The research of functional stimulation of neural tissue is of great interest within the field of clinical neuroscience to further develop new neural prosthetics. A technique which has gained increased interest during the last couple of years is the stimulation of nervous tissue using infrared laser light. Successful results have been reported, such as stimulation of cells in both the central nervous system, and in the peripheral nervous system, and even cardiomyocytes. So far, the details about the stimulation mechanism have been a question of debate as the mechanism is somewhat hard to explain. The mechanism is believed to have a photo-thermal origin, where the light from the laser is absorbed by water, thus increasing the temperature inside and around the target cell. Despite the mechanism questions, the technique holds several promising features compared to traditional electrical stimulation. Examples of advantages are that it is contact free, no penetration is needed, it has high spatial resolution and no toxic electrochemical byproducts are produced during stimulation. However, since the laser pulses locally increase the temperature of the tissue, there is a risk of heat induced damage. Therefore, the effect of increased temperatures must be investigated thoroughly. One method of examining the changes in temperature during stimulation is to model the heating. This thesis is based on the work from four papers with the main aim to investigate and describe the response of heating, caused by laser pulses, on central nervous system cells. In paper one, a model of the heating during pulsed laser stimulation is established and used to describe the dynamic temperature changes occurring during functional stimulation of cerebral cortex cells. The model was used in all four papers. Furthermore, single cell responses, as action potentials, as well as network responses, as activity inhibition, were observed. In paper two, the response of rat astrocytes exposed to laser induced hyperthermia was investigated. Cellular migration was observed and the migration limit was used to calculate the kinetic parameters for the cells, i.e., the reaction activation energy, Ea (321.4 kJ⋅mol-1), and the frequency factor, Ac (9.47 x 1048 s-1). Furthermore, a damage signal ratio (DSR) for calculating a threshold for cellular damage was defined, and calculated to six percent. In paper three, the response of hyperthermia to cerebral cortex cells was investigated, in the same way as in the second paper. Fluorescence staining of the metabolic activity was used to reveal the heat response, and by using the limit of the observed increased fluorescence the kinetic parameters, Ea (333.6 kJ⋅mol-1), and Ac (9.76 x 1050 s-1), were calculated. The DSR for the cells was calculated to five percent. In paper four, the behavior of action potentials triggered by laser stimulation was investigated. More specifically, the time delay from the start of a laser pulse to the detection of an action potential, delta-t, were investigated. Two different behaviors for the initial action potentials were observed: fast decreasing delta-t and slow decreasing delta-t. The results show the dynamic behavior of action potential responses to infrared light. The work of this thesis show the dynamic changes of the temperature during optical stimulation, using an infrared laser working at 1,550 nanometers. It also shows how the changes cause astrocytes to migrate for pulses several seconds long, and neurons to fire action potentials for pulses in the millisecond range. Furthermore, a damage signal ratio was defined and calculated for the cell systems.

In an effort to improve the auditory brainstem implant, a prosthesis in which user outcomesare modest, infrared neural stimulation (INS) was applied to the cochlear nucleus in a ratanimal model. Pulsed INS, delivered to the surface of the cochlear nucleus via an opticalfibre, evoked auditory brainstem responses (ABR) and generated broad neural activation inthe inferior Colliculus (IC). Varying the parameters of the laser stimulation revealed laserpeak power to be the dominating parameter for both ABR and IC responses. Strongestresponses were recorded when the fibre was placed at lateral positions on the cochlearnucleus, close to the temporal bone. After deafening by auditory nerve section, ABR andIC responses to INS disappeared, consistent with a reported "optophonic" effect, a laser-inducedacoustic artifact. Thus, for deaf individuals who use the auditory brainstemimplant, INS alone does not appear promising as a new approach.

No description available.

L’exposition aux radiations laser infrarouge peut être utilisée afin de dépolariser des neurones et stimuler l’activité neuronale. Le mécanisme sous-jacent d’une telle stimulation est supposé résulter d’une interaction photothermique. En effet, l’absorption de la radiation infrarouge par le tissu biologique cible, et l’eau qu’il contient, induit une augmentation de température de manière localisée, qui soit influencerait directement les propriétés membranaires de la cellule soit agirait par le biais de l’activation de canaux ioniques thermo-sensibles. Dans la plupart des cas, l’activité électrique des neurones est mesurée électriquement à l’aide de microélectrodes, mais elle peut également être sondée par le biais de la microscopie de fluorescence faisant intervenir des indicateurs calciques. Dans ce travail, l’impact de l’exposition à la radiation infrarouge sur les signaux calciques de neurones a été étudié dans le but d’éclaircir et de préciser le mécanisme résultant de l’interaction photothermique. Des neurones HT22, issus d’hippocampe de souris, et des cellules U87, issues d’un glioblastome humain, ont été utilisés en tant qu’exemples de cellules électriquement excitables et non excitables respectivement. Afin de mesurer la température et les signaux calciques au niveau cellulaire, les fluorophores Rhodamine B et Fluo-4 ont été employés. Le montage, par conséquent tout optique, pour étudier l’influence de l’exposition infrarouge sur l’activité neurale est donc présenté, ainsi que la démarche scientifique menant à l’identification de l’implication de l’activité de la phospholipase C dans le mécanisme étudié. La possibilité de stimuler l’activité neurale in vivo, dans le cerveau d’une souris, avec une mesure simultanée des signaux calciques, est également démontrée à l’aide de souris transgéniques exprimant le GCaMP6S. / Infrared laser light radiation may be used to depolarize neurons and to stimulate neural activity. The underlying mechanism of such stimulation is believed to happen due to a photothermal interaction. The absorption of the infrared radiation by the targeted biological tissue inducing a local temperature increase which either directly influence membrane properties or act via temperature sensitive ion channels. Action potentials are typically measured electrically in neurons with microelectrodes, but they can also be observed using fluorescence microscopy techniques that use synthetic or genetically encoded calcium indicators. In this work, we studied the impact of infrared laser light on neuronal calcium signals to address the mechanism of these thermal effects. HT22 mouse hippocampal neurons and U87 human glioblastoma cells were used loaded with the fluorescent calcium dye Fluo-4 and with the temperature sensitive fluorophore Rhodamine B to measure calcium signals and temperature changes at the cellular level. Here we present our all-optical strategy for studying the influence of infrared laser light on neural activity, and the scientific approach leading to conclusion of the involvement of Phospholipase C activity during infrared neural stimulation. The ability of infrared exposure to trigger neural activity in mice brain in vivo is also investigated with the use of GCaMP6s transgenic mice.

No description available.

Signalverarbeitung im Nervensystem hängt sowohl von der Netzwerkstruktur, als auch den zellulären Eigenschaften der Nervenzellen ab. In dieser Abhandlung werden zwei zelluläre Eigenschaften im Hinblick auf ihre funktionellen Anpassungsmöglichkeiten untersucht: Es wird gezeigt, dass neuronale Morphologie die Signalweiterleitung unter Berücksichtigung energetischer Beschränkungen verstärken kann, und dass selbst kleine Änderungen in biophysikalischen Parametern die Aktivierungsbifurkation in Nervenzellen, und damit deren Informationskodierung, wechseln können. Im ersten Teil dieser Abhandlung wird, unter Verwendung von mathematischen Modellen und Daten, die Hypothese aufgestellt, dass Energie-effiziente Signalweiterleitung als starker Evolutionsdruck für unterschiedliche Zellkörperlagen bei Nervenzellen wirkt. Um Energie zu sparen, kann die Signalweiterleitung vom Dendrit zum Axon verstärkt werden, indem relativ kleine Zellkörper zwischen Dendrit und Axon eingebaut werden, während relativ große Zellkörper besser ausgelagert werden. Im zweiten Teil wird gezeigt, dass biophysikalische Parameter, wie Temperatur, Membranwiderstand oder Kapazität, den Feuermechanismus des Neurons ändern, und damit gleichfalls Aktionspotential-basierte Informationsverarbeitung. Diese Arbeit identifiziert die sogenannte "saddle-node-loop" (Sattel-Knoten-Schlaufe) Bifurkation als den Übergang, der besonders drastische funktionale Auswirkungen hat. Neben der Änderung neuronaler Filtereigenschaften sowie der Ankopplung an Stimuli, führt die "saddle-node-loop" Bifurkation zu einer Erhöhung der Netzwerk-Synchronisation, was möglicherweise für das Auslösen von Anfällen durch Temperatur, wie bei Fieberkrämpfen, interessant sein könnte. / Signal processing in nervous systems is shaped by the connectome as well as the cellular properties of nerve cells. In this thesis, two cellular properties are investigated with respect to the functional adaptations they provide: It is shown that neuronal morphology can improve signal transmission under energetic constraints, and that even small changes in biophysical parameters can switch spike generation, and thus information encoding. In the first project of the thesis, mathematical modeling and data are deployed to suggest energy-efficient signaling as a major evolutionary pressure behind morphological adaptations of cell body location: In order to save energy, the electrical signal transmission from dendrite to axon can be enhanced if a relatively small cell body is located between dendrite and axon, while a relatively large cell body should be externalized. In the second project, it is shown that biophysical parameters, such as temperature, membrane leak or capacitance, can transform neuronal excitability (i.e., the spike onset bifurcation) and, with that, spike-based information processing. This thesis identifies the so-called saddle-node-loop bifurcation as the transition with particularly drastic functional implications. Besides altering neuronal filters and stimulus locking, the saddle-node-loop bifurcation leads to an increase in network synchronization, which may potentially be relevant for the initiation of seizures in response to increased temperature, such as during fever cramps.