Ipsilateral cortical projections to areas 3a, 3b,and 4 in the macaque monkey

In the macaque monkey area 3a of the cerebral cortex separates area 4, a primary motor cortical field, from somatosensory area 3b, which has a subcortical input mainly from cutaneous mechanoreceptive neurons. That each of these cortical areas has a unique thalamic input was illustrated in the preceding paper. In the present experiments the cortical afferent projections to these 3 areas of the sensorimotor cortex monkey were visualized and compared, using 4 differentiable fluorescent dyes as axonal retrogradely transported labels. The cortical projection patterns to areas 3a, 3b, and 4 were similar in that they each consisted of (a) a “halo” of input from the immediately surrounding cortex, and (b) discrete projections from one or more remote cortical areas. However, the pattern of remote inputs from precentral, mesial, and posterior parietal cortex was different for each of the 3 cortical target areas. The cortical input configuration was least complex for area 3b, its remote input projecting mainly from insular cortex. The pattern of discrete cortical inputs to the motor area 4, however, was more complex, with projections from the cingulate motor area (24c/d), the supplementary motor area, postarcuate cortex, insular cortex, and postcentral areas 2/5. Area 3a, in addition to the proximal projections from the immediately surrounding cortex, also received input from the supplementary motor area, cingulate motor cortex, insular cortex, and areas 2/5. Thus, this pattern of cortical input to area 3a resembled more closely that of the adjacent motor rather than that of the somatosensory area 3b. Contrasting with this, however, the thalamic input to area 3a was largely from somatosensory VPLc (abbreviations from Olszewski [1952] The Thalamus of the Macaca mulatta. Basel: Karger) and not from VPLo (with input from cerebellum, and projecting to precentral motor areas). © 1993 Wiley-Liss, Inc.     

HRP studies on thalamocortical neurons related to the cerebellocerebral projection in the monkey

In monkeys (Macaca fuscata and mulatta), horseradish peroxidase (HRP) was injected in the cerebral cortices, areas 4 (medial and lateral parts), 5, 6 and 9 (a part just rostral to area 6), in which stimulation of cerebellar nuclei was known to elicit superficial or deep thalamocortical responses. Retrogradely labelled thalamic neurons consisted of two separate clusters of neurons, as judged by the continuity of labelled thalamic cells; lateral (VApc, VLo, VLm, VPLo, VPLc, LP) and medial (VAmc, X, Pcn, Cl, Pulo, Pulm, Pull, MD) ones. In each cluster, neurons in the medial portion tend to project on more rostral cerebral cortices than those in the lateral portion. In the lateral cluster, medially and laterally located thalamic neurons tend to project, respectively, on lateral and medial parts of a cortical area. Retrograde and anterograde labelling revealed that thalamocortical and corticothalamic projections are mostly reciprocal except CM which receives massive terminals from area 4 but projects scarcely on to the area. According to the references which have demonstrated terminations of cerebellothalamic neurons, thalamocortical neurons relaying the cerebellocerbral projection are presumed to be included in both the medial and lateral thalamic cell clusters mentioned above. The organization of cerebellothalamocortical projections appears more complicated and elaborated than that reported before.     

Thalamic input to inferior area 6 and area 4 in the macaque monkey

Recent cytoarchitectonic, histochemical, and hodological studies in primates have shown that area 6 is formed by three main sectors: the supplementary motor area, superior area 6, which lies medial to the spur of the arcuate sulcus, and inferior area 6, which is located lateral to it. Inferior area 6 has been further subdivided into two histochemical areas: area F5, located along the inferior limb of the arcuate sulcus, and area F4, located between area F5 and area 4 (area F1). The present study traced the thalamocortical projections of inferior area 6 and the adjacent part of area 4 by injecting small amounts of WGA-HRP in specific sectors of the agranular frontal cortex. Our data showed that each histochemical area receives a large projection from one nucleus of the ventrolateral thalamus (motor thalamus) and additional projections from other nuclei of this thalamic sector. Area F5 receives a large projection from area X of Olszewski ('52) and additional projections from the caudal part of the nucleus ventralis posterior lateralis, pars oralis (VPLo), and the nucleus ventralis lateralis, pars caudalis (VLc) (VPLo-VLc complex). Area F4 receives a large projection from the nucleus ventralis lateralis, pars oralis (VLo), and additional projections from area X and the VPLo-VLc complex. The rostral part of area F1 is innervated chiefly by VLo, plus smaller contributions from rostral VPLo and the VPLo-VLc complex. The caudal part of F1 receives its greatest input from VPLo, with a small contribution from VLo. In addition, each histochemical area receives projections originating from the intralaminar thalamic nuclei, the posterior thalamus, and--for area F4 and area F5--also from the nucleus medialis dorsalis (MD). Analysis of the physiological properties of the various histochemical areas in relation to their main thalamic input showed that those cortical fields in which distal movements are predominant (area F5, caudal part of area F1) are innervated chiefly by area X and VPLo, whereas those cortical fields in which proximal movements are predominant receive their main input from VLo. Because VPLo and area X are targets of cerebellothalamic pathways, whereas VLo receives a pallidal input, we propose that the cortical fields in which distal movements are most heavily represented are mainly under the influence of the cerebellum, whereas the cortical fields in which proximal movements are most heavily represented are mainly under the influence of the basal ganglia.     

Comparing thalamocortical and corticothalamic microstructure and spatial reciprocity in the macaque ventral posterolateral nucleus (VPLc) and medial pulvinar.

The detailed morphology of thalamocortical (TC) and corticothalamic (CT) pathways connecting the ventral posterolateral nucleus (VPLc) with the primary somatosensory cortex (areas 3b and 1) and the thalamic pulvinar with the posterior parietal cortex (primarily area 7a), was compared. Each pathway processes information relevant to directed reaching tasks, but whereas VPLc receives its major input from the spinal cord and external environment, the primary afferent to the pulvinar is cortical. Using combined tracer and thick fixed slice procedures, the soma/dendritic morphology of TC neuron populations (with known destination) was shown to be quantitatively similar within VPLc and the pulvinar. This implies that differences in information processing in VPLc (a primary relay) and the pulvinar (an integrative thalamic nucleus) are not defined by a distinctive TC morphology, but rather by the connections of these neuron populations. Two morphologically distinct types of CT axon were observed within the medial pulvinar and VPLc. The more common "Type E" were fine, had boutons en passant and diffuse terminal bifurcations ending in masses of tiny boutons. "Type R" axons were thicker, smooth, and terminated in localised clusters of large terminal boutons. Each type had a unique pattern of termination reflecting a distinct action on target neuron populations. The spatial relationship between TC distribution territories and CT terminal fields was examined within the medial pulvinar and VPLc by using anterograde and retrograde tracers injected together within cortical areas 7a, and 3b/1, respectively. Spatial overlap was incomplete within both thalamic nuclei. Our findings show a more complex relationship between TC and CT neuron populations than previously demonstrated.     

Thalamic projections to sensorimotor cortex in the macaque monkey: use of multiple retrograde fluorescent tracers.

We used several fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine Latex Microspheres, Evans Blue, and Fluoro-Gold) in each of eight macaques, to examine the patterns of thalamic input to the sensorimotor cortex of macaques 12 months or older. Inputs to different zones of motor, premotor, and postarcuate cortex, supplementary motor area, and areas 3b/1 and 2/5 in the postcentral cortex, were examined. Coincident labeling of thalamocortical neuron populations with different dyes (1) increased the precision with which their soma distributions could be related within thalamic space, and (2) enabled the detection by double labeling, of individual thalamic neurons that were common to the thalamic soma distributions projecting to separate, dye-injected cortical zones. Double-labeled thalamic neurons projecting to sensorimotor cortex were rarely seen in mature macaques, even when the injection sites were only 1-1.5 mm apart, implying that their terminal arborizations were quite restricted horizontally. By contrast, separate neuron populations in each thalamic nucleus with input to sensorimotor cortex projected to more than one cytoarchitecturally distinct cortical area. In ventral posterior lateral (oral) (VPLo), for example, separate populations of cells sent axons to precentral medial, and lateral area 4, medial premotor, and postarcuate cortex, as well as to supplementary motor area. Extensive convergence of thalamic input even to the smallest zones of dye uptake in the cortex (approximately 0.5 mm3) characterized the sensorimotor cortex. The complex forms of these projection territories were explored using 3-dimensional reconstructions from coronal maps. These projection territories, while highly ordered, were not contained by the cytoarchitectonic boundaries of individual thalamic nuclei. Their organization suggests that the integration of the diverse information from spinal cord, cerebellum, and basal ganglia that is needed in the execution of complex sensorimotor tasks begins in the thalamus.     

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.     

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.     

How do the basal ganglia and cerebellum gain access to the cortical motor areas?

We have used retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase to examine the origin of the thalamic input to the two premotor areas with the densest projections to the motor cortex. These are: arcuate premotor area (APA) and the supplementary motor area (SMA). Retrograde transport demonstrated that the two premotor areas and the motor cortex each receive thalamic input from separate, cytoarchitectonically well-defined subdivisions of the ventrolateral thalamus. According to the nomenclature of Olszewski (1952), input to the APA originates largely from area X; input to the SMA originates largely from the pars oralis subdivision of the nucleus ventralis lateralis (VLo); and that to the motor cortex is largely from the pars oralis subdivision of the nucleus ventralis posterior lateralis (VPLo). These observations, when combined with prior studies on the termination of various subcortical efferents in the thalamus, lead to the following scheme of projections: rostral portions of the deep cerebellar nuclei project to motor cortex via VPLo, caudal portions of the deep cerebellar nuclei project to the APA via area X; and the globus pallidus projects to the SMA via VLo. Thus each thalamocortical pathway is associated with a distinct subcortical input.     

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.     

Brainstem and spinal projections of the deep cerebellar nuclei in the monkey, with observations on the brainstem projections of the dorsal column nuclei

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.     

Spatial distribution of thalamic projections to the supplementary motor area and the primary motor cortex: A retrograde multiple labeling study in the macaque monkey

The exact knowledge on spatial organization of information sources from the thalamus to the supplementary motor area (SMA) and to the primary motor cortex (MI) has not been established. We investigated the distribution of thalamocortical neurons projecting to forelimb representations of the SMA and the MI using a multiple retrograde labeling technique in the monkey. The forelimb area of the SMA, and the distal and proximal forelimb areas of the MI were identified by electrophysiological techniques of intracortical microstimulation and single neuron recording. Injections were made into these three representations with three different dyes in the same animal (horseradish peroxidase conjugated to wheat germ agglutinin, diamidino yellow, and fast blue), and the thalamic neurons were retrogradely labeled. Injections into the SMA densely labeled thalamic neurons in nuclei ventralis lateralis pars oralis (VLo), ventralis lateralis pars medialis (VLm) and ventralis lateralis pars caudalis (VLc), but not in nucleus ventralis posterior lateralis pars oralis (VPLo). Injections into the MI labeled thalamic neurons primarily in VLo, VLc, and VPLo. We found that the distribution of projection neurons to the three areas was largely separate in the thalamus. However, in the middle part of VLo, and in a limited portion of VLc, thalamic neurons projecting to the SMA partially overlapped with those to the distal forelimb area of the MI. They overlapped little with those to the proximal forelimb area of the MI. We noted no overlap between the distributions of thalamic projection neurons to the distal and proximal forelimb areas of the MI. These findings suggest that the SMA and MI receive separate information from the thalamus, while sharing minor sources of common inputs. © 1995 Wiley-Liss, Inc.     

Thalamic input to mesial and superior area 6 in the macaque monkey

The aim of the present study was to define the origin of the thalamocortical projections to each of the mesial and superior area 6 areas. To this purpose, restricted injections of neuronal tracers were made into areas F3, F6, F2, and F7 after physiological identification of the injection sites. The results showed that each of these areas receives afferents from a set of thalamic nuclei and that this set differs, qualitatively and quantitatively, according to the injected area. The main inputs to F3 [supplementary motor area properly defined (SMA-proper)] originate in the nuclei ventral lateral, pars oralis (VLo), ventral posterior lateral, pars oralis (VPLo), and ventral lateral, pars caudalis (VLc) as well as in caudal parts of the VPLo and VLc (VPLo/VLc complex). F6 (pre-SMA) is mainly the target of nucleus ventral anterior, pars parvocellularis (VApc), and area X of Olszewski. The input to F2 originates mainly in the VPLo/VLc complex, in VLc, and in VLo. The dorsal part of F7 (supplementary eye field) mainly receives from area X, VApc, and nucleus ventral anterior, pars magnocellularis (VAmc), whereas the ventral F7 is connected with VApc, area X, VLc, and the VPLo/VLc complex. All of the injected areas receive a strong projection from the medial dorsal nucleus (MD). It is concluded that each cortical area is a target of both cerebellar and basal ganglia circuits. F3 and F2 are targets of the so-called “motor” basal ganglia circuit and a cerebellar circuit originating in dorsorostral sectors of dentate and interpositus nuclei. In contrast, F6 and ventral F7 receive a basal ganglia input mainly from the so-called “complex” circuit and a cerebellar input originating in the ventrocaudal sectors of dentate and interpositus nuclei. Finally, with respect to the rest of F7, dorsal F7 also receives a basal ganglia input from the “oculomotor circuit.” © 1996 Wiley-Liss, Inc.     

The origin of thalamic inputs to the arcuate premotor and supplementary motor areas

We have used retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase to examine the origin of thalamic input to the two premotor areas with the densest projections to the motor cortex: the arcuate premotor area (APA) and the supplementary motor area (SMA). Retrograde transport demonstrated that the two premotor areas and the motor cortex each receive thalamic input from separate, cytoarchitectonically well defined subdivisions of the ventrolateral thalamus. According to the nomenclature of Olszewski (Olszewski, J. (1952) The Thalamus of the Macaca mulatta. An Atlas for Use with the Stereotaxic Instrument, S. Karger, AG, Basel), input to the APA originates largely from area X, input to the SMA originates largely from the pars oralis subdivision of nucleus ventralis lateralis (VLo), and that to the motor cortex originates largely from the pars oralis subdivision of nucleus ventralis posterior lateralis (VPLo). These observations, when combined with prior studies on the termination of various subcortical efferents in the thalamus, lead to the following scheme of connections: (1) rostral portions of the deep cerebellar nuclei project to the motor cortex via VPLo; (2) caudal portions of the deep cerebellar nuclei project to the APA via area X; and (3) the globus pallidus projects to the SMA via VLo. Thus, each thalamocortical pathway is associated with a distinct subcortical input.     

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.     

Thalamic connections of the vestibular cortical fields in the squirrel monkey (Saimiri sciureus).

The afferent thalamic connections to cortical fields important for control of head movement in space were analysed by intracortical retrograde tracer injections. The proprioceptive/vestibular area 3aV, the neck-trunk region of area 3a, receives two thirds of its thalamic projections from the oral and superior ventroposterior nucleus (VPO/VPS), which is considered as the proprioceptive relay of the ventroposterior complex (Kaas et al., J. Comp. Neurol. 226:211-240, 1984). The parieto-insular vestibular cortex (PIVC, area retroinsularis, Ri) receives its main thalamic input from posterior parts of the ventroposterior complex and from the medial pulvinar. Anatomical evidence is presented that the posterior region of the ventroposterior complex is a special compartment within this principal somatosensory relay complex. The parietotemporal association area T3, mainly involved in visual-optokinetic signal processing, receives a substantial input from the medial, the lateral, and the inferior pulvinar. Dual tracer experiments revealed that about 5% of the thalamic neurons projecting to 3aV were spatially intermingled with neurons projecting to areas PIVC or T3. This spatial intermingling was distributed over small but numerous, circumscribed thalamic regions, called "common patches," which were found mainly in the intralaminar nuclei, the posterior group of thalamic nuclei, and the caudal parts of the ventroposterior complex. The "common patches" may indicate a functional coupling of area 3aV with the PIVC or area T3 on the thalamic level. In control experiments thalamic projections to the granular insula Ig and the anterior part of area 7, two cerebral structures connected with the vestibular cortical areas, were studied. Some overlap in the thalamic relay structures projecting to these areas with those projecting to the vestibular cortices was found. A quantitative evaluation of thalamic regions projecting to different cortical structures was performed by constructing so-called "thalamograms." A scheme was developed that describes the afferent thalamic connections by which vestibular, visual-optokinetic, and proprioceptive signals reach the vestibular cortical areas PIVC and 3aV.     

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.     

Sagittal cytoarchitectonic maps of the Macaca mulatta thalamus with a revised nomenclature of the motor-related nuclei validated by observations on their connectivity

Cytoarchitectonic atlas plates of the Macaca mulatta thalamus are presented in the sagittal plane of section with a revised nomenclature of the motor thalamic region. The proposed changes in nomenclature are based on the analysis of topographical relationships between nigral, pallidal, and cerebellar projections to the thalamus studied in 13 rhesus monkeys with the use of autoradiography technique. Mapping of the projection zones of these motor-related systems in serial sagittal sections revealed that they are completely segregated with each honoring cytoarchitectonic boundaries of specific nuclear subdivisions. The available data on thalamic connectivity together with the results of the present study allowed us to divide the primate "motor" thalamus into two major territories: (1) the ventral anterior region (VA) and (2) the ventral lateral region (VL). Although the designation of these two areas of the motor thalamus is the same as the classic one, the nuclear subdivisions that compose them differ significantly from those described in previous classifications. As is delineated in the maps, VA represents the basal ganglia territory of the motor thalamus where nigral projections coincide with its magnocellular part (VAmc), and pallidal projections occupy densicellular (VAdc) and parvicellular (VApc) subdivisions. VAdc corresponds closely to VLo of Olszewski; however, we prefer the new term in order to avoid possible conceptual confusions with the ventral lateral region (VL), which does not receive basal ganglia projections. The VL region is characterized as a distinct cytoarchitectonic entity of the motor thalamus that receives cerebellar projections and includes area X, VPLo, VLc, and VLps of Olszewski. The ventral medial region (VM in the present study or VLm in Olszewski terminology) is usually considered together with the basal ganglia territory on a common connectional basis. However, we did not obtain convincing data to support this view, since evidence of terminal labeling was observed only in (or around) fiber bundles passing through the nucleus with other areas free of label. Rather, in this study VM was treated as an intermediate zone between the subthalamus and motor thalamus where fiber bundles from basal ganglia and cerebellum are organized in a topographical manner before reaching their destinations in the VA and VL regions, respectively. Other major thalamic regions represented in the maps were delineated purely on cytoarchitectonic grounds and their traditional nomenclature was maintained.(ABSTRACT TRUNCATED AT 400 WORDS)     

The origin of thalamic inputs to the "hand" representation in the primary motor cortex.

We used retrograde transport of WGA-HRP to examine the origin of thalamic inputs to the "hand" representation in the primary motor cortex of macaques (Macaca nemestrina). Injections were placed in either the crest of the precentral gyrus or the rostral bank of the central sulcus. The sites for injection in the sulcus were determined by using intracortical stimulation to map the location of hand representation. We found that the precentral gyrus and central sulcus receive their predominant input from different subdivisions of the ventrolateral thalamus. Ventralis posterior lateralis pars oralis (VPLo) provides the most substantial input to a portion of the hand representation on the gyrus. In contrast, Ventralis lateralis pars oralis (VLo) provides the most substantial input to a portion of the hand representation in the sulcus. Prior studies have shown that VPLo is a major site of termination of cerebellar efferents and that VLo is a major site of termination of pallidal efferents. Thus, our results indicate that both the basal ganglia and the cerebellum "directly" influence the "hand" representation of the primary motor cortex.     

Projections of auditory cortex to the medial geniculate body of the cat

The corticofugal projection from 12 auditory cortical fields onto the medial geniculate body was investigated in adult cats by using wheat germ agglutinin conjugated to horseradish peroxidase or biotinylated dextran amines. The chief goals were to determine the degree of divergence from single cortical fields, the pattern of convergence from several fields onto a single nucleus, the extent of reciprocal relations between corticothalamic and thalamocortical connections, and to contrast and compare the patterns of auditory corticogeniculate projections with corticofugal input to the inferior colliculus. The main findings were that (1) single areas showed a wide range of divergence, projecting to as few as 5, and to as many as 15, thalamic nuclei; (2) most nuclei received projections from approximately five cortical areas, whereas others were the target of as few as three areas; (3) there was global corticothalamic-thalamocortical reciprocity in every experiment, and there were also significant instances of nonreciprocal projections, with the corticothalamic input often more extensive; (4) the corticothalamic projection was far stronger and more divergent than the corticocollicular projection from the same areas, suggesting that the thalamus and the inferior colliculus receive differential degrees of corticofugal control; (5) cochleotopically organized areas had fewer corticothalamic projections than fields in which tonotopy was not a primary feature; and (6) all corticothalamic projections were topographic, focal, and clustered, indicating that areas with limited cochleotopic organization still have some internal spatial arrangement. The areas with the most divergent corticothalamic projections were polysensory regions in the posterior ectosylvian gyrus. The projection patterns were indistinguishable for the two tracers. These findings suggest that every auditory thalamic nucleus is under some degree of descending control. Many of the projections preserve the relations between cochleotopically organized thalamic and auditory areas, and suggest topographic relations between nontonotopic areas and nuclei. The collective size of the corticothalamic system suggests that both lemniscal and extralemniscal auditory thalamic nuclei receive significant corticofugal input.     

Organization of corticothalamic projections from parietal cortex in cat

Corticothalamic projections from areas 5a, 5b, and 7 of cat parietal cortex were studied with autoradiographic techniques. Each cortical area was identified by its cytoarchitectural characteristics and the patterns of termination were related to the thalamic nuclear groups. Injections of 3H-leucine in cortical area 5a were associated with terminal labeling primarily in the spinal recipient zone of the ventral lateral nucleus (VLsp) and the medial division of the posterior group (POm). The corticothalamic projections of area 5a are loosely topographically organized; medial parts of 5a project heavily to rostral and lateral parts of VLsp and sparsely to POm, while lateral parts of 5a project to more medial and caudal parts of VLsp and heavily to POm. Cortical area 5b projects primarily to the rostral portions of the lateral posterior nucleus (LP). These projections also appear to be topographically organized. The part of area 5b on the marginal gyrus projects to more ventral parts of rostral LP, while area 5b on the middle suprasylvian gyrus projects to more dorsal and lateral parts of rostral LP. Cortical area 7 projects to LP and the pulvinar (Pul). Rostral parts of area 7 project heavily to dorsal and lateral parts of LP and lightly to Pul; more caudal portions of area 7 projects relatively more heavily to Pul. The reticular, central lateral, and paracentral nuclei also receive projections, especially from the suprasylvian gyrus. The results are discussed with regard to putative sensory response characteristics of these cortical areas and to general thalamocortical organization.