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Mechanisms of anaesthetic depression of neocortical arousalEl-Beheiry, Hossam El-Dean Mohamed January 1990 (has links)
The most widely accepted hypotheses suggest that general anaesthetics interrupt conscious processes in the brain by decreasing synaptic excitation or by potentiating synaptic inhibition, especially in the neocortex. The putative transmitters in the neurological systems that generate neocortical arousal include acetylcholine, glutamate and γ-aminobutyrate (GABA). The primary objective here was to determine the neuronal mechanisms by which anaesthetics may obtund this arousal.
The majority of the investigations were carried out on pyramidal neurons in layers IV and V of guinea pig neocortex (in vitro slices), using intracellular
recording and pharmacological, including microiontophoretic, techniques. Bath applications of structurally dissimilar anaesthetics, isoflurane - a halogenated ether, and Althesin - a steroidal preparation, in concentrations of 0.5-2.5 minimum alveolar concentration (MAC) and 10-1300 μM, respectively, produced a small hyperpolarization (3-5 mV) which was associated with an increase in input conductance (10-30%). The lower concentrations (0.5-1.5 MAC and 10-200 μM) of these agents which are most relevant to the production of unconsciousness did not significantly affect the passive membrane properties. However, they produced striking decreases in spontaneous activities and the repetitive spike firing evoked by orthodromic (electrical) stimulation or intracellular current injections.
Because the observed changes in membrane properties could not explain the reduction in neuronal excitability, the effects of anaesthetics were investigated extensively on excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). The application of isoflurane or Althesin induced a dose-dependent, reversible depression in the amplitude of EPSPs,
with EC₅₀s of 1 MAC and ~50 μM, respectively. The IPSPs also were reduced in a dose-dependent manner. In order to eliminate possible shunting of the EPSPs by the GABA-activated Cl-conductance that produces the IPSP in the observed EPSP-IPSP sequence, a GABA[symbol omitted]-antagonist, bicuculline, was additionally
applied. Despite this IPSP-blockade, the anaesthetics strongly depressed the EPSPs as well as epileptiform activities evoked by subpial electrical stimulation.
In cognizance of the possibility that a postsynaptic attenuation of responsiveness to transmitter substances may be involved in the EPSP depression, the neuronal sensitivities to acetylcholine, glutamate. and GABA were determined. Anaesthetic administration markedly reduced the depolarizations and associated conductance changes evoked by dendritic applications of acetylcholine, glutamate and N-methyl-D-aspartate (NMDA). The hyperpolarizing responses to somatic applications of GABA were not affected significantly whereas the depolarizing effects observed with its dendritic application were slightly depressed. Same degree of selectivity also was evident from the lower EC₅₀s for the isoflurane- and Althesin-induced depressions of responses to acetylcholine compared with glutamate. Under in vitro conditions of hypomagnesia the responses to acetylcholine were totally blocked and the order of depression in the responses to GABA and glutamate was reversed; this may be of importance in the mechanism for the known increase in anaesthetic requirements in clinical syndromes associated with hypomagnesaemia.
Because the genesis of synaptic transients is affected by Ca²⁺ influx
or disposition, the interactions of anaesthetics were investigated on spike
afterhyperpolarizations (AHPs). The AHPs which are produced specifically by a Ca²⁺ -activated K⁺ -conductance were suppressed by the anaesthetics in a
dose-dependent manner under conditions where contaminating IPSPs had been
blocked by bicuculline. Since the passive membrane properties were
unaffected, an interference with a transmembrane Ca⁺ -influx may be involved
in the anaesthetic actions.
The effects of anaesthetics on glutamate-induced and voltage-dependent increases in intraneuronal Ca²⁺ ([Ca²⁺]i) were determined in cultured
hippocampal neurons with a Ca-sensitive probe (Fura-2) and microspectro-
fluorometric techniques. Isoflurane application depressed the increases in [Ca²⁺]i. produced by application of glutamate under conditions where its
actions would be favoured at NMDA- and quisqualate-subtypes of receptors. K⁺ -induced increases in [Ca²⁺]i also were reduced by application of isoflurane, probably due to actions on voltage-dependent Ca-channels in the membrane.
These investigations have provided evidence for the first time that excitatory transmitter actions in neocortex are selectively depressed by anaesthesia. A plausible mechanism would include suppression of the postsynaptic Ca-conductances associated with the AHPs and glutamatergic, as well as cholinergic interactions at pre- and post-synaptic sites on neurons involved in neocortical arousal. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate
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Are there order specific patterns of cortical gyrification and if so why?Pillay, Praneshri 10 December 2008 (has links)
Abstract (for Chapter 2)
Objective: The aim was to test the hypothesis that the order is a significant phylogenetic
grouping in terms of quantifiable gyrification indices. Method: The gyrification index
(GI) was measured from serial sections of the brain of twenty five different mammalian
species, representing the different orders i.e. primates, carnivores, artiodactyls and
rodents. Image J analysis was used to measure the contours of the cerebral cortex and the
GI was calculated using three different methods of analysis i.e. complete vs outer; gyral
vs sulcal and outer vs inner surface contours. The measurements were then computed
against the brain weights of each species within the order. Results: An increasing GI
correlates with an increasing brain weight in all the mammalian orders. Each order has its
own specific allometric patterns that are significantly different from the other orders
examined. The artiodactyls were the mammals with the most gyrencephalic brains, these
species being significantly more gyrencephalic than all other mammals when species of
similar brain weights are compared. The North American beaver has an atypically
lissencephalic brain for its size, differing from the trend for increased gyrencephaly found
in the other rodent species examined. Conclusions: Our results show definite trends and
patterns specific to each order. So it would seem that the order is a significant
phylogenetic grouping in terms of this neural parameter, from which we can predict with
a reasonable degree of certainty, the GI of any species of a particular order, if we know
the brain weight.
Abstract (for Chapter 3)
The mammalian order has proven to be a significant phylogenetic grouping in terms of
gyrification from which we can predict with a reasonable degree of certainty, the GI of
any species of a particular order, if we know the brain weight. We have attempted in the
present study to identify potential causes for gyrification at the class level by
investigating relationships at the level of the order. It appears that clues to the extent and
pattern of gyrification in the different mammalian orders might be related to the bones
that constitute the braincase. The external surface areas of the bones of the cranial vault
of seventeen different mammalian species were measured using a microscribe digitiser.
These values were plotted against brain weight from which we could then calculate
residual values, determining if there was more or less external cranial vault area than
expected for the size of the brain. These residuals were then plotted against the
gyrification indices determined in a previous study for the species examined. Results
indicated that for the primates and artiodactyls the skull may potentially be considered as
a limiting factor on the expansion of the cerebral cortex; however, the carnivore and
rodent orders show conflicting results which suggest that the relative surface area of the
skull appears to have no effect on the quantitative extent of gyrencephaly. These
inconclusive findings suggest that causes contributing to the quantitative extent of
gyrification across mammals may be multifactorial, and more parameters may need to be
included in the analysis to arrive at an answer.
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MULTISCALE FUNCTIONAL ARCHITECTURE OF NEOCORTEX: FROM CLUSTERS TO COLUMNSUnknown Date (has links)
The physical architecture of neural circuits is thought to underlie the computations that give rise to higher order feature sensitivity in the neocortex. Recent technological breakthroughs have allowed the structural and functional investigation of the basic computational units of neural circuits; individual synaptic connections. However, it remains unclear how cortical neurons sample and integrate the thousands of synaptic inputs, supplied by different brain structures, to achieve feature selectivity. Here, I first describe how visual cortical circuits transform the elementary inputs supplied by the periphery into highly diverse, but well-organized, feature representations. By combining and optimizing newly developed techniques to map the functional synaptic connections with defined sources of inputs, I show that the intersection between columnar architecture and dendritic sampling strategies can lead to the selectivity properties of individual neurons: First, in the canonical feedforward circuit, the basal dendrites of a pyramidal neuron utilize unique strategies to sample ON (light increment) and OFF (light decrement) inputs in orientation columns to create the distinctive receptive field structure that is responsible for basic sensitivity to visual spatial location, orientation, spatial frequency, and phase. Second, for long-range horizontal connections, apical dendrites unbiasedly integrate functionally specialized and spatially targeted inputs in different orientation columns, which generates specific axial surround modulation of the receptive field. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection
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Neuromodulation of neocortical long-term potentiation in the adult, freely moving rat /Boyd, Tiffany E. January 2001 (has links)
Thesis (Ph.D.) -- McMaster University, 2001. / Includes bibliographical references (leaves 142-170). Also available via World Wide Web.
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Lamination and within-area integration in the neocortex /Robert, Adrian, January 1999 (has links)
Thesis (Ph. D.)--University of California, San Diego, 1999. / Vita. Includes bibliographical references (leaves 154-171).
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An architectonic study of three mammals grey squirrels (Sciurus carolinesis), tree shrews (Tupaia belangeri) and galagos (Otolemur garnetti) /Wong, Peiyan. January 2009 (has links)
Thesis (Ph. D. in Psychology)--Vanderbilt University, Dec. 2009. / Title from title screen. Includes bibliographical references.
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Regulation of interneuronal voltage-gated potassium channels Kv3.1b and Kv3.2 and the calcium-binding protein parvalbumin in the rat visual cortexGrabert, Jochen. January 2005 (has links) (PDF)
Bochum, Univ., Diss., 2005.
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The role of calcium spikes in neocortical pyramidal cell dendrites : implications for the transduction of dendritic current into spike output /Oakley, John Christopher. January 1999 (has links)
Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (leaves 78-82).
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Constraining the function of CA1 in associative memory models of the hippocampusLongden, Kit January 2005 (has links)
CA1 is the main source of afferents from the hippocampus, but the function of CA1 and its perforant path (PP) input remains unclear. In this thesis, Marr’s model of the hippocampus is used to investigate previously hypothesized functions, and also to investigate some of Marr’s unexplored theoretical ideas. The last part of the thesis explains the excitatory responses to PP activity in vivo, despite inhibitory responses in vitro. Quantitative support for the idea of CA1 as a relay of information from CA3 to the neocortex and subiculum is provided by constraining Marr’s model to experimental data. Using the same approach, the much smaller capacity of the PP input by comparison implies it is not a one-shot learning network. In turn, it is argued that the entorhinal-CA1 connections cannot operate as a short-term memory network through reverberating activity. The PP input to CA1 has been hypothesized to control the activity of CA1 pyramidal cells. Marr suggested an algorithm for self-organising the output activity during pattern storage. Analytic calculations show a greater capacity for self-organised patterns than random patterns for low connectivities and high loads, confirmed in simulations over a broader parameter range. This superior performance is maintained in the absence of complex thresholding mechanisms, normally required to maintain performance levels in the sparsely connected networks. These results provide computational motivation for CA3 to establish patterns of CA1 activity without involvement from the PP input. The recent report of CA1 place cell activity with CA3 lesioned (Brun et al., 2002. Science, 296(5576):2243-6) is investigated using an integrate-and-fire neuron model of the entorhinal-CA1 network. CA1 place field activity is learnt, despite a completely inhibitory response to the stimulation of entorhinal afferents. In the model, this is achieved using N-methyl-D-asparate receptors to mediate a significant proportion of the excitatory response. Place field learning occurs over a broad parameter space. It is proposed that differences between similar contexts are slowly learnt in the PP and as a result are amplified in CA1. This would provide improved spatial memory in similar but different contexts.
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Diversité des interneurones dans le cortex en tonneau de la sourisPerrenoud, Quentin 24 November 2011 (has links) (PDF)
Les interneurones GABAergiques sont des composants cruciaux du réseau neocortical et la caractérisation fonctionnelle du neocortex a été fortement ralentie par l'absence de consensus concernant leur classification. Les interneurones se différencient du point de vue électrophysiologique et morphologique ainsi que par l'expression de marqueurs moléculaires. Une controverse demeure cependant, pour déterminer si la combinaison de ces caractéristiques définit des classes séparées ou au contraire, un continuum phénotypique où chaque cellule est unique. Pendant ma thèse, je me suis proposé d'étudier la diversité des interneurons neocorticaux en prenant en compte l'ensemble de ces critères. Des enregistrements de patch-clamp couplés à la technique de PCR sur cellule unique ont été réalisés sur un échantillon de plus de 300 interneurones et l'arborisation de près de 200 d'entre eux a été reconstruites en 3 dimensions. Les phénotypes électrophysiologiques, morphologiques et moléculaires de notre échantillon ont été quantifiés au travers d'un ensemble de 56 paramètres quantitatifs. Cet échantillon a d'abord été utilisé pour caractériser les interneurones dans la couche VI du neocortex, une région où ils n'avaient été que partiellement décrits. En utilisant une approche non supervisée, 4 classes d'interneurones ont pu être identifiées sur la base de propriétés électrophysiologiques, morphologiques et moléculaires. Additionnellement, en utilisant des immunomarquages sur des souris GAD67::GFP Knock-In, les distributions d'interneurones exprimant des marqueurs caractéristiques ont été cartographiées à travers la couche VI, mettant en évidence que des populations distinctes s'accumulent dans des sous-couches particulières. Ces analyses ont été soumises pour publication. Nous avons ensuite entrepris de caractériser la diversité des interneurones à travers l'ensemble des couches du neocortex. En utilisant des méthodes non supervisées reposant sur les propriétés électrophysiologiques et moléculaires des interneurones, nous avons montré que les classes que nous avions caractérisées dans la couche VI pouvaient être identifiées sur l'ensemble de notre échantillon. Toutefois, en analysant la séparation de ces classes avec des méthodes analytiques, nous avons trouvé que, si certaines formaient des archétypes distincts, une fraction substantielle de notre échantillon présentait des phénotypes intermédiaires à des classes particulières. Ce travail suggère donc que les interneurones du neocortex se séparent bien en plusieurs populations, mais que ces groupes ne constituent pas des classes distinctes mais plutôt des archétypes phénotypiques. Ces résultats supportent une nouvelle façon de regarder la diversité des interneurones GABAErgiques et seront rapportés dans un second article.
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