Collagen fibrils and elastic fibers in rat-tail tendon: an electron microscopic investigation

Earlier studies by the authors showed that the collagen fibrils in rat-tail tendon have a bi-modal distribution of fibril diameters from a time shortly after birth through to the onset of maturity at about 3–4 months. Present work has extended those observations for rats up to the age of 2 years. Histograms of the fibril diameter distributions for mature tail tendon and direct electron microscope observations show that the fibrils break down as the tendon ages. Further work on the constant diameter subfibrils of diameter 140 Å described previously, has confirmed that these are part of the elastic fibers present in tendon at all ages. It has been shown that there is relatively little variation in the collagen fibril diameter distribution as a function of the position of the specimen in the tail, and as the measured percentage of the area taken by the collagen fibrils present at any particular point. Estimation of the fibrillar collagen content of rat-tail tendon as a function of age indicates that it increases steadily from birth and reaches a maximum at the onset of maturity, beyond which the fibrillar collagen content appears to remain constant.     

The dynamics of the DNA hydration shell at gigahertz frequencies

We have used Brillouin scattering to measure the linewidths and frequencies of GHz acoustic phonons in Na- and Li-DNA films as a function of temperature between 300 and 140 K for samples that were dry, lightly, and heavily hydrated. The linewidths decrease with falling temperature and water contents, indicating that coupling to a water relaxation is the main source of phonon damping. The strength of the relaxation was determined using measurements of the phonon linewidth as a function of frequency, and confirmed by comparison of measured and calculated spectral profiles. The relaxation strength is anisotropic, being greater for phonons propagating perpendicular to the helix axis. The hydrated DNA exhibits both a rapid relaxation (≤ 10^?11 s per radian) giving rise to a classical f² damping, and a slower motion with a relaxation time that varies from ～ 4 × 10^?11 s per radian (primary hydration shell) to ～ 2 × 10^?12 s per radian (secondary hydration shell) at room temperature. In the frequency interval that bounds these relaxation times (～ 4 to 80 GHz) we expect degrees of freedom associated with the primary hydration shell to be important. The sample with primary hydration follows a simple Arrhenius behavior with ΔH ～ 5 kcal mole^?1. The effective activation energy for the sample with secondary hydration is somewhat higher (indicating a more cooperative water relaxation) and varies strongly with temperature. The elastic moduli change much more than can be accounted for by relaxation, indicating the importance of water motion in softening interatomic potentials. The extent of the softening caused by the “unfreezing” of water motion is similar to the degree of softening caused by hydrating the sample.     

Quantitative electron microscope observations of the collagen fibrils in rat-tail tendon

An electron microscope study of collagen fibrils from fixed tail tendons of rats has revealed that from some time shortly after birth until maturity, the fibril diameters have a bimodal distribution. The “two” types of fibril are indistinguishable in both transverse and longitudinal section. Unfixed specimens of eight-week-old-tail tendon showed a similar bimodal distribution of diameters though the positions of the peak values compared to fixed specimens of an eight-week-old-tail tendon were shifted upwards by about 30%. It has also been shown quantitatively that the polar collagen fibrils are directed randomly “up” and “down” with respect to their neighbors. Whilst it has been suggested by others that anastomosis is a feature of collagen structure, the results presented here do not support this hypothesis. Fibrillar units ～ 140 Å in diameter have been observed and the possibilities that these are elastic fibers or the breakdown products of collagen fibrils have been considered.     

Role of water content in dielectric properties and delayed luminescence of bovine Achilles' tendon

In order to investigate the role of water network in collagen structure, measurement of dielectric permittivity was performed on bovine Achilles' tendon as a function of water content. The data show a sudden decrease of the permittivity at each measured frequency value when the tendon humidity decreases. A similar behaviour is shown by the total number of photons emitted in delayed luminescence (DL) experiments. The comparison of the two results is in agreement with the hypothesis that DL is connected to the excitation and subsequent decay of collective electronic states, whose properties depend on the organized structure of the system.     

Intracellular PO2 kinetics at different contraction frequencies in Xenopus single skeletal muscle fibers.

Increasing contraction frequency in single skeletal muscle fibers has been shown to increase the magnitude of the fall in intracellular Po(2) (Pi(O(2))), reflecting a greater metabolic rate. To test whether Pi(O(2)) kinetics are altered by contraction frequency through this increase in metabolic stress, Pi(O(2)) was measured in Xenopus single fibers (n = 11) during and after contraction bouts at three different frequencies. Pi(O(2)) was measured via phosphorescence quenching at 0.16-, 0.25-, and 0.5-Hz tetanic stimulation. The kinetics of the change in Pi(O(2)) from resting baseline to end-contraction values and end contraction to rest were described as a mean response time (MRT) representing the time to 63% of the change in Pi(O(2)). As predicted, the fall in Pi(O(2)) from baseline following contractions was progressively greater at 0.5 and 0.25 Hz than at 0.16 Hz (32.8 +/- 2.1 and 29.3 +/- 2.0 Torr vs. 23.6 +/- 2.2 Torr, respectively) since metabolic demand was greater. The MRT for the decrease in Pi(O(2)) was progressively faster at the higher frequencies (0.5 Hz: 45.3 +/- 4.5 s; 0.25 Hz: 63.3 +/- 4.1 s; 0.16 Hz: 78.0 +/- 4.1 s), suggesting faster accumulation of stimulators of oxidative phosphorylation. The MRT for Pi(O(2)) off-kinetics (0.5 Hz: 84.0 +/- 11.7 s; 0.25 Hz: 79.1 +/- 8.4 s; 0.16 Hz: 81.1 +/- 8.3 s) was not different between trials. These data demonstrate in single fibers that the rate of the fall in Pi(O(2)) is dependent on contraction frequency, whereas the rate of recovery following contractions is independent of either the magnitude of the fall in Pi(O(2)) from baseline or the contraction frequency. This suggests that stimulation frequency plays an integral role in setting the initial metabolic response to work in isolated muscle fibers, possibly due to temporal recovery between contractions, but it does not determine recovery kinetics.     

On the velocity of conduction in nerve fibers with saltatory transmission

Using an electrical model to represent certain features of a nerve fiber together with a one-factor theory of excitation, an expression is obtained for the velocity of propagation of a nerve impulse along a nerve fiber exhibiting saltatory transmission. The velocity shows a maximum with respect to internodal length. A critical internodal length, a critical radius, and a critical value for the resistance of the external medium are required in order to make transmission of the impulse possible.     

Coding of acoustic information in cockroach giant fibers

Interneurons in the ventral nerve cord of Periplaneta americana are excited by sound stimuli to the cerci. The responsiveness of giant fibers in the nerve cord generally declines with increasing sound frequency but the frequency-response curve is complex with small sensitivity peaks along its course. The frequency-response curve for smaller interneurons differs from that of the largest giant fibers in having a pronounced sensitivity peak near 300 Hz. At sound frequencies below about 200 Hz, giant fiber spikes occur at the same frequency as impinging sound waves. Thus information about the frequency of sound stimuli is present in the nerve cord in the temporal pattern of activity in giant fibers at low sound frequencies, and in the spatial pattern of activity between large and small units of the nerve cord at higher sound frequencies.     

Connective-tissue macromolecules in Golgi chicken tendon organs and at their interface with muscle fibers and adjoining tendinous structures.

Tendon organs from leg and forearm muscles of white leghorn chickens were examined with a library of monoclonal antibodies to determine the composition of their connective-tissue framework and the types of connective-tissue macromolecules that occur at the sites where muscle fibers attach to the receptors. The capsules of the tendon organs were positive for connective-tissue macromolecules typical of basal lamina (collagen type IV, laminin, and heparin sulfate proteoglycan) and for tenascin, collagen types III and VI, and fibronectin. Connective-tissue bundles in the lumen of a receptor reacted primarily with antibodies against collagen type I and 4-chondroitin sulfate. The narrow partitions that divide each lumen into compartments stained for collagen type III. Toward its tendinous end, a receptor made few contacts with muscle fibers. Instead, the capsule and the collagenous bundles blended gradually with the intermuscular portions of tendons. At the muscular end, the connections were more complex. Muscle fibers that attached in series to tendon organs split to produce basal lamina-covered, finger-like extensions, which were separated from each other by fissures. Tongues of connective tissue containing tenascin, collagen types I and VI, and fibronectin extended into the fissures. Distally the tongues were continuous with the tenascin in the capsule and just internal to the capsule, fibronectin and basal lamina macromolecules in the capsule, and collagen type I in the collagenous bundles. The uninterrupted presence of these macromolecules around terminating muscle fibers and in the capsule and/or the intraluminal collagen bundles suggests that muscle fibers that attach in series at the muscular end exert a force during muscular contraction on the intraluminal collagen bundles and on the receptor capsule.     

Ion velocity distribution in a plasma with low-intensity ion acoustic turbulence

The quasi-steady ion distribution in a plasma with a single ion species and with low-intensity ion acoustic turbulence is found. Conditions are determined under which the stimulated scattering of ion acoustic waves by ions leads to the formation of a superthermal ion distribution function that decreases with increasing velocity more gradually than does a Maxwellian distribution function. It is found that the plasma conductivity increases as a result of a decrease in the turbulence level due to an enhancement of the Cherenkov damping of ion acoustic waves by resonant ions, whose number increases because of the formation of a gradually decreasing distribution of superthermal ions.     

Evaluation of leaf water status by means of permittivity at terahertz frequencies

We present an electromagnetic model of plant leaves which describes their permittivity at terahertz frequencies. The complex permittivity is investigated as a function of the water content of the leaf. Our measurements on coffee leaves (Coffea arabica L.) demonstrate that the dielectric material parameters can be employed to determine the leaf water status and, therefore, to monitor drought stress in plant leaves. The electromagnetic model consists of an effective medium theory, which is implemented by a third order extension of the Landau, Lifshitz, Looyenga model. The influence of scattering becomes important at higher frequencies and is modeled by a Rayleigh roughness factor.     

夏玉米叶片水分变化与光合作用和土壤水分的关系

叶片是光合作用的重要器官,其含水量的变化必将影响光合作用,但关于叶片水分变化对光合作用的影响报道较少。以华北夏玉米为研究对象,利用三叶期不同水分梯度的持续干旱模拟试验资料,分析夏玉米叶片水分变化及其与叶片净光合速率和土壤水分的关系。结果表明:夏玉米叶片净光合速率对叶片水分变化的响应显著且呈二次曲线关系,叶片含水量约为70.30%时,叶片净光合速率为零;叶片含水量与土壤相对湿度呈非直角双曲线关系,叶片最大含水量约为85.14%。研究结果可为准确描述叶片水分变化对光合作用的影响及客观辨识夏玉米干旱的发生发展及监测预警提供参考。     

A study of conduction velocity in nonmyelinated nerve fibers.

By treating a nonmyelinated nerve fiber as a continuous cable consisting of three distinct zones (Resting, transitional, and excited), the following mathematical expression was derived: (formula: see text) where v is the conduction velocity, d the diameter of the fiber, R the resistance of the membrane of unit area at the peak of excitation, rho the resistivity of the medium inside the fiber, and C the capacity of membrane per unit area. The validity of this expression was demonstrated by using squid giant nerve fibers intracellularly perfused with dilute salt solutions. The relationship between these results and previous theories and experiments on conduction velocity is discussed.