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R Lakes

Publications and source records attributed to R Lakes.

15 recordsLinked to original sources
PubMed2001

Nonlinear ligament viscoelasticity.

Ligaments display time-dependent behavior, characteristic of a viscoelastic solid, and are nonlinear in their stress-strain response. Recent experiments (25) reveal that stress relaxation proceeds more rapidly than creep in medial collateral ligaments, a fact not explained by linear viscoelastic theory but shown by Lakes and Vanderby (17) to be consistent with nonlinear theory. This study tests the following hypothesis: nonlinear viscoelasticity of ligament requires a description more general than the separable quasilinear viscoelasticity (QLV) formulation commonly used. The experimental test for this hypothesis involves performing both creep and relaxation studies at various loads and deformations below the damage threshold. Freshly harvested, rat medial collateral ligaments (MCLs) were used as a model. Results consistently show a nonlinear behavior in which the rate of creep is dependent upon stress level and the rate of relaxation is dependent upon strain level. Furthermore, relaxation proceeds faster than creep; consistent with the experimental observations of Thornton et al. (25) The above results from rat MCLs are not consistent with a separable QLV theory. Inclusion of these nonlinearities would require a more general formulation.

Animals↗
PubMed2000

Viscoelastic dissipation in compact bone: implications for stress-induced fluid flow in bone.

Viscoelastic properties of wet and dry human compact bone were studied in torsion and in bending for both the longitudinal and transverse directions at frequencies from 5 mHz to 5 kHz in bending to more than 50 kHz in torsion. Two series of tests were done for different longitudinal and transverse specimens from a human tibia. Wet bone exhibited a larger viscoelastic damping tan delta (phase between stress and strain sinusoids) than dry bone over a broad range of frequency. All the results had in common a relative minimum in tan delta over a frequency range, 1 to 100 Hz, which is predominantly contained in normal activities. This behavior is inconsistent with an optimal "design" for bone as a shock absorber. There was no definitive damping peak in the range of frequencies explored, which could be attributed to fluid flow in the porosity of bone.

Aged↗
PubMed1996

Observation of the retina using the tandem scanning confocal microscope.

A tandem scanning confocal microscope (TSCM) is currently being used to obtain high-resolution images of the human cornea in vivo. Advantages of confocal microscopy for in vivo imaging include optical sectioning and increased contrast through removal of scattered light. We have adapted to TSCM to view the retina in vivo by constructing an applanating lens and fitting the microscope with an imaging-intensifying camera of increased sensitivity. The microscope uses a spinning disc with 40,000 holes, each of 30 microns diameter, and a 100 W mercury arc lamp light source with a 455 nm long pass filter. The applanating lens is composed of three elements, two of which are movable for focusing. Images of a rabbit retina were obtained in vivo revealing the nerve fiber layer and blood vessels around the optic disc. The power density at the retina was calculated to be 3 mW/cm2, which is well below the power levels of a direct or indirect ophthalmoscope. Magnification of the retinal image was approximately 60 x and a 1 mm wide area of retina was in view. This prototype TSCM system demonstrates that images of a retina in vivo are obtainable with confocal microscopy and that the sharpness is comparable to standard fundus camera photography. Further modifications to improve the light level and alterations in the design of the objective should improve the quality of the images obtained and achieve the enhanced resolution of which, in theory, the confocal microscope is capable.

Animals↗
PubMed1992

Structure, receptor cell arrangement and function of the auditory organs in the foreleg tibia of three bushcricket species.

Comparative physiological and morphological studies were carried out on the auditory receptor organs of the forelegs in three closely related tettigoniid species (Psorodonotus illyricus, Decticus albifrons and Decticus verrucivorus). Functional adaptations of the receptor organs to the carrier frequencies of the conspecific song were found. The proportions of low- to high-frequency receptor cells within the organs of the three species are clearly different. This proportion is large in Psorodonotus illyricus, smaller in Decticus albifrons, and smallest in Decticus verrucivorus. These proportions are responsible for the different "frequency weighting" found in the activity of the receptor populations of the organs. The physiology of the receptor cells of the three species have been correlated with the dimensions of the critical stimulus-transforming structures inside the organs. This leads to an explanation of the different frequency selectivity of the receptor cells.

Acoustic Stimulation↗
PubMed1991

Regeneration of the projection and synaptic connections of tympanic receptor fibers of Locusta migratoria (Orthoptera) after axotomy.

The tergite nerve N6 of the first abdominal segment of the locust Locusta migratoria contains receptor fibers, from the tympanic organ, and hair sensilla as well as motoric axons. The nerve was axotomized in nymphal instars or adults, and the regeneration of nerve fibers was studied. The sensory fibers regrow and regenerate their projection pattern within the central nervous system. They recognize their specific neuropile areas even after entering the ganglion through different pathways. The receptor fibers of the tympanic organ reestablish synaptic connections to auditory interneurons, even though the physiological characteristics of the interneurons are not fully restored. This regenerative capability contrasts with the lack of regeneration of peripheral structures in locusts, but supports the described plasticity in the auditory system of monaural locusts (Lakes, Kalmring, and Engelhard, 1990). The motor fibers do not regenerate nerves innervating muscles of the body wall.

Animals↗
PubMed1990

The development of the sensory organs of the legs in the blowfly, Phormia regina.

The development of the sensory neurons of the legs of the blowfly, Phormia regina has been described from the third instar larva to the late pupa using immunohistochemical staining. The leg discs of the third instar larva contain 8 neurons of which 5 come to lie in the fifth tarsomere of the developing leg. Whereas 2 neurons persist at least to the late pupa, the other cells degenerate. The first neurons of gustatory sensilla arise in the fifth tarsomere at about 1.5 h after formation of the puparium. Most of these sensilla, however, appear within a short time period beginning at about 18 h. The femoral chordotonal sensory neurons first appear at the time of formation of the puparium, as a mass of cells situated in the distal femur. During later pupal development 2 groups of these cells come to lie at the femur-trochanter border, where they become the proximal femoral chordotonal organ of the adult; the remaining cells become the distal femoral chordotonal organ. Other scolopidial neurons appear later in development. The nerve pathways of the late pupal leg are established either by the axons of the cells that are present in the larval leg disc or by new outgrowing processes of sensory neurons. In the tibia, the initial direction of new outgrowth differs in different regions of the segment: proximal tibial neurons grow distally, while distal tibial neurons grow initially proximally.

Animals↗
PubMed1986

Ultrasonic wave propagation and attenuation in wet bone.

The propagation of ultrasonic longitudinal waves in bovine plexiform and human Haversian bone has been studied over the range 0.5-16 MHz. In wet bone little velocity dispersion was observed, in contrast to the results of earlier studies on dry bone. Large values of attenuation were observed.

Animals↗
PubMed1983

Slow compressional wave propagation in wet human and bovine cortical bone.

Ultrasonic wave propagation in bovine plexiform and human Haversian bone was studied in the range 0.5 to 15 megahertz. A new longitudinal wave was observed which traveled more slowly than the ordinary longitudinal wave. The slow wave was associated with the dynamics of fluid motion in the pores of bone.

Animals↗
PubMed1980

Long-term torsional creep in compact bone.

Bovine bone specimens loaded in torsion for periods of time exceeding one month exhibit pronounced time-dependent effects. The final strain exceeded the initial strain by a factor of more than four in some cases. Creep failure was observed in two cases. The effects are not attributable to prolonged immersion in Ringer's solution.

Animals↗
PubMed1979

Cement line motion in bone.

Compact bovine bone subjected to constant torsional load for long periods of time exhibits large anelastic effects. Displacements occur at the cement lines and are responsible for part or all of the long-term deformation. The absence of an asymptotic creep strain is consistent with an interpretation of the cement line as a viscous interface.

Animals↗