Somatosensory areas S2 and PV project to the superior colliculus of a prosimian primate, Galago garnetti

As part of an effort to describe the connections of the somatosensory system in Galago garnetti, a small prosimian primate, injections of tracers into cortex revealed that two somatosensory areas, the second somatosensory area (S2) and the parietal ventral somatosensory area (PV), project densely to the ipsilateral superior colliculus, while the primary somatosensory area (S1 or area 3b) does not. The three cortical areas were defined in microelectrode mapping experiments and recordings were used to identify appropriate injection sites in the same cases. Injections of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) were placed in S1 in different mediolateral locations representing body regions from toes to face in five galagos, and none of these injections labeled projections to the superior colliculus. In contrast, each of the two injections in the face representation of S2 in two galagos and three injections in face and forelimb representations of PV in three galagos produced dense patches of labeled terminations and axons in the intermediate gray (layer IV) over the full extent of the superior colliculus. The results suggest that the higher-order somatosensory areas, PV and S2, are directly involved in the visuomotor functions of the superior colliculus in prosimian primates, while S1 is not. The somatosensory inputs appear to be too widespread to contribute to a detailed somatotopic representation in the superior colliculus, but they may be a source of somatosensory modulation of retinotopically guided oculomotor instructions.     

Areal Distributions of Cortical Neurons Projecting to Different Levels of the Caudal Brain Stem and Spinal Cord in Rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Ag_m), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb.

The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI. Injections in lateral SI representing the face produced dense terminal label in the contralateral trigeminal complex. Injections in cortex devoted to the forelimb and forepaw labeled the contralateral cuneate nucleus and parts of the dorsal horn of the spinal cord. The cortical injections also demonstrated interconnections of parts of SI with some of the other regions of cortex with projections to the spinal cord, and provided further evidence for the existence of PV in rats.     

Areal distributions of cortical neurons projecting to different levels of the caudal brain stem and spinal cord in rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Agm), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb. The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI.(ABSTRACT TRUNCATED AT 400 WORDS)     

Areal and callosal connections in the somatosensory cortex of the star-nosed mole

Star-nosed moles have a well-developed somatosensory cortex with multiple cortical areas representing the behaviorally important tactile star. In each of three cortical representations, the 11 mechanosensory appendages from the contralateral nose are represented in a series of dark cytochrome oxidase modules. Here the connections of this complex cortical network were explored with injections of the neuroanatomical tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-H RP). The main goal was to determine the connection patterns of the somatosensory areas that represent the star. Injections of tracer made in or around the primary somatosensory representation (S1) of the star allowed us to determine the topography of local cortical connections and the projection and termination sites of corresponding interhemispheric connections. The results revealed precise topographic corticocortical connections reciprocally interconnecting the S1 star representation with its counterparts in S2 and in a third representation (S3) unique to star-nosed moles. Callosal connections from a widespread area of the contralateral hemisphere terminated primarily in the septa between cytochrome oxidase dark modules and in areas of cortex surrounding the star representations. However, midline structures of the star represented in S1 and S2 exhibited a high level of callosally labeled cells and terminals. This included label both within septa and within the centers of cytochrome oxidase dense modules representing midline appendages.     

Tectal and Related Target Areas of Spinal and Dorsal Column Nuclear Projections in Hedgehog Tenrecs

The terminal distributions of spinal and dorsal column nuclear projections to tectum, pretectum, and central gray of hedgehog tenrecs (Echinops telfairi and Setifer setosus) were investigated using anterograde axonal flow and various tracer substances. In the inferior colliculus, the densest and most extensive mesencephalic projections were found within the pericentral regions. One target area, referred to as the external portion of the inferior colliculus, was represented as a semicircle of grain patches lateral and caudal to the central nucleus. This region received somesthetic afferents from the dorsal column nuclei and from spinal segments at various levels. In contrast, after high cervical injections, the pericentral portion dorsomedial to the rostral half of the central nucleus was labeled almost exclusively. This area of labeling was distinct from the labeling in the central gray and might be best compared with the intercollicular zone in other species. The superior colliculus received projections predominantly from the high cervical cord; minor projections also arose from lumbar spinal segments and the dorsal column nuclei. The terminal field covered roughly the caudal half of the colliculus and involved the stratum griseum intermediale in a patch-like fashion. Some labeling was also found in the stratum griseum profundum and in the stratum griseum superficiale. Other than in the colliculi, weak pretectal projections were observed following dorsal column nuclear injections, while the nucleus of Darkschewitsch was labeled best following lumbosacral injections. All mesencephalic target areas were labeled consistently on the contralateral side, while their ipsilateral side was involved to a varying degree: The relatively most prominent ipsilateral labeling was seen in the central gray, being roughly similar on both sides; scarcely any labeling was noted in the ipsilateral superior colliculus. Tectal injections of retrograde tracer, in addition, revealed a considerable number of labeled neurons in a relatively cell-poor region immediately ventral to the high cervial dorsal horn. This region might correspond to the lateral cervical nucleus, an aggregation of neurons that so far has only been demonstrated in higher mammals.     

Cortical organization in moles: evidence of new areas and a specialized S2

The somatosensory cortex of several mole species (family Talpidae), with different peripheral sensory adaptations, was investigated and compared to determine common and specialized features of cortical organization. Previously unidentified medial representations of the trunk and limbs were found in all species, indicating that S1 in moles occupies a medial to lateral strip of cortex as in most other mammals. This finding suggests a large lateral forelimb representation, previously attributed to S1, is actually part of S2. In the face representation, evidence was found for three representations of the unusual nose of the star-nosed mole ( Condylura cristata ). Each of these areas was divided into a series of modules (visible in cytochrome oxidase processed tissue) representing individual nasal appendages on the star. In the closely related but less specialized eastern mole ( Scalopus aquaticus ) and coast mole ( Scapanus orarius ), only two nose representations were identified in an area of cortex with a more uniform histological appearance. The results indicate that moles have enlarged somatosensory representations of the glabrous nose as compared to shrews and rats that instead have large vibrissal representations. In addition moles have a very large and specialized representation of the digging forepaw in S2. Since this part of S2 projects directly to the cervical spinal cord, the specialization may provide adaptive sensorimotor functions related to digging.     

Cortical-organization in moles: evidence of new areas and a specialized S2

The somatosensory cortex of several mole species (family Talpidae), with different peripheral sensory adaptations, was investigated and compared to determine common and specialized features of cortical organization. Previously unidentified medial representations of the trunk and limbs were found in all species, indicating that S1 in moles occupies a medial to lateral strip of cortex as in most other mammals. This finding suggests a large lateral forelimb representation, previously attributed to S1, is actually part of S2. In the face representation, evidence was found for three representations of the unusual nose of the star-nosed mole (Condylura cristata). Each of these areas was divided into a series of modules (visible in cytochrome oxidase processed tissue) representing individual nasal appendages on the star. In the closely related but less specialized eastern mole (Scalopus aquaticus) and coast mole (Scapanus orarius), only two nose representations were identified in an area of cortex with a more uniform histological appearance. The results indicate that moles have enlarged somatosensory representations of the glabrous nose as compared to shrews and rats that instead have large vibrissal representations. In addition moles have a very large and specialized representation of the digging forepaw in S2. Since this part of S2 projects directly to the cervical spinal cord, the specialization may provide adaptive sensorimotor functions related to digging.     

A Comparison of the Ipsilateral Cortical Projections to the Dorsal and Ventral Subdivisions of the Macaque Premotor Cortex

The cortical connections of the dorsal (PMd) and ventral (PMv) subdivisions of the premotor area (PM, lateral area 6) were studied in four monkeys (Macaca fascicularis) through the use of retrograde tracers. In two animals, tracer was injected ventral to the arcuate sulcus (PMv), in a region from which forelimb movements could be elicited by intracortical microstimulation (ICMS). Tracer injections dorsal to the arcuate sulcus (PMd) were made in two locations. In one animal, tracer was injected caudal to the genu of the arcuate sulcus (in caudal PMd [cPMd], where ICMS was effective in eliciting forelimb movements); in another animal, it was injected rostral to the genu of the arcuate sulcus (in rostral PMd [rPMd], where ICMS was ineffective in eliciting movements). Retrogradely labeled neurons were counted in the ipsilateral hemisphere and located in cytoarchitectonically identified areas of the frontal and parietal lobes. Although both PMv and PMd were found to receive inputs from other motor areas, the prefrontal cortex, and the parietal cortex, there were differences in the topography and the relative strength of projections from these areas.

There were few common inputs to PMv and PMd; only the supplementary eye fields projected to all three areas studied. Interconnections within PMd or PMv appeared to link hindlimb and forelimb representations, and forelimb and face representations; however, connections between PMd and PMv were sparse. Areas cPMd and PMv were found to receive inputs from other motor areas—the primary motor area, the supplementary motor area, and the cingulate motor area—but the topography and strength of projections from these areas varied. Area rPMd was found to receive sparse inputs, if any, from these motor areas. The frontal eye field (area 8a) was found to project to PMv and rPMd, and area 46 was labeled substantially only from rPMd. Parietal projections to PMv were found to originate from a variety of somatosensory and visual areas, including the second somatosensory cortex and related areas in the parietal operculum of the lateral sulcus, as well as areas 5, 7a, and 7b, and the anterior intraparietal area. By contrast, projections to cPMd arose only from area 5. Visual areas 7m and the medial intraparietal area were labeled from rPMd. Relatively more parietal neurons were labeled after tracer injections in PMv than in PMd. Thus, PMv and PMd appear to be parts of separate, parallel networks for movement control.     

An examination of somatosensory area SIII in ferret cortex

A somatotopically organized region on the suprasylvian gyrus of the ferret was examined using multiunit recordings and anatomical tracer injections. This area, which contains a representation of the face, was bordered by the primary somatosensory area (SI), anteriorly, and by the visually responsive rostral posterior parietal cortex (PPr), posteriorly. Anatomical tracers revealed connections to this region from cortical areas MI, SI, MRSS, PPr, and the thalamic posterior nucleus. These results are consistent with previous work in ferrets as well as with the location, physiology, and connectivity of area SIII in cats. Given its associations, functional properties, location, and homology, it is proposed that this region represents the third cortical somatosensory area (SIII) in ferrets.     

Overview of Sensory Systems of <Emphasis Type="Italic">Tarsius</Emphasis>

Tarsiers form the sister taxon to anthropoid primates, and their brains possess a mix of primitive and specialized features. We describe architectonically distinct subdivisions of the somatosensory, auditory, and visual systems for tarsiers, as well as nocturnal New World owl monkeys (Aotus) and strepsirhine galagos (Otolemur) for comparison. In general, the dorsal column nuclei, the ventroposterior nucleus, and primary somatosensory cortex are somewhat less distinctly differentiated in tarsiers, suggesting that the somatosensory system is less specialized for somatosensory processing. Although the inferior colliculus and the medial geniculate complex of the auditory system are architectonically similar across the 3 primates, the primary auditory cortex of tarsiers is more distinct, suggesting a greater role in auditory cortical processing. In the visual system, the differentiation of the superior colliculus is similar in all 3 primates, whereas the laminar pattern in the lateral geniculate nucleus and the subdivisions of the inferior pulvinar in tarsiers resemble those of anthropoid primates rather than strepsirhines, in agreement with the evidence that tarsiers form the sister clade for anthropoids. In addition, primary visual cortex has more distinct sublayers in tarsiers than other primates, attesting to its importance in this visual predator. Overall, tarsiers have well developed visual and auditory systems, and a less well developed somatosensory system, suggesting an enhanced reliance on the visual and auditory senses, rather than somatosensory sense.     

The organization and connections of somatosensory cortex in the brush-tailed possum (Trichosurus vulpecula): evidence for multiple, topographically organized and interconnected representations in an Australian marsupial

Microelectrode mapping techniques were used to determine the organization of somatosensory cortex in the Australian brush-tailed possum (Trichosurus vulpecula). The results of electrophysiological mapping were combined with data on the cyto- and myeloarchitecture, and patterns of corticocortical connections, using sections cut tangential to the pial surface. We found evidence for three topographically organized representations of the body surface that were coextensive with architectonic subdivisions. A large, discontinuous cutaneous representation in anterior parietal cortex was termed the primary somatosensory area (SI). Lateral to SI we found evidence for two further areas, the second somatosensory area (SII) and the parietal ventral area (PV). While neurones in all of these areas were responsive to cutaneous stimulation, those of SI were non-habituating, whereas those in SII and PV often habituated to the stimuli. Moreover, neuronal receptive fields in SII and PV were, in general, larger than those in SI. Neurones in cortex adjacent to the rostral and caudal boundaries of SI, including cortex that interdigitated between the discontinuous SI head and body representations, required stimulation of deep receptors in the periphery to elicit responses. Within the region of cortex containing neurones responsive to stimulation of deep receptors, body parts were represented in a mediolateral progression. Injections of anatomical tracers placed in electrophysiologically identified locations in SI revealed ipsilateral connections with other parts of SI, as well as cortex rostral to, caudal to, and interdigitating between, SI. Injections in SI also resulted in labelling in PV, SII, motor cortex, posterior parietal cortex and perirhinal cortex. The patterns of contralateral projections reflected those of ipsilateral projections, although they were relatively less dense. The present findings support recent observations in other marsupials in which multiple representations of the body surface were described, and suggest that multiple interconnected sensory representations may be a common feature of cortical organization and function in marsupials.     

The use of hand morphology in the taxonomy of galagos

Measurements of volar (hand) pad area were made for 244 specimens, representing 12 species and 4 genera of galagos (sub-family Galaginae). When corrected for body weight, statistically significant differences were identified, at both the genus and species levels in the areas of the volar pads. Most informative, in terms of taxonomic differences, were measurements of two of the five pads; interdigjtal pad number 4 and thenar pad number 5. Closely related species were distinguishable on the basis of these measurements. The thick-tailed galago (Otolemur crassicaudatus) was separate from Garnett's galago (O. garnettii) and the specific status of the silver greater galago (O. argentatus) was supported. Likewise, the two needle-clawed galagos (Euoticus elegantulus andE. pallidus), recently separated on mitochodrial DNA grounds, were found to differ significantly in their volar pad morphology. These studies show that comparative studies of volar hand pad morphology provide a novel approach to the re-assessment of galago taxonomy, and may be applicable also in taxonomic studies of other prosimian groups.     

Topography of the thalamo-cortical projections on trunk representation demonstrated by fluorescent neuro-tracers

With the aim to study the detailed topography of the thalamo-cortical neurones projecting to the trunk representation zone of the first somatosensory area (SI), punctate injections of three different fluorescent tracers (Evans Blue, Nuclear Yellow and Fast Blue) were performed in the three physiologically defined subareas forming the trunk region of SI. These injections resulted in the labelling of three different cell aggregates, narrow in dorsoventral and mediolateral extent but elongated rostrocaudally, located in topographically distinct regions of the nucleus ventralis posterio-lateralis. The results suggest that the highly organized topography of the trunk representation of area SI is imposed by the thalamo-cortical input from VPL.     

Ablations of areas 3b (SI proper) and 3a of somatosensory cortex in marmosets deactivate the second and parietal ventral somatosensory areas

Partial ablations of specific parts of cortical areas 3b (SI proper) and 3a in marmosets were found to render somatotopically equivalent parts of two other cortical somatosensory fields, the second somatosensory area (SII) and the parietal ventral area (PV), unresponsive to peripheral stimulation. Microelectrode recordings in anesthetized marmosets first established the responsiveness and locations of the representations of body parts, including the hand in areas 3a and 3b, SII, and in some cases PV. The hand representations in areas 3a and 3b were then removed by aspiration. Immediately afterwards, additional recordings established that regions of SII and PV that formerly represented the hand were no longer responsive to cutaneous stimulation of the hand (or any other skin surface). Other parts of these fields, representing parts of the body other than the hand, remained responsive to stimulation of the previously effective receptive fields. We conclude that SII and PV depend on inputs (either direct or indirect) from areas 3a and 3b for their activation.     

环鸽丘脑听区传入神经投射的研究

应用神经示踪物ＰＨＡＬ和ＢＤＡ对环鸽丘脑听区的传入神经投射进行了研究。结果发现中脑外侧核背部和丘间核交界内缘区的神经元发出纤维投射至丘脑卵形核周围形成卵形壳;尾部Ｏｖ壳和Ｏｖ交界面区域接受前峡核浅区的投射;尾部Ｏｖ壳不但接受ＩＣＭ神经元的传出投射,而且有神经发出的传出纤维参与了Ｏｖ壳的形成。     

Retinohypothalamic projection in the mouse: Electron microscopic and iontophoretic investigations of hypothalamic and optic centers

The problem of the direct retinohypothalamic projection in mammals (Moore, 1973) was reinvestigated in the laboratory mouse by electron microscopy and cobalt chloride-iontophoresis. The time-course of the axonal degeneration in the suprachiasmatic nucleus was studied 3, 6 and 12 h, 1, 2, 4, 6, 9 and 12 days after unilateral retinectomy. Specificity of the degenerative changes was controlled by investigation of the superficial layers of the superior colliculus. The ratio of crossed to uncrossed optic fibers could could be determined by counting degenerating structures (axons and terminals) in the optic chiasma and the ipsilateral and contralateral areas of the optic tract, the suprachiasmatic nucleus, and the superior colliculus. The number of degenerating axons in the suprachiasmatic nucleus showed a maximum one day after unilateral retinectomy and was, at all stages studied, two to three times higher in the contralateral than in the ipsilateral nuclear area. In the optic tract and in the superior colliculus the number of degenerating profiles was three times higher in the contralateral than in the ipsilateral area. Retinohypothalamic connections and crossing pattern of retinal fibers were studied light microscopically using impregnation with cobalt sulfide in whole mounts of brains. Most of the optic fibers in the laboratory mouse are crossed crossed (70-80%). A bundle of predominantly crossed optic fibers runs to the suprachiasmatic nucleus.     

Effects of ovariectomy and estradiol replacement therapy upon the sexual and aggressive behavior of the greater galago (Galago crassicaudatus crassicaudatus)

The effects of ovariectomy and estradiol treatment upon sexual and aggressive behavior were studied in a prosimian primate, the greater galago. Ovariectomized galagos were sexually unreceptive and frequently aggressive, but retained their sexual attractiveness to males. When females were treated with estradiol monobenzoate, however, their aggression and refusals of males' mounting attempts decreased markedly. Although males mounted these females, they usually failed to copulate, possibly because the females did not perform certain postural adjustments which assist males to intromit. Estradiol benzoate alone, even in large doses, does not fully restore patterns of mating behavior in ovariectomized female greater galagos. These observations on a prosimian primate are in striking contrast to the results of similar work on Old World monkeys and chimpanzees.     

Differentiation of vocalizations in bushbabies (Galaginae, Prosimiae, Primates) and the significance for assessing phylogenetic relationships

A comparative bioacustic analysis of vocalizations of the prosimian subfamily Galaginae revealed that morphologically similar sibling taxa within the main groups of the lesser galagos and the greater galagos can be reliably identified phenotypically on the basis of the acoustic structure of their loud call or advertisement call. Results confirm the separation of two distinct species of greater galagos, Galago crassicaudatus and Galago garnettii, and strongly suggest the discrimination of three distinct species from the senegalensis lesser bushbaby group, Galago senegalensis, Galago moholi and Galao zanzibaricus. An investigation of the ontogenetic development of the loud call indicated that it is derived from the infant's isolation call, displaying in all studied bushbaby taxa a fairly similar acoustic pattern. Shared acoustic characters of the loud call among the different taxa as well as the infant's isolation call were used to propose a hypothesis about the phylogenetic affinities in bushbabies. The results seem to be supported by recent fossil records.     

The Corticocuneate Pathway in the Cat: Relations among Terminal Distribution Patterns,Cytoarchitecture, and Single Neuron Functional Properties

A combined anatomical and physiological strategy was used to investigate the organization of the corticocuneate pathway in the cat. The distribution of the corticocuneate projection was mapped by means of the anterograde horseradish peroxidase (HRP) labeling technique and correlated with the nuclear cytoarchitecture in Nissl and Golgi material, the distribution of retrogradely labeled relay cells after HRP injections in the ventrobasal complex of the thalamus, and the topographic organization derived from single-and multiunit recordings in the decerebrate, unanesthetized cat. This approach provided details about the arrangement of the corticocuneate pathway that were not available from previous studies with anterograde degeneration methods.

On the basis of cytoarchitectonic and connectional features, a number of subdivisions are identified in the cuneate nucleus, each of which is associated with characteristic functional properties. In agreement with previous studies, it is found that a large portion of the cuneate nucleus, the middle dorsal part (MCd), is exclusively devoted to the representation of cutaneous receptive fields on the digits. This “core” region contains more thalamic projecting neurons than any other subdivision of the cuneate nucleus. A topographic arrangement also exists in the subdivisions of the rostral cuneate and of the nuclear region ventral to MCd, although in these, receptive fields are larger and predominantly, but not exclusively, related to deep receptors and involve the arm, shoulder, and trunk.

Observations on corticocuneate projections were based on injections, mainly focused on functional subdivisions of the primary somatosensory cortex (SI) as described by McKenna et al (1981). Although cortical projections are mainly to cuneate regions other than its core, a significant proportion of fibers from the region of SI where the digits are represented (particularly area 3b) do project to the MCd region of the cuneate nucleus. Similarly, nuclear areas associated with receptive fields on the arm and trunk are labeled after injection in SI arm and trunk regions, respectively. Thus, a close topographic relationship appears to exist between the somatosensory cortex and cuneate regions related to the same body representation, although nuclear regions in which receptive fields on the neck area are represented receive very sparse or no detectable cortical projections even when the injection of the tracer involves the entire sensorimotor cortex. The topographic arrangement of SI projections upon the cuneate nucleus suggests that a similar pattern exists in both structures with regard to the relative representations of distal versus proximal and deep versus cutaneous receptive fields (e.g., “core” vs. “shell” organization), and that cuneate regions preferentially related to either of these classes of receptive fields receive direct connections from the corresponding regions in SI.

A comparison of the results from cats with tracer injections in areas 4 and 3b reveals that the projections from the former is denser than that arising from the latter and that their territories of termination largely overlap in the ventral portions of the cuneate nucleus. However, cortical projections to MCd may be derived from the somatosensory cortex with no contribution from area 4. The demonstration of the relative selectivity of cortical projections from different cytoarchitectonic and functional cortical areas to cuneate regions identified here provides a structural basis for the elucidation of the physiological and behavioral observations, particularly on cortical modulation of somatosensory transmission during movements.     

Multisensory integration in the superior colliculus: a neural network model

Neurons in the superior colliculus (SC) are known to integrate stimuli of different modalities (e.g., visual and auditory) following specific properties. In this work, we present a mathematical model of the integrative response of SC neurons, in order to suggest a possible physiological mechanism underlying multisensory integration in SC. The model includes three distinct neural areas: two unimodal areas (auditory and visual) are devoted to a topological representation of external stimuli, and communicate via synaptic connections with a third downstream area (in the SC) responsible for multisensory integration. The present simulations show that the model, with a single set of parameters, can mimic various responses to different combinations of external stimuli including the inverse effectiveness, both in terms of multisensory enhancement and contrast, the existence of within- and cross-modality suppression between spatially disparate stimuli, a reduction of network settling time in response to cross-modal stimuli compared with individual stimuli. The model suggests that non-linearities in neural responses and synaptic (excitatory and inhibitory) connections can explain several aspects of multisensory integration.