Dissertations / Theses on the topic 'Mormyrid'

Le glycogene represente dans les cellules sensorielles des electrorecepteurs cutanes d'un normyride gnathonemus petersii, une source energetique de reserve destinee a soutenir l'autoactivite electrique de ces cellules pourvues de connexions anatomo-fonctionnelles intactes

The histopathological effects of three heavy metals, cyanide and atrazine on the gill tissues of the mormyrid fish, Gnathonemus petersii, were determined primarily by electron microscopy. Replicate samples of fish were exposed for 6 hours to acute and subacute concentrations of each toxicant in a continuous flow system comprising aerated, dechlorinated and heated water. Specific alterations to gill ultrastructure were found to be associated with each toxicant. Cadmium damage at 1.0 mg 1.1 was associated with the formation of large subepithelial spaces within the secondary lamellae and at 10.0 mg 1,1 with lamellar aneurism. Chromium caused epithelial hyperplasia at 1.0 mg 1'1 and extensive fusion of secondary lamellae at 10.0 mg 1'1. Copper exposure stimulated hyperactivity of mucous cells at 0.1 mg 1'1 which resulted in the accumulation of a thick layer of mucus on the gills at 0.5 mg 1,1. Cyanide caused desquamation of filamental epithelium at 0.05 mg 1'1 and stripping of the epithelium and underlying tissue down to the supporting cartilage at 0.1 mg 1'1. Atrazine damage was characterized by the presence of breaks in the gill epithelium at 0.5 mg 1,1 which developed into deep pits at 5.0 mg 1'1. Morphometric measurements of the structural components of secondary lamellae revealed quantitative differences in tissue alterations among fish exposed to different toxicants. A significant increase in the volume of secondary lamellae has been found in fish exposed to heavy metals. On the other hand, fish exposed to atrazine and 0.1 mg 1,1 cyanide showed a sharp decrease in lamellar volume. The increase in lamellar volume in fish exposed to heavy metals was also accompanied by a reduction in the interlamellar water spaces and a significant increase in the water-blood diffusion distance. In cadmium-exposed fish the increase in lamellar volume was mainly attributed to the increase in the subepithelial spaces at 1.0 mg 1'1 or in the volume of blood components due to aneurism at 10.0 mg 1'1. In'fish exposed to chromium it was attributed to the thickening of the epithelial layer as a result of epithelial hyperplasia; or epithelial hypertrophy in copper-exposed fish. On the other hand, the decrease in lamellar volume in fish exposed to 0.10 mg 1'1 cyanide was mainly attributed to the decrease in the volume of the tissue-occupied region outside the basement membrane due to the stripping of lamellar epithelium. In both concentrations ofatrazine (O.S and S.O mg 1'1) the different components of the secondary lamella contributed to the decrease in lamellar volume. These morphometric analyses confirm observations made by electron microscopy. The present study, therefore, demonstrates the potential of electron microscopy, in particular SEM, as a tool for detecting pollutant-induced damage to the surface of fish gills. Preliminary work with chromium has indicated that comparative ultrastructural studies are now needed to further our understanding of the chronic exposure of fish to these toxic chemicals.

The astounding capacity for learning ranks among the nervous system’s most impressive features. This thesis comprises studies employing varied approaches to improve understanding, at the level of neural circuits, of the brain’s capacity for learning. The first part of the thesis contains investigations of hippocampal circuitry – both theoretical work and experimental work in the mouse Mus musculus – as a model system for declarative memory. To begin, Chapter 2 presents a theory of hippocampal memory storage and retrieval that reflects nonlinear dendritic processing within hippocampal pyramidal neurons. As a prelude to the experimental work that comprises the remainder of this part, Chapter 3 describes an open source software platform that we have developed for analysis of data acquired with in vivo Ca2+ imaging, the main experimental technique used throughout the remainder of this part of the thesis. As a first application of this technique, Chapter 4 characterizes the content of signaling at synapses between GABAergic neurons of the medial septum and interneurons in stratum oriens of hippocampal area CA1. Chapter 5 then combines these techniques with optogenetic, pharmacogenetic, and pharmacological manipulations to uncover inhibitory circuit mechanisms underlying fear learning. The second part of this thesis focuses on the cerebellum-like electrosensory lobe in the weakly electric mormyrid fish Gnathonemus petersii, as a model system for non-declarative memory. In Chapter 6, we study how short-duration EOD motor commands are recoded into a complex temporal basis in the granule cell layer, which can be used to cancel Purkinje-like cell firing to the longer duration and temporally varying EOD-driven sensory responses. In Chapter 7, we consider not only the temporal aspects of the granule cell code, but also the encoding of body position provided from proprioceptive and efference copy sources. Together these studies clarify how the cerebellum-like circuitry of the electrosensory lobe combines information of different forms and then uses this combined information to predict the complex dependence of sensory responses on body position and timing relative to electric organ discharge.

Simple circuits perform simple tasks. Complex circuits can perform more complicated tasks. This is true for artificial circuits and for brain circuits. As is known from artificial networks, a complexity that makes circuits substantially more powerful is distributing learning across multiple layers. In fact, most brain circuits in vertebrate systems are multi-layer circuits (but for few that perform simple reflexes) in which learning is distributed across layers. Despite the crucial contribution of learning in middle layer neurons to the output of the circuits they are embedded in, there is little understanding of the principles defining this contribution. A very common feature in brain circuits is that middle layer neurons generate two types of signals, known as spikes. These middle layer neurons commonly have long dendrites where they generate dendritic spikes. As well, like most neurons, they generate axonal spikes near the cell body. Neurons exhibiting these two spike types include pyramidal cells in the neo-cortex and the hippocampus, the Purkinje cells in the cerebellum and many more. In this thesis I study another circuit that contains middle layer neurons, the electrosensory lateral lobe (ELL) of the mormyrid fish. The ELL is a tractable brain circuit in which the middle layer neurons generate dendritic and axonal spikes. In this thesis I show that these spike types are not two different expressions of the same inputs. Rather, they have a symbiotic relationship. Instead of all inputs triggering both spikes, some inputs can selectively drive dendritic spikes. The dendritic spikes in return modify the synaptic strength of another set of inputs. The modified inputs are then transmitted to downstream neurons via the axonal spikes, which contributes a desired signal to the output of the circuits. Effectively there is a separation of learning and signaling in the middle layer neurons through the two spike types. Having two types of spikes in the same neuron doing different computations enormously expands the computational power of the neuron. But, being in the same neuron means the separation of function is constrained and needs to be supported by biophysical principles. I have thus built a biophysical model to understand the biophysical principles underlying the separation of function. I show that in the middle layer neurons of the ELL, the axonal spikes are strongly reduced in amplitude as they backpropagate to the apical dendrites, yet they remain crucial in driving dendritic spikes. Critically, modulation of inhibitory inputs can selectively dial up or down the ability of the backpropagating axonal spikes to drive dendritic spikes. Thus, a set of inhibitory modulating inputs can selectively modulate dendritic spikes. Having learning in different layers contributing to the outcome of the circuit, naturally leads to asking how the work is divided across layers and neuron types within the circuit. In this thesis I answer this question in the context of the outcome of the ELL circuit. Finally, another signature of a complex circuit is the ability to integrate many different inputs, usually in middle layer neurons, to generate sophisticated outputs. A goal for scientists studying systems neuroscience is to understand how this integration works. In this thesis I provide a coherent model of a learning behavior called vestibulo occular reflex (VOR) adaptation, that depends on the integration of separate inputs to yield a learned behavior. The VOR is a simple reflex generated in the brain stem. Inputs from the brain stem are also sent to an area in the cerebellar cortex called the flocculus. The flocculus also receives another set of inputs that generate a different behavior, called smooth-pursuit. The integration of VOR inputs with smooth-pursuit inputs in the flocculus generate VOR adaptation. Understanding complex circuits is one of the greatest challenges for today's neuroscientists. In this thesis I tackle two such circuits and hope that through a better understandings of these circuits we gain principles that apply to other circuits and thereby advance our understanding of the brain.

Dissertação de mest. em Estudos Marinhos e costeiros, Unidade de Ciências e Tecnologias dos Recursos Aquáticos, Univ. do Algarve, 1998<br>No presente estudo descreve-se a ecologia alimentar de três espécies de Sparídeos da costa Algarvia, cuja importância económica é elevada e que se encontram comercialmente ameaçadas, analisando e comparando os seus hábitos alimentares através de métodos quantitativos (coeficiente de vacuidade e índice de replecção) e métodos qualitativos (percentagem em número, frequência de ocorrência e percentagem em peso). As presas preferenciais foram determinadas pelo coeficiente alimentar de HUREAU (Q) e pelo índice de importância relativa (I.R.I.). Foi analisado um total de 105 indivíduos (46 de Pagrus pagrus, 37 de Lithognathus mormyrus e 22 de Diplodus sargus) capturados por redes de emalhar entre Abril de 1995 e Maio de 1997. As espécies P. pagrus e L. mormyrus evidenciaram um carácter generalista no seu padrão de alimentação, ingerindo um elevado número de taxa distintos, enquanto que a espécie D. sargus apresentou uma dieta mais restrita, composta maioritariamente de moluscos bivalves. O estudo da sobreposição dos regimes alimentares destas três espécies revelou a existência de um grau de sobreposição significativo. As espécies em estudo podem ser consideradas como consumidores secundários, que se alimentam de organismos planctonívoros e herbívoros, podendo ser incluídas, por isso, no nível trófico 3 de LINDEMAN.<br>This study presents data on the diet of three Sparid species from the South coast of Portugal (Algarve), that are commercially important, but also in danger of over-exploitation. Data were analyzed with numeric methods (fullness Index) and qualitative methods (percentage of occurrence and weight and number percentage). The index of relative importance (I.R.I.) and the HUREAU s feeding coefficient (Q) were used to determine prey importance. A total of 105 fish (46 Pagrus pagrus, 37 Lithognathus mormyrus and 22 Diplodus sargus) were obtained from local gillnet fishery between April 1995 and May 1997. P. pagrus and L mormyrus were relatively euryphagous, preying upon a high number of taxa, while D. sargus had a more specialized diet, composed mainly of mollusc bivalves. Overlap analysis revealed that there was a significant overiap between the diet of this three species. Ali species were considered as secondary predators and therefor included in LINDEMAN's trophic levei 3.<br>Programa PRAXIS XXI(BM /7082/95)