Cytoarchitectonic delineation of the ventral lateral thalamic region in the monkey

The cytoarchitecture of the ventral lateral region of the primate thalamus has been appraised in the frontal, parasagittal and horizontal planes. A morphologically distinct region, possessing a sparse and diffuse distribution of large and small neurons is identified. The region includes several nuclei previously separately named by Olszewski⁴⁵. These are nuclei VPLo, VLc, X, VLps, and some cellular extensions into the VLo nucleus. The whole zone is continuous, and it is shown that no clear separation exists between any of the previously identified sub-nuclei. Connectional grounds are given for suggesting that this region should be considered as a common cerebellar relay nucleus to motor cortex.Morphological criteria for distinguishing the cellsparse nucleus from adjacent nuclei are given. These cytological criteria provide a basis for the experimental analysis of cortical and subcortical connectivity of the ventral lateral thalamic region. Close attention was paid to the border between the VPLo nucleus and the VPLc nucleus. VPLc is separated from VPLo by a clear border, and no transitional zone can be detected in the parasagittal or horizontal planes. Previous ambiguities in the delineation of the VPLo-VPLc border probably stem from analysis in the frontal plane, in which the border is not clear.     

Distribution of cerebellar terminations and their relation to other afferent terminations in the ventral lateral thalamic region of the monkey

The efferent projections of the deep cerebellar nuclei were studied and their fiber trajectories and thalamic termination zones described. The thalamic termination zones for the dentate, interposed and fastigial nuclei are identical and coincide with the cytoarchitectonically unique cell-sparse region of the ventral lateral complex. This region includes nuclei VPLo, VLc, VLps, X and extensions of these between the cell clusters of nucleus VLo. The inputs from dentate and interpositus are contralateral, dense, and their termination patterns extend continuously throughout all nuclear components of the cell-sparse zone. Interdigitation of these two inputs within the cell-sparse region is directly demonstrated. The fastigial input is more restricted but bilateral. Each of the deep cerebellar nuclei also projects to the central lateral nucleus of the intralaminar complex. The strong interconnection of the cell-sparse zone with cortical area 4 is confirmed. The patterns of retrogradely labeled thalamocortical cells and of anterogradely labeled corticothalamic terminations following cortical injections of horseradish peroxidase and of tritiated amino acids, extend continuously through the VPLo-VLc region and its extensions, but do not invade the posteriorly situated VPLc nucleus. Thalamic inputs from the dorsal column nuclei terminate independently within the morphologically distinct VPLc nucleus adjacent to the cell-sparse cerebellar terminal zone. The dorsal column-lemniscal terminations do not overlap the cerebellar terminations. The clear segregation of the two sets of terminations is demonstrated directly using an anterograde double labeling method. Spinothalamic terminations end in VPLc but extend into the cerebellar terminal zone. Another ascending input, from the vestibular nuclei, is also shown to terminate within the cell-sparse zone. Comparison with other studies implies that cerebellar, pallidal and substantia nigral inputs do not converge in the monkey thalamus and that the nuclei in which they terminate project to different cortical areas. The relation of these and of sensory influences ascending to motor cortex from the periphery are discussed.     

Projections of the cerebellar and dorsal column nuclei upon the thalamus of the rhesus monkey

Projections from the cerebellar and dorsal column nuclei to the midbrain and thalamus of the rhesus monkey were traced with anterograde autoradiographic techniques, or, in a few cases, with the Fink-Heimer method. The cerebellar nuclei give rise to a massive projection to the contralateral midbrain and thalamus via the ascending limb of the superior cerebellar peduncle. Cerebellar efferent fibers terminate contralaterally in both divisions of the red nucleus, and bilaterally in the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the oculomotor nucleus, and the central gray. All the deep cerebellar nuclei project upon a broad area of the contralateral ventral thalamus as well as certain intralaminar nuclei. Corresponding ipsilateral thalamic terminations are sparse. The topograpic organization of cerebellothalamic fibers does not correspond to individual cerebellar nuclei or to cytoarchitectonic divisions of the ventral thalamic nuclei. Rather there are longitudinally oriented strips of terminal labeling which extend through all divisions of the ventral lateral nucleus, i.e., the VLps, the VLc, the VLo, as well as nucleus X, the oral division of the ventral posterolateral nucleus (VPLo), the central lateral nucleus (CL), and the most caudal region of the ventral anterior nucleus (VA). The topography of the cerebellothalamic fibers is arranged in a mediolateral pattern with fibers originating from anterior zones of the dentate and interpositus ending most laterally and those from posterior dentate and interpositus terminating most medially. The fastigial contribution is relatively sparse. The longitudinal strips of terminal labeling in the ventral thalamic nuclei are made up of still smaller terminal units consisting of disk-like aggregates of silver grains separated from one another by grain-free spaces. The dorsal column nuclei terminate primarily in the contralateral caudal division of the VPL (VPLc) and never extend rostrally into VPLo. These results demonstrate a segregation of cerebellar and dorsal columnar inputs to motor and sensory regions of the thalamus, respectively. Since these regions are separate and discrete in their cortical associations as well (Kalil, 1976), it seems unlikely that fast afferent pathways relaying to motor cortex (Lemon and Porter, 1976) could arise from the dorsal column nuclei.     

Fastigial efferent projections in the monkey: an autoradiographic study.

Because fastigial efferent fibers partially decussate within the cerebellum and cerebellar corticovestibular projections pass near, or through, the fastigial nucleus (FN), degeneration studies based on lesions in the nucleus leave unresolved questions concerning fastigial projections. Attempts were made to determine fastigial projections in the monkey using autoradiographic tracing technics. Cells in rostral, caudal and all parts of the FN were labeled with [³H] amino acids. Selective labeling of neurons in either rostral or caudal parts of the FN results in transport of isotope primarily via fibers of the contralateral uncinate fasciculus (UF) and the ipsilateral juxtarestiform body (JRB). Fastigial projections to the vestibular nuclei are mainly to ventral portions of the lateral (LVN) and inferior (IVN) vestibular nuclei, are nearly symmetrical and are quantitatively similar on each side. Fastigiovestibular projections to cell groups f and x arise from all parts of the FN and are mainly crossed; modest projections to the medial vestibular nucleus are uncrossed. No fastigial efferent fibers end in the superior vestibular nucleus on either side, or in dorsal regions of the LVN. Crossed fibers descending in IVN terminate in the nucleus parasolitarius. Fastigioreticular fibers arise predominately from rostral regions of the FN, are entirely crossed and project mainly to: (1) medial regions of the nucleus reticularis gigantocellularis, (2) the dorsal paramedian reticular nucleus and (3) the magnocellular part of the lateral reticular nucleus. Fastigiopontine fibers, emerge with the UF, bypass the vestibular nuclei and terminate upon the contralateral dorsolateral pontine nuclei. Crossed fastigiospinal fibers separate from fastigiopontine fibers and descend in the ventrolateral tegmentum beneath the spinal trigeminal tract; in the medulla and upper cervical spinal cord these fibers are intermingled with those of the vestibulospinal tract. Fastigiospinal fibers terminate in the anterior gray horn at C-1 and probably descend further. Ascending fastigial projections arise from caudal parts of the FN, are entirely crossed and ascend in dorsal parts of the midbrain tegmentum. Label is transported bilaterally to the superior colliculi and the nuclei of the posterior commissure. Contralateral fastigiothalamic projections terminate in the ventral posterolateral (VPLc and VPLo) and in parts of the ventral lateral (VLo) thalamic nuclei. The major region of termination of fastigiothalamic fibers is in VPLo. Fastigiothalamic projections, probably conveying impulses concerned with equilibrium and somatic proprioception, appear to impinge upon thalamic neurons receiving inputs from less specialized receptors that signal information concerning position sense and body movement. More modest fastigial projections to VLo could directly influence activity of neurons in the primary motor cortex.     

Comparison of cerebellothalamic and pallidothalamic projections in the monkey (Macaca fuscata): A double anterograde labeling study

To address the question of segregated projections from the internal segment of the globus pallidus (GPi) and the cerebellar nuclei (Cb) to the thalamus in the monkey, we employed a double anterograde labeling strategy combining the anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) with biotinylated dextran amine (BDA) transport. The tissue was processed sequentially for WGA-HRP, and then BDA immunohistochemistry using two different chromogens. Since the two labels were easily distinguishable on the same histological section, the interrelationship between the cerebellar and pallidal projection systems could be directly evaluated. We found that both the cerebellothalamic and pallidothalamic label consisted of dense plexuses of labeled fibers and swellings in a patch-like configuration. The patches or foci of labeling were distributed either as dense single label or as interdigitating patches of double label. We found dense single label in the central portion of the ventral anterior nucleus pars principalis (VApc) and the ventral lateral nucleus pars oralis (VLo) following the GPi injections or in the central portion of the ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X (X) following the cerebellar nuclei injections. Complementary interdigitating patches of WGA-HRP and BDA labeling were found primarily in transitional border regions between thalamic nuclei. On occasion, we found overlap of both labels. We observed a gradient pattern in the density of the pallidothalamic and cerebellothalamic projections. The pallidothalamic territory included VApc, VLo, and the ventral lateral nucleus pars caudalis (VLc), with the density of these projections decreasing along an anterior to posterior gradient in the thalamus. Occasional patches of pallidal label were found in VPLo and nucleus X. Conversely, the density of cerebellothalamic projections increased along the same gradient, with the cerebellothalamic territory extending anteriorly beyond the cell-sparse zones of VPLo, X, and VLc to include VLo and VApc also. These data suggest that although the cerebellar and pallidal projections primarily occupy separate thalamic territories, individual thalamic nuclei receive differentially weighted inputs from these sources. © 1996 Wiley-Liss, Inc.     

Subcortical projections to the frontal pole in the marmoset monkey

The subcortical projections to the marmoset frontal pole were mapped with the use of fluorescent tracer injections. The main thalamic projections, which originated in both the magnocellular and parvocellular subdivisions of the mediodorsal nucleus, were topographically organized. Our results suggest the existence of a third, caudal subdivision of this nucleus, which is likely to be homologous to the macaque's pars densocellularis. A substantial, but not topographically organized, projection to Brodmann's area 10 originated in the medial part of the ventral anterior nucleus. Minor thalamic projections originated in the medial pulvinar nucleus and in the midline/intralaminar nuclei. Finally, the posterior thalamic group (including the limitans and suprageniculate nuclei) sent a small projection to rostral area 10 that has not previously been documented in primates. The main extrathalamic projections stemmed from the claustrum, which contained as many as 50% of all subcortical labelled neurons. Minor connections originated in the hypothalamus (mainly in the lateral anterior and lateral tuberal regions), dorsal periaqueductal grey matter, basal forebrain (nucleus basalis of Meynert and horizontal limb of the diagonal band of Broca), and amygdala (basal, accessory basal and lateral nuclei). The present results, combined with recent data on the cortical projections to area 10, reveal the frontal pole as a region that integrates information from multiple neural processing systems, including high-level sensory, limbic and working memory-related structures. Although the pattern of subcortical projections is similar to that previously described in the macaque, suggesting a homologous organization, the present data also suggest functional distinctions between medial and lateral sectors of area 10.     

Thalamic projections to areas 3a, 3b,and 4 in the sensorimotor cortex of the mature and infant macaque monkey

Area 3a in the macaque monkey, located in the fundus of the central sulcus, separates motor and somatosensory cortical areas 4 and 3b. The known connections of areas 4 and 3b differ substantially, as does the information which they receive, process, and transfer to other parts of the central nervous system. In this analysis the thalamic projections to each of these three cortical fields were examined and compared by using retrogradely transported fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine and Green latex microspheres) as neuron labels. Coincident labeling of projections to 2–3 cortical sites in each monkey allowed the direct comparison of the soma distributions within the thalamic space of the different neuron populations projecting to areas 3a, 3b, and 4, as well as to boundary zones between these cortical fields. The soma distribution ofthalamic neurons projecting to a small circumscribed zone (diameter = 0.5–1.0 mm) strictly within cortical area 3a (in region of hand representation) filled out a “territory” traversing the dorsal half of the cytoarchitectonically defined thalamic nucleus, VPLc (abbreviations as in Olszewski [1952] The Thalamus of the Macaca mulatta. Basel: Karger). This elongate, rather cylindrical, territory extended caudally into the anterior pulvinar nucleus, but not forward into VPLo. The rostrocaudal extent of the thalamic territory defining the soma distribution of neurons projecting to small zones of cortical area 3b was similar, but typically extended into the ventral part of VPLc, filling out a medially concavo-convex laminar space. Two such territories projecting to adjacent zones of areas 3a and 3b, respectively, overlapped and shared thalamic space, but not thalamic neurons. Contrasting with the 3a and 3b thalamic territories, the soma distribution of thalamic neurons projecting to a circumscribed zone in the nearby motor cortex (area 4) did not penetrate into VPLc, but instead filled out a mediolaterally flattened territory extending from rostral VLo, VLm, VPLo to caudal and dorsal VLc, LP, and Pulo. These territories skirted around VPLc. All three cortical areas (4, 3a, and 3b) also received input from distinctive clusters of cells in the intralaminar Cn.Md. It is inferred that, in combination, the thalamic territories in areas 3a, 3b, and 4 (and also area 1 and 2), which would be coactive during the execution of a manual task, constituted a lamellar space extending from VLo rostrally to Pul.o caudally. How Pul.o neuron populations relate to the more rostral populations within the same thalamic territory projecting to a localized cortical zone remains uncertain. Within the medially located territories the distribution of the neuron population in Pul.o was spatially continuous with the more rostral thalamic cells projecting to the same localized cortex, but in lateral thalamic territories these 2 populations were usually spatially discontinous. In the newborn macaque an orderly change in the territorial projections to localized zones in area 4, 3a, and 3b was also demonstable. However, thalamic nuclear projections were more expansive than in the mature animal. As well as the VPLc input, a third of the thalamic input to area 3a was now from VLo, VPLo, and VLm. Area 4 also had a significant input from VPLc, an input not observed in the mature macaque. © 1993 Wiley-Liss, Inc.     

Primate supplementary eye field. II. Comparative aspects of connections with the thalamus, corpus striatum, and related forebrain nuclei

The supplementary eye field (SEF) was defined electrophysiologically in behaving monkeys to study its connections with the diencephalon and corpus striatum. The specificity of SEF pathways was determined with horseradish peroxidase (HRP) histochemistry to compare its connections with those of the arcuate frontal eye field (FEF), contiguous dorsocaudal area 6 (6DC), and primary motor cortex (M1, arm/hand region). Results indicate that patterns of SEF connectivity were similar to the FEF and markedly different from areas 6DC and M1. Primary reciprocal thalamic pathways of the SEF were with the magnocellular ventral anterior (VA) nucleus, medial parvicellular VA, medial area X, and paralaminar medialis dorsalis (multiformis and parvicellularis). FEF showed similar connections but its most robust pathway was with MD rather than VA. In contrast, area 6DC showed the most extensive reciprocal connections with lateral VApc and lateral area X with only sparse connections with paralaminar MD. Area 6DC also exhibited reciprocal connections with the ventral lateral (VL) complex and the ventral posterior lateral nucleus, pars oralis (VPLo). M1 showed dense bidirectional connections with VPLo, and to a lesser extent, with VL. M1 pathways with the medial dorsal nucleus were negligible. All areas exhibited connections with the paracentral and central lateral nuclei and only M1 lacked connections with the central superior lateral nucleus. SEF and FEF exhibited similar efferent projections to the caudate and putamen. In the caudate, terminal fields were restricted to a central longitudinal core while those from area 6DC were more widely distributed. Eye field efferents were restricted to the putamen's face region while 6DC projections were more exuberant. The arm/hand region of M1 projected to the arm/hand region of the putamen. Pathways are discussed with respect to their significance in oculomotor control.     

Topographic organization of subcortical projections to the anterior thalamic nuclei in the rat.

Subcortical projections to the anterior thalamic nuclei were studied in the rat, with special reference to projections from the mammillary nuclei, by retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The medial mammillary nucleus (MM) projects predominantly ipsilaterally to the entire anterior thalamic nuclei, whereas the lateral mammillary nucleus projects bilaterally to the anterodorsal nucleus (AD) of the anterior thalamic nuclei. A topographic relationship was recognized between the MM and the anterior thalamic nuclei. The dorsal region of the pars mediana of the MM projects to the interanteromedial nucleus (IAM), whereas the ventral region projects to the rostral part of the anteromedial nucleus (AM). The dorsal and the ventral regions of the pars medialis project to the dorsomedial part of the AM at its caudal and rostral levels, respectively. The dorsomedial region of the pars lateralis projects to the ventral AM. The ventrolateral region of the pars lateralis projects to the ventral part of the anteroventral nucleus (AV) in such a manner that rostral cells project rostrally and caudal cells project caudally. The pars basalis projects predominantly ipsilaterally to the dorsolateral AV and bilaterally to the AD. The rostrolateral region of the pars posterior projects to the lateral AV, whereas the medial and the caudal regions of the pars posterior project to the dorsomedial AV. The rostrodorsal part of the nucleus reticularis thalami was found to project to the anterior thalamic nuclei; cells located rostrally in this part project to the IAM and AM, whereas cells located caudodorsally project to the AV and AD. The laterodorsal tegmental nucleus projects predominantly ipsilaterally to the AV, especially to its dorsolateral part. The present study demonstrates that subdivisions of the subcortical structures are connected to the subnuclei of the anterior thalamic nuclei, with a clear-cut topography arranged in the dorsoventral and the rostrocaudal dimensions.     

A PHA-L analysis of ascending projections of the dorsal raphe nucleus in the rat.

Ascending projections from the dorsal raphe nucleus (DR) were examined in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). The majority of labeled fibers from the DR ascended through the forebrain within the medial forebrain bundle. DR fibers were found to terminate heavily in several subcortical as well as cortical sites. The following subcortical nuclei receive dense projections from the DR: ventral regions of the midbrain central gray including the 'supraoculomotor central gray' region, the ventral tegmental area, the substantia nigra-pars compacta, midline and intralaminar nuclei of the thalamus including the posterior paraventricular, the parafascicular, reuniens, rhomboid, intermediodorsal/mediodorsal, and central medial thalamic nuclei, the central, lateral and basolateral nuclei of the amygdala, posteromedial regions of the striatum, the bed nucleus of the stria terminalis, the lateral septal nucleus, the lateral preoptic area, the substantia innominata, the magnocellular preoptic nucleus, the endopiriform nucleus, and the ventral pallidum. The following subcortical nuclei receive moderately dense projections from the DR: the median raphe nucleus, the midbrain reticular formation, the cuneiform/pedunculopontine tegmental area, the retrorubral nucleus, the supramammillary nucleus, the lateral hypothalamus, the paracentral and central lateral intralaminar nuclei of the thalamus, the globus pallidus, the medial preoptic area, the vertical and horizontal limbs of the diagonal band nuclei, the claustrum, the nucleus accumbens, and the olfactory tubercle. The piriform, insular and frontal cortices receive dense projections from the DR; the occipital, entorhinal, perirhinal, frontal orbital, anterior cingulate, and infralimbic cortices, as well as the hippocampal formation, receive moderately dense projections from the DR. Some notable differences were observed in projections from the caudal DR and the rostral DR. For example, the hippocampal formation receives moderately dense projections from the caudal DR and essentially none from the rostral DR. On the other hand, virtually all neocortical regions receive significantly denser projections from the rostral than from the caudal DR. The present results demonstrate that dorsal raphe fibers project significantly throughout widespread regions of the midbrain and forebrain.     

Parabrachial nucleus projections to midline and intralaminar thalamic nuclei of the rat

The projections from the parabrachial nucleus to the midline and intralaminar thalamic nuclei were examined in the rat. Stereotaxic injections of the retrograde tracer cholera toxin-beta (CTb) were made in each of the intralaminar nuclei of the dorsal thalamus (the lateral parafascicular, medial parafascicular, oval paracentral, central lateral, paracentral, and central medial nuclei), as well as the midline thalamic nuclei (the paraventricular, intermediodorsal, mediodorsal, paratenial, rhomboid, reuniens, parvicellular part of the ventral posterior, and caudal ventral medial nuclei). The retrograde cell body labeling pattern within the parabrachial subnuclei was then analyzed. The paracentral thalamic nucleus received an input only from the internal lateral parabrachial subnucleus. However, this subnucleus also projected to all the other intralaminar thalamic nuclei, except for the central lateral thalamic nucleus, which received no parabrachial afferent inputs. The external lateral parabrachial subnucleus projected to the lateral parafascicular, reuniens, central medial, parvicellular part of the ventral posterior, and caudal ventromedial thalamic nuclei. Following CTb injections in the paraventricular thalamic nucleus, retrogradely labeled cells were found in the central lateral, dorsal lateral, and external lateral parabrachial subnuclei. The medial and ventral lateral parabrachial subnuclei projected to the oval paracentral, parafascicular, and rhomboid thalamic nuclei. Finally, the waist area of the parabrachial nucleus was densely labeled after CTb injections in the parvicellular part of the ventral posterior thalamic nucleus. Nociceptive, visceral, and gustatory signals may reach specific cortical and other forebrain sites via this parabrachial-thalamic pathway.     

Importance of the projection from the sensory to the motor cortex for recovery of motor function following partial thalamic lesion in the monkey

Motor deficits produced by thalamic lesions were studied using adult cynomolgus monkeys. Lesioned areas included n. ventralis anterior (VA), ventralis lateralis (VL), n. ventralis posterolateralis pars oralis (VPLo), pars caudalis (VPLc) n. subthalamus (STN) and n. centrum medianum (CM). When the lesion included VA, VL and VPLo, there was a cerebellar syndrome, i.e., ataxia and dysmetria. When the lesion included VPLo and VPLc, the animal was paralyzed. When the lesion included VPLo and rostral part of VPLc, there was loss of orientation in hand movement and clumsiness of finger manipulation. These motor deficits gradually disappeared within 1-2 weeks and the function recovered near to normal except for when VPLo and VPLc were totally destroyed. After recovery of motor function, the somatic sensory cortex (areas 1, 2, 3b) ipsilateral to the thalamic lesion was removed. Removal of the sensory cortex resulted in abolition of the recovered function, but when the border area between VPLo and VPLc was intact, the function recovered again. On the other hand, when the thalamic lesion included this border area, succeeding cortical lesion permanently abolished the recovered function or the reappeared function was substantially worse than that before the cortical lesion. Neuronal mechanisms subserving these differences are discussed and it is concluded that when direct sensory input to the motor cortex was interrupted by lesion of the border area between VPLo and VPLc, the lost function was compensated by reorganization of the projection from the sensory cortex to the motor cortex.     

Differential projections of habenular nuclei

Lesions were made in the lateral and medial habenular nuclei of the cat. Subsequent degeneration of nerve fibers and terminalis was studied using Nauta-Gygax silver technique. The medial and lateral habenular nuclei project differentially to the septum, olfactory, tubercle, thalamus, midbrain tegmentum and tectum. The diffuse part of the habenulopeduncular tract rises from the lateral habenular nucleus and the compact part rises from both nuclei. Degenerating terminals were seen caudally in the following nuclei: interpeduncular, central superior, dorsal raphae, ventral tegmental (from the medial habenular nucleus), dosral tegmental (from the lateral habenular nucleus), pretectal area, superior colliculus and inferior colliculus (from the lateral habenular nucleus). Rostral projections course in the medial part of the stria medullaris from the medial habenular nucleus and in the lateral part of the stria medullaris from the lateral habenular nucleus: Degenerating terminals were seen rostrally in the following nuclei: dorsomedial, anteroventral, anterodorsal, paraventricular, posterior medial septal (from the medial habenular nucleus) and preoptic area (from the lateral habenular nucleus). Projections occur from the medial habenular nucleus to the amygdala via the stria terminalis. The habenular nuclei are considered to be structures of the limbic system which are differentially related to midbrain, thalamic, amygdaloid, septal and preoptic structures via feedback circuits.     

Autoradiographic studies of the projections of the midbrain reticular formation: ascending projections of nucleus cuneiformis.

The ascending projections of the cuneiform nucleus in the cat were traced by autoradiography in the transverse and sagittal planes following stereotaxically placed injections of (3)H-leucine. The ascending fibers are almost exclusively ipsilateral and enter the diencephalon as a wide radiation. At the mesodiencephalic junction fibers enter the nucleus of the posterior commissure and pretectal nuclei, and others cross in the posterior commissure to distribute to these structures on the contralateral side. More ventrally directed fibers distribute to the fields of Forel and then spread into the posterior hypothalamus and zona incerta. At the caudal level of the ventral thalamic group, the ascending fibers diverge and follow two separate courses. One division of fibers continues forward beneath the ventral thalamic group and distributes to the zpna incerta and dorsal hypothalamic area. It rapidly diminishes in size as it attains more rostral levels where it is found in the bed nuclei of the stria terminalis and the anterior commissure. Other fibers of this division spread laterally to innervate the ventral lateral geniculate nucleus, the lateral hypothalamus, and preoptic area, and still others follow the entire confirmation of the thalamic reticular nucleus. The second division of fiber ascends through midline and intralaminar nuclei, completely encircling the mediodorsal nucleus, which is uninnervated except for a small ventral region. The distribution of this division is heaviest to the paraventricular, parafascicular, and central dorsal nuclei. Neither division is conspicuous rostral to the anterior commissure. No projections to neostriatum or specific thalamic nuclei were evident.     

Entopeduncular nucleus projections to the contralateral thalamic nuclei: an HRP study

Neurons of the entopeduncular nucleus projecting to the ventral anterior, ventral lateral and centromedian nuclei of the thalamus were identified on the contralateral, as well as the ipsilateral side, by the retrograde transport of horseradish peroxidase in the cat.     

Subcortical projections to lateral geniculate and thalamic reticular nuclei in the hooded rat

Restricted injections either of horseradish peroxidase conjugated with wheat germ agglutinin, or of unconjugated horseradish peroxidase were made into hooded rats in order to distinguish subcortical sources of afferents to dorsal lateral geniculate nucleus from those to the adjacent visually responsive thalamic reticular nucleus, which modulates geniculate activity. Five “nonvisual” brainstem regions project to the dorsal lateral geniculate nucleus: mesencephalic reticular formation, dorsal raphe nucleus, periaqueductal gray matter, dorsal tegmental nucleus, and locus coeruleus. Projections are generally bilateral, but ipsilateral projections dominate. Of these regions, three also project ipsilaterally to the thalamic reticular nucleus: mesencephalic reticular formation, periaqueductal gray matter, and dorsal tegmental nucleus. Similar discrete injections of horseradish peroxidase into ventral lateral geniculate nucleus allowed a comparison of afferents to dorsal and ventral lateral geniculate nuclei. In addition to the five nonvisual brainstem regions which project to the dorsal division, the ventral lateral geniculate nucleus receives afferents from the perirubral reticular formation and the central gray matter at the thalamic level. The dorsal and ventral lateral geniculate nuclei receive substantially different afferents from subcortical visual centres. The dorsal division receives projections from superior colliculus, pretectum, and parabigeminal nucleus whereas the ventral division receives afferents from superior colliculus, additional pretectal nuclei, lateral terminal nucleus of the accessory optic system, and the contralateral ventral lateral geniculate nucleus.     

Ascending projections to the mammillary nuclei in the rat: a study using retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase

Cells of origin of ascending afferents to the mammillary nuclei and the afferents' fields of termination within these nuclei were studied by using retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase in the rat. The pars compacta of the superior central nucleus projects bilaterally to the median region of the medial mammillary nucleus. The ventral tegmental nucleus projects ipsilaterally to the medial mammillary nucleus, except for its median region, in a topographic manner such that the rostrodorsolateral part of the ventral tegmental nucleus projects to the medial quadrant of the medial mammillary nucleus; the rostroventromedial part projects to the dorsal quadrant; the caudodorsolateral part projects to the ventral quadrant; and the caudoventromedial part projects to the lateral quadrant. These projection fields extend throughout the longitudinal axis of the medial mammillary nucleus, except for its most caudal region, to which only the dorsolateral part of the ventral tegmental nucleus projects. This nucleus also projects topographically to the ipsilateral dorsal premammillary nucleus; the rostral part of the ventral tegmental nucleus projects to the dorsal part of the dorsal premammillary nucleus, whereas the caudal part projects to the ventral part. The periaqueductal gray around the dorsal tegmental nucleus projects bilaterally to the supramammillary nucleus. The pars alpha of the pontine periaqueductal gray projects bilaterally to the peripheral part of the lateral mammillary nucleus, whereas the pars ventralis of the dorsal tegmental nucleus projects ipsilaterally to the lateral mammillary nucleus. The results show that the tegmentomammillary projections are organized in a gradient fashion, with the rostral to caudal position of cells of origin within the tegmental nuclei of Gudden being reflected by the medial to lateral position of fields of termination within the mammillary nuclei.     

Afferent projections to the dorsal and ventral respiratory nuclei in the medulla oblongata of the cat studied by the horseradish peroxidase technique

The central afferent projections to cells in the region of the dorsal and ventral groups of respiratory neurones of the medulla were studied in the cat using the retrograde axonal transport of horseradish peroxidase (HRP).HRP was injected electrophoretically either in the ventrolateral nucleus of the solitary tract (the dorsal group), or into the nucleus ambiguus and retroambigualis (the ventral group). Microelectrode recordings of the activity of the respiratory neurones in these locations were obtained prior to the iontophoretic injections of the enzyme.Projections from the parabrachial region of the pons (nucleus locus coeruleus and subcoeruleus, lateral and medial parabrachial nuclei, Kölliker-Fuse nucleus), nucleus reticularis pontis oralis, retrofacial nucleus, nucleus paragigantocellularis lateralis and ventrolateral nucleus of the solitary tract to the nucleus ambiguus and retroambigualis were identified.The ventrolateral nucleus of the solitary tract was found to receive an input from the retrofacial and lateral paragigantocellular nuclei, and to have strong reciprocal connections with the nucleus paragigantocellularis dorsalis of the medulla upon which pontine Kölliker-Fuse nucleus projects, and some of the primary respiratory and cardiovascular afferences are thought to converge.     

Organization of thalamic projections to the ventral striatum in the primate

Although thalamic projections to the dorsal striatum are well described in primates and other species, little is known about thalamic projections to the ventral or “limbic” striatum in the primate. This study explores the organization of the thalamic projections to the ventral striatum in the primate brain by means of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and Lucifer yellow (LY) retrograde tracer techniques. In addition, because functional and connective differences have been described for the core and shell components of the nucleus accumbens in the rat and are thought to be similar in the primate, this study also explores whether these regions of the nucleus accumbens can be distinguished by their thalamic input. Tracer injections are placed in different portions of the ventral striatum, including the medial and lateral regions of the ventral striatum; the central region of the ventral striatum, including the dorsal part of the core of the nucleus accumbens; and the shell region of the nucleus accumbens. Retrogradely labeled neurons are located mainly in the midline nuclear group (anterior and posterior paraventricular, paratenial, rhomboid, and reuniens thalamic nuclei) and in the parafascicular thalamic nucleus. Additional labeled cells are found in other portions of the intralaminar nuclear group as well as in other thalamic nuclei in the ventral, anterior, medial, lateral, and posterior thalamic nuclear groups. The distribution of labeled cells varies depending on the area of the ventral striatum injected. All regions of the ventral striatum receive strong projections from the midline thalamic nuclei and from the parafascicular nucleus. In addition, the medial region of the ventral striatum receives numerous projections from the central superior lateral nucleus, the magnocellular subdivision of the ventral anterior nucleus, and parts of the mediodorsal nucleus. After injection into the lateral region of the ventral striatum, few labeled neurons are seen scattered in nuclei of the intralaminar and ventral thalamic groups and occasional labeled cells in the mediodorsal nucleus. The central region of the ventral striatum, including the dorsal part of the core of the nucleus accumbens, receives a limited projection from the midline thqlamic, predominantly from the rhomboid nucleus. It receives much smaller projections from the central medial nucleus and the ventral, anterior, and medial thalamic groups. The shell of the nucleus accumbens receives the most limited projection from the thalamus and is innervated almost exclusively by the midline thalamic nuclei and the central medial and parafascicular nuclei. The shell is distinguished from the rest of the ventral striatum in that it receives the fewest projections from the ventral, anterior, medial, and lateral thalamic nuclei. © 1995 Wiley-Liss, Inc.     

Organization of cortical and subcortical projections to area 6m of the cat

By analyzing regional variations of afferent connectivity, we have identified a medial subdivision of feline area 6 (area 6m) which differs from all surrounding sectors of the frontal lobe in its pattern of inputs. Area 6m is located in the ventral bank of the cruciate sulcus and on the adjacent medial face of the frontal lobe and is partially coextensive with the medial frontal eye field as identified previously in electrophysiological experiments. Area 6m is innervated by axons from visual, association, and oculomotor areas and does not receive projections from somesthetic or somatomotor areas. Cortical sources of input to area 6m include several retinotopically organized extrastriate visual areas (AMLS, ALLS, and PLLS), association areas with strong links to the visual system (area 7, granular insula, posterior ectosylvian gyrus, and cingulate gyrus), and a lateral division of area 6 (area 61) with oculomotor functions. Thalamic afferents of area 6m derive from the paralamellar ventral anterior nucleus, from a dorsolateral division of the mediodorsal nucleus, and from the rostral intralaminar nuclei. The claustrum and the basolateral nucleus of the amygdala project to area 6m. Projections from area 7, the posterior cingulate area, the ventral anterior nucleus, and the mediodorsal nucleus are spatially ordered in a pattern such that parts of area 6 close to the fundus of the cruciate sulcus receive input from neurons positioned anteriorly in the cortical areas, dorsolaterally in the ventral anterior nucleus, and ventrolaterally in the mediodorsal nucleus. Our results indicate that area 6m probably is involved in the voluntary control of gaze and attention rather than in skeletomotor functions.