of relaxed frog skeletal muscle fibres were measured in response to imposed length stretches. The tension response to a constant-velocity stretch exhibited a clear discontinuity. Tension rose more rapidly during the initial approximately 0.4 % L0 of the stretch than during the latter stages (where L0 is the resting length of the fibre). This initial tension response is attributed to the short-range elastic component (SREC). 2. The use of paired triangular stretches revealed that the maximum tension produced during the SREC response of the second stretch was significantly reduced by the first stretch. This history-dependent behaviour of the SREC reflects thixotropic stiffness changes that have been previously described in relaxed muscle. 3. The biphasic nature of the SREC tension response to movement was most apparent during the first imposed length change after a period at a fixed length, irrespective of the direction of movement. 4. If a relaxed muscle was subjected to an imposed triangular length change so that the muscle was initially stretched and subsequently shortened back to its original fibre length, the resting tension at the end of the stretch was reduced relative to its initial pre-stretch value. Following the end of the stretch, tension slowly increased towards its initial value but the tension recovery was not accompanied by a contemporaneous increase in sarcomere length. This finding suggests that the resting tension of a relaxed muscle fibre is not entirely due to passive elasticity. The results are compatible with the suggestion that a portion of the resting tension - the filamentary resting tension (FRT) - is produced by a low level of active force generation. 5. If a second identical stretch was imposed on the muscle at a time when the resting tension was reduced by the previous stretch, the maximal tension produced during the second stretch was the same as that produced during the first, despite the second stretch commencing from a lower initial resting tension. 6. In experiments using paired triangular length changes, an inter-stretch interval of zero did not produce a substantially greater thixotropic reduction in the second stretch elastic limit force than an inter-stretch interval in the range 0.5-1 s. 7. A theoretical model was developed in which the SREC and FRT arise as manifestations of a small number of slowly cycling cross-bridges linking the actin and myosin filaments of a relaxed skeletal muscle. The predictions of the model are compatible with many of the experimental observations. If the SREC and FRT are indeed due to cross-bridge activity, the model suggests that the cross-bridge attachment rate must increase during interfilamentary movement. A mechanism (based on misregistration between the actin binding sites and the myosin cross-bridges) by which this might arise is presented.

Thixotropy in skeletal muscle and in muscle spindles: A review

UWE PROSKE,t DAVID L. MORGAN* and J. EDMUND GREGORY'} *Department of Electrical and Computer Systems Engineering, Monash University, Clayton, Victoria, 3168, Australia ~ Department of Physiology, Monash University, Clayton, Victoria, 3168, Australia CONTENTS 1. Extrafusal muscle 1.1. Introduction 1.2. Mechanism 1.2.1. Basic observation 1.2.2. The role of calcium 1.2.3. Relation to 'weakly attached state' 1.3. Slack in muscle fibers 1.4. Measurements using intact joints 1.5. Conclusions 2. Thixotropy in muscle spindles 2.1. Historical observations 2.2. Mechanism 2.3. Stretch excitation 2.4. Two forms of muscle conditioning 2.5. Time-course 2.6. Spindles and extrafusal thixotropy 2.7. Fusimotor after-effects 2.8. Human spindles 2.9. Thixotropy as a tool for studying spindle mechanisms 2.9.1. Two kinds of spindles 2.9.2. Selective conditioning with single fusimotor fibers 2.9.3. Secondary endings 2.10. Thixotropy and reflex action 2.11. Proprioception 2.12. Conclusions References 705 705 705 705 706 706 706 7O7 708 708 709 710 710 711 712 712 712 713 713 713 714 716 716 718 718 719 1. EXTRAFUSAL MUSCLE 1.1. INTRODUCTION The term thixotropy comes from the Greek words 'thixis' meaning touch and 'tropos' meaning trans- formation. It is applied to substances which change their physical properties as a result of being mechan- ically shaken. More formally, it applies to materials which behave as solids below a certain applied shear force and as fluids at higher forces. It is commonly applied to polymeric molecules which on mechanical agitation detach weak bonds. In biology it was first used by Peterfi (1927) who noted a reduction in viscosity of the cytoplasm of sea urchin eggs follow- ing disturbance with a needle. Thixotropy has been described for suspensions of red blood cell aggregates and for synovial fluid (quoted from Lackie et al., 1984). The term thixotropy has been used in striated muscle to describe the dry friction-like behavior of passive muscle to movement, as distinct from its elastic (length dependent) or viscous (velocity depen- dent) behavior. Thus the element described by Hill (1968) as a short range elastic component (SREC) could be classified as thixotropic, in that the tension did not vary directly with length or velocity, but increased with length up to a limit of about 0.2% of muscle length and then remained constant for the remainder of the movement. Manifestations of the same phenomenon, but ap- plied to whole animals, have also been called thixotropy. The term is used to describe the resistance of a muscle or limb to imposed motion which is high for small movements but less for larger movements. Transformation from the high resistance to the low resistance state is possible by giving a brief large amplitude 'stirring' movement. 1.2. MECHANISM 1.2.1. Basic observation The basic observations on skeletal muscle and the resulting hypothesis are both due to Hill (1968). He showed that the tension response of passive frog muscle to a slow stretch was biphasic, with an initial, stiff, elastic region at the onset of the movement, which gave way to a nearly constant tension due to motion over the remainder of the stretch. The prop- erty of the muscle responsible for this response was 705

Thixotropy: the effect of stretch size in relaxed frog muscle.

Small forces were applied to isolated frog muscle; the resulting displacements were used to calculate muscle stiffness (elastic modulus, E). Stiffness is much greater for small forces within the range of the Short Range Elastic Component (SREC; Hill, 1968). 'Stirring' the muscle greatly reduces the stiffness, but only when the applied force is small. Stiffness subsequently returns to its original level. This strongly suggests that muscle thixotropy (Lakie, Walsh & Wright, 1984) is a property of the SREC.

Short‐range elasticity and resting tension of relaxed human lower leg muscles.

1. Tension responses of relaxed human lower leg muscles were recorded during applied triangular stretch cycles at a range of constant angular velocities which were below the threshold for inducing stretch reflexes. 2. Length-tension curves so obtained begin with a steep initial rising phase which is demonstrated to be equivalent to the 'short-range elastic component' (s.r.e.c.) observed in resting frog muscle (Hill, 1968). The s.r.e.c. develops only gradually following a stretch-release cycle. The time course can be described by a saturation function. It is suggested that this can be explained as resulting from a slow shift of the kinetic equilibrium between attached and detached cross-bridges towards attached cross-bridges. A kinetic model is presented, by which the sum of the cross-bridge formation and decay rate constants can be calculated from the saturation curves. 3. The recovery of resting tension following a stretch-release cycle proceeds at a significantly higher rate than the development of the s.r.e.c. Resting tension is also selectively influenced by passive movements of very small amplitude. This further suggests that the s.r.e.c. and resting tension are generated by different sources.

The stiffness of frog skinned muscle fibres at altered lateral filament spacing.

When the surface membrane is removed from a frog muscle fibre the myofibrils swell, so that the spacing between the filaments increases by 10-30%. In this study, the stiffness of skinned fibres was measured when the lateral spacing of the filament lattice was reduced osmotically using polyvinyl pyrrolidone (PVP), a synthetic linear polymer. In the absence of PVP, the apparent stiffness of relaxed skinned fibres measured during ramp stretches was about 0.05-0.1 of the stiffness of intact fibres. The stiffness increased when the lattice spacing was decreased. The mechanical characteristics of resting stiffness in the presence of PVP (or bovine serum albumin) were similar to those of the short-range elastic component of resting intact fibres. However, when the spacing of skinned fibres was reduced osmotically to that of the intact lattice, the stiffness of skinned fibres was about 1.6 times higher. In the absence of PVP, Ca2+-activated skinned fibres were less stiff than are fully active intact fibres during isometric tetani. The skinned fibre force-extension relation was markedly curved. As the filament spacing was reduced, the stiffness of the Ca2+-activated skinned fibres increased, with little change in isometric tension, and the force-extension curve became more linear. Experiments at varied filament overlap and during feed-back control of sarcomere length, monitored by laser diffraction, showed that PVP increased the stiffness of the sarcomeres. The increase of active stiffness in the presence of PVP could be partially dissociated from the increase of resting stiffness by inclusion of Mg tripolyphosphate in the bathing solutions. It is concluded that the low stiffness and the non-linearity of the force-extension curve observed in fully activated skinned fibres are due primarily to the increase of filament separation that occurs when a fibre is skinned. The mechanism may be related to an increased angle between the subfragment-2 part of the cross-bridge and the backbone of the thick filament, perhaps leading to buckling of cross-bridges under compression.

The initial burst of impulses in responses of toad muscle spindles during stretch.

The responses of muscle spindles in the iliofibularis muscle of the cane toad Bufo marinus were examined during constant velocity stretch of the passive muscle. Spindles were found to show an 'initial burst' of high frequency impulses at the onset of stretch. Associated with the initial burst was a steep passive tension rise in the whole muscle, the short-range elastic component (Hill, 1968), called here the passive stiffness. The size of the initial burst was found to depend on muscle length in a similar way as whole-muscle tetanic tension. Repetitive stretch was found to reduce both the initial burst and passive stiffness. The time taken for both to return to their control values was 3 and 10 s respectively. If immediately following repetitive stretch the muscle, and hence the spindle, was held stretched for 3 s, the initial burst in response to a subsequent stretch from a shorter length remained reduced in size for 300 s. The depression could be reversed by a brief period of fusimotor stimulation. Hypertonic Ringer solutions were found to increase the initial burst and passive stiffness, while both were reduced in hypotonic solutions. Low concentrations of caffeine (1.5 mM) produced a similar decrease in both the initial burst and the passive stiffness. Calcium-free Ringer solution left the stiffness unchanged, and increased the whole dynamic response of the spindle. Metabolic exhaustion and poisoning of the muscle caused the initial burst to increase while decreasing the active tension. It is concluded that the initial burst is an intrafusal manifestation of the passive short-range stiffness of extrafusal muscle which is thought to be due to the formation of stable cross-bridges between the actin and myosin filaments of myofibrils.

Series elasticity in frog muscle as revealed by optical diffraction.

Using an optical diffraction technique, the series elasticity of frog striated muscle fibres was investigated. One source of series elasticity was located in the cross-bridges during the application of either quick stretches or releases of muscle fibres. Evidence is presented here for a second component attributable to a small population of slowly activated sarcomeres. The size of the second component was progressively reduced until it virtually disappeared at a sarcomere length of 3 microns. A third component appears to reside in the thick filaments. Calculation of the elastic energy in the muscle fibres enabled an identification of the source of the energy to be made in terms of the components of the series elasticity. Evidence is presented of a short-range elastic component present in resting fibres.

The dependence of the short-range elasticity on sarcomere length in resting isolated frog muscle fibres.

The tension response of a resting muscle fibre to a sudden small stretch (ca. 1% of the fibre length) at constant velocity was analyzed according to a mechanical model in which two components are arranged in parallel: (i) The parallel elastic component (PEC) which is responsible for the resting tension of the fibre. (ii) The short-range elastic component (SREC) which has a highly non-linear tension response: Initially, the tension increases linearly with stretch, but with further stretch the increase in tension subsides and a steady tension level is attained. Both the initial short-range elastic stiffness and the steady state tension level increased with increasing sarcomere length up to 2.9-3.3 micrometer where both variables reached maximum values. The maximum value of the steady tension level was about 1% of the maximum twitch tension. With further increase of sarcomere length both the short-range elastic stiffness and the steady tension level decreased and approached zero at a sarcomere length of about 3.7 micrometer. The structural basis for the SREC is considered to be myosin heads which are cross-linking the thick and the thin filaments even in the resting state of the fibre.

The effect of a small stretch on the latency relaxation and the short-range elastic stiffness in isolated frog muscle fibres.

The amplitude of the latency relaxation and the stiffness of the short-range elastic component (SREC) were determined in single muscle fibres before and immediately after subjecting the fibre to a sudden small stretch at constant velocity. After the stretch the amplitude of the latency relaxation was reduced compared to the value obtained before the stretch provided that the velocity of stretch exceeded about 1 mm . ms-1 per half-sarcomere and that the amplitude of the stretch was sufficient to reach the stationary response of the SREC. The effect of the stretch on the amplitude of the latency relaxation subsided after about 4 ms. A reduction of the amplitude of the latency relaxation was accompanied by a reduction of the short-range elastic stiffness. This result is discussed in terms of the various hypotheses put forward to account for the generation of the latency relaxation.

Calcium activation produces a characteristic response to stretch in both skeletal and cardiac muscle.

RESTING frog skeletal muscle, when stretched at a constant velocity, exhibits a force response which, for length changes less than about 0.2%, is elastic in nature. First observed by Hill1, this response was attributed to a short range elastic component (SREC) that was thought to be a property of a small number of cross bridges which, even in the resting state, extend from the myosin-containing thick filaments to interact with sites on the actin-containing thin filaments. This view was criticised2 on the grounds that the SREC stiffness remained approximately constant as the amount of thick and thin filament overlap was decreased by increasing muscle length. On the basis of the sliding filament theory3,4, SREC stiffness would be expected to decrease because reduced amounts of thick and thin filament overlap would result in a decreased number of cross bridges available in interact with thin filament sites. Flitney and Hirst5 have reported that in twitch contractions of frog skeletal muscle a short range elastic response is present and is similar in form to that of resting muscle. In this case, however, a directly proportional relationship was found between the short range stiffness and the amount of overlap between the thick and thin filaments, which suggests that this stiffness is a property of attached cross bridges. To investigate the possible presence of attached cross bridges in resting muscle, we have measured the calcium dependence of the short range stiffness in skeletal and cardiac muscle preparations in which the myofibrils were directly exposed to the bathing medium.

Light scattering associated with tension changes in the short-range elastic component of resting frog's muscle.

1. A study has been made of some optical and associated mechanical properties of resting frog's sartorius muscles in isotonic and hypertonic solutions. Tension and transparency changes accompanying small alterations of muscle length (smaller than 1-5%) were recorded simultaneously. 2. The form of the transparency change is complex. It has three phases, two of which occur during the length change and the third (delayed phase) after it is complete. Directional recording of the response reveals both scattering and diffraction components. 3. The change of light scattering is associated with tension changes in the short-range elastic component (SREC) of the muscle. Its magnitude is related to the stiffness of the SREC; both increase when the osmotic strength of the external solution is raised and when muscle length is increased. 4. The change of light scattering is not much affected by the velocity of the applied length change. This is true also of the mechanical stiffness of the SREC. 5. The origin of the diffraction change is not known. 6. It is concluded that the scattering effect is caused by conformational changes in the SREC.

The Effect of Bathing Solution Tonicity on Resting Tension in Frog Muscle Fibers

Resting tension and short-range elastic properties of isolated twitch muscle fibers of the frog have been studied while bathed by solutions of different tonicities. Resting tension in isotonic solution at 2.3-microm sarcomere spacing averaged 0.46 mN.mm(-2) and was proportional to the fiber cross-section area. Hypertonic solutions, containing 0.1-0.5 mM tetracaine to block contracture tension, caused a small sustained tension increase, which was proportional to the fiber cross-section area and which reached 0.9 mN.mm(-2) at two times normal tonicity (2T). Further increases in tonicity caused little increase in tension. Hypotonic solutions decreased tension. Thus, tension at 2.3 microm is a continuous, direct function of tonicity. The dependence of tension on tonicity lessened at greater sarcomere lengths. At 3.2 microm either a very small rise or, in some fibers, a fall in tension resulted from an increase in tonicity. Hypertonic solutions also decreased the tension of extended sarcolemma preparations. In constant-speed stretch experiments the elastic modulus, calculated from the initial part of the stretch response, rose steeply with tonicity over the whole range investigated (1-2.5T). The results show that tension and stiffness of the short-range elastic component do not increase in parallel in hypertonic solutions.

Tension due to interaction between the sliding filaments in resting striated muscle. The effect of stimulation.

1. A resting sartorius muscle of the frog or toad possesses a special kind of elasticity which is shown to be due to a component lying between the two sets of filaments. The elastic effect is seen only for very small length changes, up to about 0.2% of the muscle length, and the ;elastic limit' is then reached. If the length change then continues at a constant velocity the tension developed is maintained at a fixed level, producing a sort of frictional resistance. The component responsible is called the ;short-range elastic component', or SREC.2. It is also shown that a small part of the permanent tension of a resting muscle is probably due to ;active' interaction between the filaments. This is called the ;filamentary resting tension', or FRT. For a sarcomere length of 2.0 mu the FRT amounts to about 150 mg in a muscle weighing 100 mg.3. The stiffness of the SREC and the magnitude of the FRT are shown to be related to one another. They are both increased, and may rise to high values, by making the external solution hypertonic.4. The working hypothesis is as follows. In a resting muscle the cross-bridges on the myosin filaments are not entirely inactive, but a very small proportion of them are cross-linked with the actin filaments. The links are very stable and have a long ;life'. The elastic behaviour is due to the flexural rigidity, or spring-like properties of these bridges. The elasticity is ;short-range' because the bridges can be bent, or stretched, only a small way from the steady-state position before the contacts ;slip'. The ;filamentary resting tension' (which is present in the absence of any external length change) is attributed to an ;active process', which operates by imparting ;organized' potential energy to the participating cross-bridges, by potentiating their attachment, against their elastic resistance, to sites on the actin filaments which are displaced towards the Z line.5. It is shown that the ability of the muscle to maintain a ;frictional' resistance to a continuing, slow, length change is suppressed during a maintained tetanic contraction. The process of suppression starts during the period of the latency relaxation.6. It is suggested that the latency relaxation may be due to a reduction of the filamentary resting tension.7. The experiments with the latency relaxation were facilitated by the use of strongly hypertonic solutions. The positive twitch tension is then greatly reduced, or may be practically abolished, while the latency relaxation remains at about its normal size and is extended in duration.