Convergence and branching patterns of round, type 2 corticopulvinar axons

Abstract

Corticopulvinar connections consist of at least two morphologically distinct subpopulations. In one subgroup (E, type 1), axons have an "elongated" terminal field and thin, spinous terminations; in the other (R, type 2), axons have a small, round arbor and large, beaded terminations. Previous work (Rockland, 1996) indicates that E-type axons from several occipitotemporal areas branch extensively within and sometimes between pulvinar subdivisions, but that R-type axons tend to have spatially delimited arbors. The present report is a further investigation of R-type axons from areas V1 and MT and was initiated to test the generality of the previous findings. There are four main results: 1) By serial section reconstruction of anterogradely labeled axons, 10 of 25 axons originating in area V1 had two or three spatially separate arbors (8 and 2 axons, respectively). Sixteen axons analyzed from area MT, however, all had single arbors, although the arbors were often formed by the convergence of widely separate branches. 2) Multiple (at least 2-5) R-type corticopulvinar axons, from V1 or from MT, can converge in a single focus. 3) R-type axons originating from both areas V1 and MT can branch to other structures; namely, the superior colliculus, the pretectal area, and/or the reticular nucleus of the thalamus. 4) Finally, corticopulvinar terminations from area V1 are predominantly R-type, whereas those from MT are more predominantly E-type. These results thus provide additional evidence of the special relationship of area V1 to the pulvinar. They also emphasize that the idea of corticopulvinocortical "feedback loops," although convenient as a shorthand nomenclature, does not adequately convey the full complexity of the system.

Spikes, synchrony, and attentive learning by laminar thalamocortical circuits

“…Most of its inputs originate from the cerebral cortex and the superior colliculus (SC). The LGN receives a massive cortical projection from V1 cortical layer 6, and the PULV receives afferents from layers 5 and 6 of several cortical areas (Rockland, 1998;Wang et al, 2002;Shipp, 2003 Fitzpatrick at al. (1985); Callaway and Wiser (1996) Layer 2/3 cells V1 → Layer 2/3 cells V1 D Recurrent connections (grouping) in 2/3.…”

“…The longer and smaller the dendrite, the more attenuated the post-synaptic current will be at the soma. Differential dendritic termination and synaptic weight magnitudes can be used to simulate the proposed functional differentiation between driving, large, round (R-type) thalamic vesicles occurring at retinothalamic and thalamocortical synapses, and elongated, small vesicles that characterize many corticothalamic terminations (Rockland, 1998;Sherman and Guillery, 2001). Elongated and round-type synapses also widely occur at the level of corticocortial synapses (Rockland, 2002).…”

Fewer driver synapses in higher order than in first order thalamic relays

“…We have associated RL terminals in higher order thalamic nuclei as emanating from layer 5 of the cortex. Evidence that this occurs is available from many examples (reviewed in Sherman, 2005;Sherman and Guillery, 2006): from visual cortex to the pulvinar Rockland, 1998;Rouiller and Welker, 2000), from somatosensory cortex to the posterior nucleus (Hoogland et al, 1991;; and from the auditory cortex to the magnocellular part of the medial geniculate nucleus (Ojima, 1994;Bartlett et al, 2000). Evidence for this also exists for the medial dorsal nucleus (Schwartz et al, 1991).…”

Distribution, morphology, and synaptic targets of corticothalamic terminals in the cat lateral posterior-pulvinar complex that originate from the posteromedial lateral suprasylvian cortex

“…Tracer injections confined to areas 17 and 18 label type II corticothalamic terminals in the LP nucleus that originate from layer V cells and type I corticothalamic terminals in the dorsal lateral geniculate nucleus (dLGN) that originate from layer VI cells (Ojima et al, 1996). In the LP nucleus, area 17 corticothalamic terminals are RL profiles that participate in glomeruli (Vidnyánszky et al, 1996;Feig and Harting, 1998), whereas, in the dLGN, area 17 corticothalamic terminals are RS profiles that contact small caliber dendrites outside of glomeruli (Jones and Powell, 1969;Vidnyánszky and Hamori, 1994;Vidnyánszky et al, 1996;Erisir et al, 1997).The recognition that corticothalamic terminals originating from layers V or VI of the striate cortex may serve different functions within the thalamus has led to a renewed interest in the cortical inputs to the LP-pulvinar complex (Rockland, 1994(Rockland, , 1996(Rockland, , 1998Bourassa and Deschênes, 1995;Vidnyánszky et al, 1996; Anderson et al, 1998;Feig and Harting, 1998;Li et al, 2003). Because the synaptic connections of large type II corticothalamic terminals in the LP-pulvinar nucleus are similar to the synaptic connections of retinal terminals in the dLGN, it has been suggested that the type II corticothalamic terminals may provide the primary or driving input to the LP-pulvinar complex (Guillery, 1995; Rodrigo-Angulo and ReinosoSuarez, 1995;Sherman and Guillery, 1996;Feig and Harting, 1998).…”