The thalamic matrix and thalamocortical synchrony

High-frequency synchronous activity of neurons in the cerebral cortex and thalamus is a concomitant of discrete conscious events. In the primate thalamus, a newly identified population of neurons provides a basis for this synchronization. A matrix of calbindin-immunoreactive neurons extends throughout the thalamus and projects to superficial layers of cortex over wide areas, unconstrained by boundaries between areas. In some nuclei, a core of parvalbumin-immunoreactive neurons is superimposed upon the matrix. Core neurons project in a topographically ordered fashion to middle layers of the cortex in an area-specific manner. Matrix neurons, recruited by corticothalamic connections, can disperse activity across cortical areas and thalamic nuclei. Their superficial terminations can synchronize specific and nonspecific elements of the thalamocortical network in coherent activity that underlies cognitive events.     

Crossed corticothalamic and thalamocortical connections of macaque prefrontal cortex

We have conducted a systematic comparison of the ipsilateral (uncrossed) and contralateral (crossed) thalamic connections of prefrontal cortex in macaque monkeys, using cortical implants of horseradish peroxidase pellets and tetramethyl benzidine histochemistry to demonstrate anterograde and retrograde thalamic labeling. Contrary to the prevailing belief that thalamocortical projections are entirely uncrossed, our findings indicate that a modest crossed projection to prefrontal cortex arises from the mesial thalamus, principally the anteromedial and midline nuclei. Also, while confirming that corticothalamic projections are bilateral, we found that the pattern of crossed projections differs from that of uncrossed projections. Projections to mesial thalamic nuclei, specifically to the anteromedial nucleus, the midline nuclei, and the magnocellular part of the mediodorsal nucleus are bilateral, the contralateral projection being nearly as dense as the ipsilateral projection. Projections to the parvicellular part of the mediodorsal and ventral anterior nuclei are also bilateral, but the contralateral projection is much weaker than the ipsilateral projection. Prefrontal projections to the reticular nucleus, medial pulvinar, suprageniculate nucleus, and limitans nucleus appear to be exclusively ipsilateral. These results indicate that prefrontal cortex has prominent bilateral and reciprocal connections with the nuclei of the mesial thalamic region. As this region of the diencephalon has been implicated by anatomical and behavioral studies in memory functions, our findings suggest that prefrontal cortex, through its connections with this region, may be involved in the bilateral integration of mnemonic systems.     

A population of nicotinic receptors is associated with thalamocortical afferents in the adult rat: Laminal and areal analysis

In the adult rat brain, a prominent population of nicotinic cholinoceptors binds ³H-nicotine with nanomolar affinity. These receptors are abundant in most thalamic nuclei and in neocortical layers 3/4, which receive a major thalamic input. To test whether cortical nicotinic receptors are associated with thalamocortical afferents, unilateral excitotoxic (N-methyl-D-aspartate) lesions were made in one of four thalamic nuclear groups (anterior, ventral, medial geniculate, or dorsal lateral geniculate) or in temporal cortex. After 1 or 4 weeks of survival, cortical ³H-nicotine binding was quantified via autoradiography. Thalamic lesions resulted in a partial loss of ³H-nicotine binding in ipsilateral cerebral cortex. In each thalamic lesion group, the greatest decrease (35–45%) occurred within the cortical layers and area (i.e., cingulate, parietal, temporal, or occipital cortex) receiving the densest thalamocortical innervation. Binding of ³H-nicotine was also reduced within the thalamus local to the lesion, particularly at the longer survival time. Saturation analysis, performed in frontoparietal cortical tissue homogenates following ventral thalamic lesions, revealed a significant (34%) reduction in receptor density but not affinity. Direct excitotoxic lesions of the neocortex (temporal cortex) tended to preserve ³H-nicotine binding in layers 3/4, despite local neuronal loss. These results, taken with other published findings, suggest that some nicotinic cholinoceptors in adult rat cerebral cortex are located on thalamocortical terminals. This organizing principle appears to apply not only to sensory and motor relay projections but also to association nuclei that project to allocortical areas. These receptors may provide a local mechanism for nicotinic cholinergic modulation of thalamocortical input. J. Comp. Neurol. 380:175–190, 1997. © 1997 Wiley-Liss, Inc.     

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.     

Anterior thalamic afferents from the mamillary body and the limbic cortex in the rat

Anterior thalamic afferents from the mamillary body and the limbic cortex were studied by using single and double retrograde transport methods in the rat. The medial mamillary nucleus was divided on the basis of the cytoarchitecture into four subnuclei: the pars medialis centralis, pars medialis dorsalis, pars lateralis, and pars basalis. Extensive connections were seen between each of these subdivisions of the mamillary body and the anterior thalamic nuclei, topographically organized so that the anteromedial thalamic nucleus receives projections exclusively from the pars medialis centralis, while the anteroventral thalamic nucleus receives projections from the pars medialis dorsalis and pars lateralis. Nuclei in the dorsal half of these two mamillary subdivisions project predominantly to the medial half of the anteroventral thalamic nucleus, and those in the ventral half to the lateral half of the nucleus. The pars basalis was found to have numerous projections to the magnocellular part of the anteroventral nucleus. All limbic cortical areas send projections bilaterally to all regions of the anteromedial nucleus as well as to the parvicellular parts of the anteroventral thalamic nucleus, while the anterodorsal nucleus receives ipsilateral projections originating exclusively from the preagranular, anterior limbic, and cingular regions. The magnocellular part of the anteroventral nucleus, however, receives only ipsilateral projections from all of the limbic cortex. Some neurons in the infralimbic region also project bilaterally to all of the anterior thalamic nuclei except the anterodorsal nucleus. All of these cortical projections to the anterior thalamus originate in layers V and VI of the limbic cortex.     

Connections of the retrosplenial dysgranular cortex in the rat.

Although the retrosplenial dysgranular cortex (Rdg) is situated both physically and connectionally between the hippocampal formation and the neocortex, few studies have focused on the connections of Rdg. The present study employs retrograde and anterograde anatomical tracing methods to delineate the connections of Rdg. Each projection to Rdg terminates in distinct layers of the cortex. The thalamic projections to Rdg originate in the anterior (primarily the anteromedial), lateral (primarily the laterodorsal), and reuniens nuclei. Those from the anteromedial nucleus terminate predominantely in layers I and IV-VI, whereas the axons arising from the laterodorsal nucleus have a dense terminal plexus in layers I and III-IV. The cortical projections to Rdg originate primarily in the infraradiata, retrosplenial, postsubicular, and areas 17 and 18b cortices. The projections arising from visual areas 18b and 17 predominantly terminate in layer I of Rdg, axons from contralateral Rdg form a dense terminal plexus in layers I-IV, with a smaller number of terminals in layers V and VI, afferents from postsubiculum terminate in layers I and III-V, and the projection from infraradiata cortex terminates in layers I and V-VI. The efferent projections from Rdg are widespread. The major cortical projections from Rdg are to infraradiata, retrosplenial granular, area 18b, and postsubicular cortices. Subcortical projections from Rdg terminate primarily in the ipsilateral caudate and lateral thalamic nuclei and bilaterally in the anterior thalamic nuclei. The efferent projections from Rdg are topographically organized. Rostral Rdg projects to the dorsal infraradiata cortex and the rostral postsubiculum, while caudal Rdg axons terminate predominantely in the ventral infraradiata and the caudal postsubicular cortices. Caudal but not rostral Rdg projects to areas 17 and 18b of the cortex. The Rdg projections to the lateral and anterior nuclei also are organized along the rostral-caudal axis. Together, these data suggest that Rdg integrates thalamic, hippocampal, and neocortical information.     

Connections of the posterior thalamic region with the auditory ectosylvian cortex in the dog

The purpose of the present study was to define auditory cortical areas in the dog on the basis of thalamocortical connectivity patterns. Connections between the posterior thalamic region and auditory ectosylvian cortex were studied using axonally transported tracers: fluorochromes and biotinylated dextran amine. Cyto- and chemoarchitecture provided grounds for the division of the posterior thalamic region into three complexes, medial geniculate body (MGB), posterior nuclei (Po), and lateromedial and suprageniculate nuclei (LM-Sg). Distinctive cytoarchitectonic features and the distribution of dominant thalamocortical connections (determined quantitatively) allowed us to define four ectosylvian areas: middle (EM), anterior (EA), posterior (EP), and composite (CE). We found that each area was a place of convergence for projections from five to eleven nuclei of the three thalamic complexes, with dominant projections derived from one or two nuclei. Dominant topographical projections from the ventral nucleus to area EM confirmed physiological reports that it may be considered a primary auditory area (AI). We found the anterior part of the EM to be distinct in having unique strong connections with the deep dorsal MGB nucleus. Area EA, which receives dominant projections from the lateral Po (Pol) and medial MGB nuclei, as well as area EP, which receives dominant connections from the dorsal caudal MGB nucleus, compose two parasensory areas. Area CE receives dominant projections from the extrageniculate nuclei, anterior region of the LM-Sg, and Pol, supplemented with an input from the somatosensory VP complex, and may be considered a polymodal association area.     

Adult rat motor cortex connections to thalamus following neonatal and juvenile frontal cortical lesions: WGA-HRP and amino acid studies

Frontal cortex was removed in 1- and 30-day-old rats. When both groups reached 90 days of age, the forelimb motor/sensory cortex in the unlesioned hemisphere was injected with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or tritiated leucine. Thalamic neurons were retrogradely labeled only ipsilateral to the WGA-HRP injection site in both neonatally and juvenile-lesioned subjects. Ventrolateral (VL), ventromedial (VM), centromedial (CM), centrolateral (CL), parafascicular (PF), posteromedial (POm), and posterior (PO) thalamic nuclei were labeled. This and the demonstration of only ipsilateral thalamocortical connections at birth helped explain the marked thalamic atrophy which developed ipsilateral to neonatal frontal cortex lesions. Death of thalamic neurons after neonatal removal of their normal cortical target could be due to their failure to sprout into the opposite cortex because that cortex was already innervated by the opposite thalamus at birth. Leucine motor/sensory cortex injections in both neonatally and juvenile-lesioned subjects labeled the ipsilateral VL, VM, CM, CL, PF, POm, and PO thalamic nuclei; contralateral CM, CL, and PF thalamic nuclei; ipsilateral medial, ventral, and lateral pontine nuclei; and parts of the contralateral pontine nuclei. The ipsilateral connections were always more robust than the contralateral connections. The contralateral corticothalamic and corticopontine projections, however, were much more numerous and widespread in neonatally compared to juvenile-lesioned subjects. The greater sparing of some motor functions said to occur in neonatal compared to adult motor cortex-lesioned subjects could be due to the plasticity of corticothalamic, corticopontine, and other corticofugal pathways, but not to the plasticity of thalamocortical pathways.     

The contribution of the dorsal lateral geniculate nucleus to the total pattern of thalamic terminations in striate cortex of the Virginia opossum

These studies were carried out to show the manner of projection of the dorsal lateral geniculate nucleus and other thalamic nuclei to striate cortex in the Virginia opossum. In order to demonstrate these projections, lesions were made in the dorsal lateral geniculate nucleus, in the ventral lateral geniculate nucleus, in most of the thalamus on one side except for the dorsal lateral geniculate nucleus, and in the entire unilateral thalamus. Following various survival times, usually seven days, the brains were appropriately prepared and stained with procedure I of the Fink-Heimer technique. Dorsal lateral geniculate neurons project in a topographical manner only to certain layers of striate cortex. These projections from the lateral geniculate are compared with the same system in other mammals, and it is concluded that it is similar in all mammals studied, except for the cat. In the cat the lateral geniculate projects beyond the border of striate cortex, but even in the cat the layers of termination within striate cortex are apparently similar. The ventral lateral geniculate nucleus does not project to visual cortex. Dorsal thalamic nuclei other dian the lateral geniculate project to peristriate cortex and to layers VI and I of striate cortex. The finding that thalamic nuclei, other than the lateral geniculate nucleus, project to striate cortex has never been described as part of the visual pathways in other mammals. It is suggested that these additional projections arise mainly from the lateral nuclear group of the thalamus in the opossum, and must be considered in relation to any response characteristics and organization of striate cells determined from physiological studies. These multiple thalamic projections can provide the substrate for more than one representation or “map” of sensory information in striate cortex.     

Orderly anomalous retinal projections to the medial geniculate,ventrobasal, and lateral posterior nuclei of the hamster

Experiments were performed to determine (1) under what conditions early brain surgery can cause sensory afferents to the thalamus to form connections at abnormal thalamic sites and (2) the extent to which such ectopic projections are receptotopically organized. In newborn Syrian hamsters, two of the retina's principal synaptic targets, the superior colliculus and dorsal lateral geniculate nucleus, were destroyed, respectively, by a direct lesion and by retrograde degeneration following a lesion of the occipital cortex. In the same brains, alternative terminal space for the retinofugal axons was made available in auditory (medial geniculate) or somatosensory (ventrobasal) thalamic nuclei by lesions of ascending auditory or somatosensory pathways, respectively; additional terminal space was made in the lateral posterior nucleus by degeneration of afferents from the superior colliculus. The projections of the contralateral retina were traced in neonatally operated adults by making one or two small peripheral retinal lesions and intraocular injections of ³H-proline 5 days and 1 day, respectively, prior to sacrifice. The neonatal surgery reliably produced anomalous crossed retinal projections to the partially deafferented structures. These projections terminate preferentially at the nuclear surfaces. Computer reconstructions from serial sections demonstrated several signs of spatial order suggestive of receptotopic organization in the anomalous retinothalamic projections. In order of increasing stringency, these signs (which are not mutually exclusive) are: (1) In each nucleus, a restricted retinal sector gives rise to a limited part of the abnormal projection. (2) In each nucleus, different parts of the retina give rise to different parts of the anomalous projection. (3) In each nucleus, there is a more or less consistent polarity of the anomalous connection. Each small retinal sector appears to be represented along a “line of projection” in each of its abnormal thalamic targets, as it normally is in the dorsal and ventral lateral geniculate nuclei and in the superior colliculus. In some brains, some of the abnormal projections produce only a partial representation of the retina. However, in a single animal, a retinal sector not represented in the anomalous projections to one nucleus can contribute to the abnormal connections with another nucleus. In additional experiments, an attempt was made to direct developing auditory and somatosensory fibers normally terminating in the medial geniculate and ventrobasal nuclei, respectively, to anomalous thalamic targets. The axons were deprived of some of their normal thalamic sites of termination and alternative terminal space was made available in another thalamic sensory nucleus. These experiments failed to produce reliable evidence of ectopic auditory or somatosensory thalamic projections. The anomalous retinal projections to nuclei that normally recieve little (lateral posterior) or no (medial geniculate, ventrobasal) optic tract input, show that the preference of retinal axons for their normal targets is relative, not absolute. The orderliness of the ectopic projections opposes the hypothesis that the formation of retinotopic connections depends upon the matching of a set of signals distributed among the retinofugal fibers and a corresponding set of cues unique to the normal terminal fields of optic axons. The results are consistent with the formation of receptotopic connections by interactions among developing axons and suggest the action of additional factors that determine the terminal sites and organization of central neuronal connections.     

Mediodorsal nucleus: areal, laminar, and tangential distribution of afferents and efferents in the frontal lobe of rhesus monkeys

The terminal distribution of thalamic afferents in primate prefrontal cortex has never been examined in any detail. In the present study, WGA-HRP was injected into major subdivisions of the mediodorsal nucleus (MD) in the rhesus monkey in order to determine 1) The areal distribution of MD projections, 2) the layer(s) in which MD afferents terminate, 3) the tangential pattern of the MD axonal terminals, 4) the cells of origin of the reciprocal corticothalamic pathway, and 5) the degree of reciprocity between the corticothalamic and thalamocortical pathways in the different regions of the prefrontal cortex. As expected on the basis of retrograde degeneration and transport studies, injections centered in the magnocellular (MDmc) subnucleus of MD labeled cells and terminals in the ventral and medial prefrontal cortex. Injections involving ventral MDmc labeled the more lateral of these areas (Walker's areas 11 and 12); injections of the dorsal MDmc labeled the ventromedial regions (areas 13 and 14). In contrast, injections involving mainly the lateral, parvicellular (MDpc) moiety labeled cells and terminals in dorsolateral and dorsomedial areas (Walker's 46, 9, and 8B). Area 8A was labeled most prominently when injections included the multiform portion of MD (MDmf) and area 10 had connections with anterior portions of MD. A dorsal-ventral topography for MDpc exists with dorsal MDpc labeling dorsal and dorsomedial prefrontal areas and ventral MDpc labeling dorsolateral prefrontal cortex. Our findings with respect to MD are consistent with a nucleus-to-field organization of its thalamocortical projection system. Outside of the traditional boundaries of prefrontal cortex, lateral MD projections extended to the supplementary motor area (SMA) and the dorsal part of the anterior cingulate (AC) whereas the medial MD projection targeted the ventromedial cingulate cortex and spared SMA. In addition, a few labeled cells and sparse terminals were found in the inferior parietal lobule, the superior temporal sulcus, and the anterior part of the insula after injections that involved the medial part of MD. Labeled terminals were invariably confined to layer IV and adjacent deep layer III. No terminal label was ever observed in layers I, II, superficial III, V, or VI in any part of the cerebral cortex following injections confined to any part of MD.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cross-modal plasticity of the corticothalamic circuits in rats enucleated on the first postnatal day

Reorganization of the reciprocal corticothalamic connections was studied as a possible anatomical substrate of the cross-modal compensation of the missing visual input of the visual cortex by somatosensory-evoked activities in neonatally enucleated rats. The use of quantitative retrograde tract-tracing techniques revealed that the contribution of the lateral posterior thalamic nucleus (LP) is significantly increased following enucleation, while that of the dorsolateral geniculate and the lateral dorsal nuclei is decreased in the thalamocortical afferentation of a region in visual cortical area 17. In contrast with the control rats, a dense terminal arborization of afferents was labelled in the LP after the injection of anterograde tracer into the barrel cortex of the enucleated rats. The injection of anterograde tracer into the visual cortex also demonstrated a massive afferentation into the LP of the enucleated rats. Visual and somatosensory corticothalamic afferents exhibited similar ultrastructural features in the LP after enucleation, but their synaptic organizations differed as regards the diameter of the postsynaptic dendrites. Taken together with the previous observations, these results suggest a central role for the LP in the transmission of the somatosensory-evoked activities to the visual cortex after early blindness.     

Corticothalamic neurons and thalamocortical terminal fields: an investigation in rat using horseradish peroxidase and autoradiography.

Subsequent to thalamic injections in rats of horseradish peroxidase (HRP) alone or HRP and [3H]leucine in combination, the cells of origin of the corticothalamic projections and the terminal fields of the thalamocortical projections were identified. HRP-labeled corticothalamic neurons were uniformly found in layers V and VI. They were medium to small in size and always pyramidal in shape with the larger neurons being found in layer V. On the other hand, 3 different patterns for the distribution of thalamocortical terminal fields were observed. The autoradiographic material indicated that in prefrontal cortex the bulk of thalamocortical fibers terminate in layer III while in motor cortex they terminate primarily in layer V. A third pattern was shared by temporal, occipital and parietal cortex where the bulk of thalamocortical fibers terminate preferentially in layer IV. The data derived from the rats which had received thalamic injections of HRP and [3H]leucine in combination indicated that the connections between cortex and thalamus are in general reciprocal. These results are discussed with regard to earlier studies using classical or more recently developed neuroanatomical methods.     

Place of the thalamus in the network of interconnections between the association and limbic cortices in the monkey

Anatomophysiological criteria underlying the definition of associative cortex as well as limbic cortex include some imprecise data. The original notion of "cortical association spheres" (Flechsig) with no connections with the thalamus has rightly been abandoned, and that of the macroscopic "large limbic lobe" (Broca) fails to stand up to histologic or hodologic findings. However, the concept of cortical areas implicated specifically in multiple sensorial integration, sensory-motor coupling and control of behavior lasts due to necessity. In the monkey, the posterior parietal cortex of area 7 (PG area), the cortex of the upper slope of the superior temporal sulcus (STS) and the prefrontal cortex anterior to the sulcus arcuatus exchange direct corticocortical connections, receive afferents from sensory cortex and are not connected to specific thalamic relays. The term "associative" in its widest sense applies more particularly therefore to these cortical areas organized in networks. On the internal surface of the hemisphere, the cingular gyrus, retrosplenial cortex and parahippocampic gyrus (TF and TH areas) which occupy the major part of the limbic lobe, participate in the formation of this network and exchange direct cortico-cortical connections with the associative cortex defined above. The use of anterograde (labelled aminoacids) and retrograde (peroxidases) markers and of fluorescent dyes, allowing double retrograde labelling, demonstrates that the median pulvinar nucleus is connected with the knots of the associative cortical network. This thalamic nucleus, of a relatively increased size from phylogenetic evolution, is therefore excluded from the classification opposing specific and diffuse projection nuclei. In contrast to the thalamic reticular nucleus, which lacks cortical projections, and to the nuclei of the internal medullary band, which have the striatum as main target, the median pulvinar is a thalamic structure connected directly and specifically with each of the cortical areas, lesions of which result in negligence behavior.     

Structure and connections of the thalamic reticular nucleus: Advancing views over half a century

The advance of knowledge of the thalamic reticular nucleus and its connections has been reviewed and Max Cowan's contributions to this knowledge and to the methods used for studying the nucleus have been summarized. Whereas 50 years ago the nucleus was seen as a diffusely organized cell group closely related to the brain stem reticular formation, it can now be seen as a complex, tightly organized entity that has a significant inhibitory, modulatory action on the thalamic relay to cortex. The nucleus is under the control, on the one hand, of topographically organized afferents from the cerebral cortex and the thalamus, and on the other of more diffuse afferents from brain stem, basal forebrain, and other regions. Whereas the second group of afferents can be expected to have global actions on thalamocortical transmission, relevant for overall attentive state, the former group will have local actions, modulating transmission through the thalamus to cortex with highly specific local effects. Since it appears that all areas of cortex and all parts of the thalamus are linked directly to the reticular nucleus, it now becomes important to define how the several pathways that pass through the thalamus relate to each other in their reticular connections.     

A comparative analysis of the morphology of corticothalamic projections in mammals

Recent anatomical tracing methods have revealed new principles underlying the organization of corticothalamic connections in the mammalian nervous system. These data demonstrated the distribution of two types of synaptic contacts in the corticothalamic projection: small (<1 microm) and giant (2-10 microm) axon terminals. We compare the organization of corticothalamic projections in the auditory, somatosensory, visual, and motor systems of a variety of mammalian species, including the monkey. In all these systems and species, both types of corticothalamic terminals have been observed. Small endings formed the major corticothalamic terminal field, whereas giant terminals were less numerous and formed additional terminal fields together with small terminals. After comparing their spatial distribution, as well as the degree of reciprocity between the corticothalamic and thalamocortical projections, different roles are proposed for small and giant endings. Small terminals are typically present in the projection serving the feed-back control of the cerebral cortex on the thalamic nucleus from which it receives its main projection. In contrast, giant terminals are involved in feed-forward projections by which activity from a cortical area is distributed, via the thalamus, to other parts of the cerebral cortex. The cross-species and cross-systems comparison reveals differences in the mode of feed-forward projection, which may be involved in the activation of other parts of the same cortical area or form part of a projection that activates other cortical areas.     

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.     

Cortical and thalamic afferent connections of the insular and adjacent cortex of the rat

Thalamic and cortical afferents to the insular and perirhinal cortex of the rat were investigated. Unilateral injections of horseradish peroxidase (HRP) were made iontophoretically along the rhinal sulcus. HRP injections covered or invaded areas along the rhinal fissure from about the level of the middle cerebral artery to the posterior end of the fissure. The most anterior injection labeled a few cells in the mediodorsal nucleus. More posterior injections labeled neurons in the basal portion of the nucleus ventralis medialis, thus suggesting that this cortical region constitutes the rat's gustatory (insular) cortex. We consider the cortex situated posterior to the gustatory cortex in and above the rhinal sulcus as the core region of the rat's (associative) insular cortex, as this cortex receives afferents from the regions of and between the nuclei suprageniculatus and geniculatus medialis, pars magnocellularis. It includes parts of the cortex termed perirhinal in other studies. The cortex dorsal and posterior to the insular cortex we consider auditory cortex, as it receives afferents from the principal part of the medial geniculate nucleus, and the cortex ventral to the insular cortex (below the fundus of the rhinal sulcus) we consider to constitute the prepiriform cortex, which is athalamic. The posterior part of the perirhinal cortex (area 35) receives afferents from nonspecific thalamic nuclei (midline nuclei). Cortical afferents to the injection loci arise from a number of regions, above all from regions of the medial and sulcal prefrontal cortex. Those injections confined to the projection cortex of the suprageniculate-magnocellular medial geniculate nuclear complex also led to labeling in contralateral prefrontal regions, particularly in area 25 (infralimbic region). A comparison of our results with those on the insular cortex of cats and monkeys suggests that on the basis of thalamocortical connections, topographical relations, and involvements of neurons in information processing and overt behavior, the insular cortex has to be regarded as a heterogeneous region which may be separated into prefrontal insular, gustatory (somatosensory) insular, and associative insular portions.