<p> The spontaneous oscillations of a fluidized bed with a variable volume below the bed support are investigated.</p> <p> The pressure vs. time dependence of the fixed bed is determined for glassbeads of 3 size ranges and for hematite.</p> <p> For each case several different bed masses were employed. For the oscillating bed the influence on the frequency of the oscillations of the materials fluidized, the size of the particles, the chamber volume and the superficial gas velocity are studied.</p> <p> Models of five different authors have been reviewed and a model to describe the relationship between the frequency and various parameters such as chamber volume, bed mass, gas velocity and the pressure drop characteristic (Δp vs. μ) is derived. The latter together with three other models have been evaluated by means of the experimental data.</p> / Thesis / Master of Engineering (MEngr)

L’interaction d’assemblées de moteurs et de filaments du cytosquelette donne naissance à des comportements actifs qui demeurent peu compris, malgré la large caractérisation de leurs molécules individuellement. En contrôlant la géométrie de polymérisation de l’actine via des micropatrons surfaciques de nucléation, nous avons observé in vitro l’émergence de battements de faisceaux de filaments parallèles d’actine en présence de myosines en solution (myosine V ou HMM II (Heavy MeroMyosine II)). La forme du battement est similaire pour les deux types de moteurs, mais avec des oscillations un ordre de grandeur plus rapides avec la myosine II qu’avec la myosine V. Dans les deux cas, une onde de déformation transverse se propage à vitesse uniforme de la base à la pointe du faisceau d’actine. Avec la polymérisation, les faisceaux d’actine s’allongent à vitesse constante : la période croît, mais la vitesse de l’onde mécanique reste inchangée. L’utilisation de myosines-GFP a révélé un pic de concentration et un recrutement localisé des myosines au sein du faisceau d’actine, avant qu’une onde de concentration ne se propage vers la pointe de concert avec l’onde mécanique de l’actine. Ces résultats présentent une nouvelle forme de couplage entre l’affinité des myosines à l’actine et la forme du faisceau d’actine. Ce travail de thèse décrit l’émergence de battements actifs imitant ceux des flagelles comme une propriété intrinsèque de l’interaction de filaments polaires et de moteurs moléculaires. Le contrôle de la structure lors du processus d’auto-organisation fournit des informations clés pour étudier les principes physiques génériques du battement flagellaire. / The emergent active behaviors of molecular motors assemblies and cytoskeletal filaments systems remain poorly understood, though individual molecules have been extensively characterized. By controlling the geometry of actin polymerization with surface micropatterns of a nucleation promoting factor, we were able to demonstrate in vitro the emergence of flagellar-like beating of bundles of parallel actin filaments in the presence of myosin motors. We worked with both myosin V and heavy-meromyosin II. The waveform of oscillation was similar for the two types of motors, but oscillations with myosin II were one order of magnitude faster than with myosin Va. In both cases, a bending wave traveled at a uniform speed from the anchored base of the actin bundle towards the tip. As polymerization occurred, the actin bundle elongated at a constant speed, resulting in an increase of the oscillation period, but the speed of the traveling bending wave remains constant. GFP-tagged myosin V revealed the presence of a myosin concentration peak within the actin bundle. Strikingly, myosin V motors were locally recruited within the actin bundle, before a concentration wave propagated towards the bundle’s tip in concert with the actin bending wave. These results revealed a novel form of coupling between the myosin affinity for actin and the actin bundle shape. Our work demonstrates that active flagellar-like beating emerges as an intrinsic property of polar bundles of filaments in interaction with molecular motors. Structural control over the self-assembly process provides key information to clarify the underlying physical principles of flagellar-like beating.

L'oubli est une perte temporaire ou permanente de mémoire, que l'on perçoit souvent de manière déplaisante lorsqu'elle nous empêche d'accéder à un savoir que l'on a acquis. Cependant, des découvertes récentes suggèrent que l'oubli peut aussi être un processus adaptatif permettant d'optimiser notre mémoire, en effaçant des informations non pertinentes susceptibles d'interférer avec le stockage ou le rappel de nouvelles informations. Ainsi, l'oubli adaptatif est particulièrement essentiel au fonctionnement de notre mémoire à court terme ou mémoire de travail (MT), car les informations qui y sont stockées doivent être oubliées une fois utilisées. A l'inverse, des informations peuvent être stockées pendant toute une vie dans la mémoire à long terme ou mémoire de référence (MR) chez l'animal. Les mécanismes cellulaires et moléculaires sous-tendant le stockage des informations en mémoire mais également leur oubli adaptatif restent mal connus. Au cours de cette thèse, nous avons adopté une approche comparative et utilisé trois tâches comportementales chez le rat au sein d’un même labyrinthe radial : une tâche de MR et deux tâches de MT impliquant un oubli adaptatif plus ou moins efficace. Nous avons étudié la transmission synaptique à la synapse entre le cortex entorhinal et le gyrus denté (voie d’entrée de l’hippocampe, structure clé de la mémoire) entre les deux jours d’apprentissage de ces trois tâches. Nous avons montré que la consolidation mnésique (en MR) induit un phénomène de potentialisation synaptique à long terme (proche d'une LTP), comme attendu d’après la littérature. A l'inverse, nous avons montré pour la première fois que l’oubli adaptatif en MT induirait une dépression synaptique à long terme. De plus, de nombreuses études suggèrent l’implication du sommeil dans la mémoire, mais le rôle des différentes phases de sommeil dans la consolidation mnésique ainsi que leur rôle dans l’oubli adaptatif reste ambigu. Nous avons donc également réalisé des enregistrements polysomnographiques (entre les deux jours des tâches), afin de quantifier la durée des états sommeil et la puissance des oscillations cérébrales. Nous avons ainsi confirmé le rôle du sommeil paradoxal, et plus particulièrement de ses oscillations gamma, pour la consolidation mnésique en MR. A l’inverse, l’oubli adaptatif en MT serait favorisé par les oscillations lentes du sommeil lent. Ces résultats représentent une contribution significative non seulement aux mécanismes neuronaux sous-tendant la mémoire et l’oubli adaptatif, mais également aux modulations de ces mécanismes par le sommeil. Nous avons donc montré que la consolidation mnésique induit un phénomène physiologique de potentialisation synaptique proche d'une LTP. Or l’induction artificielle de LTP par stimulation tétanique est considérée comme un modèle cellulaire de la mémoire. Notre second objectif a été d'évaluer l'impact de la modulation des états de sommeil sur une LTP, cette-fois-ci induite artificiellement (dans les mêmes conditions à la même synapse chez le rat vigile). Nous voulions ainsi comparer l'effet de la modulation du sommeil sur une potentialisation physiologique (après apprentissage) ou sur une LTP artificielle. Nous avons montré de nombreuses similitudes entre ces deux situations de potentialisation synaptique, notamment en ce qui concerne le rôle favorable du sommeil paradoxal, ce qui confirme l’intérêt de la LTP artificielle pour l’étude de la mémoire. Enfin, notre étude montre que non seulement le sommeil, mais également les oscillations de l'éveil contribuent à la mémoire et l’oubli. Nous avons analysé les oscillations locales dans le gyrus denté au cours des trois tâches comportementales déjà décrites. L'importante résolution spatiale et temporelle de cette analyse nous a permis d'identifier l'implication de certaines oscillations locales à des moments cruciaux de prise de décision dans le labyrinthe, au cours de l'encodage et du rappel d'informations stockées en MR ou en MT / Forgetting is a temporary or permanent loss of memory, often perceived as deleterious to our cognitive abilities, especially when it prevents us from accessing information we previously acquired. However, recent studies in Neurosciences suggest that forgetting could also be an adaptive phenomenon that would optimize memory function by erasing non relevant information, that could otherwise interfere with the storage or recall of new information. Therefore, adaptive forgetting would particularly be necessary for daily activities relying on short-term or working memory (WM), as information temporarily stored in WM need to be forgotten once used, so that this temporary information does not interfere in the future with the storage and recall of newer information. On the contrary, information can be stored for a lifetime in long-term or reference memory (RM) in animals. The molecular and cellular mechanisms underlying memory storage of information, but also adaptive forgetting, are still unclear. During this thesis, we used a comparative approach by training rats in three different behavioral tasks in the same radial maze: one RM task and two WM tasks involving a more or less effective adaptive forgetting process of previously stored information. We studied synaptic transmission at the synapse between the entorhinal cortex and the dentate gyrus (gating hippocampus, a key structure for memory) between two days of training in these three tasks. Our results show that memory consolidation (in RM) induces a form of long-term potentiation (LTP-like), confirming previous published work from the literature. However, we showed for the first time that adaptive forgetting in WM could trigger long-term synaptic depression. Moreover, numerous studies suggest that sleep is important for optimal memory processing, but the role of the different sleep phases in memory consolidation and in adaptive forgetting remains to be elucidated. We thus also performed polysomnographic recordings (between the two trainings days in the three behavioral tasks), in order to measure sleep state durations and sleep oscillations associated with these processes. Our results confirm the essential role of paradoxical sleep, and more specifically gamma oscillations during this state, for memory consolidation in RM. On the contrary, we also found that adaptive forgetting in WM would benefit from slow oscillations during slow wave sleep. We believe that these results contribute significantly to our understanding of neuronal mechanisms underlying memory and forgetting, especially concerning the modulation of these mechanisms by the different sleep states following training. On the one hand, we thus here showed that memory consolidation induces an LTP-like physiological phenomenon. On the other hand, the induction of an artificial form of LTP by tetanic stimulation is considered a cellular model of memory. Our second goal was to assess the modulation of an artificial LTP (at the same synapse, in the same conditions, on freely-moving rats) by sleep states. We also wanted to compare the impact of sleep states on a physiological LTP-like process (after learning) or on an artificial LTP. Our results showed many similarities between these two situations of synaptic potentiation, in particular concerning the positive role of paradoxical sleep, confirming the relevance of artificial LTP as a model to study memory processes. Finally, our study shows that not only sleep, but also oscillations during waking, could contribute to memory and forgetting. We therefore analyzed local spontaneous oscillations in the dentate gyrus while rats were performing the three behavioral tasks previously described. The high spatial and temporal resolution of this analysis allowed us to show the role of different local spontaneous oscillations at critical moments of training in the maze, in particular during decision making, and during encoding or retrieval of information stored in RM or WM

Cilia and flagella are hair-like appendages of eukaryotic cells. They are actively bending structures that exhibit regular beat patterns and thereby play an important role in many different circumstances where motion on a cellular level is required. Most dramatic is the effect of nodal cilia whose vortical motion leads to a fluid flow that is directly responsible for establishing the left-right axis during embryological development in many vertebrate species, but examples range from the propulsion of single cells, such as the swimming of sperm, to the transport of mucus along epithelial cells, e.g. in the ciliated trachea. Cilia and flagella contain an evolutionary highly conserved structure called the axoneme, whose characteristic architecture is based on a cylindrical arrangement of elastic filaments (microtubules). In the presence of a chemical fuel (ATP), molecular motors (dynein) exert shear forces between neighbouring microtubules, leading to a bending of the axoneme through structural constraints. We address the following two questions: How can these organelles generate regular oscillatory beat patterns in the absence of a biochemical signal regulating the activity of the force generating elements? And how can the beat patterns be so different for apparently very similar structures? We present a theoretical description of the axonemal structure as an actively bending elastic cylinder, and show that in such a system bending waves emerge from a non-oscillatory state via a dynamic instability. The corresponding beat patterns are solutions to a set of coupled partial differential equations presented herein.

Cilia and flagella are hair-like appendages of eukaryotic cells. They are actively bending structures that exhibit regular beat patterns and thereby play an important role in many different circumstances where motion on a cellular level is required. Most dramatic is the effect of nodal cilia whose vortical motion leads to a fluid flow that is directly responsible for establishing the left-right axis during embryological development in many vertebrate species, but examples range from the propulsion of single cells, such as the swimming of sperm, to the transport of mucus along epithelial cells, e.g. in the ciliated trachea. Cilia and flagella contain an evolutionary highly conserved structure called the axoneme, whose characteristic architecture is based on a cylindrical arrangement of elastic filaments (microtubules). In the presence of a chemical fuel (ATP), molecular motors (dynein) exert shear forces between neighbouring microtubules, leading to a bending of the axoneme through structural constraints. We address the following two questions: How can these organelles generate regular oscillatory beat patterns in the absence of a biochemical signal regulating the activity of the force generating elements? And how can the beat patterns be so different for apparently very similar structures? We present a theoretical description of the axonemal structure as an actively bending elastic cylinder, and show that in such a system bending waves emerge from a non-oscillatory state via a dynamic instability. The corresponding beat patterns are solutions to a set of coupled partial differential equations presented herein.