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No description available.

No description available.

Behaviors are a product of underlying neural circuits, yet there is a paucity of mechanistic information about how nervous systems contribute to the repeated evolution of similar behaviors. Theoretical studies have predicted that the same behavioral output can be generated by neural circuits with different properties. Here, we test the theory in biological circuits by comparing the central pattern generator (CPG) circuits underlying swimming behaviors in nudibranchs (Mollusca, Gastropoda, Euthyneura, Nudipleura). In comparative studies of neural circuits, neurotransmitter content can serve as landmarks or molecular markers for neuron types. Here, we created a comprehensive map of GABA-immunoreactive neurons in six Nudipleura species. None of the known swim CPG neurons were GABA-ir, but they were located next to identifiable GABA-ir neurons/clusters. Despite strong conservation of the GABA-ergic system, there were differences, particularly in the buccal ganglia, which may represent adaptive changes. We applied our knowledge of neurotransmitter distribution along with morphological traits to identify the neuron type Si1 in Flabellina, a species that swims via whole body left-right (LR) flexions and in Tritonia, a dorsal-ventral (DV) swimming species. Si1 is a CPG member of the LR species Melibe, whereas its homologue in the LR species Dendronotus is not. In Flabellina, Si1 was part of the LR CPG and despite having similar synaptic connections as Flabellina and Melibe, Si1 in Tritonia was not part of its DV swim CPG. Side by side circuit comparison of Flabellina, Melibe and Dendronotus revealed different combinations of circuit architecture and modulation resulting in different circuit configurations for LR swimming. This includes differences in the role and activity pattern of Si1, sensitivity to curare and the effect of homologues of C2, a DV CPG neuron, on the LR motor pattern. These results collectively reveal three different circuit variations for generating the same behavior. It suggests that the neural substrate from which behaviors arise is phylogenetically constrained. While this neural substrate can be configured in multiple different ways to generate the same outcome, the possibilities are finite and, as seen here, similar structural and functional neural motifs are used in the evolution of these circuits.

In mammals, spatial learning and memory depend on neural processing carried out in the hippocampal formation. Interestingly, extracellular recordings from behaving animals have shown that cells in this region exhibit spatially modulated activity patterns, thus providing insights into the neural activity underlying spatial behaviour. One area within the hippocampal formation, layer II of the medial entorhinal cortex, houses cells that encode a grid-like map of space using a firing rate code. At the same time, oscillatory signals at distinct theta (4–12 Hz) and gamma (30–120 Hz) frequencies are also present in layer II, providing a substrate for a timing code. To understand how layer II of the medial entorhinal cortex produces these outputs I sought to characterise the electrical properties and functional computational architecture of its microcircuitry. The functionality of any neural circuit depends on the electrical properties of its constituent cells. Because the grid cells in layer II are likely to be stellate cells, I used the perforated patch-clamp technique to accurately assess the intrinsic excitable properties of this cell type. Compared to whole-cell recordings, these recordings indicate that some intrinsic properties of stellate cells, such as spike clustering, which is revealed to be robust, are more likely to play a functional role in circuit computation. Conversely, other intrinsic properties, such as spontaneous membrane potential fluctuations, which are confirmed to be insufficiently stable to support reliable interference patterns, are revealed to be less likely than other, more robust electrical properties to play a direct role in circuit function. The characteristic connectivity profiles of different cell types are also critical for circuit function. To investigate cell type-specific connectivity in layer II I used optogenetic stimulation in combination with in vitro electrophysiology to record synaptic activity in different cell types while selectively activating distinct subpopulations of cells with light. Using this method I found that connections between stellate cells are absent or very rare and that communication between stellate cells is instead mediated by strong feedback inhibition from fast-spiking interneurons. Dissecting oscillatory activity in neural circuits may be important for establishing functionally relevant circuit architecture and dynamics but is difficult in vivo. I accomplished this in vitro by recapitulating the interacting theta and gamma rhythms that are observed in vivo with an optogenetic method. I found that locally driving a subset of neurons in the layer II microcircuit at theta frequency with a light stimiulus produced a nested field rhythm at gamma frequency that was also evident as rhythmic inhibition onto stellate cells. Critically, these interacting rhythms closely resembled those recorded from behaving animals. In addition, I found that this thetanested gamma is sufficiently regular to act as a clock-like reference signal, indicating its potential role in implementing a timing code. To functionally dissect the circuit I performed multiple simultaneous whole-cell patch-clamp recordings during circuit activation. These recordings revealed how feedback interactions between stellate cells and fast-spiking interneurons underpin the theta-nested gamma rhythm. Together, these results suggest that feedback inhibition in layer II acts as a common substrate for theta-nested gamma oscillations and possibly also grid firing fields, thereby providing a framework for understanding how computations are carried out in layer II of the medial entorhinal cortex.

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Interruption of DC is more challenging than that of AC because of the absence of DC zero-crossings. This thesis proposes an alternative counter-voltage (ACV) DC hybrid circuit breaker (DC-HCB). This DC-HCB can interrupt the DC up to 5 kA with the source voltage of up to 1 kV and it can meet the general requirements i.e., interrupting the DC within 5ms, and limiting over-voltages due to the CB operation. The existing/investigated DC circuit breakers, i.e., active mode DC-HCB and the traditional counter-voltage (TCV) DC-HCB are also investigated and compared with the proposed DC-HCB. The investigations are based on the time-domain simulation studies in the PSCAD/EMTDC. The studies show that the ACV DC-HCB is the preferred option to both the existing DC-HCBs. The main features of the proposed ACV DC-HCB include lower capacitance in the commutation path and the need for a smaller dielectric strength for the primary-path circuit breaker.

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This thesis prompt a theoretical analysis of the hybrid plasmonic waveguide (HPWG) and a TE-pass polarizer based on HPWG has been designed, fabricated and characterized. A combination of low propagation loss, high power density, and large confinement is useful for many applications. The analysis results in this thesis show that the HPWG offers a better compromise between loss and confinement as compared to pure plasmonic waveguides. Another interesting property of the HPWG is its polarization diversity. In the HPWG the transverse electric and the transverse magnetic modes reside in different layers. We have designed a very compact hybrid TE-pass polarizer using this property. The polarizer was fabricated and characterized. The device shows low insertion loss for the TE mode with a high extinction ratio at telecommunication wavelength range for a 30 µm long HPWG section. Its performance compares favorably against previously reported silicon based integrated optic TE-pass polarizers.

This thesis prompt a theoretical analysis of the hybrid plasmonic waveguide (HPWG) and a TE-pass polarizer based on HPWG has been designed, fabricated and characterized. A combination of low propagation loss, high power density, and large confinement is useful for many applications. The analysis results in this thesis show that the HPWG offers a better compromise between loss and confinement as compared to pure plasmonic waveguides. Another interesting property of the HPWG is its polarization diversity. In the HPWG the transverse electric and the transverse magnetic modes reside in different layers. We have designed a very compact hybrid TE-pass polarizer using this property. The polarizer was fabricated and characterized. The device shows low insertion loss for the TE mode with a high extinction ratio at telecommunication wavelength range for a 30 µm long HPWG section. Its performance compares favorably against previously reported silicon based integrated optic TE-pass polarizers.