Common fur and mystacial vibrissae parallel sensory pathways: 14 C 2-deoxyglucose and WGA-HRP studies in the rat

Stimulation of mystacial vibrissae in rows A,B, and C increased (14C) 2-deoxyglucose (2DG) uptake in spinal trigeminal nucleus pars caudalis (Sp5c) mostly in ventral portions of laminae III-IV with less activation of II and V. Stimulation of common fur above the whiskers mainly activated lamina II, with less activation in deeper layers. The patterns of activation were compatible with an inverted head, onion skin Sp5c somatotopy. Wheatgerm Agglutinin-Horseradish Peroxidase (WGA-HRP) injections into common fur between mystacial vibrissae rows A-B and B-C led to anterograde transganglionic labeling only of Sp5c, mainly of lamina II with less label in layer V, and very sparse label in III and IV. WGA-HRP skin injections appear to primarily label small fibers, which along with larger fibers, were metabolically activated during common fur stimulation. Mystacial vibrissae stimulation increased 2DG uptake in ventral ipsilateral spinal trigeminal nuclei pars interpolaris (Sp5i) and oralis (Sp5o) and principal trigeminal sensory nucleus (Pr5). Common fur stimulation above the whiskers slightly increased 2DG uptake in ventral Sp5i, Sp5o, and possibly Pr5. The most dorsal aspect of the ventroposteromedial (VPM) nucleus of thalamus was activated contralateral to whisker stimulation. Stimulation of the common fur dorsal to the whiskers activated a region of dorsal VPM caudal to the VPM region activated during whisker stimulation. This is consistent with previous data showing that ventral whiskers and portions of the face are represented rostrally in VPM, and more dorsal whiskers and dorsal portions of the face are represented progressively more caudally in VPM. Mystacial vibrissae stimulation activated the contralateral primary sensory SI barrelfield cortex and a separate region in the second somatosensory SII cortex. Common fur stimulation above the whiskers activated a cortical region between the SI and SII whisker activated regions of cortex. It is proposed that this region represented the combined SI and SII common fur regions of somatosensory neocortex. Both whisker and common fur stimulation activated all layers of cortex, with layer IV being most activated followed by II-III, V, and VI. These data indicate that sensory input from the mystacial vibrissae in the adult rat is processed in brainstem, thalamic, and cortical pathways which are predominantly parallel to those which process information from the neighboring common fur sensory receptors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Multiple vibrissae sensory regions in rat cerebellum: a (14C) 2-deoxyglucose study

Repetitive tactile sensory stimulation of the right mystacial vibrissae (whiskers) was performed in awake, adult rats. Regions of increased (14C) 2-deoxyglucose (2DG) uptake were mapped autoradiographically in cerebellum. Predominantly ipsilateral activation of multiple discrete granule cell regions occurred in paramedian lobule, crus 2, crus 1, lobulus simplex, and anterior lobe hemisphere. Bilateral and contralateral activation of cerebellum did occur. Multiple small patches, as well as large granule cell regions, were activated. Mossy fiber afferents from the spinal trigeminal nuclei (particularly interpolaris), principal trigeminal sensory nucleus, and superior colliculus could account for metabolic activation of the granular layer. The slight metabolic activation of the molecular layer could have occurred from climbing or parallel fibers. Comparisons of the paramedian lobule granule cell regions activated during repetitive sensory stimulation of the vibrissae (RSSV) to those activated during vibrissae motor cortex stimulation (VMIS) showed regions only activated by RSSV, regions only activated by VMIS, and regions activated by both RSSV and VMIS. The granule cell regions activated during RSSV and VMIS were usually adjacent to or overlapping each other. Regions only activated during RSSV or only during VMIS could represent technical problems in trying to compare vibrissae motor and sensory pathways in two different groups of animals. Alternatively, cerebellar regions activated only during RSSV could process vibrissae tactile inputs. Regions activated only during VMIS could process vibrissae motor and perhaps proprioceptive sensory input. Regions activated during both RSSV and VMIS might process vibrissae proprioceptive sensory input and/or might represent loci where vibrissae motor, proprioceptive sensory, and tactile sensory convergence occur. The results raise the possibility that vibrissae motor, proprioceptive sensory, and tactile sensory pathways could be processed in separate granule cell patches in parts of cerebellum and in the same granule cell patches in other parts of cerebellum.     

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: Cortex,diencephalon, midbrain

The caudal forelimb region of right “motor” cortex was repetitively stimulated in normal, conscious rats. Left forelimb movements were produced and (¹⁴C) 2-deoxyglucose (2DG) was injected. After sacrifice, regions of increased brain (¹⁴C) 2DG uptake were mapped autoradiographically. Uptake of 2DG increased about the stimulating electrode in motor (MI) cortex. Columnar activation of primary (SI) and second (SII) somatosensory neocortex occurred. The rostral or second forelimb (MII) region of motor cortex was activated. Many ipsilateral subcortical structures were also activated during fore-limb MI stimulation (FLMIS). Rostral dorsolateral caudate-putamen (CP), central globus pallidus (GP), posterior entopeduncular nucleus (EPN), sub-thalamic nucleus (STN), zona incerta (ZI), and caudal, ventrolateral substantia nigra pars reticulata (SNr) were activated. Thalamic nuclei that increased (¹⁴C) 2DG uptake included anterior dorsolateral reticular (R), ventral and central ventrolateral (VL), lateral ventromedial (VM), ventral ventrobasal (VB), dorsolateral posteromedial (POm), and the parafascicular-centre median (Pf-CM) complex. Activated midbrain regions included ventromedial magnocellular red nucleus (RNm), posterior deep layers of the superior colliculus (SCsgp), lateral deep mesencephalic nucleus (DMN), nucleus tegmenti pedunculopontinus (NTPP), and anterior pretectal nucleus (NCU). Monosynaptic connections from MI or SI to SII, MII, CP, STN, ZI, R, VL, VM, VB, POm, Pf-CM, RNm, SCsgp, SNr, and DMN can account for ipsilateral activation of these structures. GP and EPN must be activated polysynaptically, either from MI stimulation or sensory feedback, since there are no known monosynaptic connections from MI and SI to these structures. Most rat brain motor-sensory structures are somatotopically organized. However, the same regions of R, EPN, CM-Pf, DMN, and ZI are activated during FLMIS compared to VMIS (vibrissae MI stimulation). Since these structures are not somatopically organized, this suggests they are involved in motor-sensory processing independent of which body part is moving. VB, SII, and MII are activated during FLMIS but not during VMIS.     

Functional development of the vibrissae somatosensory system of the rat: (14C) 2-deoxyglucose metabolic mapping study

Functional development of the rat whisker somatosensory system was studied by using the (¹⁴C) 2-deoxyglucose (2DG) metabolic mapping technique. Restrained rat pups had their left mystacial vibrissae stroked for 30 minutes and their brains harvested, sectioned, and autoradiographed from the level of the lower medulla to the frontal cortex. Subjects were tested at postnatal days (PNDs) 0–9 and 21. At birth, all subjects exhibited a significant increase of 2DG uptake in the left spinal trigeminal nuclei, the principal trigeminal sensory nucleus, and a portion of the right ventral posteromedial thalamic nucleus. The primary somatosensory cortex exhibited significant 2DG uptake contralateral to stimulation by PND 6, followed by the secondary somatosensory cortex at PND 7. The pattern of 2DG uptake in the somatosensory cortices became more intense and well defined by PND 9. Given that the somatosensory system develops in an orderly fashion from the periphery to higher brain structures, the present results show that brain structures mediating whisker sensory input are not metabolically active until projections from lower somatosensory centers are established. Neurons become responsive to whisker stimulation in the subcortical structures at birth and in the somatosensory cortex a few days later. This cortical activity follows the organization of the upper tier of thalamocortical fibers into a “barrelfield.” Moreover, there is a gradual enhancement in functional activity of the vibrissa neurons at different somatosensory nuclei as rats mature. The present study elucidates the time course of functional development in the rat somatosensory system. J. Comp. Neurol. 384:323–336, 1997. © 1997 Wiley-Liss, Inc.     

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: Pons,cerebellum, medulla,spinal cord,muscle

Electrical stimulation of the right forelimb motor (MI) sensory (SI) cortex in normal, adult rats produced repetitive left forelimb movements. Regions of increased (¹⁴C) 2-deoxyglucose (2DG) uptake were mapped autoradiographically during these movements. MI stimulation activated the ipsilateral reticular tegmental pontine nucleus (RTF) and the middle (rostral-caudal) third of the pontine nuclei including pyramidal (P), medial (POM), ventral (POV), and lateral (POL) pontine nuclei. The ipsilateral inferior olivary complex was activated including dorsal accessory olive (DAO), principal olive (PO), and medial accessory olive (MAO). The contralateral lateral reticular (LR) nucleus and nucleus cuneatus (CU) were activated. Lateral vermal, paravermal, and hemispheric portions of the contralateral cerebellum were also activated. Parts of vermian lobules IV, V, VI, VII, and VIII, and lobulus simplex, crus I, crus II, paramedian lobule, and copula pyramidis were activated. Granule cell layers were activated much more than molecular layers. Discrete microzones of high granule cell 2DG uptake alternated with zones of low uptake in left paramedian lobule and copula pyramidis and may correlate with the fractured cerebellar somatotopy described physiologically by Welker and his associates. Portions of the left lateral and interpositus nuclei were metabolically activated. Medial portions of laminae I-VI were activated in the dorsal horn of cervical spinal cord. The 2DG uptake was either unchanged or decreased in the ventral horn. Thoracic and lumbar spinal cord were not activated. Monsynaptic MI and SI connections to P, POM, POV, POL, RTF, DAO, PO, MAO, LR, CU, and spinal cord could account for activation of those structures. However, there are no direct MI or SI connections to the deep cerebellar nuclei, the cerebellar hemisphere, or the muscles. Activation of these structures must be due to activation of polysynaptic pathways, sensory feedback from the moving forelimb, or both. The present experiments cannot distinguish these possibilities. Comparison of the regions activated during forelimb MI stimulation (FLMIS) to those activated during vibrissae MI stimulation (VMIS) suggests that the pontine nuclei, cerebellar hemisphere, and possibly the deep cerebellar nuclei are somatotopically organized. RTP, LR, CU, and spinal cord were activated during FLMIS but were not activated during VMIS. The failure to activate the ventral horn of cervical spinal cord may be due to known inhibition of alpha-motor neurons during motor cortex stimulation.     

Sensorimotor corticocortical projections from rat barrel cortex have an anisotropic organization that facilitates integration of inputs from whiskers in the same row

We used a dual anterograde-tracing paradigm to characterize the organization of corticocortical projections from primary somatosensory (SI) barrel cortex. In one group of rats, biotinylated dextran amine (BDA) and Fluoro-Ruby (FR) were injected into separate barrel columns that occupied the same row of barrel cortex; in the other group, the tracers were deposited into barrel columns residing in different rows. The labeled corticocortical terminals in the primary motor (MI) and secondary somatosensory (SII) cortices were plotted, and digital reconstructions of these plots were quantitatively analyzed. In all cases, labeled projections from focal tracer deposits in SI barrel cortex terminated in elongated, row-like strips of cortex that corresponded to the whisker representations of the MI or SII cortical areas. When both tracers were injected into separate parts of the same SI barrel row, FR- and BDA-labeled terminals tended to merge into a single strip of labeled MI or SII cortex. By comparison, when the tracers were placed in different SI barrel rows, both MI and SII contained at least two row-like FR- and BDA-labeled strips that formed mirror image representations of the SI injection sites. Quantitative analysis of these labeling patterns revealed three major findings. First, labeled overlap in SII was significantly greater for projections from the same barrel row than for projections from different barrel rows. Second, in the infragranular layers of MI but not in the supragranular layers, labeled overlap was significantly higher for projections from the same SI barrel row. Finally, in all layers of SII and in the infragranular layers of MI, the amount of labeled overlap was proportional to the proximity of the tracer injection sites. These results indicate that SI projections to MI and SII have an anisotropic organization that facilitates the integration of sensory information received from neighboring barrels that represent whiskers in the same row.     

Cortical-thalamic relationships in the rat

Single electrolytic lesions were made in the ventrobasal or posterior thalamic nuclear complex of 12 rats with a microelectrode after having first recorded the activity evoked by light tactile stimulation of various body parts. Resultant cortical axonal degeneration was studied with the Fink-Heimer I technique. Lesions in a region of the ventrobasal complex responding to stimulation of contralateral mystacial vibrissae led to somatotopically organized degeneration in the barrel field of the first somatic sensory cortex (SI), i.e., caudoventral vibrissae were represented dorsocaudally in the barrel field while rostrodorsal vibrissae were represented in the rostroventral barrel field. Degenerating axons ascended through deep cortical layers in bundles and terminated densely on the cells of lamina IV, involving neurons within the core and walls of the barrels as well as those in septal areas between barrels. A lesion in a lateral portion of the ventrobasal complex, Emmers' ventrobasal complex SII area, which was responsive to bilateral stroking of the hairs of the rat's dorsum, produced degeneration caudal to and overlapping cortical SI, but not in SII. Lesions of the posterior complex led to axonal degeneration caudal to SI, overlapping both visual and auditory cortices. A lesion in the medial portion of the ventrobasal complex, the thalamic gustatory nucleus, resulted in dense degeneration in a region of cortex dorsal to the rhinal fissure at the rostrocaudal level of the middle cerebral artery. This study answers the question of the cortical target of a portion of Emmers' ventrobasal complex SII region and of the posterior complex and confirms the position of the gustatory area and somatotopic representation of mystacial vibrissae.     

Areal and laminar organization of corticocortical projections in the rat somatosensory cortex

The present study examines patterns of connectivity between the primary somatosensory cortex of the rat (SI) and surrounding cortical areas also implicated in the processing of somatosensory information. The impetus for the study was the recent reports of major differences in the organization of cortex lateral and caudal to the SI in two other rodent species; the mouse (Carvell and Simons, '86: Somatosens. Res. 3:213-237; '87: J. Comp. Neurol. 265:409-427) and the grey squirrel (Krubitzer et al., '86: J. Comp. Neurol 250: 403-430). Corticocortical connections between the somatosensory areas of the rat parietal cortex were examined by using the combined retrograde and anterograde transport of horseradish peroxidase as well as the retrograde transport of fluorescent tracers. Tracer injections were made into different locations within SI and dysgranular cortex as well as into more lateral regions of parietal cortex. The tangential patterns of distribution both of callosal connections and of cytochrome oxidase activity together provided points of reference in determining the relation between injection sites and the resultant patterns of label. The results indicate that two distinct somatosensory areas, SI and the dysgranular cortex, are interconnected with a further lateral somatosensory area referred to as the second somatosensory area (SII). These projections are organized in a topographic fashion, which we interpret as evidence for a single representation of the body surface in SII. The three somatosensory areas each exhibit unique laminar patterns of ipsilateral corticocortical projection neurons and terminations. In SI, projection neurons are found mainly in layers II, III, and Va, and terminations are largely restricted to the infragranular layers. In the dysgranular cortex, projection neurons and terminations are found in all layers except layer I in which only terminal label is detectable and layer Vb in which notably fewer neurons are labelled. In SII, projection neurons and terminations are found in all layers except layer I and are particularly dense in lower layer III and layer IV. Further, whereas the laminar and areal distributions of ipsilateral and contralateral corticocortical projections largely overlap in both SI and the dysgranular cortex, in SII they tend to be areally segregated. Neurons projecting bilaterally to both ipsilateral and contralateral somatosensory cortex were equally rare in all three somatosensory areas. These results are discussed in relation to the organization of SII in other rodent species, and it is concluded that in the rat, like the mouse, cortex lateral and caudal to SI contains a single representation of the body surface.     

Organization of the spinal trigeminal nucleus in star‐nosed moles

Somatosensory inputs from the face project to multiple regions of the trigeminal nuclear complex in the brainstem. In mice and rats, three subdivisions contain visible representations of the mystacial vibrissae, the principal sensory nucleus, spinal trigeminal subnucleus interpolaris, and subnucleus caudalis. These regions are considered important for touch with high spatial acuity, active touch, and pain and temperature sensation, respectively. Like mice and rats, the star‐nosed mole (Condylura cristata) is a somatosensory specialist. Given the visible star pattern in preparations of the star‐nosed mole cortex and the principal sensory nucleus, we hypothesized there were star patterns in the spinal trigeminal nucleus subnuclei interpolaris and caudalis. In sections processed for cytochrome oxidase, we found star‐like segmentation consisting of lightly stained septa separating darkly stained patches in subnucleus interpolaris (juvenile tissue) and subnucleus caudalis (juvenile and adult tissue). Subnucleus caudalis represented the face in a three‐dimensional map, with the most anterior part of the face represented more rostrally than posterior parts of the face. Multiunit electrophysiological mapping was used to map the ipsilateral face. Ray‐specific receptive fields in adults matched the CO segmentation. The mean areas of multiunit receptive fields in subnucleus interpolaris and caudalis were larger than previously mapped receptive fields in the mole's principal sensory nucleus. The proportion of tissue devoted to each ray's representation differed between the subnucleus interpolaris and the principal sensory nucleus. Our finding that different trigeminal brainstem maps can exaggerate different parts of the face could provide new insights for the roles of these different somatosensory stations. J. Comp. Neurol. 522:3335–3350, 2014. © 2014 Wiley Periodicals, Inc.     

Functional metabolic mapping during forelimb movement in rat. II. Stimulation of forelimb muscles

Repetitive electrical stimulation of wrist extensor muscles in rat was combined with quantitative 14C-deoxyglucose autoradiography to study sensory systems functionally activated during forelimb movement. Metabolism increased ipsilaterally in the wrist extensors, the dorsal horn of the cervical spinal cord, the cuneate nucleus and cerebellar hemisphere. The metabolic activation in cerebellum occurred in cortex surrounding the primary fissure anteriorly (lobules simplex and V), and the prepyramidal fissure posteriorly (lobules paramedian and copula pyramis). Metabolism was increased in both granule cell and molecular layers and was uniform throughout the zone of activation. Hindlimb stimulation primarily activated the medial aspect of copula pyramis, demonstrating the somatotopic specificity of changes. Forelimb stimulation also activated contralateral sites in the dorsal accessory nucleus of the inferior olive, ventrobasal thalamus, and SI and SII in cortex. Studies of the relationship between the magnitude of the response and the frequency of the stimulation revealed a positive correlation in muscle, dorsal horn and cuneate nucleus. Other activated sites only showed a significant change at the highest rates of stimulation. Comparison of the pattern of metabolic activation during forelimb movements induced centrally (Collins et al., 1986) with the pattern induced peripherally revealed overlap primarily in the paramedian zone of anterior and posterior cerebellum, and the granular cortex of SI and SII. These studies suggest that forelimb movement initiated centrally would have considerable influence on feedback sensation from the moving limb in these sites.     

The development of vibrissae representation in subcortical trigeminal centers of the neonatal rat.

In every station of the trigeminal system of the young rat, the segmented activity of the mitochondrial enzyme succinic dehydrogenase (SDH) clearly delineates the representation of the mystacial vibrissae. In the trigeminal complex of the medulla, three parallel representation can be seen, two in the spinal trigeminal nucleus and one in the principal trigeminal nucleus. In the next station, the ventrobasal complex of the thalamus, a single representation occurs. Likewise, layer IV of somatosensory cortex contains one representation of the vibrissae. Further, neonatal damage to the mystacial vibrissae results in anomalies within each representation. The present study delineates both the normal development of subcortical trigeminal stations and the aberrant organization seen after vibrisse removal. The results of a similar study on somatosensory cortex (Killackey and Belford, '79) and the present data allow the comparison of the development of each of the five vibrissae representations in the trigeminal system. In the brainstem, each of the three trigeminal complex representations are present at birth, although the pattern becomes more distinct over the first several days of life. Interestingly, vibrissae removal at birth induces an aberrant pattern that is distinct by postnatal Day 3. Although details are not equally discernible in each representation, the abnormalities appear to be similar. The SDH segmentation in the ventrobasal complex develops during postnatal Days 1 through 4. At Day 1, portions of the matrix of high density SDH activity break up into bands. Clusters can be discerned within these bands on Day 2. By Day 4 the pattern is sharply delineated. Vibrissae removal at birth results in anomalies that are a part of the initial development of segmentation, not a later reorganization. Comparison of the present data with that of our previous studies indicates that there is a sequential development of the central somatosensory structures related to the vibrissae, beginning with the most peripheral station. Further, there are many similarities in the development of each station. There are also differences which are particularly important in comparing the trigeminal nuclei with the later stations. The unique features in the abnormal development of the trigeminal nuclei are likely due to their direct connections with the periphery.     

Overlapping corticostriatal projections from the rodent vibrissal representations in primary and secondary somatosensory cortex

To determine whether the neostriatum receives overlapping projections from two somatosensory cortical areas, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into the whisker representations of primary (SI) and secondary (SII) somatosensory cortex. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to compare the extent of overlapping projections to both subcortical structures. Corticostriatal projections from focal sites in both somatosensory areas exhibited substantial amounts of divergence within the dorsolateral neostriatum. Most of the labeled terminals were concentrated in densely packed arborizations that occupied lamellar-shaped regions along the dorsolateral edge of the neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the thalamus, especially in the ventrobasal complex (VB) and in the medial part of the posterior (POm) nucleus. Because these thalamic regions are topographically organized and have reciprocal connections with corresponding representations in both SI and SII, the amount of labeled overlap in the thalamus was used to indicate the degree of somatotopic correspondence at the SI and SII injection sites. We found that the proportion of overlapping projections to the neostriatum was moderately correlated with the amount of overlap observed in the thalamus. This result strongly indicates that specific sites in the dorsolateral neostriatum receive convergent projections from corresponding somatotopic representations in SI and SII, but also suggests that some of the corticostriatal divergence may reflect neostriatal integration of somatosensory information from noncorresponding representations in SI and SII.     

Projections on the cortical somatic I barrel subfield from ipsilateral vibrissae in adult rodents.

In adult rats and mice, ipsilateral projections to SI posteromedial barrel subfield (PMBSF) are demonstrated for mystacial vibrissae (sinus hairs). Mechanical stimulation with angular deflections in the 2-5 degrees range, elicits potentials in limited areas of SI whose geometric centers coincide for two homologous (contra- and ipsilateral) vibrissae. The cortical domains for two adjacent vibrissae overlap one another slightly. Phase reversal of potentials and unit discharges are also present within the cortex. Ablation of the SI area contralateral to the SI area under study completely abolishes the ipsilaterally projected potentials. It is proposed that ipsilateral responses are mediated via the corpus callosum with an extra delay of 4-5 msec, thus giving each hemisphere the opportunity to compare vibrissal information originating from the two mystacial pads.     

Overlapping corticostriatal projections from the rodent vibrissal representations in primary and secondary somatosensory cortex

To determine whether the neostriatum receives overlapping projections from two somatosensory cortical areas, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into the whisker representations of primary (SI) and secondary (SII) somatosensory cortex. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to compare the extent of overlapping projections to both subcortical structures. Corticostriatal projections from focal sites in both somatosensory areas exhibited substantial amounts of divergence within the dorsolateral neostriatum. Most of the labeled terminals were concentrated in densely packed arborizations that occupied lamellar-shaped regions along the dorsolateral edge of the neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the thalamus, especially in the ventrobasal complex (VB) and in the medial part of the posterior (POm) nucleus. Because these thalamic regions are topographically organized and have reciprocal connections with corresponding representations in both SI and SII, the amount of labeled overlap in the thalamus was used to indicate the degree of somatotopic correspondence at the SI and SII injection sites. We found that the proportion of overlapping projections to the neostriatum was moderately correlated with the amount of overlap observed in the thalamus. This result strongly indicates that specific sites in the dorsolateral neostriatum receive convergent projections from corresponding somatotopic representations in SI and SII, but also suggests that some of the corticostriatal divergence may reflect neostriatal integration of somatosensory information from noncorresponding representations in SI and SII.     

Altered somatosensory processing in trigeminal neuralgia

Trigeminal neuralgia (TN) is a pain state characterized by intermittent unilateral pain attacks in one or several facial areas innervated by the trigeminal nerve. The somatosensory cortex is heavily involved in the perception of sensory features of pain, but it is also the primary target for thalamic input of nonpainful somatosensory information. Thus, pain and somatosensory processing are accomplished in overlapping cortical structures raising the question whether pain states are associated with alteration of somatosensory function itself. To test this hypothesis, we used functional magnetic resonance imaging to assess activation of primary (SI) and secondary (SII) somatosensory cortices upon nonpainful tactile stimulation of lips and fingers in 18 patients with TN and 10 patients with TN relieved from pain after successful neurosurgical intervention in comparison with 13 healthy subjects. We found that SI and SII activations in patients did neither depend on the affected side of TN nor differ between operated and nonoperated patients. However, SI and SII activations, but not thalamic activations, were significantly reduced in patients as compared to controls. These differences were most prominent for finger stimulation, an area not associated with TN. For lip stimulation SI and SII activations were reduced in patients with TN on the contra‐ but not on the ipsilateral side to the stimulus. These findings suggest a general reduction of SI and SII processing in patients with TN, indicating a long‐term modulation of somatosensory function and pointing to an attempt of cortical adaptation to potentially painful stimuli. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.     

Reactions of neurons in the primary and secondary somatosensory zones of the cortex to stimulation of the ventral posterior nucleus of the thalamus]

Neuronal responses in the first and second somatosensory cortex (SI and SII) to stimulation of the ventroposterior nucleus of the thalamus (VP) were studied in experiments on cats immobilized with d-tubocurarine. 12.0% responding neurons in SI and 9.5% in SII were activated antidromically by VP stimulation. In the majority of antidromic responses the latencies did not exceed 1.0 ms. The minimal latency of orthodromic spikes was 1.5 ms in SI and 1.7 ms in SII. In SI the number of neurons whose orthodromic spike latencies did not exceed 3.0 ms was larger than neurons activated with latencies of 3.1-4.5 ms. In SII an inverse quantitative relationship between those two neuronal groups was observed. In SII a significantly larger number of neurons was excited with latencies of EPSPs ranged between 1.1-9.0 ms in SI and between 1.4-6.6 ms in SII and the latencies of IPSPs between 1.5-6.8 ms in SI and 2.2-9.4 ms in SII. The importance of different pathways for excitatory and inhibitory VP influences to neurons of SI and SII is discussed.     

Three-dimensional topography of corticopontine projections from rat sensorimotor cortex: comparisons with corticostriatal projections reveal diverse integrative organization

The major cortical-subcortical re-entrant pathways through the basal ganglia and cerebellum are considered to represent anatomically segregated channels for information originating in different cortical areas. A capacity for integrating unique combinations of cortical inputs has been well documented in the basal ganglia circuits but is largely undefined in the precerebellar circuits. To compare and quantify the amount of overlap that occurs in the first link of the cortico-ponto-cerebellar pathway, a dual tracing approach was used to map the spatial relationship between projections originating from the primary somatosensory cortex (SI), the secondary somatosensory cortex (SII), and the primary motor cortex (MI). The anterograde tracers biotinylated dextran amine and Fluoro-Ruby were injected into homologous whisker representations of either SI and SII, or SI and MI. The ensuing pontine labeling patterns were analyzed using a computerized three-dimensional reconstruction approach. The results demonstrate that whisker-related projections from SI and MI are largely segregated. At some locations, the two projections are adjoining and partly overlapping. Furthermore, SI contributes significantly more corticopontine projections than MI. By comparison, projections from corresponding representations in SI and SII terminate in similar parts of the pontine nuclei and display considerable amounts of spatial overlap. Finally, comparison of corticopontine and corticostriatal projections in the same experimental animals reveals that SI-SII overlap is significantly larger in the pontine nuclei than in the neostriatum. These structural differences indicate a larger capacity for integration of information within the same sensory modality in the pontocerebellar system compared to the basal ganglia.     

Interhemispheric connections of the somatosensory cortex in the rabbit

Corpus callosal connections of somatosensory cortex were studied in rabbits by combining anatomical tracing and electrophysiological mapping in the same animals. The results show that callosal connections are unevenly distributed in SI and SII. In SI, the representations of all body surfaces caudal to the neck and midline structures of the head have dense callosal connections. Conversely, connections are sparse to absent within representations of laterally positioned surfaces of the head, such as the sinus hairs, vibrissae, and nonmidline portions of the lips. Almost all of SII has dense callosal connections; only the representations of the vibrissae and sinus hairs have moderate callosal connections. The laminar distribution of callosal connections in rabbit SI and SII is similar to that observed in other mammals. Callosal terminations extend from the inner portion of layer I to the outer portion of layer VI, are moderately denser in the supragranular layers, and are sparse in layer IV. Callosally projecting cells are found predominantly in layers II, III, and V and are sparse in layers IV and VI. These data further emphasize the direct correspondence between the pattern of callosal connections in SI and the functional importance of particular body surfaces. Hence, representations of body surfaces important in the exploration of the environment are relatively free of callosal connections, whereas representations of midline and more lateral surfaces, less significant in tactile exploration, receive dense callosal connections. Callosal connections in rabbits are distributed extensively throughout responsive koniocortical regions rather than being relegated to distinct, specialized regions of "unresponsive" dysgranular cortex as in rodents.     

Thalamic and corticocortical connections of the second somatic sensory area of the mouse

Thalamic and corticocortical connections of the second somatic sensory area (SII) in the mouse cerebral cortex were investigated by means of the retrograde transport of horseradish peroxidase. Focal injections of the enzyme were made in physiologically determined locations within the parietal cortex. Results show that SII receives substantial inputs from topographically appropriate regions within the ipsilateral ventrobasal nucleus and from the ipsilateral posterior group. The limb representation, which was previously found to be responsive to auditory stimulation, received inputs also from the medial division of the medial geniculate body. The SII face representation, which is largely unresponsive to auditory stimuli, received little or no input from the medial geniculate body. SII injections yielded retrograde labeling in the topographically appropriate region in the first somatic sensory area (SI), and SI injections retrogradely labeled cells in SII in a pattern consistent with previous electrophysiological maps. Homotypical regions within SI and SII therefore appear to be reciprocally interconnected. SII also receives inputs from the ipsilateral motor cortex and from contralateral SI and SII. Finally, injections into the SI paw but not face regions yielded retrograde labeling in the thalamic ventrolateral nucleus. Thus, the distal limb representations in SI and SII each receive inputs from a third major relay nucleus (i.e., medial geniculate to SII, ventrolateral nucleus to SI) whereas the face representations do not. These results indicate a close functional interrelationship between homotypical areas in SI and SII, though the two areas differ in several important respects. It is proposed that SII in mice may complement the function of SI by helping to define the overall sensory context in which detailed tactile discriminations are made.     

Electrophysiological study of the topography of the vibrissae projections to the tactile thalamus and cerebral cortex in mutant mice with hair defects

Projections of the mystacial vibrissae to the tactile thalamus and somatosensory neocortex were studied in mutant mice with hair defects, of the Mottled ($\text{Mo}\text{Mo}$), Hairless ($\text{hr}\text{hr}$), and Nude ($\text{Nu}\text{Nu}$) types. Results show that Mottled mice have projections of the same kind as normal mice. In Hairless mice there are silent zones between the projections of intact vibrissae. In Nude mice which lack hairs since birth, the most important change is that the vibrissae project to regions of the thalamus and the SI cerebral cortex ordinarily innervated from the common fur of the muzzle. Thus Nude mice seem to compensate for the absence of hairs of the common fur. We hypothesize that this change represents the counterpart of changes observed in normal mice whose vibrissae follicles were destroyed at birth. Nude mice lack hairs since birth and their vibrissae project to the fur regions. In normal mice whose vibrissae were destroyed at birth, common hairs of the mystacial pad project to the vibrissa region.