Tectal mosaic: Organization of the descending tectal projections in comparison to the ascending tectofugal pathway in the pigeon

Abstract

The optic tectum of vertebrates is an essential relay station for visuomotor behavior and is characterized by a set of connections that comprises topographically ordered input from the eyes and an output that reaches premotor hindbrain regions. In the avian tectofugal system, different ascending cell classes have recently been identified based on their dendritic and axonal projection patterns, although comparable information about the descending cells is missing. By means of retrograde tracing, the present study describes the detailed morphology of tectal output neurons that constitute the descending tectobulbar and tectopontine pathways in pigeons. Descending cells were more numerous in the dorsal tectum and differed in terms of 1) their relative amount of ipsi- vs. contralateral projections, 2) the location of the efferent cell bodies within different tectal layers, and 3) their differential access to visual input via dendritic ramifications within the outer retinorecipient laminae. Thus, the descending tectal system is constituted by different cell classes presumably processing diverse aspects of the visual environment in a visual field-dependent manner. We demonstrate, based on a careful morphological analysis and on double-labeling experiments, that the descending pathways are largely separated from the ascending projections even when they arise from the same layers. These data support the concept that the tectum is arranged as a mosaic of multiple cell types with diverse input functions at the same location of the tectal map. Such an arrangement would enable the tectal projections onto diverse areas to be both retinotopically organized and functionally specific.

Organization of telencephalotectal projections in pigeons: Impact for lateralized top-down control

“…Injections into all these target layers could be verified by labeling of other efferent and afferent tectal connections. i) Tracer application within layer 13 was confirmed by bilateral fiber staining within the nucleus rotundus (Hellmann and Güntürkün, 1999), within the mainly contralaterally descending tectobulbar as well as the mainly ipsilaterally projecting tectopetal fiber tracts (Hellmann et al, 2004). ii) Tracer application into the superficial laminae was confirmed by labeled cells located within Ipc which projects onto layers 2-13 (Wang et al, 2006), Imc (nucleus isthmi magnocellularis) which terminates in layers 10 -12 (Wang et al, 2004), Slu which project onto the layers 4 -13 Wang et al, 2006), PT (nucleus pretectalis) which projects onto layer 5 (Gamlin et al, 1996), and VLT (nucleus ventrolateralis thalami) which projects onto layers 11-14 (Hunt and Brecha, 1984;Schulte et al, 2006).…”

Comparative Morphology of the Avian Cerebellum: II. Size of Folia

“…This visual input arises from not only the tectofugal system, via the pontine nuclei, but also the nucleus lentiformis mesencephali (LM) of the pretectum [Pakan et al, 2005;Pakan and Wylie, in press]. Although the tecto-pontine system is involved in processing local motion [e.g., Frost and Nakayama, 1983] for orientation and avoidance [Ewert, 1970;Ingle, 1970;Hellmann et al, 2004], the LM processes 'optic flow' and is important for generating optomotor responses [e.g., Gioanni et al, 1983;Winterson and Brauth, 1985;McKenna and Wallman, 1985;Wylie and Crowder, 2000]. It has been suggested that an integration of local motion analysis and optic flow analysis is important for 'steering behaviour' during locomotion in complex environments [Sherk and Fowler, 2001;Elder et al, 2005;Page and Duffy, 2005;Sato et al, 2005;Logan and Duffy, in press].…”

Visual responses and afferent connections of the n. ventrolateralis thalami (VLT) in the pigeon (Columba livia)

“…Dendritic arborizations could not be detected higher than up to layer 12. Thus, this deep population resembles cells of the descending tectal output [20]. Moreover, massive labeling of multipolar cells was observed mainly in the ventral part of the ipsilateral nucleus isthmi, pars semilunaris (SLu, A 1.75-A 2.25; Fig.…”

“…5). These deep tectal laminae are in part the source of the diverse descending tectomotor output pathways [20]. Since VLT neurons are partly GABAergic [9], this diencephalic structure could inhibit some tectal circuits while activating others.…”