4,163 research outputs found
Learning intrinsic excitability in medium spiny neurons
We present an unsupervised, local activation-dependent learning rule for
intrinsic plasticity (IP) which affects the composition of ion channel
conductances for single neurons in a use-dependent way. We use a
single-compartment conductance-based model for medium spiny striatal neurons in
order to show the effects of parametrization of individual ion channels on the
neuronal activation function. We show that parameter changes within the
physiological ranges are sufficient to create an ensemble of neurons with
significantly different activation functions. We emphasize that the effects of
intrinsic neuronal variability on spiking behavior require a distributed mode
of synaptic input and can be eliminated by strongly correlated input. We show
how variability and adaptivity in ion channel conductances can be utilized to
store patterns without an additional contribution by synaptic plasticity (SP).
The adaptation of the spike response may result in either "positive" or
"negative" pattern learning. However, read-out of stored information depends on
a distributed pattern of synaptic activity to let intrinsic variability
determine spike response. We briefly discuss the implications of this
conditional memory on learning and addiction.Comment: 20 pages, 8 figure
Designing biodegradable alternatives to commodity polymers
The development and widespread adoption of commodity polymers changed societal landscapes on a global scale. Without the everyday materials used in packaging, textiles, construction and medicine, our lives would be unrecognisable. Through decades of use, however, the environmental impact of waste plastics has become grimly apparent, leading to sustained pressure from environmentalists, consumers and scientists to deliver replacement materials. The need to reduce the environmental impact of commodity polymers is beyond question, yet the reality of replacing these ubiquitous materials with sustainable alternatives is complex. In this tutorial review, we will explore the concepts of sustainable design and biodegradability, as applied to the design of synthetic polymers intended for use at scale. We will provide an overview of the potential biodegradation pathways available to polymers in different environments, and highlight the importance of considering these pathways when designing new materials. We will identify gaps in our collective understanding of the production, use and fate of biodegradable polymers: from identifying appropriate feedstock materials, to considering changes needed to production and recycling practices, and to improving our understanding of the environmental fate of the materials we produce. We will discuss the current standard methods for the determination of biodegradability, where lengthy experimental timescales often frustrate the development of new materials, and highlight the need to develop better tools and models to assess the degradation rate of polymers in different environments
Highest weight Macdonald and Jack Polynomials
Fractional quantum Hall states of particles in the lowest Landau levels are
described by multivariate polynomials. The incompressible liquid states when
described on a sphere are fully invariant under the rotation group. Excited
quasiparticle/quasihole states are member of multiplets under the rotation
group and generically there is a nontrivial highest weight member of the
multiplet from which all states can be constructed. Some of the trial states
proposed in the literature belong to classical families of symmetric
polynomials. In this paper we study Macdonald and Jack polynomials that are
highest weight states. For Macdonald polynomials it is a (q,t)-deformation of
the raising angular momentum operator that defines the highest weight
condition. By specialization of the parameters we obtain a classification of
the highest weight Jack polynomials. Our results are valid in the case of
staircase and rectangular partition indexing the polynomials.Comment: 17 pages, published versio
Alkaline-Earth Derivatives of Diphenylphosphine-Borane
Treatment of β-diketiminato (BDI = HC{C(CH3)Ndipp)}2 where dipp = 2,6-iPr2C6H3) magnesium butyl [(BDI)MgBu] (I) and calcium hexamethyldisilazide [(BDI)Ca{N(SiMe3)2}] (II) complexes with equimolar quantities of diphenylphosphine-borane, Ph2PH·BH3, results in the formation of the respective alkaline earth (Ae) phosphidoborane derivatives [(BDI)Mg(Ph2PBH3)]2 (6a) and [(BDI)Ca(Ph2PBH3)] (7a). Although satisfactory single crystals of 7a could not be obtained, 6a was crystallographically characterized and both compounds display similar NMR spectra. The dimeric Ae-hydride complexes [(BDI)AeH]2 (IIIa, Ae = Mg; IIIb, Ae = Ca) react with substoichiometric quantities of Ph2PH·BH3, allowing the crystallization of the dimeric Mg and trimeric Ca phosphidoborane species [(BDI)Mg(H)(H3BPPh2)Mg(BDI)] (8) and [{(BDI)Ca}3(H)(H3BPPh2)2] (9). In the absence of coordinating Lewis bases, compounds 6a, 7a, 8, and 9 display dynamic solution-state behavior (in benzene and toluene), while addition of THF furnishes the monomeric adducts [(BDI)Mg(H3BPPh2)·THF] (6b) and [(BDI)Ca(H3BPPh2)·THF] (7b). Addition of Ph2PH·BH3 to compound 6a results in BH3 transfer to eliminate Ph2PH and generate the phosphinodiboronate complex [(BDI)Mg{(H3B)2PPh2}]2 (10) in preference to dehydrocoupling of the phosphidoborane and phosphine-borane reagents.</p
Vip3A Resistance Alleles Exist at High Levels in Australian Targets before Release of Cotton Expressing This Toxin
Crops engineered to produce insecticidal crystal (Cry) proteins from the soil bacterium Bacillus thuringiensis (Bt) have revolutionised pest control in agriculture. However field-level resistance to Bt has developed in some targets. Utilising novel vegetative insecticidal proteins (Vips), also derived from Bt but genetically distinct from Cry toxins, is a possible solution that biotechnical companies intend to employ. Using data collected over two seasons we determined that, before deployment of Vip-expressing plants in Australia, resistance alleles exist in key targets as polymorphisms at frequencies of 0.027 (n = 273 lines, 95% CI = 0.019–0.038) in H. armigera and 0.008 (n = 248 lines, 0.004–0.015) in H. punctigera. These frequencies are above mutation rates normally encountered. Homozygous resistant neonates survived doses of Vip3A higher than those estimated in field-grown plants. Fortunately the resistance is largely, if not completely, recessive and does not confer resistance to the Bt toxins Cry1Ac or Cry2Ab already deployed in cotton crops. These later characteristics are favourable for resistance management; however the robustness of Vip3A inclusive varieties will depend on resistance frequencies to the Cry toxins when it is released (anticipated 2016) and the efficacy of Vip3A throughout the season. It is appropriate to pre-emptively screen key targets of Bt crops elsewhere, especially those such as H. zea in the USA, which is not only closely related to H. armigera but also will be exposed to Vip in several varieties of cotton and corn
Aminopiperidine based complexes for lactide polymerisation
Herein we report the synthesis and characterisation of a series of salalen and salan ligands derived from 2-(aminomethyl)piperidine. Depending on the choice of starting salicylaldehyde, a bicyclic salan type ligand (1-3H2) or imino salalen type ligand (4-6H, 7-9H2) were prepared. The ligands were successfully complexed to group 4 metals and aluminium; with hafnium and zirconium octahedral complexes, M(1-3)2, were realized; whilst with aluminium tetrahedral and trigonal bipyramidal complexes, Al(1-9)Mex (x = 1,2), were isolated. The complexes have been characterised in solution via 1H and 13C{1H} NMR spectroscopy and in the solid state by X-ray crystallography. The group 4 complexes were observed to have a fac-fac arrangement of ligands and there were two isomers present when 3H2 was ligated. The imino aluminium complexes Al(7-9)Me were isolated as a mixture of diastereoisomers. The resultant complexes were trialed in the ring opening polymerisation of rac-lactide with both heterotactic and isotactic PLA being demonstrated. Tacticity was found to be dependent on the nature of the ligand and metal used; the M(1-3)2 complexes were generally found to have a heterotactic preference (Pr = 0.67-0.76) and the aluminium polymerisation outcome was dictated more by the steric influence of the ligand, particularly for Al(4-6)Me2/Al(7-9)M
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