The Somatotopic Organization Within the Rabbit's Thalamic Reticular Nucleus

The organization of the somatosensory representation within the rabbit's thalamic reticular nucleus (TRN) was studied. Focal injections of horseradish peroxidase (HRP), wheatgerm agglutinin conjugated to HRP, or [³H]proline were made into somatosensory cortical area 1 (S1). The resultant labelling in the thalamus was analysed. Single injections into S1 result in single zones of terminal labelling in TRN that are restricted to the centroventral part of the sheet-like nucleus. In reconstructions from horizontal sections these zones of labelling resemble 'slabs', which lie in the plane of the nucleus parallel to its borders, occupy only a fraction of the thickness of the reticular sheet, and are elongated in the dorsoventral and oblique rostrocaudal dimensions. Thus, the slabs of S1 terminals, which represent various loci of the body surface, and the main distribution of the reticular dendrites have a similar orientation. In comparisons of the zones of labelling following single or double injections at different cortical sites in S1, an inner (medial) to outer (lateral) shift in labelling in the ventrobasal complex (VB) is accompanied by an inner (medial) to outer (lateral) shift in labelling along the thickness of the reticular sheet; a rostral to caudal shift in labelling in VB is accompanied by a rostral to caudal shift in labelling along the plane of the reticular sheet. Thus, like VB, the reticular nucleus receives a topographically accurate projection from S1. Further, the somatotopic map conveyed from S1 to TRN lies perpendicular to the plane of the nucleus and repeats the spatial organization of the map in VB. These findings, together with those for the visual sector of the rabbit's TRN, indicate that the representation of the cortical sheet is broken up into significant parcels at the inner and outer borders of the reticular sheet.     

The Topographic Organization and Axis of Projection within the Visual Sector of the Rabbit's Thalamic Reticular Nucleus

The organization of the visual field representation within the thalamic reticular nucleus (TRN) of the rabbit was studied. Focal injections of horseradish peroxidase (HRP) and/or [3H]proline were made into visuocortical areas V1 and V2 and the dorsal lateral geniculate nucleus (dLGN). The resultant labelling in the thalamus was analysed. A single injection in V1 or V2 results in a single zone of terminal label within the TRN that is restricted to the dorsocaudal part of the sheet-like nucleus. In comparisons of the zones of label following injections at two different cortical sites in V1, a medial to lateral shift in label across the thickness of the TRN sheet is accompanied by a medial to lateral shift in label in the dLGN; a dorsal to ventral shift in label within the plane of the TRN sheet is accompanied by a dorsal to ventral shift in label in the dLGN. Thus, like the dLGN the TRN receives a precise topographic projection from V1. In reconstructions from horizontal sections the zones of label within the TRN resemble 'slabs', which lie within the plane of the nucleus parallel to its borders. Thus, the slabs of visuocortical terminals and reticular dendrites are similarly oriented. As revealed by the orientation of the slabs, the lines of projection representing points in visual space are represented by the oblique rostrocaudal dimension of the TRN. Injections restricted to V1 produce terminal labelling that is confined to the outer two-thirds of the TRN across its thickness, whilst those involving V2 result in terminal labelling within the inner one-third of the nucleus. Thus, the adjacent cortical areas V1 and V2 project in a continuous fashion across the mediolateral dimension of the TRN. The organization of the map within the TRN, which was revealed by visuocortical injections, was confirmed by the pattern of retrograde labelling within the nucleus following geniculate injections of HRP. On the basis of these findings and those in other mammalian species, two major conclusions can be reached that alter our view of the TRN. First, rather than mapping onto the whole nucleus in a continuous fashion, the cortical projection to the TRN has significant discontinuities. Second, rather than integrating efferents from widespread cortical areas, the reticular dendrites are related to focal areas of cortex.     

Organization in the somatosensory sector of the cat's thalamic reticular nucleus

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the somatosensory part of the dorsal thalamus in the cat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the ventrobasal complex (VB) and the medial division of the posterior complex (POm) of the thalamus. The resultant retrograde labelling in TRN was analyzed. Large injections of a tracer in VB label many reticular cells that are restricted to a centroventral, or somatosensory, sector of TRN. Small injections of a tracer in VB produce narrow zones of labelled cells in this sector. In reconstructions these zones resemble thin “slabs,” which lie parallel to the plane of TRN along its oblique rostrocaudal dimension and occupy only a fraction of its thickness. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different nuclei of VB, inner cells, intermediate cells, and outer cells across the thickness of TRN project to the ventral posteromedial, the medial division of the ventral posterolateral, and the lateral division of the ventral posterolateral nuclei, respectively. Furthermore, shifts in injected areas along the dorsoventral dimension of VB produce similar shifts in zones of labelled cells in TRN. Thus, reticular cells form an accurate map on the basis of their connections with VB. Large injections of a tracer in the ventral subdivision of POm label many reticular cells that are also restricted to the centroventral sector of TRN. Small injections of a tracer in ventral POm produce broad zones of labelled cells in this sector. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different regions of ventral POm, cells that project to these regions are scattered across the thickness of TRN and occupy overlapping territories. Large injections of a tracer in either VB or ventral POm also label cells in a restricted centroventral region of the perireticular nucleus. Double injections of different tracers in VB and ventral POm produce many cells in TRN that are labelled from both of these dorsal thalamic structures and fewer cells that are labelled from only one or the other of these structures. These results indicate that there is a dual organization in the projections of cells in the somatosensory sector of TRN to dorsal thalamus: Projections to VB are topographically organized whereas those to ventral POm lack a topographical organization. Furthermore, both of these mapped and nonmapped projections can arise from single reticular cells in the somatosensory sector. © 1996 Wiley-Liss, Inc.     

Organization in the auditory sector of the cat's thalamic reticular nucleus

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the auditory part of the cat's dorsal thalamus. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the medial geniculate complex (MG). The resultant retrograde labelling in the TRN was analyzed. Injections of WGA-HRP into the ventral (MGv), dorsal (MGd), or medial (MGm) nuclei of the MG label zones of cells that are restricted to a caudoventral sector of the TRN. In reconstructions, these zones resemble “slabs” that are elongated in the dorsoventral and oblique rostrocaudal dimensions of the nucleus. In comparisons of the zones of labelling in the TRN following tracer injections into different nuclei of the MG, inner and caudal cells project to the pars lateralis of the MGv (MGvl) or to the MGd, and outer and rostral cells project to the pars ovoidea of the MGv (MGvo) or to the MGm. Thus, cells projecting to the MGvl or MGd or to the MGvo or MGm occupy overlapping territories. Double injections of different fluorescent dyes into selected pairs of MG nuclei result in reticular cells that are labelled from either both nuclei or only one or the other nucleus in each pair. These results indicate that the projections of cells in the auditory sector of the TRN to the MGvl or MGvo or to the MGd or MGm are topographically organized. Furthermore, projections to more than one MG nucleus can arise from single reticular cells. J. Comp. Neurol. 390:167–182, 1998. © 1998 Wiley-Liss, Inc.     

Organization of connections between the thalamic reticular and the anterior thalamic nuclei in the rat

The thalamic reticular nucleus (TRN) receives topographically organized input from specific sensory nuclei such as the lateral geniculate nucleus. The present study shows this in the rat. However, the pattern of thalamic connections to the limbic reticular sector is unknown. Injecting biocytin into the ventral parts of anteroventral and anteromedial nuclei labeled neurons and axons in the rostral TRN. Filled axon collaterals and their terminals occupied a rectangular sheet in a plane close to the horizontal, and were confined to the inner zone (the medial portion) of the limbic TRN. Retrogradely filled cells were in the middle of the rostral pole in the same horizontal plane, receiving synapses from surrounding labeled boutons. In electron micrographs, thalamic terminals were found to contain round, densely packed synaptic vesicles and formed asymmetrical synapses onto reticular somata and dendritic profiles. Displacing the injection site along the dorso-ventral and rostro-caudal axs in the anterior nuclei produced corresponding shifts of antero- and retrograde labeling within the inner reticular zone. Projections from the dorsal portions of the anterior nuclei did not follow this pattern. Axons from the anterodorsal nucleus occupied the rostralmost tip of both inner and outer zones of the dorsal limbic sector. In accordance with earlier reports, the limbie sector was found to represent several dorsal thalamic nuclei parallel to each other medio-laterally. A topography is described for the limbic reticulo-thalamic connections, suggesting that the rostral TRN is able to influence circumscribed areas of the limbic thalamus. © 1995 Wiley-Liss, Inc.     

Thalamic connectivity of the second somatosensory area and neighboring somatosensory fields of the lateral sulcus of the macaque

The thalamocortical relations of the somatic fields in and around the lateral sulcus of the macaque were studied following cortical injections of tritated amino acids and horseradish peroxidase (HRP). Special attention was paid to the second somatosensory area (S2), the connections of which were also studied by means of thalamic isotope injections and retrograde degeneration. S2 was shown to receive its major thalamic input from the ventroposterior inferior thalamic nucleus (VPI) and not, as previously reported, from the caudal division of the ventroposterior lateral nucleus (VPLc). Following small injections of isotope or HRP into the hand representation of S2, only VPI was labeled. Larger injections, which included the representations of more body parts, led to heavy label in VPI, with scattered label in VPLc, the central lateral nucleus (CL), and the posterior nucleus (Po). In addition, small isotope injections into VPLc did not result in label in S2 unless VPI was also involved in the injection site, and ablations of S2 led to cell loss in VPI. Comparison of injections involving different body parts in S2 suggested a somatotopic arrangement within VPI such that the trunk and lower limb representations are located posterolaterally and the hand and arm representations anteromedially. The location of the thalamic representations of the head, face, and intraoral structures that project to S2 may be in the ventroposterior medial nucleus (VPM). The granular (Ig) and dysgranular (Id) fields of the insula and the retroinsular field (Ri) each receive inputs from a variety of nuclei located at the posteroventral border of the thalamus. Ig receives its heaviest input from the suprageniculate-limitans complex (SG-Li), with additional inputs from Po, the magnocellular division of the medial geniculate n. (MGmc), VPI, and the medial pulvinar (Pulm). Id receives its heaviest input from the basal ventromedial n. (VMb), with additional inputs from VPI, Po, SG-Li, MGmc, and Pulm. Ri receives its heaviest input from Po, with additional input from SG-Li, MGmc, Pulm, and perhaps VPI. Area 7b receives its input from Pulm, the oral division of the pulvinar, the lateral posterior n., the medial dorsal n., and the caudal division of the ventrolateral n. These results indicate that the somatic cortical fields, except for those comprising the first somatosensory area, each receive inputs from an array of thalamic nuclei, rather than just one, and that individual thalamic somatosensory relay nuclei each project to more than one cortical field.     

Auditory thalamic reticular nucleus of the rat: anatomical nodes for modulation of auditory and cross-modal sensory processing in the loop connectivity between the cortex and thalamus

The auditory sector of the thalamic reticular nucleus (TRN) plays a pivotal role in gain and/or gate control of auditory input relayed from the thalamus to cortex. The TRN is also likely involved in cross-modal sensory processing for attentional gating function. In the present study, we anatomically examined how cortical and thalamic afferents intersect in the auditory TRN with regard to these two functional pathways. Iontophoretic injections of biocytin into subregions of the auditory TRN, which were made with the guidance of electrophysiological recording of auditory response, resulted in retrograde labeling of cortical and thalamic cells, indicating the sources of afferents to the TRN. Cortical afferents from area Te1 (temporal cortex, area 1), which contains the primary and anterior auditory fields, topographically intersected thalamic afferents from the ventral division of the medial geniculate nucleus at the subregions of the auditory TRN, suggesting tonotopically organized convergence of afferents, although they innervated a given small part of the TRN from large parts. In the caudodorsal and rostroventral parts of the auditory TRN, cortical afferents from nonprimary visual and somatosensory areas intersected thalamic afferents from auditory, visual, and somatosensory nuclei. Furthermore, afferents from the caudal insular cortex and the parvicellular part of the ventral posterior thalamic nucleus, which are associated with visceral processing, converged to the rostroventral end of the auditory TRN. The results suggest that the auditory TRN consists of anatomical nodes that mediate tonotopic and/or cross-modal modulation of auditory and other sensory processing in the loop connectivity between the cortex and thalamus.     

Topographic organization of projections from the thalamic reticular nucleus to the anterior thalamic nuclei in the rat

We have investigated connections between the thalamic reticular nucleus (TRN) and the anterior thalamic nuclei (ATN) in the rat, following injections of horseradish peroxidase (HRP) into subnuclei of the ATN and different regions of the rostral TRN. Three nonoverlapping groups of neurons in the dorsal part of the ipsilateral rostral TRN project to, and receive reciprocal projections from, specific subnuclei of the ATN. A vertical sheet of neurons in the most dorsal part of the rostral TRN projects to the dorsal half of the posterior subdivision of the anteroventral thalamic nucleus (AVp), the dorsal region of the medial subdivision of the anteroventral thalamic nucleus (AVm), and the dorsolateral part of the rostral anterodorsal thalamic nucleus (AD). Immediately ventral to this part of TRN, but still within its dorsal portion, are a lateral cluster of neurons and a medially located vertical sheet of neurons. The lateral cluster projects to the ventral part of AVp and to the dorsomedial part of rostral AD. The medial sheet projects to the ventral part of AVm, the ventral part of rostral AD, and to the caudal portions of both AV and AD. There appears to be no input to the anteromedial thalamic nucleus (AM) from the TRN. These findings shed new light on the anatomy of the rostral TRN, the ATN, and the connections between the two, and are relevant to emerging hypotheses about the functional organization of the TRN and reticulo-thalamic projections.     

Cells of the perireticular nucleus project to the developing neocortex of the rat

The perireticular nucleus is a recently described thin sheet of small cells among the fibres of the internal capsule, lying lateral to the thalamic reticular nucleus and medial to the globus pallidus (Clemence and Mitrofanis [1992]. J. Comp. Neurol. 322:167–180). During development, the perireticular nucleus is relatively large, lying in the path of the growing corticofugal and thalamocortical axons and filling the area of the internal capsule lateral to the thalamic reticular nucleus. After these axons have formed their connections, the perireticular nucleus rapidly decreases in size, leaving only a few cells in the adult (Mitrofanis [1992] J. Comp. Neurol. 320:161–181). In this study, we aiwed to investigate the connections between the developing cortex and thalamus by making injections of tracer into the cortical plate. Injections of Horse Radish Peroxidase (HRP), Wheat Germ Agglutinin bound to HRP (WGA-HRP) and l'dioctadecyl 3, 3, 3′, 3 tetramethycarbocyanine perchlorate (DiI) were made in vivo between embryonic day (E)18 and adult and DiI was placed in the fixed brains of rats aged between E16 and postnatal day (P)1. Between E17 and P10, the retrograde perikaryal labelling resulting from these injections revealed a transient projection from the perireticular nucleus to the ipsilateral cortical plate. No cells were labelled in the thalamic reticular nucleus. This suggests that the perireticular nucleus must be regarded as a group of cells distinct from the thalamic reticular nucleus and having a separate role in development. Comparisons between the perireticular cells and the cells of the cortical subplate suggest that both may be playing comparable roles in early development, possibly guiding fibres towards their end stations or serving to rearrange the complex mapped projections linking thalamus and cortex. © 1995 Wiley-Liss, Inc.     

Axonal projections of single auditory neurons in the thalamic reticular nucleus: implications for tonotopy-related gating function and cross-modal modulation

Tonotopically comparable subfields of the primary auditory area (AI) and nonprimary auditory areas (non-AI), i.e. posterodorsal area (PD) and ventral auditory area (VA), in the rat cortex have similar topographies in the projection to the ventral division of the medial geniculate nucleus (MGV), but reverse topographies in the projection to the thalamic reticular nucleus (TRN). In this study, we examined axonal projections of single auditory TRN cells, using juxtacellular recording and labeling techniques, to determine features of TRN projections and estimate how the TRN mediates corticofugal inhibition along with the reverse topographies of cortical projections to the TRN. Auditory TRN cells sent topographic projections to limited parts of the MGV in a manner that relays cortical inputs from tonotopically comparable subfields of the AI and non-AI (PD and VA) to different parts of the MGV. The results suggest that corticofugal excitations from the AI and non-AI modulate thalamic cell activity in the same part of the MGV, whereas corticofugal inhibitions via the TRN modulate cell activity in different parts of the MGV with regard to tonotopic organization. The AI and non-AI could serve distinctive gating functions for auditory attention through the differential topography of inhibitory modulation. In addition, we obtained an intriguing finding that a subset of auditory TRN cells projected to the somatosensory but not to the auditory thalamic nuclei. There was also a cell projecting to the MGV and somatosensory nuclei. These findings extend the previously suggested possibility that TRN has a cross-modal as well as an intramodal gating function in the thalamus.     

Morphology of neurons in the thalamic reticular nucleus (TRN) of mammals as revealed by intracellular injections into fixed brain slices

I have investigated the morphology of neurons in the thalamic reticular nucleus (TRN) by means of intracellular injections in fixed tissue in order to study whether neurons in visual (dorsocaudal part), somatosensory (intermediate part), or limbic/motor (rostral part) sectors in the rat, rabbit, and cat differ morphologically in relation to their different sensory cortical or thalamic inputs. In addition, I have compared the different mammalian species to ask whether there is a morphological difference of TRN neurons according to reported differences in the intrinsic thalamic organisation, for example, due to the presence of GABAergic local circuit neurons in the majority of thalamic nuclei in the cat and the lack of those neurons in most of the rat thalamic nuclei, and presynaptic dendrites in the cat but not in the rat. In all animals investigated so far, neurons in the caudal (visual) and intermediate (somatosensory) part of the TRN have an elongated dendritic morphology in all three species, but some neurons in the rostral part, in particular in dorsal sections, have a distinctive multipolar morphology. Neurons have round, ovoid, or elongated somata ranging in area between 150 and 860 μm². In general 4–8 first order dendrites emerge directly from the two poles of the soma or from a thick stem segment. Most of the dendrites then run parallel to the borders of the nucleus extending for relatively long distances, up to 450 μm, but remain inside the border of the nucleus. Only a few (1–3) dendrites could be observed to run perpendicular to the border of the nucleus and generally only for a short distance (20–70 μm). Some of the smooth first order dendrites give rise to second order dendrites (up to 200 μm in length), which then branch into short (15–70 μm) third order dendrites. Dendritic spines and varicosities, spine-like protusions and/or hair-like processes are mainly found on second and third order dendrites. Surprisingly, the shape, arrangement, and the size of the dendritic field are not strictly related to the shape and size of the nucleus. In mammalian species with a comparatively narrow TRN (rat and cat) the dendritic field size was similar to that in the rabbit with a broad TRN. There was considerable variability in dendritic morphology in the caudal and intermediate parts of TRN. However, in contrast to two recent studies in the rat TRN I have found no obvious basis for classification of neurons in the mammalian TRN according to dendritic morphology. In addition, there seems to be no difference in neuronal morphology of TRN neurons in relation to different intrinsic thalamic organisation within or between species. © 1993 Wiley-Liss, Inc.     

The organization of projections to selected points of somatosensory cortex from the cat ventrobasal complex

The organization of neurons in the cat ventrobasal complex (VB) which project to somatosensory cortex (SI) was investigated by the use of the retrograde transport of the enzyme horseradish peroxidase (HRP). Two histochemical procedures were used to visualize retrogradely transported HRP. Injections of HRP in electrophysiologically characterized points of SI cortex labeled distinctive zones of neurons in VB ipsilateral to the injections. Injections placed in the forelimb or hindlimb cortical areas labeled laminar-like aggregates of neurons have long axis corresponded to the long axis of VB. Injections of the SI trigeminal representation resulted in very compact aggregates of HRP positive neurons which were less clearly laminar. The density of projection from VB to various portions of SI paralleled the general innervation density of the peripheral skin. Injections of the cortical vibrissal, face and forepaw representations labeled a greater number of neurons in VB per unit area of cortex injected than did injections of the hindpaw or trunk representations. For a given somatotopic area, the number of labeled neurons in VB increased linearly as the area of the cortical HRP injection increased. Differences in the sensitivity of each histochemical procedure and the relationship of differing sensitivities to the observed results are also discussed.     

Afferent projections from the brainstem to the three floccular zones in cats. II. Mossy fiber projections

Mossy fiber projections from the brainstem to the flocculus were studied following injections of horseradish peroxidase (HRP) into the flocculus and following microinjections of HRP into each of the three zones of the flocculus. It has been found that the flocculus receives mossy fiber projections from 4 main sources. (1) Perihypoglossal nucleus--dense projections originate from discrete areas of the rostral pole of the intercalated nucleus, the ventral part of the prepositus hypoglossal nucleus and the adjacent reticular formation. (2) Vestibular nuclear complex--secondary vestibular fibers come from discrete areas in the vestibular nuclei: the ventromedial and dorsomedial parts of the medial and inferior nucleus, the central area of the superior nucleus, the ventromedial part of the lateral nucleus, the group y and the interstitial nucleus of the vestibular nerve. (3) Medullary reticular formation--the strongest projection of mossy fibers arises from the accessory group of the paramedian reticular nucleus. (4) Pontine reticular formation and raphe nucleus--dense projections originate from a narrow zone which involves the caudal part of the dorsal nucleus of the raphe, the inferior and superior central nucleus of the raphe and the medial part of the nucleus reticularis tegmenti pontis. No clear indication of a different mossy fiber projection from the nuclei in the brainstem to the 3 zones of the flocculus was found.     

The Course and Termination of Corticothalamic Fibres Arising in the Visual Cortex of the Rat

Corticothalamic axons have been studied in adult Lister hooded rats with single or dual injections of tracers into the visual cortex. Labelled axons leave medial and lateral injection sites in separate or partially overlapping bundles along parallel trajectories in the subcortical white matter. In the internal capsule they converge and both bundles enter roughly the same sector of the thalamic reticular nucleus (TRN). Their reticular terminal fields, however, differ. Axons from a medial injection site innervate more lateral parts of the TRN than do the axons from lateral injection sites. The most medial third of the TRN is not innervated from area 17 but receives a topographically arranged input from peristriate cortex (Crabtree and Killackey, 1989, Eur J. Neurosci., 1, 94-109; Coleman and Mitrofanis, 1996, EWK J. Neurosci., 8, 388-404). The two groups of axons then separate in the dorsal thalamus, axons from medial parts of visual cortex turning caudally into lateral regions of the lateral geniculate nucleus, whereas fibres from more lateral cortex continue into medial parts of the nucleus. Connolly and van Essen (1984, J. Comp. Neurol., 226, 544-564) and Nelson and LeVay (1985, J. Comp. Neurol., 240, 322-330) have shown that in the geniculocortical pathway the two groups of fibres cross over in the subcortical white matter, probably in the region of the subplate. We show that the corticothalamic pathway also has a crossing, but it occurs in, or close to, the diencephalon itself, in the region of the perireticular nucleus. This result suggests that each of these pathways, the geniculocortical and the corticogeniculate, may undergo reorganization within distinct cerebral zones, one diencephalic for the corticothalamic axons and the other telencephalic for the thalamocortical axons.     

Organization of the Visual Reticular Thalamic Nucleus of the Rat

The visual sector of the reticular thalamic nucleus has come under some intense scrutiny over recent years, principally because of the key role that the nucleus plays in the processing of visual information. Despite this scrutiny, we know very little of how the connections between the reticular nucleus and the different areas of visual cortex and the different visual dorsal thalamic nuclei are organized. This study examines the patterns of reticular connections with the visual cortex and the dorsal thalamus in the rat, a species where the visual pathways have been well documented. Biotinylated dextran, an anterograde and retrograde tracer, was injected into different visual cortical areas [17; rostral 18a: presumed area AL (anterolateral); caudal 18a: presumed area LM (lateromedial); rostral 18b: presumed area AM (anteromedial); caudal 18b: presumed area PM (posteromedial)] and into the different visual dorsal thalamic nuclei (posterior thalamic, lateral posterior, lateral geniculate nuclei), and the patterns of anterograde and retrograde labelling in the reticular nucleus were examined. From the cortical injections, we find that the visual sector of the reticular nucleus is divided into subsectors that each receive an input from a distinct visual cortical area, with little or no overlap. Further, the resulting pattern of cortical terminations in the reticular nucleus reflects largely the patterns of termination in the dorsal thalamus. That is, each cortical area projects to a largely distinct subsector of the reticular nucleus, as it does to a largely distinct dorsal thalamic nucleus. As with each of the visual cortical areas, each of the visual dorsal thalamic (lateral geniculate, lateral posterior, posterior thalamic) nuclei relate to a separate territory of the reticular nucleus, with little or no overlap. Each of these dorsal thalamic territories within the reticular nucleus receives inputs from one or more of the visual cortical areas. For instance, the region of the reticular nucleus that is labelled after an injection into the lateral geniculate nucleus encompasses the reticular regions which receive afferents from cortical areas 17, rostral 18b and caudal 18b. These results suggest that individual cortical areas may influence the activity of different dorsal thalamic nuclei through their reticular connections.     

Are there connections between the thalamic reticular nucleus and the brainstem reticular formation?

Increasing awareness that the thalamic reticular nucleus (TRN) plays an important role in controlling the output of cortically projecting cells in nuclei of the dorsal thalamus has focused attention on the question of whether there exist ascending projections to the TRN from the mesencephalic or other parts of the brainstem reticular formation (BRF). We have examined this and the related question of whether the neurons of TRN project to the BRF, by anterograde and retrograde tracing experiments with horseradish peroxidase (HRP) and HRP conjugated to wheat germ agglutinin. Injections of tracer were placed stereotaxically in the BRF at various depths and rostrocaudal and mediolateral coordinates, and the TRN and adjacent nuclei were examined in serial coronal sections, using tetramethylbenzidine as the principal chromogen. Retrogradely labelled cell bodies were consistently seen in hypothalamus and zona incerta but never in TRN, suggesting that, in the rat, TRN neurons do not project caudal to the thalamus. After 54 out of 60 injections, no terminal label was detected in any part of the TRN although such label was present in other parts of the thalamus, including the intralaminar nuclei, in the same sections. We therefore conclude that direct projections from the BRF to the TRN must be extremely sparse, and that those effects of BRF stimulation upon thalamocortical transmission that are mediated by the TRN (rather than by direct projections to dorsal thalamic nuclei) probably depend chiefly on indirect polysynaptic pathways.     

Connections between the reticular nucleus of the thalamus and pulvinar-lateralis posterior complex: A WGA-HRP study

The present study utilises the capacity of wheat germ agglutinin-conjugated horseradish peroxidase to label both afferent and efferent projections from selected regions of the thalamic reticular nucleus (TRN) to the pulvinar lateralis-posterior complex (Pul-LP) of the cat. Fourteen injections into the TRN located between anterior-posterior levels 8.5 and 4.5 were analysed. The projection of the TEN to the Pul-LP complex is roughly organised in a topographic manner and is not widespread within the thalamus. Anterograde labelling in the Pul-LP extended rostrocaudally with a slight oblique dorsoventral orientation. Projections to the medial LP were predominantly but not exclusively from rostral areas of TRN, while projections to the lateral LP were largely from caudal areas of the TRN. Projections to other areas of the Pul-LP were sparse. The connections between TRN and Pul-LP were reciprocal, although the distribution of labelled cells and anterograde labelling was not completely overlapping. Reciprocal connections with the dorsal lateral geniculate nucleus were largely with the C-laminae and the medial interlaminar nucleus. The results are discussed with reference to the corticothalamic projections and the visuotopy of the Pul-LP. © 1995 Wiley-Liss, Inc.     

Morphology of visual sector thalamic reticular neurons in the macaque monkey suggests retinotopically specialized,parallel stream‐mixed input to the lateral geniculate nucleus

The thalamic reticular nucleus (TRN) is a unique brain structure at the interface between the thalamus and the cortex. Because the TRN receives bottom‐up sensory input and top‐down cortical input, it could serve as an integration hub for sensory and cognitive signals. Functional evidence supports broad roles for the TRN in arousal, attention, and sensory selection. How specific circuits connecting the TRN with sensory thalamic structures implement these functions is not known. The structural organization and function of the TRN is particularly interesting in the context of highly organized sensory systems, such as the primate visual system, where neurons in the retina and dorsal lateral geniculate nucleus of the thalamus (dLGN) are morphologically and physiologically distinct and also specialized for processing particular features of the visual environment. To gain insight into the functional relationship between the visual sector of the TRN and the dLGN, we reconstructed a large number of TRN neurons that were retrogradely labeled following injections of rabies virus expressing enhanced green fluorescent protein (EGFP) into the dLGN. An independent cluster analysis, based on 10 morphological metrics measured for each reconstructed neuron, revealed three clusters of TRN neurons that differed in cell body shape and size, dendritic arborization patterns, and medial‐lateral position within the TRN. TRN dendritic and axonal morphologies are inconsistent with visual stream‐specific projections to the dLGN. Instead, TRN neuronal organization could facilitate transmission of global arousal and/or cognitive signals to the dLGN with retinotopic precision that preserves specialized processing of foveal versus peripheral visual information. J. Comp. Neurol. 525:1273–1290, 2017. © 2016 Wiley Periodicals, Inc.     

Anatomical organization of cortico-mesencephalo-olivary pathways in the cat as demonstrated by axonal transport techniques

Cortical projections arising from areas 4 and 6 and terminating in midbrain cell groups known to project to the inferior olive (IO) have been studied in the cat. Injections of the bidirectional tracers horseradish peroxidase (HRP) and wheat germ agglutinin (WGA) conjugated to HRP were made into the midbrain. All cases of lateralized midbrain injections resulted in virtually ipsilateral labelling of lamina V cortical neurons. Retrogradely labelled neurons in cortical areas 4 and 6 were found after injections located in the interstitial nucleus of Cajal (INC), nucleus of Darkschewitsch (ND), and in the caudal parafascicular (Pf) and subparafascicular (sPf) nuclei (perifascicular region, PF). Injections that were more caudal and within the parvi- and magnocellular red nucleus (RNp and RNm) labelled cells not only in areas 4 and 6 but also in portions of adjacent areas 3a, 3b, 5a, and 7. These midbrain injections also resulted in the anterograde labelling of projections observed to terminate in the ipsilateral IO. The distribution in the midbrain of projections arising from cortical areas 4 and 6, and portions of areas 3a and 3b, was studied with autoradiographic methods. After injections of tritium-labelled amino acids in those cortical areas, a pattern of largely ipsilateral terminations was revealed. Whereas all cortical areas studied labelled the PF, differential grain distributions in central mesencephalic nuclei were apparent after injections in various portions of the motor and adjacent somatosensory cortex. Injections involving the frontal eye fields (FEF) labelled the INC bilaterally, but ipsilateral terminations were densest. These cases also labelled the region of the fields of Forel. When the neck region of the cortex was involved in the injections, the more caudal aspects of the INC (INCc) and the RN were labelled. The cortical areas related to the upper limb gave rise to terminations in the ND and the RNp. Contributions to both ND and RNp inputs from injections in the FEF and neck regions were also occasionally but not consistently noted. A relatively discrete injection in the vibrissae field weakly labelled ND. Additional components of the motor cortical projections to the superior colliculus (SC) and pretectal nuclei were also analysed since those regions also project to the IO. Cortical regions involving the representation of the neck musculature were shown to project principally ipsilaterally to lamina IV of the SC as well as to the anterior pretectal nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neurons at the origin of the medical component of the bulbopontine spinoreticular tract in the rat: An anatomical study using horseradish peroxidase retrograde transport

An anatomical technique based on the retrograde transport of horseradish perodidase (HRP) was used to investigate the progections of sponal cord neurions to the reticular formations in the rat. Both large and restricted injections were staggered all along the bulbar and pontine levels, involving the nucleus gigantocellularis, the nuclei reticularis pontis, pars oralis and cauladis and in some cases the nucleus raphé magnus. Labeled cells we constantly encountered in the reticular part of the neck of the dorsal horn throughout the whole length of the cord, mainly contralateral to the central core of the injection iste. This area was taken as the equivalent of lamina V in the cat. Other labeled cells were observed in the medial arts of the intermediate and ventral horns, in areas considered similar to laminae VII and VIII in the cat. The two most rostral cervical nating form the dorsolateral part of ventral horns. Thus, this study is a clear confirmation that the bulbopontine reticular formations constitute a target for various somatosensory inputs originating in spinal cord. It demonstrates that the medial spinoreticular tract (mSRT) differs from the other main ascending tracts by the absence of projections from (1) superficial layers and nucleus of the dorsolateral funiculus contrary to the spinomesencephalic tract; (2)ventromedial zone of the lumber dorsal horn unlike the spinothalamic tact; (3) the neck of the doral hor in its medial portuion contrary to the spinoreticular component reaching the lateral reticular nucleus; and (4) central cervical nucleus and Clarke's columns, unlike the spinocerebellar tracts. The difficulty in demonstrating retrograde labeling from discrete injections coulde result from the fact that mSRT neu-rons have sparsely ramified collaterals on their terminal zones.