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The autoradiographic demonstration of axonal connec - tions in the central nervous sytem
... At the beginning of the period, our understanding of brain circuitry was rudimentary -a problem exacerbated by how older 'lesion' techniques did not distinguish between damage to cells of origin and to fibres of passage. A crucial first step was the introduction of axonal trac- ing techniques such as autoradiography and horseradish peroxi- dase ( Cowan et al., 1972;LaVail and LaVail, 1972), which heralded a quiet revolution that continues to this day with new single-cell connectional techniques (Figure 2(a)). ...
This review brings together past and present achievements in memory research, ranging from molecular to psychological discoveries. Despite some false starts, major advances include our growing understanding of learning-related neural plasticity and the characterisation of different classes of memory. One striking example is the ability to reactivate targeted neuronal ensembles so that an animal will seemingly re-experience a particular memory, with the further potential to modify such memories. Meanwhile, human functional imaging studies can distinguish individual episodic memories based on voxel activation patterns. While the hippocampus continues to provide a rich source of information, future progress requires broadening our research to involve other sites. Related challenges include the need to understand better the role of glial–neuron interactions and to look beyond the synapse as the sole site of experience-dependent plasticity. Unmet goals include translating our neuroscientific knowledge in order to optimise learning and memory, especially among disadvantaged populations.
... In spite of their promise, the Fink-Heimer methods were destined to compose a brief last chapter in the 90-year-long history of tracing fiber pathways by means of experimentally induced Wallerian degeneration. By the time the methods were coming into wide use, Maxwell Cowan, Anita Hendrickson, and their associates (Cowan et al., 1972) were already adapting for routine use the fundamentally different strategy, introduced by Iasek et al. (1968), of tracing nerve fibers by autoradiographic recording of the uptake and intra-axonal transport of radioactively labeled amino acids. This strategy offered enough advantages to replace the Wallerian degeneration methods almost completely. ...
... A radiolabeled amino acid (usually leucine or proline) is injected intracortically into the zone of interest. After 5 days, the animal is killed and the distribution of this amino acid (the axonal transport) in other brain regions is studied [12]. This method cannot be used to study the human brain because of ethical reasons. ...
Aim:
To study the peri-insular association tract anatomy and define the permissible anatomical boundaries for resection of glial insular tumors with allowance for the surgical anatomy of the peri-insular association tracts.
Material and methods:
In an anatomic study of the superior longitudinal fascicle system (SLF I, SLF II, SLF III, arcuate fascicle), we used 12 anatomical specimens (6 left and 6 right hemispheres) prepared according to the Klingler's fiber dissection technique. To confirm the dissection data, we used MR tractography (HARDI-CSD-tractography) of the conduction tracts, which was performed in two healthy volunteers.
Results:
Except the SLF I (identified in 7 hemispheres by fiber dissection), all fascicles of the SLF system were found in all investigated hemispheres by both fiber dissection and MR tractography. The transcortical approach to the insula through the frontal and (or) parietal operculum is associated with a significant risk of transverse transection of the SLF III fibers passing in the frontal and parietal opercula. The most optimal area for the transcortical approach to the insula is the anterior third of the superior temporal gyrus that lacks important association tracts and, consequently, a risk of their injury. The superior peri-insular sulcus is an intraoperative landmark for the transsylvian approach, which enables identification of the SLF II and arcuate fascicle in the surgical wound.
Conclusion:
Detailed knowledge of the peri-insular association tract anatomy is the prerequisite for neurosurgery in the insular region. Our findings facilitate correct identification of both the site for cerebral operculum dissection upon the transcortical approach and the intraoperative landmarks for locating the association tracts in the surgical wound upon the transsylvian approach to the insula.
... One reason is that the majority of these studies from 1970s and on, utilized anterogradely transported, radioactively labeled amino acids to label connections across the entire monkey brain, where the emphasis was usually on finding terminal fields (e.g., Rosene and Van Hoesen 1977;Pandya and Yeterian 1996;Seltzer and Pandya 2009) rather than following fiber trajectories. When attention was directed to the fiber pathways themselves, the methodological approach using tritiated amino acids (autoradiogaphic studies; Cowan et al. 1972), made it possible to identify the trajectories of fiber aggregates, or bundles of fibers coursing through the white matter toward their destinations. These studies provided new insights into the overall structure of white matter connectivity at the mesoscale level, and often included reconstructions of the paths as artistic renderings that assumed smoothly curved trajectories (e.g., Pandya and Rosene 1985;Demeter et al. 1990;Pandya 1992, 2006;Petrides and Pandya 1999). ...
Brain fiber pathways are presumed to follow smooth curves but recent high angular resolution diffusion MRI (dMRI) suggests that instead they follow 3 primary axes often nearly orthogonal. To investigate this, we analyzed axon pathways under monkey primary motor cortex with (1) dMRI tractography, (2) axon tract tracing, and (3) axon immunohistochemistry. dMRI tractography shows the predicted crossings of axons in mediolateral and dorsoventral orientations and does not show axon turns in this region. Axons labeled with tract tracer in the motor cortex dispersed in the centrum semiovale by microscopically sharp axonal turns and/or branches (radii ≤15 µm) into 2 sharply defined orientations, mediolateral and dorsoventral. Nearby sections processed with SMI-32 antibody to label projection axons and SMI-312 antibody to label all axons revealed axon distributions parallel to the tracer axons. All 3 histological methods confirmed preponderant axon distributions parallel with dMRI axes with few axons (<20%) following smooth curves or diagonal orientations. These findings indicate that axons navigate deep white matter via microscopic sharp turns and branches between primary axes. They support dMRI observations of primary fiber axes, as well as the prediction that fiber crossings include navigational events not yet directly resolved by dMRI. New methods will be needed to incorporate coherent microscopic navigation into dMRI of connectivity.
... One case (Case 6) was cut in the parasagittal plane. Every fifth section was mounted onto gelatinized slides, dehydrated, defatted, and processed for autoradiography according to the procedures of Cowan et al. (1972). These sections were dipped in Kodak NTB2 emulsion and exposed at 4 C for at least 12 weeks. ...
Area V4 has numerous, topographically organized connections with multiple cortical areas, some of which are important for spatially organized visual processing, and others which seem important for spatial attention. Although the topographic organization of V4's connections with other cortical areas has been established, the detailed topography of its connections with subcortical areas is unclear. We therefore injected retrograde and anterograde tracers in different topographical regions of V4 in nine macaques to determine the organization of its subcortical connections. The injection sites included representations ranging from the fovea to far peripheral eccentricities in both the upper and lower visual fields. The topographically organized connections of V4 included bidirectional connections with four subdivisions of the pulvinar, two subdivisions of the claustrum, and the interlaminar portions of the lateral geniculate nucleus, and efferent projections to the superficial and intermediate layers of the superior colliculus, the thalamic reticular nucleus, and the caudate nucleus. All of these structures have a possible role in spatial attention. The nontopographic, or converging, connections included bidirectional connections with the lateral nucleus of the amygdala, afferent inputs from the dorsal raphe, median raphe, locus coeruleus, ventral tegmentum and nucleus basalis of Meynert, and efferent projections to the putamen. Any role of these structures in attention may be less spatially specific.
... † Imaging methods (not connectivity methods per se) that can be used in conjunction with dyes and tracers, etc. for detailed reconstruction of microcircuitry. acids[20] Medium In vivo rodent, slice culture No cell morphology visible, limited compatibility with histological methodsTable 2: Comparison of conventional neuronal tracer substances. In columns titled Labeling efficacy, subcolumn A indicates anterograde direction, R retrograde direction, and the gray-level indicates efficacy with black strongest. ...
Survey of methods relevant for determining neuronal connectivity. To supplement the discussion provided in the main article, here we provide a brief general overview of experimental methods for determining and imaging neuronal connection patterns.
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... In particular, little is known about the projections and terminations of the amygdalotegmental pathway. Therefore, in the present study in the cat horseradish peroxidase (HRP) was injected into the brain stem to supplement previous data (Hopkins, 1975) on the cells of origin of the amygdalotegmental projection and 3H-leucine (Cowan et al., 1972) was injected into the amygdala to determine the exact brain stem projections of the central nucleus (CE), Parts of these results have been reported briefly (Hopkins and Holstege, 1976). ...
Amygdalotegmental projections were studied in 26 cats after injections of horseradish peroxidase (HRP) in the diencephalon, midbrain and lower brain stem and in 6 cats after injection of 3H-leucine in the amygdala. Following HRP injections in the posterior hypothalamus, periaqueductal gray (PAG) and tegmentum many retrogradely labeled neurons were present in the central nucleus (CE) of the amygdala, primarily ipsilaterally. Injections of HRP in the posterior hypothalamus and mesencephalon also resulted in the labeling of neurons in the basal nucleus, pars magnocellularis. Following 3H-leucine injections in CE and adjacent structures autoradiographically labeled fibers were present in the stria terminalis and ventral amygdalofugal pathways. In the mesencephalon heavily labeled fiber bundles were located lateral to the red nucleus. Labeled fibers and terminals were distributed to the mesencephalic reticular formation, substantia nigra, ventral tegmental area and PAG. In the pontine and medullary tegmentum the bulk of passing fibers was located laterally in the reticular formation. Many labeled fibers and terminals were distributed to the parabrachial nuclei, locus coeruleus, nucleus subcoeruleus and lateral tegmental fields. Many terminals were also present in the solitary nucleus and dorsal motor nucleus of the vagus nerve. The location of the cells of origin and the distribution of the terminals of the amygdalotegmental projection suggest that this pathway plays an important role in the integration of somatic and autonomic responses associated with affective defense.
... ????m thick coronal sections. The transport of 3 H-proline was revealed in sections processed for autoradiography [40], with an exposure time of 6 weeks at 4 ? C, while HRP labeling was revealed using tetramethylbenzidine as the chromogen [41]. ...
Previous studies in rabbits identified an array of extrastriate cortical areas anatomically connected with V1 but did not describe their internal topography. To address this issue, we injected multiple anatomical tracers into different regions in V1 of the same animal and analyzed the topography of resulting extrastriate labeled fields with reference to the patterns of callosal connections and myeloarchitecture revealed in tangential sections of the flattened cortex. Our results extend previous studies and provide further evidence that rabbit extrastriate areas resemble the visual areas in rats and mice not only in their general location with respect to V1 but also in their internal topography. Moreover, extrastriate areas in the rabbit maintain a constant relationship with myeloarchitectonic borders and features of the callosal pattern. These findings highlight the rabbit as an alternative model to rats and mice for advancing our understanding of cortical visual processing in mammals, especially for projects benefiting from a larger brain.
... With the purpose studying the cortical connections without the need for histology, Cowan et al. [1972] developed autoradiographic tracing. This technique consists in the injection of a radioactive chemical tracer is used in combination with radiography to visualize axonal pathways. ...
The motivation of this thesis is the in vivo dissection of the human brain's white matter from diffusion magnetic resonance imaging. This procedure isolates the human brain's white matter tracts that play a role in a particular function or disorder of the brain so they can be analysed. Manually performing this task requires a great knowledge of brain anatomy and several hours of work. Hence, the development of a technique to automatically perform the identification of white matter structures is of utmost importance. The brain is organized in networks that are made up of tracts connecting different regions. These networks are important for the development of brain functions such as language or vision. Moreover, lesions and cognitive disorders are sometimes better explained by disconnection mechanisms between cerebral regions than by damage of those regions. Despite several decades of tracing these networks in the brain, our knowledge of cerebral connections has progressed very little since the beginning of the last century. Recently, we have seen a spectacular development of magnetic resonance imaging (MRI) techniques for the study of the living human brain. One technique for exploring white matter tissue characteristics and pathways in vivo is Diffusion MRI (dMRI). Particularly, dMRI-based tractography facilitates tracing the white matter tracts in vivo. Overall, dMRI is a promising technique to explore the anatomical basis of human cognition and its disorders. This thesis has several contributions. We develop the means for the automatic dissection of WM tracts from dMRI, this is based on a mathematical framework for the WM and its tracts based on the Gaussian process formalism. Using this framework, we develop techniques to find group differences in the white matter, particularly between healthy and schizophrenic subjects and to improve the visualization and representation of spinal cord MRI images.
... A Cowan et al., 1972 andInsausti andAmaral, 2008 for details). After a period of 48 hours for WGA-HRP, or 14 days for fluorescent tracers and BDA, animals were tranqui- lized with ketamine (8 mg/kg i.m.), deeply anesthetized with sodium thiopental (200 mg/kg, sodium thiopen- thal, Abbott, West Berkshire, UK), and perfused trans- cardially. ...
The anatomical organization of the lateral prefrontal cortex (LPFC) afferents to the anterior part of the temporal lobe (ATL) still needs to be clarified. The LPFC has two subdivisions, dorsal (dLPFC) and ventral (vLPFC) which have been linked to cognitive processes. The ATL includes several different cortical areas, namely temporal polar cortex and rostral parts of perirhinal, inferotemporal and anterior tip of the superior temporal gyrus cortices. Multiple sensory modalities converge in the ATL. All of them (except rostral inferotemporal and superior temporal gyrus cortices) are components of the medial temporal lobe, critical for long-term memory processing. We studied the LPFC connections with the ATL by placing retrograde tracer injections into the ATL: temporal polar (n=3), perirhinal (areas 35 and 36, n=6) and inferotemporal cortices (area TE, n=5), plus one additional deposit in the posterior parahippocampal cortex (area TF, n=1). Anterograde tracer deposits into the dLPFC (A9 and A46, n=2) and vLPFC (A46v, n=2) and orbitofrontal cortex (OFC, n=2) were placed for confirmation of those projections. Results showed that vLPFC displays a moderate projection to rostral area TE and dorsomedial portion of temporal polar cortex; in contrast, the dLPFC connections with the ATL were weak. By comparison, OFC and medial frontal cortices (MFC) showed dense connectivity with the ATL, namely A13 with temporopolar and perirhinal cortices. All areas of MFC projected to temporopolar cortex, albeit with a lower intensity. The functional significance of such paucity of LPFC afferents is unknown. This article is protected by copyright. All rights reserved.
© 2015 Wiley Periodicals, Inc.
... Following perfusion, the brains were stored in 10% formalin for 2 weeks, then embedded in paraffin, and cut into 10-µm coronal sections. Sections were mounted on glass slides coated with Kodak NTB2 emulsion, stored at 4°C in the dark, and subsequently processed using a method modified from Cowan et al. (1971). For each animal, there was more than 1 series of sections. ...
The projections from the amygdala and hippocampus (including subiculum and presubiculum) to prefrontal cortex were compared using anterograde tracers injected into macaque monkeys (Macaca fascicularis, Macaca mulatta). Almost all prefrontal areas were found to receive some amygdala inputs. These connections, which predominantly arose from the intermediate and magnocellular basal nucleus, were particularly dense in parts of the medial and orbital prefrontal cortex. Contralateral inputs were not, however, observed. The hippocampal projections to prefrontal areas were far more restricted, being confined to the ipsilateral medial and orbital prefrontal cortex (within areas 11, 13, 14, 24a, 32, and 25). These hippocampal projections principally arose from the subiculum, with the fornix providing the sole route. Thus, while the lateral prefrontal cortex essentially receives only amygdala inputs, the orbital prefrontal cortex receives both amygdala and hippocampal inputs, though these typically target different areas. Only in medial prefrontal cortex do direct inputs from both structures terminate in common sites. But, even when convergence occurs within an area, the projections predominantly terminate in different lamina (hippocampal inputs to layer III and amygdala inputs to layers I, II, and VI). The resulting segregation of prefrontal inputs could enable the parallel processing of different information types in prefrontal cortex.
© The Author 2015. Published by Oxford University Press.
... One case (Case 6) was cut in the parasagittal plane. Every fifth section was mounted onto gelatinized slides, dehydrated, defatted and processed for autoradiography according to the procedures of Cowan et al. (1972). These sections were dipped in Kodak NTB2 emulsion and exposed at 4 @BULLET C for at least 12 weeks. ...
The claustrum is a surprisingly large, sheet-like neuronal structure hidden beneath the inner surface of the neocortex. We found that the portions of the claustrum connected with V4 appear to overlap considerably with those portions connected with other cortical visual areas, including V1, V2, MT, MST and FST, TEO and TE. We found extensive reciprocal connections between V4 and the ventral portion of the claustrum (vCl), which extended through at least half of the rostrocaudal extent of the structure. Additionally, in approximately 75% of the cases, we found reciprocal connections between V4 and a more restricted region located farther dorsal, near the middle of the structure (mCl). Both vCl and mCl appear to have at least a crude topographic organization. Based on the projection of these claustrum subdivisions to the amygdala, we propose that vCl and mCl are gateways for the transmission of visual information to the memory system. In addition to these crude visuotopically organized regions, there are other parts of the claustrum that obey the topographical proximity principle, with considerable overlap of their connections. There is only an overall segregation of claustrum regions reciprocally connected to the occipital, parietal, temporal and frontal lobes. The portion of the claustrum connected to the visual cortex is located ventral and posterior; the one connected to the auditory cortex is located dorsal and posterior; the one connected to the somatosensory cortex is located dorsal and medial; the one connected to the frontal premotor and motor cortices is located dorsal and anterior; while the one connected to the temporal cortex is located ventral and anterior. The extensive reciprocal connections of the claustrum with almost the entire neocortex and its projections to the hippocampus, amygdala and basal ganglia prompt us to propose its role as a gateway for perceptual information to the memory system.
... Sections collected for the analysis of the 3 H-amino acid injections were processed according to the protocol of Cowan et al. (1972) for autoradiographic demonstration of the anterogradely transported isotope. Sections were counterstained with thionin to allow the determination of cytoarchitectonic boundaries of different cortical areas (Lavenex et al., 2002). ...
The entorhinal cortex is the primary interface between the hippocampal formation and neocortical sources of sensory information. While much is known about the cells of origin, termination patterns and topography of the entorhinal projections to other fields of the adult hippocampal formation, very little is known about the development of these pathways, particularly in the human or nonhuman primate. We have carried out experiments in which the anterograde tracers (3) H-amino acids, biotinylated dextran amine and Phaseolus vulgaris leucoagglutinin were injected into the entorhinal cortex in 2-week-old rhesus monkeys (Macaca mulatta). We found that the three fiber bundles originating from the entorhinal cortex (the perforant path, the alvear pathway and the commissural connection) are all established by two weeks of age. Fundamental features of the laminar and topographic distribution of these pathways is also similar to those in adults. There is evidence, however, that some of these projections may be more extensive in the neonate than in the mature brain. The homotopic commissural projections from the entorhinal cortex, for example, originate from a larger region within the entorhinal cortex and terminate much more densely in layer I of the contralateral entorhinal cortex than in the adult. These findings indicate that the overall topographical organization of the main cortical afferent pathways to the dentate gyrus and hippocampus are established by birth. These findings add to the growing body of literature on the development of the primate hippocampal formation and will facilitate further investigations on the development of episodic memory. J. Comp. Neurol., 2013. © 2013 Wiley Periodicals, Inc.
... After survival periods ranging from 7 to 10 days, the animals were overdosed with sodium pentobarbital and perfused transcardially with saline followed by a 10% formalin solution. The brains were subsequently processed for autoradiography according to the technique described by Cowan et al. (1972). A series of coronal sections of the individual hemispheres in which an injection had been placed were examined microscopically under darkfield illumination. ...
The present study investigated the origin, course, and terminations of the association fiber system linking the frontal cortex with the hippocampal system by means of the cingulum bundle. Injections of tritiated amino acids were placed within individual cytoarchitectonic areas of the frontal cortex in the rhesus monkey. It was demonstrated that the mid-dorsolateral frontal cortex (areas 46, 9/46, and 9) and its medial extension (medial areas 9 and 9/32) is the origin of a specific fiber pathway, running posteriorly as part of the cingulum bundle, and terminating mainly in the retrosplenial area 30 and the posterior presubiculum. This fiber bundle therefore provides the anatomical substrate of a functional interaction between the mid-dorsolateral frontal cortex and the hippocampal memory system for the monitoring of information within working memory. J. Comp. Neurol. 407:183–192, 1999. © 1999 Wiley-Liss, Inc.
... To define the boundaries of the cortical injection sites, area 17 was sectioned at 40 µm on a freezing microtome. The 50-µm thalamic sections and the cortical sections were mounted on gelatin-coated slides and processed according to standard autoradiographic procedures (Cowan et al., 1972). The protocol used to process the thalamic (LPl and IPm) tissue for electron microscopic autoradiography has been described in previous studies (Harting et al., 1991Harting et al., , 1997 Harting, 1992, 1994;). ...
Electron microscopic anterograde autoradiography has been used to analyze the morphology and postsynaptic relationships of area 17 cortical terminals in the lateral division of the lateral posterior nucleus (LPl) of the cat and medial division of the inferior pulvinar nucleus (IPm) of the owl monkey. Such terminals are thought to arise exclusively from layer 5 in the cat and primate (Lund et al. [1975] J. Comp. Neurol. 164:287–304; Abramson and Chalupa [1985] Neuroscience 15:81–95). All labeled terminals in both nuclei exhibited the morphology of ascending “lemniscal” afferents. That is, they contained round vesicles, were large, made asymmetrical synaptic and filamentous nonsynaptic contacts, and were classified as RLs. These cortical RLs also exhibited the postsynaptic relationships of lemniscal afferents. Thus, they were presynaptic to large dendrites within glial encapsulated glomeruli, where a majority was involved in complex synaptic arrangements called triads. They also were found adjacent to terminal profiles with pleomorphic vesicles but never adjacent to small terminals containing round vesicles.Our results suggest that the layer 5 projection from area 17 provides a functional “drive” for some LPl and IPm neurons. Information carried over this “re-entrant” pathway (Guillery [1995] J. Anat. 187:583–592) could be modified within the LPl and IPm by both cortical and subcortical pathways and subsequently conveyed to higher visual cortical areas, where it could be integrated with messages carried through the well-documented corticocortical pathways (Casagrande and Kaas [1994] Cerebral cortex New York: Plenum Press). J. Comp. Neurol. 395:281–295, 1998. © 1998 Wiley-Liss, Inc.
... The autoradiographic method was used to trace the anterograde connections of the posteroventral part of retrosplenial area 30 and posterior cingulate area 23 (Cowan et al., 1972). These cortical areas were injected with tritiated amino acids (leucine and proline; volume 0.4 mL; speci®c activity range, 40±80 mCi, aqueous solution). ...
Because of the sharp curvature of the retrosplenial region around the splenium of the corpus callosum, standard coronal sections are not appropriate for architectonic analysis of its posteroventral part. In the present study, examination of the posteroventral retrosplenial region of the rhesus monkey in sections that were orthogonal to its axis of curvature (and therefore appropriate for architectonic analysis) has permitted definition of its architecture and precise extent. This analysis demonstrated that areas 29 and 30 of the retrosplenial cortex, as well as adjacent area 23 of the posterior cingulate cortex, extend together as an arch around the splenium of the corpus callosum and maintain their topographical relationship with one another throughout their entire course. Injections of anterograde and retrograde tracers confined to retrosplenial area 30 revealed that this area has reciprocal connections with adjacent areas 23, 19 and PGm, with the mid-dorsolateral part of the prefrontal cortex (areas 9, 9/46 and 46), with multimodal area TPO in the superior temporal sulcus, as well as the posterior parahippocampal cortex, the presubiculum and the entorhinal cortex. There are also bidirectional connections with the lateroposterior thalamic nucleus, as well as the laterodorsal and the anteroventral limbic thalamic nuclei. The connectivity of area 30 suggests that it may play a role in working memory processes subserved by the mid-dorsolateral frontal cortex in interaction with the hippocampal system.
... Tritiated proline/leucine was injected into two more hemispheres to allow for comparison of axoplasmic transport and bulk filling methods. These were also processed by a standard method (Cowan et al., 1972). ...
Thalamocortical projections were studied in adult cats using biotinylated dextran amines, wheat germ agglutinin conjugated to horseradish peroxidase, and autoradiography with tritiated leucine and/or proline. The input from 7 architectonically defined nuclei to 14 auditory cortical fields was characterized qualitatively and quantitatively. The principal results were that 1) every thalamic nucleus projected to more than 1 field (range, 4–14 fields; mean, 7 fields); 2) only the projection from the ventral division to some primary fields (primary auditory cortex and posterior auditory cortex) had a periodic, clustered distribution, whereas the input from other divisions to nonprimary areas was continuous; 3) layers III–V received >85% of the total axonal profiles; 4) in most experiments, five or more layers were labeled; 5) the projections to nonprimary auditory areas had many laterally oriented axons; 6) the heaviest input to layer I in all experiments was usually in its upper half, suggesting a sublaminar arrangement; 7) the largest axonal trunks (up to 6 μm in diameter) arose from the medial division and ended in layer Ia, where they ran laterally for long distances; 8) there were three projection patterns: type 1 had its peak in layers III–IV with little input to layer I, and it arose from the ventral division and the dorsal superficial, dorsal, and suprageniculate nuclei of the dorsal division; type 2 had heavy labeling in layer I and less in layers III–IV, arising from the dorsal division nuclei primarily, especially the caudal dorsal and deep dorsal nuclei; and type 3 was a trimodal concentration in layers I, III–IV, and VI that originated chiefly in the medial division and had the lowest density of labeling; and 9) the quantitative profiles with the three methods were very similar. The results suggest that the subdivisions of the auditory thalamus have consistent patterns of laminar distribution to different cortical areas, that an average of five or more layers receive significant input in a specific area, that a given thalamic nucleus can influence areas as far as 20 mm apart, that the first information to arrive at the cortex may reach layer I by virtue of the giant axons, and that several laminar patterns of auditory thalamocortical projection exist. The view that the auditory thalamus (and perhaps other thalamic nuclei) serves mainly a relay function underestimates its many modes for influencing the cortex on a laminar basis. J. Comp. Neurol. 427:302–331, 2000. © 2000 Wiley-Liss, Inc.
... Because autoradiographic silver grains are present on the surface of a tissue slide, whereas neural structures (such as swellings) are generally within a tissue section, visualizing a structure and the corresponding silver grains required taking images at multiple focal planes. Double labeling of axons/swellings with BDA and 3 H-leucine, as shown inFigure 4, provides evidence that the tracers were anterogradely transported, because 3 H-leucine is specifically transported anterogradely (Cowan et al., 1972), whereas BDA is transported both antero-and retrogradely. These images demonstrate that the colors (brown and black) of the two reaction products (CTB and BDA) are clearly distinguishable. ...
The marginal shell of the anteroventral cochlear nucleus is anatomically and physiologically different from its central core. Previous studies suggest that neurons in the marginal shell are well suited to encode the intensity of acoustic stimuli. To investigate the projections of the marginal shell, a focal injection (<100 nl) of a mixture of biotinylated dextran amine (BDA) and 3H-leucine was made into the marginal shell of the cat combined with injection of cholera toxin subunit-B (CTB) into the cochleas. Following a 7-day survival, the cats were perfused. Axons and swellings labeled with BDA and olivocochlear neurons labeled with CTB were immunocytochemically stained black and brown, respectively. 3H-leucine labels were visualized by autoradiography. Labeled neural structures were examined via light microscopy. We found that swellings labeled with BDA, sometimes doubly labeled with BDA and 3H-leucine, were in close apposition with dendrites and/or somata of olivocochlear neurons identified with CTB labeling. Double labeling with BDA and 3H-leucine signifies that the label was anterogradely transported. The results support the conclusion that the anteroventral cochlear nucleus projects to medial olivocochlear neurons bilaterally and to lateral olivocochlear neurons ipsilaterally. Furthermore, the results are consistent with the interpretation that the marginal shell provides a source of the above-mentioned projections. Together with information in the literature, the present anatomical results support a hypothesis that the marginal shell provides information about stimulus intensity as a part of a reflex (or feedback gain control) system comprising the cochlea, cochlear neurons, cochlear nucleus, medial olivocochlear neurons, and cochlear outer hair cells. J. Comp. Neurol. 420:127–138, 2000. © 2000 Wiley-Liss, Inc.
The organization of projections from the macaque monkey hippocampus, subiculum, presubiculum and parasubiculum to the entorhinal cortex was analyzed using anterograde and retrograde tracing techniques. Projections exclusively originate in the CA1 field of the hippocampus and in the subiculum, presubiculum and parasubiculum. The CA1 and subicular projections terminate most densely in layers V and VI of the entorhinal cortex, with sparser innervation of the deep portion of layer III and layer I. Entorhinal projections from CA1 and the subiculum are topographically organized such that a rostrocaudal axis of origin is related to a medial‐to‐lateral axis of termination. A proximodistal axis of origin in CA1 and distoproximal axis in subiculum are related to a rostrocaudal axis of termination in the entorhinal cortex. The presubiculum sends a dense, bilateral projection to caudal parts of the entorhinal cortex. This projection terminates most densely in layer III with sparser termination in layers I, II and V. The same parts of entorhinal cortex receive a dense projection from the parasubiculum. This projection terminates in layers III and II. Both presubicular and parasubicular projections demonstrate the same longitudinal topographic organization as the projections from CA1 and the subiculum. These studies demonstrate that: 1) hippocampal and subicular inputs to the entorhinal cortex in the monkey are organized similar to those described in non‐primate species; 2) the topographic organization of the projections from the hippocampus and subicular areas matches that of the reciprocal projections from the entorhinal cortex to the hippocampus and the subicular areas.
Certain organizational features of brain networks present in the individual are lost when central tendencies are examined in the group. Here we investigated the detailed network organization of four individuals each scanned 24 times using MRI. We discovered that the distributed network known as the default network is comprised of two separate networks possessing adjacent regions in eight or more cortical zones. A distinction between the networks is that one is coupled to the hippocampal formation while the other is not. Further exploration revealed that these two networks were juxtaposed with additional networks that themselves fractionate group-defined networks. The collective networks display a repeating spatial progression in multiple cortical zones, suggesting that they are embedded within a broad macroscale gradient. Regions contributing to the newly defined networks are spatially variable across individuals and adjacent to distinct networks, raising issues for network estimation in group-averaged data and applied endeavors, including targeted neuromodulation.
Arbeiten mit drei und mehr Autoren wurden im Text mit „Nomen et al.“ und Jahreszahl zitiert. Da der zweite Autor in den Textzitaten nicht genannt ist, sind diese „et al.-Zitate“ auch nachstehend nicht alphabetisch nach dem Zweitautor, sondern chronologisch geordnet. Hat der erste Autor (nachstehend als „Nomen A“ bezeichnet) auch allein publiziert und/oder mit nur einem weiteren Autor, dann sind diese Literaturangaben wie folgt geordnet:
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Nomen A (chronologisch geordnet).
2.
Nomen A und Nomen B (alphabetisch nach Nomen B geordnet).
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Nomen A et al. (chronologisch geordnet).
In recent years, careful utilization of experimental and clinical data has permitted new insights into the basic organization of mammalian forebrain. The intent of the present chapter is to examine one aspect of this inquiry: the structure of auditory forebrain and its role in mammalian hearing. To achieve this goal, the discussion is built around three issues presented in three separate sections.
Most of the sensory systems of vertebrates have an evolutionary history that stretches back to before the origin of the vertebrates themselves. If this long history were available for close inspection, a number of fundamental questions about the physiology of the senses and their primary behavioral contributions could be quickly answered and these answers could be expected to bring with them a kind of insight into sensory system function not possible to gain by experiment alone. Even as incompletely known as it now is, the evolutionary history of the sensory systems remains a source of new and relatively independent ideas about structure-function relationships that serve to augment the range of plausible hypotheses fueling direct physiological and behavioral experimentation (Tucker and Smith, 1976; Wever, 1976; Stebbins, 1970; Glickstein, 1976; Berkley, 1976). It is for this reason that the conclusions of the comparative and paleontological sciences are of particular value to those interested in the neural mechanisms of sensory integration.
The second somatic sensory area (S II) was originally described on the basis of evoked potential mapping studies (1, 64, 65). This type of physiological study and the more detailed microelectrode mapping studies that followed have provided important data concerning the functional organization of this region of the cerebral cortex. Unfortunately these physiological data have not been correlated with connectional or cytoarchitectonic data. The need for such correlative information is suggested by studies such as that of Carreras and Anderson (7), whose division of the anterior ectosylvian gyrus in the cat into functional areas on the basis of the vascular pattern of the cortex is still widely quoted.
The identification of multiple cortical areas involved in different aspects of somatic sensory and motor function has naturally led to interest in the relations between these areas. At the level of the cortex itself, connections between the different areas are formed by extensive ipsilateral intracortical pathways, which allow interactions between different areas, directly or via less direct routes through the basal ganglia, thalamus and cerebellum.
Our understanding of the detailed organization of the brain and other nervous tissue has been largely dependent on the development of technologies within the past 150 years. This article presents the most influential neuroanatomical procedures commonly used now, including classical histochemical techniques, modern molecular techniques, confocal microscopy, quantitative techniques, intracellular labeling and three-dimensional reconstruction techniques, neuroanatomical tract-tracing techniques, electron microscopy, and live imaging techniques. Interestingly, despite the great advances in modern in vivo imaging techniques and informatics, some neuroanatomical procedures developed more than 100 years ago are still central to current investigations of the structural organization of nervous systems.
The description of an anatomical specimen may look straightforward, but it appears that it depends in fact of several intermingled factors: technical methods for conservation, dissection and vascular injection and the anatomist skills are of course important. This is especially true when the studied organ, as for instance the brain, is subject to rapid putrefaction after death without any preservation technique. Nevertheless the possibility to reject, or at least criticize, the dominant paradigm is probably as important as these technical considerations: important changes occurred in brain representation between the early Middle Ages and the Early Modern Times, without major improvements of cadaveric preservation or dissection methods; Vesalius rejected the existence of the rete mirabile in human not only because he was a talented anatomist but also because he accepted and had the courage to fight the dominant tradition inherited from Galen. Such difficulties in the scientific approach obviously remain vivid, and shouldn't be forgotten despite the development of modern tools for studying brain morphology and function.
The nervous system is complex not simply because of the enormous number of neurons it contains but by virtue of the specificity with which they are connected. Unraveling this specificity is the task of neuroanatomy. In this endeavor, neuroanatomists have traditionally exploited an impressive array of tools ranging from the Golgi method to electron microscopy. An ideal method for studying anatomy would label neurons that are interconnected, and, in addition, allow expression of foreign genes in these neurons. Fortuitously, nature has already partially developed such a method in the form of neurotropic viruses, which have evolved to deliver their genetic material between synaptically connected neurons while largely eluding glia and the immune system. While these characteristics make some of these viruses a threat to human health, simple modifications allow them to be used in controlled experimental settings, thus enabling neuroanatomists to trace multi-synaptic connections within and across brain regions. Wild-type neurotropic viruses, such as rabies and alpha-herpes virus, have already contributed greatly to our understanding of brain connectivity, and modern molecular techniques have enabled the construction of recombinant forms of these and other viruses. These newly engineered reagents are particularly useful, as they can target genetically defined populations of neurons, spread only one synapse to either inputs or outputs, and carry instructions by which the targeted neurons can be made to express exogenous proteins, such as calcium sensors or light-sensitive ion channels, that can be used to study neuronal function. In this review, we address these uniquely powerful features of the viruses already in the neuroanatomist's toolbox, as well as the aspects of their biology that currently limit their utility. Based on the latter, we consider strategies for improving viral tracing methods by reducing toxicity, improving control of transsynaptic spread, and extending t
The connections of the cortical dysgranular “unresponsive zone” (UZ) (Sur et al.: J. Comp. Neurol. 179:425–450, '78) in the grey squirrel were studied with horseradish peroxidase and autoradiographic techniques. The results of these experiments show that the major subcortical connections of the unresponsive zone are in large part reciprocal. Connections are distributed within the thalamus in a poorly defined region including restricted portions of several nuclei that lie along the rostral, dorsal, and caudal borders of the ventral posterior nucleus. Additional thalamic connections of the UZ terminate in the reticular nucleus and are reciprocally related to the paralaminar and central median nuclei. Extrathalamic terminations were observed in the zona incerta, the intermediate and deep layers of the superior colliculus, the red nucleus, and several subdivisions of the pontine nuclei. The similarity between the pattern of subcortical connections of the UZ in the grey squirrel and patterns reported for the parietal septal region in rats (Chapin and Lin: J. Comp. Neurol. 229:199–213, '84) and for area 3a in primates (Friedman and Jones: J. Neurophysiol. 45:59–85, '81), suggests that the UZ in the grey squirrel may represent a counterpart of at least part of area 3a as described in primates. The results are further discussed with respect to a possible role of the thalamus in control or modulation of interhemispheric circuits and of the UZ in the modulation of nociceptive and kinesthetic pathways through the thalamus. Finally, the term parietal dysgranular cortex (PDC) is proposed as an alternative to denote the region currently called the unresponsive zone.
Despite their invariable coexistence in the mammalian brain, limbic system (hippocampus and amygdala) and corpus striatum (striatum or caudatoputamen, and pallidum or globus pallidus) have long made the impression of being two mutually isolated neural mechanisms. Until about twenty-five years ago, these two major components of the forebrain seemed to lack any direct interconnection; for an even longer time they appeared to have no common sources of afferent supply, and their respective efferent fiber pathways until very recently, seemed to have no points of convergence anywhere along their course. To be more specific: until about twenty years ago, known or suspected neocortical afferents to the limbic system were limited to the cingulo-hippocampal connection suggested by Cajal (1911) and later by Papez (1937), whereas cortical afferents to the corpus striatum were generally believed to originate largely or even entirely from the sensorimotor cortex; neither were any other sources of afferents known to be shared by limbic system and corpus striatum. As to the efferent connections of these two forebrain mechanisms: those of the corpus striatum until only a few years ago were thought to be distributed exclusively to the substantia nigra, subthalamic nucleus, centrum medianum and VA-VL complex of the thalamus, and to certain mesencephalic regions (see Nauta and Mehler, 1966, for a review). In none of these distributions did the projections of the corpus striatum seem to overlap the efferents of the limbic system. The latter, instead, have been traced to the anterior and mediodorsal nuclei of the thalamus, as well as to the subcortical continuum formed by the septum, preoptic region and hypothalamus, and extend caudally beyond the hypothalamus over the ventral tegmental area throughout the paramedian region of the midbrain, partly by way of the medial forebrain bundle, partly also by a more dorsal route composed of the stria medullaris, habenular nuclei, and fasciculus retroflexus. It is important to note that a substantial second component of the medial forebrain bundle deviates laterally from the main bundle and distributes itself largely to more lateral regions of the midbrain tegmentum (see below).
Summary Descending projections from the mesencephalon and superior colliculus to the inferior olive were analyzed by an autoradiographic tracing method. Injections of tritium-labelled leucine were placed in regions which had previously been identified as sources of afferents to the olive. These were located adjacent to the central gray and extended from the rostral red nucleus to the posterior thalamus. Additional injections were made in the superior colliculus. Other injections were placed in the basal ganglia and thalamus. Injections restricted to one side of the central mesencephalon resulted in predominantly ipsilateral labelling of the olive. After injections in the caudo-medial parafascicular and subparafascicular nuclei and rostral nucleus of Darkschewitsch, deposits of grains were observed in the rostral pole of the medial accessory olive and adjacent ventral lamella of the principal olive. The medial accessory olive contained grains into its middle third. More caudal injections which involved the interstitial nucleus of Cajal as well as the nucleus of Darkschewitsch and rostral red nucleus resulted in the dense labelling of the entire principal olive (except the dorsal cap), the entire medial acessory olive (except subnucleus ß and the caudo-medial pole) and the caudo-dorsal accessory olive. Injections centered in the caudal magnocellular red nucleus and extending into the rostral parvocellular division labelled the dorsal lamella of the principal olive almost exclusively. When only the caudal part of the red nucleus was involved in the injection, the olive was entirely clear of grains. Minor contralateral distributions were observed in the dorsomedial cell column, the medial tip of the dorsal lamella and in the caudal medial accessory olive. The deep layers of the superior colliculus were found to project strongly to the contralateral medial accessory olive immediately beside subnucleus ß and weakly to the same area ipsilaterally.
Deoxyglucose labelled with 14C is utilized for the determination of regional glucose metabolism in the brain. In this paper, procedures are described in which 3H replaces the 14C marker for film autoradiography. A significant improvement of resolution is obtained as a result of the use of tritium and of various precautions to avoid diffusion of the soluble deoxyglucose. In addition, a method using a fine-grain emulsion (NTB-2) is presented. In this case, definition of labelled sites at a cytological level is obtained. Precise quantification is also possible through grain counting. © 1980 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.
Cross-sectional areas of somata in the medial interlaminar nucleus (MIN) and lamina A of the cat were measured at different ages to determine neuronal growth characteristics. Normal and monocularly lid-sutured kittens of ages 2 - 16 weeks plus several adults were studied. During development of normal kittens, we observed that MIN cells grew until about 8 weeks of age and were larger, on average, than cells in lamina A at all ages. The growth pattern was similar for cells in both MIN and lamina A. In mono- cularly deprived kittens, a deprived/nondepr ived cell size difference began to appear between 3 and 5 weeks of age and con- tinued to increase until about 8 weeks of age. We conclude that monocular lid-suturing affects the size of MIN neurons much as it does neurons in lamina A, and the effect is concurrent with the period of normal growth.
Behavioral and functional studies in humans suggest that attention plays a key role in activating the primary olfactory cortex through an unknown circuit mechanism. We report that a novel pathway from the anterior cingulate cortex, an area which has a key role in attention, projects directly to the primary olfactory cortex in rhesus monkeys, innervating mostly the anterior olfactory nucleus. Axons from the anterior cingulate cortex formed synapses mostly with spines of putative excitatory pyramidal neurons and with a small proportion of a neurochemical class of inhibitory neurons that are thought to have disinhibitory effect on excitatory neurons. This novel pathway from the anterior cingulate is poised to exert a powerful excitatory effect on the anterior olfactory nucleus, which is a critical hub for odorant processing via extensive bilateral connections with primary olfactory cortices and the olfactory bulb. Acting on the anterior olfactory nucleus, the anterior cingulate may activate the entire primary olfactory cortex to mediate the process of rapid attention to olfactory stimuli.
One of the main characteristics of brains is their profuse connectivity at different spatial scales. Understanding brain function evidently first requires a comprehensive description of neuronal anatomical connections. Not surprisingly a large number of histological markers were developed over the years that can be used for tracing mono- or polysynaptic connections. Biocytin is a classical neuroanatomical tracer commonly used to map brain connectivity. However, the endogenous degradation of the molecule by the action of biotinidase enzymes precludes its applicability in long-term experiments and limits the quality and completeness of the rendered connections. With the aim to improve the stability of this classical tracer, two novel biocytin-derived compounds were designed and synthesized. Here we present their greatly improved stability in biological tissue along with retained capacity to function as neuronal tracers. The experiments, 24 and 96 h postinjection, demonstrated that the newly synthesized molecules yielded more detailed and complete information about brain networks than that obtained with conventional biocytin. Preliminary results suggest that the reported molecular designs can be further diversified for use as multimodal tracers in combined MRI and optical or electron microscopy experiments.
The significance of axonal branches in maintaining the integrity of the neuronal soma after axon section has been studied in the lateral mammillary nucleus (LMN) of the cat. Earlier studies with the Golgi and reduced silver methods have shown that the axons of most, if not all, of the cells of this nucleus which enter the principal mammillary tract (PMT) bifurcate with one branch entering the mammillo-thalamic tract (MThT) and the other the mammillo-tegmental tract (MTgT). Some of the fibers in the MThT may again bifurcate before ending bilaterally in the anterodorsal nucleus of the thalamus (AD).
Unilateral destruction of the AD results in no detectable retrograde cell degeneration in the LMN. Lesions of the MThT (below the interanteromedial nucleus) cause a slight (∼ 15%) cell loss in the LMN, while isolated lesions of the MTgT have no detectable effect upon the cells of the nucleus. Combined lesions of the MThT and the MTgT cause severe retrograde degeneration in the LMN: approximately 60% of the cells disappear and many of the surviving neurons are frankly atrophic. Lesions of the PMT involving the parent axons of the cells in the LMN result in about the same degree of cell loss as combined lesions of the MThT and MTgT, but the persisting neurons are more severely shrunken and pyknotic.
The implications of these findings for the organization of the connections of the LMN and for the interpretation of retrograde cell degeneration are discussed.
The distribution of neurons in the basal telencephalon, the diencephalon, and the brainstem that project to the hippocampal formation has been analyzed in mature cynomolgus monkeys (Macaca fascicularis) by the injection of horseradish peroxidase into different rostro-caudal levels of the hippocampal formation. After injections which involve Ammon's horn, the dentate gyrus, and the subicular complex, retrogradely labeled neurons are found in the following regions: in the amygdala (specifically in the anterior amygdaloid area, the basolateral nucleus, and the periamygdaloid cortex); in the medial septal nucleus and the nucleus of the diagonal band; in the ventral part of the claustrum; in the substantia innominata and the basal nucleus of Meynert; in the rostral thalamus (specifically in the anterior nuclear complex, the laterodorsal nucleus, the paraventricular and parataenial nuclei, the nucleus reuniens, and the nucleus centralis medialis); in the lateral preoptic and lateral hypothalamic areas, and especially in the supramammillary and retromammillary regions; in the ventral tegmental area, the tegmental reticular fields, the raphé nuclei (specifically in nucleus centralis superior and the dorsal raphé nucleus), in the nucleus reticularis tegmenti pontis, the central gray, the dorsal tegmental nucleus, and in the locus coeruleus.
The projections to the basis pontis from cytoarchitectonically defined subregions of the superior (SPL) and inferior (IPL) parietal lobules were investigated in 14 rhesus monkeys by using the anterograde tracing techniques of autoradiography and horseradish peroxidase histochemistry. The results of our study confirm and complement available information regarding the parietopontine projections. The projections are found in clusters distributed in lamellae approximately concentric to the peduncle. They are directed most heavily towards the peripeduncular and lateral nuclei of the pons. There are also lesser, but nevertheless substantial projections to other nuclei including the intrapeduncular, ventral, dorsolateral, extreme dorsolateral, and dorsal nuclei. The dorsomedial, paramedian, and NRTP nuclei receive only minor projections. The SPL projections are relatively widespread with respect to the more focussed IPL projections. The IPL projections are, in general, situated more laterally and at more rostral levels of the pontine nuclei than are those of the SPL.
The sulcal cortex of the SPL (area PEa) favors the dorsolateral, extreme dorsolateral, and ventral nuclei compared to the light projections to these nuclei from the convexity of the SPL. The sulcal cortex of the IPL, area POa, differs from the gyral cortex in favoring the ventral and extreme dorsolateral nuclei. The rostral IPL differs from the caudal IPL in that the intrapeduncular nucleus receives projections only from rostral regions, while the lateral nucleus receives projections preferentially from caudal regions. The pontine projections from the medial SPL, area PGm, are unique in the parietal lobe in that they include the paramedian nucleus. Projections arising from multimodal regions located caudally in the SPL (areas PEa and PGm) and IPL (areas PG and Opt) are more strongly represented and more laterally placed within the pontine nuclei than projections arising from more rostral, unimodal, posterior parietal regions.
The heavy projections to the pontine nuclei from the posterior parietal cortex, and particularly from those caudal parietal regions that have prominent associative and limbic connections, seem to suggest that the corticopontocerebellar pathways permit a cerebellar contribution not only to the coordination of movement, but also to the modulation and integration of higher function.
The medial interlaminar nucleus (MIN) of the cat lies medial to the laminated region of the dorsal lateral geniculate (lamLGN). This latter region includes the A and C laminae. As does lamLGN, MIN receives direct retinal input and projects to various visual cortical areas. We examined the MIN of 15 normal adult cats with electrophysiological and anatomical techniques.
Autoradiographs processed from cats that had one eye injected with tritiated fucose and proline indicate that MIN is composed of at least two laminae, one for each eye. The area which receives input from the ipsilateral eye is a small central region surrounded dorsally, medially, and ventrally by a larger crescent shaped region that receives input from the contralateral eye. This pattern was also evident from electrophysiological recording experiments.
Extracellular recordings from 102 single-units in MIN indicate that these cells have properties essentially identical to lamLGN Y-cells. That is, they had short latencies to orthodromic stimulation of the optic chiasm and antidromic stimulation of the visual cortices, responded in a phasic manner to the presentation of a standing contrast within the receptive field center, responded to rapidly moving visual stimuli, and showed non-linear spatial summation properties typical of lamLGN Y-cells. We discovered two difference between MIN cells and lamLGN Y-cells. First the mean receptive field center size of MIN cells is considerably larger than that of lamLGN Y-cells, and second, MIN cells do not have the non-dominant eye inhibitory receptive fields found for many lamLGN Y-cells.
Cell size measurements indicate that while the mean cell size in MIN is approximately 30% greater than in the A laminae of lamLGN, the distribution of MIN cell sizes extends over the full range of cell sizes in the A laminae. Since the A laminae are comprised mostly of X- and Y-cells, this suggests that, although Y-cells on average are larger than X-cells, considerable overlap exists in their size distribution. No differences between the ipsilateral and contra lateral terminal zones were found on any measure.
Since MIN cells share most or all the fundamental features of lamLGN Y-cells, we suggest that these cell groups should be considered subpopulations of a more general group of geniculate Y-cells. Accordingly, we refer to these two subpopulations as lamLGN Y-cells and MIN Y-cells.
In this study, tritiated-uridine incorporation was autoradiographically examined following axotomy of hamster facial motor neurons (HFMN) at the critical development age of 15 days postnatal and in the adult. The postoperative times selected were 0.5, 1, 2, and 4 days. In the 15-day operative series, no changes in incorporation were observed at any of the post-operative times, except at 4 days postoperative, when there was a decrease in tritiated-uridine incorporation in the axotomized neurons relative to the controls. In the adult operative series there were no changes in incorporation at 0.5 or 1 day postoperative, relative to the controls. At 2 days postoperative in the adult, there was a transient increase in tritiated-uridine incorporation that returned to control levels by 4 days postoperative. When axotomized and control cytoplasmic/nuclear grain densities were compared, no changes were found in either operative series. These results of the time course of axotomy-induced changes in RNA synthesis in HFMN corroborate our previous findings of an age-dependent reactive sequence in HFMN and lend support to the hypothesis that the young neurons are synthesizing at peak capacity related to final growth and cannot be stimulated further by axotomy. As discussed, the transient increase in RNA levels in the adult, the lack of any changes in the rate of transfer of RNA from the nuclcus to the cytoplasm, and the decrease in RNA levels in the 15-day neurons may be related to the presence of an unusual intranucleolar body within the nucleolus of HFMN that contains ribosomal precursors.
1. The distribution of labeled macromolecules was studied within the dorsolateral thalamic nuclei of the pigeon after unilateral intraocular injection of either 3H-proline, 3H-leucine or 3H-fucose. The highest densities of grains were found in nucleus dorsolateralis anterior, pars lateralis at its dorsolateral aspect (DLLd) and in nucleus lateralis anterior (LA) whereas moderate labeling was observed in the ventral aspect of DLL (DLLv) and in nucleus dorsolateralis anterior, pars magnocellularis (DLAmc). No significant label was found on the ipsilateral side. 2. After circumscribed unilateral ablation of the visual wulst, the cells in DLLv were most severely affected by retrograde degeneration, whereas DLLd, DLAmc and LA remained intact. Bilateral ablation of the wulst or combination of wulst damage with section of the supraoptic decussation gave rise to additional degeneration in DLLd, thus suggesting a contralateral retinotelencephalic pathway via DLLv and an ipsilateral retino-telencephalic pathway via DLLd and recrossing via DSO. LA and DLAmc represent relays in retinothalamo-telencephalic pathways of unknown destination. 3. DLLv was confirmed as relay in the contralateral retino-thalamo-hyperstriatal pathway in a combined series of experiments, where the autoradiographic and the retrograde degeneration techniques were applied in the same animal.
To compare the distributions of normal and regenerated optic axons in the goldfish tectum, small groups of axons crossing the rostromedial tectum were cut and filled with horseradish peroxidase which subsequently revealed the retinal locations of their somata.
In normal fish, the peroxidase-filled ganglion cells were virtually confined to a narrow arc spanning the ventronasal quadrant of the retina. In fish with regenerated visual projections (50–736 days after optic nerve transection, optic nerve crush or deflection of optic axons to the ipsilateral tectum) the filled cells were distributed across the full extent of the retina from centre to periphery and were less rigidly confined within appropriate quadrants. The absence of any detectable arc of filled cells in the ventronasal quadrant after regeneration showed that few, if any, of the regenerated axons followed their original paths across the tectum. Quantitative analysis of local cell distributions indicated that axons were re-routed independently rather than in groups. Nevertheless, axons consistently displayed a crude bias towards appropriate tectal regions, even in ipsilateral tecta where the relative positions of these regions are inverted.
These results imply that regenerating optic axons are widely scattered by the effects of surgery. They may subsequently show preferences for appropriate central paths but with a resolution too low to define much more than the orientation of the retino-tectal map. Since there is both anatomical and electrophysiological evidence that regenerated optic terminal arborizations eventually adopt a precise retinotopic arrangement, this arrangement must chiefly reflect ordering mechanisms which act in the final stages of axon growth or synapsis.
Fluorescent somatopetal tracers were used to infiltrate, by diffusion rather than injections, the dorsolateral cortex of one hemisphere in rats. In different animals the tracers penetrated into the cortex to different depths. We found several interesting features of the commissural system: first, there were no areas without commissural neurons. At least a few labelled cell bodies were present in a single-cell layer also in acallosal cortical areas. Secondly, there is a considerable variety of laminar distribution patterns of labelled perikarya in different areas. Thirdly, some cortical fields, which cytoarchitecturally appear uniform, can be subdivided according to different distributions of cell bodies with commissural projections. Fourthly, when only supragranular layers were infiltrated, labelled cell bodies were present mainly in the supragranular layers of the contralateral cortex. Infiltration of the first layer alone did not label any neurons in the contralateral cortex but did label neurons in layer VIb ipsilaterally. In the subcortex, the labelled perikarya were found in the structures already known to project directly to the cortex. In rats with the tracer restricted mainly to the supragranular layers, a conspicuously reduced labelling was found in the basal forebrain and the thalamus. In the thalami of those animals, labelled neurons were found only in paralamellar nuclei. The high sensitivity of the tracer used, together with infiltration of the entire dorsolateral cortex, allows us to conclude that probably all sources of innervation of the isocortex in rats have been seen.
Injections of 3H-leucine were made into the region of the central cervical nucleus (CCN) in the C1–C4 segments of the spinal cord in 19 adult cats. In the cerebellum labelled mossy fibre terminals were found bilaterally concentrated in lobule I and adjoining parts of lobule II of the anterior lobe. In addition, a fair number of terminals were found in the basal parts of lobules III–VIII. The terminals were mainly found in the vermal zone. Only a minor proportion was observed in the intermediate zone of the anterior lobe and only occasionally were terminals seen in the hemispheral parts. No labelled climbing fibres were observed. Axons probably derived from the CCN were found to ascend in the brain stem, mostly contralateral to the injection site. They passed close to the vestibular nuclear complex and some fibres appeared to terminate in the nucleus x of Brodal and Pompeiano. The majority of axons entered the cerebellum via the superior cerebellar peduncle, a few via the restiform body. The findings suggest that lobules I–II are important recipients of information from the neck.
The retinal projections were studied in the black piranah (Serrasalmus niger) with degeneration and autoradiographic methods. The projections are bilateral to the hypothalamic optic nucleus, the dorsomedial optic nucleus, corpus geniculatum ipsum of Meader (1934) and the optic tectum. Unilateral, crossed projections were traced to the pretectal nucleus and the cortical nucleus. The visual system of the black piranah is exceptionally well developed but has retained many primitive features including the extensive bilateral projections.
Injection of radioactive leucine in various regions of the brain stem reticular formation has revealed the presence of ample crossed reticulo-reticular connections in the cat. The terminal area for the crossed fibers are almost mirror images of the injected sites. The findings made is another example that hitherto unknown fiber connections can be demonstrated by axoplasmic protein tracing.
Retinal projections were studied in the tree shrew, Tupaia glis, by means of thaw-mount autoradiography. In this technique, unfixed and unembedded frozen sections are directly mounted on photographic emulsion coated slides. Loss of radiolabeled material through tissue processing is avoided, probably resulting in increased discriminatory sensitivity. Together with multiple injections of precursor cocktail it is possible to demonstrate at the light microscopic level (1) fibers in passage and axon terminals simultaneously, and (2) preferentially labeled axon terminals in the projection field as areas of greater grain density.
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