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H Machemer

Publications and source records attributed to H Machemer.

At least 19 recordsLinked to original sources
PubMed2000

[Biomechanics of interlocked nailing in humeral shaft fractures. Comparison of 2 nail systems and the effect of interfragmentary compression with the unreamed humeral nail].

In this biomechanical study the implanted Unreamed Humeral Nail (UHN) has been tested concerning bending and torsional stiffnesses. In literature other intramedullary implants have been criticized for insufficient rotatory stability especially in transverse and short oblique fractures of the humeral shaft. This study examined, whether the implanted UHN, as well as the UHN implanted with interfragmentary compression through a specific compression device, is able to augment torsional stiffness significantly. To evaluate bending and torsional stiffnesses, the UHN has been compared biomechanically to the Russell-Taylor humeral nail (RT) in paired mid-shaft osteotomized cadaveric humeri. Identic paired comparison has been performed with the UHN without and UHN with interfragmentary compression. In anterior-posterior, as well as medio-lateral direction stiffness under four-point-bending is significantly higher in stabilizing with the RT. Under torsional loading with moments of 4 Nm, 6 Nm and 8 Nm the UHN reached more than the double torsional stiffness. The RT, which is only dynamically interlocked, owns a high initial "play" between bolts and nail itself. Through additional interfragmentary compression stiffness of the UHN under four-point-bending in anterior-posterior, as well as medio-lateral direction augments significantly. Also under torsional loading with moments of 4 Nm, 6 Nm und 8 Nm torsional stiffness increases with interfragmentary compression significantly. In comparison to other biomechanical studies of different authorship it is clear, that this statically interlocked intramedullary nailing of the humeral shaft is superior to non-statically interlocked types of nailing concerning their stabilizing potency in torsion and serves especially for fracture types, which are critically under rotation, as transverse or short oblique humeral shaft fractures.

Biomechanical Phenomena↗
PubMed2000

Physical and physiological components of the graviresponses of wild-type and mutant Paramecium Tetraurelia.

Wild-type and the morphological mutant kin 241 of Paramecium tetraurelia showed improved orientation away from the centre of gravity (negative gravitaxis) when accelerations were increased from 1 to 7 g. Gravitaxis was more pronounced in the mutant. A correlation between the efficiency of orientation and the applied g value suggests a physical basis for gravitaxis. Transiently enhanced rates of reversal of the swimming direction coincided with transiently enhanced gravitaxis because reversals occurred more often in downward swimmers than in upward swimmers. The results provide evidence of a physiological modulation of gravitaxis by means of the randomizing effect of depolarization-dependent swimming reversals. Gravity bimodally altered propulsion rates of wild-type P. tetraurelia so that sedimentation was partly antagonized in upward and downward swimmers (negative gravikinesis). In the mutant, only increases in propulsion were observed, although the orientation-dependent sensitivity of the gravikinetic response was the same as in the wild-type population. Observed swimming speed and sedimentation rates in the wild-type and mutant cells were linearly related to acceleration, allowing the determination of gravikinesis as a linear (and so far non-saturating) function of gravity.

Animals↗
PubMed1999

The gravikinetic response of Paramecium is based on orientation-dependent mechanotransduction.

Paramecium generates persistent shifts of the membrane potential of a few millivolts depending on its orientation with respect to the gravity vector. The resulting potential-induced modulation of the speed of propulsion is called gravikinesis because it acts to neutralize, fully or in part, sedimentation. Gravisensitivity is maximal at neutral orientation, i.e., in horizontally swimming cells, when the gravitational force per unit membrane area is at minimum. Stimulus-response relationships and energetic considerations show that sensing of the gravity vector by a nonspecialized, single-cell organism ranks among the most sensitive mechanoreceptors known in nature.

Animals↗
PubMed1999

The identification of gravikinesis from ciliates: methods and experience.

Recent advances in the gravitational physiology of ciliates are reported: the theoretical and experimental assessment of gravikinesis and sedimentation, calculation of gravikinesis using slopes of observed swimming and sedimentation data under hypergravity, orientational distributions of gravikinesis, central and membrane-associated gravitransduction, and the kinetics of activation and relaxation of gravikinesis.

Animals↗
PubMed1998

Mechanisms of graviperception and response in unicellular systems.

This introduction to a symposium considers established principles of mechanoreception and the physiology of excitable cells as a background of gravireception. From the perspective of experimental work in protists, current developments in the treatment and interpretation of graviresponses are addressed.

Animals↗
PubMed1998

Electric potentiation of gravikinesis in Paramecium is possibly mediated by filaments.

Sensitivity of Paramecium to mechanical stress including gravitational force is organized along two opposing gradients of membrane channel distribution: depolarizing Ca channels and hyperpolarizing K channels. Mechanoreceptor channels reside in the membrane of the cell soma and are activated, when the weight of the cytoplasm deforms the "lower" plasma membrane. Channel distribution is such as to generate ciliary activation which can counteract sedimentation of the cells: a reduction in downward swimming rate and an augmentation in upward swimming rate. Application of weak DC fields does not only induce the well-known cathodal orientation and swimming of Paramecium toward the cathode (galvano-taxis). We document that swimming velocity is augmented up to 175% as a function of the voltage gradient between 0.3 V/cm and 0.8 V/cm (galvanokinesis). A gradient of 0.3 V/cm was highly effective in raising the common negative gravikinesis of downward swimmers threefold. The gravikinesis of upward swimmers reversed polarity under field stimulation inducing cells to augment sedimentation effects (positive gravikinesis). Both effects of electric-field stimulation on ciliary activation are of the depolarizing type: reduction in the frequency of normally beating cilia. Analysis of the data shows that a voltage-sensitivity of gravireceptor channels would not account for the observed potentiation of negative gravikinesis. It is suggested that a previously described voltage-dependent Ca channel of the soma membrane interferes with a Ca(2+)-sensitive, peripheral filament system, which directly connects to gravireceptor channels.

Animals↗
PubMed1998

Relaxation and activation of graviresponses in Paramecium caudatum.

The kinetics of gravitaxis and gravikinesis in Paramecium caudatum were investigated by employing (1) step transitions from normal gravity (1 g) to weightlessness (microgravity) and (2) turns of the experimental chambers from the horizontal to the vertical position at 1 g. The transition to microgravity left existing cell orientations unchanged. Relaxation of negative gravitaxis under microgravity took longer than 10 s and may be described by the time constant of the decay of orientation coefficients. Gravitaxis was started at 1 g by turning the experimental chamber from a horizontal to a vertical position. Gravitaxis activated rapidly during the turning procedure and relaxed to an intermediate level after the turning had stopped. Gravity-induced regulation of swimming speed (gravikinesis) at 1 g had reached a steady state after 1 min; at this point, gravikinesis counteracted the effects of sedimentation (negative gravikinesis). A step transition to microgravity initially reversed the sign of the gravikinesis (positive gravikinesis). The relaxation of this kinetic response was not completed during 10 s of microgravity. The data suggest that gravikinesis is functionally unrelated to gravitaxis and is strongly affected by the rate of change in acceleration. We present a model explaining why gravikinesis reverses sign upon the onset of a step from 1 g to microgravity.

Animals↗
PubMed1997

Assessment of g-dependent Cellular Gravitaxis: Determination of Cell Orientation from Locomotion Track

Movement of cells in the gravity field is principally affected in two ways: velocity and orientation. Experimental observation of gravitaxis in large cell populations can document the velocity and orientation of swimming tracks, but orientations of individual cells are not represented at low magnifications. Cell orientations may depart from track orientations due to superposition of sedimentation on cellular propulsion. Here, we show that determination of the sedimentation rate in addition to cell track parameters allows a reconstitution of cell orientation employing geometric principles. Published and original cellular data indicate that gravitactic orientation of cells swimming in the gravity field is superior to that suggested from the experimental tracks. Similar conclusions apply to cells which walk or glide along substrate surfaces. Calculation of cell orientation coefficients provides a basis for determinations of the acceleration-dependence of gravitaxis and for quantitative tests on physical and/or physiological principles of cellular gravitaxis. Copyright 1997 Academic Press Limited

Journal Article↗
PubMed1997

Is there an orientation-dependent excursion of the Muller body in the "statocystoid" of Loxodes?

The ciliate Loxodes possesses a number of vesicles at its anterior dorsal margin. These so-called Muller vesicles contain a spherical inclusion (Muller body) in which lie crystals of barium salts. The Muller body is connected via a stalk to the wall of its vesicle. It is presumed to function as a stato-organelle and to respond by visible motions to changes of the direction of gravity. We have attempted to document the motion of the Muller body with respect to the direction of the gravity vector. Living cells moving in a horizontal or a vertical plane have been viewed under the light microscope with differential interference contrast, documented on video film, and sequences of single frames have been evaluated. Apparent excursions of the Muller body by about 1.5 μm, corresponding to a deviation of 10° at the base of the stalk, are observed in cells moving in a horizontal plane. No larger excursions have been seen in the vertical plane. Implications of this result for a model of the stato-organelle are discussed.

Animals↗
PubMed1996

A theory of gravikinesis in Paramecium.

The archaic eukaryote unicellular microorganism, Paramecium, is propelled by thousands of cilia, which are regulated by modulation of the membrane potential. Ciliates can successfully cope with gravity, which is the phylogenetically oldest stimulus for living things. One mechanism for overcoming sedimentation is negative gravitaxis, an orientational response antiparallel to the gravity vector. We have postulated the existence of a negative gravikinesis in Paramecium, i.e. a modulation of swimming speed as a function of cellular orientation in space. With negative gravikinesis, an upward oriented cell actively augments the rate of forward swimming and depresses active locomotion during downward orientation. A brief outline of the gravikinesis hypothesis is given on a quantitative basis and experimental data are presented which have confirmed the major assumptions.

Animals↗
PubMed1996

Gravitaxis screened for physical mechanism using g-modulated cellular orientational behaviour.

Advanced methods of recording cellular orientation with respect to the gravity vector are yielding increasingly wellfounded data on gravitaxis. The present study introduces a quantitative method which allows us to predict the precision of orientational behaviour as a function of acceleration assuming static buoyancy as a hypothetical physical principle of gravitaxis. The precision of orientation is expressed by the orientation coefficient as derived from circular statistics. Orientation coefficients calculated from experimental data at various g-values are tested for fit with a sigmoidal orientation coefficient-g-transfer function including a proportionality factor k. Residual orientation values in the low-hypogravity range obey a reciprocal function between k and g. Intersection of this residual-g function with the orientation coefficient-g-relationship gives the minimal acceleration to generate cellular orientation. Those data which clearly diverge from the orientation coefficient-g-curve bear some probability that the observed gravitaxis was guided in part by a physiological mechanism of gravireception and active graviorientation. Data which fit the orientation coefficient-g-curve qualify as being in agreement with a mechanical basis of cellular gravitaxis. Examples from the literature are presented and discussed in the light of our scheme of gravitaxis screening.

Acceleration↗
PubMed1995

Fluorometric measurement of the intracellular free Ca(2+)-concentration in the ciliate Didinium nasutum using Fura-2.

We developed an experimental approach to measure somatic and ciliary Ca(2+)-signals in the ciliate Didinium under voltage clamp conditions using the dye Fura-2. Intracellular pressure injection of Fura-2 molecules did not alter electrophysiological membrane properties besides an expected buffering effect. The intracellular free Ca(2+)-concentration was determined at 2 x 10(-7) M. During membrane excitation, this resting value increased in the cilia; a quantification was not feasible. Within the cell soma, however, the Ca(2+)-level was unchanged within the physiological range of the membrane potential (-70 mV to 0 mV). Increasing the driving force for Ca(2+)-ions via strong hyperpolarization (potentials negative to -200 mV) a centripetal increase in the somatic Ca2+ concentration was found. Our results support the hypothesis that Ca2+ is the intracellular messenger in rapid electromotor coupling in ciliates.

Animals↗
PubMed1995

Voltage-dependence of ciliary activity in the ciliate Didinium nasutum

In the gymnostome ciliate Didinium nasutum, swimming behaviour depends upon the cyclic activity of about 3000 cilia. The normal beating mode, resulting in forward swimming of the cell, is characterized by a posteriad effective beat (18 left of the longitudinal axis) at a frequency of approximately 15 Hz. Activation of depolarization-sensitive ciliary Ca2+ channels leads to an increase in intracellular Ca2+ concentration and a change in the beating mode. Following rapid reorientation, the effective stroke is anteriad (24 ° right of the longitudinal axis) and the beating frequency is about 26 Hz, resulting in fast backward swimming of the cell. In response to minor depolarizations, and hence small increases in cytoplasmic Ca2+ concentration, the cilia inactivate. Frequency increase and reversal in beat orientation share a single threshold level of membrane potential, since both changes of the beating mode occur simultaneously.

Journal Article↗
PubMed1994

Analysis of three-dimensional ciliary beating by means of high-speed stereomicroscopy.

Results are presented on the analysis of three-dimensional motion of compound cilia or cirri in voltage-clamped specimens of the protozoan Stylonychia mytilus. Time series of three-dimensional data were obtained by using the anaxial illumination method for simultaneous recording of stereoscopic video images. Data processing involved the following steps: determination of a reference coordinate system based solely on features present in each stereo-pair; tracing of cirral axes in digitized images, conversion to parameter curves by means of least-squares polynomial approximation, conversion of pairs of two-dimensional data to a series of three-dimensional data; correction for distortion due to projective shortening and conversion to a series of polynomial triplets, and analysis of the periodical components of the motion pattern in the frequency domain. Reconstructed beating cycles show typical differences between hyperpolarization-induced ciliary activity and depolarization-induced ciliary activity. Reconstructions of the motion of the basal segment of a cirrus are in agreement with existing data. Analysis of the curvature and torsion of a cirral axis during beating does not reveal any simple pattern of propagated activity within the axoneme. The return stroke may be subdivided into two phases. First, a curvature peak develops proximally. Secondly, a region with increased torsion arises more distally and spreads out in proximal direction. Both curvature and torsion return to minimal values by the beginning of the power stroke.

Animals↗
PubMed1994

GRAVIRESPONSES IN PARAMECIUM CAUDATUM AND DIDINIUM NASUTUM EXAMINED UNDER VARIED HYPERGRAVITY CONDITIONS

The swimming behaviours of two species of ciliates characterized by different mechanosensory and ciliary motor properties were investigated under hypergravity up to 5.4 g. The experiments were designed to examine large numbers of cells using video recording, digital data processing and statistics for the documentation of the rates and orientations of swimming. The gravikinetic responses (change in active swimming rates) were calculated from (1) the velocities of vertical swimming in the gravity field, (2) sedimentation of Ni2+-immobilized cells and (3) the intrinsic rate of propulsion, independent of gravity. Propulsion was determined from the intersection of regression lines of the gravity-dependent upward and downward swimming velocities. The rates of swimming and sedimentation, and consequently the gravikineses, were linear functions of gravitational acceleration. Comparisons of cell populations from different cultures suggest that there is an age-dependent change in gravikinesis. In starved Paramecium caudatum (7-day cultures), the kinetic responses antagonizing sedimentation (negative gravikinesis) increased with acceleration. In Didinium nasutum, negative gravikinesis was documented at 1 g in downward-swimming specimens only, which agrees with the mechanosensory organization of this cell. Hypergravity induced the gravikinesis of Didinium to change sign. In both species, and at all accelerations tested, a neutral gravitaxis was documented. Such behaviour incorporates distinct acceleration-dependent orientational and velocity responses, keeping populations of cells stationary in the gravity field (taxis coefficients close to zero).

Journal Article↗
PubMed1992

Ciliary beating in three dimensions: steps of a quantitative description.

We document a novel approach for quantitative assessment of ciliary activity, exemplified in rapid three-dimensional cyclic motion of the frontal cirri of Stylonychia. Cells held under voltage-clamp control are stimulated by step pulses to elicit reproducible hyperpolarization- or depolarization-induced ciliary motor responses. High-speed video recording at 200 fields per second is used for imaging ciliary organelles of the same cell in two perspectives: the axial view and, following cell rotation by 90 degrees, the lateral view. From video sequences of typically 1 s, the contours of the cirral images are determined and digitized. Computer programs are established to (1) reduce an observed image to a "ciliary axis", (2) sort series of axes by template to generate an averaged ciliary cycle in 2D-projection, and (3) to associate the generalized axial and lateral 2D-images for generation of a sequence of three-dimensional images, which quantitatively represent the cycle in space and time. The method allows us to produce predetermined perspectives of images selected from the ciliary cycle, and to generate stereo views for graphical representation of ciliary motion. The approach includes a potential for extraction of the complete microtubular sliding program of a cilium under reproducible electric stimulation of the ciliary membrane.

Animals↗
PubMed1992

Short-term microgravity to isolate graviperception in cells.

In the fall of 1991 a series of drop-tower experiments in ZARM (Bremen) was devoted to behavioural responses of unicellular organisms to step-type transition from normal gravity to microgravity. Modules for simultaneous 4-fold video-recording were incorporated into the flight capsule. In the course of 25 flights, 100 sets of experiments, each holding 100 to 200 cells, were flown under various conditions with a technical success rate of 94% and about 80% of the cells accessible to evaluation in the laboratory. A major goal of the experiments was the assessment of parameters of locomotion (velocity, orientation) in the absence of the gravity vector. The data show that in two species, Paramecium and Loxodes, the properties of steady-state microgravity-swimming correspond to horizontal swimming under 1g-conditions. In a third species, Didinium, microgravity-swimming velocity exceeds 1 g-horizontal rates. The data are in agreement with an electrophysiological hypothesis of graviperception in cells.

Animals↗