Arterial Supply of the Thalamus: A Comprehensive Review.

The thalamus is a deep cerebral structure that is crucial for proper neurological functioning as it transmits signals from nearly all pathways in the body. Insult to the thalamus can, therefore, result in complex syndromes involving sensation, cognition, executive function, fine motor control, emotion, and arousal, to name a few. Specific territories in the thalamus that are supplied by deep cerebral arteries have been shown to correlate with clinical symptoms. The aim of this review is to enhance our understanding of the arterial anatomy of the thalamus and the complications that can arise from lesions to it by considering the functions of known thalamic nuclei supplied by each vascular territory.

Parallel but separate inputs from limbic cortices to the mammillary bodies and anterior thalamic nuclei in the rat.

The proposal that separate populations of subicular cells provide the direct hippocampal projections to the mammillary bodies and anterior thalamic nuclei was tested by placing two different fluorescent tracers in these two sites. In spite of varying the injection locations within the mammillary bodies and within the three principal anterior thalamic nuclei and the lateral dorsal thalamic nucleus, the overall pattern of results remained consistent. Neurons projecting to the thalamus were localized to the deepest cell populations within the subiculum while neurons projecting to the mammillary bodies consisted of more superficially placed pyramidal cells within the subiculum. Even when these two cell populations become more intermingled, e.g., in parts of the intermediate subiculum, almost no individual cells were found to project to both diencephalic targets. In adjacent limbic areas, i.e., the retrosplenial cortex, postsubiculum, and entorhinal cortex, populations of cells that project to the anterior thalamic nuclei and mammillary bodies were completely segregated. This segregated pattern included afferents to those nuclei comprising the head-direction system. The sole exception was a handful of double-labeled cells, mainly confined to the ventral subiculum, that were only found after pairs of injections in the anteromedial thalamic nucleus and mammillary bodies. The projections to the anterior thalamic nuclei also had a septal-temporal gradient with relatively fewer cells projecting from the ventral (temporal) subiculum. These limbic projections to the mammillary bodies and anterior thalamus comprise a circuit that is vital for memory, within which the two major components could convey parallel, independent information.

Connections of auditory and visual cortex in the prairie vole (Microtus ochrogaster): evidence for multisensory processing in primary sensory areas.

In prairie voles, primary sensory areas are dominated by neurons that respond to one sensory modality, but some neurons also respond to stimulation of other modalities. To reveal the anatomical substrate for these multimodal responses, we examined the connections of the primary auditory area + the anterior auditory field (A1 + AAF), the temporal anterior area (TA), and the primary visual area (V1). A1 + AAF had intrinsic connections and connections with TA, multimodal cortex (MM), V1, and primary somatosensory area (S1). TA had intrinsic connections and connections with A1 + AAF, MM, and V2. Callosal connections were observed in homotopic locations in auditory cortex for both fields. A1 + AAF and TA receive thalamic input primarily from divisions of the medial geniculate nucleus but also from the lateral geniculate nucleus (LGd), the lateral posterior nucleus, and the ventral posterior nucleus (VP). V1 had dense intrinsic connections and connections with V2, MM, auditory cortex, pyriform cortex (Pyr), and, in some cases, somatosensory cortex. V1 had interhemispheric connections with V1, V2, MM, S1, and Pyr and received thalamic input from LGd and VP. Our results indicate that multisensory integration occurs in primary sensory areas of the prairie vole cortex, and this may be related to behavioral specializations associated with its niche.

Modular organization in area 21a of the cat revealed by optical imaging: comparison with the primary visual cortex.

Area 21a, located on the cat's lateral suprasylvian cortex, is considered as a higher-order cortical area. Little is known about its specific role in visual processing. In this study, the functional organization of area 21a was investigated by optical imaging of intrinsic signals and was compared to that of primary visual areas. We found a clear modular pattern for orientation selectivity in area 21a, with signal amplitude being four times lower than that in primary visual areas. There were no significant differences between the domains' characteristics, nor the tuning bandwidth, in areas of the primary visual cortex (17 and 18) and 21a. This suggests that the basic cortical structure is independent of the hierarchical level or function of one area. A uniform representation of spatial frequency was found in areas 17 and 18, as well as in area 21a. The mean preferred spatial frequency in area 21a was 0.30 c/deg. In contrast to area 18, no direction maps were observed in area 21a whether drifting gratings or random dot kinematograms were used. This study supports the proposal that area 21a plays a pivotal role along the ventral processing stream and is mainly involved in form processing.

Cortical connections of the rat lateral posterior thalamic nucleus.

Spatial processing related to directed attention is thought to be mediated by a specific cortical-basal ganglia-thalamic-cortical network in the rat. Key components of this network are associative cortical areas medial agranular cortex (AGm) and posterior parietal cortex (PPC), dorsocentral striatum (DCS), and lateral posterior (LP) thalamic nucleus, all of which are interconnected. Previously, we found that thalamostriatal projections reaching DCS arise from separate populations of neurons of the mediorostral part of LP (LPMR). The far medial LPMR (fmLPMR) terminates in central DCS, a projection area of AGm, whereas central LPMR terminates in dorsal DCS, a projection area of PPC. This represents segregated regional convergence in DCS from different sources of thalamic and cortical inputs. In the present study, thalamocortical and corticothalamic projections arising from and terminating in LPMR and neighboring thalamic nuclei were studied by anterograde and retrograde tracing techniques in order to further understand the anatomical basis of this neural circuitry. A significant finding was that within LPMR, separate neuronal populations provide thalamic inputs to AGm or PPC and that these cortical areas project to separate regions in LPMR, from which they receive thalamic inputs. Other cortical areas adjacent to AGm or PPC also demonstrated reciprocal connections with LP or surrounding nuclei in a topographic manner. Our findings suggest that the cortical-basal ganglia-thalamic network mediating directed attention in the rat is formed by multiple loops, each having reciprocal connections that are organized in a precise and segregated topographical manner.

Thalamic nuclei in the opossum Monodelphis domestica.

We investigated nuclear divisions of the thalamus in the gray short-tailed opossum (Monodelphis domestica) to gain detailed information for further developmental and comparative studies. Nissl and myelin staining, histochemistry for acetylcholinesterase and immunohistochemistry for calretinin and parvalbumin were performed on parallel series of sections. Many features of the Monodelphis opossum thalamus resemble those in Didelphis and small eutherians showing no particular sensory specializations, particularly in small murid rodents. However, several features of thalamic organization in Monodelphis were distinct from those in rodents. In the opossum the anterior and midline nuclear groups are more clearly separated from adjacent structures than in eutherians. The dorsal lateral geniculate nucleus (LGNd) starts more rostrally and occupies a large part of the lateral wall of the thalamus. As in other marsupials, two cytoarchitectonically different parts, alpha and beta are discernible in the LGNd of the opossum. Each of them may be subdivided into two additional bands in acetylcholinesterase staining, while in murid rodents the LGNd consists of a homogeneous mass of cells. Therefore, differentiation of the LGNd of the Monodelphis opossum is more advanced than in murid rodents. The medial geniculate body consists of three nuclei (medial, dorsal and ventral) that are cytoarchitectonically distinct and stain differentially for parvalbumin. The relatively large size of the MG and LGNd points to specialization of the visual and auditory systems in the Monodelphis opossum. In contrast to rodents, the lateral dorsal and lateral posterior nuclei in the opossum are poorly differentiated cytoarchitectonically.

Striatal projections from the rat lateral posterior thalamic nucleus.

The dorsocentral striatum (DCS) has been implicated as an associative striatal area receiving inputs from several cortical areas including medial agranular cortex (AGm), posterior parietal cortex (PPC), and visual association cortex to form a cortical-subcortical circuit involved in directed attention and neglect. The lateral posterior thalamic nucleus (LP) may also play a role in directed attention and neglect because LP has robust reciprocal connections with these cortical areas and projects to DCS. We used anterograde axonal tracing to map thalamostriatal projections from LP and surrounding thalamic nuclei, with a focus on projections to DCS. The thalamic nuclei investigated included LP, laterodorsal thalamic nucleus (LD), central lateral nucleus (CL), and posterior thalamic nucleus (Po). We found that the mediorostral part of LP (LPMR) projects strongly to DCS as well as to the dorsal peripheral region of the striatum. Further, there is topography within LPMR and DCS such that the far medial LPMR projects to the central region of DCS (projection area of AGm) and the central LPMR projects to the dorsal region of DCS (projection area of PPC and Oc2M). In contrast, the laterorostral part of LP (LPLR) and other thalamic nuclei surrounding LP project to dorsolateral to dorsomedial peripheral regions of the striatum but do not project to DCS. These findings indicate that DCS is a region of convergence for thalamostriatal and corticostriatal projections from regions that are themselves interconnected, serving as the key element of the corticostriatal-thalamic network mediating spatial processing and directed attention.

Laterodorsal nucleus of the thalamus: A processor of somatosensory inputs.

The laterodorsal (LD) nucleus of the thalamus has been considered a "higher order" nucleus that provides inputs to limbic cortical areas. Although its functions are largely unknown, it is often considered to be involved in spatial learning and memory. Here we provide evidence that LD is part of a hitherto unknown pathway for processing somatosensory information. Juxtacellular and extracellular recordings from LD neurons reveal that they respond to vibrissa stimulation with short latency (median = 7 ms) and large magnitude responses (median = 1.2 spikes/stimulus). Most neurons (62%) had large receptive fields, responding to six and more individual vibrissae. Electrical stimulation of the trigeminal nucleus interpolaris (SpVi) evoked short latency responses (median = 3.8 ms) in vibrissa-responsive LD neurons. Labeling produced by anterograde and retrograde neuroanatomical tracers confirmed that LD neurons receive direct inputs from SpVi. Electrophysiological and neuroanatomical analyses revealed also that LD projects upon the cingulate and retrosplenial cortex, but has only sparse projections to the barrel cortex. These findings suggest that LD is part of a novel processing stream involved in spatial orientation and learning related to somatosensory cues.

Striatal projections from the lateral and posterior thalamic complexes. An anterograde tracer study in the cat.

Striatal projections from the lateral intermediate (LI) and posterior (Po) thalamic complexes were studied with the anterograde tracers wheat germ agglutinin-horseradish peroxidase and Phaseolus vulgaris leucoagglutinin. Projections to the lateral part of the head and body of the caudate nucleus (CN) and to the putamen (Pu) were found to arise from the ventral parts of the caudal subdivision of the LI besides the well established sources in the intralaminar and ventral thalamic nuclei. No projections to the CN and only a few to the Pu were found to arise from the medial division of the Po. The presence of terminal and intercalated varicosities in the thalamostriatal fibers suggests that they form both terminal and en passant synapses. Thalamostriatal fibers from these thalamic sectors were unevenly distributed within the CN, with patches of either low-density innervation or with no projections at all interspersed within irregular, more densely innervated areas. The former coincided with the acetylcholinesterase-poor striosomes and the latter areas of dense projection with the extrastriosomal matrix.

Neuronal pathway finding: from neurons to initial neural networks.

Neuronal pathway finding is crucial for structured cellular organization and development of neural circuits within the nervous system. Neuronal pathway finding within the visual system has been extensively studied and therefore is used as a model to review existing knowledge regarding concepts of this developmental process. General principles of neuron pathway finding throughout the nervous system exist. Comprehension of these concepts guides neuroscience nurses in gaining an understanding of the developmental course of action, the implications of different anomalies, as well as the theoretical basis and nursing implications of some provocative new therapies being proposed to treat neurodegenerative diseases and neurologic injuries. These therapies have limitations in light of current ethical, developmental, and delivery modes and what is known about the development of neuronal pathways.

Presence and connections of auditory neurons in the rostrodorsal and rostrolateral parts of the thalamic reticular nucleus.

OBJECTIVE: Auditory neurons have been identified in the caudoventral part of the thalamic reticular nucleus (TRN). We examined the acoustic input to single cells in the rostrodorsal part of the TRN. MATERIAL AND METHODS: In alpha-chloralose-anesthetized cats, we extracellularly recorded the responses of single neurons in the rostral TRN to acoustic and light stimuli. Next, to examine efferent projections of auditory neurons in the rostral TRN, we injected wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into other thalamic nuclei where auditory neurons were detected, including the lateral posterior nucleus (LP), the lateral medial and suprageniculate nuclei and the centromedian nucleus. Finally, intracortical microstimulation of the LP was performed to demonstrate antidromic activation of the auditory neurons in the rostral TRN. RESULTS: In the rostral TRN, 2 types of response to auditory stimuli were observed: brief, short-latency bursts (13-20 ms; mean 16.5 ms) and longer bursts with a long latency (38.8-50 ms; mean 44.8 ms). Injection of WGA-HRP into the medial LP labeled cells only in the rostrodorsal TRN, while extending the injection to the other nuclei labeled cells in the rostrodorsal and rostrolateral parts of the nucleus. Auditory neurons in the rostral TRN were activated antidromically by microstimulation of auditory neurons in the LP, with a latency of 1.2 ms. CONCLUSIONS: These results strongly suggest that auditory neurons in the rostrodorsal TRN project to auditory neurons in the LP. The rostral auditory TRN may be involved in transmission of auditory information via the non-specific association system of the thalamus.

Retinal projections to the lateral posterior-pulvinar complex in intact and early visual cortex lesioned cats.

In intact cats, it is generally considered that the lateral posterior-pulvinar complex (LP-pulvinar) does not receive direct retinal terminals, with the exception of the retino-recipient zone known as the geniculate wing. There is, however, some evidence that early lesions of the visual cortex can occasionally induce the formation of novel retinal projections to the LP nucleus. Given the importance of knowing the connectivity pattern of the LP-pulvinar complex in intact and lesioned animals, we used the B fragment of cholera toxin, a sensitive anterograde tracer, to reinvestigate the retinal projections to the LP-pulvinar in normal cats and in cats with early unilateral lesions of the visual cortex (areas 17 and 18). Immunohistochemical localization of the toxin was performed to show the distribution and morphology of retinofugal terminals. A direct bilateral but predominantly contralateral retinal projection reached the caudal portion of LPl and LPm in the form of patches located mainly along its dorsomedial surface and many scattered terminals. The distribution of retinal projections to LP-pulvinar in intact and operated cats did not differ. Contrary to what had been previously reported, we found no evidence for lesion-induced sprouting of retinal axons in these higher-order thalamic nuclei. Retinal input to the LP-pulvinar might modulate visual responses driven by primary visual cortex or superior colliculus.

Differential projections from the mediodorsal and centrolateral thalamic nuclei to the frontal cortex in rats.

The aim of the present study was to investigate afferent projections from the medial thalamic nuclei (MT) to the frontal cortical areas using a single small iontophoretic injection of biotinylated dextran amine (BDA) and analysis of the anterogradely labeled fibers and varicosities. Projections from the mediodorsal (MD) nuclei were found primarily and extensively in the anterior cingulate cortex (ACC), whereas those from the centrolateral (CL) thalamic nucleus were found in the frontal motor cortex. The density of terminals in the ACC was high in layers II and III and sparse in layer I. The majority of projected fibers from the CL were found at a high density in layer V, with a moderate density in the superficial layers. The differential projection patterns were topographically organized in the medial prefrontal cortex and sensory motor cortex. These findings support the results of our previous electrophysiological studies suggesting that neurons in the medial thalamic nuclei relay nociceptive information to the limbic or sensory motor cortical areas. The present results agree with the current notion that the medial thalamo-frontal cortical network circuitry plays an important role in processing the emotional aspect of nociception.

The role of the laterodorsal nucleus of the thalamus in spatial learning and memory in the rat.

The anterior thalamic nuclei appear to play an important role in learning and memory. Connectionally and structurally, the lateral dorsal nucleus is similar to the anterior nuclei. This study tested the hypothesis that the laterodorsal thalamic nucleus (LD) also contributes to these functions. Adult Sprague-Dawley rats received bilateral ibotenic acid lesions of LD, and 2 weeks later the rats were tested in a repeated acquisition water maze task. The control groups displayed a short final escape latency and showed a preference for the correct quadrant in the probe trial. Rats with a lesion restricted to LD (LDL) were mildly impaired in the task, but rats with lesions that destroyed LD and also significantly (50%) damaged the adjacent anterior thalamic nuclei (LDL+) were severely impaired, displaying no improvement in performing the spatial task. In a second experiment, training in the same paradigm for 2 weeks resulted in improved final performance by LDL and control rats but not by LDL+ rats. These findings support the hypothesis that together with the anterior thalamic nuclei, LD plays a role in spatial learning and memory.

Cerebellar input to the posterior parietal cortex in the rat.

The cerebello-thalamo-parietal projections were investigated in rats by means of a multiple anterograde-retrograde tracing technique. Retrograde fluorescent tracers were injected in different loci of the parietal cortex. Injected areas were verified cytoarchitectonically and confirmed by analyzing the retrograde thalamic labeling pattern obtained. Anterograde fluorescent tracers were placed in the intermediate and lateral deep cerebellar nuclei. The topographical overlap between cerebellar terminals and parietal-projecting thalamic neurons was analyzed. In the central lateral (CL) and ventrolateral (VL) thalamic nuclei, cells projecting to anterior somatosensory (S1) and posterior parietal (PPC) cortices were demonstrated to receive direct cerebellar input. In particular, two patterns of organization were revealed. In CL, the PPC- and S1-projecting neurons, both receiving cerebellar fibers, were intermingled. In VL, PPC, and S1-projecting neurons were instead segregated, and the areas containing labeled neurons received separate contingents of cerebellar fibers. These patterns suggest that in CL axons originating from the same or neighboring cerebellar neurons can terminate on both PPC- and S1-projecting neurons, while in VL cerebellar information is funneled to S1 and PPC through two segregated parallel pathways. The significance of the observed organization is discussed comparing findings in other species and in relation with electrophysiological and functional studies on cerebelloparietal interrelationships.

An architectonic comparison of the ventrobasal complex of two megachiropteran and one microchiropteran bat: implications for the evolution of Chiroptera.

The architectonic features of the thalamic ventrobasal complex (Vb) of two species of Megachiropteran (Grey-headed flying fox, Pteropus poliocephalus, and the Eastern tube-nosed bat, Nyctimene robinsoni) are compared with those of a Microchiropteran (Australian ghost bat, Macroderma gigas). The somatosensory system was chosen for comparison as it represents a sensory system that has undergone analogous modifications in both Chiropteran lineages (the evolution of the wing). The components of Vb were examined as there are taxon-specific features in this region of the brain. Within the Megachiropteran Vb, four subnuclei were recognized: the ventral posterior medial (VPM), the ventral posterior lateral (VPL), the ventral posterior inferior (VPI), and the basal ventral medial (VMb). In the ghost bat only VPM and VPL were identified with certainty. No VPI was evident in the ghost bat, however a putative VMb was observed. Vb of the ghost bat also lacked the arcuate lamina, which distinguishes VPM from VPL in the Megachiropterans and many other mammals. These taxon-specific differences lend support to the proposal that the order Chiroptera has a diphyletic origin.

Intrasexual and intersexual dimorphisms of the red salmon prosencephalon.

Intrasexual as well as intersexual dimorphisms were found in the prosencephalon and mesencephalon of adult Oncorhynchus nerka (red/sockeye salmon). These dimorphisms are concerned with the position of the preoptic nucleus, nucleus lateralis tuberis, habenula, third ventricle, tectal ventricles, preoptic recess, recessus lateralis, horizontal commissure, posterior commissure, and toral commissure. The intrasexual dimorphism was characterized by either a rostral ("r"-pattern) or a caudal ("c"-pattern) position of the preoptic region as well as varying locations of other structures within the prosencephalon. As compared to "c"-pattern fish, the preoptic nucleus and nucleus lateralis tuberis were located more rostral, and the habenula was positioned further caudal, in "r"-type animals. The intersexual dimorphism was also characterized by different positions of the structures listed above. With the exception of the preoptic nucleus, all of these were located further rostral in "r"-pattern females than in type "r" males. In "c"-pattern females, they were positioned further caudal than in type "c" males. The number of neurons in the parvocellular and in the magnocellular portion of the preoptic region differed in the two genders with respect to "r"- as well as "c"-pattern fish. Males had more neurons than females in both the magno- and the parvocellular subdivisions of the preoptic region. In "r"- and "c"-pattern fish, the average size of magnocellular preoptic neurons was larger in females than in males. The observed intersexual variations may reflect gender-specific differences in the control of the pituitary. Functional correlates of intrasexual dimorphism are obscure.

Unequal representation of the temporal and nasal retina in an anomalous projection to the lateral thalamus.

Study of an anomalously regenerated, nontopographically organized retinal projection in the frog olfactory cortex revealed that the temporal retina is the main source of this projection, suggesting the existence of specific temporal fiber-directed attractant or trophic influences. In the present study, we examined the organization of an anomalous retinal projection that forms in the frog thalamus after ablation of the optic tectum. The projections from different sectors of the retina were studied by means of the anterograde transport of biotinylated dextran-amine (BDA) delivered to incisions made across the nerve fiber layer in frogs surviving ablation of the contralateral tectal hemisphere for 13-46 weeks. The projections from nasal retinal sectors were always lightly constructed in the aberrant terminal field, whereas their projections to the lateral geniculate complex remained reasonably strong. In contrast, the projections from temporal retinal sectors, though also weak initially, in time became robust and filled the aberrant field over most of its extent. The specific amplification of the temporal fiber projection now observed in two foreign targets provides further evidence for the existence of target-based, attractant/trophic molecules with functional specificity for temporal retinal fibers. That such agents can exist or be inducible in a foreign area would suggest that they belong to a family of molecules having natural biological activity in normal development or regeneration. However, the possibility that the augmented role of the temporal retina in these projections is a result of experience-based plasticity is also discussed.