1,720,973 research outputs found
The movement of spermatozoa with helical head: theoretical analysis and experimental results.
No full textThe present work is concerned with the study of the swimming of flagellated microscopic organisms with a helical head and a helical pattern of flagellar beating, such as Xenopus sperms. The theoretical approach is similar to that taken by Chang and Wu (1971) in the study of helical flagellar movement. The model used in the present study allows us to determine the velocity of propulsion (U) and the frequency of rotation of the sperm head (fh) as a function of the frequency of the wave of motion (ft) traveling along the tail. The results relative to the case of helical and planar flagellar waves are compared. Our main finding is that the helical shape of the head seems to increase the efficiency of propulsion of the spermatozoon when compared with the more commonly shaped spherical head. Experimentally measured values of fh versus U may be fitted by a linear plot whose slope is much higher than that corresponding to the case of planar flagellar beating. This fact is consistent with an effectively three-dimensional (nonplanar) movement of the flagellar tail. However, the results do not fit those predicted from a circular helix, suggesting that a different shape of the flagellar beating should be considered
Computer reconstruction of the spread of excitation in nerve terminals with inhomogeneous channel distribution.
No full textA direct numerical integration method, as modified by Du Fort and Frankel (1953), has been used to solve the partial differential equation system which describes the spread of action potential in a mammalian nerve terminal. Branching of the terminal as well as inhomogeneous distributions of Na+ and K+ voltage-dependent channels (Brigant and Mallart 1982) have been incorporated in the model. Using the channel densities and the kinetic parameters measured in the node of Ranvier, the depolarization in the terminal branches has an amplitude of only 60% of the action potential in the node. Furthermore, the time courses of the calculated membrane currents differ considerably from the ones measured by Brigant and Mallart (1982) and by Konishi and Sears (1984). Increasing the Na+ and K+ channel densities may considerably increase the terminal depolarization and also reproduce qualitatively the current wave-forms observed experimentally. The model can also reproduce some of the effects of pharmacological channel blocks. The simulation allows a new interpretation of the different components of membrane current along the terminal
Segmented and equivalent representation of the cable equation.
No full textThe linear cable theory has been applied to a modular structure consisting of n repeating units each composed of two subunits with different values of resistance and capacitance. For n going to infinity, i.e., for infinite cables, we have derived analytically the Laplace transform of the solution by making use of a difference method and we have inverted it by means of a numerical procedure. The results have been compared with those obtained by the direct application of the cable equation to a simplified nonmodular model with "equivalent" electrical parameters. The implication of our work in the analysis of the time and space course of the potential of real fibers has been discussed. In particular, we have shown that the simplified ("equivalent") model is a very good representation of the segmented model for the nodal regions of myelinated fibers in a steady situation and in every condition for muscle fibers. An approximate solution for the steady potential of myelinated fibers has been derived for both nodal and internodal regions. The applications of our work to other cases dealing with repeating structures, such as earthworm giant fibers, have been discussed and our results have been compared with other attempts to solve similar problems
Exact solution of the unidimensional Poisson-Boltzmann equation for a 1:2 (2:1) electrolyte.
No full textThe unidimensional Poisson-Boltzmann equation for a 1:2 (2:1) electrolyte has been solved analytically. The results have been compared with those obtained from the linearized equation. It is shown that in physiological conditions the difference may be greater than 10%. The value of the derivative of the potential in x=0, (dpsi/dx)x=0, has been used by many authors in the evaluation of the superficial charges of biological membranes. The value of (dpsi/dx)x-0 have also been compared with the ones derived from the linearized equation. The difference may be greater than 25%. Our results suggest that the linearization of the Poisson-Boltzmann equation for a 1:2(2:1) electrolyte may be greatly misleading
Quantitative data concerning the oviposition of Bombylius fimbriatus Meigen (Diptera, Bombyliidae), a parasite of Andrena agilissima (Scopoli) (Hymenoptera, Andrenidae).
No full textAnalysis of lumped and distributed elements models of cut muscle fibers in vaseline or sucrose gap preparations.
No full textNest provisioning and stinging pattern in Ammophila sabulosa (Hymenoptera, Sphecidae): influence of prey size
No full textNesting habits of two spider wasps: Anoplius infuscatus and Episyron sp. (Hymenoptera: Pompilidae), with a review of the literature.
No full textMany behavioural aspects of Anoplius infuscatus (Vander Linden) and Episyron sp. Schioedte were examined in detail, both on a qualitative and, when possible, on a quantitative level, during a long-term fi eld study in Northern Italy. Both wasps dig unicellular nests on bare soil, which are fi lled with a single spider to feed the brood. The main differences regard the way to hunt their prey, that of burrowing, that of transporting and introducing the spider into the nest, the habit of amputating spider legs and the abdominal movements during nest closing. On the whole, starting from prey capture to nest closure, A. infuscatus is signifi cantly slower than Episyron. The observed ethological differences
are well fi tted to the exploitation of the different preys consisting in wolf spiders (Lycosidae) and orb weaving spiders (Araneidae), respectively. Some fl exibility in the observed behaviour is described.
A critical re-examination of the literature shows that, even if most results agree with those of previous authors, two controversial points remain open regarding Episyron species: the position of the egg (on the prey or on the cell wall) and the way to grasp the spider during tranport and introdution into the nest
Temporal transition of nesting activities in the digger wasp Bembecinus tridens (Hymenoptera, Crabronidae).
No full textA nest aggregation of Bembecinus tridens (Hymenoptera: Crabronidae) was investigated through observations of marked
individual females in Northern Italy, in order to quantify frequencies, duration, and sequence of the nesting activities. Wasp
females dug unicellular (in one case multicellular) nests preferably on oblique surfaces. Nest digging occurred mainly in the
morning, while definitive nest closing was performed only in the afternoon, and nests were frequented by the owners for
3.52 days on average. One to eight nests were dug by single females in rapid succession in the area observed, revealing a
rapid dispersion towards other sites. Provisioning and non-provisioning (control) flights were positively correlated and
occurred mainly in the late morning. A number of orientation flights followed most exits from the nests but was higher at the
end of nest digging. Nest closing lasted more at the end of provisioning and at the end of digging. Provisioning began more
frequently one day after nest digging (and oviposition), less frequently two days after, and the number of prey per day was
negatively correlated with the number of provisioning days. Data are consistent with a situation of primitive progressive
provisioning, and match that of other species of the same genus
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