Internal Shortening Research Articles

Regional Structural Synthesis, Wyoming Salient of Western Overthrust Belt

Surface geologic mapping, regional and high-density reflection seismic data and information from approximately 370 wells have been combined to describe geometrically that area of the Western Overthrust belt between the Snake River plain and the Uinta uplift. Particular care has been taken to verify two-dimensional interpretations of multiple thrust sheets by linear restoration. By establishing equivalence of pre-thrust lengths of affected beds, one can gain confidence in interpreted structural geometries, as well as generate data in regard to internal shortening and deformation intensity. Because of irresoluable geometric problems within sections, 47 cross sections were developed at roughly 6-mi (10 km) spacing to help verify changes in displacement or placement of key st uctural elements. Displacements of 0 to 32 mi (0 to 51 km) have been demonstrated. Jurassic to Paleocene (Sevier) thrusts were active across a previously deformed cratonic shelf terrane and interacted with active structural elements such as the Uinta uplift, Gros Ventre Range, and Moxa arch. Thrusts are progressively younger to the east except for the Darby-Prospect pair in which the Darby system is younger.

Contractility and the length-tension relation of the dog anterior tibial artery.

Active force (S) and rate of change of force (S) throughout the time of isometric contraction in electrically stimulated, circumferentially cut segments of dog anterior tibial artery were analyzed for dynamic indices of the contractile state. Previous works have determined indices that change with temperature but not with length (L) or calcium (Ca) and were designated indices of crossbridge dynamics. They assumed that changes in L and Ca change only the number of crossbridges between contractile filaments and temperature affects only the dynamics of individual crossbridges. We have shown: (1) specific ratios of S/S to be independent of L; (2) S/S at the same instant of time (VCE) is related to an index from previous work which was defined as a velocity of internal shortening; (3) specific indices of S/S were decreased with lower temperature but increased with time in a Ca-free solution. Our results for a Ca-free solution do not agree with previous work but can be explained by changes of the smooth muscle cell membrane. This suggests that L-independent indices are useful for characterizing changes in the overall dynamic state of the tissue but are not necessarily representative of a change in crossbridge dynamics.

Do laser diffraction studies on striated muscle indicate stepwise sarcomere shortening?

ILLUMINATION of striated muscle with a laser beam and measurement of the ensuing diffraction angles is a widely used method for the determination of sarcomere lengths. Pollack et al.1 recently presented results showing that during internal shortening of single muscle fibres the angle of the diffracted first order beam changes in stepwise fashion. As the most likely explanation for this finding they suggested that the majority of the sarcomeres in the field illuminated by the laser beam, after having started shortening, suddenly cease shortening for several milliseconds before continuing the shortening process. The authors point out that this possibility would impose serious complications on the cross-bridge theory as presently envisaged2,3, and in fact, on any proposed molecular theory of contraction. On the basis of diffraction studies using an experimental arrangement similar to that of Pollack et al. we now propose that the steps in the records of sarcomere length during contraction are an expression of slight differences in sarcomere length existing in myofibrillar clusters within the muscle fibre and therefore do not conflict with present contraction theories.

Absence of a plateau in length-tension relationship of rabbit papillary muscle when internal shortening is prevented.

IN the length–tension relationship of the isolated papillary muscle there is a region in which increased muscle length is associated with an increased force of contraction. Several problems make it difficult to determine the physiological basis for this relationship. Thick and thin filament lengths and sarcomere lengths have not been determined with the same accuracy in heart muscle as in skeletal, though there is evidence that these lengths are nearly the same in both muscles1–3. Papillary muscle cannot normally be tetanised or made to contract at low temperatures, and there is reason to believe that it is not fully activated by a single stimulus in the usual bathing medium4,5. In at least some papillary muscles there is considerable internal shortening during a twitch contraction6,7. Such length changes during contraction could change the amount of force developed by affecting the amount of thick and thin filament overlap and by producing shortening deactivation8,9. We have developed a photoelectronic servo apparatus similar to that developed by Gordon et al. (see refs 10 and 11 for details of operation) for use with papillary muscles to hold constant the length of a segment of preparation during contraction.

Usefulness and Limitations of Assessment of Internal Shortening Velocity by Ultrasound in Man

Measurement of circumferential shortening velocity offers an attractive method for assessing left ventricular performance during ejection since both the extent of shortening and the time factor ( i e , the rate of shortening) are considered. Initially, a complex angiographic approach utilizing frame-byframe analysis of the left ventricular cavity silhouette was employed.' It was postulated that at the time of maximum wall stress during ejection, the rate of series elastic extension should-be zero, and therefore the instantaneous shortening velocity at peak wall tension should be equal to contractile element velocity ( Vce ) . Since intraventricular pressure, chamber radius and wall thickness can be measured throughout systole, instantaneous velocity for any given wall stress can be derived. In each instance, velocity was by dividing the corresponding instantaneous circumference so that chambers of different size could be compared. This method proved fruitful and it was possible to separate patients with normal from those with impaired left ventricular performance. Mirsky andhis colleagues2 have provided a mathematical rationale for the normalized velocitv concept. The behavior of heart muscle can be described using exponential stress-strain characteristics

The compliance of contracting skeletal muscle.

1. The method of controlled releases was used to obtain tension-extension curves in toad (Bufo bufo) sartorii under a variety of conditions at 0 degrees C.2. The curves obtained were approximately linear over a considerable range of force (0.4P(0) to P(0)) if the releases were given from the plateau of tetanic tension. The slope of this linear region was little affected by changes of release velocity in the range 10-120 mm/sec.3. Such changes as did occur with alterations in release velocity could be quantitatively accounted for in terms of the internal shortening predicted by A. V. Hill's two-component model.4. As the muscles were stretched above l(0), we found that the maximum stiffness of the tetanized muscles fell in much the same way as the maximum developed force, P(0).5. In another series of experiments we found a rapid change in the overall shape of the tension-extension curve during the early phase of force development in an isometric tetanus. The stiffness of the muscle increased with increasing developed force during this period.6. The force-velocity curve in these muscles was measured by two methods, both giving a similar result. Surprisingly, toad muscle appears to have about the same intrinsic speed as frog muscle at 0 degrees C. The a.b product from our experiments is considerably greater than the reported values for the maintenance heat rate at 0 degrees C in these muscles.7. The probable site of the variable compliance in active muscle is discussed. It seems most likely that this is within the A-band, perhaps in the cross-bridges themselves.

The metabolism of phosphocreatine during an isometric tetanus in the frog sartorius muscle

1. 1. A study was made of the phosphocreatine breakdown in anaerobic iodoacetate-poisoned muscle associated with one or several isometric tetani at o°. 2. 2. The rate of phosphocreatine breakdown in the course of tetanization, apart from the earliest moments, was 0.28 μmole/g/sec. This was constant for up to 60 sec until the near-exhaustion of all phosphocreatine, although the tension decreased by 0.89% per sec, and was not affected by dividing the period of stimulation into several separate tetani. 3. 3. This rate of phosphocreatine breakdown is consistent with known values of the maintenance heat if the apparent enthalpy change for phosphocreatine hydrolysis and associated ionization changes amounts to —10 000 cal/mole. 4. 4. An extra-metabolism was found which was proportional to the number of tetani and not to their durations, and which averaged 0.33 μmole/g/tetanus. 5. 5. A small part of this extra-metabolism is due to elastic work and internal shortening during tension generation, and a larger part may represent the residual activation heat during relaxation. The remainder appears insufficient to account for the “labile part” of the maintenance heat, at the present state of knowledge. However, several aspects of this problem require further examination.

The metabolism of phosphocreatine during an isometric tetanus in the frog sartorius muscle

1. 1. A study was made of the phosphocreatine breakdown in anaerobic iodoacetate-poisoned muscle associated with one or several isometric tetani at o°. 2. 2. The rate of phosphocreatine breakdown in the course of tetanization, apart from the earliest moments, was 0.28 μmole/g/sec. This was constant for up to 60 sec until the near-exhaustion of all phosphocreatine, although the tension decreased by 0.89% per sec, and was not affected by dividing the period of stimulation into several separate tetani. 3. 3. This rate of phosphocreatine breakdown is consistent with known values of the maintenance heat if the apparent enthalpy change for phosphocreatine hydrolysis and associated ionization changes amounts to —10 000 cal/mole. 4. 4. An extra-metabolism was found which was proportional to the number of tetani and not to their durations, and which averaged 0.33 μmole/g/tetanus. 5. 5. A small part of this extra-metabolism is due to elastic work and internal shortening during tension generation, and a larger part may represent the residual activation heat during relaxation. The remainder appears insufficient to account for the “labile part” of the maintenance heat, at the present state of knowledge. However, several aspects of this problem require further examination.

The influence of temperature on the tension developed in an isometric twitch

In muscular contraction the development of tension requires that the contractile component should shorten and stretch the series elastic component. In an isometric twitch the maximum tension is reached as a balance between two opposing processes, internal shortening on the one hand and decay of the active state (relaxation) on the other. The fact that the maximum tension in a twitch is considerably less than in a tetanus has been attributed to oncoming relaxation allowing insufficient time for internal shortening to be completed. The maximum tension in a twitch is considerably reduced by a rise of temperature, while that in a tetanus is somewhat increased. This would require that the temperature coefficient of the velocity of shortening should be substantially less than that of the decay of activity. Evidence for this exists. On this view the effect of a quick stretch, applied during the early stage of a twitch, in increasing the tension ratio, stretch/isometric, should be much greater at a higher temperature. This expectation is confirmed on frogs’ muscles over the range 0 to 20°C. The effect of temperature, therefore, on the size of a twitch can be attributed to the difference between the temperature coefficients of velocity of shortening and rate of relaxation.

The energetics of relaxation in a muscle twitch

When a muscle relaxes under a load heat is produced which is equivalent, or nearly equiva­lent, to the mechanical energy which disappears. The appearance of this heat is simultaneous with the disappearance of the mechanical energy. When a muscle is without load, or under zero tension, during the interval normally occupied by relaxation, there is no measureable heat after shortening is complete. This is true whatever the initial load, or the work done. If chemical changes occur during relaxation their net thermal effect is negligibly small. The heat produced in an isometric contraction is of a very complex nature, involving internal shortening, the transfer of work from one part to another and the dissipation of mechanical energy as heat. The nature of relaxation is discussed, and an analogy described with the process of ʻcold­ drawing’ of a crystalline long-chain polymer.