It is dogma that action potentials are initiated at the soma/axon hillock of neurons. However, dendrites often exhibit conductances necessary for spike generation and represent functionally independent processing compartments within neurons. GnRH neurons provide an interesting neuronal phenotype with simple, relatively unbranched, unipolar or bipolar dendrites of extensive lengths (>1000 microm) covered in spines. These neurons control fertility and must integrate a variety of internal homeostatic and external environmental cues. We used imaging, electrophysiological, and modeling studies to understand how they integrate and process information along dendrites. Simultaneous recordings from distal dendrites and somata of individual GnRH neurons indicate distal dendrites are the primary site of spike initiation in these cells. Compartmental modeling indicates that sites of spike initiation depend upon location of excitatory input and dendrite geometry. Together, these studies demonstrate a novel pattern of spike generation in mammalian neurons and indicate that afferent inputs within distal dendritic microdomains directly initiate action potentials.
The activity of hypothalamic GnRH neurons results in the intermittent release of GnRH required for reproductive function. This intermittent neurosecretory activity has been proposed to reflect integration of intrinsic properties of and synaptic input to GnRH neurons. Determining the relative impact of synaptic inputs at different locations on the GnRH neuron is difficult, if not impossible, using only experimental approaches. Thus, we used electrophysiological recordings and neuronal reconstructions to generate computer models of GnRH neurons to examine the effects of synaptic inputs at varying distances from the soma along dendrites. The parameters of the models were adjusted to duplicate measured passive and active electrophysiology of cells from mouse brain slices. Our morphological findings reinforce the emerging picture of a complex dendritic structure of GnRH neurons. Furthermore, analysis of reduced morphology models indicated that this population of cells is unlikely to exhibit low-frequency tonic spiking in the absence of synaptic input. Finally, applying realistic patterns of synaptic input to modeled GnRH neurons indicates that synapses located more than about 30% of the average dendrite length from the soma cannot drive firing at frequencies consistent with neuropeptide release. Thus, processing of synaptic input to dendrites of GnRH neurons is probably more complex than simple summation.
Silicon carbide is a wide-band-gap semiconductor suitable for high-power high-voltage devices and it has excellent properties for use in photoconductive semiconductor switches ͑PCSSs͒. PCSS were fabricated as planar structures on high-resistivity 4H-SiC and tested at dc bias voltages up to 1000 V. The typical maximum photocurrent of the device at 1000 V was about 49.4 A. The average on-state resistance and the ratio of on-state to off-state currents were about 20 ⍀ and 3ϫ10 11 , respectively. Photoconductivity pulse widths for all applied voltages were 8-10 ns. These excellent results are due in part to the removal of the surface damage by high-temperature H 2 etching and surface preparation. Atomic force microscopy images revealed that very good surface morphology, atomic layer flatness, and large step width were achieved.
We report the results of drone lidar survey at a high-elevation archaeological site in the Chachapoyas region of Peruvian Amazonia. Unlike traditional airborne remote sensing, drone lidar produces very high-density measurements at a wide range of scan angles by operating at low altitudes and slow flight speeds. These measurements can resolve near vertical surfaces and novel dimensions of variability in architectural datasets. We show in a case study at Kuelap that the number of detected structures almost exactly matches the number reported from previous ground level surveys, and we use these data to quantify the relative circularity and size frequency distribution of architectural structures. We demonstrate variability in domestic architecture that was obscured in previous models produced using low-resolution remote sensing. Spatial analysis of these attributes produces new hypotheses about the site's construction history and social organization.
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