Cerebellocerebral projection from the fastigial nucleus onto the frontal cortex in the cat

Two frontal cortical areas hidden in sulci were found to be responsive to stimulation of the cerebellar fastigial nucleus under pentobarbital anesthesia: one is the ventral bank of the cruciate sulcus and the other is the area surrounding the fundus of the presylvian sulcus which corresponds to subregions of the frontal eye field. The fastigial projection onto these areas via the thalamic ventromedial (VM) nucleus was identified electrophysiologically and morphologically.     

Organization of cortical and subcortical projections to the feline insular visual area, IVA.

Extracellular recordings with carbon fiber-filled microelectrodes were used to identify the visually responsive area within the insular cortex (referred to hereafter as the insular visual area, IVA) of anaesthetized cats. Broadly speaking, IVA comprises the cortex surrounding the anterior ectosylvian sulcus (AEs) along its ventral bank and the major portion of the anterior sylvian gyrus. Visually sensitive cells were recorded along the whole length of the AEs. In the same animals, the afferent connections of IVA were studied through the use of the retrograde tracers wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) and fluorescent Diamidino yellow (DY), in combination with standard electrophysiological stimulation and recording techniques. The results indicate that: (1) the IVA receives a wide variety of telencephalic inputs, not only from visual, sensorimotor, auditory, limbic and association cortical areas, and from the claustrum, amygdala and basal nucleus of Meynert, as well, but also from the diencephalic projections arising mainly from the lateralis medialis-suprage niculate nuclear complex (LM-Sg) and the ventral medial nucleus (VM). (2) The gyral part of IVA (gIVA) receives afferents mainly from the lateral part of the lateral suprasylvian visual area (LS) throughout almost its entire length, as well as from area 20, the posterior suprasylvian sulcal area (PS), the frontal eye fields, areas 6 and 36, and almost the whole length of the cortical area lying along the anterior ectosylvian sulcus (AEs). (3) By contrast with (2), the sulcal part of IVA (sIVA) which corresponds to the anterior part of the anterior ectosylvian visual area (AEV) of Norita et al. ('86), receives cortical projections mainly from the lateral and medial parts of the anterior half of LS, area 20, PS, the frontal eye fields, area 36, and most parts of the cortical area extending along the AEs. (4) Subcortically, IVA receives thalamic afferents mainly from VM and LM-Sg. The connections between IVA and LM-Sg are organized topographically, with the more anterior part of IVA being related to the more ventral portion of LM-Sg, and with sIVA being related chiefly to the mid-portions of LM-Sg. These results thus suggest that IVA may function as an integrative centre among structures belonging to the extrageniculostriate system, the sensorimotor system, as well as to the limbic system. Furthermore, our electrophysiological and anatomical findings, together with previous reports concerning AEV, suggest that the posterior part of AEV (AEV proper) is distinctive from gIVA, and that the sIVA apparently serves as a transitional region between AEV and gIVA.     

Olivocerebellar and cerebelloolivary connections of the oculomotor region of the fastigial nucleus in the macaque monkey

Anatomical connections of the caudal portion of the fastigial nucleus (FN) with the inferior olive (IO) were studied in macaque monkeys with wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA/HRP) and HRP. When injected HRP was confined to a caudal portion of the FN, retrogradely labeled Purkinje cells (P cells) appeared in the oculomotor vermis. We defined the area that receives the projection from vermal lobule VII as the fastigial oculomotor region. The same HRP injection resulted in retrograde labeling of IO neurons in an area of group b (of Bowman and Sladek: J. Comp. Neurol. 152:299-316, '73) of the contralateral medial accessory olive (MAO). This area was designated as the Z-portion because in the coronal section it appears like the letter "Z." Retrogradely labeled IO neurons were also found in the Z-portion when HRP was injected into the oculomotor vermis, indicating that neurons in this portion project to both the fastigial and vermal oculomotor regions. Anterogradely labeled axons from the contralateral fastigial oculomotor region also terminated in the Z-portion. When the effective site included a region anterior to the fastigial oculomotor region, labeled P cells appeared in lobule V and labeled IO neurons appeared in group a. Labeled terminals of fastigial fibers were also found in group a. When the effective site included a region ventral to the oculomotor region, labeled P cells appeared in vermal lobules VIII and IX and labeled IO neurons appeared in caudal parts of a and b, in addition to group c. HRP injection into the posterior interposed nucleus (PIN) resulted in labeling of P cells in the paravermal zone and of IO neurons in the rostral two-thirds of the MAO and the dorsal accessory olive (DAO). The location of the labeled terminals coincided with the region where the densest labeling of IO neurons was found. Thus, the olivary projections to both the cerebellar cortex and deep cerebellar nuclei and the nucleoolivary projection exhibited a closely related topographical organization.     

Cerebral cortical projections to the reticular regions around the trigeminal motor nucleus in the cat

Cerebral cortical regions which send projection fibers to the reticular regions around the trigeminal motor nucleus were identified in the cat by the horseradish peroxidase (HRP) method. The reticular region around the trigeminal motor nucleus are known to contain many interneurons for masticatory motoneurons. After injections of HRP into the reticular regions around the trigeminal motor nucleus, HRP-labeled neuronal cell bodies in the cerebral cortex were found in layer V. They were distributed bilaterally in the orbitofrontal cortical regions, mainly in the rostral extension of the orbital gyrus close to the presylvian sulcus; more were located in the floor and lateral bank of the presylvian sulcus than in the crown of the orbital gyrus. After injections of HRP conjugated with wheat germ agglutinin (WGA-HRP) into these cortical regions, many labeled presumed axon terminals were distributed bilaterally in the reticular regions around the trigeminal motor nucleus; mainly in the region ventral to the trigeminal motor nucleus and in the intertrigeminal region between the main sensory trigeminal nucleus and the trigeminal motor nucleus. Terminal labeling in these regions was more prominent after WGA-HRP injection into the lateral bank of the presylvian sulcus than after WGA-HRP injection into the crown of the orbital gyrus. Thus, the present results indicate that the main part of the cortical region projecting directly to the reticular regions around the trigeminal motor nucleus in the cat is folded into the presylvian sulcus.     

Direct cortical projections to the parabrachial nucleus in the cat

Direct projections from the cerebral cortex to the parabrachial nucleus in the cat were examined by the horseradish peroxidase (HRP)method. When HRP was injected into the parabrachial nucleus, retrogradely labeled neuronal cell bodies were seen, bilaterally with an ipsilateral predominance, mainly in the orbital gyrus, the lateral bank of the presylvian sulcus, and a restricted region in the infralimbic cortex on the medial surface of the frontal lobe (stereotaxic coordinates; Fr: 22, L: 1, H: -1); all labeled neurons were in deep pyramidal cell layer. After injecting HRP conjugated to wheat germ agglutinin (WGA-HRP) into the cortical regions where retrogradely labeled neurons were found after injecting HRP into the parabrachial nucleus, anterogradely labeled cortical fibers were traced to the parabrachial nucleus. Corticoparabrachial fibers originating from the orbital gyrus and the lateral bank of the presylvian sulcus ran ipsilaterally through the internal capsule and the cerebral peduncle down to the lower brainstem, whereas those from the infralimbic cortex coursed down ipsilaterally through the medial forebrain bundle. These cortical fibers to the parabrachial nucleus were distributed bilaterally with an ipsilateral predominance. Cortical fiber terminals in the parabrachial nucleus were topographically arranged: Corticoparabrachial fibers from the lateral bank of the presylvian sulcus ended most massively in the dorsal part of the lateral parabrachial nucleus. Corticoparabrachial fibers from the orbital gyrus ended most heavily in the medial parabrachial nucleus and less heavily in the lateral parabrachial nucleus. Corticoparabrachial fibers from the infralimbic cortex ended mostly in the parabrachial regions surrounding the brachium conjunctivum.     

Reciprocal parabrachial-cortical connections in the rat

The projection from the parabrachial nucleus (PB) to the cerbral cortex in the rat was studied in detail using the autoradiographic method for tracing anterograde axonal transport and the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) method for both anterograde and retrograde tracing. PB innervates layers I, V and VI of a continuous sheet of cortex extending from the posterior insular cortex caudally, through the dorsal agranular and the granular anterior insular cortex and on rostrally into the lateral prefrontal cortex. Within the prefrontal area, PB fibers innervate primarily layer V of the ventrolateral cortex caudally, but more rostrally the innervated region includes progressively more dorsal portions of the prefrontal area, until by the frontal pole the entire lateral half of the hemisphere is innervated. This projection originates for the most part in a cluster of neurons in the caudal ventral part of the medial PB subdivision, although a few neurons in the adjacent parts of the PB, the Kolliker-Fuse nucleus and the subcoeruleus region also participate.After injection of WGA-HRP into the PB region, retrogradely labeled neurons were found in layer V of the same cortical areas which receive PB inputs. The importance of this monosynaptic reciprocal brainstem-cortical projection as a possible anatomical substrate for the regulation of cortical arousal is discussed.     

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.     

Projections of thalamic gustatory and lingual areas in the monkey, Macaca fascicularis

The efferent projections of the parvicellular division of the ventroposteromedial nucleus of the thalamus (VMPpc; thalamic taste area) were traced to cortex in Macaca fascicularis by using tritiated amino acid autoradiography. Labeled fascicles could be traced from VPMpc to two discrete regions of cortex. The primary efferent projection was located on ipsilateral insular-opercular cortex adjacent to the superior limiting sulcus and extended as far rostrally as the posterior lateral orbitofrontal cortex. An additional projection was located within primary somatosensory (SI) cortex subjacent to the anterior subcentral sulcus. Following autoradiographic injections in VPM, the trigeminal somatosensory relay, a dense terminal plexus was labeled on SI cortex of both pre- and postcentral gyri, but not within insular-opercular cortex. The autoradiographic data were verified by injecting each cortical projection area with horseradish peroxidase (HRP) and observing the pattern of retrogradely labeled somata within the thalamus. Injections in the precentral gyrus near the anterior subcentral sulcus retrogradely labeled neurons within VPMpc, whereas injections further caudally near the floor of the central sulcus labeled neurons within VPM. Injections of HRP within opercular, insular, or posterior lateral orbitofrontal cortex retrogradely labeled neurons within VPMpc.     

Cortical and thalamic afferent connections of the insular and adjacent cortex of the rat

Thalamic and cortical afferents to the insular and perirhinal cortex of the rat were investigated. Unilateral injections of horseradish peroxidase (HRP) were made iontophoretically along the rhinal sulcus. HRP injections covered or invaded areas along the rhinal fissure from about the level of the middle cerebral artery to the posterior end of the fissure. The most anterior injection labeled a few cells in the mediodorsal nucleus. More posterior injections labeled neurons in the basal portion of the nucleus ventralis medialis, thus suggesting that this cortical region constitutes the rat's gustatory (insular) cortex. We consider the cortex situated posterior to the gustatory cortex in and above the rhinal sulcus as the core region of the rat's (associative) insular cortex, as this cortex receives afferents from the regions of and between the nuclei suprageniculatus and geniculatus medialis, pars magnocellularis. It includes parts of the cortex termed perirhinal in other studies. The cortex dorsal and posterior to the insular cortex we consider auditory cortex, as it receives afferents from the principal part of the medial geniculate nucleus, and the cortex ventral to the insular cortex (below the fundus of the rhinal sulcus) we consider to constitute the prepiriform cortex, which is athalamic. The posterior part of the perirhinal cortex (area 35) receives afferents from nonspecific thalamic nuclei (midline nuclei). Cortical afferents to the injection loci arise from a number of regions, above all from regions of the medial and sulcal prefrontal cortex. Those injections confined to the projection cortex of the suprageniculate-magnocellular medial geniculate nuclear complex also led to labeling in contralateral prefrontal regions, particularly in area 25 (infralimbic region). A comparison of our results with those on the insular cortex of cats and monkeys suggests that on the basis of thalamocortical connections, topographical relations, and involvements of neurons in information processing and overt behavior, the insular cortex has to be regarded as a heterogeneous region which may be separated into prefrontal insular, gustatory (somatosensory) insular, and associative insular portions.     

The afferent and efferent connections of the ventromedial thalamic nucleus in the rat

The afferent and efferent connections of the ventromedial (VM) nucleus of the thalamus in the rat were studied by experiments using the methods of retrograde cell marking by horseradish peroxidase (HRP) and anterograde fiber tracing by autoradiography. Tritiated amino acids deposited microelectrophoretically into VM label a cortical projection that is distributed to a sharply defined superficial portion of layer I of almost the entire extent of the ipsilateral neocortex. The labeling is most dense at frontal cortical levels, where fibers radiate through the deeper layers to terminate in the outer one-quarter of layer I throughout all neocortex rostral to the genu of the corpus callosum. A lesser number of labeled fibers extends caudally in a supracallosal location to innervate parieto-occipital cortical areas. Labeled collaterals ascend through the cortical layers to reach layer I, where grains in the superficial portion are found in a gradually decreasing rostrocaudal gradient of density that reaches the caudal pole of the hemisphere. Coronal sections at most levels contain a band of labeling in layer I that extends uninterrupted from the callosal sulcus at the midline to the banks of the rhinal sulcus laterally. Caudal retrosplenial and ventral temporal areas appear to be the only sectors of neocortex spared by the ubiquitous projection. Evidence for additional terminal distribution in deeper layers is found only in the dorsal and lateral sectors of the cortex rostral to the genu where sparsely labeled bands appear in layers III and V. The nearly exclusive distribution of VM's cortical afferents to layer I is compared and contrasted with multilaminar distributions of other “unspecific” cortical afferent fibers. HRP injected into VM labels neurons in a variety of structures at levels ranging from the frontal cortex to caudal medulla. Cell labelling in the globus pallidus, deep layers of the superior colliculus, cerebellar nuclei and the substantia nigra, pars reticulata suggest that VM is a point of convergence for several components of the extrapyramidal motor system. The nigrothalamic projection is topographic: medial and lateral districts of the pars reticulata are connected to medial and lateral districts of VM, respectively. A dorsal-ventral association may also obtain. Cell labeling in the prefrontal cortex, the cortex along the rhinal sulcus, the lateral habenular nucleus, tegmental and medullary reticular formations, and parabrachial nuclei indicates that VM also receives projections from more heterogeneous sources.     

Connections of area 8 with area 6 in the brain of the macaque monkey

The corticocortical connections between the arcuate area (Walker's areas 8A and 45 or Brodmann's area 8) and the premotor and supplementary motor areas (Vogts' area 6) in the brain of the macaque monkey were studied microscopically with wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), which was injected into Brodmann's area 8 primarily to elucidate the projections of area 8 into area 6. In addition, in the same material, the pattern of connections between the arcuate area and area 46 of the prefrontal cortex was studied. On the basis of the findings of the present study, an anatomical cortical hierarchy for areas 8, 6, and 46 is discussed. Anterogradely labeled axon terminals and retrogradely labeled cells appeared in the premotor area, in the supplementary motor area, and in area 46 on the side of injection; sites containing labeled axon terminals also contained labeled cells. In other words, the examined connections were reciprocal. In the labeled areas, labeled terminals and cells coexisted, and most formed radial columns. However, no labeling of cells and terminals was seen in the motor area (Brodmann's area 4). After injection of WGA-HRP into area 8A, labeled terminals and cells appeared predominantly in the superior premotor area (a region of the premotor area above the arcuate spur, Vogt and Vogt's upper areas 6a alpha and 6a beta), forming one, two, or three bands of label in the anteroposterior direction, whereas labeling occurred to a lesser extent in the inferior premotor area (a region of the premotor area below the arcuate spur, Vogt and Vogt's areas 4c, lower 6a alpha and 6b). In contrast, injection of WGA-HRP into area 45 resulted in the predominant labeling of the inferior premotor area with scant labeling in the superior premotor area. In the premotor area, labeled terminals were distributed over the entire cortical depth, although few were found in the lower half of layer III, and labeled cells were distributed mainly in layer III. In area 46 of the banks of the principal sulcus in the prefrontal cortex, labeled terminals were distributed in all cortical layers or over the entire cortical depth with a lower concentration in layer IV; labeled cells were found mostly in layers III and V, with a relatively high density in layer V.     

Insular cortex and neighboring fields in the cat: A redefinition based on cortical microarchitecture and connections with the thalamus

The insular areas of the cerebral cortex in carnivores remain vaguely defined and fragmentarily characterized. We have examined the cortical microarchitecture and thalamic connections of the insular region in cats, as a part of a broader study aimed to clarify their subdivisions, functional affiliations, and eventual similarities with other mammals. We report that cortical areas, which resemble the insular fields of other mammals, are located in the cat's orbital gyrus and anterior rhinal sulcus. Our data suggest four such areas: (a) a “ventral agranular insular area” in the lower bank of the anterior rhinal sulcus, architectonically transitional between iso- and allocortex and sparsely connected to the thalamus, mainly with midline nuclei; (b) a “dorsal agranular insular area” in the upper bank of the anterior rhinal sulcus, linked to the mediodorsal, ventromedial, parafascicular and midline nuclei; (c) a “dysgranular insular area” in the anteroventral half of the orbital gyrus, characterized by its connections with gustatory and viscerosensory portions of the ventroposterior complex and with the ventrolateral nucleus; and (d) a “granular insular area”, dorsocaudal in the orbital gyrus, which is chiefly bound to spinothalamic-recipient thalamic nuclei such as the posterior medial and the ventroposterior inferior. Three further fields are situated caudally to the insular areas. The anterior sylvian gyrus and dorsal lip of the pseudosylvian sulcus, which we designate “anterior sylvian area”, is connected to the ventromedial, suprageniculate, and lateralis medialis nuclei. The fundus and ventral bank of the pseudosylvian sulcus, or “parainsular area”, is associated with caudal portions of the medial geniculate complex. The rostral part of the ventral bank of the anterior ectosylvian sulcus, referred to as “ventral anterior ectosylvian area”, is heavily interconnected with the lateral posterior-pulvinar complex and the ventromedial nucleus. Present results reveal that these areas interact with a wide array of sensory, motor, and limbic thalamic nuclei. In addition, these data provide a consistent basis for comparisons with cortical fields in other mammals. J. Comp. Neurol. 384:456–482, 1997. © 1997 Wiley-Liss, Inc.     

Multiple cortical targets of one thalamic nucleus: the projections of the ventral medial nucleus in the cat studied with retrograde tracers

The organization of the cortical projections of the ventral medial thalamic nucleus (VM) was studied in the cat with retrograde tracers. The extent of the VM-cortical projections was first investigated with horseradish peroxidase injected in different cortical fields. The results obtained in the experiments indicated that the main target of VM efferents is represented by a large territory anterior to the cruciate sulcus involving area 6 and the gyrus proreus and extending into the anterior part of the medial cortical surface. The afferents to these precruciate fields arise from throughout the VM. In addition, the lateral third of VM projects upon the lateral precruciate cortex that is coextensive with the precruciate part of area 4, whereas VM efferents do not extend into the posterior sigmoid gyrus. A second major target of VM efferents is represented by the insular cortex in the anterior sylvian gyrus. VM projections also reach the prepyriform cortex and the cingulate gyrus. An anteroposterior decrease of density was found in the VM-cingulate projections. Sparse VM projections reach the temporal cortex, the adjacent posterior sylvian and ectosylvian fields, and the anterior ectosylvian gyrus. No VM projections were found either upon the visual areas 17 and 18 or upon the primary auditory cortex. The interrelations between some VM-cortical cell populations and their divergent collateralization were studied by using double retrograde labeling with fluorescent tracers. The results of these experiments demonstrated that a relatively high number (at least 20%) of VM cells projecting to the insula are also connected to the precruciate fields by means of axon collaterals. This finding indicates that VM is a highly collateralized structure of the cat's thalamus. Very few branched cells were found in the other combinations of cortical fields here examined (precruciate vs. posterior sylvian fields, lateral precruciate vs. proreal cortex, anterior vs. posterior cingulate fields). Altogether these data indicate that VM branched cells preferentially interconnect the two main cortical targets of the nucleus, i.e., precruciate and insular fields. The results of the present study are discussed in regard to the literature on the VM projections in the rat and the previously available data in the cat, to the afferent VM organization in the cat, to the relationships between VM and the nucleus submedius, and to the anatomical and functional role of VM in relation to the so-called "nonspecific" thalamocortical system.     

Insular cortex projection to the nucleus of the solitary tract and brainstem visceromotor regions in the mouse

Anterograde transport of horseradish peroxidase (HRP) and HRP conjugated to wheat germ agglutinin (WGA-HRP) demonstrated a substantial, bilateral projection from insular cortex to the nucleus of the solitary tract (NTS) in the mouse. Injections that labeled the projection were restricted to the cortical sector homologous to taste-visceral cortex in the rat and label was antero- and retrogradely transported to several subcortical structures along ascending taste-visceral pathways. Multiple, small injections in this cortical region labeled fibers and terminals throughout the rostro-caudal extent of NTS. Small, single injection showed that the projection is topologically organized: Rostral points along the cortical strip project to rostral parts of NTS, intermediate points to the intermediate levels of NTS and caudal parts of the cortical field to caudal parts of NTS. In NTS the primary cranial nerve afferents distribute along a rostral to caudal gradient with the VII nerve rostral, the IX intermediate and the Xth caudal [2,6,37]. The present results indicate that the cortical sensory representation of these cranial nerve afferents reflects their topographic distribution in NTS. This suggests that there is an organized anatomical substrate by which the cerebral cortex may selectively influence the central processing of both gustatory and visceral afferent information in the primary CNS relay for these modalities. This insulofugal pathway also terminates in parts of NTS and additional medullary areas that contain preganglionic parasympathetic motoneurons. This appears to be the first anatomical demonstration of a projection from any part of the cerebral cortex to parasympathetic motor nuclei. The pathway provides a substantial direct channel by which higher cortical activity may modulate parasympathetic function.     

The anterior ectosylvian sulcal auditory field in the cat: II. A horseradish peroxidase study of its thalamic and cortical connections.

The thalamic and cortical projections to acoustically responsive regions of the anterior ectosylvian sulcus were determined by identifying retrogradely labelled cells after physiologically guided iontophoretic injections of horseradish peroxidase. The medial division of the medial geniculate nucleus, the intermediate division of the posterior nuclear group, the principal division of the ventromedial nucleus, and the lateroposterior complex were consistently labelled after these injections, although each animal showed slightly different patterns of labelling. The suprageniculate nucleus and the lateral and medial divisions of the posterior nuclear group were also labelled in most experiments. The cortex of the suprasylvian sulcus was the most consistently and densely labelled cortical region; each experiment showed a slightly different pattern of labelling throughout the suprasylvian sulcus, with an overall tendency for greater labelling in the ventral (lateral) bank of the middle region of the sulcus. Other cortical regions labelled less consistently included the anterior ectosylvian sulcus itself, the insular cortex of the anterior sylvian gyrus, and the posterior rhinal sulcus. In three experiments the contralateral cortex was examined and a small number of labelled cells was located in the anterior ectosylvian and suprasylvian sulci. Input from extralemniscal auditory thalamus is compatible with previously described auditory response properties of anterior ectosylvian sulcus neurons. The results also confirm the presence of input from visual and multimodal regions of thalamus and cortex, and therefore support claims of overlap of modalities within the sulcus. This overlap, as well as input from motor regions, suggests that the anterior ectosylvian sulcal field serves a sensorimotor role.     

The thalamo-cortical projection of the nucleus submedius in the cat

The cortical projection of the nucleus submedius (Sm) was studied in the cat with the autoradiographic and horseradish peroxidase (HRP) methods. The results indicate that Sm projects topographically on to layer 3 of a distinct agranular cortical field that occupies the posterolateral gyrus proreus, the adjacent fundus of the rhinal sulcus, and the postero-ventral portion of the medial wall of the presylvian sulcus. This cortical field is denoted the ventrolateral orbital cortex (VLO), consonant with previous nomenclature in the rat (Krettek and Price, '77a). The more ventral part (VLOβ) is cytoarchitectonically distinct from the dorsal part (VLOα); the former receives input from the anterior part of Sm (Sm_a), while the latter receives superficial layer 1 of VLO probably also arises from Sm, and there may be an input to layers 5 and 6. The corticothalamic projection from VLO to Sm reciprocates the ipsilateral thalamocortical projection and also has a moderate contralateral component. A dense, subpial layer 1 input to VLO arises from cells of the ventromedial nucleus (VM) subjacent to Sm. The present experiments also indicate that clusters of cells in VM probably provide input to layer 3 of the cortex in the fundus of the presylvian sulcus, as well as area 6aβ in the lateral wall of the presylvian sulcus and the ventral bank of the cruciate sulcus. Results from the HRP experiments additionally indicate that VLOβ and the anteroventral (Sm_v) portion of VLOα are reciprocally connected with the ventral agranular insular cortex and the cingulate cortex, ipsilaterally, while the posterodorsal (Sm_d) portion of VLOα is instead connected with specific portions of the somatosensory cortical areas bilaterally. All portions of VLOα appear to project to the ventrolateral periaqueductal gray. In light of the recent suggestion that (Sm_d) is involved with nociception (Craig and Burton, '81), the present results suggest that the related portion of VLOα may serve as a cortical representation for noxious stimuli.     

Afferents to a midbrain periaqueductal grey region involved in the ‘defence reaction’ in the cat as revealed by horseradish peroxidase. I. The telencephalon

Horseradish peroxidase injections were made at sites, within the midcollicular portion of the midbrain periaqueductal grey region (PAG), at which both electrical stimulation and subsequent microinjections of excitatory amino acids elicited defensive behaviour. Since excitatory amino acids depolarize cell bodies and dendrites located in the vicinity of the injection site but not axons of passage, the injections were centred within a PAG region known to contain neurones whose excitation elicited defensive behaviour. The telencephalic afferents to these sites were then determined. Sixty percent of the labelled telencephalic neurones were found in the frontal cortex, specifically in the medial frontal cortex along the banks of the rostral two-thirds of the cruciate sulcus, primarily area 6 and area 4, and the medial frontal cortex ventral to area 6 (area 32). Twenty-five percent of the labelled telencephalic neurones were found in the orbito-insular cortex while 8% were found in the parietal cortex surrounding the anterior ectosylvian sulcus. Although the functional significance of these projections remains to be established, available data suggest that these projections to the PAG arise from frontal 'oculomotor' and motor cortices, a polysensory insular cortical region and somatosensory, visual and auditory parietal cortical areas.     

Cortical topography of thalamic intralaminar nuclei

The coritical projection of the thalamic intralaminar nuclei (ILN) has been studied by injecting little amounts of horseradish peroxidase (HRP) on the cerebral cortex of the rat. All of the cortical areas (except area 17) receive ipsilateral projections from at least one nucleus of the ILN. The nucleus centralis had the largest number of labeled neurons, principally after injections in frontal, temporal and occipital cortical areas. The nucleus paracentralis presented only moderate numbers of HRP positive neuronrs from the frontal cortex, and very few from parietal and temporal areas. The nucleus parafascicularis showed labeled somata after frontal injections as well as parietal and temporal areas. In comparison to the other ILN, the amount of labeled neurons in this nucleus is relatively small. The nucleus centralis medialis presented the least number of labeled neurons regardless of injected area. Its cortical efferents remain restricted to small areas of the dorsal aspects of frontal, anterior cingular and temporal cortices. Each of the ILN contains neurons which connect with more than one cortical zone, according to a characteristics topographic distribution.     

The cortical projections of the mediodorsal nucleus and adjacent thalamic nuclei in the rat.

The mediodorsal nucleus of the rat thalamus has been divided into medial, central and lateral segments on the basis of its structure and axonal connections, and these segments have been shown by experiments using the autoradiographic method of demonstrating axonal connections to project to seven distinct cortical areas covering most of the frontal pole of the hemisphere. The position and cytoarchitectonic characteristics of these areas are described. The medial segment of the nucleus projects to the prelimbic area (32) on the medial surface of the hemisphere, and to the dorsal agranular insular area, dorsal to the rhinal sulcus on the lateral surface. The lateral segment projects to the anterior cingulate area (area 24) and the medial precentral area on the dorsomedial shoulder of the hemisphere, while the central segment projects to the ventral agranular insular area in the dorsal bank of the rhinal sulcus, and to a lateral part of the orbital cortex further rostrally. (The term "orbital" is used to refer to the cortex on the ventral surface of the frontal pole of the hemisphere.) A ventral part of this orbital cortex also receives fibers from the mediodorsal nucleus, possibly its lateral segment, but the medial part of the orbital cortex, and the ventrolateral orbital area in the fundus of the rhinal sulcus receive projections from the paratenial nucleus and the submedial nucleus, respectively. All of these thalamocortical projections end in layer III, and in the outer part of layer I. The basal nucleus of the ventromedial complex (the thalamic taste relay) has been shown to have a similar laminar projection (layer I and layers III/IV) to the granular insular area immediately dorsal to, but not overlapping, the mediodorsal projection field. However, the principal nucleus of the ventromedial complex appears to project to layer I, and possibly layer VI, of the entire frontal pole of the hemisphere. The anteromedial nucleus does not appear to project to layer III of the projection field of the mediodorsal nucleus, although it may project to layers I and VI, especially in the anterior cingulate and medial precentral areas. A thalamoamygdaloid projection from the medial segment of the mediodorsal nucleus to the basolateral nucleus of the amygdala has also been demonstrated, which reciprocates an amygdalothalamic projection from the basolateral nucleus to the medial segment. The habenular nuclei also appear to project to the central nucleus of the amygdala. These results are discussed in relation to the delineation and subdivision of the prefrontal cortex in the rat, and to amygdalothalamic and amygdalocortical projections which are described in a subsequent paper (Krettek and Price, '77).     

Connections between the cerebellum and hypothalamus in the tree shrew (Tupaia glis)

Following injections of a wheat germ agglutinin-horseradish peroxidase (WGA-HRP) conjugate into representative areas of cerebellar cortex of tree shrew (Tupaia glis) retrogradely labeled cells were found in posterior, lateral and dorsal hypothalamic areas and in the lateral mammillary nucleus. These represent monosynaptic hypothalamocerebellar cortical projections. Anterogradely labeled axons were also traced into various areas of the contralateral hypothalamus subsequent to injections of WGA-HRP into the cerebellar nuclei. These results identify a direct cerebellar nucleohypothalamic pathway. These hypothalamocerebellar and cerebellohypothalamic connections represent potential circuits through which the cerebellum may interact with visceral centers.