| 1. |
- Blesic, S., et al.
(författare)
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A random walk model analysis of spinal dorsalhorn neuron discharges
- 2000
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Konferensbidrag (refereegranskat)abstract
- Methods of statistical physics have been recently successfully ap- plied to the study of spatial ánd temporal randomness in various biological systems, like the analýsts of the DNA nucleotide sequences and the heartbeat time series. We study the interspike intervals (ISI) time series of the spinal dorsal horn nociceptive- responsive neurons (DHN) activity in decerebrate cats applying the detrended fluctuatìon analysis (DFA) that is a modification of the standard random walk model analysis. Specifically, we focus on ISI variability as an important quantity to help elucidate sensory coding and signal processing performed by DHN. DFA has been applied for it permtts quantification of correlation properties of a nonstationary time series of neuronal discharge, Changes in DHN activity were extracellularly recorded with high impedance glass microelectrodes from superficial laminae of the dorsal horn, during different experimentally simúlated conditions. We háve analyzed DHN discharge patterns during spontaneous activity, as well as in the presence of diflerent noxious and non- noxious mechanical stimuli. Application of DFA method showed significant changes in dynamics of neural discharge when the ex- ternal stimulus is appliéd. These findings demonstrate the relevance of the application of methods of statistical. physics to identify the changes in aflerent inflow, and to characterize temporal patterns of activity of neurons under study
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| 2. |
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| 4. |
- Björklund, Andreas, et al.
(författare)
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Approximating longest directed paths and cycles
- 2004
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Ingår i: Lecture Notes in Computer Science. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1611-3349. ; 3142, s. 222-233
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the hardness of approximating the longest path and the longest cycle in directed graphs on n vertices. We show that neither of these two problems can be polynomial time approximated within n(1-epsilon)for any epsilon > 0 unless P = NP. In particular, the result holds for digraphs of constant bounded outdegree that contain a Hamiltonian cycle. Assuming the stronger complexity conjecture that Satisfiability cannot be solved in subexponential time, we show that there is no polynomial time algorithm that finds a directed path of length Omega(f(n) log(2) n), or a directed cycle of length Omega(f(n) log n), for any nondecreasing, polynomial time computable function f in w(1). With a recent algorithm for undirected graphs by Gabow, this shows that long paths and cycles are harder to find in directed graphs than in undirected graphs. We also find a directed path of length Omega(log(2) n/log log n) in Hamiltonian digraphs with bounded outdegree. With our hardness results, this shows that long directed cycles are harder to find than a long directed paths. Furthermore, we present a simple polynomial time algorithm that finds paths of length Omega(n) in directed expanders of constant bounded outdegree.
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| 6. |
- Lee, Chong S., et al.
(författare)
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Embryonic ventral mesencephalic grafts improve levodopa-induced dyskinesia in a rat model of Parkinson's disease
- 2000
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Ingår i: Brain. - 0006-8950. ; 123:7, s. 1365-1379
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the role of dopamine neurons in the manifestation of levodopa-induced dyskinesia in a rat model of Parkinson's disease. Daily treatment with a subthreshold dose of levodopa gradually induced abnormal involuntary movements (AIM) in 6-hydroxydopamine-lesioned rats, which included stereotypy and contraversive rotation. After 4 weeks of levodopa treatment, rats with mild and severe AIM were assigned to two treatment subgroups. The graft subgroup received embryonic ventral mesencephalic tissue into the striatum, whilst the sham-graft subgroup received vehicle only. Rats continued to receive levodopa treatment for 3 months post-graft. Brain sections at the level of the basal ganglia were processed for autoradiography using a ligand for dopamine transporter, and in situ hybridization histochemistry for mRNAs encoding postsynaptic markers. Levodopa-induced AIM significantly improved in grafted rats. The severity of AIM correlated inversely with the density of dopamine nerve terminals in the striatum (P < 0.001), with almost no AIM when the density of dopamine nerve terminals was > 10-20% of normal. Embryonic dopamine neuronal grafts normalized not only mRNA expression for preproenkephalin (PPE) in the indirect pathway, but also mRNA expression for prodynorphin (PDyn) in the direct pathway, which was upregulated by levodopa treatment. AIM scores correlated linearly with expression of PPE mRNA in the indirect pathway (P < 0.001) and also with PDyn mRNA in the direct pathway (P < 0.001). We conclude that embryonic dopamine neuronal grafts may improve levodopa-induced dyskinesia by restoring altered activities of postsynaptic neurons, resulting not only from dopamine denervation, but also from levodopa therapy, provided that the density of striatal dopaminergic nerve terminals is restored above a 'threshold' level.
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| 7. |
- Marshall, M. B., et al.
(författare)
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Ultrasonic characterisation of a wheel/rail contact
- 2004
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Ingår i: Transient Processes in Tribology. - : Elsevier. - 0444517065 ; , s. 151-158
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Konferensbidrag (refereegranskat)abstract
- A quantification of stress in a wheel/rail contact is essential information required in fatigue and wear calculations for determining design life, regrinding and maintenance schedules. The aim of this work was to use ultrasound to non-destructively determine wheel/rail contact pressures. A wheel/rail interface behaves like a spring. If the pressure is high there are few air gaps; so it is very stiff and allows transmission of an ultrasonic wave. If the pressure is low then interface stiffness is lower and most ultrasound is reflected. A spring model was used to determine maps of contact stiffness from wheel/rail contact ultrasonic reflection data. A calibration procedure was then used to determine the pressure. Measured contact pressure contours are compared with those predicted by various contact theories.
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| 8. |
- Olofsson, Ulf, et al.
(författare)
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Simulation of mild wear in boundary lubricated spherical roller thrust bearings
- 2000
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 241:2, s. 180-185
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Tidskriftsartikel (refereegranskat)abstract
- Owing to the curved contact surfaces in a spherical roller thrust beating, the rollers will undergo sliding. For an unskewed roller there will be two points along each contact where the sliding velocity is zero. At all other points along the contact, sliding is present. Under boundary lubricated conditions the sliding can give rise to mild wear. Experimental results show that this wear can cause a significant change in the surface profile outside the zero sliding points. The mild wear in the contact was simulated using Archard's wear law. An iterative wear model is described in which the normal load distribution, the tangential tractions and the sliding distances are repeatedly calculated to simulate the changes in surface geometry due to wear. Good qualitative agreement was achieved between the simulation results and the previously presented experimental results.
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| 9. |
- Schölin, Anna, et al.
(författare)
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Islet antibodies and remaining beta-cell function 8 years after diagnosis of diabetes in young adults : a prospective follow-up of the nationwide Diabetes Incidence Study in Sweden
- 2004
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Ingår i: Journal of Internal Medicine. - : Wiley. - 0954-6820 .- 1365-2796. ; 255:3, s. 384-391
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- ObjectivesTo establish the prevalence of remaining β-cell function 8 years after diagnosis of diabetes in young adults and relate the findings to islet antibodies at diagnosis and 8 years later.DesignPopulation-based cohort study.SettingNationwide from all Departments of Medicine and Endocrinology in Sweden.SubjectsA total of 312 young (15–34 years old) adults diagnosed with diabetes during 1987–88.Main outcome measurePlasma connecting peptide (C-peptide) 8 years after diagnosis. Preserved β-cell function was defined as measurable C-peptide levels. Three islet antibodies – cytoplasmic islet cell antibodies (ICA), glutamic acid decarboxylase antibodies and tyrosine phosphatase antibodies – were measured.ResultsAmongst 269 islet antibody positives (ab+) at diagnosis, preserved β-cell function was found in 16% (42/269) 8 years later and these patients had a higher body mass index (median 22.7 and 20.5 kg m−2, respectively; P = 0.0003), an increased frequency of one islet antibody (50 and 24%, respectively; P = 0.001), and a lower prevalence of ICA (55 and 6%, respectively; P = 0.007) at diagnosis compared with ab+ without remaining β-cell function. Amongst the 241 patients without detectable β-cell function at follow-up, 14 lacked islet antibodies, both at diagnosis and at follow-up.ConclusionsSixteen per cent of patients with autoimmune type 1 diabetes had remaining β-cell function 8 years after diagnosis whereas 5.8% with β-cell failure lacked islet autoimmunity, both at diagnosis and at follow-up.
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| 10. |
- Sellgren, U., et al.
(författare)
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A finite element-based model of normal contact between rough surfaces
- 2003
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 254:11, s. 1180-1188
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Tidskriftsartikel (refereegranskat)abstract
- Engineering surfaces can be characterized as more or less randomly rough. Contact between engineering surfaces is thus discontinuous and the real area of contact is a small fraction of the nominal contact area. The stiffness of a rough surface layer thus influences the contact state as well as the behavior of the surrounding system. A contact model that takes the properties of engineering surfaces into account has been developed and implemented using finite element software. The results obtained with the model have been verified by comparison with results from an independent numerical method. The results show that the height distribution of the topography has a significant influence on the contact stiffness but that the curvature of the roughness is of minor importance. The contact model that was developed for determining the apparent contact area and the distribution of the mean contact pressure could thus be based on a limited set of height parameters that describe the surface topography. By operating on the calculated apparent pressure distribution with a transformation function that is based on both height and curvature parameters, the real contact area can be estimated when the apparent contact state is known. The model presented is also valid for cases with local plastic flow in the bulk material.
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