Effects of 1.9 MeV monoenergetic neutrons onVicia faba chromosomes: Microdosimetric considerations

Summary AeratedVicia faba root meristems were irradiated with 1.9 MeV monoenergetic neutrons. This source of neutrons optimally provides one class of particles (recoil protons) with ranges able to traverse cell nuclei at moderate to high-LET. The volumes of theVicia faba nuclei were log-normally distributed with a mean of 1100 µm³. The yield of chromatid-type aberrations was linear against absorbed dose and near-constant over 5 collection periods (2–12 h), after irradiation. Energy deposition events (recoil protons) determined by microdosimetry were related to cytological changes with the finding that 19% of incident recoil protons initiate visible changes inVicia faba chromosomes. It is probable that a substantial fraction of recoil proton track length and deposited energy is in insensitive (non-DNA containing) portions of the nuclear volume.     

Microdosimetry model for boron neutron capture therapy: I. Determination of microscopic quantities of heavy particles on a cellular scale

Due to the limitations of existing microdosimetry models, a new model called MICOR has been developed to analyze the spatial distribution of microscopic energy deposition for boron neutron capture therapy (BNCT). As in most existing models, the reactions independent of the incident neutron energy such as the boron and the nitrogen capture reactions can be considered. While other models do not include reactions that are dependent on the neutron energy such as the proton recoil reaction, the present model is designed so that the energy deposition resulting from these reactions is included. The model MICOR has been extended to enable the determination of the biological effects of BNCT, which cannot be done with the existing models. The present paper describes the determination of several microscopic quantities such as the number of hits, the energy deposition in the cell nucleus, and the distribution of lineal and specific energy deposition. The companion paper (Radiat. Res. 155, 000-000 2001) deals with the conversion of these microscopic quantities into biological effects. The model is used to analyze the results of a radiobiological experiment performed at the HB11 facility in the HFR in Petten. This analysis shows the value of the model in determining the dose depositions on a cellular scale and the importance of the extension to the energy deposition of the proton recoil.     

Hydrodynamic stability in posthatchling loggerhead (Caretta caretta) and green (Chelonia mydas) sea turtles

Swimming animals may experience a wide range of destabilizing forces resulting from the movements of their propulsors. These forces often cause movements in directions other than the intended trajectory (i.e., recoil motions), potentially increasing locomotor costs. We quantified rectilinear swimming stability for posthatchling loggerhead (Caretta caretta) and green turtles (Chelonia mydas). Sea turtles predominantly swim via “aquatic flight”, which is characterized by synchronous dorsoventral flapping of their forelimbs. We tested four predictions about the effects of “aquatic flight” on stability: (1) it would produce little lateral recoil; (2) lateral recoil motions would be non-cyclic; (3) vertical recoil motions would be larger than lateral recoil motions; and (4) vertical recoil motions would be cyclic. Additionally, because posthatchling loggerheads possess dorsal keels on the shell that are absent in green turtles, we evaluated whether such keels might improve stability in swimming turtles. While our expectations for patterns of cyclicity in recoil motions (predictions 2 and 4) were met, our expectations for differences in their absolute and relative magnitudes (predictions 1 and 3) were not. We suggest that lateral recoil motions were greater than predicted due to slight asynchronies between the motions of the left and right foreflippers. Additionally, although minimum lateral recoil motions were smaller than minimum vertical recoil motions, maximum recoil motions were greater in the lateral direction, so that average recoil did not differ significantly between these directions. Finally, because loggerheads did not display higher levels of stability compared to green turtles, there is little evidence to support a stabilizing role for dorsal keels in loggerhead turtles.     

Wheat seed proteins exhibit a complex mechanism of protein elasticity

Elastomeric proteins are found in a number of animal tissues (elastin, abductin and resilin), where they have evolved to fulfil a range of biological functions. All exhibit rubber-like elasticity, undergoing deformation without rupture, storing the energy involved in deformation, and then recovering to their initial state when the stress is removed. The second part of the process is passive, entropy decreasing when the proteins are deformed, with the higher entropy of the relaxed state providing the driving force for recoil. In plants there is only one well-documented elastomeric protein system, the alcohol-soluble seed storage proteins (gluten) of wheat. The elastic properties of these proteins have no known biological role, the proteins acting as a store for the germinating seed. Here we show that the modulus of elasticity of a group of wheat gluten subunits, when cross-linked by gamma-radiation, is similar to that of the cross-linked polypentapeptide of elastin. However, thermoelasticity studies indicate that the mechanism of elastic recoil is different from elastin and other characterized protein elastomers. Elastomeric force, f, has two components, an internal energy component, f(e), and an entropic component, f(s). The ratio f(e)/f can be determined experimentally; if this ratio is less than 0.5 the elastomeric force is predominantly entropic in origin. The ratio was determined as 5.6 for the cross-linked high M(r) subunits of wheat glutenin and near zero for the cross-linked polypentapeptide of elastin. Tensile stress must be entropic or energetic in origin, the results would suggest that elastic recoil in the wheat gluten subunits, in part, may be associated with extensive hydrogen bonding within and between subunits and that entropic and energetic mechanisms contribute to the observed elasticity.     

Study of microdosimetric energy deposition patterns in tissue-equivalent medium due to low-energy neutron fields using a graphite-walled proportional counter

To improve radiation protection dosimetry for low-energy neutron fields encountered in nuclear power reactor environments, there is increasing interest in modeling neutron energy deposition in metrological instruments such as tissue-equivalent proportional counters (TEPCs). Along with these computational developments, there is also a need for experimental data with which to benchmark and test the results obtained from the modeling methods developed. The experimental work described in this paper is a study of the energy deposition in tissue-equivalent (TE) medium using an in-house built graphite-walled proportional counter (GPC) filled with TE gas. The GPC is a simple model of a standard TEPC because the response of the counter at these energies is almost entirely due to the neutron interactions in the sensitive volume of the counter. Energy deposition in tissue spheres of diameter 1, 2, 4 and 8 μm was measured in low-energy neutron fields below 500 keV. We have observed a continuously increasing trend in microdosimetric averages with an increase in neutron energy. The values of these averages decrease as we increase the simulated diameter at a given neutron energy. A similar trend for these microdosimetric averages has been observed for standard TEPCs and the Rossi-type, TE, spherical wall-less counter filled with propane-based TE gas in the same energy range. This implies that at the microdosimetric level, in the neutron energy range we employed in this study, the pattern of average energy deposited by starter and insider proton recoil events in the gas is similar to those generated cumulatively by crosser and stopper events originating from the counter wall plus starter and insider recoil events originating in the sensitive volume of a TEPC.     

Is the spring quality of muscle plastic?

During locomotion, major muscle groups are often activated cyclically. This alternate stretch-shorten pattern of activity could enable muscle to function as a spring, storing and recovering elastic recoil potential energy. Because the ability to store and recover elastic recoil energy could profoundly affect the energetics of locomotion, one might expect this to be an adaptable feature of skeletal muscle. This study tests the hypothesis that chronic eccentric (Ecc) training results in a change in the spring properties of skeletal muscle. Nine female Sprague-Dawley rats underwent chronic Ecc training for 8 wk on a motorized treadmill. The spring properties of muscle were characterized by both active and passive lengthening force productions. A single "spring constant (Deltaforce/Deltalength) from the passive length-tension curves was calculated for each muscle. Results from measurements on long heads of triceps brachii muscle indicate that the trained group produced significantly more passive lengthening force (P = 0.0001) as well as more active lengthening force (P = 0.0001) at all lengths of muscle stretch. In addition, the spring constants were significantly different between the Ecc (1.71 N/mm) and the control (1.31 N/mm) groups. A stiffer spring is capable of storing more energy per unit length stretched, which is of functional importance during locomotion.     

Tendon elasticity and muscle function

Vertebrate animals exploit the elastic properties of their tendons in several different ways. Firstly, metabolic energy can be saved in locomotion if tendons stretch and then recoil, storing and returning elastic strain energy, as the animal loses and regains kinetic energy. Leg tendons save energy in this way when birds and mammals run, and an aponeurosis in the back is also important in galloping mammals. Tendons may have similar energy-saving roles in other modes of locomotion, for example in cetacean swimming. Secondly, tendons can recoil elastically much faster than muscles can shorten, enabling animals to jump further than they otherwise could. Thirdly, tendon elasticity affects the control of muscles, enhancing force control at the expense of position control.     

Elastic structures in the back and their rôle in galloping in some mammals

A study of Fallow deer ( Dma dama ) and Domestic dog ( Canis familiaris ) leads to identification of an aponeurosis in the back as an important elastic strain energy store in galloping. Kinetic energy lost by the body, as the forelegs end their backward swing and the hind legs end their forward swing, is stored briefly as elastic strain energy, and recovered in an elastic recoil. Thus energy is saved, making galloping the most economical gait for high speeds. Some strain energy is also stored in muscle fibres and in the vertebral column. Mechanical tests on the aponeurosis and vertebrae lead to estimates of the quantities of energy involved.     

Dependence of calculus retropulsion dynamics on fiber size and radiant exposure during Ho:YAG lithotripsy

During pulsed laser lithotripsy, the calculus is subject to a strong recoil momentum which moves the calculus away from laser delivery and prolongs the operation. This study was designed to quantify the recoil momentum during Ho:YAG laser lithotripsy. The correlation among crater shape, debris trajectory, laser-induced bubble and recoil momentum was investigated. Calculus phantoms made from plaster of Paris were ablated with free running Ho:YAG lasers. The dynamics of recoil action of a calculus phantom was monitored by a high-speed video camera and the laser ablation craters were examined with Optical Coherent Tomography (OCT). Higher radiant exposure resulted in larger ablation volume (mass) which increased the recoil momentum. Smaller fibers produced narrow craters with a steep contoured geometry and decreased recoil momentum compared to larger fibers. In the presence of water, recoil motion of the phantom deviated from that of phantom in air. Under certain conditions, we observed the phantom rocking towards the fiber after the laser pulse. The shape of the crater is one of the major contributing factors to the diminished recoil momentum of smaller fibers. The re-entrance flow of water induced by the bubble collapse is considered to be the cause of the rocking of the phantom.     

Evidence for a vertebrate catapult: elastic energy storage in the plantaris tendon during frog jumping

Anuran jumping is one of the most powerful accelerations in vertebrate locomotion. Several species are hypothesized to use a catapult-like mechanism to store and rapidly release elastic energy, producing power outputs far beyond the capability of muscle. Most evidence for this mechanism comes from measurements of whole-body power output; the decoupling of joint motion and muscle shortening expected in a catapult-like mechanism has not been demonstrated. We used high-speed marker-based biplanar X-ray cinefluoroscopy to quantify plantaris muscle fascicle strain and ankle joint motion in frogs in order to test for two hallmarks of a catapult mechanism: (i) shortening of fascicles prior to joint movement (during tendon stretch), and (ii) rapid joint movement during the jump without rapid muscle-shortening (during tendon recoil). During all jumps, muscle fascicles shortened by an average of 7.8 per cent (54% of total strain) prior to joint movement, stretching the tendon. The subsequent period of initial joint movement and high joint angular acceleration occurred with minimal muscle fascicle length change, consistent with the recoil of the elastic tendon. These data support the plantaris longus tendon as a site of elastic energy storage during frog jumping, and demonstrate that catapult mechanisms may be employed even in sub-maximal jumps.     

Mechanically unfolding protein L using a laser-feedback-controlled cantilever

Force spectroscopy using the atomic force microscope (AFM) can yield important information on the strength and lifetimes of the folded states of single proteins and their complexes when they are loaded with force. For example, by mechanically unfolding concatenated proteins at different velocities, a dynamic force spectrum can be built up that allows reconstruction of the energy landscape that the protein traverses during unfolding. To characterize fully the unfolding landscape, however, it is necessary both to explore the entire force spectrum and to characterize each species populated during unfolding. In the conventional AFM apparatus, force is applied to the protein construct through a compliant cantilever. This limits the dynamic range of the force spectrum that can be probed, and the cantilever recoil after unfolding may mask the presence of metastable intermediates. Here, we describe to our knowledge a new technique—constant-deflection AFM—in which the compliance of the AFM cantilever is removed. Using this technique, we show that protein L exhibits a more complex unfolding energy landscape than previously detected using the conventional technique. This technique is also able to detect the presence of a refolding intermediate whose formation is otherwise prevented by cantilever recoil.     

Indirect evidence for elastic energy playing a role in limb recovery during toad hopping

Elastic energy is critical for amplifying muscle power during the propulsive phase of anuran jumping. In this study, we use toads (Bufo marinus) to address whether elastic recoil is also involved after take-off to help flex the limbs before landing. The potential for such spring-like behaviour stems from the unusually flexed configuration of a toad''s hindlimbs in a relaxed state. Manual extension of the knee beyond approximately 90° leads to the rapid development of passive tension in the limb as underlying elastic tissues become stretched. We hypothesized that during take-off, the knee regularly extends beyond this, allowing passive recoil to help drive limb flexion in mid-air. To test this, we used high-speed video and electromyography to record hindlimb kinematics and electrical activity in a hindlimb extensor (semimembranosus) and flexor (iliofibularis). We predicted that hops in which the knees extended further during take-off would require less knee flexor recruitment during recovery. Knees extended beyond 90° in over 80% of hops, and longer hops involved greater degrees of knee extension during take-off and more intense semimembranosus activity. However, knee flexion velocities during recovery were maintained despite a significant decrease in iliofibularis intensity in longer hops, results consistent with elastic recoil playing a role.     

A strain energy function for lung parenchyma

The strain energy for the air-filled lung is calculated from a model of the parenchymal microstructure. The energy is the sum of the surface energy and the elastic energies of two tissue components. The first of these is the peripheral tissue system that provides the recoil pressure of the saline-filled lung, and the second is the system of line elements that form the free edges of the alveolar walls bordering the alveolar ducts. The computed strain energy is consistent with the observed linear elastic behavior of parenchyma and the data on large deformations around blood vessels.     

Contribution of tree structures in the lung to lung elastic recoil

The direct contribution of forces in tree structures in the lung to lung recoil pressure and changes in recoil pressure induced by alterations of the forces are analyzed. The analysis distinguishes the contributions of axial and circumferential tensions in the trees and indicates that only axial tensions directly contribute to static recoil. This contribution is derived from analysis of the axial forces transmitted across a random plane transecting the lung. The change in recoil pressure induced by changes in axial tension is similarly derived. Alterations of circumferential tensions in the trees indirectly change recoil by causing nonuniform deformations of the surrounding lung parenchyma, and a continuum elasticity solution for the stress induced by the deformations is derived. Sample calculations are presented for the airway tree based on available data on airway morphometric and mechanical properties. The increase in recoil pressure accompanying increases in axial and circumferential tensions with contraction of airway smooth muscle is also analyzed. The calculations indicate that axial stresses in the airway tree out to bronchioles directly contribute only a small fraction of the static recoil pressure. However, it is found that contraction of smooth muscle in these airways can increase recoil pressure appreciably (10-20%), mainly by the deformation of the parenchyma with increases in circumferential tension in smaller airways. The results indicate that the geometric and mechanical properties of the airway tree are such that only peripheral elements of the tree can substantially affect the elastic properties of the lung. The possible contributions of vascular trees for which data on mechanical and morphometric properties are more limited are also discussed.     

Effect of bronchomotor tone on static mechanical properties of lung and ventilation distribution

To study the relationship between bronchomotor tone, static mechanical properties of the lung, and ventilation distribution, we measured the pressure-volume (P-V) curve of the lung and several ventilatory indexes before and after intravenous atropine in eight normal subjects. The indexes of ventilation distribution were derived from multiple breath N2 washouts by a recently developed analysis (7,8). The latter not only provides a sensitive measure of overall ventilation inhomogeneity but distinguishes between the convection-dependent inhomogeneity (CDI) among larger lung units and that due to the interaction of convection and diffusion (DCDI) within the lung periphery. Atropine decreased lung elastic recoil but distensibility, as defined by the exponent (K) in the monoexponential analysis of the P-V data, was unchanged. The overall ventilation inhomogeneity increased by 37% after atropine (P less than 0.02) due to an increase in the CDI component. More importantly, there was a significant correlation between the loss of lung recoil (but not K) and each of the indexes of CDI among the subjects. There was no correlation between the changes in lung recoil and in DCDI. Our findings indicate that normal bronchomotor tone contributes to the elastic recoil of the lung. Furthermore, the tone is distributed in a way that enhances the uniformity of ventilation distribution among diffusion-independent lung units. Presumably this is achieved by minimizing interacinar intrinsic inequalities in static mechanical properties.     

Elastic strain energy storage in the feet of running monkeys

Monkeys are 'flat–footed' in comparison to humans, but they are still able to utilize elastic strain energy stores in their feet to reduce the metabolic energy cost of running. During contact with the ground, bending moments act on the foot to produce a 'reversed arch', storing strain energy which is returned in the subsequent elastic recoil. This energy–saving mechanism has been investigated in dynamic bending tests and is discussed in relation to the arched foot of humans.     

Lung elastic recoil during breathing at increased lung volume.

During dynamic hyperinflation with induced bronchoconstriction, there is a reduction in lung elastic recoil at constant lung volume (R. Pellegrino, O. Wilson, G. Jenouri, and J. R. Rodarte. J. Appl. Physiol. 81: 964-975, 1996). In the present study, lung elastic recoil at control end inspiration was measured in normal subjects in a volume displacement plethysmograph before and after voluntary increases in mean lung volume, which were achieved by one tidal volume increase in functional residual capacity (FRC) with constant tidal volume and by doubling tidal volume with constant FRC. Lung elastic recoil at control end inspiration was significantly decreased by approximately 10% within four breaths of increasing FRC. When tidal volume was doubled, the decrease in computed lung recoil at control end inspiration was not significant. Because voluntary increases of lung volume should not produce airway closure, we conclude that stress relaxation was responsible for the decrease in lung recoil.     

Muscle power attenuation by tendon during energy dissipation

An important function of skeletal muscle is deceleration via active muscle fascicle lengthening, which dissipates movement energy. The mechanical interplay between muscle contraction and tendon elasticity is critical when muscles produce energy. However, the role of tendon elasticity during muscular energy dissipation remains unknown. We tested the hypothesis that tendon elasticity functions as a mechanical buffer, preventing high (and probably damaging) velocities and powers during active muscle fascicle lengthening. We directly measured lateral gastrocnemius muscle force and length in wild turkeys during controlled landings requiring rapid energy dissipation. Muscle-tendon unit (MTU) strain was measured via video kinematics, independent of muscle fascicle strain (measured via sonomicrometry). We found that rapid MTU lengthening immediately following impact involved little or no muscle fascicle lengthening. Therefore, joint flexion had to be accommodated by tendon stretch. After the early contact period, muscle fascicles lengthened and absorbed energy. This late lengthening occurred after most of the joint flexion, and was thus mainly driven by tendon recoil. Temporary tendon energy storage led to a significant reduction in muscle fascicle lengthening velocity and the rate of energy absorption. We conclude that tendons function as power attenuators that probably protect muscles against damage from rapid and forceful lengthening during energy dissipation.     

Contribution of the tendinous tissue to force enhancement during stretch-shortening cycle exercise depends on the prestretch and concentric phase intensities.

When the prestretch intensity and concentric work are increased in stretch-shortening cycle (SSC) exercises, the utilization of the elastic energy can increase during the concentric phase. In order to further understand this process during SSC exercises, the interaction between fascicle-tendinous tissues (TT) of the vastus lateralis (VL) muscle was examined under different prestretch and rebound intensity drop jumps. Ten male subjects participated in the study. Direct VL fascicle lengths (N = 10) and in vivo patellar tendon force (N = 1) were measured together with the electromyographic (EMG) activity of VL during the trials. With increasing drop height but the same rebound height condition, the TT stretch increased during the early braking phase with a subsequent increase in its recoil during the early push-off phase. This occurred concomitantly with decreased fascicle shortening and EMG activation. However, with the increased rebound height but the same drop height condition, the fascicles were stretched less during the late braking phase with higher EMG activation. In this situation, TT could be stretched more by the tension provided by fascicles. Consequently, the TT recoil increased during the late push-off phase. These observations confirm that there can be an intensity specific fascicle-TT interaction during SSC exercises.     

Elastic mechanisms in primate locomotion

Tendons that stretch elastically and recoil, as the forces on them rise and fall, can save energy in running by enabling the animal to make do with shorter or slower muscle fascicles, that can generate force more economically. Non-human primates have rather long fascicles and thick tendons in their distal leg muscles and so seem poorly adapted to save energy in this way. Additional savings are made possible by the elastic compliance of ligaments in the foot. Though tendon and ligament compliance tend to save energy, the compliance of branches tends to increase the energy cost of arboreal locomotion.