The organization of mechanoreceptive projections in the ventrobasal thalamus of sheep

Somatosensory nuclei of the manatee brainstem and thalamus

Florida manatees have an extensive, well-developed system of vibrissae distributed over their entire bodies and especially concentrated on the face. Although behavioral and anatomical assessments support the manatee's reliance on somatosensation, a systematic analysis of the manatee thalamus and brainstem areas dedicated to tactile input has never been completed. Using histochemical and histological techniques (including stains for myelin, Nissl, cytochrome oxidase, and acetylcholinesterase), we characterized the relative size, extent, and specializations of somatosensory regions of the brainstem and thalamus. The principal somatosensory regions of the brainstem (trigeminal, cuneate, gracile, and Bischoff's nucleus) and the thalamus (ventroposterior nucleus) were disproportionately large relative to nuclei dedicated to other sensory modalities, providing neuroanatomical evidence that supports the manatee's reliance on somatosensation. In fact, areas of the thalamus related to somatosensation (the ventroposterior and posterior nuclei) and audition (the medial geniculate nucleus) appeared to displace the lateral geniculate nucleus dedicated to the subordinate visual modality. Furthermore, it is noteworthy that, although the manatee cortex contains Rindenkerne (barrel-like cortical nuclei located in layer VI), no corresponding cell clusters were located in the brainstem ("barrelettes") or thalamus ("barreloids").

Representation of the face and intraoral structures in area 3b of the squirrel monkey (Saimiri sciureus) somatosensory cortex, with special reference to the ipsilateral representation

Studies of the representation of the trigeminal nerve in the thalamus and cerebral cortex of mammals have revealed representations of both contra- and ipsilateral intraoral structures. However, the relative extent of both representations is subject to considerable species variation. The present study employed microelectrode mapping and anatomical tracing to investigate the location and extent of the ipsilateral representation in area 3b of the somatosensory cortex of squirrel monkeys. A small region, approximately 2 mm2, was found to be responsive to stimulation of ipsilateral intraoral structures. This region was located on the anteromedial border of area 3b, surrounded by the representation of the contralateral roof of the mouth. This region corresponded to areas of intense anterograde labeling following injections placed in the ventromedial portion of the ventral posterior medial nucleus of the thalamus at the only sites where neural responses could be elicited by stimulation of ipsilateral intraoral structures. The amount of thalamus and cortex given over to the ipsilateral representation in the squirrel monkey is small compared with that of the macaque monkey. This difference may be related to the lack of cheek pouches in the squirrel monkey, and therefore a different strategy for eating. The representation of the contralateral lower lip in area 3b was split by the representation of the contralateral upper lip. This split representation is in agreement with previous studies of the trigeminal representation in area 3b of the macaque monkey and may be a general feature of the representation of the trigeminal nerve in area 3b of primate cerebral cortex.

Kinesthetic cortical area anterior to primary somatic sensory cortex in the raccoon (Procyon lotor)

Extracellular microelectrode recording of cortical unit activity, with subsequent histological examination, was used to determine the extent, organization, and cytoarchitecture of the zone of muscle afferent projections (kinesthetic cortex) anterior to the primary somatic sensory cortex in anesthetized raccoons. Activity was evoked in response to mechanical stimulation of muscles from which the overlying skin had been dissected away. Most kinesthetic responses were elicited in a contiguous cortical area, which included: the anterior bank of the lateral arm, and the fundus and posterior bank of the medial arm of the medial central sulcus; and the anterior two-thirds of the interfundic rise within the interbrachial sulcus. Some responses were recorded in a separate small area of the anterior bank at the medial end of the lateral central sulcus. Somatotopy was evident with forelimb represented lateral to hindlimb. Proximal limb muscles were represented in the center of the medial central sulcus; distal muscle projections were medial (hindlimb) or lateral (forelimb) in the same sulcus. Most representations were of flexor and extensor muscles of the contralateral carpus and forepaw digits. Activity at a given recording locus in the kinesthetic area could be elicited by both flexor and extensor muscles, which acted about a common joint. Low amplitude units evoked by cutaneous stimulation of the dissected skin were recorded in the kinesthetic area; these were from receptive fields of skin that normally overlay the muscles whose higher-amplitude evoked kinesthetic units were represented in that same recording locus. The kinesthetic zone was anterior to primary somatic sensory cortex, where the outer stripe of Baillarger and granular layer IV become attenuated. In the hindlimb muscle representation area, the additional criterion of area 3a (large pyramidal cells in layer V) was seen. However, no cytoarchitecture could be identified that was consistently associated with the kinesthetic cortex.

Organization of postcranial kinesthetic projections to the ventrobasal thalamus in raccoons

To determine the presence and organization of kinesthetic, as compared with other mechanosensory projection zones in the thalamus of raccoons, unit-cluster responses to mechanical stimulation of the postcranial body were mapped electrophysiologically in the thalami of 14 raccoons anesthetized with Dial-urethane. A distinct zone of kinesthetic projections (from receptive fields in muscles, tendons, and joints) was found in the rostral and dorsal aspects of the mechanosensory projection zone. These projections are somatotopically organized: those from axial structures lie dorsalmost and those from successively more distal limb regions are successively more caudoventral. The kinesthetic forelimb representation is large and lies rostrodorsal to a large central core of cutaneous projections from the forepaw digits. A few scattered kinesthetic projections were found at the caudal edge of the sensory thalamic region. The large, spatially and somatotopically distinct kinesthetic projection zone in the thalamus parallels those seen in the cortex and medulla of raccoons. Similar findings in monkeys, and suggestions from data in cats and humans support the hypothesis of a distinct pathway to the cortex for kinesthetic information in all mammals.

Vibrissae representation in subcortical trigeminal centers of the neonatal rat

In the neonatal rat differential activity levels of the metabolic enzyme succinic dehydrogenase (SDH) reveal intricately detailed sgementation in the neuropil of the spinal and principal trigeminal nuclei of the brainstem and in the ventrobasal complex of the thalamus. The segmentation occurs in the portions of these nuclei that electrophysiological evidence has indicated to be related to the mystacial vibrissae and sinus hairs on the face of the rat. Indeed, the pattern of segmentation in each nucleus replicates the topographic distribution of the vibrissae and sinus hairs. Further, within the spinal trigeminal nucleus, there appear to be two distinct representations of the vibrissae, one in the subnucleus caudalis and a second in the subnucleus interpolaris. Examination of these patterns of segmentation indicates that the large mystacial vibriaase and sinus hairs on the face of the young rat are somatotopically represented three times within the trigeminal complex, as straight cylinders of neuropil, and once in the ventrobasal complex, as curved cylinders of neuropil. Neonatal vibrissae damage leads to an aberrant organization of the segmentation in the spinal trigeminal nucleus and the ventrobasal complex. In the spinal trigeminal nucleus, the SDH activity in areas associated with damaged vibrissae is of a lower than normal density, and patterns are indistinct. However, rows of clusters associated with the adjacent normal vibrissae are apparent and appear to be enlarged. In the ventrobasal complex, vibrissae damage results in bands of normal density SDH activity where rows of segmented clusters would normally be present. Comparison of these data to the cortical data in the previous paper (Killackey and Belford, '79) indicates that cortical and nuclear structures can have aspects of their development controlled by similar mechanisms.

Axonal projections and connections of the principal sensory trigeminal nucleus in the monkey

To date, anatomical studies of ascending principal sensory trigeminal nuclear (PrV) axons in the monkey have been restricted to few incomplete investigations utilizing the Marchi method. In the present study total or partial unilateral stereotaxic lesions of PrV were made in cebus and rhesus monkeys and analyzed with the aid of a variety of Nauta silver impregnation techniques applied to frozen sections. Analysis of the fiber degeneration emanating from PrV lesions indicates that PrV fibers form an ascending system composed of two distinct components. Most PrV axons project ventromedially from PrV through the ventral pontine tegmentum and gradually decussate across the midline in the mesencephalic tegmentum up to the level of the caudal pole of nucleus interpeduncularis. These decussated fibers form the trigeminal lemniscus, which courses dorsomedial to the medial lemniscus during its ascent into the diencephalon. A few whorls of preterminal fiber degeneration separating from the trigeminal lemniscus first appear in the magnocellular area of the thalamus medial to the medial geniculate body. The lemniscal PrV axons terminate densely throughout most, but not all, of the magnocellular part of nucleus ventralis posteromedialis (VPM) contralateral to the side of their origin. Some collateral-like fibers from the trigeminal lemniscus also were observed ending in the ventral segment of the zona incerta. Other axons, arising chiefly from the dorsal one-third of PrV form a smaller ipsilateral trigeminothalamic projection. These fibers all remain on the side of their origin and terminate consistently in a discrete dorsomedial paralaminar portion of VPM that does not receive lemniscal PrV connections. A commissural fiber system at the pontine level connects parvicellular reticular cells with their counterparts and the motor trigeminal nucleus of the opposite side. These interconnections appear to provide an anatomical link for the integration of bilateral trigeminal sensory information and motor function.

Species differences in mechanosensory projections from the mouth to the ventrobasal thalamus

To determine whether the largely ipsilateral, inverted representation of mouth parts in the ventrobasal thalamus of sheep was unique to that species or an expansion of a general mammalian pattern, the corresponding thalamic projections were mapped electrophysiologically in a selected series of mammals (oppossums, agoutis, squirrel monkeys, cats, raccoons, and sheep) representing major branches of evolution among therian mammals. In mapping, tungsten microelectrodes were used to record multi-unit discharges in the thalamus in response to mechanical stimulation of oral surfaces. The pattern of projections seen in sheep is not a general mammalian pattern; there is extensive variability among mammals in the laterality and internal orgainzation of the projections from the mouth. In spite of the great variability, the results suggest an hypothesis concerning phylogenetic trends: descendants of palaeoryctoid insectivores (cats, raccoons, and sheep in our sample) have extensive ipsilateral projections from the mouth, in other therian mammals (opossums, agoutis, and squirrel monkeys in our sample) the ipsilateral component is small or absent.