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PubMed1996

Transneuronal changes of the inhibitory circuitry in the macaque somatosensory thalamus following lesions of the dorsal column nuclei.

The inhibitory circuitry of the ventroposterolateral nucleus (VPL) of the macaque somatosensory thalamus was analyzed in normal animals and in those surviving for a few days or several weeks following a unilateral lesion of the cuneate nucleus, the source of medial lemniscal (ML) axons carrying information from the contralateral upper extremity. Inhibitory synaptic terminals in the VPL were defined as those that contain flattened or pleomorphic synaptic vesicles and that can be shown to be immunoreactive for gamma-aminobutyric acid (GABA). There are two types of these profiles: F axon terminals that arise from neurons of the thalamic reticular nucleus, and perhaps from VPL local circuit neurons (LCNs); and the dendritic appendages of LCNs that form presynaptic dendrites (PSDs). ML terminals normally have extensive synaptic interactions with PSDs but not with F axon terminals. Electron microscopic analyses revealed that cuneatus lesions resulted in a rapid loss of ML terminals and a statistically significant reduction in both F and PSD synaptic profiles. Confocal scanning microscopy also demonstrated a profound loss of GABA immunoreactivity in the deafferented VPL. These changes persisted for more than 20 weeks, without any evidence of reactive synaptogenesis of surviving sensory afferents or of inhibitory synapses. The changes in GABA circuitry are transneuronal, and the possible mechanisms that may underlie them are discussed. It is suggested that the altered GABAergic circuitry of the VPL in the monkey may serve as a model for understanding changes in somatic sensation in the human following peripheral or central deafferentation.

Afferent Pathways↗
PubMed1997

Insular cortex and neighboring fields in the cat: a redefinition based on cortical microarchitecture and connections with the thalamus.

The insular areas of the cerebral cortex in carnivores remain vaguely defined and fragmentarily characterized. We have examined the cortical microarchitecture and thalamic connections of the insular region in cats, as a part of a broader study aimed to clarify their subdivisions, functional affiliations, and eventual similarities with other mammals. We report that cortical areas, which resemble the insular fields of other mammals, are located in the cat's orbital gyrus and anterior rhinal sulcus. Our data suggest four such areas: (a) a "ventral agranular insular area" in the lower bank of the anterior rhinal sulcus, architectonically transitional between iso- and allocortex and sparsely connected to the thalamus, mainly with midline nuclei; (b) a "dorsal agranular insular area" in the upper bank of the anterior rhinal sulcus, linked to the mediodorsal, ventromedial, parafascicular and midline nuclei; (c) a "dysgranular insular area" in the anteroventral half of the orbital gyrus, characterized by its connections with gustatory and viscerosensory portions of the ventroposterior complex and with the ventrolateral nucleus; and (d) a "granular insular area", dorsocaudal in the orbital gyrus, which is chiefly bound to spinothalamic-recipient thalamic nuclei such as the posterior medial and the ventroposterior inferior. Three further fields are situated caudally to the insular areas. The anterior sylvian gyrus and dorsal lip of the pseudosylvian sulcus, which we designate "anterior sylvian area", is connected to the ventromedial, suprageniculate, and lateralis medialis nuclei. The fundus and ventral bank of the pseudosylvian sulcus, or "parainsular area", is associated with caudal portions of the medial geniculate complex. The rostral part of the ventral bank of the anterior ectosylvian sulcus, referred to as "ventral anterior ectosylvian area", is heavily interconnected with the lateral posterior-pulvinar complex and the ventromedial nucleus. Present results reveal that these areas interact with a wide array of sensory, motor, and limbic thalamic nuclei. In addition, these data provide a consistent basis for comparisons with cortical fields in other mammals.

Animals↗
PubMed1997

Multiarchitectonic and stereotactic atlas of the human thalamus.

To improve anatomical definition and stereotactic precision of thalamic targets in neurosurgical treatments of chronic functional disorders, a new atlas of the human thalamus has been developed. This atlas is based on multiarchitectonic parcellation in sections parallel or perpendicular to the standard intercommissural reference plane. The calcium-binding proteins parvalbumin (PV), calbindin D-28K (CB), and calretinin (CR) were used as neurochemical markers to further characterize thalamic nuclei and delimit subterritories of functional significance for stereotactic explorations. Their overall distribution reveals a subcompartmentalization of thalamic nuclei into several groups. Predominant PV immunostaining characterizes primary somatosensory, visual and auditory nuclei, the ventral lateral posterior nucleus, reticular nucleus (R), and to a lesser degree also, lateral part of the centre median nucleus, and anterior, lateral, and inferior divisions of the pulvinar complex. In contrast, CB immunoreactivity is prevalent in medial thalamic nuclei (intralaminar and midline), the posterior complex, ventral posterior inferior nucleus, the ventral lateral anterior nucleus, ventral anterior, and ventral medial nuclei. The complementary distributions of PV and CB appear to correlate with distinct lemniscal and spinothalamic somatosensory pathways and to cerebellar and pallidal motor territories, respectively. Calretinin, while overlapping with CB in medial thalamic territories, is also expressed in R and limbic associated anterior group nuclei that contain little or no CB. Preliminary analysis indicates that interindividual nuclear variations cannot easily be taken into account by standardization procedures. Nevertheless, some corrections in antero-posterior coordinates in relation to different intercommissural distances are proposed.

Adult↗
PubMed2000

Opiate receptor avidity in the thalamus is sexually dimorphic in the elderly.

Opiate receptor avidity (B(')(max)/K(D)), was measured in the subcortex of nine females (five healthy subjects, four Alzheimer patients) and 15 males (seven healthy subjects, eight Alzheimer patients), 51-75 years of age, with the opiate receptor antagonist 6-deoxy-6-beta-[(18)F]fluoronaltrexone (cyclofoxy, CF) and a positron emission tomograph. CF avidity was 27.5% less in the thalamus of healthy women compared to healthy men and 48.5% less in Alzheimer disease female patients compared to male patients.

Aged↗
PubMed2004

Glucose hypometabolism of hypothalamus and thalamus in narcolepsy.

It has been hypothesized that hypothalamus is involved in narcolepsy. The relative difference between cerebral glucose metabolism of 24 narcoleptic patients and 24 normal controls was studied using 18F-fluorodeoxy glucose positron emission tomography. Patients with narcolepsy showed significantly reduced cerebral glucose metabolism in bilateral rectal and subcallosal gyri, the medial convexity of right superior frontal gyrus, bilateral precuneus, right inferior parietal lobule, and in left supramarginal gyrus (uncorrected p < 0.001). Bilateral posterior hypothalami and mediodorsal thalamic nuclei showed hypometabolism with significance at the level of corrected p < 0.05, with small volume correction. This study showed cerebral glucose hypometabolism of the hypothalamus-thalamus-orbitofrontal pathways in the narcoleptic brain.

Adolescent↗
PubMed1987

Magnetic resonance imaging in multiple sclerosis: decreased signal in thalamus and putamen.

High-field strength (1.5 Tesla) magnetic resonance imaging in 15 patients with multiple and extensive white-matter lesions and clinically definite multiple sclerosis delineated a previously undescribed finding of abnormally decreased signal intensity on T2-weighted images in the thalamus and putamen. The decreased signal intensity (preferential decreased T2 relaxation time) is most likely to be related to abnormally increased iron accumulation causing local magnetic field heterogeneities.

Humans↗
PubMed2002

Distribution and morphology of nigral axons projecting to the thalamus in primates.

This study presents an analysis of the distribution and organization pattern of axons originating from the substantia nigra pars reticulata and projecting to the thalamus in monkeys. Biotin dextran amine was iontophoretically injected into different parts of the substantia nigra pars reticulata of monkeys (African green monkeys and macaques). In whatever part of the substantia nigra the injection was made, numerous axonal endings were found to be distributed within different thalamic regions: the ventral anterior nucleus and mainly its magnocellular part, the most ventromedial part of the ventral lateral nucleus, and the mediodorsal and parafascicular nuclei. Moreover, the nigrothalamic projection appeared to be topographically organized. Ten anterogradely labeled axons were reconstructed from serial sections. The axons terminated in three to six terminal fields per axon located in a relatively small portion of only one thalamic region. These terminal fields were variable in size and comprised 4-43 very thin, varicose branches. They consisted either of different axonal branches of the same axon or of different axons and covered 10-31 thalamic cell bodies. These findings demonstrate that the overall morphological organization of individual nigral axons is complex and allows single axons to influence thalamic neurons via a combination of divergent, convergent, and amplification processes.

Animals↗
PubMed2004

Cortical connections of the lateral mediodorsal thalamus in cynomolgus monkeys.

The prefrontal cortex has been defined as that cortical territory that has "essential or sustaining" connections with the mediodorsal (MD) nucleus of the thalamus. However, recent studies in the monkey have documented projections from MD to the more caudal, agranular regions of the frontal cortex, suggesting that the connections of MD may be characterized by a breadth of distribution and diversity of functional roles too great to be useful as a unifying and defining feature for a specific cortical territory. In this study, we placed tracer injections in the lateral divisions of MD in cynomolgus monkeys (Macaca fascicularis) to assess the relative proportions of connections devoted to diverse regions of the frontal cortex (FC). Three different patterns of label were observed in the cortex, associated with different locations within lateral MD. We have designated these as the ventrolateral MD-arcuate FC circuit, having most label in areas 8 and 6; the caudoventral MD-dorsomedial FC circuit, having most label in areas 24 and presupplementary motor area (SMA); and the anterodorsal MD-anterior FC circuit, with the most label in areas 9, 46, 12, and 10. Only two of the nine cases injected in lateral MD were predominantly connected with the anterior FC. Thus, particular locales within lateral MD are connected with multiple, functionally diverse cortical regions, including several not classically recognized as "prefrontal" areas. This divergence may distinguish MD-frontocortical and reciprocal corticothalamic pathways from the largely segregated pathways arising from the other thalamic nuclei that are interconnected with the frontal cortex, such as those from the ventrolateral nuclear group.

Animals↗
PubMed1978

An autoradiographic study of the projections of visual cortical area 1 to the thalamus, pretectum and superior colliculus of the rabbit.

The projections of visual cortical area 1 (vl) to the thalamus, pretectum and superior colliculus of the rabbit have been studied by Giolli and Guthrie ('67, '71) using the Nauta and Fink-Heimer methods to determine the course and distribution of degenerating nerve fibers. The present study represents a reinvestigation of these same projections utilizing the tracing method of autoradiography. An injection of 3H leucine was produced within a small region of vl in each of 18 adult albino rabbits, and the brains were subsequently processed for autoradiography by the method of Cowan et al. ('72). The results have confirmed the observations of Giolli and Guthrie ('67, '71) (1) by showing that vl of the rabbit projects to the thalamic reticular nucleus, the ventral lateral geniculate nucleus, the dorsal lateral geniculate nucleus; the pulvinar, the anterior and posterior pretectal nuclei and the superior colliculus and (2) by showing that a particular retinotopic organization is present in each of these projections. However, unlike Giolli and Guthrie ('67, '71), the present autoradiographic study has further revealed (1) that both the ventrolateral and the posterior thalamic nuclei receive inputs from vl and (2) that the nucleus of the optic tract is not innervated by axons originating from vl.

Animals↗
PubMed1978

A comparison of projections of entopeduncular neurons to the thalamus, the midbrain and the habenula in the cat.

Anatomical studies have demonstrated that the output of the striopallidal system is distributed to two areas of the thalamus: the ventrolateral-ventroanterior and the centromedian nuclei. The two areas are involved in different ways in the control of somatic motor activity. Pallidal efferents are also distributed to a still obscure tegmental area in the midbrain, the pedunculopontine nucleus, and to the lateral habenular nucleus, a structure of the limbic system. The present study compares the projections of entopeduncular neurons to the four sites in cats. The comparison is based on an estimation of the number of entopeduncular neurons sending fibers to each site and branching to more than one site. The four projection sites were stimulated electrically in anesthetized cats and the number of entopeduncular neurons excited antidromically were counted. At least 68% of entopeduncular neurons were excited antidromically by stimulation of the ventrolateral nucleus, an equal number were excited antidromically by stimulation of the nucleus centromedian and slightly fewer but still more than 50% by stimulation of the pedunculopontine nucleus. The three sites gave rise to antidromic responses of the same entopeduncular neuron in at least 33% of the cases. Only 25% of entopeduncular neurons responded antidromically to stimulation of the lateral habenular nucleus exclusively (one-third) or not (two-thirds). Some neurons recorded incidentally in the globus pallidus responded antidromically to the stimulation sites. Neurons were also recorded in the preoptico-hypothalamic area and 67% responded antidromically exclusively to the stimulation of the lateral habenular nucleus.

Action Potentials↗
PubMed1979

A golgi study of the class V cell in the visual thalamus of the cat.

Golgi methods were used to study class V cells within the cat visual thalamus. Counterstaining was combined with Golgi staining to assess the distribution of dendrites relative to cytoarchitectural boundaries. Class V cells were encountered within all laminae of the lateral geniculate nucleus, the medial interlaminar nucleus, and the lateral posterior complex. The cells possess medium-sized perikarya and smooth and varicose or moniliform dendrites. Dendritic appendages are sparse and occur as single or serial swellings on thin processes. Many class V cells exhibit large, sparse dendritic arbors which span laminar or nuclear borders; dendrites were seen to lie within and to cross the interlaminar zones of the lateral geniculate nucleus, and extend beyond this nucleus into the perigeniculate nucleus and medial interlaminar nucleus. Class V cells of the lateral posterior complex send dendrites into the external medullary lamina. Indirect evidence favors the interpretation that the class V cells are thalamo-cortical relay cells.

Animals↗
PubMed1987

Demarcations of the mechanosensory projection zones in the raccoon thalamus, shown by cytochrome oxidase, acetylcholinesterase, and Nissl stains.

To determine anatomically the boundaries and internal organization of the kinesthetic and cutaneous mechanosensory regions of the ventrobasal thalamus, alternate section series from electrophysiologically mapped tissues from 14 raccoons were stained for cytochrome oxidase, myelinated fibers, acetylcholinesterase, and Nissl substance. Microelectrode tracks, along with electrolytic lesions placed as tissue markers, reveal that the mechanoreceptor projection zones have higher cytochrome oxidase and lower acetylcholinesterase staining than some neighboring regions. Both these enzymatic stains reveal particularly sharp boundaries separating the mechanoresponsive region, from the lateral posterior nucleus dorsally and from the ventroposterior inferior nucleus ventrally. The kinesthetic projection zone is often separated from other mechanoreceptor projections by bundles as well as laminae of myelinated fibers, similar to those separating cutaneous projections from distinct body parts. These subdivisions are particularly well marked by the cytochrome oxidase stain. The combination, in neighboring sections, of the use of the several stains adds considerably to the visible delineation of these functionally distinct regions, beyond what can be seen in Nissl-stained sections.

Acetylcholinesterase↗
PubMed1988

Vestibular projections to the thalamus of the pigeon: an anatomical study.

In a series of retrograde tracing studies involving the injection of WGA-HRP into the thalamus of the pigeon, labeled neurons were consistently observed in anterior regions of the vestibular nuclei. Following small dorsal thalamic injections, labeled neurons were located predominantly in rostroventrolateral regions of the superior vestibular nucleus, less numerously within the ventral part of the lateral vestibular nucleus, and least numerously within the medial vestibular nucleus. Following large dorsal thalamic injections, many more vestibular neurons were labeled, and these were distributed more extensively throughout anterior parts of the superior, lateral, and medial nuclei. No labeled neurons were found in the descending nucleus. Injections of tritiated amino acids into vestibular nuclei revealed a terminal field within the dorsal thalamic nucleus: dorsolateralis posterior, pars rostralis. The location of this field between auditory, somatosensory, and paleostriatally and neostriatally projecting nuclei suggests a general similarity to the organization of vestibulothalamic projections in mammals.

Animals↗
PubMed1988

A cytoarchitectonic atlas of the medial geniculate body of the opossum, Didelphys virginiana, with a comment on the posterior intralaminar nuclei of the thalamus.

The organization of the medial geniculate body and adjacent posterior thalamus of the Virginia opossum was studied in Nissl-, Golgi-, reduced silver, and myelin-stained preparations. Our chief goals were to define the cytoarchitectonic subdivisions and boundaries in Nissl preparations and to reconcile these with those observed with the Golgi method and in experimental material, to present these results in an atlas of Nissl-stained sections, and to compare the chief nuclear groups in the opossum and the cat medial geniculate body. In the opossum, the ventral division consists chiefly of the ventral nucleus. The ventral nucleus is divided into two main parts: the pars lateralis and the pars ovoidea, the former being relatively smaller in the opossum. The ventral nucleus of both species contains large principal neurons with bushy, tufted dendrites and smaller Golgi type II cells. However, the opossum has far fewer Golgi type II cells, and the texture of the neuropil is correspondingly different, although the primary ascending input from the midbrain arises from the central nucleus of the inferior colliculus in both species. The dorsal division consists of the dorsal nuclei, including the suprageniculate nucleus and the caudal part of the lateral posterior nucleus, the marginal zone, and the posterior limitans nucleus. These nuclei are identified in both species, although they are much smaller in the opossum. The neurons consist of medium-size and small somata with a predominantly radiate mode of dendritic branching and a lower cell concentration than in the ventral division. In both species the afferent brain stem input comes from the inferior colliculus, the lateral tegmental area, the intercollicular tegmentum, and the superior colliculus. The medial division contains several types of cells, which are heterogeneous in form and size, most having radiating dendrites and a low cellular concentration. This division is especially smaller in the opossum, although comparable inputs arise from various auditory and non-auditory sources in the midbrain and spinal cord in both species. A large intralaminar complex of nuclei occurs in the opossum, which have a more extensive distribution than previously appreciated. They not only occupy the intramedullary laminae but form a shell around the medial geniculate nuclei and adjoining main sensory nuclei. The intralaminar complex includes the posterior limitans, posterior intralaminar, posterior, parafascicular, posterior parafascicular, central intralaminar, limitans, and central medial nuclei, and the marginal zone of the medial geniculate body.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Development of the rat thalamus: II. Time and site of origin and settling pattern of neurons derived from the anterior lobule of the thalamic neuroepithelium.

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, and neuroepithelial site of origin of the anterior thalamic nuclei--the lateral dorsal (lateral anterior), anterodorsal, anteroventral and anteromedial nuclei--and of two rostral midline structures--the anterior paraventricular and paratenial nuclei. The neurons of the lateral dorsal nucleus are generated over a 3-day period between days E14-E16 and their settling pattern displays a combined lateral-to-medial and dorsal-to-ventral neurogenetic gradient. The bulk of the neurons of the anteroventral nucleus are generated over a 3-day period between days E15-E17 and settle with an oblique lateral-to-medial and ventral-to-dorsal neurogenetic gradient. The bulk of the neurons of the anteromedial nucleus are generated over a 2-day period between days E16-E17 and show the same settling pattern as the anteroventral nucleus. The neurons of the anterodorsal nucleus are generated over a 3-day period between days E15-E17 and show a lateral-to-medial neurogenetic gradient. The bulk of the neurons of the central part and lateral part of the paraventricular nucleus are generated over a 2-day period (E16-E17 and E17-E18, respectively) and each part displays a ventral-to-dorsal neurogenetic gradient. Finally, the bulk of the neurons of the paratenial nucleus are generated over a 4-day period between days E15-E18 and settle with a lateral-to-medial neurogenetic gradient. Observations are presented that the anterior thalamic nuclei, constituting the distinct "limbic thalamus," derive from a discrete neuroepithelial source. This is the crescent-shaped germinal matrix lining the diencephalic (medial) wall of the hitherto unrecognized anterior transitional promontory, which we call the anterior thalamic neuroepithelial lobule. On day E16 three migratory streams leave the anterior neuroepithelial lobule and, on the basis of their labeling pattern in relation to the neurogenetic gradients of the anterior thalamic nuclei, they are identified, from dorsal to ventral, as the putative migratory streams of the anterodorsal, anteroventral, and lateral dorsal nuclei. On day E17 the putative migratory stream of the anteromedial nucleus appears to leave the same neuroepithelial region that on the previous days was the source of the anteroventral nucleus. Dorsally, two neuroepithelial patches persist after day E17 and these are identified as the putative cell lines of the anterior paraventricular and paratenial nuclei.

Animals↗
PubMed1989

Development of the rat thalamus: IV. The intermediate lobule of the thalamic neuroepithelium, and the time and site of origin and settling pattern of neurons of the ventral nuclear complex.

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, migratory route, and site of origin of neurons of the ventral nuclear complex of the thalamus. Quantitative examination of long-survival radiograms established that the bulk of the neurons of the ventral nuclear complex are generated between days E14 and E16 but with statistically significant differences between its three nuclei. The ventrobasal nucleus is the oldest component (97% of the cells are generated on days E14 and E15); the ventrolateral nucleus is next (82% of the cells are generated on days E14 and E15); and the ventromedial nucleus is last (51% of the cells are generated on days E14 and E15). In addition to this caudal-to-rostral (from the ventrobasal nucleus to the ventrolateral nucleus) and lateral-to-medial (from the ventrobasal nucleus to the ventromedial nucleus) internuclear gradients, there are lateral-to-medial and ventral-to-dorsal intranuclear neurogenetic gradients within the ventrobasal and ventrolateral nuclei. Qualitative examination of short and sequential survival thymidine radiograms indicate that the neurons of the ventral nuclear complex originate in the unique intermediate thalamic neuroepithelial lobule, which is distinguished from the rest of the thalamic neuroepithelium by the presence of a mitotically active secondary neuroepithelial matrix. Two sublobules can be distinguished in the intermediate lobule during the early stages of thalamic development. On the basis of their location and chronological pattern of cell production and differentiation, it is inferred that the neurons of the ventrobasal nucleus originate in the earlier differentiating, posteroventrally situated inverted sublobule, and the neurons of the ventrolateral nucleus are produced in the later differentiating, anterodorsally situated everted sublobule. The neurons of the ventromedial nucleus appear to originate from the intermediate neuroepithelial lobule after its two sublobules are no longer distinguishable. The heavily labeled neurons generated soon after injection on day E15 form a wave front that translocates in a lateral direction at a steady rate of 215 microns/day. Examination of methacrylate-embedded materials showed that, in day E15 rats the actively migrating cells are spindle-shaped, with their long axis oriented horizontally. The far-laterally situated differentiating cells (the oldest neurons) become vertically oriented by day E16. Associated with this change in polarity, vertically oriented fibers appear among the cells. These fibers can be traced to the inte

Animals↗
PubMed1989

Central acoustic tract in an echolocating bat: an extralemniscal auditory pathway to the thalamus.

To determine the sources and targets of auditory pathways that bypass the inferior colliculus in the mustache bat, we injected WGA-HRP in the medial geniculate body and related auditory nuclei of the thalamus as well as in the lower brainstem. We used electrophysiological methods to verify that the injection electrode was in an area responsive to sound. The only thalamic injections that produced retrograde transport to cells in auditory nuclei caudal to the inferior colliculus were those that included the suprageniculate nucleus. These injections labeled a group of large multipolar cells lying between the ventral nucleus of the lateral lemniscus and the superior olivary complex. Neurons in this cell group have also been shown to project to the deep layers of the superior colliculus in the mustache bat. The pathway revealed by these studies is almost identical to the "central acoustic tract" in which fibers course medial to the lateral lemniscus and bypass the inferior colliculus to reach the deep superior colliculus and the suprageniculate nucleus.

Animals↗
PubMed1990

Anatomical investigation of projections from thalamus to posterior parietal cortex in the rhesus monkey: a WGA-HRP and fluorescent tracer study.

The parietothalamic projections have been shown to be heterogeneous and appear to be a reflection of the detailed architectonic parcellation of the parietal lobe. In the present study WGA-HRP injections were placed in the different subdivisions of the posterior parietal cortex of the rhesus monkey to determine whether a similarly complex pattern also exists in the thalamocortical pathway. Additionally, in an attempt to determine whether there is an intranuclear specificity of projections from individual thalamic nuclei to different subdivisions of the parietal lobe, multiple retrograde fluorescent tracers were injected into the rostral to caudal sectors of the parietal lobe of the same animal. Different subdivisions of the parietal lobe appear to receive different sets of thalamic input. Thus the superior parietal lobule (SPL) projections are derived from more lateral regions in the thalamus, arising predominantly from the lateral posterior (LP) and pulvinar oralis (PO) nuclei, with additional contributions from the pulvinar lateralis (PL) and pulvinar medialis (PM) nuclei. The inferior parietal lobule (IPL), by contrast, receives its projections from more medial thalamic regions, its main thalamic input originating from PM, and aided by LP, PL, and PO. Both the SPL and IPL also receive projections from the mediodorsal (MD), ventroposterior, ventrolateral, intralaminar, and limbic nuclei, albeit from different components within these nuclei. A topographical arrangement also exists in the thalamic projections to the rostral versus the caudal subdivisions of both the SPL and the IPL. Thus, in the SPL, the ventral posterolateral nucleus, pars oralis (VPLo), ventral lateral nucleus, pars oralis (VLo), and ventral lateral nucleus, pars medialis (VLm) project to rostral regions, whereas the PM and limbic nuclei, anteroventral (AV), and anteromedial (AM), project to area PGm on the medial convexity of the SPL. With respect to projections to the IPL, the ventral posteromedial (VPM) and PO nuclei project to rostral regions, whereas the limbic nuclei lateral dorsal (LD), AM and AV project only to the caudal most area, Opt. A rostrocaudal difference is reflected also within certain nuclei (LP, PO, and PM) that project to the SPL or IPL. Thus rostral parietal subdivisions receive projections from ventral regions within these thalamic nuclei, whereas caudal parietal afferents arise from the dorsal parts of these nuclei. Intervening cortical levels receive projections from intermediate positions within the nuclei. It therefore seems that the increasing architectonic and functional complexity as one moves from rostral to caudal in the SPL and IPL appear to be reflected in the thalamic afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗