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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Synaptic transmission in cortical networks : focus on the corpus callosum and CA1 hippocampal area of rodents

Karayannis, Theofanis January 2007 (has links)
No description available.
22

Computational models of invariant object representation in the inferotemporal cortex

Perry, Gavin January 2005 (has links)
No description available.
23

Quantitative methods to assess cerebral haemodynamics

Hall, Emma Louise January 2012 (has links)
In this thesis methods for the assessment of cerebral haemodynamics using 7 T Magnetic Resonance Imaging (MRI) are described. The measurement of haemodynamic parameters, such as cerebral blood flow (CBF), is an important clinical tool. Arterial Spin Labelling (ASL) is a non-invasive technique for CBF measurement using MRI. ASL methodology for ultra high field (7 T) MRI was developed, including investigation of the optimal readout strategy. Look-Locker 3D-EPI is demonstrated to give large volume coverage improving on previous studies. Applications of methods developed to monitor functional activity, through flow or arterial blood volume, in healthy volunteers and in patients with low grade gliomas using Look-Locker ASL are described. The effect of an increased level of carbon dioxide in the blood (hypercapnia) was studied using ASL and functional MRI; hypercapnia is a potent vasodilator and has a large impact on haemodynamics. These measures were used to estimate the increase in oxygen metabolism associated with a simple motor task. To study the physiology behind the hypercapnic response, magnetoencephalography was used to measure the impact of hypercapnia on neuronal activity. It was shown that hypercapnia induces widespread desynchronisation in a wide frequency range, up to ~ 50 Hz, with peaks in the sensory-motor areas. This suggests that hypercapnia is not iso-metabolic, which is an assumption of calibrated BOLD. A Look-Locker gradient echo sequence is described for the quantitative monitoring of a gadolinium contrast agent uptake through the change in longitudinal relaxation rate. This sequence was used to measure cerebral blood volume in Multiple Sclerosis patients. Further development of the sequence yielded a high resolution anatomical scan with reduced artefacts due to field inhomogeneities associated with ultra high field imaging. This allows whole head images acquired at sub-millimetre resolution in a short scan time, for application in patient studies.
24

Sex hormonal modulation of hemispheric asymmetry and interhemispheric crosstalk

Bayer, Ulrike January 2009 (has links)
Fluctuating levels of sex hormones (estrogen, E and progesterone, P) during the menstrual cycle have been shown to affect fundamental principles of brain organization, that is functional cerebral asymmetries (FCAs). Regarding a possible underlying mechanism, it seems likely that dynamics in FCAs are driven by hormonal modulations of interhemispheric crosstalk (i.e., interhemispheric inhibition). Whether other aspects of interhemispheric interaction, such as interhemispheric integration (IHI), are also susceptible to menstrual cycle-related hormonal changes has not yet been examined. Moreover, most of the findings come from studies investigating younger women during hormonal distinct cycle phases. This approach, however, does not allow conclusions about causal relationships between hormonal changes and functional brain organization. It seems, thus, necessary to directly manipulate the hormonal status of participants via exogenous hormone therapy (HT).The present thesis focused on sex hormonal changes in IHI and FCAs in normally cycling women and postmenopausal women with and without HT. Younger women were tested twice, once during the low-hormone menstrual phase and once during the high-P luteal phase. Postmenopausal women were tested in a between-participants design differentiating between postmenopausal women using E therapy (ET), those using E plus synthetic progestins, and postmenopausal controls without HT. The results show that IHI in normally cycling women fluctuates across the menstrual cycle with an enhanced interhemispheric processing during the luteal phase. Thus, it seems that aspects of interhemispheric interaction (i.e., IHI) other than those involved in FCAs are also affected by the menstrual cycle and cycle-related hormonal changes. In contrast, HT, and ET in particular, after the menopause seems to affect intrahemispheric processing whereas intrahemispheric was essentially unaffected by HT. A modulation of intrahemispheric functioning (i.e. right hemisphere functioning) which was related to estradiol-levels also became evident when postmenopausal women were tested on a right hemisphere dominated asymmetry task. The findings can be explained by a faster and more pronounced age-related decline in interhemispheric relative to intrahemispheric processing which seems to be accompanied by a higher sensitivity to HT. Aging processes together with differences in the hormonal status (exogenous changes as a result of HT vs. endogenous changes during the menstrual cycle) may also explain divergent behavioural outcomes in postmenopausal women and younger women. Taken together, the findings show that the female brain retains its plasticity even after reproductive ages and remains susceptible to the effects of sex hormones throughout lifetime.
25

The effect of direct current electric fields on hippocampal neurons in vitro

Yao, Li January 2005 (has links)
Initiation and effectiveness of neuron migration to the right places are critical for the development and repair of central nervous system. It is now confirmed that mammalian, including human central nervous system (CNS) contains stem cells and nerve progenitor cells. Upon brain injury either from ischemia or trauma, those cells must be able to proliferate and migrate to the damaged parts in order to repair the damage. To understand the underlying mechanisms and find new techniques of directing neuron growth are of both scientific and medical significance. Endogenous electric fields (EFs) are widespread in developing and regenerating tissues. During embryonic development, endogenous EFs exist rostrocaudally and mediolaterally at the neural plate and neural fold stages. The size, location and developmental timing of EFs correlate with active neuron migration. Therefore, electric fields may provide a signal to direct cellular behavior of nerve cells during nervous system development and reparation. Application of EFs directs cell migration of many types of cells. However, whether neurons migrate directionally in applied electric fields has not been demonstrated. I have investigated the effects of applied physiological EFs on cultured hippocampal neurons and the underlying mechanisms. Hippocampal neurons from rats and mice were cultured on poly-L-lysine and laminin coated dishes and were identified with MAP-2 (a specific marker for neuron) staining. The neurons were exposed to small applied electric fields and the migration and division were recorded with an imaging system. 1. In a DC electric field, dissociated rat hippocampal neurons migrate to the cathode. The migration direction was reversed when the EF polarity was reversed. Neuronal migration in EFs involes the same steps as that in non-EF cultures, namely leading process extension, nuclear translocation and retraction of trailing process. 2. The cathodal migration of hippocampal neurons is voltage and time dependent. A DC EF does not have significant influence on neuronal migration speed. This result shows that the effect of EFs is different on neuronal migration speed and direction of migration. 3. The guidance effect of EFs is also seen in neuronal migration from hippocampal micro-explants and neuronal migration along glial process. Both types of migration are towards the cathode. 4. Electric fields direct growth cone path finding and neurite orientation. EF induced neurite orientation showed time and voltage dependence. Leading process branching, turning and swapping over of leading and trailing processes are the main types of neurite re-orientation. Guided neuronal migration and neuron leading process orientation are not accompanied with neuronal size change. 5. In an applied EF, MAP-2 (microtubule associated protein 2), p-Akt, Golgi apparatus and centrosome redistribute asymmetrically to the cathode facing side of hippocampal neurons. 6. Inhibition of ROCK (Rho-associated protein kinase) and Pi3k (phosphoinositide-3 kinase) with inhibitors decreases leading neurite orientation and Golgi polarization in the neurons in response to the EFs. They also decrease the directedness and speed of guided neuronal migration in EFs. 7. Null mutation of p110gamma, which encodes the catalytic domain of Pi3k gamma significantly decreased the electric field directed neuron migration. These results suggest that ROCK and Pi3k regulate EFs directed neuronal migration. 8. Application of electric fields also affects cell division of postnatal hippocampal neurons. The neuronal phenotype of divided cells was confirmed with MAP-2 and GFAP staining. Most cells divided with a cleavage plane perpendicular to the EF vector. 9. Using vibrating probe techniques, we show that there is a significant change in electric currents upon wounding hippocampus in vitro. In conclusion, this study demonstrates that hippocampal neurons from both rat and mice respond to applied EFs by directional migration and directional division. The electric field directed neuronal migration involves ROCK and Pi3k signaling. This raises the possibilities that 1) there may be a neglected role for endogenous electric fields in directing neuron migration and division; 2) electric fields may be used as a potential cue to direct neuronal migration in repair of the central nervous system.
26

An investigation of the association between handedness, cognition, brain structure and function

Powell, Joanne Lindsey January 2011 (has links)
Left- and right-handers show functional and structural brain differences. However, the literature on the relationship between handedness and cognitive ability is inconsistent. Moreover, possible differences in the neuroanatomical correlates of cognitive ability, including regional grey matter (GM) volume, between left- and right-handers have not been explored. This thesis describes work with two main aims: (i) to explore differences in brain structure and function between left- and right-handers using MRI on a sample of left- (n=40) and right- (n=42) handers, and (ii) to explore the effect of handedness on the neuroanatomical correlates of cognitive capacity on the same cohort. The effect of sex and handedness on pars opercularis (PO) and pars triangularis (PTR) volume and the sulcal contours defining these regions are described in Chapter 5. PO volume asymmetry is leftward (left-greater-than-right) in right-handed males, non-asymmetrical in right-handed females and rightward in left-handed males and females. PTR volume is rightward in right-handers and non-asymmetrical in left-handers. The inferior frontal sulcus is discontinuous more often in the right than left hemisphere in right-handers and discontinuous more often in the left than right hemisphere in left-handers. The probability of presence of diagonal sulcus is higher in the right than the left hemisphere for left-handers. A second part to this study found a significant effect of handedness on foot preference for kicking and parental handedness. In Chapter 6 fractional anisotropy (FA) asymmetry across the whole brain is explored using voxel-wise statistics on FA maps obtained from diffusion weighted images: increased FA is found in right-handers, and FA asymmetry along the uncinate fasciculus and arcuate fasciculus in both groups. Chapter 7 shows significantly greater leftward language laterality in right-handers and greater activation in right IFG in response to a language production task in left- compared to right-handers. Working memory score is higher in right-handers is associated with increased leftward language laterality. Subjects with opposed language and spatial laterality perform better in tests assessing verbal comprehension and perceptual organization. Next, relationships between GM volume and cognitive ability is explored for fluid and crystallised intellectual functioning using voxel-based morphology (Chapter 8). Significant differences in the GM correlates of fluid and crystallised intelligence were found between the handedness groups. Lastly, Chapter 9 explores the relationship between prefrontal cortex (PFC) volume and intentionality in left- and right-handers using stereological volume estimates from T1-weighted MR images. Although no significant difference in intentionality score was found between the handedness groups, higher scores of intentionality were associated with larger orbital PFC volume in right-handers, but with larger dorsal PFC volume in left-handers. This research extends the literature demonstrating differences in brain structure and function between left- and right-handers. Overall, the results suggest that individuals may achieve similar cognitive ability scores with different brain designs. Future research should consider the effect of group differences in the population and how this might influence brain ‘design’ and cognitive ability.
27

The role of oscillation population activity in cortico-basal ganglia circuits

Sharott, Andrew David January 2006 (has links)
The basal ganglia (BG) are a group of subcortical brain nuclei that are anatomically situated between the cortex and thalamus. Hitherto, models of basal ganglia function have been based solely on the anatomical connectivity and changes in the rate of neurons mediated by inhibitory and excitatory neurotransmitter interactions and modulated by dopamine. Depletion of striatal dopamine as occurs in Parkinson's Disease (PD) however, leads primarily to changes in the rhythmicity of basal ganglia neurons. The general aim of this thesis is to use frontal electrocorticogram (ECoG) and basal ganglia local field potential (LFP) recordings in the rat to further investigate the putative role for oscillations and synchronisation in these structures in the healthy and dopamine depleted brain. In the awake animal, lesion of the SNc lead to a dramatic increase in the power and synchronisation of P-frequency band oscillations in the cortex and subthalamic nucleus (STN) compared to the sham lesioned animal. These results are highly similar to those in human patients and provide further evidence for a direct pathophysological role for p-frequency band oscillations in PD. In the healthy, anaesthetised animal, LFPs recorded in the STN, globus pallidus (GP) and substantia nigra pars reticulata (SNr) were all found to be coherent with the ECoG. A detailed analysis of the interdependence and direction of these activities during two different brain states, prominent slow wave activity (SWA) and global activation, lead to the hypothesis that there were state dependant changes in the dominance of the cortico-subthalamic and cortico-striatal pathways. Multiple LFP recordings in the striatum and GP provided further evidence for this hypothesis, as coherence between the ECoG and GP was found to be dependent on the striatum. Together these results suggest that oscillations and synchronisation may mediate information flow in cortico-basal ganglia networks in both health and disease.
28

Dynamic and stochastic behaviour of neocortical synapses

Bremaud, Antoine January 2012 (has links)
The neocortex receives inputs from many other brain regions, it contains many different types of neurones in 6 layers and processes large volumes of information. This thesis deals with some of the properties of the local synaptic circuitry of the neocortex. Dual intracellular recordings with biocytin labelling were performed in slices of adult rat neocortex in vitro using conventional sharp micro-electrodes. Responses of postsynaptic cells to trains of presynaptic action potentials were recorded. Histological processing identified the cells recorded and laminar location. The amplitude of each excitatory postsynaptic potential (EPSP), in each sweep was measured. Subsets of measurements for which conditions were deemed to be stable were selected. For recordings that included multiple data subsets whose amplitudes differed primarily because of differences in presynaptic release probability (p), the binomial parameters n (number of synapses), p and q (quantal amplitude) were estimated by fitting relationships between EPSP coefficient of variation, variance or proportion of failures of release and mean amplitude, with equations based on simple binomial models. Striking differences in the binomial parameters estimated for different classes of connections were found. To determine how far the outcomes of this analysis depended on the assumption of a simple binomial model in which p and q are identical at all synapses, Monte-Carlo simulations of simple and more complex binomial models of synaptic release were generated. These models demonstrated the wide range of conditions under which analysis based on simple binomial models can provide reliable estimates of n, p and q. Computational models (NEURON) that integrate short term synaptic dynamics with a stochastic simulation of synaptic transmission were developed. These models display properties similar to those displayed by synapses, but not observed in traditional deterministic models of release. For example, recovery from synaptic depression has peaks and troughs superimposed on a smooth exponential decay.
29

Corticosterone-induced changes in N-methyl-D-aspartate receptor-mediated transmission in the hippocampus

Riedemann, Maria-Therese January 2012 (has links)
Due to its great importance in learning and memory, the hippocampus has received a lot of attention from studies investigating synaptic plasticity, the molecular correlates of learning and memory. Corticosterone (CaRT) is the endogenous glucocorticoid in rodents and is of high physiological significance to the organism. In response to stress the glucocorticoid concentration rises rapidly, releasing the hormone into the blood and allowing it to exert its actions on the level of the pituitary, the hypothalamus and the hippocampus. Interestingly, learning and memory are also subject to glucocorticoid actions in the hippocampus and the duration of increased glucocorticoid exposure has opposite effects on these measures: while long- term CORT exposure or chronic stress often leads to impairments of learning or memory, short-term CORT exposure has been shown to enhance or facilitate learning and memory. All of these studies have ascribed an important role to the N-methyl-D-aspartate receptor (NMDAR) in modulating CORT-induced changes in synaptic plasticity. Against this background this project aimed at the understanding of rapid CORT-induced effects on synaptic NMDAR-mediated transmission in the rat hippocampus. Using patch-clamp technique it could be shown that CORT rapidly caused an increase in NMDAR-mediated transmission in the hippocampus and that this effect was independent of protein synthesis but dependent on calcium and CaMKII activation. Accordingly, increased levels of phosphorylated CaMKII at its autophosphorylation site Thr286 following CaRT exposure were monitored. Furthermore, increased CaMKII-dependent surface expression of NMDARs following CORT exposure was observed, indicating that CORT induces trafficking of NMDARs. Accordingly, it could be shown that CORT exposure led to an increased association of GluN2B-containing receptors with the motor protein KIF-17. This finding was further corroborated by increased CORT-induced binding of NMDARs to the postsynaptic anchor protein PSD-95. Furthermore, increases in basal NMDAR-mediated transmission were accompanied by changes in the synaptic cluster size of PSD-95 in response to CORT treatment.
30

The mechanisms of plasticity between the lateral amygdala and the perirhinal cortex

Laing, Michael January 2013 (has links)
A key feature of memory is that experiences with emotional significance are better remembered than those which lack emotional significance (Cahill 1994, Roozendaal 2008). The amygdala which is located within the medial temporal lobe, projects to multiple brain regions and via these projections the amygdala has the capacity to modulate the activity of other brain regions (Kajiwara 2003, Roozendaal 2008). There are robust reciprocal connections between the amygdala and the perirhinal cortex and activation of the lateral amygdala has been demonstrated to enhance learning and memory (Roozendaal 2008) and synaptic plasticity (Perugini 2012) within the perirhinal cortex. However, the mechanisms by which the amygdala regulates perirhinal cortex plasticity are still largely unknown. By studying synaptic communication and plasticity between the lateral amygdala and the perirhinal cortex is it possible to determine how the amygdala modulates activity within the perirhinal cortex? Horizontal slices were taken from juvenile male Lister Hooded rats and recordings were made in layer 111111 of the perirhinal cortex. Two stimulation electrodes were placed on the slice, one in the lateral amygdala and the other in layer 111111 of the perirhinal cortex. Extracellular field recordings demonstrated that blockade of the ~-adrenoceptors prevented the induction of lateral-amygdala (LA-PRh) long-term potentiation specifically. However the chemical activation of ~-adrenoceptors , by bath application of the J3- adrenoceptor agonist isoprenaline, induced perirhinal cortex (PRh-PRh) long-term potentiation when coupled with basal stimulation. Application of isoprenaline induced only a transient potentiation of the LA-PRh response however this was converted to long-term potentiation when delivered with a subthreshold long-term potentiation protocol. Voltage clamp recordings demonstrated that long-term depression at both the PRh-PRh and LA-PRh is NMDA-R dependent and -adrenoceptor independent, sensitive to NR2A but not NR2B inhibition and the long-term depression is induced presynaptically.

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