The ultrastructure of the subnucleus gelatinosus of the nucleus of the tractus solitarius in the cat

The dorsomedial region of the nucleus of the tractus solitarius termed the subnucleus gelatinosus (SNG) was studied at the light and electron microscopic level in the cat. In cresyl violet and luxol fast blue stained sections the SNG contained small neuronal somata that were scattered throughout a pale-staining neuropil containing few myelinated fibers. These neurons were difficult to impregnate with Golgi staining techniques, but in successful impregnations the somata were observed to be 10--19 micrometers in diameter and bore few sparsely branching primary dendrites. Spines were present on the dendrites of some neurons and were more numerous on distal portions of the dendritic tree. Ultrastructural examination of the SNG revealed that the neuronal complement consisted of round, oval, or spindle shaped neurons with little or no organized Nissl substance. Rare myelin-like ensheathments of neuronal perikarya were also observed. Bundles of fine unmyelinated axons that coursed mainly longitudinally were a prominent feature of the area. The most common type of axon terminal observed contained mainly round clear vesicles, approximately 31 nm in diameter, and made asymmetrical synaptic contact with a dendritic profile. Pleomorphic vesicle-containing terminals involved in symmetrical synaptic contact were also commonly seen. Axodendritic and axosomatic synapses were associated with terminals containing either round clear vesicles or pleomorphic vesicles. Less commonly, dendrodendritic and dendrosomatic synapses were seen, the presynaptic elements of which contained pleomorphic vesicles. Following removal of a nodose ganglion, degenerating terminals of vagal afferent fibers were observed throughout the neuropil. Such terminals contained round, clear vesicles with an occasional large, dense-cored vesicle, and made axodendritic and axosomatic synaptic contacts.     

The distribution and synaptic organization of serotoninergic elements in the inferior olivary complex of the opossum

Immunohistochemistry and high-resolution autoradiography were used to analyze the distribution and synaptic organization of serotonin (5HT) - containing elements in the inferior olivary complex of the opossum. Immunoreactive beaded varicosities are present throughout the olivary complex. The densest 5HT immunostaining is present in subnucleus b of the caudal medial accessory nucleus. The rostral principal olive is sparsely populated with immunoreactive elements. Fine beaded fibers arborize throughout the neuropil of all the olivary nuclei except in subnucleus b of the caudal medial accessory nucleus where they also circumscribe neuronal cell bodies. In addition, a distinct population of large beaded fibers are occasionally encountered in the neuropil of the medial accessory nucleus. Ultrastructurally, labeled profiles that correspond in size to the smaller beads (less than 1 micron) contain tubulovesicular elements, large dense-cored vesicles, and clear vesicles. In contrast, larger profiles (greater than 2 microns) are characterized by numerous clear synaptic vesicles. Synaptic junctions were encountered in only 2% of the labeled elements. The majority of the labeled profiles were in juxtaposition to small-diameter dendrites (less than 2 microns) except in the caudal medial accessory nucleus, where they also were found in apposition to olivary cell bodies. Our results, when compared with other accounts, indicate that rather than major differences in the nuclear distribution of serotonin between species, there are differences in the density of serotoninergic elements in specific nuclei of the mammalian inferior olive. Based on the size of the labeled profiles and the distinct vesicle populations, our data suggest there are at least two populations of 5HT varicosities that are in juxtaposition to olivary neurons. Further, boutons containing 5HT primarily interact with the distal dendrites of olivary neurons except in the caudal medial accessory nucleus where cell bodies are in apposition to 5HT varicosities.     

A note on the synapses between vesicle containing profiles in the pontine nuclei of the cat

In the cat synapses between vesicle containing profiles were observed in ventral and dorsolateral pontine nuclei. The presynaptic elements consisted of two types of axon terminals: axon terminals characterized by a population of small (38-40 nm) round synaptic vesicles (SSV) and axon terminals containing pleomorphic synaptic vesicles (PSV). The postsynaptic pale elements (PP) had pleomorphic vesicles and some features attributed to dendrites. In the dorsolateral pontine nucleus most of PP profiles took part in serial synapses, usually as an intermediate component, they were rarely observed in triads. On the basis of their electron microscopical appearance and synaptic relations they might be considered to represent a dendritic part of putative interneurons.     

Intracellular labeling of neurons in the medial accessory olive of the cat: II. Ultrastructure of dendritic spines and their GABAergic innervation

In order to describe the morphology of dendritic spines of identified neurons in the cat inferior olive together with their gamma-aminobutyric acid (GABA) synaptic input, a technique was used combining intracellular labeling of horseradish peroxidase with postembedding gold-immunocytochemistry. With this technique physiologically identified olivary cells were reconstructed with the light microscope, and the horseradish peroxidase reaction product and immunogold labeling were subsequently examined in serial sections at the ultrastructural level. In addition, a degenerating neuron was observed, resulting in a triple labeling in single ultrathin sections. Quantitative and three-dimensional analysis showed that the dendritic spines were composed of long, thin stalks ending in one or more spine heads. The spines of cells located in the caudal half of the medial accessory olive (type I cells, characterized by dendrites which run away from the soma) were found to be less complex than those of cells located rostrally in this olivary subnucleus (type II cells, characterized by dendrites which tend to turn back towards the soma). Most, if not all, of the spines of both cell types were located within glomeruli. On average, the spines within individual glomeruli originated from 6 different dendrites (with a maximum of 8). Different spines within the same glomerulus were never derived from different dendrites of the same olivary neuron, but single spines frequently gave rise to several spine heads, which could be located either within different glomeruli or inside a single glomerulus. The glomerular spine heads originating from the same spine were rarely located near one another. All spines and most of the spine heads were contacted by both GABAergic and non-GABAergic terminals. Most of the GABAergic terminals contained pleomorphical vesicles and displayed symmetric synapses whereas the non-GABAergic terminals showed usually round to oval vesicles and asymmetric synapses.     

Fine structure and synaptic connections of the common spiny neuron of the rat neostriatum: A study employing intracellular injection of horseradish peroxidase

Medium-sized spiny neurons of the rat neostriatum, identified by intracellular injection of horseradish peroxidase, were examined at both light and electron microscopic levels. These neurons were characterized by their heavy investment of dendritic spines, beginning about 20 μm from the soma and continuing to the tips of the dendrites. Their axons arose from the soma or from a large dendritic trunk very near the soma, and tapered rapidly to form a main axonal branch from which arose several smaller initial collaterals. These arborized extensively throughout an area of about the same size as, and highly overlapping with, the dendritic field of the cell, while the main axon could be followed for distances of up to 1 mm in the direction of the globus pallidus. Three major synaptic types were seen in contact with spiny neurons. Boutons containing small round synaptic vesicles formed synapses exclusively with spiny regions of the dendrites, and most of these were axo-spinous. Small, very pleomorphic synaptic vesicles characterized a second bouton type of unknown origin, which made contacts with somata, initial segments, and dendrites, but not dendritic spines. Boutons containing large pleomorphic synaptic vesicles had the most widespread distribution, contacting all regions including dendritic spines. Spines receiving these contacts also were postsynaptic to boutons containing small round vesicles. Axon collaterals of spiny cells formed synapses with large pleomorphic vesicles and made synapses with somata, initial segments of axons, dendrites, and dendritic spines of striatal neurons, including other spiny cells.     

Axonal projections from the lateral and medial superior olive to the inferior colliculus of the cat: A study using electron microscopic autoradiography

The superior olivary complex is the first site in the central auditory system where binaural interactions occur. The output of these nuclei is direct to the central nucleus of the inferior colliculus, where binaural inputs synapse with monaural afferents such as those from the cochlear nuclei. Despite the importance of the olivary pathways for binaural information processing, little is known about their synaptic organization ir the colliculus. The present study investigates the structure of the projections from the lateral and medial superior olivary nuclei to the inferior colliculus at the electron microscopic level. Stereotaxic placement and electrophysi ological responses to binaural sounds were used to locate the superior olive. Anterograde axonal transport of 3H-leucine was combined with light and electron microscopic autoradiography to reveal the location and morphology of the olivary axonal endings. The results show that the superior olivary complex contributes different patterns of synaptic input to the central nucleus of the inferior colliculus. Each projection from the superior olivary complex to the colliculus differs in the number and combinations of endings. Axonal endings from the ipsilateral medial superior olive were exclusively the round (R) type that contain round synaptic vesicles and make asymmetrical synaptic junctions. This morpholo is usually associated with excitatory synapses and neurotransmitters such as glutamate. Endings from medial superior olive terminate densely in the central nucleus. The projection from the contralateral lateral superior olive also terminates primarily as R endings. This projection also includes small numbers of pleomorphic (PL) endings that contain pleomorphic synaptic vesicles and usually make symmetrical synaptic junctions. The PL morpholo is associated with inhibitory synapses and transmitters such as gamma-aminobutyric acid and glycine. All endings from the contralateral lateral superior olive terminate much less densely than endings from the medial olive. In contrast, the projection from the ipsilateral lateral superior olive contributes both R and PL endings in roughly equal proportions. These ipsilateral afferents are heterogeneous in density and can terminate in lower or higher concentrations than endings from the contralateral side. These data show that the superior olive is a major contributor to the synaptic organization of the centr nucleus of the inferior colliculus. The ipsilateral projections of the medial and lateral superior olive may produce higher concentrations of R endings than other inputs to the central nucleus. Such endings may participate in excitatory synapses. The highest concentra tions of PL endings come from the ipsilateral lateral superior olive. In combination with inputs from the contralateral dorsal nucleus of the lateral lemniscus, PL endings from the superior olive may participate in many inhibitory synapses found in the central nucleus. These different patterns of synaptic input from the superior olivary complex will influence how binaural information is transmitted to the inferior colliculus. © 1995 Wiley-Liss, Inc.     

Ultrastructural studies of the primate parabigeminal nucleus: electron microscopic autoradiographic analysis of the tectoparabigeminal projection in Galago crassicaudatus

The normal ultrastructure of the parabigeminal nucleus and the morphology and synaptic relationships of tectoparabigeminal terminals have been examined. Five different morphological types of terminals have been observed within the parabigeminal nucleus. Three of these profiles contain round vesicles and make asymmetrical synapses, while two contain pleomorphic vesicles and make symmetrical synapses. Electron microscopic autoradiographic data indicate that labeled tectoparabigeminal terminals represent only one of the three profiles containing round vesicles. Such terminals are primarily presynaptic to dendritic shafts, and several labeled profiles have been observed presynaptic to the same dendrite.     

Localization of glutamic-acid-decarboxylase-immunoreactive axon terminals in the inferior olive of the rat, with special emphasis on anatomical relations between GABAergic synapses and dendrodendritic gap junctions

Immunocytochemical and electron microscopic methods were used to examine the GABAergic innervation of the inferior olivary nucleus in adult rats. This neuronal system was visualized with an antibody against glutamic acid decarboxylase (GAD, EC 4.1.1.15), the GABA-synthesizing enzyme. A GAD-positive reaction product was encountered only in short segments of preterminal axons and in axon terminals. Their relative number per unit area of neuropil was very similar in all olivary subnuclei. Despite this homogeneity in density, obvious intraregional differences existed. Some regions were strongly immunoreactive (the "c" subgroup, the beta nucleus, and the mediolateral outgrowth of the medial accessory olive), whereas others were weakly labeled (the dorsomedial cell column and the central zones of the medial accessory and principal olives). The strongly immunoreactive areas contained the largest and most intensively labeled axon terminals. Areas of weak labeling were filled with small, weakly immunoreactive nerve terminals. Thus, variations in size and in intensity of labeling create a specific pattern of GABA innervation, revealed by an almost continuous gradient between the above-mentioned extremes. The GAD-positive axon terminals established conventional synapses with dendrites (94% of the samples) or with cell bodies (6%). The vast majority of these synapses were type II (84%) and only a small proportion formed type I synaptic contacts (16%), regardless of the nature of the postsynaptic element. Immunoreactive terminals were also involved in the complex synaptic arrangements--the glomeruli, which characterize the olivary neuropil. Within these formations, olivary neurons were electrotonically coupled through dendrodendritic gap junctions. There was a constant association between GAD-positive axon terminals and small dendritic appendages linked by gap junctions. This association was revealed not only by the systematic presence of immunolabeled terminals directly apposed to the dendritic appendages but, more importantly, by the frequent presence of type II synapses straddling both elements. These synapses were in close proximity to the low-resistance pathways represented by the gap junctions. The strategic location of these GABA synapses is discussed in relation to recent findings indicating the possibility of a synaptic modulation of the electrical coupling: the release of GABA, by increasing nonjunctional membrane conductance, could shunt the coupling between olivary neurons. The functional decoupling of selected gap junctions would be responsible for the spatial organization of the olivary electrotonic coupling.     

Ultrastructure and synaptology of the nucleus ambiguus in the rat: The compact formation

The fine structure of the esophagomotor compact formation of the nucleus ambiguus was studied. Esophageal motoneurons are atypical in that they have extensive direct somato somatic and somato-dendritic appositions without intervening glial processes. A unique feature is the presence of finger- and leaf-like somatic protrusions which partially wrap longitudinally oriented dendrites and occasionally, small groups of dendrites and axons. The neuropil contains many longitudinally oriented, small-diameter dendrites of relatively uniform size (1.1 ± 0.4 S. D. μm in diameter). Motoneuronal somatic profiles have 0–5 synapses per profile which represents a synaptic density of 10.6 synapses per soma. Axodendritic synapses measure 0.5 × 0.7 μm in the transverse plane and are up to 3.0 μm long in the sagittal plane. Many axon terminals contact both a soma and dendrite in close apposition. Most axon terminals (>90%) contain round vesicles and form asymmetric junctions with somata and dendrites. Axon terminal degeneration after electrolytic lesions and labelling after injections of wheat germ agglutinin-horseradish peroxidase in the nucleus of the tractus solitarius show that afferent connections to the compact formation form axodendritic synapses. The ultrastructure and synaptology of esophageal motoneurons is characterized by the close apposition of somata and dendrites (somatic-dendritic bundling), and the longitudinal orientation of dendrites (dendritic bundling), axons and axon terminals in the neuropil. These features may be important morphological substrates for synchronization and coordination of esophageal motoneuronal activity and esophageal peristalsis. © 1995 Wiley-Liss, Inc.     

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.     

Ultrastructure and synaptology of the nucleus ambiguus in the rat: The semicompact and loose formations

The fine structure of the pharyngomotor semicompact and laryngomotor loose format formations of the rat nucleus ambiguus was studied in single and serial sections by means of light and electron microscopy. Motoneurons and their dendrites were identified after retrograde labelling by injections of neuroanatomical tracers into pharyngeal and laryngeal muscles or nerves. Pharyngeal motoneurons measured 39 × 29 μm and had 2–25 axosomatic synapses per somatic profile, representing an estimated average of 1S2 synapses per soma. Laryngeal motoneurons measured 42 × 30 μm with 6–33 synapses per profile, or an average of 339 synapses per soma. In both subdivisions, axon terminals that contained round vesicles and formed asymmetric junctions and terminals that contained pleomorphic vesicles and formed symmetric junctions were distributed in approximately equal proportions on somata and dendrites, forming over 90% of the synapse population. A small percentage (2–8%) of synapses had a subsurface cistern situated below the axon terminal (C type). Small, atypical motoneurons measuring 15 × 5 μm with an invaginated nucleus were also present in both subdivisions. The ultrastructure and synaptology of pharyngeal and laryngeal motoneurons are characterized by similarities to those of spinal motoneurons and by their relatively large numbers of axosomatic synapses in comparison to esophageal motoneurons of the compact formation of the nucleus ambiguus. © 1996 Wiley-Liss, Inc.     

Neuronal and synaptic composition of the mediodorsal thalamic nucleus in the rat: a light and electron microscopic Golgi study.

The distribution and dendritic domain of neurons in each segment of the mediodorsal thalamic nucleus (MD) have been studied in the rat with the Golgi technique. In addition, a combined Golgi method-electron microscopic (Golgi-EM) study was undertaken to determine the distribution of morphologically distinct synapse types along the dendrites of individual identified neurons in MD. All the subdivisions or "segments" of MD (medial, central, lateral) contained both stellate and fusiform cells. The dendritic domain of both types of cells was predominantly restricted to the same segment of MD that contained the cell body of the neuron. Typical stellate neurons were found near the center of each segment, with radiating dendrites that extended to but not across the boundaries of the segment. Fusiform cells were usually located close to the segmental or nuclear boundaries and tended to have dendrites oriented parallel to those borders; again, the dendrites tended not to extend across borders between segments or at the outer edge of MD. In the medial segment of MD many fusiform cells had especially bipolar dendritic configurations, generally with a dorsoventral orientation. Because no small neurons were identified that might correspond to thalamic interneurons, all the impregnated cells in MD are presumed to be thalamocortical projection neurons. These results indicate that cells and their major dendrites are confined to a single segment of MD, with little dendritic overlap across segmental or nuclear borders. The segments of MD may therefore be considered to be relatively independent subnuclei. The distribution of the four types of synapses previously identified in MD (Kuroda and Price, J. Comp. Neurol., 303:513-533, 1991) was determined along several identified dendrites studied with the Golgi-EM method. Primary dendrites were contacted mostly by large axon terminals, including both large, round vesicle (LR) terminals and large, pleomorphic vesicle (LP) terminals, as well as a few small to medium sized terminals with pleomorphic vesicles (SMP). No small terminals with round vesicles (SR terminals) were observed to make synapses with primary dendrites. Secondary and tertiary dendrites received synapses from all types of axon terminals. Higher order dendrites were contracted predominantly by SR boutons, but they also carried some LR and SMP terminals. In addition, SMP boutons were often found to form symmetric contacts with cell somata.     

Synaptology of the cerebello-olivary pathway. Double labelling with anterograde axonal tracing and GABA immunocytochemistry in the rat

The dentato-olivary projection has been ultrastructurally studied in rats that received a wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injection in the nucleus lateralis. Ultrathin sections containing the inferior olive have been double-labelled with the GABA-immunogold method. About 97% of the WGA-HRP labelled axon terminals are GABA-immunopositive. Most of them belong to a single type consisting of small boutons establishing symmetrical synapses on dendrites. Nevertheless, there is some morphological and neurochemical diversity among the labelled terminals, and particularly, a small contingent are GABA-immunonegative. Of the GABAergic dentato-olivary boutons, 4% occupy a privileged position, with synaptic contacts straddling two dendritic profiles linked by gap junctions. The strategic location of these inhibitory dentato-olivary synapses suggests that they can modulate the electrotonic coupling rate between sets of inferior olivary neurons.     

Synaptic circuitry of physiologically identified W-cells in the cat's dorsal lateral geniculate nucleus

The cat's retinogeniculocortical system is comprised of at least 3 parallel pathways, the W-, X-, and Y-cell pathways. Prior studies, particularly at the level of the lateral geniculate nucleus, have focused on X- and Y-cells. In the present study, we describe the synaptic inputs for 2 geniculate W-cells from the parvocellular C-laminae after these neurons were physiologically identified and intracellularly labeled with HRP. For each of the W-cells, we examined electron micrographs taken from over 500 consecutive thin sections; we reconstructed the entire soma plus roughly 15% of the dendritic arbor and determined the pattern of synaptic inputs to these reconstructed regions of each neuron. In several ways, each W-cell exhibits a similar pattern of synaptic inputs. First, we estimate that each W-cell receives approximately 3000-4000 synaptic contacts, which occur most densely on dendrites 50-150 microns from each soma. Second, axosomatic contacts are extremely rare, and most derive from terminals with flattened or pleomorphic vesicles (F terminals). Third, terminals with round vesicles, large profiles, and pale mitochondria (RLP terminals), which are presumed to be retinal terminals, form only about 2-4% of all synapses onto these W-cells; these synapses occur on proximal dendrites. Fourth, F terminals, which provide roughly 15-20% of all synaptic input to these cells, occupy the same region of proximal dendritic arbor as do the RLP terminals. Fifth, and finally, terminals with round vesicles, small profiles, and dark mitochondria (RSD terminals) provide the majority of synapses along all portions of the dendritic arbor. Compared with geniculate X- and Y-cells of the A-laminae (Wilson et al., 1984), these W-cells are innervated by fewer synapses overall and, in particular, by dramatically fewer synapses from RLP (or retinal) terminals. This paucity of direct retinal input to geniculate W-cells might explain the remarkably poor responsiveness of these neurons to visual stimuli and to electrical activation of the optic chiasm.     

The inferior olivary complex of guinea pig: cytoarchitecture and cellular morphology

The inferior olivary complex (I.O.C.) of the guinea pig can be divided into three primary subdivisions: the principal olive (PO), the medial accessory olive (MAO), and the dorsal accessory olive (DAO). In Nissl-stained preparations, the PO possessed darker staining cells than did the MAO and DAO and was the most densely populated with cells. All neuronal somata in the I.O.C. were oblique-spheroid in profile (mean size: coronal = 18.3 microns, parasagittal = 15.8 microns). Based on Golgi impregnations, it was apparent that inferior olive cells were of two unique radiate-cell types (I and II). Type I neurons had relatively diffuse, sparsely branched dendritic arbors, whereas type II cells had dendrites which were highly branched and massed about the cell body, at times creating complex spirals. Type II cells were further categorized into types IIa and IIb based on geometric variations of the type II dendritic arbors. Indices of branching and tortuosity, together with estimates of dendritic arbor volume, were quite helpful in distinguishing cell types. The cell types were differentially distributed across the subdivisions with type I neurons being encountered in the MAO while type II cells were found in all three subdivisions. Within the neuropil of the I.O.C., three different afferent axonal arbors were identified, as was the presence of dendrites from surrounding reticular formation cells. Neuronal aggregates creating a possible electrical syncytium within the I.O.C. are consistent with the dendroarchitectonics of the cells.     

The anatomical organization of the cerebello-olivary projection in the cat.

The cerebello-olivary pathway in the cat has been examined using orthograde and retrograde neuroanatomical tracing techniques. The orthograde transport of 3H-leucine from injection sites in the deep cerebellar nuclei labeled dentate and interpositus projections to the rostral two-thirds of the contralateral inferior olivary complex. These projections are topographically organized, with the dentate nucleus projecting to the principal olivary nucleus and the posterior and anterior interpositus nuclei projecting to the medial and dorsal accessory olives respectively. Fibers from the ventral half of the dentate nucleus terminate in the lateral bend and ventral lamina of the principal olive, whereas the medial and lateral parts of the dorsal half of the nucleus project to the medial and lateral regions of the dorsal lamina respectively. It is apparent that the more caudal parts of the interpositus nuclei project to areas of the medial and dorsal accessory olives near the caudal end of the principal olivary nucleus, whereas neurons in the more rostral parts of the interpositus nuclei project to the more rostral areas of the accessory olivary nuclei. A connection between the fastigial ncleus and the inferior olive could not be demonstrated. The retrograde transport of horseradish peroxidase (HRP) from injections sites in the inferior olive labeled cells throughout the contralateral dentate and interpositus nuclei. The labeled cells were especially numerous in the ventral parts of the dentate and posterior interpositus nlclei. These HRP-positive neurons were consistently small (10--15 mu) ovoid or spindle-shaped cells, with relatively large nuclei and light-staining Nissl substance. This evidence strongly suggests that the cerebello-olivary pathway originates from a population of small neurons in the dentate and interpositus nuclei and projects to specific, topographically defined areas in the contralateral inferior olive.     

Structure of the piriform cortex of the opossum. II. Fine structure of cell bodies and neuropil

The fine structure of cell bodies and neuropil in the piriform cortex of the opossum has been examined. A close similarity in ultrastructure of many features has been demonstrated between this pylogenetically old cortex in a primitive mammal and the neocortex of higher mammals. Cell bodies of pyramidal cells are very similar to those in the neocortex: The nucleus is pale with a smooth surface, the cytoplasm has a modest number of organelles, and the soma receives a small number of exclusively symmetrical synapses. Semilunar cells, which have apical but no basal den-drites, are very similar to pyramidal cells in ultrastructure of their cell bodies. Two populations of neurons with nonpyramidal ultrastructural features have been distinguished: (1) cells in layer III that closely resemble the well-known large multipolar cells in neocortex by virtue of a large number of symmetrical and asymmetrical somatic synapses and long cisterns of rough endoplasmic reticulum (ER); and (2) large cells in layer I with very few somatic synapses, a large number of mitochondria, and short cisterns of rough ER that may correspond to cells with somatic appendages described with the Golgi method. Large numbers of profiles are found in all layers that contain round vesicles and make asymmetrical synapses onto dendritic spines, and occasionally, dendritic shafts. Theseprofileshavedistinctly different morphological features in layer Ia, in which olfactory bulb afferents are concentrated, and in layers Ib, II, and III, which contain terminals of association and commis-sural fibers. A smaller number of profiles containing pleomorphic vesicles make symmetrical contacts onto initial segments, dendritic shafts, cell bodies, and occasionally, dendritic spines. Most dendritic spines in all layers are small to medium in size (0.3–1.2 μm) and presumably originate from pyramidal cells. In layer Ia, however, large, flattened spines are also present which appear to originate from semi-lunar cells. In layer III, and to a lesser extent other layers, large irregular spines are present that may be branched appendages on dendrites of complex appendage cells (Haberly, 1983).     

Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. I. Substance P immunoreactivity

The ultrastructural localization of substance P-like immunoreactivity (SPLI) in lamina I (marginal zone) and lamina II₀ (outer substantia gelatinosa) of the dorsal horn of the macaque monkey was examined by the indirect antibody peroxidase-antiperoxidase method. SPLI was found in small unmyelinated and finely myelinated axons and a variety of terminal types. The majority of SPLI terminals contained a few to many large granular vesicles (mean diameter 90 nm) in addition to a population of small clear round vesicles. A very few terminals contained mainly small round vesicles. SPLI terminals were presynaptic in axosomatic, axodendritic and axospinous contacts forming, in all but the axosomatic junctions, asymmetrical synapses. Some axosomatic junctions were symmetrical. SPLI terminals also formed the center of glomeruli with unlabeled dendrites and dendritic spines; some of the unlabeled dendrites contained a few small scattered vesicles and large dense-core vesicles. In more complex formations 2 to 4 SPLI terminals were associated with one another and linked by desmosomal contacts. The individual terminals in the complexes or ‘congregate terminals’ were simple large granular vesicle containing terminals (LGV), LGV-central terminals of associated glomeruli, or terminals containing mainly small round vesicles. In the apical region of lamina I an unlabeled terminal was found occasionally in contact with an SPLI terminal, which in turn synapsed onto a dendrite. These contacts have some synaptic characteristics and the SPLI terminal was possibly postsynaptic. Most of the types of SPLI terminals resemble closely terminal types shown to be of primary afferent origin. These terminals which make direct contact with dorsal horn dendrites may be the morphological substrate for postsynaptic excitation of dorsal horn neurons by substance P. The contacts of unlabeled terminals with SPLI terminals may represent a morphological substrate by which other neurochemical substances such as enkephalin or serotonin may modulate the substance P effects on dorsal horn neuronal activity.     

An EM-autoradiographic analysis of the projection from cortical areas 17, 18, and 19 to the superior colliculus in the cat

The electron microscopic autoradiographic method was used to identify terminals of axons from cortical areas 17, 18, and 19 in the superficial layers of the superior colliculus. The results show that terminals of area 17 neurons contain round vesicles and made asymmetrical synaptic contacts predominantly onto one or more dendrites or dendritic appendages. Some profiles postsynaptic to labeled terminals contain vesicles and presumably are involved in serial synaptic arrangements. Terminals of area 18 and 19 neurons in the superficial collicular layers appear to comprise two populations, one similar in most respects to area 17 terminals, containing round vesicles and making asymmetrical contacts. The other contains pleomorphic vesicles and makes symmetrical contacts upon dendrites and dendritic appendages. These terminals rarely contact more than one postsynaptic profile, and rarely do the postsynaptic profiles contain vesicles. The two populations of area 18 and 19 terminals containing round and pleomorphic vesicles, respectively, are present in the ratio of approximately 3:1, although this ratio varies throughout the sublaminae of the superficial collicular layers. The presence of two distinct types of cortical terminals in the colliculus suggests that cortical modulation of collicular processing is more complex than was previously conceived.