Terminal arbors of axons projecting to the somatosensory cortex of the adult rat. I. The normal morphology of specific thalamocortical afferents

Abstract

A characteristic feature of the rat somatosensory neocortex is a discrete topographic representation of the facial whiskers. Afferent fibers projecting to this vibrissae representation were "bulk-labeled" by injecting horseradish peroxidase into the white matter. Terminal arbors with the morphological characteristics of Lorente de No's (1949) "specific" thalamocortical afferents were then reconstructed through serial sections. These terminal arbors, characterized by the discrete organization of their dense plexus in layer IV, have a laminar distribution of boutons that parallels the laminar pattern of terminal degeneration resulting from lesions of the ventral posterior nucleus of the thalamus. The regional distribution of different-sized arbors corresponds to the distribution of vibrissae-related clusters of different sizes. Larger arbors were found in the posteromedial region corresponding to the mystacial vibrissae representation, while smaller arbors were found in the anterolateral region corresponding to the representation of the anterior sinus hairs. Terminal arbors were also reconstructed from sections stained simultaneously to demonstrate the pattern of vibrissae-related clusters. The greatest concentration of boutons on these axons occurred within a single vibrissae-related cluster. Furthermore, when 2 fibers terminated within a single cluster, their terminal arbors appeared to be largely coextensive. The morphology, size, and distribution of these terminal arbors support the hypothesis that the layer IV plexus of a single specific thalamocortical afferent tends to fill a vibrissae-related cluster. Thus, the organization of specific thalamocortical afferents may be responsible for clustered organization within the somatotopic map of the rodent neocortex.

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

“…Nevertheless, the complexity of single POm TCA arbors in adult rats (Deschenes et al, 1998) far exceeds the typical morphology we observed at P8. This difference likely reflects the continued maturation of POm TCA arbors beyond the times we examined, similar to the extended period of elaboration of VPM TCA arbors (Jensen and Killackey, 1987; Agmon et al, 1993; Catalano et al, 1996; Rebsam et al, 2002). …”

A New Thalamic Pathway of Vibrissal Information Modulated by the Motor Cortex

“…If cells in the head and core of the barreloids form separate pathways of vibrissal information, one would expect both populations of neurons to also differ in the way they innervate barrel cortex. Studies that examined the terminal arbors of single thalamocortical fibers in the barrel cortex reported terminal fields that consist of a dense plexus in layers 3-4 of a barrel column and of a sparse terminal field in the upper layer 6 of the same column (Jensen and Killackey, 1987;Pierret et al, 2000;Arnold et al, 2001). That later study also presented several examples of fibers that did not match this prototypical projection pattern, either by the spatial or laminar distribution of their terminal branches.…”

Derivation and Analysis of Basic Computational Operations of Thalamocortical Circuits

“…The projection patterns described are thus arranged into two large thalamocortical circuits: 1) topographic projection of thalamic core cells > middle cortical layers > superficial layers > deep layers, with reciprocal topographic feedback from layer VI back to both the core cells and to the overlying portion of nucleus reticularis; 2) matrix cells projecting nontopographically to layer I and receiving projections back from layer V, without interposed nucleus reticularis projections. Evidence suggests that the repeating thalamocortical, cortico-cortical, and corticothalamic projection patterns hold not only for primary sensory areas including VPM/VPL, LGd, and MGv to layer IV, and Pom, LP/Pul, and MGm to layer I of somatosensory, visual and auditory cortices, respectively (Killackey and Ebner, 1972;Ryugo and Killackey, 1974;Ribak and Peters, 1975;Herkenham, 1980;Kelly and Wong, 1981;Swadlow, 1983;Rieck and Carey, 1985;Herkenham, 1986;Jensen and Killackey, 1987;Winer and Larue, 1987;Scheel, 1988;Conley and Diamond, 1990;Rouiller and Welker, 1991;Bourassa and Deschenes, 1995;Huang and Winer, 2000), but also for a wide array of thalamic nuclei, intralaminar and nonintralaminar alike (Jones and Hendry, 1989;Rausell et al, 1992;Molinari et al, 1994;Molinari et al, 1995;Kuroda et al, 1998;Mitchell and Cauller, 2001;Rauschecker et al, 1997;Jones, 1998;Reep and Corwin, 1999;Linke and Schwegler, 2000;Jones, 2001)). For those cortical areas not receiving topographic projections from thalamus, the extensive topography-preserving cortico-cortical projections from superficial layers to recipient middle layers with reciprocal projections from the target's deep layers back to the source's superficial layers, may subserve a related function …”