With the recent discoveries of phenomena such as plateau potentials, bistability, and synaptic amplification the focus of motoneuron research has been directed to the dendritic processes giving rise to these latent behaviors. The common consensus is that the mechanism behind bistability (an L-type calcium channel generating a persistent inward current, PIC; Schwindt and Crill 1980, Hounsgaard and Kiehn 1985, 1989) is also responsible for the amplification of synaptic input in motoneurons. However, modeling studies utilizing only calcium-based PICs (Powers 1993, Booth et al. 1997, Elbasinouy et al. 2005) have been unable to reproduce the high degree of synaptic amplification observed in experimental preparations (Prather et al. 2001, Lee et al. 2003, Hultborn et al. 2003). The present work examines a theoretical amplification mechanism (electrotonic compression), based on a sodium PIC of dendritic origin, which acts to supplement the synaptic amplification due to the calcium PIC. The current goal is to test the "goodness-of-fit" of electrotonic compression with established mechanisms and behaviors. The findings of this modeling study support the concept of a dendritic sodium PIC which acts to reduce the attenuation of synaptic currents enroute to the motoneuron soma. Furthermore, it is suggested that the ratiometric expression of ion channels giving rise to this mechanism takes the form of a distribution "rule" applied ubiquitously across the dendritic tree, while the plateau-producing L-type calcium channels undergo a more discretized or regional distribution. This study demonstrates the power inherent to the controlled expansion of morphological complexity in an already complex model. While modeling studies are suitable testbeds for the evaluation of theoretical and/or experimentally intractable facets of physiology, great care and consideration should be given to the specification of models with high dimensionality. With the continual progression of our knowledge-base and computational capabilities, we can expect that more and more empirical observations will find their way into models of increasing complexity wherein the layers of embedded hypotheses are frequently implicit. It is therefore imperative that the neural modeling discipline adopt more rigorous methodologies to both accommodate and rein-in this growing complexity.

In this research an approach to all optical delta sigma modulator (ADSM) has been elaborated. Two important components of ADSM; "leaky integrator" and "inverted bi-stable quantizer" were modeled, on the basis of cross gain modulation of the Semiconductor Optical Amplifier (SOA). The simulations (via VPI photonics) were all in micrometer scale (suitable for chip fabrication). By simulating each element of ADSM the whole circuit was simulated and results have been showed and analyzed. By investigating the ADSM, the limiting factor for reaching higher frequencies (THz) was recognized to be the quantization device. Thus a new optical switch was introduced, for the first time so called "proteresis." By applying proteretic bi-stable device in the delta sigma modulator, the resonance frequency was improved minimum two fold from 295MHz to 575MHz without making any change in hysteretic bi-stable switch. The broad impact of this research is on the digital technologies that can be utilized in high-speed signal processing. The prime examples are the RF technologies used in military and civilian applications. Furthermore introduction of proteresis opens a new research gate for compensating delay in almost every system.

Neurons can encode information using the rate of their action potentials, making the relation between input rate and output rate a prominent feature of neuronal information processing. This relation, known as I{O function, can rapidly change in response to various factors or neuronal processes. Most noticeably, a neuron can undergo a multiplicative operation, resulting in a change of the slope of its I{O curve, also know as gain change. Gain changes represent multiplicative operations, and they are wide- spread. They have been found to play an important role in the encoding of spatial location and coordinate transformation, to signal amplification, and other neuronal functions. One of the factors found to introduce and control neuronal gain is synaptic Short Term Depression (STD). We use both integrate-and- re and conductance based neuron models to identify the effect of STD in excitatory and inhibitory modulatory input. More specifically, we are interested in the effect of STD at the inhibitory synapse from Purkinje cells to cerebellar nucleus neurons. Using a previously published, biologically realistic model, we find that the presence of STD results in a gain change. Most importantly we identify STD at the inhibitory synapse to enable excitation-mediated gain control. To isolate the mechanism that allows excitation to control gain, even though STD is applied at a different synapse, we first show that the overall effect is mediated by average conductance. Having done this, we find that the effect of STD is based on the non-linearity introduced in the relation between input rate and average conductance. We find this effect to vary, depending on the position of the I{O function on the input rate axis. Modulatory input shifts the I{O curve along the input rate axis, consequently shifting it to a position where STD has a different effect. The gain differences in the STD effects between the two positions enable excitation to perform gain control.

[pt] A conversão de comprimento de onda de sinais por meio de técnicas totalmente ópticas é um assunto inovador e de extrema necessidade para as redes com roteamento de comprimento de onda; a técnica de conversão de comprimentos de onda por modulação de ganho cruzado é uma das mais simples, em princípio, que atinge este objetivo. Duas modalidades são apresentadas neste trabalho: a clássica, também conhecida por pump & probe, e uma nova, a de modulação de ganho cruzado do espectro da ASE em um SOA. A técnica pump e probe é apresentada, assim como um experimento baseado nesta. A técnica de modulação de ganho cruzado da ASE é explorada como alternativa à técnicas de conversão de comprimento de onda que necessitam de outra fonte de luz, para a qual o sinal deve ser convertido. Na modulação de ganho cruzado da ASE, o sinal é convertido de luz coerente para incoerente; e, uma vez modulado o espectro da ASE do SOA, este é filtrado no comprimento de onda que se deseja obter a conversão. Assim, este conversor pode ser sintonizável, já que não é um parâmetro de entrada que define o comprimento de onda convertido, e sim um filtro passa-faixa ao fim do dispositivo. Para se avaliar os tempos de resposta da técnica, a conversão é feita utilizando-se pulsos elétricos ultracurtos (50 ps), o que não havia sido feito até então. / [en] Wavelength conversion of optical signals by all-optical techniques is an innovative and necessary technology for wavelength routed networks in the near future; the cross-gain modulation method is one of the simplest, in form, to attain this goal. Two categories of the main technique are presented: the classic, also known as pump and probe, and a novel one, named cross- gain modulation of the ASE spectrum of a SOA. The cross-gain modulation of the ASE spectrum is explored here as an alternative to previous all-optical wavelength conversion techniques that require another light source, to which the incoming signal is to be converted; the signal is converted from coherent to incoherent light; and, once modulated throughout the SOA`s ASE spectrum, the signal is then filtered at the central wavelength it is desired to be converted. Thus, this particular wavelength converter can be tunable, in the sense that it is reconfigurable, since a band pass filter located at the end of the device selects what wavelength the signal will be converted to. In order to assess the response times of the technique, the conversion is made for ultra short electrical pulses (50 ps), a feature unknown until now.