Synaptic organization in the dorsal cochlear nucleus of the cat: a light and electron microscopic study

This report describes some observations of the synaptic organization of one region of the cat dorsal cochlear nucleus (DCN). The large “fusiform cell” and its innervation from the cochlea are emphasized. The morphology of the mature fusiform cell and its postnatal development are described in rapid Golgi impregnations of perfusion-fixed littermate cats. The mature features are correlated with profiles of fusiform cell bodies, apical dendrites, and basal dendritic trunks in electron micrographs from adult cat brains. Small neurons and granule cells are also identified in electron micrographs. In Golgi impregnations, axons of small cells and granule cells may terminate upon fusiform cells. Six classes of axons can be distinguished in rapid Golgi impregnations of the DCN. Two classes are of cochlear origin. One axonal class arises from small cells. The sources of the remaining axonal classes have not been identified in this study. Primary afferents can terminate as large, mossy endings in the DCN neuropil. They can also participate in axonal nests along with axons and dendrites of small cells. In electron micrographs, four synaptic endings can be distinguished. Primary cochlear fibers end in large terminals with asymmetrical synaptic complexes and round, clear vesicles. Primary axons can end in glomeruli, resembling those of the cerebellum, or in synaptic nests which are conglomerates of neuronal processes including other types of endings. The origins of the other synaptic types are not yet known. According to this study, primary afferent input could influence fusiform cells directly or indirectly, via small cells and granule cells.     

A Golgi and ultrastructural study of the monkey globus pallidus

Golgi preparations reveal that the most frequent type of pallidal neuron (principal cell), which has been recognized in all previous reports, is large (20–50 (Am)), fusiform, with dendrites up to 700 μm long. Large neurons of globular shape are less frequently impregnated. The morphology of dendrites varies considerably within the same neuron. Some exhibit numerous spines and protrusions and are seen to terminate in elaborate arborizations. A small interneuron (12 (μm)), with relatively short dendrites, up to 150 μm, and a short sparsely branching axon is observed less frequently. At least two types of afferent axons are present. A small-diameter fiber from the neostriatum enters the pallidum in bundles and gives rise to numerous thin branching processes with varicosit es about 1 μm in size. The axon collaterals are oriented orthogonal to the main axon and parallel to the dendrites of principal cells. A large-caliber fiber with clusters of 2–3-μm swellings can also be seen in close proximity to large pallidal dendrites. Ultrastructurally, principal cell dendrites (trunks, spines, and protrusions) are totally covered by synapsing axon terminals. In contrast, some small dentrites, presumed to belong to interneurons, form very few synapses. At least six categories of profiles containing vesicles are observed. One group has cytologic features of dendrites and participates in serial and triadic synapses with other profiles in the pallidal neuropil. Results suggest that the synaptic organization of the globus pallidus may be viewed as a repetitive, geometric arrangement of striatal and other afferent axons ensheathing and synapsing with the dendrites of principal cells. This pattern is interrupted by the presence of presynaptic dendrites, probably belonging to interneurons, which participate in complex synaptic arrangements.     

The structure of the dorsal lateral geniculate nucleus in the mouse. A Golgi and electron microscopic study

Two types of neurons have been distinguished in Golgi and electron microscopic preparations of the dorsal lateral geniculate nucleus of young mice. In addition to a thalamo-striate relay cell (TSR neuron) with brush-like dendritic arbors and a thick, unbranched axon, a small tomedium size cell (PA neuron) of oval or spindle shaped body and few, long and seldom branched dendrites is frequently identified in our Golgi preparations. This second type of cell may exhibit none, one or several sparsely branched axon-like processes which terminate in the vicinity of the cell body. The dendrites of the PA neurons show characteristically large, spheroidal processes (p) 1–3 m? in diameter issuing forth singly, in clusters or as a “string of beads” from delicate, often long, pedicles attached to the dendritic shafts. Profiles comparable to these processes and in apparent continuity with PA dendrites have been identified with the electron microscope and show synaptic vesicles and a system of sacs of smooth E.R. The portions of the dendrites from which these vesicle-containing processes issue also show clusters of vesicles, ribosomes and an orderly array of microtubules. Golgi impregnated axons are followed from the optic tract and seen terminating as irregular enlargements (2–6 m?) on proximal dendrites of relay cells and on distal dendrites of the PA neuron. The intimate contact of the terminal branches of an optic collateral with a distal PA dendrite is carefully illustrated. Small calibered axons are likewise traced from the internal medullary lamina and seen to end by means of end-knobs on distal dendrites of both types of neurons. Electron microscopic observations substantiate the Golgi images and reveal three different types (I, II, III) of endings in the geniculate neuropil. The large type I endings correspond to the retinal afferents which generally make asymmetric synapti contact with type II profiles and/or with clusters of microspines on the TSR dendrites. Type II, thought to be the spheroidal dendritic appendages of the PA neurons, form symmetrical synaptic contacts with profiles of its own kind or more commonly with dendrites of the TSR neuron. The type III ending, probably cortical in origin, establishes asymmetrical synaptic contacts with small dendritic profiles. Only profiles of types I and II endings, together with those of other dendritic profiles, form part of the nest-like junctions known as the synaptic glomeruli. The significance of the unusual polarization of the geniculate interneurons is discussed.     

Electron microscope study of Golgi-stained cells in lamina II of the rat spinal dorsal horn

Seven Golgi-stained cells in lamina II of the rat spinal dorsal horn were examined by electron microscopy. One of the cells studied was an islet cell, two were stalked cells, and the remaining four cells could not be classed into either group. The islet cell and three of the unclassified cells possessed presynaptic dendrites and formed synapses onto a variety of dendritic spines and shafts within lamina II. The axons of the islet cell and of one of the unclassified cells formed symmetric axodendritic synapses mainly onto dendritic shafts. The two stalked cells and the remaining unclassified cell did not possess vesicle-containing dendrites. This last cell bore some resemblance to a stalked cell and may have represented an atypical example of one. Most of the synapses involving the cells took place outside synaptic glomeruli, but all seven cells were postsynaptic to central axons within glomeruli and in most cases to both type I and type II central axons, suggesting a monosynaptic input from myelinated and unmyelinated primary afferent axons. In addition, most of the cells were postsynaptic to vesicle-containing dendrites. It is concluded that certain cells, which do not belong to the stalked or islet classes, possess presynaptic dendrites and function as presumed inhibitory interneurones within lamina II. The target of the cells with presynaptic dendrites includes other cells within lamina II and may also include cells in deeper laminae of the dorsal horn. Further evidence will be needed in order to determine whether all cells in lamina II that do not possess presynaptic dendrites form a single functional class.     

Synaptic ultrastructure of functionally and morphologically characterized neurons of the superficial spinal dorsal horn of cat

Recordings of neuronal unitary discharges evoked by primary afferent input were made in the superficial part of the spinal cord's dorsal horn, the marginal zone and substantia gelatinosa (also known as laminae I and II), using fine micropipette electrodes filled with HRP. After physiological characterization with respect to primary afferent input, HRP was injected intracellularly iontophoretically into the recorded neuron. Following histochemical processing, the neurons so delineated were studied at the light and electron microscopic levels. No clear relationship between function and either general cellular configuration or synaptic ultrastructure appeared in these analyses, although the concentration of dendritic distribution could be related to the nature of primary afferent excitation. Nocireceptive cells had dendrites mostly branching and ending in lamina I and IIo, while the dendrites of innocuous mechanoreceptive cells arborized principally in lamina II and III. Glomerular synaptic complexes (large, complex arrays of axonic and dendritic profiles with synaptic interconnections) were found to contact a few neurons of both the nocireceptive and mechanoreceptive classes. All neurons received large numbers of simple axonic contacts (small axonic boutons with only 1 or 2 synaptic contacts with a single postsynaptic profile). A degree of specificity in the presynaptic articulations appeared to be reflected by the observations that (1) nocireceptive neurons were never found to receive synaptic contacts from boutons which resembled the known ultrastructure of peripheral innocuous mechanoreceptors, and (2) mechanoreceptive neurons were never seen to receive synaptic contacts from boutons which resembled the known ultrastructure of primary afferent nocireceptors. The axons of the labeled neurons of both nocireceptive and mechanoreceptive classes terminated in simple axonic synapses. All classes of neurons participated in dendrodendritic contacts; however, only some mechanoreceptive neurons had dendrites containing vesicles that were presynaptic to other profiles. No nocireceptive neurons, regardless of gross configuration, were found to have vesicles in their dendrites, but 3 nocireceptive neurons received synapses from presynaptic dendritic profiles.     

The dorsal cochlear nucleus of the mouse: a light microscopic analysis of neurons that project to the inferior colliculus

In the mouse dorsal cochlear nucleus (DCN), all members of a distinct class of large multipolar neurons were shown to project to the contralateral inferior colliculus by using retrograde horseradish peroxidase techniques. Typically, these multipolar neurons have the largest cell bodies in the nucleus and are distributed in layers II, III, and IV. Each contains a round, pale nucleus with a prominent nucleolus and conspicuous Nissl bodies. In Golgi preparations, however, two types of large cells could be distinguished on the basis of dendritic characteristics. Pyramidal cells form relatively flattened, slablike dendritic fields whose alignment contributes to the laminar organization of the DCN. They represent 75-80% of the large cell population and are found in layer II and the superficial region of layer III. Giant cells represent the other type of large multipolar neuron and are distributed in the deeper regions of layer III and in layer IV. Their ellipsoidal dendritic fields are formed by long and relatively unbranched dendrites that project across the laminae. The differences in dendritic morphology imply that each cell class segregates its afferent input in distinct ways and subserves different auditory functions.     

Morphology of afferent synapses in the Mauthner cell of larval Xenopus laevis

The fine structure of the afferent synapses on the Mauthner cell of larval Xenopus laevis has been studied as a first step toward comparing the fine structure of the afferent synaptic apparatus before and after metamorphosis. There are various types of afferent endings on this cell, some of which are confined to specific cellular regions, while others are distributed over most of the large surface of the neuron. Four different main types of endings have been observed: club endings, round-vesicle end bulbs, flattened-vesicle end bulbs and spiral fibers endings. While the myelinated club endings and the spiral fibers endings are located at the distal end of the lateral dendrite and in the axon cap, respectively, the end bulbs are widely distributed over the whole cell. A further type of ending has been observed, although rarely, on the Mauthner cell soma and dendrites: end bulbs characterized by an unusually dense presynaptic substance. Results obtained in the present research suggest that, as in fish, different endings on the anuran Mauthner neuron correspond to different synaptic inputs. The possible origin of some of these inputs is discussed.     

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.     

Anatomy of the gustatory system in the hamster: synaptology of facial afferent terminals in the solitary nucleus

The solitary nucleus is the first level of the central nervous system where processing of taste information can occur. A structural basis for that processing was investigated. Facial taste afferent axons were labelled by application of horseradish peroxidase to either the chorda tympani or the geniculate ganglion. The labelled afferent fibers in the rostral solitary nucleus were studied with light and electron microscopy. Preterminal facial taste afferent axons enter the nucleus from the solitary tract with a pronounced lateral to medial trajectory. The axons bear numerous preterminal and terminal swellings that, with the electron microscope, were identified as synaptic endings located in glomeruli. The endings are ovoid or scalloped, indented by structures that surround them. The primary afferent endings contain large, round vesicles and synapse, by means of slightly asymmetrical junctional complexes, on small dendrites and spines. Two types of unlabelled endings, surrounding the labelled ones, contact the dendrites receiving taste afferent input or contact the endings of taste afferent axons themselves. One type is variable in size and contains scattered large round vesicles. It resembles a presynaptic dendrite. The other is a small axonal ending packed with small, pleomorphic vesicles, that engages in symmetrical junctions. The synaptic milieu of the taste endings allows for the possibility of modulation of taste-elicited activity in afferent endings or second-order neurons by other, possibly interneuronal, inputs.     

Retinal and visual cortical projection to the superior colliculus of the rabbit

The rabbit superior colliculus was examined using a variety of light- and electron-microscopic techniques. Golgi study showed that there is one principal type of neuron whose axon leaves the superficial gray—the vertical cell. The remaining varieties of neurons possess heavily spine-studded dendrites and local axons. Among these types are the marginal cell, the stellate cell, and the pyriform cell. Using ultrastructural techniques, we found contralateral optic input is confined to a zone 100 to 350 μm below the surface of the colliculus. Terminals are presynaptic to both axons and dendrites and enter into serial synapses. They degenerate by passing through a filamentous to a dense phase. Visual cortical axon terminals are 300 to 600 μm below the surface and usually contact only one postsynaptic profile. They do not participate in serial synapses. F axon terminals (symmetric thickening, flattened vesicles) are also numerous in the upper collicular layers, but do not degenerate after enucleation or visual cortex ablation. We conclude that considerable evidence exists for a large degree of horizontal and vertical organization in the upper layers of the superior colliculus. These data are compared to the tectum in other species and to our ongoing experiments on the development of the superior colliculus.     

Dendroaxonic synapses in the substantia gelatinosa glomeruli of the spinal trigeminal nucleus of the cat

The glomeruli in the substantia gelatinosa layer of the spinal trigeminal nucleus of the cat contain three kinds of dendritic processes. One of these, the type 2 dendrite, contains large synaptic vesicles in its spine heads and in its shaft. The type 2 dendrite receives axodendritic synapses from primary trigeminal afferent (C) axons and an occasional axodendritic synapse from small axonal (P) endings with small synaptic vesicles. The type 2 dendrites in turn form dendroaxonic synapses on the C endings. The dendroaxonic synapse and the axodendritic synapse of the C ending typically occur in reciprocal pairs. The axodendritic synapse usually lies in the depths of scalloped depressions in the surface of the C ending while the dendroaxonic synapse is found on the rim of the depression. Type 1 spines, i.e., dendritic spines receiving axodendritic synapses from the primary ending and lacking synaptic vesicles, also receive dendrodendritic synapses from type 2 dendrites. The types 2 dendrite with its large, rounded synaptic vesicles is considered to be excitatory at its dendroaxonic and dendrodendritic synapses. The type 2 dendrites course from glomerulus to glomerulus receiving their excitatory input through the axodendritic synapses of C axons. A type 2 dendrite, in response to C axon excitation would activate type 1 spines directly through their dendrodendritic synapses (C→2→1) and indirectly by increasing transmitter release at the axodendritic synapses of the C axonal endings through their dendroaxonic synapses (2→C→1). The type 2 dendrites could serve two functions. First, they may prolong transmitter release from the axodendritic synapses of C axonal endings beyond the time of arrival of incoming action potentials because of the reciprocal pairing of dendroaxonic and axodendritic synapses (C?2). Second, they may extend the spatial range of the excitatory output of active primary afferent axons to type 1 spines of glomeruli whose primary afferent axons may be inactive (C→2→1).     

The organization of the posterior ventral cochlear nucleus in the rat

The ventral cochlear nucleus was examined in 31 rats' brains prepared according to the protargol method of Bodian. The following regions were delimited on the basis of synaptic, cellular and axonal criteria. Region II, which occupies the central part of the nucleus, consists principally of cells (type g)which receive modified bulbs of Held from the acoustic nerve and which send large axons to the trapezoid body. The cells were graded in size, the smallest (high frequency) being located dorsally and the larges (low frequency) being located ventrally. Associated with region II are the cells (type b) of the acoustic nerve nucleus. These cells receive boutons from the acoustic nerve and send very large axons to the trapezoid body. Region IV is located in the posterior part of the ventral cochlear nucleus and is composed of large multipolar cells (type k). The dendrites of these cells are specifically organized with respect to the fibers of the acoustic nerve. Synaptic endings, from the descending branch of the acoustic nerve, consist of boutons on cell body and dendrites. The axons of these cells compose the stria of Held. Region V is located in the posterior part of the ventral cochlear nucleus and appeared to have only intrinsic connections. All regions received synaptic endings in addition to those which arose from the acoustic nerve.     

Targets of olivocochlear collaterals in cochlear nucleus of rat and guinea pig

Descending auditory pathways can modify afferent auditory input en route to cortex. One component of these pathways is the olivocochlear system which originates in brainstem and terminates in cochlea. Medial olivocochlear (MOC) neurons also project collaterals to cochlear nucleus and make synaptic contacts with dendrites of multipolar neurons. Two broadly distinct populations of multipolar cells exist: T-stellate and D-stellate neurons, thought to project to inferior colliculus and contralateral cochlear nucleus, respectively. It is unclear which of these neurons receive direct MOC collateral input due to conflicting results between in vivo and in vitro studies. This study used anatomical techniques to identify which multipolar cell population receives synaptic innervation from MOC collaterals. The retrograde tracer Fluorogold was injected into inferior colliculus or cochlear nucleus to label T-stellate and D-stellate neurons, respectively. Axonal branches of MOC neurons were labeled by biocytin injections at the floor of the fourth ventricle. Fluorogold injections resulted in labeled cochlear nucleus multipolar neurons. Biocytin abundantly labeled MOC collaterals which entered cochlear nucleus. Microscopic analysis revealed that MOC collaterals made some putative synaptic contacts with the retrogradely labeled neurons but many more putative contacts were observed on unidentified neural targets. This suggest that both T- and D-stellate neurons receive synaptic innervation from the MOC collaterals on their somata and proximal dendrites. The prevalence of these contacts cannot be stated with certainty because of technical limitations, but the possibility exists that the collaterals may also make contacts with neurons not projecting to inferior colliculus or the contralateral cochlear nucleus.     

Cochlear nerve projections to the small cell shell of the cochlear nucleus: The neuroanatomy of extremely thin sensory axons

Labeling cochlear nerve fibers in the inner ear of chinchillas with biotinylated dextran polyamine was used to trace the thin fibers (Type II), which likely innervate outer hair cells. These axons, 0.1–0.5 μm in diameter, were distinguished from the thicker Type I, fibers innervating inner hair cells, and traced to small‐cell clusters in the cochlear nucleus. This study provided two major new insights into the outer hair cell connections in the cochlear nucleus and the potential significance of very thin axons and synaptic nests, which are widespread in the CNS. 1) EM serial reconstructions of labeled and unlabeled material revealed that Type II axons rarely formed synapses with conventional features (vesicles gathered at junctions). Rather, their endings contained arrays of endoplasmic reticulum and small spherical vesicles without junctions. 2) Type II axons projected predominantly to synaptic nests, where they contacted other endings and dendrites of local interneurons (small stellate and mitt cells, but not granule cells). Synaptic nests lacked intrinsic glia and, presumably, their high‐affinity amino acid transporters. As functional units, nests and their Type II inputs from outer hair cells may contribute to an analog processing mode, which is slower, more diffuse, longer‐lasting, and potentially more plastic than the digital processors addressed by inner hair cells. Synapse 33:83–117, 1999. © 1999 Wiley‐Liss, Inc.     

A cytoarchitectonic atlas of the medial geniculate body of the opossum, Didelphys virginiana, with a comment on the posterior intralaminar nuclei of the thalamus

The organization of the medial geniculate body and adjacent posterior thalamus of the Virginia opossum was studied in Nissl-, Golgi-, reduced silver, and myelin-stained preparations. Our chief goals were to define the cytoarchitectonic subdivisions and boundaries in Nissl preparations and to reconcile these with those observed with the Golgi method and in experimental material, to present these results in an atlas of Nissl-stained sections, and to compare the chief nuclear groups in the opossum and the cat medial geniculate body. In the opossum, the ventral division consists chiefly of the ventral nucleus. The ventral nucleus is divided into two main parts: the pars lateralis and the pars ovoidea, the former being relatively smaller in the opossum. The ventral nucleus of both species contains large principal neurons with bushy, tufted dendrites and smaller Golgi type II cells. However, the opossum has far fewer Golgi type II cells, and the texture of the neuropil is correspondingly different, although the primary ascending input from the midbrain arises from the central nucleus of the inferior colliculus in both species. The dorsal division consists of the dorsal nuclei, including the suprageniculate nucleus and the caudal part of the lateral posterior nucleus, the marginal zone, and the posterior limitans nucleus. These nuclei are identified in both species, although they are much smaller in the opossum. The neurons consist of medium-size and small somata with a predominantly radiate mode of dendritic branching and a lower cell concentration than in the ventral division. In both species the afferent brain stem input comes from the inferior colliculus, the lateral tegmental area, the intercollicular tegmentum, and the superior colliculus. The medial division contains several types of cells, which are heterogeneous in form and size, most having radiating dendrites and a low cellular concentration. This division is especially smaller in the opossum, although comparable inputs arise from various auditory and non-auditory sources in the midbrain and spinal cord in both species. A large intralaminar complex of nuclei occurs in the opossum, which have a more extensive distribution than previously appreciated. They not only occupy the intramedullary laminae but form a shell around the medial geniculate nuclei and adjoining main sensory nuclei. The intralaminar complex includes the posterior limitans, posterior intralaminar, posterior, parafascicular, posterior parafascicular, central intralaminar, limitans, and central medial nuclei, and the marginal zone of the medial geniculate body.(ABSTRACT TRUNCATED AT 400 WORDS)     

GABA- and glycine-like immunoreactivity in axons and dendrites contacting the central terminals of rapidly adapting glabrous skin afferents in rat spinal cord

The object of the present study was to determine the nature and distribution of synaptic contacts on the terminals of rapidly adapting mechanosensory afferents innervating the glabrous skin of the rat foot. Afferents were physiologically characterized by intracellular recording, before injection with neurobiotin and preparation for electron microscopy. Axon terminals were serially sectioned and immunolabeled with antibodies against GABA and glycine using a postembedding immunogold method. Afferent boutons in lamina III were often surrounded by several presynaptic axons and postsynaptic dendrites (thus forming type II glomeruli), while boutons in laminae IV-V had only simple, nonglomerular interactions. In both regions triadic synaptic arrangements where presynaptic interneurons contact both afferent boutons and their postsynaptic dendrites were present in 50-75% of boutons. Approximately three-quarters of presynaptic axons were immunoreactive for both GABA and glycine and most of the remainder for GABA alone. Most postsynaptic dendrites were not immunoreactive. Comparisons are made with information from similar studies of other rat and cat afferents conducting in the Aalphabeta range. This demonstrates that although the principles of control may be similar for cutaneous afferents of this type there are significant differences between cutaneous and 1a muscle afferents in the rat. There are also differences in detail between the interactions of afferents of the same modality in rat and cat; in the rat there are greater numbers of presynaptic axons per bouton and a greater proportion of boutons receive axo-axonic contacts and are involved in synaptic triads.     

Fine structure of rat locus coeruleus

Locus coeruleus of the rat was studied in material prepared by aldehyde-osmium fixation. Cell bodies of locus coeruleus neurons possess large nuclei with a prominent nucleolus, a homogeneous karyoplasm of moderate density, and occasional indentations of the nuclear membrane. The cytoplasm is rich in organelles, including an extensive network of endoplasmic reticulum which forms well organized Nissl bodies. The highly developed Golgi apparatus surrounds the nucleus and extends into large dendritic trunks. In coronal section, cell bodies appear elongated along an approximate dorso-ventral axis, and most dendrites as well as axons appear in cross-section. In parasagittal sections the cells are very elongate, with dendrites and axons in the neuropil mostly cut longitudinally. Thus, locus coeruleus neurons possess disc-shaped dendritic fields parallel to the anterior-posterior axis of the brainstem, with predominantly longitudinal axo-dendritic synaptic configurations. Presynaptic profiles in locus coeruleus neuropil were classified according to the characteristics of their vesicle populations and other features. The most frequently encountered synaptic ending was characterized by small, round, densely packed synaptic vesicles, and comprised approximately 41% of the total sample of 775 synapses. Another group having large, rounded synaptic vesicles, which could be traced in a number of instances to large myelinated axons, accounted for 20% of the sample. Synaptic endings having large, flattened vesicles were also numerous, comprising 23% of the total. Another category of presynaptic endings was identified as those possessing numerous, small, flattened vesicles and comprising about 11% of the sample. Presynaptic endings having many vesicles of mixed sizes accounted for 2% of the total, and another group of the same proportion having small, rounded synaptic vesicles but also an unusually large number of larger, dense-cored vesicles was also present. Two other categories of synaptic endings were encountered, each comprising less than 1% of the total. One of these was derived from small, unmyelinated axons and contained clusters of pleomorphic synaptic vesicles. The other consisted of dendro-dendritic synapses between locus coeruleus neurons and also displayed small clusters of pleomorphic synaptic vesicles near the zone of synaptic apposition. Quantitative analysis revealed that most afferents to the nucleus synapse onto dendrites ranging between 0.5 and 2.5 micrometers in diameter and onto spine-like appendages derived from somata and dendrites. There were no significant differences between different categories of afferent terminals and their spatial distribution onto various postsynaptic targets of locus coeruleus neurons.     

The cytoarchitecture and some efferent projections of the centromedian-parafascicular complex in the lesser bushbaby (Galago senegalensis)

The morphology of neurons in the centromedian (CM) and parafascicular (PF) nuclei in the lesser bushbaby (Galago senegalensis) is described in coronal and horizontal brain sections using Golgi-, horseradish peroxidase (HRP)-, and Nissl-staining procedures. The CM contains two types of cells referred to as principal neurons and Golgi type II (like) neurons. Cell bodies of principal neurons are relatively large in cross-sectional area (mean = 130.42 micron2), round to spindle in shape, support short somatic spines, and give rise to three to five primary dendrites. The dendrites branch in a "radiate" pattern and possess numerous appendages consisting of narrow, stalk-supported swellings. The presumed axons of these cells are impregnated only in their initial segments. On the basis of the similarity of principal neuron soma shapes and cross-sectional areas with those of HRP-reactive somata following cortical HRP implantation, it is concluded that at least some of the principal neurons in Galago CM project to somatic sensory-motor cortex. Golgi type II (like) neurons have small (mean = 79.43 micron2), round somata which support several spines and give rise to three to four small-diameter dendrites. The dendrites are infrequently branched, sinuous in their courses, and give rise to complex appendages and beaded processes. However, the axons of these cells could not be seen to ramify in the immediate vicinity of the dendritic field or soma, and there is considerable overlap in the cross-sectional areas of Golgi type II (like) neurons seen in Golgi preparations and HRP-stained cells following cortical implant of HRP pellets. Consequently, although Golgi type II (like) cells have traits characteristic of classically described intrinsic neurons, a cortical projection of these cells cannot be ruled out by the present study. The parafascicular nucleus contains two groups of large, radiate cells characterized by the presence or absence of somatic spines. Cells with somatic spines also contain numerous appendages on the dendrites. Cells without somatic spines support only a few, isolated, short dendritic appendages. Numerous small cell-bodied neurons are present in Nissl-stained sections of PF; however, cells which resemble Golgi type II neurons were not observed in the PF in the present Golgi-impregnated material. In contrast to the CM, the large cell-bodied neurons in PF were not found to project to somatic sensory-motor cortex in Galago.     

Spatial relationships between the terminations of somatic sensory motor pathways in the rostral brainstem of cats and monkeys. II. Cerebellar projections compared with those of the ascending somatic sensory pathways in lateral diencephalon

In order to classify the presynaptic elements contacting the principle class of globus pallidus neurons, electron microscopic examination of serial sections made from a medially located large globus pallidus neuron, labeled with intracellular horseradish peroxidase, was undertaken. In addition, the use of labeled and light microscopically reconstructed material allowed us to quantitatively determine the distribution of each bouton type along the soma and dendrites. Six types of presynaptic terminals contacting the labeled cell have been recognized. Type 1 endings, the most numerous (84%), make symmetrical contacts on all portions of the cell, except spines, contain large pleomorphic, and a few large dense-core vesicles. Type 2 endings are filled with small spherical-to-ellipsoidal synaptic vesicles. They make asymmetrical contacts only with higher-order dendrites and account for 12% of synaptic contacts onto the labeled neuron. Type 3 endings are large, contain sparsely distributed large pleomorphic vesicles, and make two symmetrical synapses per bouton, one onto a spine head and the other onto the underlying dendritic shaft. They are infrequent (0.2%), being found only in association with dendritic spines. Type 4 endings contain large pleomorphic synaptic vesicles and no dense-core vesicles. They make symmetrical contacts with the short primary dendrites. Type 5 endings contain a mixture of small clear pleomorphic vesicles and numerous large dense-core vesicles. They contact only the cell body and the short primary dendrites, making up 20% of somatic synaptic contacts but less than 1% of contacts onto dendrites. Type 6 boutons contain oval and flattened synaptic vesicles and establish symmetrical contacts with higher-order dendritic branches and the cell body.     

An EM study of the dorsal nucleus of the lateral lemniscus: inhibitory, commissural, synaptic connections between ascending auditory pathways

The dorsal nucleus of the lateral lemniscus (DNLL) and its connections constitute one of the ascending auditory pathways to the inferior colliculus. One notable feature of this nucleus is the heavy commissural connections between DNLL on opposite sides of the midbrain. These commissural connections may have a significant impact on the ascending pathway. In this study, the fine structure of DNLL in the cat and its commissural connections were examined. Both anterograde and retrograde transport methods were used simultaneously at the EM level. Injections of 3H-leucine mixed with WGA-HRP were made in one DNLL. After axonal transport, EM autoradiographic methods were used to identify the anterogradely labeled axonal endings from the opposite DNLL. In the same location, retrogradely labeled neurons with crossed connections were identified with HRP histochemistry. Two types of axonal endings were found in DNLL, those with round synaptic vesicles forming asymmetrical synaptic junctions and those with pleomorphic vesicles and symmetrical synapses. Both types were equally common. However, only endings with pleomorphic vesicles were labeled after injections in the contralateral DNLL. The labeled endings from the opposite DNLL appeared to represent a homogeneous population, even though a number of variations in the 2 types of endings were found. Labeled endings were presynaptic to all parts of neurons in DNLL, but a large proportion of the synapses were on cell bodies and large dendrites. Two patterns of nuclear morphology and distribution of rough endoplasmic reticulum were identified and may represent different cell types. Examples of both cell types were observed to project to the contralateral side and received labeled synaptic endings. The major finding of this study is that the crossed connections between DNLL exhibit the morphology associated with inhibitory function. Since neurons in DNLL are thought to use GABA as a neurotransmitter, the crossed connections could provide inhibitory inputs to DNLL on each side. Since some neurons receive numerous axosomatic inputs from the contralateral DNLL and also project to the opposite side, they may participate in direct reciprocal, inhibitory connections between the nuclei. Crossed inhibitory connections in the DNLL pathway may be important in regulating the flow of ascending auditory information.