2,247 research outputs found

    Chemotaxis of Bracken Spermatozoids : Implications of Electrochemical Orientation

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    1. The chemotaxis of bracken spermatozoids involves their precise orientation in a gradient of bimalate or a few other similar ions. When a voltage gradient is established in a sperm suspension containing bimalate or other chemotactically active ions, a similar orientation is observed, causing the spermatozoids to swim towards the anode. 2. Photographic records of sperm responses reveal a linear relationship between turning rate and the component of the gradient perpendicular to the direction of swimming. 3. Estimates have been obtained of the ratio of the magnitudes of the two types of gradient required to produce an equal tactic response. 4. The results suggest that the sensory elements of the spermatozoids adsorb bimalate ions. The reversible adsorption of bimalate ions on ā€˜bimalate-combining sitesā€™ on the anterior end of a spermatozoid might fully explain its tactic behaviour, without requiring any modification of flagellar activity by bimalate

    CO_2-inhibition of the amplitude of bending of triton-demembranated sea urchin sperm flagella

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    Demembranated sea urchin spermatozoa were reactivated in solutions containing KHCO_3 and observed in a covered well slide. Although KHCO_3 itself causes a small inhibition of flagellar beat frequency, the results confirm previous observations of a direct inhibition of flagellar bend angle by CO_2 with no effect of CO_2 on frequency. Observation of the effect of pH on the inhibition of bend angle in solutions containing KHCO_3 indicates that a given concentration of OH^- has a similar effect to the same concentration of HCO^(-)_3, as would be expected if CO_(2-) inhibition results from reaction of CO_2 with protein-NH^(+)_(3) groups to form carbamates. CO2 may interfere with a control mechanism which selectively suppresses dynein cross-bridge activity in order to generate rhythmic bending. This control mechanism may incorporate a feedback control involving a measure of flagellar amplitude, which fails to operate successfully when the amplitude is reduced below a critical level

    Flagellar propulsion

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    In this JEB Classics paper, Sir James Gray and G. J. Hancock explained how spermatozoa are propelled by flagellar bending waves (Gray and Hancock, 1955). This paper was a lasting success because it provided an easy-to-understand solution to a complicated hydrodynamic problem, and because it provided a quantitative prediction of the swimming speed that was almost identical to the swimming speed measured in Gray's accompanying paper on the movement of sea urchin spermatozoa (Gray, 1955)

    Symmetry Breaking in a Model for Nodal Cilia

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    Nodal cilia are very short cilia found in the embryonic node on the ventral surface of early mammalian embryos. They create a right to left fluid flow that is responsible for determining the normal asymmetry of the internal organs of the mammalian body. To do this, the distal end of the cilium must circle in a counterclockwise sense. Computer simulations with 3-dimensional models of flagella allow examination of 3-dimensional movements such as those of nodal cilia. 3-dimensional circling motions of short cilia can be achieved with velocity controlled models, in which dynein activity is regulated by sliding velocity. If dyneins on one outer doublet are controlled by the sliding velocity experienced by that doublet, the system is symmetric, and the 3-dimensional models can show either clockwise or counterclockwise circling. My computer simulations have examined two possible symmetry breaking mechanisms: 1) dyneins on doublet N are regulated by a mixture of the sliding velocities experienced by doublets N and N+1 (numbered in a clockwise direction, looking from the base). or 2) symmetry is broken by an off-axis force that produces a right-handed twist of the axoneme, consistent with observations that some dyneins can rotate their substrate microtubules in a clockwise direction

    Simple method for predicting viscosity of gas mixtures

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    Method is derived from the Chapman-enskog theory which describes viscosities at low-to-moderate pressures. Mixtures of nonpolar gases require the viscosities and molecular weights of the constituents in addition to the mixture composition. Dipole moments, boiling points and liquid boiling point densities are also needed with polar gases

    Effects of Increased Viscosity on the Movements of Some Invertebrate Spermatozoa

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    In the wave pattern of a flagellum generating planar undulatory bending waves, bending and unbending are represented by rather abrupt transitions between bent and unbent states (Brokaw & Wright, 1963; Brokaw, 1965). The magnitude of these transitions is indicated by the radius of curvature of the bent regions of the wave pattern, but their detailed time course cannot be resolved. The propagated bending waves required for propulsion are generated by the progression of these transitions along the flagellum

    Flagellar oscillation: new vibes from beads

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    Calcium-induced asymmetrical beating of triton-demembranated sea urchin sperm flagella

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    Asymmetrical bending waves can be obtained by reactivating demembranated sea urchin spermatozoa at high Ca2+ concentrations. Moving-film flash photography shows that asymmetrical flagellar bending waves are associated with premature termination of the growth of the bends in one direction (the reverse bends) while the bends in the opposite direction (the principal bends) grow for one full beat cycle, and with unequal rates of growth of principal and reverse bends. The relative proportions of these two components of asymmetry are highly variable. The increased angle in the principal bend is compensated by a decreased angle in the reverse bend, so that there is no change in mean bend angle; the wavelength and beat frequency are also independent of the degree of asymmetry. This new information is still insufficient to identify a particular mechanism for Ca2+-induced asymmetry. When a developing bend stops growing before initiation of growth of a new bend in the same direction, a modification of the sliding between tubules in the distal portion of the flagellum is required. This modification can be described as a superposition of synchronous sliding on the metachronous sliding associated with propagating bending waves. Synchronous sliding is particularly evident in highly asymmetrical flagella, but is probably not the cause of asymmetry. The control of metachronous sliding appears to be unaffected by the superposition of synchronous sliding

    Effects of viscosity and ATP concentration on the movement of reactivated sea-urchin sperm flagella

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    1. Spermatozoa from the sea urchin, Lytechinus pictus, can be demembranated with solutions containing Triton X-100 and 5mM-CaCl2 and reactivated in ATP solutions containing low concentrations (10^(āˆ’9)M) Of free Ca^(2+) ion to give symmetrical bending wave movements, even at very low ATP concentrations. At ATP concentrations of 0.01-0.02 mM the reactivated spermatozoa have beat frequencies near 1 Hz, nearly normal bend angles, and wave-lengths about 50% longer than normal. 2. The effects of increased viscosity, obtained by addition of methyl cellulose to the reactivation solutions, on bend angle and beat frequency decrease with decreasing ATP concentration, and become almost undetectable at 0.01 mM ATP. On the other hand, the effect of increased viscosity on wavelength shows relatively little change with ATP concentration, although it is noticeably reduced at 0.01 mM ATP. 3. These observations suggest that the beat frequency at low ATP concentrations is determined by an intrinsic rate-limiting process, in contrast to the viscocity-limited behaviour at high beat frequencies. Quantitative agreement with the experimental results is obtained with a model in which ATP concentration and viscosity each determine the rates of one step in a two-step reaction cycle which determines the beat frequency. 4. Two other models which can qualitatively explain the effects of ATP concentration and viscosity on flagellar beat frequency fail to show quantitative agreement with the experimental results. In one of these models, ATP concentration determines the maximum rate of shear between filaments. In the other, ATP concentration determines a time delay which is required to bring the active moment into phase with the elastic moments which would be expected to dominate the bending resistance at low beat frequencies

    Movement of the Flagella of Polytoma Uvella

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    1. Dark-field photographic records of the wave patterns of moving flagella have been made using multiple-flash exposures at flash rates of up to 50 per sec. Patterns obtained from ATP-reactivated isolated flagella show reduced amplitude of bending, but are otherwise similar to those obtained from flagella under normal conditions. The co-ordination required to produce propagated waves of active bending appears to be preserved after isolation and reactivation. 2. Addition of methyl cellulose to the medium to increase the viscosity reduces the frequency of beat much more than the amplitude. This behaviour can be partially explained by an analysis of the equations for sinusoidal wave movement of flagella which shows that maximum efficiency of forward swimming will be obtained if the amplitude of beat is maintained greater than 1/2{pi} times the wavelength, and variations in available power or viscosity are compensated by changes in beat frequency. 3. Wave patterns at low frequencies in low ATP concentrations are unlike those obtained when the frequency is reduced by increased viscosity. The effect of ATP concentration on beat frequency is not explained by an effect on the power available for beating or by an effect on the 'internal viscosity' of the flagella
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