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PubMed1996

Distinct lateral and medial projections of the spinohypothalamic tract of the rat.

We recently described a direct nociceptive projection from the spinal cord to the hypothalamus in the rat. Several electrophysiological studies of this projection indicated that the axons of some spinohypothalamic tract neurons (SHT) reach the hypothalamus either by a lateral or by a medial route. The purpose of this study was to determine the origin of all SHT neurons that reach the hypothalamus through the lateral and the medial projections, and to investigate the possibility of ablating the SHT without damaging other important sensory and motor tracts by combining retrograde tracing techniques with axonal ablation. As compared with control cases, significant (P < .05) reductions in the number of labeled SHT neurons were encountered, 26% in the ipsilateral spinal cord following lesions of the medial projection, 67% in the contralateral spinal cord following lesions of the lateral projection, and 94% in both contra- and ipsilateral sides following lesions of both the medial and lateral projections. Bilateral lesions of the lateral projections had no effect on the distribution of labeled neurons in the spinal cord and dorsal column nuclei following injections of Fluoro-Gold (FG) into the thalamus, and a small unilateral lesion of the lateral projection reduced the ipsilateral labeling in the motor cortex following injections of FG into the pyramidal decussation. These findings suggest that most SHT neurons ascend through the contralateral lateral projection and that less than half continue in the medial projection to the ipsilateral side. They also suggest a site that can be lesioned without affecting other ascending sensory spinal pathways.

Afferent Pathways↗
PubMed1997

Projections of the nucleus of the basal optic root in pigeons (Columba livia) revealed with biotinylated dextran amine.

The nucleus of the basal optic root (nBOR) of the accessory optic system is known to be involved in the analysis of the visual consequences of self-motion. Previous studies have shown that the nBOR in pigeons projects bilaterally to the vestibulocerebellum, the inferior olive, the interstitial nucleus of Cajal, and the oculomotor complex and projects unilaterally to the ipsilateral pretectal nucleus lentiformis mesencephali and the contralateral nBOR. By using the anterograde tracer biotinylated dextran amine, we confirmed these projections and found (previously unreported) projections to the nucleus Darkshewitsch, the nucleus ruber, the mesencephalic reticular formation, and the area ventralis of Tsai as well as ipsilateral projections to the central gray, the pontine nuclei, the cerebellar nuclei, the vestibular nuclei, the processus cerebellovestibularis, and the dorsolateral thalamus. In addition to previous studies, which showed a projection to the dorsomedial subdivision of the contralateral oculomotor complex, we found terminal labelling in the ventral and dorsolateral subdivisions. Individual fibers were reconstructed from serial sections, and collaterals to various nuclei were demonstrated. For example, collaterals of fibers projecting to the vestibulocerebellum terminated in the vestibular or cerebellar nuclei; collaterals of fibers to the inferior olive terminated in the pontine nuclei; many individual neurons projected to the interstitial nucleus of Cajal, the nucleus Darkshewitsch, and the central gray and also projected to the nucleus ruber and the mesencephalic reticular formation; collaterals of fibers to the contralateral nucleus of the basal optic root terminated in the mesencephalic reticular formation and/or the area ventralis of Tsai; neurons projecting to the nucleus lentiformis mesencephali also terminated in the dorsolateral thalamus. The consequences of these data for understanding the visual control of eye movements, neck movements, posture, locomotion, and visual perception are discussed.

Animals↗
PubMed1997

Quantitative and neurogenic analysis of neurons with supraspinal projections in the superficial dorsal horn of the rat lumbar spinal cord.

Dual retrograde axonal tracers, Fluoro-Gold (FG) and true blue (TB), were used in conjunction with [3H]thymidine autoradiography to determine the number and neurogenic pattern of neurons with supraspinal projections in the superficial dorsal horn (SDH), i.e., laminae I and II, in spinal segment L1 of the rat. FG was injected into rostral brain centers (dorsal thalamus and midbrain), and TB was injected into the caudal brainstem (medulla) in young adult rats previously administered [3H]thymidine in utero. Following stereological correction, each dorsal horn had an average of 1.22 neurons in lamina I and 0.24 neurons in lamina II that had supraspinal projections per 10-microm transverse section. In the SDH, 52% of the neurons with supraspinal projections were found to project to rostral brain centers alone, 3.0% only to the caudal brainstem, and 45% to both areas. There was no significant difference in the percentage distribution of each of the three groups of neurons between lamina I and lamina II. Cell counts in the present study, in conjunction with previous observations in the literature, suggest that the majority of supraspinal projection neurons in the SDH fall into two groups: 1) spinomesencephalic neurons with collaterals to the medulla and 2) spinothalamic neurons with collaterals to the midbrain. The neurogenesis of supraspinal projection neurons in the SDH proceeded along an axon-length gradient, whereby neurons with the longest axons, those with projections to rostral brain centers, completed neurogenesis prior to neurons with shorter axons, those with projections only to the caudal brainstem. The generation of all SDH neurons with supraspinal projections was completed on embryonic day 14 (E14), 2 days prior to the completion of neurogenesis for SDH neurons with intraspinal projections.

Animals↗
PubMed1999

Relationships between cytochrome oxidase (CO) blobs in primate primary visual cortex (V1) and the distribution of neurons projecting to the middle temporal area (MT).

The cytochrome oxidase (CO) blobs and interblobs in layer 3B of primate visual cortex have different sets of corticocortical connections. Cortical layers below layer 3B also project corticocortically, but the relationship of efferent projections from the deeper layers to the overlying blob/interblob architecture is less clear. We studied the tangential organization of neurons projecting from primary visual cortex (V1) to the middle temporal visual area (MT) and their relationship to the CO blobs. MT-projecting neurons in two primate species, bush babies and owl monkeys, were retrogradely labeled, then charted in tangential sections, and compared to the positions of the overlying CO blobs. In both primate species, MT-projecting neurons in layer 3C were unevenly distributed in the tangential plane, with dense patches of labeled cells that were aligned with the CO blobs. A novel two-dimensional spatial correlation method was used to show the colocalization of MT-projecting cells with the overlying blobs. Chi-square analyses performed with the cortical surface equally divided into compartments of blob, interblob, and blob/interblob borders showed that blob columns tended to have about 1.5 times more MT-projecting cells (P < 0.0001) than interblob columns. Similar analyses were applied to published data on V1 cells projecting to area MT in macaque monkey (Shipp and Zeki [1989] Euro J Neurosci 1:310-332). Again, the results showed a significant correlation between the cell distribution and CO blobs. Taken together, these results suggest that layer 3C is not uniform but is made up of a mosaic of cells that project to area MT and cells that project to some other location. These findings also indicate that the mosaic organization of layer 3C is related in some unique way to the overlying CO architecture.

Animals↗
PubMed2002

The alpha-factor receptor C-terminus is important for mating projection formation and orientation in Saccharomyces cerevisiae.

Successful mating of MATa Saccharomyces cerevisiae cells is dependent on Ste2p, the alpha-factor receptor. Besides receiving the pheromone signal and transducing it through the G-protein coupled MAP kinase pathway, Ste2p is active in the establishment and orientation of the mating projection. We investigated the role of the carboxyl terminus of the receptor in mating projection formation and orientation using a spatial gradient assay. Cells carrying the ste2-T326 mutation, truncating 105 of the 135 amino acids in the receptor tail including a motif necessary for its ligand-mediated internalization, display slow onset of projection formation, abnormal shmoo morphology, and reduced ability to orient the mating projection toward a pheromone source. This reduction was due to the increased loss of mating projection orientation in a pheromone gradient. Cells with a mutated endocytosis motif were defective in reorientation in a pheromone gradient. ste2-Delta296 cells, which carry a complete truncation of the Ste2p tail, exhibit a severe defect in projection formation, and those projections that do form are unable to orient in a pheromone gradient. These results suggest a complex role for the Ste2p carboxy-terminal tail in the formation, orientation, and directional adjustment of the mating projection, and that endocytosis of the receptor is important for this process. In addition, mutations in RSR1/BUD1 and SPA2, genes necessary for budding polarity, exhibited little or no defect in formation or orientation of mating projections. We conclude that mating projection orientation depends upon the carboxyl terminus of the pheromone receptor and not the directional machinery used in budding.

Cell Differentiation↗
PubMed2003

Tectothalamic visual projections in turtles: their cells of origin revealed by tracing methods.

In two species of turtle (Emys orbicularis and Testudo horsfieldi), retrograde and anterograde tracer techniques were used to study projections from the optic tectum to the nucleus rotundus (Rot) and to the dorsal lateral geniculate nucleus (GLd). The ipsilateral Rot received the most massive tectal projections, stemming from numerous neurons located in the stratum griseum centrale (SGC). These neurons varied in size and shape, many of them having a wide zone of dendritic arborization within both the (SGC) and the stratum griseum et fibrosum superficiale (SGFS). Projections from the tectum to the GLd were ipsilateral, were extremely scarce, and arose from a small number of neurons of various shapes situated in the SGFS; these cells were, as a rule, smaller than those projecting to the Rot. For the most part, these neurons were radially oriented, with rather restricted dendritic arborizations in the most superficial sublayers of the SGFS; smaller numbers of projection neurons were horizontally oriented, with long dendrites branching throughout the layer. Some neurons located in the stratum griseum periventriculare (SGP) projected to both the Rot and the GLd. Most of these neurons had dendritic arborizations within the retinorecipient zone of the SGFS. We were unable to rule out the possibility that some cells projecting to the GLd were situated in the SGC. Both the GLd and the main body of the Rot did not contain neurons projecting to the optic tectum. Thalamic neurons projecting to the tectum were observed in the ventral lateral geniculate nucleus, the intergeniculate leaflet and the interstitial nuclei of the tectothalamic tract, and the nucleus of the decussatio supraoptica ventralis. The question of whether variation in the laminar organization of the tectorotundal and tectogeniculate projection neurons in reptiles, birds, and mammals may be related to different degrees of differentiation of the tectal layers is discussed.

Animals↗
PubMed2003

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the fire-bellied toad Bombina orientalis.

Intracellular recording and biocytin labeling were carried out in the fire-bellied toad Bombina orientalis to study the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation. Labeled neurons (n = 142) were divided into the following groups: TH1 neurons projecting to the dorsal striatum; TH2 neurons projecting to the amygdala, nucleus accumbens, and septal nuclei; TH3 neurons projecting to the medial or dorsal pallium; TH4 neurons with projections ascending to the dorsal striatum or ventral striatum/amygdala and descending to the optic tectum, tegmentum, and rostral medulla oblongata; TH5 neurons with projections to the tegmentum, rostral medulla oblongata, prectectum, or tectum; and TH6 neurons projecting to the hypothalamus. TH1 neurons are found in the central, TH2 neurons in the anterior and central, TH3 neurons in the anterior dorsal nucleus, and TH4 and TH5 neurons in the posterior dorsal or ventral nucleus. Neurons with descending projections arborize in restricted parts of retinal afferents; neurons with ascending projections do not substantially arborize within retinal afferents. At electrical optic nerve stimulation, neurons in the ventral thalamus respond with excitation at latencies of 10.8 msec; one-third of them follow repetitive stimulation and possibly are monosynaptically driven. Neurons in the dorsal thalamus respond mostly with inhibition at latencies of 42.3 msec and are polysynaptically driven. This corroborates the view that neurons in the dorsal thalamus projecting to the telencephalon receive no substantial direct retinal input and that the thalamopallial pathway of amphibians is not homologous to the mammalian retinogeniculocortical pathway.

Animals↗
PubMed2001

Orbitomedial prefrontal cortical projections to hypothalamus in the rat.

A previous study in the rat revealed that distinct orbital and medial prefrontal cortical (OMPFC) areas projected to specific columns of the midbrain periaqueductal gray region (PAG). This study used anterograde tracing techniques to define projections to the hypothalamus arising from the same OMPFC regions. In addition, injections of anterograde and retrograde tracers were made into different PAG columns to examine connections between hypothalamic regions and PAG columns projected upon by the same OMPFC regions. The most extensive patterns of hypothalamic termination were seen after injection of anterograde tracer in prelimbic and infralimbic (PL/IL) and the ventral and medial orbital (VO/MO) cortices. Projections from rostral PL/IL and VO/MO targeted the rostrocaudal extent of the lateral hypothalamus, as well as lateral perifornical, and dorsal and posterior hypothalamic areas. Projections arising from caudal PL/IL terminated within the dorsal hypothalamus, including the dorsomedial nucleus and dorsal and posterior hypothalamic areas. There were also projections to medial perifornical and lateral hypothalamic areas. In contrast, it was found that anterior cingulate (AC), dorsolateral orbital (DLO), and agranular insular (AId) cortices projected to distinct and restricted hypothalamic regions. Projections arising from AC terminated within dorsal and posterior hypothalamic areas, whereas DLO and AId projected to the lateral hypothalamus. The same OMPFC regions also projected indirectly, by means of specific PAG columns, to many of the same hypothalamic fields. In the context of our previous findings, these data indicate that, in both rat and macaque, parallel but distinct circuits interconnect OMPFC areas with specific hypothalamic regions, as well as PAG columns.

Animals↗
PubMed2004

Motor projections to the basis pontis in rhesus monkey.

Motor corticopontine studies suggest that the pons is topographically organized, but details remain unresolved. We used physiological mapping in rhesus monkey to define subregions in precentral motor cortex (M1), injected isotope tracers into M1 and the supplementary motor area (SMA), and studied projections to the basis pontis. Labeled fibers descend in the internal capsule (SMA in anterior limb and genu; M1 in posterior limb) and traverse the midsection of the cerebral peduncle, where SMA fibers are medial, and face, arm, and leg fibers are progressively lateral. Each motor region has unique terminations in the ipsilateral basis pontis and nucleus reticularis tegmenti pontis. Projections are topographically organized, preferentially in the caudal half of the pons, situated in close proximity to traversing corticofugal fibers. In nuclei that receive multiple inputs, terminations appear to interdigitate. Projections from the SMA-face region are most medial and include the median pontine nucleus. M1-face projections are also medial but are lateral to those from SMA-face. Hand projections are in medially placed curved lamellae in mid- and caudal pons. Dorsal trunk projections are in medial and ventral locations. Ventral trunk/hip projections encircle the peduncle in the caudal pons. Foot projections are heaviest caudally in laterally placed, curved lamellae. These results have relevance for anatomical clinical correlations in the human basis pontis. Furthermore, the dichotomy of motor-predominant caudal pons projections to cerebellar anterior lobe, contrasted with associative-predominant rostral pons projections to cerebellar posterior lobe, is consistent with new hypotheses regarding the cerebellar contribution to motor activity and cognitive processing.

Animals↗
PubMed1975

The projections of cells in different layers of the cat's visual cortex.

The projection of cells in different layers of several cortical visual areas in the cat were studied using the method of retrograde transport of horseradish peroxidase. Injections of the enzyme were made through a recording micropipette, making it possible to localize the injection site by physiological criteria. We found that layer VI cells projected to the alteral geniculate nucleus, while a distinct population of cells in layer V projected to the superior colliculus. Cells in layers II and III were tha major sources of ipsilateral cortico-cortical connections. This pattern of projection was consistent from one visual area to another. Pyramidal cells appeared to be the source of cortico-geniculate, cortico-collicular and cortico-cortical projections. The proportion of cells within a layer that terminated in a given site varied from layer to layer: apparently all of the large pyramids in layer V had terminals in the superior colliculus, about half of the pyramids in layer VI had terminals in the lateral geniculate nucleus, while only a small proportion of the pyramids in layers II and III had terminals in any single cortical area. The results indicated a remarkable specificity in the projections of the cortical layers. The cortical connections of the different cell types in layers A and A1 of the lateral geniculate nucleus were also examined: the cells that projected to area 17 were much more numerous and were on the average smaller than those that projected to area 18. Projections to the cortex were also found from the pulvinar, the medial interlaminar nucleus and the posterior nucleus. Direct connections were observed to the lateral geniculate nucleus from several midbrain reticular nuclei. Finally, projections were found to the superior colliculus from the zona incerta, the reticular nucleus of the thalamus and the ventral lateral geniculate nucleus.

Animals↗
PubMed1976

The posterior thalamic region and its cortical projection in New World and Old World monkeys.

The posterior nuclear complex of the thalamus in rhesus, pigtailed and squirrel monkeys consists of the combined suprageniculate-limitans nucleus and an ill defined region of heterogeneous cell types extending anteriorly from the dorsal lobe of the medial geniculate body towards the posterior pole of the ventral nuclear complex. This region is referred to as the posterior nucleus. It is directly continuous with the ventroposteroinferior nucleus. The cortical projections of each of these nuclei, together with those of the adjacent ventral, pulvinar and medial geniculate complexes, have been studied by means of the autoradiographic tracing technique. The suprageniculate-limitans nucleus, the main input to which is the superior colliculus, projects upon the granular insular area of the cortex. The medial portion of the posterior nucleus projects to the retroinsular field lying posterior to the second somatic sensory area. There is clinical and electrophysiological evidence to suggest that the retroinsular area may form part of a central pain pathway. The lateral portion of the posterior nucleus which is closely related to certain elements of the medial geniculate complex, projects to the postauditory cortical field. The ventroposterioinferior nucleus, which may be involved in vestibular function, projects to the dysgranular insular field. The principal medial geniculate nucleus can be subdivided into a ventral division that projects to field AI of the auditory cortex and a dorsal division that merges with the posterior nucleus; it is further subdivided into an anterodorsal component that projects to two fields on the superior temporal gyrus, together with a posterodorsal component in which separate cell populations project to areas lying anterior and medial to AI. The magnocellular medial geniculate nucleus, sometimes considered a part of the posterior complex, appears to project diffusely to layer I of all the auditory fields. The auditory fields are bounded on three sides by the projection field of the medial nucleus of the pulvinar which also extends into the upper end of the lateral sulcus to bound the fields receiving fibers from the posterior nucleus. The topography of the areas receiving fibers from the posterior, medial geniculate and pulvinar complexes, taken in conjunction with the rotation of the primate temporal lobe, permits all of these fields to be compared with similar, better known areas in the cat brain.

Animals↗
PubMed1977

The projection from the lateral geniculate nucleus onto the visual cortex in the cat. A quantitative study with horseradish-peroxidase.

Horseradish-peroxidase (HRP) was injected (9-18 microng in 0.03-0.06 micronl) into cortical areas 17, 18 or 19 of 11 adult cats. After survival times of 17 hours to 7 days, the thalamus was examined for retrogradely HRP labelled nerve cells in serial transverse sections. From these sections, the percentage of labelled cells occurring in each subdivision of the dorsal lateral geniculate nucleus (LGNd) was calculated for each animal. One case each for injections in areas 17, 18, and 19 was then chosen for nerve cell size measurements in each LGNd subdivision. The perikaryal area of each labelled cell (N=689), and of representative samples of unlabelled cells (N=1137), was measured by planimetry. Size distribution histograms, mean values, standard deviation, and statistical significance levels were obtained by computer. It was found that area 17 receives a projection almost exclusively from laminae A and Al, and that the projecting cells belong to all cell size classes. Area 18 receives a projection mainly from laminae C and Al, and from the medial interlaminar nucleus (MIN). The projecting cells belong mainly to the large cell size classes. Area 19 receives a projection largely from MIN, and also from the C-laminae and extrageniculate cell groups. The projecting cells belong to all cell size classes, with some emphasis on the large cells of lamina C. A significant projection was found to exist from the parvocellular laminae of LGNd onto area 19 and, to a lesser degree, area 18. In conclusion, as one goes from area 17 and 18 and to 19 the projection source shifts from the A-lamine through the C-laminae on to MIN and extrageniculate cell groups. The cells which project to area 18 are on the whole larger than those which project to areas 17 and 19. A significant proportion of the contralateral visual input to area 18 is relayed via lamina C. These results provide a quantitative confirmation and extension of previous anatomical findings, and are in close relationship with physiological results regarding parallel channel processing in the visual system.

Animals↗
PubMed1978

Projections to the inferior olive of the cat. II. Comparisons of input from the gracile, cuneate and the spinal trigeminal nuclei.

The present experiments compared the projections to the inferior olive of the cat from the gracile, cuneate and spinal trigeminal nuclei. A differential labeling strategy was used for these comparisons. It was found that all three somatic sensory nuclei project to portions of all three major divisions of the contralateral inferior olive. The spinal trigeminal n. may also project less densely to the ipsilateral medial accessory olive. Projections to the dorsal accessory nucleus (DAO) and the medially-adjacent ventral lamella of the principal nucleus are roughly somatotopically organized. Although there is considerable overlap between the projection zones, the gracile n. projects predominantly to lateral DAO, the cuneate n. projects predominantly to medial DAO, and the spinal trigeminal nucleus pars caudalis projects predominantly to the most medial portions of DAO and the ventral lamella of principal olive. Projections to the medial accessory olive, on the other hand, are not as highly organized. Instead, they overlap extensively within a small egg-shaped area in the middle of the caudal half of the nucleus. Whereas all portions of the gracile and cuneate nuclei project to the inferior olive, only the pars caudalis of the spinal trigeminal nucleus appears to do so. These results were compared with the three available olivocerebellar maps as well as with the available behavioral and electrophysiological evidence on cerebellar somatotopic organization. This comparison indicated that the inputs to the cerebellum from the three second-order somatosensory nuclei via the inferior olive appear to be generally consistent with cerebellar somatotopic organization. This consistency is apparent not only with respect to the longitudinally-organized, vermal and paravermal differences in the anterior lobe, but also with respect to the transversely-organized specific somatotopy of the intermediate zone of the anterior lobe and the paramedian lobule.

Animals↗
PubMed1979

Projections from the spinal and the principal sensory nuclei of the trigeminal nerve to the cerebellar cortex in the cat, as studied by retrograde transport of horseradish peroxidase.

Projections from the spinal (Vsp) and the principle sensory (Vp) nuclei of the trigeminal nerve to the cerebellar cortex were studied by means of retrograde transport of horseradish peroxidase in the cat. Neurons projecting to the simple lobule and the dorsal part of the paramedian lobule (PMD) were located mainly in the dorsal part of the nucleus interpolaris (Vi) and of the caudal one third of the nucleus oralis (Vo) and in the rostralmost part of the Vo. Neurons projecting to the medial part of the posterior folia of crus II (crus IIp) were located in the dorsal to ventral parts of the Vi and of the caudal one third of the Vo and in the rostralmost part of the Vo, while those projecting to the lateral part of crus IIp were confined to the ventral part of the Vi and of the caudal one third of the Vo. Neurons of the Vp also projected to all of these cortical areas. They were relatively confined to the ventral part of this nucleus. Thest trigeminocerebellar projections were exclusively ipsilateral to the cell origin. There were sparse projections from the Vi and Vo to lobules V to VIIIa. In addition, a small group of neurons in the subnucleus magnocellularis of the nucleus caudalis of the Vsp also projected to the above cortical areas. No projections were, however, observed to the anterior portion of the anterior lobe, crus I, the anterior folia of crus II, paraflocculus, flocculus and the ventral part of the PMD. The majority of these cerebellar projection neurons were medium-sized and triangular, fusiform or ovoid in shape. There were small neurons of similar types and large multipolar neurons as well.

Animals↗
PubMed1979

Topographic projections of the retina and optic tectum upon the ventral lateral geniculate nucleus in the chick.

The topographic projections of the retina upon the optic tectum and ventral lateral geniculate nucleus (GLv) of the chick were investigated by making small intraretinal injections of 3H-proline. The retinotectal projection pattern was similar to that described for the pigeon. The retinal projection to the GLv was also topographic and was restricted to the outermost lamina of the nucleus. The anteroposterior retinal axis was reversed in the GLv relative to its orientation in the tectum but the superoinferior axis was oriented identically in both. Furthermore, the posterior retina had an enlarged area of projection in the GLv similar to the enlarged area of retinotectal projection for the "red field" found in pigeons. The tectogeniculate projection was topographic and was confined to the outermost geniculate lamina. The second-order retinotopic map made by the tectogeniculate projections was in register with the retinogeniculate projection. Although the retinal and tectal projection areas were coextensive in the outermost geniculate lamina, the grain density distributions peaked at different points along a radial path through the geniculate laminae. Injections of HRP into the optic tectum led to very light retrograde labeling of a small population of GLv cells topographically corresponding to the tectogeniculate projection zone of the injection site. The data suggest that the chick GLv is comparable to the GLv of other non-primate mammals.

Animals↗
PubMed1980

Radial organization of thalamic projections to the neocortex in the mouse.

The intracortical distributions of the thalamic projections to a large number of neocortical fields are studied by the anterograde degeneration methods in the mouse. The basic radial distribution of terminating thalamofugal axons is uniform throughout the mouse cortex and is essentially the same as that encountered in other mammalian species. Terminating axons are concentrated in three tiers: an outer tier in layer I, a middle tier in layers IV and/or III, and an inner tier in layer VI. In most fields, terminating axons also extend, to some extent, into layer V. Variations are encountered from field to field, particularly in the density and degree of divergence of projections and in the radial extent of individual tiers with respect to cytoarchitectonic layers. In accord with other studies, the thalamic projections to each field appear to be composed of two general axon classes. Class I axons terminate densely in the middle tier, seem to be of large caliber, and often have collaterals to the other tiers. Class II axons do not terminate densely in the middle tier and seem to be of small caliber. Terminating class II axons may be distributed to one or more tiers and may be concentrated in the inner and/or outer tiers. The thalamic projection to each field has its origin in multiple nuclei. All thalamic nuclei projecting to the neocortex appear to have class II projections and many also have class I projections. Patterns of degeneration in the cortex associated with lesions in different positions in many nuclei suggest that thalamic relay neurons are organized along "lines of projection"--neurons in the same line projecting to the same tangentially restricted cortical region. The neurons of origin of class I and class II axons are intermixed along the lines of projection.

Afferent Pathways↗
PubMed1981

The tectopontine projection the the rat with comments on visual pathways to the basilar pons.

The projection from the superior and inferior colliculi to the basilar pons in the rat was studied with the technique of orthograde transport of labeled amino acids and autoradiography. Injections restricted to the medial or lateral regions of the superior colliculus gave rise to grain labeling representing terminal fields over the ipsilateral peduncular, dorsolateral, and ventrolateral regions of the caudal basilar pons and over the dorsomedial area of the contralateral nucleus reticularis tegmenti pontis (NRTP). The pontine projection from the superior colliculus to the lateral basilar pons is topographically organized; the medial superior colliculus projects primarily to the peduncular region, whereas the lateral superior colliculus terminates chiefly in ventrolateral pontine areas. A projection from the superior colliculus to the contralateral dorsomedial pontine and medial peduncular pontine regions, a previously undescribed finding, has also been shown. Descending fibers from the inferior colliculus do not appear to terminate extensively within the basilar pons but rather course adjacent to pontine cells of the dorsolateral region in the caudal pons. Pretectal nuclei project ipsilaterally to medial and lateral nuclei in the rostral and middle basilar pons, respectively. A rostrocaudal topography exists in the tectopontine projection; the pretectum projects to rostromiddle basilar pons, the superior colliculus to more caudal pontine regions, and the inferior colliculus (although sparsely) to further caudal areas. The pontine projection pattern from the colliculi and pretectum differs from the pontine afferents from the visual cortices. The findings of this study, when compared to our results from previous investigations on the pontocerebellar projection system, suggest that the tectal inputs to certain lateral cerebellar lobules are relayed primarily through NRTP rather than the basilar pons. The collicular projection to midvermal lobules of the cerebellum appear to be mediated in part by both NRTP and lateral pontine nuclei.

Animals↗
PubMed1981

Efferent projections of the medial preoptic nucleus and medial hypothalamus in the pigeon.

The efferent projections of the medial preoptic nucleus (POM), anterior-medial hypothalamic area (AM), and the posteromedial hypothalamic nucleus (PMH) in the pigeon were traced by the autoradiographic technique. Similar and differential connections were noted from these regions. Projections from POM and AM-PMH were traced to nucleus septalis lateralis, nucleus dorsomedialis thalami, nucleus dorsolateralis anterior thalami (pars ventralis), posterior hypothalamic and medial mammillary areas, area ventralis tegmenti (Tsai), central gray of midbrain and nucleus intercollicularis and substantia grisea periventricularis of the midbrain. The density of silver grains in these regions differed with POM and AM-PMH injections. Other projections were observed exclusively from only one or two of the nuclear regions injected. Connections from POM and the rostral part of AM were seen to the median eminence, neurohypophysis, and the nucleus of anterior pallial commissure. Only cells of the anterior part of AM project fibers to nucleus septalis medialis. In the hypothalamus, projections from POM are concentrated in the periventricular region and in the preoptic-hypophyseal tract in the extreme lateral hypothalamus, while AM-PMH projections are heaviest in the medial hypothalamus and lateral preoptic area. A major difference in the connections of PMH from POM is the more substantial PMH projection to the midbrain. A prominent projection courses dorsolaterally and posteriorly from PMH toward nucleus ovoidalis and splits into two pathways: a lateral pathway which heavily innervates n. intercollicularis and the periventricular gray and a ventrolateral projection to the midbrain tegmentum. The projections described above provide anatomical substrates for neuroendocrine, autonomic, and behavioral functions.

Animals↗