The optic tectum in birds and its homologue the superior colliculus in mammals both send major bilateral, nontopographic projections to the nucleus rotundus and caudal pulvinar, respectively. These projections originate from widefield tectal ganglion cells (TGCs) located in layer 13 in the avian tectum and in the lower superficial layers in the mammalian colliculus. The TGCs characteristically have monostratified arrays of brush-like dendritic terminations and respond mostly to bidimensional motion or looming features. In birds, this TGC-mediated tectofugal output is controlled by feedback signals from the nucleus isthmi pars parvocellularis (Ipc). The Ipc neurons display topographically organized axons that densely ramify in restricted columnar terminal fields overlapping various neural elements that could mediate this tectofugal control, including the retinal terminals and the TGC dendrites themselves. Whether the Ipc axons make synaptic contact with these or other tectal neural elements remains undetermined. We double labeled Ipc axons and their presumptive postsynaptic targets in the tectum of chickens (Gallus gallus) with neural tracers and performed an ultrastructural analysis. We found that the Ipc terminal boutons form glomerulus-like structures in the superficial and intermediate tectal layers, establishing asymmetric synapses with several dendritic profiles. In these glomeruli, at least two of the postsynaptic dendrites originated from TGCs. We also found synaptic contacts between retinal terminals and TGC dendrites. These findings suggest that, in birds, Ipc axons control the ascending tectal outflow of retinal signals through direct synaptic contacts with the TGCs.
Pubmed ID: 26224333 RIS Download
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View all literature mentionsA Windows (Win32) software application for montaging, aligning, tracing, measuring, and reconstructing objects from serial microscopic section images. The software is designed for microscopy in which section resolution is much less than section thickness, such as transmitted electron microscopy (EM) where the resolution is a few nanometers while the section thickness is many tens of nanometers. Reconstruct can easily handle series with hundreds of very large, high-resolution section images. It facilitates image cropping, scaling and alignment. Multiple images can be placed side-by-side to make a montage of a section from a mosaic of images. The alignment of adjacent sections can be rapidly compared by either blending the two sections or by flickering between them. Sections can be moved while blended. Reconstruct aids in the calibration of image size. Images taken at different magnifications can be combined, calibrated and aligned. Tools for tracing and editing of objects on sections are provided. Objects can be surfaced from the traces and previewed in an OpenGL-based 3D scene window. The 3D scene can be saved as a bitmap or as a VRML file.
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