Synaptic input to cochlear nucleus dendrites that receive medial olivocochlear synapses

Axons of olivocochlear neurons originate in the superior olivary complex and project to the cochlea. Along their course, medial olivocochlear axons give off branches to the cochlear nucleus. We labeled these branches with horseradish peroxidase and used electron microscopy to determine their target dendrites. Target dendrites were of two classes: “large” dendrites and “varicose” dendrites. Using serial sections, we reconstructed the dendrites and, in addition to the labeled olivocochlear input, we determined the synaptic profile of unlabeled inputs onto the dendrites. We classified the terminals on the basis of the shape and size of their synaptic vesicles. On large dendrites, the predominant type of unlabeled terminal had small round (SmRnd) vesicles. These terminals are likely to be excitatory, and some of them may originate from unlabeled medial olivocochlear branches. On varicose dendrites, the predominant type of terminal had pleomorphic vesicles. These terminals are likely to be inhibitory. They may be from descending inputs that arise in higher centers. A final type of terminal onto large dendrites exhibited signs of neuronal degeneration, possibly because the cell body of origin was damaged during the injection procedure. These terminals often had long, perforated synaptic densities and may originate from type II primary afferents. Thus, medial olivocochlear efferents and type II afferents, which both contact outer hair cells in the periphery, appear to synapse onto the same targets in the cochlear nucleus. In contrast, where examined, the target dendrites did not receive terminals with large vesicles from afferents that contact inner hair cells. Thus, target neurons appear to function in a neural circuit associated more closely with outer than with inner hair cells. © 1996 Wiley-Liss, Inc.     

Projections from auditory cortex to the cochlear nucleus in rats: Synapses on granule cell dendrites

Previous work has demonstrated that layer V pyramidal cells of primary auditory cortex project directly to the cochlear nucleus. The postsynaptic targets of these centrifugal projections, however, are not known. For the present study, biotinylated dextran amine, an anterograde tracer, was injected into the auditory cortex of rats, and labeled terminals were examined with light and electron microscopy. Labeled corticobulbar axons and terminals in the cochlear nucleus are found almost exclusively in the granule cell domain, and the terminals appear as boutons (1–2 μm in diameter) or as small mossy fiber endings (2–5 μm in diameter). These cortical endings contain round synaptic vesicles and form asymmetric synapses on hairy dendritic profiles, from which thin (0.1 μm in diameter), nonsynaptic “hairs” protrude deep into the labeled endings. These postsynaptic dendrites, which are typical of granule cells, surround and receive synapses from large, unlabeled mossy fiber endings containing round synaptic vesicles and are also postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles. No labeled fibers were observed synapsing on profiles that did not fit the characteristics of granule cell dendrites. We describe a circuit in the auditory system by which ascending information in the cochlear nucleus can be modified directly by descending cortical influences. © 1996 Wiley-Liss, Inc.     

Cochlear-nucleus branches of thick (medial) olivocochlear fibers in the mouse: a cochleotopic projection

Olivocochlear neurons have somata in the superior olivary complex and provide an efferent innervation to the cochlea. One subgroup of olivocochlear neurons, medial olivocochlear neurons, sends fibers to innervate the cochlear outer hair cells. En route to the cochlea, medial olivocochlear fibers give off branches to the ventral cochlear nucleus, the first auditory center of the brain. This study examines the cochlear-nucleus branches of medial olivocochlear fibers, comparing those from fibers that innervate the cochlear base with those from fibers that innervate the cochlear apex. Basal fibers give off dorsal branches to the granule cell lamina and ventral branches to the auditory nerve root. Apical fibers give off few dorsal branches but many ventral branches that terminate rostrally to the nerve root. This cochleotopic mapping of medial olivocochlear branches corresponds in a general way to that of afferent fibers. Unlike afferent fibers, however, the branches terminate primarily along the edges of the cochlear nucleus. In the mouse, the particular edges of termination are (1) the medial border of the ventral cochlear nucleus where it meets the underlying vestibular nerve root, and (2) the border between the ventral cochlear nucleus and the granule cell lamina. Neurons and dendrites of these border regions may thus integrate efferent and afferent information in a frequency-specific manner.     

Commissural axons of the mouse cochlear nucleus

The axons of commissural neurons that project from one cochlear nucleus to the other were studied after labeling with anterograde tracer. Injections were made into the dorsal subdivision of the cochlear nucleus in order to restrict labeling only to the group of commissural neurons that gave off collaterals to, or were located in, this subdivision. The number of labeled commissural axons in each injection was correlated with the number of labeled radiate multipolar neurons, suggesting radiate neurons as the predominant origin of the axons. The radiate commissural axons are thick and myelinated, and they exit the dorsal acoustic stria of the injected cochlear nucleus to cross the brainstem in the dorsal half, near the crossing position of the olivocochlear bundle. They enter the opposite cochlear nucleus via the dorsal and ventral acoustic stria and at its medial border. Reconstructions of single axons demonstrate that terminations are mostly in the core and typically within a single subdivision of the cochlear nucleus. Extents of termination range from narrow to broad along both the dorsoventral (i.e., tonotopic) and the rostrocaudal dimensions. In the electron microscope, labeled swellings form synapses that are symmetric (in that there is little postsynaptic density), a characteristic of inhibitory synapses. Our labeled axons do not appear to include excitatory commissural axons that end in edge regions of the nucleus. Radiate commissural axons could mediate the broadband inhibition observed in responses to contralateral sound, and they may balance input from the two ears with a quick time course. J. Comp. Neurol. 521:1683–1696, 2013. © 2012 Wiley Periodicals, Inc.     

Mossy fiber projections from the cuneate nucleus to the cochlear nucleus in the rat

A reciprocal connection is known to exist between the cuneate nucleus, which is a first-order somatosensory nucleus, and the cochlear nucleus, which is a first-order auditory nucleus. We continued this line of study by investigating the fiber endings of this projection in the cochlear nucleus of rats using the neuronal tracer Phaseolus vulgaris leucoagglutinin in combination with ultrastructural and immunocytochemical analyses. In the cochlear nucleus, mossy fiber terminals had been described and named for their morphologic similarity to those in the cerebellum, but their origins had not been discovered. In the present study, we determined that the axonal projections from the cuneate region gave rise to mossy fiber terminals in the granule cell regions of the ipsilateral cochlear nucleus. The cuneate mossy fibers appear to be excitatory in nature, because they are filled with round synaptic vesicles, they make asymmetric synapses with postsynaptic targets, and they are labeled with an antibody to glutamate. The postsynaptic targets of the mossy fibers include dendrites of granule cells. This projection onto the granule cell interneuron circuit of the cochlear nucleus indicates that somatosensory cues are intimately involved with information processing at this early stage of the auditory system. © 1996 Wiley-Liss, Inc.     

Brainstem branches from olivocochlear axons in cats and rodents

Horseradish peroxidase was used to label axons of olivocochlear (OC) neurons by intracellular injections in cats and extracellular injections in rodents. These axons arise from cell bodies in the superior olivary complex and project to the cochlea. En route to the cochlea, the thick axons (greater than 0.7 micron diam.) of medial olivocochlear (MOC) neurons formed collaterals that terminated in the ventral cochlear nucleus, the interstitial nucleus of the vestibular nerve (in cats), and the inferior vestibular nucleus (in rodents). The thin axons (less than 0.7 micron diam.), presumed to arise from lateral olivocochlear (LOC) neurons, did not branch near the CN. Within the CN, the MOC collaterals tended to ramify in and near regions with high densities of granule cells, regions also associated with the terminals of type II afferent axons (Brown et al.: J. Comp. Neurol. 278:581-590, '88). These results suggest that those fibers associated peripherally with outer hair cells (MOC efferents and type II afferents) are associated centrally with regions containing granule cells, whereas those fibers associated with inner hair cells peripherally (LOC efferents and type I afferents) are not.     

An electron microscopic observation of the vesicular acetylcholine transporter-immunoreactive fibers in the rat dorsal raphe nucleus

By using immunocytochemistry with an antibody directed against the vesicular acetylcholine transporter, many cholinergic neuronal processes were found to be immunopositive in the dorsal raphe nucleus. At the electron microscopic level, most of these processes were found to be axons. The immunopositive axon terminals made synapses on immunonegative dendrites and their spines whereas rare synapses were found between the immunopositive axon terminals and the immunonegative neuronal perikarya. Occasionally, the dendrites postsynaptic to an immunopositive axon terminal also received a synapse from an immunonegative axon terminal. The synapses made by the immunopositive axon terminals were usually symmetric and had a short active zone. Fewer immunostained dendrites were found, and they usually received asymmetric synapses from nonimmunostained axon terminals. The existence of cholinergic axon terminals and the synapses made by these terminals support the physiological data indicating that acetylcholine plays a role in the pain inhibition system in the dorsal raphe nucleus.     

Differential expression of the metabotropic glutamate receptor mGluR1α by neurons and axons in the cochlear nucleus: In situ hybridization and immunohistochemistry

mGluR1α is a metabotropic glutamate receptor involved in synaptic modifiability. A differential expression in specific neuronal types could reflect their different connections and response properties in central auditory processing. Using in situ hybridization and immunohistochemistry, we studied mGluR1α receptor expression throughout the cochlear nucleus. Robust labeling occurred in the dorsal cochlear nucleus and small cell shell, with less in the ventral cochlear nucleus. Among the most intensely labeled were the granule cells of the small cell shell. In the dorsal cochlear nucleus, most cell types expressed message and receptor protein, except granule cells. High levels of receptor were expressed by corn cells and cartwheel cells. The terminal dendrites and synaptic spines of cartwheel and fusiform cells contained receptor protein in the molecular layer, where they could synapse with parallel fibers. Fusiform dendrites also expressed mRNA for mGluR1α. The basal dendrites of fusiform cells contained receptor protein in the region where they receive cochlear nerve synapses. Immunostaining of terminal axons was prominent in the molecular layer and the small cell shell, where they were associated with synaptic nests, structures thought to provide long-term changes in excitability. Differential expression levels may reflect different functional requirements of specific cell types, including inhibitory interneurons, like corn cells and cartwheel cells, and excitatory interneurons, like granule cells in the small cell shell, which may participate in local circuits involved in modulatory or gating functions, such as stimulus enhancement or suppression. In presynaptic axons, mGluR1α may relate to the long-term signaling requirements of their modulatory functions. Synapse 28:251–270, 1998. © 1998 Wiley-Liss, Inc.     

F3/F11 cell surface molecule expression in the developing mouse cerebellum is polarized at synaptic sites and within granule cells.

The distribution of the F3/F11 neuronal cell surface molecule was investigated in the developing and adult mouse cerebellum by immunocytochemistry at the light and electron microscopic levels. F3/F11 was confined to subsets of neuronal types, since the Purkinje cell body and dendritic arborization as well as the stellate cells were not immunoreactive. In the young developing cerebellum, the granule cell axons strongly express F3/F11 as soon as they begin to grow, consistent with a functional role in promoting directional outgrowth of neuronal processes. In 10-d-old and adult cerebella, the granule cell bodies and dendrites were not immunoreactive whereas the parallel fibers, which are the granule cell axons, were labeled including in their presynaptic varicosities. By contrast, dendrites, cell bodies, and axons of Golgi cells were labeled by anti-F3 antibodies. Hence, F3/F11 can either be expressed throughout the cell or be polarized to the axons. This raises the question of how segregation of the glypiated F3/F11 molecule between different subcellular compartments depending on the type of neuron is achieved. F3/F11 was found to be present at three types of synaptic sites, suggesting that it might play a role in the formation and maintenance of synapses. However, in each type of synpase, F3/F11 was present at only the pre- or postsynaptic site, never at both: the parallel fiber varicosities contained F3/F11 whereas the postsynaptic compartment in contact, that is, the Purkinje cell dendritic spines, did not. The granule cell dendrites were unlabeled while the mossy fiber terminals contacting them were immunoreactive, and finally, the Golgi cell dendrites and dendritic spines were labeled while the presynaptic compartment contacting them was not. If F3/F11 functions as an adhesion molecule in vivo as indicated by in vitro assays, F3/F11-mediated adhesion is likely to be heterophilic.     

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.     

Structure of the mitral cell in the olfactory bulb of the goldfish (Carassius auratus)

The mitral cell in the olfactory bulb of the goldfish was examined by means of light microscopy, high-voltage electron microscopy, and conventional electron microscopy. Mitral cells are located rather diffusely throughout the glomerular and plexiform layers. They do not make their own discrete layer. The cell bodies are rounded or triangular, and are about 10–25 μm in diameter. In Golgi-impregnated material, thick cylindrical dendrites can be seen arising from the cell bodies and branching in the glomerular layer. Dendritic branches of some cells make two or more rather compact tufts, while the dendrites of other cells intermingle loosely with one another. In semithin and thin sections, darkly stained nodules appear to be scattered diffusely in the glomerular layer without clustering into discrete spheres, which are characteristic of the mammalian glomerulus. Hence, instead of the glomerulus, the “glomerular area” is defined as an area consisting of darkly stained nodules with rather pale granular regions surrounding them. Branches of mitral cell dendrites in the glomerular area consist of cylindrical shafts and irregular appendages arising from them. The shafts appear in the pale granular region and the appendages are found in the darkly stained nodules. Synapses can be found on all parts of the mitral cell: the soma, axon hillock, axon initial segment, thick dendritic stems, and dendritic branches. The abundance of synapses seems to vary considerably from part to part, and is highest on the dendritic branches in the glomerular area. The mitral cell is postsynaptic to olfactory nerve terminals and granule cell dendrites, and presynaptic to granule cell dendrites and some processes of unknown origin. Olfactory nerve terminals make asymmetrical synapses specifically on the appendages of the dendritic branches. Of the synapses on the shafts of the mitral cell dendritic branches in the glomerular area, 90% are with granule cell dendrites. Of the synapses between two different kinds of processes 30% are mitral-to-granule asymmetrical synapses, 20% are granule-to-mitral symmetrical synapses, and 50% are reciprocal pairs. Gap junctions and mixed synapses are also seen on branches of mitral cell dendrites. Features of the goldfish mitral cell are compared with those of the mammal. The differences in neuronal organization between the olfactory bulbs of teleosts and mammals are discussed.     

Subcellular compartmentalization of a potassium channel (Kv1.4): preferential distribution in dendrites and dendritic spines of neurons in the dorsal cochlear nucleus

Voltage-dependent ion channels have specific patterns of distribution along the neuronal plasma membrane of dendrites, cell bodies and axons, which need to be unravelled in order to understand their contribution to neuronal excitability and firing patterns. We have investigated the subcellular compartmentalization of Kv1.4, a transient, fast-inactivating potassium channel, in fusiform cells and related interneurons of the rat dorsal cochlear nucleus. A polyclonal antibody which binds to a region near the N-terminus domain of a Kv1.4 channel was raised in rabbits. Using a high-resolution combination of immunocytochemical methods, Kv1.4 was localized mainly in the apical dendritic trunks and cell bodies of fusiform cells, as well as in dendrites and cell bodies of interneurons of the dorsal cochlear nucleus, likely cartwheel cells. Quantitative immunogold immunocytochemistry revealed a pronounced distal to proximal gradient in the dendrosomatic distribution of Kv1. 4. In plasma membrane localizations, Kv1.4 was preferentially present in dendritic spines, either in the spine neck or in perisynaptic locations, always away from the postsynaptic density. These findings indicate that Kv1.4 is largely distributed in dendritic compartments of fusiform and cartwheel cells of the dorsal cochlear nucleus. Its preferential localization in dendritic spines, where granule cell axons make powerful excitatory synapses, suggests a role for this voltage-dependent ion channel in the regulation of dendritic excitability and excitatory inputs.     

The fine structure of the lateral superior olivary nucleus of the cat

The synaptic organization of the lateral superior olivary nucleus of the cat was analyzed under the electron microscope. The predominant cell type, the fusiform cell, has dendrites that extend from opposite poles of the cell body toward the margins of the nucleus, where they terminate in spinous branches. The fusiform cells are contacted by three types of synaptic terminals that can be distinguished by the size and shape of their synaptic vesicles. The somatic and proximal dendritic surfaces are apposed by synaptic terminals containing small, flat synaptic vesicles. Further from the cell body, the dendrites form numerous synaptic contacts with terminals containing large round vesicles as well as with the terminals containing small, flat vesicles. The most distal dendritic branches and their spiny appendages appear to form synapses almost exclusively with the terminals with large, round vesicles. A relatively rare type of terminal that contains small, round vesicles may form synapses with either the somatic or dendritic surfaces. A few small cells are interspersed among the fusiform cells, but they are more commonly located around the margins of the nucleus. The small cells form few axosomatic contacts. The simplest interpretation of the findings is that the terminals with small, flat vesicles arise in the medial nucleus of the trapezoid body and are inhibitory in function, whereas the terminals with large, round vesicles arise in the anteroventral cochlear nucleus and are excitatory; however, this remains to be demonstrated experimentally. In any case, the differential distribution of these two types of inputs on the somatic and dendritic surfaces must be an important determinant of the physiological response properties of the fusiform cells to binaural acoustic stimuli.     

Immunocytochemical localization of GABA in the cochlear nucleus of the guinea pig

The immunocytochemical distribution of gamma-aminobutyric acid (GABA) was determined in the cochlear nucleus of the guinea pig using affinity-purified antibodies made against GABA conjugated to bovine serum albumin. Light microscopic immunocytochemistry shows immunoreactive puncta, which appear to be GABA-positive presynaptic terminals, distributed throughout the cochlear nucleus. In the ventral cochlear nucleus, these puncta are often found around unlabeled neuronal cell bodies. While occasional labeled small cells are found in the ventral cochlear nucleus, most GABA-immunoreactive cell bodies are present in the superficial layers of the dorsal cochlear nucleus. Based on size and shape, immunoreactive cells in the dorsal cochlear nucleus are divided into 3 classes: medium round cells with diameters averaging 16 microns, small round cells with average diameters of 9 microns and small flattened cells with major and minor diameters averaging 11 and 6 microns, respectively. Labeled fusiform and granule cells are not seen. A similar distribution of label was seen using antibodies against glutamic acid decarboxylase. Electron microscopic immunocytochemistry of the anteroventral cochlear nucleus shows GABA immunoreactive boutons containing oval/pleomorphic synaptic vesicles on cell bodies and dendrites. Other major classes of terminals, including those with small round, large round and flattened synaptic vesicles are unlabeled.     

Patterns of glutamate decarboxylase immunostaining in the feline cochlear nuclear complex studied with silver enhancement and electron microscopy

The cochlear nuclear complex of the cat was immunostained with an antiserum to glutamate decarboxylase (GAD), the biosynthetic enzyme for the inhibitory neurotransmitter GABA, and studied with different procedures, including silver intensification, topical colchicine injections, semithin sections, and immunoelectron microscopy. Immunostaining was found in all portions of the nucleus. Relatively few immunostained cell bodies were observed: most of these were in the dorsal cochlear nucleus and included stellate cells, cartwheel cells, Golgi cells, and unidentified cells in the deep layers. An accumulation of immunoreactive cells was also found within the small cell cap and along the medial border of the ventral cochlear nucleus. Immunostained cells were sparse in magnocellular portions of the ventral nucleus. Most staining within the nucleus was of nerve terminals. These included small boutons that were prominent in the neuropil of the dorsal cochlear nucleus, the granule cell domain, in a region beneath the superficial granule cell layer within the small cell cap region, and along the medial border of the ventral nucleus. Octopus cells showed small, GAD-positive terminals distributed at moderate density on both cell bodies and dendrites. Larger, more distinctive terminals were identified on the large cells in the ventral nucleus, in particular on spherical cells and globular cells. There was a striking positive correlation of the size, location, and complexity of GAD-positive terminals with the size, location, and complexity of primary fiber endings on the same cells. This correlation did not hold in the dorsal nucleus, where pyramidal cells receive many large GAD-positive somatic terminals despite the paucity of primary endings on their cell bodies. The GAD-positive terminals contained pleomorphic synaptic vesicles and formed symmetric synaptic junctions that occupied a substantial portion of the appositional surface to cell bodies, dendrites, axon hillocks, and the beginning portion of the initial axon segments. Thus, the cells provided with large terminals can be subjected to considerable inhibition that may be activated indirectly through primary fibers and interneurons or by descending inputs from the auditory brainstem.     

Serotonin-containing axon terminals in the hypoglossal nucleus of the rat. An immuno-electronmicroscopic study

The morphology and distribution of serotonin-containing axon terminals in the rat hypoglossal nucleus (XII) was investigated immunocytochemically at the electron microscopic level. Serotonin-positive profiles were found throughout all regions of XII and included unmyelinated axons, varicosities and axon terminals. Most labeled profiles (68.1%) were nonsynaptic unmyelinated axons and varicosities, while synaptic profiles, ending on dendrites and somata, were seen less frequently (28.7%). The majority of labeled axon terminals (76.9%) ended on small-to-medium-sized dendrites. Most axodendritic terminals contained small, round agranular vesicles (20-55 microns), several large (60-100 microns) dense core vesicles, and were associated with a pronounced asymmetric postsynaptic specialization. By contrast, labeled axosomatic terminals were seen less often than those ending on dendrites (23.0%). Axosomatic terminals typically contained small, round, agranular and large dense core vesicles and were associated with a symmetric or no postsynaptic specialization. These results provide the structural substrates for elucidating the functional role of serotonin in tongue control.     

Noradrenergic innervation of the thalamic reticular nucleus: a light and electron microscopic immunohistochemical study in rats.

Fluoro-ruby injections in the rat locus coeruleus result in scattered chain-like arrays of varicose anterogradely labeled axons within the thalamic reticular nucleus of rats. An abundant meshwork of axons giving rise to en passant boutons is detected immunohistochemically within this thalamic nucleus by means of an antibody to dopamine-beta-hydroxylase (DBH). The density of DBH-positive axonal boutons within the reticular nucleus neuropil is greater than that found in the relay nuclei of the dorsal thalamus (with the exception of the anterior group nuclei). Single DBH-positive axons appear to contact both proximal and distal dendrites and occasionally the somata of reticular nucleus neurons. Labeled axons are seen closely juxtaposed not only to the swollen segments of the beaded reticular neuron dendrites, but to the constricted segments as well. Electron microscopic examination of DBH-positive axon terminals within the reticular nucleus neuropil indicates that many of the axonal boutons detected light microscopically participate in asymmetric synaptic contacts. The postsynaptic densities of these synapses are thicker than those of nearby symmetric synapses, but often subtend a shorter length of the postsynaptic membrane than the densities associated with other nearby asymmetric synapses. These observations indicate that the ascending noradrenergic system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, is well situated to influence signal transmission through the nuclei of the dorsal thalamus indirectly via a moderately dense terminal projection upon the thalamic reticular nucleus.     

Fine structural survey of the rat's brainstem sensory trigeminal complex

The fine structural organization of the principal sensory trigeminal nucleus was compared with that of the spinal trigeminal nucleus (subnuclei oralis, interpolaris, and the deep layers of caudalis) in adult albino rats. Direct comparisons indicate similarities between all of the subdivisions of the brainstem trigeminal complex both in the major morphological classes of neurons present and in basic patterns of synaptic connections. Major differences between the several subdivisions occur in the relative numbers and distribution of the different cell types. The spinal trigeminal nucleus is distinguished by more numerous large (22-40 micron) polygonal neurons which give rise to long straight primary dendrites. Both the perikaryal surface and the thick primary dendrites of many of these cells are densely innervated by synaptic terminals. Especially large cells of this type are a prominent feature of subnucleus oralis. By contrast, the principal sensory nucleus is distinguished by its high density of small to medium-sized (8-20 micron) round or ovoid neurons. These smaller neurons tend to receive a sparse axosomatic innervation. In addition to these differences the spinal trigeminal neuropil is distinguished by the striking manner in which it is broken up by large rostrocaudally oriented bundles of myelinated axons. Proximal dendrites of polygonal and fusiform neurons often wrap around these large axon bundles. Morphologically heterogeneous populations of synaptic terminals with round vesicles (R terminals) and terminals with predominantly flattened vesicles (F terminals) occur in all of the subdivisions of the trigeminal complex. Both types of terminal make primarily axodendritic synapses, but both also make axosomatic synapses, and axospinous synapses with somatic as well as dendritic spines. In addition, axoaxonic synaptic contacts from F terminals onto large R terminals are seen in all subdivisions. Convincing examples of presynaptic dendrites were not observed in any of the brainstem subdivisions. Synaptic glomeruli, characteristic groupings of dendrites and synaptic terminals, are found throughout the brainstem trigeminal complex. The dendritic elements in these glomeruli tend to be small-diameter dendrites, spines, and large, spinelike appendages. Within the glomerulus these elements are postsynaptic to a single large R terminal and may also be postsynaptic to smaller F terminals. In addition, axoaxonic synaptic contacts from the F terminals onto the R terminal are a consistent feature of trigeminal synaptic glomeruli.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pattern of rat olfactory bulb mitral and tufted cell connections to the anterior olfactory nucleus pars externa

The pattern of output of mitral and tufted cells of the rat olfactory bulb (OB) to layer Ia overlying the pars externa (pE) of the anterior olfactory nucleus (AON) has been studied in the rat by iontophoresis of horseradish peroxidase and Phaseolus vulgaris-leucoagglutinin. These agents labeled mitral and tufted cells and at least the proximal portion of their axons. In most cases we observed small branches from axons of the lateral olfactory tract that appear to terminate in the region of the pE AON, while the main axon could often be traced for considerable distances past these branches. These branches are assumed to terminate in the pE AON because they could not be traced to other terminal regions, because they ramify in layer Ia, and because they usually show small swellings characteristic of axons in terminal regions. Although each ramification could be extensive, we found that the positions of these small branches were related to the positions of the injections within the OB. Dorsal medial injections labeled dorsal branches. Ventral medial injections labeled ventral branches. Injections on the lateral face of the OB labeled intermediate branches. The centers of the regions within which branches were labeled were strongly correlated with the positions of the injection around the circumference. Comparison of the anterior-posterior axis of the OB produced no such strong correlation. Reconstructions of axons showed that terminal branches arise from both mitral and tufted cells, although at least some mitral cells are shown not to have such branches in the pE AON. Studies of the patterns of dendrites and terminals in the pE AON indicate that this region has the same pattern of layer Ia and Ib terminals seen in other olfactory cortical regions. The pE AON cell layer is intercalated just below the boundary between layers Ia and Ib. Since dendrites of the underlying pars lateralis of the AON (pL AON) penetrate into layer Ia over much of the pE AON, it is necessary to remember that at least part of the pL AON may also receive topographically organized inputs.     

Electron-microscopic analysis of synaptic input from the perigeniculate nucleus to the A-laminae of the lateral geniculate nucleus in cats.

The perigeniculate nucleus of carnivores is thought to be a part of the thalamic reticular nucleus related to visual centers of the thalamus. Physiological studies show that perigeniculate neurons, which are primarily GABAergic, provide feedback inhibition onto neurons in the lateral geniculate nucleus. However, little is known about the anatomical organization of this feedback pathway. To address this, we used two complementary tracing methods to label perigeniculate axons for electron microscopic study in the geniculate A-laminae: intracellular injection of horseradish peroxidase (HRP) to fill an individual perigeniculate cell and its axon; and anterograde transport of Phaseolus vulgaris leucoagglutinin to label a population of perigeniculate axons. Labeled perigeniculate terminals display features of F1 terminals in the geniculate neuropil: they are small, contain dark mitochondria, and form symmetric synaptic contacts. We found that most of the perigeniculate terminals (greater than 90%) contact geniculate cell dendrites in regions that also receive a rich innervation from terminals deriving from visual cortex (e.g., "cortico-recipient" dendrites). The remainder of the perigeniculate synapses (10%) contacted dendrites in regions that also received direct retinal input (e.g., "retino-recipient" dendrites). Serial reconstruction of segments of dendrites postsynaptic to perigeniculate terminals suggests that these terminals contact both classes of relay cell in the A-laminae (X and Y), although our preliminary conclusion is that an individual perigeniculate cell contacts only one class. Finally, our quantitative comparison between labeled perigeniculate terminals and unlabeled F1 terminals indicates that these perigeniculate terminals form a distinct subset of F1 terminals. We quantitatively compared the labeled perigeniculate terminals to unlabeled F1 terminals. Although the parameters of the perigeniculate terminals fell entirely within the range of those for the unlabeled F1 terminals, as populations, we found consistent differences between these two groups. We thus conclude that, as populations, other sources of F1 terminals are morphologically distinct from perigeniculate terminals and innervate different targets.