The partitioning of the ventral neural tube into five distinct neuronal progenitor domains is dependent on the morphogenic action of the secreted protein Sonic hedgehog (Shh). The prevailing model stipulates that Class I genes are repressed and Class II genes are activated by high levels of Shh signaling and that sharp progenitor domain boundaries are established by the mutual repression of complementary pairs of Class I and Class II transcription factors. While core elements of this model are supported by experimental evidence, a number of issues remain unresolved. Foremost of these is a more thorough understanding of the mechanism by which Class I genes are regulated. In this study, we describe the consequences of Shh misexpression on Class I and Class II gene expression in the hindbrain of ShhP1 embryos. We observed that an ectopic source of Shh in the otic vesicle of ShhP1 embryos ventralized the adjacent hindbrain by inducing, rather than repressing, the expression of several Class I genes (Pax6, Dbx1, Dbx2). The Shh dependent activation of Class I genes was mediated, in part, by Gli2. These results bear significance on the model of ventral neural tube patterning as they suggest a dual role for Shh in the regulation of Class I genes, whereby low levels of Shh signaling initiate Class I gene transcription, while higher levels restrict the domains of Class I gene expression to intermediate positions of the neural tube through the activation of Class II transcriptional regulators.
Motion planning determines trajectories for vehicles that link an initial location and heading with a final location and heading. Techniques for motion planning have been developed for two-dimensional maneuvering; however, they are less mature for three-dimensional maneuvering. The concept of motion primitives is particularly attractive for motion planning that determines trajectories as a set of maneuvers that satisfy differential constraints. This paper furthers work on a higher-level abstraction of trajectory primitives that consider sequences of motion primitives. In this paper, trajectory primitives are developed that deal with airspace constraints of an environment. The motion planning is shown to be an optimization involving a pair of trajectory primitives that is related by an intermediate waypoint. The resulting path is completely parameterized by the waypoint location.
Preliminary flight control design typically assumes that the aircraft is a rigid body; however, the aeroservoelastic effects of wing flexibility can have a large impact on flight dynamics and controller performance. The more flexible the aircraft becomes, the worse the closed loop performance can become. The actual point in which the flexibility causes the original control design to be unacceptable is typically not available to control designers early in the design process. In this paper, a parametric study is conducted that looks at several flight dynamics and control metrics pertinent to aircraft design and are studied for various flexibility configurations. These metrics are then parameterized by wing stiffness. The point at which control performance becomes unacceptable is also explored.
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