PhD

dissertationPrimary cultures containing 99% neurons, 99% non-neuronal cell (glia), or both cell types prepared from the sympathetic ganglia of 12-day chick embryos. Levels of cyclic AMP in the non-neuronal cells (~ 14 pmoles per mg protein) were approximately three-fold higher than levels in the neurons (~ 4 pmoles per mg protein). Mixed cultures had concentrations of cyclic AMP which fell between the values measured for pure neuronal and pure non-neuronal cultures. The measured cyclic AMP values of mixed cultures were indistinguishable from values predicted by summing the expected contributions of the neurons and non-neuronal cells. Thus, contact between the neurons and non-neuronal cells in these mixed cultures did not appear to alter the level of cyclic AMP in either cell type. Neuronal-glial interactions, such as the specific neuronal stimulation of non-neuronal cell proliferation, occurred independently of any changes in the level of cyclic AMP in the mixed cultures. Cell density was varied in both pure and mixed cultures, and both cyclic AMP concentrations and amounts of 3H-thymidine incorporation into DNA were measured. The cyclic AMP content of the non-neuronal cells varied inversely with cell density. 3H-Thymidine incorporation was independed of cell density in both neuronal and non-neuronal cultures. Parallel density-dependent decreases in cyclic AMP concentration and 3H-thymidine incorporation were observed in mixed cultures as cell density was increased. The data suggest that there is no relationship between changes in rate of non-neuronal cell proliferation and cyclic AMP levels in these cultures. The effects of some putative neurotransmitters, analogs of transmitters, and adenosine on levels of cyclic AMP were determined in primary neuronal, non-neural, and mixed cultures of embryonic chick sympathetic ganglion cells. Adenosine increased the level of cyclic AMP in the non-neuronal cells but had no effect on levels in either neuronal or mixed cultures. Norepinephrine increased the level of cyclic AMP in neurons, had no effect on the non-neuronal levels, and decreased the cyclic nucleotide content of mixed cultures. Dopamine, isoproterenol, and pilocarpine all caused a reduction in the cyclic AMP content of mixed cultures but had no effect on levels of either neurons on non-neuronal cells. The measured cyclic AMP levels of norepinephrine- and isoproterenol-treated mixed cultures were significantly lower than the values predicted by summing the expected individual contributions of the neurons and the non-neuronal cells. Thus, cells in mixed cultures sometimes responded differently from cells in pure cultures

The Origin of Large-scale HI structures in the Magellanic Bridge

We investigate the formation of a number of key large-scale HI features in the ISM of the Magellanic Bridge using dissipationless numerical simulation techniques. This study comprises the first direct comparison between detailed HI maps of the Bridge and numerical simulations. We confirm that the SMC forms two tidal filaments: a near arm, which forms the connection between the SMC and LMC, and a counterarm. We show that the HI of the most dense part of the Bridge can become arranged into a bimodal configuration, and that the formation of a "loop" of HI, located off the North-Eastern edge of the SMC can be reproduced simply as a projection of the counter-arm, and without invoking localised energy-deposition processes such as SNe or stellar winds.Comment: 5 Pages, 4 Figures, Accepted - MNRAS let

Measurement of the Multi-TeV Neutrino Interaction Cross-Section with IceCube Using Earth Absorption

Neutrinos interact only very weakly, so they are extremely penetrating. The theoretical neutrino–nucleon interaction cross-section, however, increases with increasing neutrino energy, and neutrinos with energies above 40 teraelectronvolts (TeV) are expected to be absorbed as they pass through the Earth. Experimentally, the cross-section has been determined only at the relatively low energies (below 0.4 TeV) that are available at neutrino beams from accelerators1,2. Here we report a measurement of neutrino absorption by the Earth using a sample of 10,784 energetic upward-going neutrino-induced muons. The flux of high-energy neutrinos transiting long paths through the Earth is attenuated compared to a reference sample that follows shorter trajectories. Using a fit to the two-dimensional distribution of muon energy and zenith angle, we determine the neutrino–nucleon interaction cross-section for neutrino energies 6.3–980 TeV, more than an order of magnitude higher than previous measurements. The measured cross-section is about 1.3 times the prediction of the standard model3, consistent with the expectations for charged- and neutral-current interactions. We do not observe a large increase in the cross-section with neutrino energy, in contrast with the predictions of some theoretical models, including those invoking more compact spatial dimensions4 or the production of leptoquarks5. This cross-section measurement can be used to set limits on the existence of some hypothesized beyond-standard-model particles, including leptoquarks

Arboricity, h-Index, and Dynamic Algorithms

In this paper we present a modification of a technique by Chiba and Nishizeki [Chiba and Nishizeki: Arboricity and Subgraph Listing Algorithms, SIAM J. Comput. 14(1), pp. 210--223 (1985)]. Based on it, we design a data structure suitable for dynamic graph algorithms. We employ the data structure to formulate new algorithms for several problems, including counting subgraphs of four vertices, recognition of diamond-free graphs, cop-win graphs and strongly chordal graphs, among others. We improve the time complexity for graphs with low arboricity or h-index.Comment: 19 pages, no figure

Generalisation : graphs and colourings

The interaction between practice and theory in mathematics is a central theme. Many mathematical structures and theories result from the formalisation of a real problem. Graph Theory is rich with such examples. The graph structure itself was formalised by Leonard Euler in the quest to solve the problem of the Bridges of Königsberg. Once a structure is formalised, and results are proven, the mathematician seeks to generalise. This can be considered as one of the main praxis in mathematics. The idea of generalisation will be illustrated through graph colouring. This idea also results from a classic problem, in which it was well known by topographers that four colours suffice to colour any map such that no countries sharing a border receive the same colour. The proof of this theorem eluded mathematicians for centuries and was proven in 1976. Generalisation of graphs to hypergraphs, and variations on the colouring theme will be discussed, as well as applications in other disciplines.peer-reviewe