The Sliding Filament Model of Muscle Contraction. Cross-Bridge Maximal force production; Speed of contraction / speed of stimulation; Muscle fiber efficiency Force-Velocity Relationship Stimulation results in reflex relaxation of muscle. Force–velocity relationships of fresh and fatigued human explain the slow relaxation from an isometric contraction. power output over a complete contraction–relaxation cycle. The approach used muscle length (the length–tension relationship), the shortening or lengthening.
Upper and lower records show fiber shortening and step changes in load, respectively. The magnitude of step changes in load is given as fractions of Po.
Muscle - Force and velocity of contraction | relax-sakura.info
As the Huxley contraction model Figure 3 only describes distribution of myosin head attached to actin filament during constant velocity shortening, Podolsky and Nolan [ 10 ] proposed another contraction model to account for the isotonic velocity transients Figure 9. In contrast with the Huxley contraction model, the Podolsky-Nolan model assumes large values of f and a very small value of g in the positive x region, so that all myosin heads passing through this region form A-M links irrespective the velocity of filament sliding.
In the negative x region, g remains to be very small over a distance from the equilibrium position 0 and then increases to a large value Figure 9. As the result, the mode of distribution of A-M link under various loads is markedly different from that in the Huxley contraction model Figure By some additional assumptions, the Podolsky-Nolan model can explain not only the isotonic velocity transients, but also other muscle contraction characteristics and also heat measurement results.
Podolsky-Nolan contraction model constructed to explain the isotonic velocity transient. Upper and middle panel show f and g, i.
Lower panel shows dependence of elastic constant k of A-M link on x. Values of P are given at the left of diagrams. In each diagram, A-M link distribution immediately after quick changes in load is given by shaded area, while the subsequent steady A-M link distribution is given by solid line.
Schematic drawing of experimental arrangement to apply quick changes in load in two arbitrary steps. A single fiber P is mounted between force transducer T and lever L with clips C1 and C2, and stimulated maximally with Pt wire electrodes.
Lever L is pivoted at A and loaded by spring F1, which is hooked to lever L and another lever K, so that the length of F1 can be changed by micromanipulator G1 carrying K. Long arms of L and K are restrained by pairs of electromagnetically controlled stops S1 and S2 and S3 and S4, respectively.
Short arm of L serves as a vane interrupting light bean C directed towards photodiode not shown to serve as displacement transducer recording fiber length changes. With a pair of additional springs F2 and F3, whose lengths are adjusted by microman3 ipulators G2 and G3, the length of F1 can be changed quickly when S3 and S4 are removed to produce movement of K.
Oscillation of K is damped with Y shaped dashpot device H. After the fiber develops the maximum isometric force Po, stops restraining lever motion are removed in various sequences, so that the amount of load on the fiber can be changed in two arbitrary steps, as shown in Figure 12 and 13 From ref.
The early time course of isotonic shortening was similar to the isotonic velocity transients reported by Civan and Podolsky [ 9 ], while the early time course of isotonic lengthening was variable. The values of P are expressed relative to Po on the left of each record. Records of experiments, in which the load on the fiber was increased quickly after a period of isotonic shortening under a load of 0.
Note marked oscillatory length changes with alternate lengthening and shortening. To account for the marked fiber length oscillations with alternate fibre lengthening and shortening shown in Figure 13, Sugi and Tsuchiya [ 11 ] constructed a contraction model shown in Figure The dependence of the values of f and g, i.
Sugi-Tsuchiya contraction model constructed to explain the marked oscillatory fiber length changes following quick increases in load. The values of rate constants f and g for the formation and the breaking of A-M links, respectively, are shown as functions of distance from the myosin head equilibrium position 0.
As described above, no definite conclusion can be reached about what is actually taking place in muscle, though various contraction models have been presented to explain mechanical responses of muscle fibers in terms of changes in the A-M link distribution. Much more experimental work is needed for the full understanding of contraction mechanism. Force-Velocity Relation in Single Skinned Muscle Fibers To obtain information about the molecular mechanism of muscle contraction, the use of intact muscle fibers has serious limitations resulting from difficulties in precisely control chemical and ionic conditions around the myofilaments.
The difficulties can be overcome by the use of skinned muscle fibers, from with surface membrane is removed by mechanical or chemical means. To eliminate the gap between contraction characteristic of intact muscle or muscle fibers and biochemical studies on actomyosin ATPase reaction steps in solution, where the three-dimensional myofilament lattice is destroyed, skinned fibers are widely used and their characteristics including the force-pCa relation and MgATP concentration dependence of force development and shortening velocity have been obtained [ 1213 ].
Another great advantage in the use of skinned fibers is that the rate of ATP hydrolysis by the contractile system can be measured simultaneously with mechanical experiments by measuring the rate of ADP production by NADH fluorescence [ 14 ]. Due to structural instability of skinned fibers, however, it is difficult to obtain enough data points to obtain P-V relations, since fiber deterioration slowly proceeds in each contraction-relaxation cycle by the application of contracting and relaxing solutions [ 12 ].
Until the early s, it was necessary for us to use hand-made mechanical or electronic elements to construct such an experimental setup such as shown in Figure 9, with enormous technical skill and patience, because the performance of commercially available electronic devices at that time were not satisfactory for our purpose. Fortunately, it is now possible to perform sophisticated mechanical experiments including multi-step load changes and ramp decreases in force with instruments commercially available Aurora Scientific Inc.
They found that, in the presence of the antibody 1. In response to ramp decreases in force, skinned fibers shortened with velocities increasing with decreasing force, reaching a maximum at zero force. The P-V relations thus obtained are presented in Figure Despite the decreased steady isometric force, the value of Vmax remained unchanged Figure 16Aand when the P-V curves were found to be identical if they were normalized with respect to the maximum steady force attained Figure 16B.
These results indicate that the decrease in isomeric force by the antibody results from decrease in the number of myosin heads involved in force generation, while all myosin heads hydrolyze ATP irrespective of whether they generate force or do not generate force; in other words, individual myosin molecules in myosin filaments no longer generate force if the antibody attach to their subfragment-2 region, while their ATPase activity remains unchanged Figures 17 and Effect of antibody to myosin subfragment-2 on the isometric force development and the MgATPase activity in skinned muscle fibers.
The records were obtained before application Aand after min B and min C after application of antibody to myosin subfragment-2 1.
Times of application of contracting and relaxing solutions are indicated by upward and downward arrows, respectively. Upper and lower records show fiber length and force changes in the fiber, respectively.
Force-Velocity Relation: Its Implications about Molecular Mechanism of Muscle Contraction
Records A were taken before, and records B, C and D were taken at 30, 60 and 90 min after application of the antibody 1. The change in the maximum velocity of shortening and economy of force development occur because each myosin cross-bridge head cycles more slowly and remains in the attached force-producing state for a longer period of time. In the thyrotoxic type of hypertrophy, calcium is removed more quickly while there is a shift in myosin.
At the molecular level there are more sarcoplasmic reticular calcium pumps, while the myosin cross-bridge head cycles more rapidly and remains attached in the force-producing state for a shorter period of time.
The result is a heart that contracts much faster but less economically than normal and can meet the peripheral need for large volumes of blood at normal pressures.
Smooth muscle Because vertebrate smooth muscle is located in the walls of many hollow organs, the normal functioning of the cardiovascular, respiratory, gastrointestinal, and reproductive systems depends on the constrictive capabilities of smooth muscle cells.
Smooth muscle is distinguished from the striated muscles of the skeleton and heart by its structure and its functional capabilities. As the name implies, smooth muscle presents a uniform appearance that lacks the obvious striping characteristic of striated muscle.
Vascular smooth muscle shortens 50 times slower than fast skeletal muscle but generates comparable force using times less chemical energy in the process. These differences in the mechanical properties of smooth versus striated muscle relate to differences in the basic mechanism responsible for muscle shortening and force production. As in striated muscle, smooth muscle contraction results from the cyclic interaction of the contractile protein myosin i. The arrangement of these contractile proteins and the nature of their cyclic interaction account for the unique contractile capabilities of smooth muscle.
High intensity, metabolically demanding exercise leads to a considerable loss of muscle power, as has been shown in fatigued animal De Haan et al.
Whilst many fundamental mechanisms are common to all skeletal muscles, the balance between potential rate limiting steps during fatigue may vary between amphibian and mammalian muscles. Examples of this are the contribution of calcium re-uptake during relaxation Allen et al. The objective of the work described here was firstly to characterise the stretch response of the human adductor pollicis stimulated to contract in vivo and, secondly, to determine to what extent the different components of the response are affected by fatigue.
METHODS Subjects The study was approved by the local ethical committee and eight healthy female subjects aged years took part after giving their written informed consent. Female subjects were selected because in male subjects, who have, in general, larger and stronger hands, the motor system was not powerful enough to stretch at high velocities the adductor pollicis muscle.
The subjects were all right handed and did not undertake any regular exercise that particularly involved the hand muscles. They visited the laboratory on three different occasions. On the first occasion they were familiarised with the procedures and electrical stimulation. The measurements reported here were made on the second and third visits. Force recording and stimulation Methods for stimulating the adductor pollicis and recording force are given in detail elsewhere De Ruiter et al.
Briefly, the subject sat in an adjustable chair with the left forearm supinated, and the hand palm up securely fixed horizontally with the thumb abducted and in contact with a vertical rod. The rod was attached to a strain gauge, mounted below the plane of the hand, which measured the force applied by the thumb.
The rod was attached by a lever system to a linear motor so that controlled shortening and lengthening of the adductor pollicis could be achieved by rotating the thumb. A thumb angle of 0 deg was defined as the position in which the thumb was fully adducted and touching the index finger.