Dual efferent projections of the trigeminal principal sensory nucleus to the thalamic ventroposteromedial nucleus in the squirrel monkey

Morphologic Characterization of Trigeminothalamic Terminal Arbors Arising From the Principal Nucleus in the Macaque

The ventral posterior medial nucleus (VPM) is amandatory relay for orofacial sensory information targeting the primary somatosensory cortex. We characterized the morphology of VPM axons arising in the principal trigeminal sensory nucleus (pV) through injections of biotinylated dextran amine (BDA) placed in pV of Macaca fascicularis and mulatta monkeys. Labeled terminals formed a patchy bilateral distribution. Within contralateral VPM, patches were found primarily, but not exclusively, within the laterally located, vertical segment, and in ipsilateral VPM, primarily, but not exclusively, in the medially located, horizontal segment. Two fiber types were labeled: thin and thick. Thin fibers were poorly branched and diffusely distributed. They were studded with small en passant boutons. Most labeled fibers were thick and they branched extensively to form distinctive terminal arbors decorated with numerous boutons that varied in size and shape. Quantitative analysis of thick fiber arbor features showed little difference between the sides, although contralateral boutons were significantly larger than ipsilateral ones. Bouton distribution with respect to counterstained somata suggests that proximal dendrites are their main target. Indeed, ultrastructural examination demonstrated that they provide large diameter dendrites with numerous contacts. Direct comparison of thick fiber terminal arbors to cytochrome oxidase (CO) staining revealed that these arbors are much smaller than individual CO-rich patches believed to designate rods containing discrete body area representations. Thus, each terminal arbor appears to heavily innervate a small number of VPM neurons within a rod. This relationship would serve to maintain relatively small receptive fields within the topographic representation of the face.

Ipsilateral representation of oral structures in human anterior parietal somatosensory cortex and integration of inputs across the midline

Anterior parietal somatosensory areas 3a, 3b, 1, and 2 generally contain cells with receptive fields that are on the contralateral body. However, inputs from midline structures such as the mouth must be uniquely integrated across the midline. This hypothesis is supported by studies of these fields from nonhuman primates that demonstrate ipsilateral representations of oral structures. We used magnetoencephalography (MEG) to examine the cortical representations of the lips and tongue in humans and to examine the time course of interaction of bilateral inputs from these structures. Ipsilateral activation was observed in response to tactile stimulation of the upper lip in 69% of cases, the lower lip in 85% of cases, and the tongue in 88% of cases. In the contralateral hemisphere, the map of oral structures tended to be in agreement with that from nonhuman primates, although variation was large and source locations were not statistically significantly different from each other. There were no differences in latency of activation for ipsi-vs. contralateral responses (about 30 msec), and cortical sources from ipsi-and contralateral stimulation tended to be located together. Differential activation for bilateral vs. unilateral stimulation occurred later than activation in S1, around 110 msec, and was localized to the upper bank of the Sylvian sulcus. Our findings indicate that, unlike nonhuman primates, humans have an ipsilateral representation of the lips in 3b/1, possibly related to the precise manipulation necessary for the articulation of speech. The distinct pattern of differential activation for uni-vs. bilateral stimulation suggests a unique neural mechanism of integration across the midline for inputs from the mouth.

Separate, parallel sensory and hedonic pathways in the mammalian somatosensory system

We propose that separate sensory and hedonic representations exist in each of the primary structures of the somatosensory system, including brainstem, thalamic and cortical components. In the dorsal horn of the spinal cord, the hedonic representation, which consists primarily of nociceptive-specific, wide dynamic range, and thermoreceptive neurons, is located in laminae I and II, while the sensory representation, composed primarily by low-threshold and wide dynamic range neurons, is found in laminae III through V. A similar arrangement is found in the caudal spinal trigeminal nucleus. Based on the available anatomical and electrophysiological data, we then determine the corresponding hedonic and sensory representations in the area of the dorsal column nuclei, ventrobasal and posterior thalamic complex, and cortex. In rodent primary somatosensory cortex, a hedonic representation can be found in laminae Vb and VI. In carnivore and primate primary and secondary somatosensory cortical areas no hedonic representation exists, and the activities of neurons in both areas represent the sensory aspect exclusively. However, there is a hedonic representation in the posterior part of insular cortex, bordering on retroinsular cortex, that receives projections from two thalamic areas in which hedonics are represented. The functions of the segregated components of the system are discussed, especially in relation to the subjective awareness of pain.

Cerebral responses to pain in patients suffering acute post-dental extraction pain measured by positron emission tomography (PET)

Previous studies with normal volunteers have demonstrated distributed cortical responses to experimental heat pain within a network of structures. The network includes the insula, anterior cingulate, prefrontal, inferior parietal and somatosensory cortices. Patients suffering from chronic nociceptive pain following rheumatoid arthritis (RA) have shown damped central responses to experimental heat pain applied to the back of the right hand. In this study of patients with acute, left-sided, post-molar-extraction (surgical) pain, we assessed the cortical responses to experimental heat pain, applied to the back of the right hand, using positron emission tomography (PET), and compared the responses with a previously reported control group and the RA group. In response to the experimental heat pain, the surgical group indicated significantly increased regional cerebral blood flow in the prefrontal cortex [Brodman's area (BA) 44] ipsilateral to the heat stimulus. Contralateral increases were detected in the putamen and transverse temporal gyrus (BA 40/41/42) with bilateral increases in the insular cortex. Compared to the control and RA group, there were significantly reduced responses in the anterior cingulate (BA 24), pre-frontal medial, and orbito-frontal (BA 9/10/32/47) cortices. These results suggest that relatively discrete regions of the cerebral cortex are responsible for acute nociceptive processing during an acute inflammatory episode. The reduced frontal and anterior cingulate responses to the experimental heat pain (applied to the right hand) during acute inflammatory pain (left jaw) illustrates cortical modulation of nociceptive processing that may be related to non-somatotopic, bilateral, nociceptive inputs to these areas. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

Trigeminal projections to thalamus and subthalamus in the hedgehog tenrec

The objective of the present study was the identification and characterization of the trigemino-diencephalic target areas in the Madagascan lesser hedgehog tenrec in order to get a more comprehensive view on the mammalian somatosensory thalamus, its evolution and representation in different species. Such an analysis has been considered important because in lower mammals the head and face are relatively well represented, but their ascending trigeminal projections have scarcely been analysed. Following injections of different tracer substances into the rostral and caudal portions of the trigeminal nuclear complex the most prominent area of termination was found in the medial ventroposterior nucleus. These projections were patchy and scarcely overlapped the region previously shown to receive spinal and dorsal column nuclear afferents. On the basis of the laterality and the intensity of the projections, two subdivisions were distinguished, the principal portion and the accessory portion receiving a dense contralateral and a weak bilateral input, respectively. They were considered equivalents to the magnocellular and parvocellular subdivisions of the medial ventroposterior nucleus in more differentiated mammals. In the latter species, however, the overlap between trigeminal and parabrachial fibres appears less extensive than in the tenrec. In addition, a weak bilateral projection was shown from the caudal trigeminal nucleus to the caudal and dorsal subdivision of the nucleus submedius. There was little, if any evidence for a trigeminal projection to the intralaminar nuclei and we failed to identify a correlate to the posterior nuclear complex of higher mammals. On the other hand, there was a distinct contralateral projection to the ventral portion of the zona incerta. This projection was of similar strength as the projection to the medial ventroposterior nucleus; it supports the notion that the zona incerta may play a crucial role in relaying trigeminal information.

Representation of face and intra-oral structures in area 3b of macaque monkey somatosensory cortex

Details of the representation of body regions innervated by the trigeminal nerve were elucidated in monkey cerebral cortex. Microelectrode recording was used to generate somatosensory maps in the posterior bank of the central sulcus and on the exposed cortical surface lateral to the lateral tip of the central sulcus in Macaca nemestrina. The area innervated by the contralateral trigeminal nerve is represented in an 8-mm mediolateral extent of area 3b lateral to the representation of the hand. Lateral to this, still within area 3b, there is an expanded representation of ipsilateral intra-oral structures measuring 6 mm in mediolateral extent. Both representations fill area 3b anteroposteriorly. The ipsilateral representation forms approximately 40% of the trigeminal representation, consistent with the amount of the ventroposterior medial (VPM) thalamic nucleus devoted to representation of ipsilateral intra-oral structures. Comparison of the present results with maps of the face representation in other species of monkey shows a consistent somatotopy of the face between species; size variations are mainly related to the enlarged ipsi- and contralateral representations of the cheek pouches in macaques. The general somatotopy of the trigeminal representation in monkeys is consistent with that in other mammalian species.

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.

Parcellated organization in the trigeminal and dorsal column nuclei of primates

The subdivisions of the brainstem trigeminal complex in non-primate mammals are characterized by aggregated or parcellated patterns of neural organization. The present studies used cytochrome oxidase histochemistry to test if parcellated organization patterns also occur in the brainstems of primates. The results demonstrate that a parcellated pattern of neural organization exists in the trigeminal nucleus principalis, but not in the spinal trigeminal nuclei, of macaque and squirrel monkeys. The results further suggest that parcellation in the nucleus principalis qualitatively resembles the aggregated organization in dorsal column nuclei. Taken together with previous findings from non-primates, these results indicate that central parcellation is an organizational feature of specific ascending somatosensory projections in many mammals including primates.

Morphological characteristics and terminating patterns of masseteric neurons of the mesencephalic trigeminal nucleus in the rat: an intracellular horseradish peroxidase labeling study

In order to study the morphological characteristics and terminating patterns of the neurons of the trigeminal mesencephalic nucleus (Vme), 55 masseteric neurons in Vme in the rat were stained by intracellular injection of horseradish peroxidase (HRP). Labeled cells were distributed throughout the nucleus. These neurons were divided into three types: uni- or pseudounipolar (type A, n = 43), bipolar (type B, n = 5), and multipolar cells (type C, n = 7). Each type was further divided into two subtypes according to the largest diameter of the perikarya (type a greater than or equal to 30 microns, type b less than 30 microns). The central processes of type Aa neurons projected to the following three groups of target nuclei: 1) nuclei functioning as interneurons, including supratrigeminal nucleus (Vsup), intertrigeminal nucleus (Vint), juxta-trigeminal region (Vjux), and parvicellular nucleus of the pontomedullary reticular formation (PcRF); 2) motor nuclei, including the trigeminal motor nucleus (Vmo), accessory facial nucleus (NVIIacs), accessory abducens nucleus (NVIacs), and a small number of labeled axons in the oculomotor nucleus and trochlear nucleus; 3) sensory nuclei, including the dorsomedial part of the principal trigeminal sensory nucleus (Vpdm) and the dorsomedial part of subnucleus oralis of the trigeminal spinal nucleus (Vodm). Labeled processes were dense in the Vsup, Vmo, and Vpdm. The proprioceptive pathway of the fifth nerve is discussed. Direct projections from type Aa neurons of Vme to the Vpdm and dorsolateral part of the Vsup contribute to conduction of the proprioceptive information from spindles of masticatory muscle to the contralateral thalamus in the rat. Different axon morphology, distribution, terminal branch density, and terminating patterns of type Aa neurons were noted in different functional groups of the projecting nuclei, especially in the Vsup, Vmo, and Vpdm. The highest terminal branching density, the most extensive distribution, and two different types of branching patterns (claw-like and comb-like) were observed in Vsup. Selective distribution and single-beaded or "Y"-shaped terminal branches were observed in Vmo. In the Vppdm the axonal branches were sparser than in the Vsup or Vmo, and had an arrangement like the branches of a weeping willow tree. These characteristics of anatomical organization might be related to the function of each projecting nucleus.

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)

Thalamocortical and thalamo-amygdaloid projections from the parvicellular division of the posteromedial ventral nucleus in the cat

Projections from the parvicellular division of the posteromedial ventral thalamic nucleus (VPMpc) of the cat were examined. After injection of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) into the VPMpc, both anterogradely labeled axon terminals and retrogradely labeled neuronal cell bodies were found ipsilaterally in three discrete regions of the cerebral cortex, i.e., in the orbital cortex, caudoventral part of the infralimbic cortex, and medial part of the fundus of the posterior rhinal sulcus (perirhinal area); in the subcortical regions, anterogradely labeled axon terminals were seen ipsilaterally in the rostrodorsal part of the lateral amygdaloid nucleus. Neuronal connections between these VPMpc-recipient regions were further verified by injecting WGA-HRP into each of the three cortical and the lateral amygdaloid regions. After injection of WGA-HRP into each of the three cortical regions, labeled neuronal cell bodies and axon terminals were seen ipsilaterally in the VPMpc, especially in its medial part, and in the other two of the three VPMpc-recipient cortical regions. In the rostrodorsal part of the lateral amygdaloid nucleus, both axon terminals and neuronal cell bodies were labeled after WGA-HRP injection into the perirhinal area, and only axon terminals were labeled after WGA-HRP injection into the orbital cortex, but no labeling was observed after WGA-HRP injection into the infralimbic cortex. After injection of WGA-HRP into the rostrodorsal portion of the lateral amygdaloid nucleus, both axon terminals and neuronal cell bodies were labeled ipsilaterally in the perirhinal area and the ectorhinal area, and only neuronal cell bodies were labeled ipsilaterally in the VPMpc (especially in its medial part) and orbital cortical region; no labeling was observed in the infralimbic cortex. The present results indicate that the VPMpc of the cat is connected reciprocally with the orbital, infralimbic, and perirhinal cortical regions on the ipsilateral side, that the three VPMpc-recipient cortical regions are reciprocally connected with each other, that the VPMpc sends fibers ipsilaterally to the rostrodorsal part of the lateral amygdaloid nucleus, which may relay information from the VPMpc to the perirhinal cortical area, and that the VPMpc-recipient area in the lateral amygdaloid nucleus receives cortical fibers from the orbital and perirhinal cortical regions.