Este trabalho de Mestrado tem como objetivo contribuir para a área de placa de circuito impresso no que se refere ao projeto de layout focado não só em satisfazer as conexões das trilhas, mas nas regras de projeto com foco na redução de curto circuito de solda para o processo de solda por onda. Projetos de PCB (Printed Circuit Board) ou PCI (Placa de Circuito Impresso) envolvem uma série de conhecimentos no que se refere ao entendimento das funcionalidades dos circuitos e para tanto é importante que se faça o correto posicionamento dos componentes em grupos de circuitos pela funcionalidade; além disso, é importante que se conheça as regras de capacidade de corrente, de distâncias de isolação em função das tensões aplicadas, características de impedância, áreas de restrição mecânica entre outras. O que será visto neste trabalho está focado na aplicação de conceitos e considerações ligadas ao processo de montagem da placa eletrônica por solda a onda. Muitos dos defeitos que ocorrem num processo de montagem da PCB são atribuídos ao processo de montagem da PCB como, por exemplo, a temperatura da solda, o tempo de solda, quantidade de fluxo aplicado na placa, altura da onda de solda, etc. Recomendações sobre posicionamento de componentes PTH (Pin Through Hole) e SMD (Surface Mounting Devise) em relação ao sentido em que a PCB entra em direção à solda a onda...
In many nervous systems, the establishment of neural circuits is known to proceed via a two-stage process; (1) early, activity-independent wiring to produce a rough map characterized by excessive synaptic connections, and (2) subsequent, use-dependent pruning to eliminate inappropriate connections and reinforce maintained synapses. In invertebrates, however, evidence of the activity-dependent phase of synaptic refinement has been elusive, and the dogma has long been that invertebrate circuits are “hard-wired” in a purely activity-independent manner. This conclusion has been challenged recently through the use of new transgenic tools employed in the powerful Drosophila system, which have allowed unprecedented temporal control and single neuron imaging resolution. These recent studies reveal that activity-dependent mechanisms are indeed required to refine circuit maps in Drosophila during precise, restricted windows of late-phase development. Such mechanisms of circuit refinement may be key to understanding a number of human neurological diseases, including developmental disorders such as Fragile X syndrome (FXS) and autism, which are hypothesized to result from defects in synaptic connectivity and activity-dependent circuit function. This review focuses on our current understanding of activity-dependent synaptic connectivity in Drosophila...
Postnatal cortical circuit development is characterized by windows of heightened plasticity that contribute to the acquisition of mature connectivity and function. What drives the transition between different critical plasticity windows is not known. Here we show that a switch in sign of inhibitory plasticity correlates with the reported transition between pre-critical (pre-CP) and critical period (CP) for ocular dominance plasticity (ODP). In layer 4 of binocular visual cortex (V1b), depression of inhibitory synapses onto pyramidal neurons is induced when rats are monocularly deprived (MD) for 2 days at the end of the third postnatal week (pre-CP); whereas potentiation is induced if the MD is started in the fourth postnatal week (CP). The magnitude of potentiation increases with deprivations started close to the peak of the CP for ODP. The direction of inhibitory plasticity depends on the differential manipulation of circuits activated by the two eyes. During development, these two forms of plasticity shift the balance between excitation and inhibition of the circuit in opposite directions, while the excitatory synaptic drive remains unaffected. Inhibitory plasticity is thus fundamental in modulating cortical circuit refinement and might be one of the mechanisms promoting OD shifts.
One unifying explanation for the complexity of Autism Spectrum Disorders (ASD) may lie in the disruption of excitatory/inhibitory (E/I) circuit balance during critical periods of development. We examined whether Parvalbumin (PV)-positive inhibitory neurons, which normally drive experience-dependent circuit refinement (Hensch Nat Rev Neurosci 6:877–888, 1), are disrupted across heterogeneous ASD mouse models. We performed a meta-analysis of PV expression in previously published ASD mouse models and analyzed two additional models, reflecting an embryonic chemical insult (prenatal valproate, VPA) or single-gene mutation identified in human patients (Neuroligin-3, NL-3 R451C). PV-cells were reduced in the neocortex across multiple ASD mouse models. In striking contrast to controls, both VPA and NL-3 mouse models exhibited an asymmetric PV-cell reduction across hemispheres in parietal and occipital cortices (but not the underlying area CA1). ASD mouse models may share a PV-circuit disruption, providing new insight into circuit development and potential prevention by treatment of autism.
Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, sensory hypersensitivity, and high incidences of autism spectrum disorders and epilepsy. These phenotypes are suggestive of defects in neural circuit development and imbalances in excitatory glutamatergic and inhibitory GABAergic neurotransmission. While alterations in excitatory synapse function and plasticity are well-established in Fmr1 knockout (KO) mouse models of FXS, a number of recent electrophysiological and molecular studies now identify prominent defects in inhibitory GABAergic transmission in behaviorally relevant forebrain regions such as the amygdala, cortex, and hippocampus. In this review, we summarize evidence for GABAergic system dysfunction in FXS patients and Fmr1 KO mouse models alike. We then discuss some of the known developmental roles of GABAergic signaling, as well as the development and refinement of GABAergic synapses as a framework for understanding potential causes of mature circuit dysfunction. Finally, we highlight the GABAergic system as a relevant target for the treatment of FXS.
A hallmark of mammalian neural circuit development is the refinement of initially imprecise connections by competitive activity-dependent processes. In the developing visual system retinal ganglion cell (RGC) axons from the two eyes undergo activity-dependent competition for territory in the dorsal lateral geniculate nucleus (dLGN). However, the direct contributions of synaptic transmission to this process remain unclear. We used a novel genetic approach to reduce glutamate release selectively from the axons of ipsilateral-projecting RGCs and found that the release-deficient axons failed to exclude competing axons from their territory in the dLGN. Nevertheless, the release-deficient axons consolidated and maintained their normal amount of dLGN territory in the face of fully-active competing axons. These results show that during visual circuit refinement glutamatergic transmission plays a direct role in excluding competing axons from inappropriate target regions, and they argue that consolidation and maintenance of axonal territory are considerably less sensitive to alterations in synaptic activity levels.
Neural activity plays an important role in development and maturation of visual circuits in the brain. Activity can be instructive in refining visual projections by directly mediating formation and elimination of specific synaptic contacts through competition-based mechanisms. Alternatively, activity could be permissive – regulating production of factors which create a favorable environment for circuit refinement. Here we used the Xenopus laevis tadpole visual system to test whether activity is instructive or permissive for shaping development of the retinotectal circuit. In vivo spike output was dampened in a small subgroup of tectal neurons, starting from developmental stages 44–46, by overexpressing Shaker-like Xenopus Kv1.1 potassium channels using electroporation. Tadpoles were then reared until stage 49, a time period when significant refinement of the retinotectal map occurs. Kv1.1 expressing neurons had significantly decreased spike output in response to both current injection and visual stimuli, compared to untransfected controls, with spiking occurring during a more limited time interval. We found that Kv1.1 expressing neurons had larger visual receptive fields, decreased receptive field sharpness and more persistent recurrent excitation than control neurons...
The neuropeptide pigment-dispersing factor (PDF) synchronizes molecular oscillations within circadian pacemakers in the Drosophila brain. It is expressed in the small ventral lateral neurons (sLNvs) and large ventral lateral neurons, the former being indispensable for maintaining behavioral rhythmicity under free-running conditions. How PDF circuits develop the specific connectivity traits that endow such global behavioral control remains unknown. Here, we show that mature sLNv circuits require PDF signaling during early development, acting through its cognate receptor PDFR at postsynaptic targets. Yet, axonal defects by PDF knockdown are presynaptic and become apparent only after metamorphosis, highlighting a delayed response to a signal released early on. Presynaptic expression of constitutively active bone morphogenetic protein (BMP) receptors prevents pdfr mutants misrouting phenotype, while sLNv-restricted downregulation of BMP signaling components phenocopied pdf01. Thus, we have uncovered a novel mechanism that provides an early “tagging” of synaptic targets that will guide circuit refinement later in development.
Early-use activity during circuit-specific critical periods refines brain circuitry by the coupled processes of eliminating inappropriate synapses and strengthening maintained synapses. We theorize these activity-dependent (A-D) developmental processes are specifically impaired in autism spectrum disorders (ASDs). ASD genetic models in both mouse and Drosophila have pioneered our insights into normal A-D neural circuit assembly and consolidation, and how these developmental mechanisms go awry in specific genetic conditions. The monogenic fragile X syndrome (FXS), a common cause of heritable ASD and intellectual disability, has been particularly well linked to defects in A-D critical period processes. The fragile X mental retardation protein (FMRP) is positively activity-regulated in expression and function, in turn regulates excitability and activity in a negative feedback loop, and appears to be required for the A-D remodeling of synaptic connectivity during early-use critical periods. The Drosophila FXS model has been shown to functionally conserve the roles of human FMRP in synaptogenesis, and has been centrally important in generating our current mechanistic understanding of the FXS disease state. Recent advances in Drosophila optogenetics...
Astrocytes, the most abundant cells in the central nervous system, promote synapse formation and help refine neural connectivity. Although they are allocated to spatially distinct regional domains during development, it is unknown whether region-restricted astrocytes are functionally heterogeneous. Here we show that postnatal spinal cord astrocytes express several region-specific genes, and that ventral astrocyte-encoded Semaphorin3a (Sema3a) is required for proper motor neuron and sensory neuron circuit organization. Loss of astrocyte-encoded Sema3a led to dysregulated α–motor neuron axon initial segment orientation, markedly abnormal synaptic inputs, and selective death of α–but not of adjacent γ–motor neurons. Additionally, a subset of TrkA+ sensory afferents projected to ectopic ventral positions. These findings demonstrate that stable maintenance of a positional cue by developing astrocytes influences multiple aspects of sensorimotor circuit formation. More generally, they suggest that regional astrocyte heterogeneity may help to coordinate postnatal neural circuit refinement.
Sensory information undergoes ordered and coordinated processing across cortical layers. Whereas cortical layer (L) 4 faithfully acquires thalamic information, the superficial layers appear well staged for more refined processing of L4-relayed signals to generate corticocortical outputs. However, the specific role of superficial layer processing and how it is specified by local synaptic circuits remains not well understood. Here, in the mouse primary auditory cortex, we showed that upper L2/3 circuits play a crucial role in refining functional selectivity of excitatory neurons by sharpening auditory tonal receptive fields and enhancing contrast of frequency representation. This refinement is mediated by synaptic inhibition being more broadly recruited than excitation, with the inhibition predominantly originating from interneurons in the same cortical layer. By comparing the onsets of synaptic inputs as well as of spiking responses of different types of neuron, we found that the broadly tuned, fast responding inhibition observed in excitatory cells can be primarily attributed to feedforward inhibition originating from parvalbumin (PV)-positive neurons, whereas somatostatin (SOM)-positive interneurons respond much later compared with the onset of inhibitory inputs to excitatory neurons. We propose that the feedforward circuit-mediated inhibition from PV neurons...
The circuitry of the olfactory bulb contains a precise anatomical map that links isofunctional regions within each olfactory bulb. This intrabulbar map forms perinatally and undergoes activity-dependent refinement during the first postnatal weeks. Although this map retains its plasticity throughout adulthood, its organization is remarkably stable despite the addition of millions of new neurons to this circuit. Here we show that the continuous supply of new neuroblasts from the subventricular zone is necessary for both the restoration and maintenance of this precise central circuit. Using pharmacogenetic methods to conditionally ablate adult neurogenesis in transgenic mice, we find that the influx of neuroblasts is required for recovery of intrabulbar map precision after disruption due to sensory block. We further demonstrate that eliminating adult-born interneurons in naive animals leads to an expansion of tufted cell axons that is identical to the changes caused by sensory block, thus revealing an essential role for new neurons in circuit maintenance under baseline conditions. These findings show, for the first time, that inhibiting adult neurogenesis alters the circuitry of projection neurons in brain regions that receive new interneurons and points to a critical role for adult-born neurons in stabilizing a brain circuit that exhibits high levels of plasticity.
The vesicular glutamate transporter 3 (VGLUT3) is expressed at several locations not normally associated with glutamate release. Although the function of this protein has been generally elusive, when expressed in non-glutamatergic synaptic terminals, VGLUT3 can not only allow glutamate co-transmission but also synergize the action of non-glutamate vesicular transporters. Interestingly, in the immature glycinergic projection between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO) of auditory brainstem, the transient early expression of VGLUT3 is required for normal developmental refinement. It has however been unknown whether the primary function of VGLUT3 in development of these inhibitory synapses is to enable glutamate release or to promote loading of inhibitory neurotransmitter through vesicular synergy. Using tissue from young mice in which Vglut3 had been genetically deleted, we evaluated inhibitory neurotransmission in the MNTB-LSO pathway. Our results show, in contrast to what has been seen at adult synapses, that VGLUT3 expression has little or no effect on vesicular synergy at the immature glycinergic synapse of brainstem. This finding supports the model that the primary function of increased VGLUT3 expression in the immature auditory brainstem is to enable glutamate release in a developing inhibitory circuit.
The ability of a neuron to alter its synaptic connections during development is essential to circuit assembly. Synapse remodeling or refinement has been observed in many species and many neuronal circuits, yet the mechanisms defining which neurons undergo remodeling are unclear. Moreover, the molecules that execute the process of remodeling are also obscure. To address this issue, we sought to identify targets of the transcription factor unc-55 COUP-TF, which acts as a cell-specific repressor of synapse remodeling in C. elegans. unc-55 COUP-TF is expressed in VD neurons, where it prevents synapse remodeling. DD neurons can remodel synapses because they do not express unc-55 COUP-TF. Ectopic expression of unc-55 COUP-TF in DD neurons prevents remodeling. We identified the transcription factor Hunchback-like hbl-1 as a target of UNC-55 COUP-TF repression. Differential expression of hbl-1 explains the cell-type specificity of remodeling. hbl-1 is expressed in the DD neurons that are capable of remodeling, and is not expressed in the VD neurons that do not remodel. In unc-55 mutants, hbl-1 expression increases in VD neurons where it promotes ectopic remodeling. Moreover, hbl-1 expression levels bidirectionally regulate the timing of DD remodeling...
This thesis describes two programs for generating tests for digital circuits that exploit several kinds of expert knowledge not used by previous approaches. First, many test generation problems can be solved efficiently using operation relations, a novel representation of circuit behavior that connects internal component operations with directly executable circuit operations. Operation relations can be computed efficiently by searching traces of simulated circuit behavior. Second, experts write test programs rather than test vectors because programs are more readable and compact. Test programs can be constructed automatically by merging program fragments using expert-supplied goal-refinement rules and domain-independent planning techniques.
Electrical circuit designers seldom create really new topologies or use old ones in a novel way. Most designs are known combinations of common configurations tailored for the particular problem at hand. In this thesis I show that much of the behavior of a designer engaged in such ordinary design can be modelled by a clearly defined computational mechanism executing a set of stylized rules. Each of my rules embodies a particular piece of the designer's knowledge. A circuit is represented as a hierarchy of abstract objects, each of which is composed of other objects. The leaves of this tree represent the physical devices from which physical circuits are fabricated. By analogy with context-free languages, a class of circuits is generated by a phrase-structure grammar of which each rule describes how one type of abstract object can be expanded into a combination of more concrete parts. Circuits are designed by first postulating an abstract object which meets the particular design requirements. This object is then expanded into a concrete circuit by successive refinement using rules of my grammar. There are in general many rules which can be used to expand a given abstract component. Analysis must be done at each level of the expansion to constrain the search to a reasonable set. Thus the rule of my circuit grammar provide constraints which allow the approximate qualitative analysis of partially instantiated circuits. Later...
Microglia, the brain's resident immune cells and phagocytes, are emerging as critical regulators of developing synaptic circuits in the healthy brain after having long been thought to function primarily during central nervous system (CNS) injury or disease. Recent work indicates that microglia engulf synapses in the developing brain; however, how microglia know which synapses to target for removal remains a major open question. For my dissertation research, I studied microglia-mediated pruning in the retinogeniculate system and sought to identify the molecules regulating microglial engulfment of synaptic inputs. I discovered that "eat me" and "don't eat me" signals, immune molecules known for either promoting or inhibiting macrophage phagocytosis of cells or debris, localize to the dorsal lateral geniculate nucleus of the thalamus (dLGN) and direct retinogeniculate refinement. We found that "eat me" signal C3 and its microglial receptor, CR3, are required for normal engulfment, and that loss of either of these molecules leads to a reduction in phagocytosis and sustained deficits in refinement. These data suggest that microglia-mediated pruning may be analogous to the removal of non-self material by phagocytes in the immune system. To test this hypothesis...
La ritmicidad comportamental en Drosophila es sostenida por una red neuronal que en el cerebro adulto comprende alrededor de 150 neuronas. Entre éstas, las sLNv han demostrado ser esenciales para el control de la actividad rítmica. Desde el punto de vista circadiano, las sLNv tienen dos características sobresalientes: expresan el neuropéptido PDF, y atraviesan cambios estructurales circadianos en sus terminales axonales, proceso denominado plasticidad estructural. En esta tesis estudiamos qué otros cambios ocurrían concomitantes con la plasticidad estructural. Encontramos que ésta es acompañada por aumentos en el número de sinapsis y disponibilidad de vesículas en momentos de mayor complejidad del árbol axonal, que luego caen durante la fase de menor complejidad. Además demostramos que la plasticidad estructural depende de los niveles de PDF y a raíz de este análisis descubrimos un nuevo rol para PDF durante el desarrollo de las sLNv. La arquitectura adulta de las sLNv necesita que PDF actúe sobre su postsinapsis temprano en el desarrollo larval, proceso que requiere su receptor canónico. Sin embargo, la ausencia de PDF produce defectos presinápticos evidentes luego de la metamorfosis. Comprobamos que previo a la metamorfosis la postsinapsis libera GBB...
This paper discusses highly general mechanisms for specifying the refinement
of a real-time system as a collection of lower level parallel components that
preserve the timing and functional requirements of the upper level
specification. These mechanisms are discussed in the context of ASTRAL, which
is a formal specification language for real-time systems. Refinement is
accomplished by mapping all of the elements of an upper level specification
into lower level elements that may be split among several parallel components.
In addition, actions that can occur in the upper level are mapped to actions of
components operating at the lower level. This allows several types of
implementation strategies to be specified in a natural way, while the price for
generality (in terms of complexity) is paid only when necessary. The refinement
mechanisms are first illustrated using a simple digital circuit; then, through
a highly complex phone system; finally, design guidelines gleaned from these
specifications are presented.; Comment: 59 pages
Recently, researchers have been working toward the development of practical
general-purpose protocols for verifiable computation. These protocols enable a
computationally weak verifier to offload computations to a powerful but
untrusted prover, while providing the verifier with a guarantee that the prover
performed the computations correctly. Despite substantial progress, existing
implementations are not yet practical. The main bottleneck is typically the
extra effort required by the prover to return an answer with a guarantee of
correctness, compared to returning an answer with no guarantee.
We describe a refinement of a powerful interactive proof protocol originally
due to Goldwasser, Kalai, and Rothblum. Cormode, Mitzenmacher, and Thaler show
how to implement the prover in this protocol in time O(S log S), where S is the
size of an arithmetic circuit computing the function of interest. Our
refinements apply to circuits whose wiring pattern is sufficiently "regular";
for these circuits, we bring the runtime of the prover down to O(S). That is,
our prover can evaluate the circuit with a guarantee of correctness, with only
a constant-factor blowup in work compared to evaluating the circuit with no
We argue that our refinements capture a large class of circuits...