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D Weihs

Publications and source records attributed to D Weihs.

At least 19 recordsLinked to original sources
PubMed2001

Head stabilization in herons.

We examined head stabilization in relation to body mass and length of legs in four heron species (little egrets, Egretta garzetta; night herons, Nycticorax nycticorax; squacco herons, Ardeola ralloides; and cattle egrets, Bubulcus ibis: Aves: Ardeidae). Head stabilization, under controlled, sinusoidal, perch perturbations was mostly elicited at frequencies lower than 1 Hz. Maximal perturbation amplitudes sustained were positively correlated with leg length and maximal perturbation frequencies sustained were negatively correlated with body mass and with leg length. The species differed significantly in average maximal perturbation amplitudes sustained. Combinations of amplitude and frequency for which stabilization was achieved were bounded by a decreasing concave "envelope" curve in the frequency-amplitude plane, with inter specific differences in "envelope". As physical constraints, we tested maximal vertical acceleration, which translates into a line defined by the product of frequency2 x amplitude, and maximal vertical velocity, which translates into a line defined by the product of frequency x amplitude. Both relations were in good agreement with the experimental results for all but squacco herons. The results support predictions based on mechanical considerations and may explain the predominance of motor patterns employed by herons while foraging.

Animals↗
PubMed2001

Boxfishes (Teleostei: Ostraciidae) as a model system for fishes swimming with many fins: kinematics.

Swimming movements in boxfishes were much more complex and varied than classical descriptions indicated. At low to moderate rectilinear swimming speeds (<5 TL s(-1), where TL is total body length), they were entirely median- and paired-fin swimmers, apparently using their caudal fins for steering. The pectoral and median paired fins generate both the thrust needed for forward motion and the continuously varied, interacting forces required for the maintenance of rectilinearity. It was only at higher swimming speeds (above 5 TL s(-1)), when burst-and-coast swimming was used, that they became primarily body and caudal-fin swimmers. Despite their unwieldy appearance and often asynchronous fin beats, boxfish swam in a stable manner. Swimming boxfish used three gaits. Fin-beat asymmetry and a relatively non-linear swimming trajectory characterized the first gait (0--1 TL s(-1)). The beginning of the second gait (1--3 TL s(-1)) was characterized by varying fin-beat frequencies and amplitudes as well as synchrony in pectoral fin motions. The remainder of the second gait (3--5 TL s(-1)) was characterized by constant fin-beat amplitudes, varying fin-beat frequencies and increasing pectoral fin-beat asynchrony. The third gait (>5 TL s(-1)) was characterized by the use of a caudal burst-and-coast variant. Adduction was always faster than abduction in the pectoral fins. There were no measurable refractory periods between successive phases of the fin movement cycles. Dorsal and anal fin movements were synchronized at speeds greater than 2.5 TL s(-1), but were often out of phase with pectoral fin movements.

Animals↗
PubMed2000

Microstructural evolution of lipid aggregates in nucleating model and human biles visualized by cryogenic transmission electron microscopy.

Obtaining reliable information on the physical state and ultrastructure of bile is difficult because of its mixed aqueous-lipid composition and thermodynamic metastability. We have used time-lapse cryogenic transmission electron microscopy (cryo-TEM) combined with video-enhanced light microscopy (VELM) to study microstructural evolution in nucleating bile. A well-characterized model bile and gallbladder biles from cholesterol and pigment gallstone patients were studied sequentially during cholesterol nucleation and precipitation. In model bile, cholesterol crystallization was preceded by the appearance of the following distinct microstructures: spheroidal micelles (3-5 nm), discoidal membrane patches (50-150 nm) often in multiple layers (2-10), discs (50-100 nm), and unilamellar (50-200 nm) and larger multilamellar vesicles (MLVs). The membrane patches and discs appeared to be short-lived intermediates in a micelle-to-vesicle transition. Vesicular structures formed by growth and closure of patches as well as by budding off from vesicles with fewer bilayers. MLVs became more abundant, uniform, and concentric as a function of time. In native bile, all the above microstructures, except discoidal membrane patches, were observed. However, native MLVs were more uniform and concentric from the beginning. When cholesterol crystals appeared by light microscopy, MLVs were always detected by cryo-TEM. Edges of early cholesterol crystals were lined up with micelles and MLVs in a way suggesting an active role in feeding crystal growth from these microstructures. These findings, for the first time documented by cryo-TEM in human bile, provide a microstructural framework that can serve as a basis for investigation of specific factors that influence biliary cholesterol nucleation and crystal formation.

Bile↗
PubMed1982

Energetic advantages of burst-and-coast swimming of fish at high speeds.

A theoretical model describes how an intermittent swimming style can be energetically advantageous over continuous swimming at high average velocities. Kinematic data are collected from high-speed ciné pictures of free swimming cod and saithe at high velocities in a burst-and-coast style. These data suggest that fish make use of the advantages shown by choosing initial and final burst velocities close to predicted optimal values. The limiting role of rapid glycogen depletion in fast white anaerobic muscle fibres is discussed.

Anaerobiosis↗
PubMed1982

[Enzymatic lecithin determination in amniotic fluid for antepartal diagnosis of lung maturity - a multi-center study (author's transl)].

A new test-combination for the enzymatic determination of lecithin in amniotic fluid for the assessment of fetal lung maturity has been developed by Boehringer Mannheim. This test was evaluated by 12 hospitals and has been compared with the L/S ratio, the foam-test or the densitometric determination of lecithin. The assay is based on the hydrolysis of lecithin by phospholipase C which starts an enzymatic chain reaction in which NADH consumption if measured photometrically. The intra- and interassay precision were characterized by CV values below 10%. Average recoveries of lecithin were 95-102%. It is recommended to centrifuge the samples (10 min, 700 g) and to start the analysis as soon as possible after receipt of the specimen. The total amount of time required is 2 hours for a single determination. Batches of up to 10 samples require little extra time. An opened test-combination can be used for a maximum of 30 single determinations. Comparison of the quantitative enzymatic lecithin determination with other methods showed that the critical value for lecithin is 5.0 mg/100 ml. Above 5.1 mg/100 ml no case respiratory distress syndrome was observed. The good precision accuracy and the simple handling make the enzymatic lecithin determination suitable for routine use.

Amniotic Fluid↗
PubMed1978

Effect of the menstrual cycle, oral contraception and pregnancy on forearm blood flow, venous distensibility and clotting factors.

Forearm blood flow, venous distensibility and various clotting factors were determined in 118 apparently healthy women (mean age 26 years), either during the menstrual cycle, or while taking a combined contraceptive (A) with high progestin:estrogen ratio (d-norgestrel 0.125 mg + ethinylestradiol 0.03 mg), or a sequential contraceptive (B) with low progestin: estrogen ratio (megestrol 0.1/1 mg + ethinylestradiol 0.1 mg), or in the 1st, 2nd and 3rd trimester of pregnancy. Venous distensibility in women taking contraceptive A was higher than in other women during the follicular phase of a normal menstrual cycle. Venous distensibility was not affected by contraceptive B. Blood flow and blood pressure remained unchanged by contraceptives A and B. Fibrinogen concentration was increased by both contraceptives, factor VII was either decreased (A) or unchanged (B), and factor X was either unchanged (A) or increased (B). The oral contraceptive with the high progestagen component appeared to increase venous capacitance and may induce venous stasis, whereas coagulability was particularly enhanced by the estrogen-type contraceptive. Pregnant women differed from women on oral contraceptives in regard to peripheral circulation; they showed a tremendous increase of blood flow with secondary vasodilation.

Adult↗