17,529 research outputs found
Unidirectional Rotation Neurones in the Optomotor System of the Crab, Carcinus
1. Among the optomotor fibres to the eye muscles in Carcinus a class was found which responds to unidirectional fast rotations around various body axes. All had large signals and are therefore of large diameter.
2. In one set of these fibres which fires especially for rotations around the dorsoventral axis, it could be shown that discharges take place especially during accelerations and that, when a rotation in the null direction is suddenly stopped, a short discharge occurs. The fibres for other axes behave in a similar manner.
3. For rotations around the ventro-dorsal axis, but not for other directions, mediumsized fibres are present which, in contrast to the fast fibres, respond to visual stimulation, as well as to body rotations in darkness, thus combining the input properties of the unidirectional fast rotatory and the unidirectional purely optokinetic small fibres. Their sensitivity to visual input is for low rotation velocities, to body rotations is for high rotation velocities
Factors Regulating the Discharge Frequency in Optomotor Fibres Of Carcinus Maenas
The influence of the excited state of the animal on various motor neurone discharges and accompanying muscle action potentials was studied in the eyestalk of the crab, Carcinus maenas.
In most cases large increases in firing frequency could be obtained during such states. An exception is the tonic eye-withdrawal system in which an inhibitory effect is caused.
A pronounced difference in habituation to constant stimuli between spring and summer was found for the position fibres; in spring it was slow and in summer much quicker
Optimization of Stone Cutting Techniques for the Seismic Protection of Archaeological Sites
Since the beginning of civilization, history tells of the movement of art pieces, monuments and manufacts from site to site. The causes are multiple: the displacements due to the "spoils of war", ordered by kings and emperors, the movements caused by the need for reuse, especially in the early Christian period, and so forth. Considerations about the events of the past, yield a possible strategy to transform this concept into a technique for earthquake prevention of archaeological sites. The seismic safety retrofits have often proven to be scarcely effective, because of the difficulties involved in complex sites. The aim of this study is to analyze an "alternative" method of preventing natural disaster like floods, eruption and earthquakes, through the movimentation of the most representative structural elements of archaeological sites by decomposition of the masonry and marbles [1]. The procedure considers a process of "cutting optimization," calibrated on the characteristics of the specific material that has to be cut and then displaced in safer places (i.e., MEP, "manufact evacuation plan"). This process should not create excessive problems to the structure, and aims to reassembly the manufact in contexts able to guarantee safety through advanced earthquake-resistant expedients. From these considerations, the work develops a procedure to safeguard the archaeological site of Pompei (Naples), through an appropriate analysis of representative portions of the site, aimed to a careful handling and to a proper reconstruction in a safe location, from the seismic point of vie
On the Decoupling of the Homogeneous and Inhomogeneous Parts in Inhomogeneous Quantum Groups
We show that, if there exists a realization of a Hopf algebra in a
-module algebra , then one can split their cross-product into the tensor
product algebra of itself with a subalgebra isomorphic to and commuting
with . This result applies in particular to the algebra underlying
inhomogeneous quantum groups like the Euclidean ones, which are obtained as
cross-products of the quantum Euclidean spaces with the quantum groups
of rotation of , for which it has no classical analog.Comment: Latex file, 27 pages. Final version to appear in J. Phys.
X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation
We have incorporated the description of the X-ray properties of Active
Galactic Nuclei (AGNs) into a semi-analytic model of galaxy formation, adopting
physically motivated scaling laws for accretion triggered by galaxy encounters.
Our model reproduces the level of the cosmic X-ray background at 30 keV; we
predict that the largest contribution (around 2/3) comes from sources with
intermediate X-ray luminosity 10^{43.5}< L_X/erg/s <10^{44.5}, with 50 % of the
total specific intensity produced at z<2. The predicted number density of
luminous X-ray AGNs (L_X>10^{44.5} erg/s in the 2-10 keV band) peaks at z
around 2 with a decline of around 3 dex to z=0; for the low luminosity sources
(10^{43}<L_X/erg/s <10^{44}) it has a broaderand less pronounced maximum around
z 1.5. The comparison with the data shows a generally good agreement. The model
predictions slightly exceed the observed number of low-luminosity AGNs at z
around 1.5, with the discrepancy progressively extending to
intermediate-luminosity objects at higher redshifts; we discuss possible
origins for the mismatch. Finally, we predict the source counts and the flux
distribution at different redshifts in the hard (20-100 keV) X-ray band for the
sources contributing to the X- ray background.Comment: 27 pages, accepted for publication in Ap
Influence of disordered porous media in the anomalous properties of a simple water model
The thermodynamic, dynamic and structural behavior of a water-like system
confined in a matrix is analyzed for increasing confining geometries. The
liquid is modeled by a two dimensional associating lattice gas model that
exhibits density and diffusion anomalies, in similarity to the anomalies
present in liquid water. The matrix is a triangular lattice in which fixed
obstacles impose restrictions to the occupation of the particles. We show that
obstacules shortens all lines, including the phase coexistence, the critical
and the anomalous lines. The inclusion of a very dense matrix not only suppress
the anomalies but also the liquid-liquid critical point
Optimal Content Downloading in Vehicular Networks
We consider a system where users aboard communication-enabled vehicles are interested in downloading different contents from Internet-based servers. This scenario captures many of the infotainment services that vehicular communication is envisioned to enable, including news reporting, navigation maps and software updating, or multimedia file downloading. In this paper, we outline the performance limits of such a vehicular content downloading system by modelling the downloading process as an optimization problem, and maximizing the overall system throughput. Our approach allows us to investigate the impact of different factors, such as the roadside infrastructure deployment, the vehicle-to-vehicle relaying, and the penetration rate of the communication technology, even in presence of large instances of the problem. Results highlight the existence of two operational regimes at different penetration rates and the importance of an efficient, yet 2-hop constrained, vehicle-to-vehicle relaying
The BeppoSAX High Energy Large Area Survey (HELLAS) - VI. The radio properties
We present results of a complete radio follow-up obtained with the VLA and
ATCA radio telescopes down to a 6 cm flux limit of about 0.3 mJy of all the 147
X-ray sources detected in the BeppoSAX HELLAS survey. We found 53 X-ray/radio
likely associations, corresponding to about one third of the X-ray sample.
Using the two point spectral index alpha_ro=0.35 we divided all the HELLAS
X-ray sources in radio quiet and radio loud. We have 26 sources classified as
radio-loud objects, corresponding to about 18% of the HELLAS sample. In
agreement with previous results, the identified radio-loud sources are
associated mainly with Type 1 AGNs with L(5-10 keV) > 10^44 erg/s, while all
the identified Type 2 AGNs and Emission Line Galaxies are radio quiet objects
with L(5-10 keV) < 10^44 erg/s. The analysis of the radio spectral index
suggests that Type 1 AGNs have a mean radio spectral index flatter than Type 2
AGNs and Emission Line Galaxies.Comment: 12 pages, 5 figures, MNRAS, accepte
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