Axodendritic Synaptic Contacts Research Articles

Calcitonin gene-related peptide- and substance P-containing primary afferent fibers in the dorsal column of the rat

The dorsal column and its nuclei exhibit a considerable number of fibers containing neuropeptides, such as calcitonin gene-related peptide (CGRP) and substance P (SP), whose origins and functional roles are as yet unknown. The present study attempts to determine the origin and nature of these fibers by means of immunohistochemistry combined with several experimental manipulations. A similar study was done on scattered substance P (SP) fibers whose presence was confirmed in this study. Transection of the upper cervical cord of rats resulted in an accumulation of CGRP, sometimes with SP also, in the caudal aspect of the lesion, thus indicating the presence of peptide-containing ascending fibers. Hemitransection of the dorsal column at the level of C 2–3 caused reduction of CGRP-containing fibers in the dorsal column and its nuclei on the operated side. Electron microscopic observation of the nucleus gracilis revealed that CGRP-like immunoreactive terminals made direct axodendritic synaptic contacts. Medium- to large-sized neurons in the dorsal root ganglia were labeled with Fast blue dye which was injected into the dorsal column nuclei. These included medium- to large-sized neurons exhibiting immunoreactivity to CGRP-like substances, and neurons of a medium size which were immunoreactive to SP-like compounds. The incidence of the former was higher at the thoracic level than at the cervical and lumbar levels, while that of the latter was very low. Electron microscopic observation of CGRP-containing fibers in the cervical region of the dorsal column revealed that 88% of these fibers were unmyelinated and the remainder were thinly myelinated. These results indicate that medium- to large-sized CGRP- (and SP)-containing neurons in the dorsal root ganglia give rise to fine fibers into the dorsal column and may terminate in the dorsal column nuclei.

A survey of the cytology and synaptic organization of the insular trigeminal-cuneatus lateralis nucleus in the rat, including an identification of spinal afferent inputs.

The cytology and synaptic organization of the insular trigeminal-cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.

Phenylethanolamine n-methyltransferase immunoreactive terminals synapse on adrenal preganglionic neurons in the rat spinal cord

Adrenergic neurons in the C1 region in the ventrolateral medulla oblongata send descending axons into spinal cord which terminate in thoracic and upper lumbar segments, overlapping the distribution of sympathetic preganglionic neurons. The present study was undertaken to determine whether adrenergic fibers synapse directly on preganglionic neurons which innervate the adrenal medulla and to examine the ultrastructure of these fibers during development. The ultrastructure and synaptology of adrenergic axons in the intennediolateral nucleus of mid-thoracic spinal cord were studied in 7-, 9-, 24-, 30-, 60-, and 90-day-old rats using immunocytochemical staining for phenylethanolamine N-methyltransferase, the epinephrine-synthesizing enzyme. Phenylethanolamine N-methyltransferase-immunoreactivity was observed in the cytoplasm of unmyelinated axonal varicosities and intervaricose segments in the neuropil of intemediolateral nucleus. Phenylethanolamine N-methyltransferase-immunoreactive synaptic boutons were filled with spherical electron-lucent vesicles and occasional larger dense-core vesicles. These boutons were observed to form symmetrical synaptic contacts with dendritic processes at all ages examined. Asymmetrical synapses on dendrites were also observed in adult rats. Axosomatic synaptic contacts were frequently observed in immature rats, but were never observed in adult rats. To determine whether adrenergic axons synapse on preganglionic neurons which project to the adrenal medulla, adrenal preganglionic neurons were retrogradely labeled with horseradish peroxidase and adrenergic axons were stained for phenylethanolamine N-methyltransferase-immunoreactivity. In young rats, phenylethanolamine N-methyltransferase-immunoreactive boutons were observed to form symmetrical axosomatic and axodendritic synaptic contacts with adrenal preganglionic neurons in intermediolateral nucleus. These contacts had already formed by postnatal day 7, the youngest age studied. In constrast, it was not possible to verify that adrenal preganglionic neurons receive adrenergic innervation in adult rats, since phenylethanolamine N-methyltransferase-immunoreactive boutons were only observed in contact with small diameter dendrites that were not retrogradely labeled by horseradish peroxidase. These studies demonstrate that adrenal preganglionic neurons receive adrenergic synapses prior to the first postnatal week. The initial synapses which form on preganglionic somata and proximal dendrites appear to reorganize late in development. It is suggested that these become more distally located as the dendritic tree matures. More generally, these observations suggest that adrenergic bulbospinal neurons are involved in central regulation of adrenal development and function.

Electron microscopical localization of guanylate cyclase activity in the neocortex of the guinea pig

The localization of the guanylate cyclase (GC) activity has been established in the neocortex of adult guinea pigs by means of electron microscopical histochemistry [the DMSO-method of Fujimoto et al. (1981)]. Reaction product was deposited within a population of large- and medium-sized cortical neurons as well as in the cytoplasm of a part of the dendrites of variable size and in the cytoplasm and the nuclear membrane of a number of protoplasmic astrocytes. In the perikarya of the positive neurons, the reaction precipitate was mainly located within the cisterns of the rough endoplasmic reticulum and on the nuclear membrane. In the dendrites, the reaction product was usually distributed in close contact with microtubules, microfilaments, and beneath the postsynaptic membranes of a number of axodendritic synaptic contacts. The axons and all presynaptic boutons were negative. Thus, the localization of the GC could be determined as exclusively postsynaptic. The results obtained support the view for the probable participation of cyclic GMP in the cholinergic, glutaminergic or GABAergic, or peptidergic transmitter mechanisms in the central nervous system.

Ultrastructure of stratum lacunosum-moleculare interneurons of hippocampal CA1 region.

Intracellular recordings were obtained from nonpyramidal neurons (interneurons) in stratum lacunosum-moleculare (L-M) of the CA1 region of guinea pig hippocampal slices. These interneurons had response characteristics that distinguish them from pyramidal cells and other interneuron types: the L-M neurons had relatively broad action potentials with large spike afterhyperpolarizations, high input resistance and little spike-firing adaptation, and low spontaneous activity. Lucifer Yellow (LY) and horseradish peroxidase (HRP) were injected intracellularly into physiologically identified L-M interneurons, and the cells were characterized morphologically using light and electron microscopy. L-M somata were fusiform-shaped (15 x 25 micron), had multiple processes, and were located at the border between stratum (str.) lacunosum-moleculare and str. radiatum. L-M dendrites coursed through str. lacunosum-moleculare and projected into str. radiatum. L-M axons made axodendritic synaptic contacts primarily in str. lacunosum-moleculare and str. radiatum, but also in str. moleculare of the dentate gyrus. These axodendritic synaptic contacts were made onto spiny dendritic processes (presumably pyramidal cell and granule cell dendrites) and onto aspinous dendrites (presumably interneuron dendrites), and appeared to be of the symmetric type (type 2), characteristic of inhibitory synapses. In separate groups of animals, selective lesions were made of afferents to the CA1 and dentate regions of hippocampus, and subsequent degeneration of contacts and L-M interneuron somata and dendrites was examined at the ultrastructural level. Fibers originating from contralateral and ipsilateral CA3 region, and from ipsilateral entorhinal cortex, were found to make synaptic contact onto presumed L-M interneurons. Degenerating terminals appeared to be of the asymmetric type (type 1), characteristic of excitatory synapses. These morphological data are consistent with electrophysiological results showing that L-M interneurons can mediate feedforward inhibition of CA1 pyramidal cells.

The GABAergic neurons and axon terminals in the lateralis medialis-suprageniculate nuclear complex of the cat: GABA-immunocytochemical and WGA-HRP studies by light and electron microscopy.

In order to get more detailed information on the neural circuit of the lateralis medialis-suprageniculate nuclear (LM-Sg) complex of the cat, the GABAergic innervation of this complex was studied by GABA immunohistochemical techniques. Small immunoreactive cells were found throughout the LM-Sg complex. On the basis of their ultrastructural features, these GABAergic cells were identified as Golgi type II interneurons. The neuropil of this nucleus displayed a conspicuous granular immunoreactivity. Ultrastructurally, the immunoreactive neural profiles in the neuropil were identified as the presynaptic dendrites of interneurons, myelinated axons, or axon terminals. The GABAergic dendritic profiles, containing pleomorphic synaptic vesicles, were involved in synaptic glomeruli. Additionally, GABAergic axon terminals containing pleomorphic synaptic vesicles formed symmetric axodendritic synaptic contacts mainly in the extraglomerular neuropil. They appeared to correspond to either axon terminals from the thalamic reticular nucleus (TRN) or the axon terminals of interneurons. The projections from the TRN to the LM-Sg complex were studied by using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Following injection of WGA-HRP into the LM-Sg complex, a number of retrogradely labeled cells were observed in the TRN. The connections between the TRN and the LM-Sg complex appeared to be topographically organized, the dorsal TRN being connected mainly with the dorsomedial portion of the LM-Sg complex, and the ventral TRN being connected chiefly with the ventrolateral portion of the LM-Sg complex. Following injection of the tracer into the TRN, ultrastructural examination of anterograde labeling in the LM-Sg complex revealed that labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts mainly with small to medium-sized dendrites. The labeled terminals were not involved in synaptic glomeruli. The present results provide anatomic support for the contention that the projection cells of the LM-Sg complex may be inhibited by both the TRN axons and interneurons, probably through the mediation of GABA.

Immunocytochemical localization of substance P in the spinal trigeminal nucleus of the rat: a light and electron microscopic study.

The neuropeptide substance P is a transmitter candidate for certain primary afferent fibers which terminate in the substantia gelatinosa. In this study the light and electron microscopic localization of substance P in the substantia gelatinosa of the spinal trigeminal nucleus of the rat has been studied using immunocytochemical procedures. Substance P immunoreactive fibers were observed mainly in lamina I and outer lamina II. Ultrastructural analysis revealed immunoreactivity in unmyelinated fibers and in axon terminals which contained agranular spherical vesicles and large dense-cored vesicles and which made predominantly simple asymmetric axodendritic synaptic contacts. Immunoreactive terminals only rarely formed the central terminal of synaptic glomeruli and in only one example was a stained terminal possibly postsynaptic to an unstained terminal. The majority of synapses were onto small dendrites in outer lamina II and in some cases these dendrites were themselves presynaptic to other dendrites. Immunoreactive terminals also synapsed with the soma and proximal dendrites of large neurons on the border of laminae I and II. The results show that there are at least two distinct targets for substance P immunoreactive terminals in the substantia gelatinosa, namely the large lamina I neurons and lamina II probable interneurons. Some of the former may be projection neurons while some of the latter may correspond to the inhibitory islet cells described by Gobel and colleagues in the cat. In addition the results indicate that few substance P immunoreactive terminals receive axoaxonic synapses and emphasize instead the role of postsynaptic interactions. In particular the results suggest several sites at which substance P might interact postsynaptically with the neuropeptide enkephalin.

Synaptology of three peptidergic neuron types in the central nucleus of the rat amygdala

The synaptology of neurotensin (NT)-, somatostatin (SS)- and vasoactive intestinal polypeptide (VIP)-immunoreactive neurons was studied in the central nucleus of the rat amygdala (CNA). Three types of axon terminals formed synaptic contacts with peptide-immunoreactive neurons in the CNA: Type A terminals containing many round or oval vesicles; Type B terminals containing many pleomorphic vesicles; and Type C terminals containing fewer, pleomorphic vesicles. Peptide-immunoreactive terminals were type A. All three types of terminals formed symmetrical axosomatic and asymmetrical axodendritic contacts. However, type B and peptide-immunoreactive terminals frequently formed symmetrical axodendritic synaptic contacts. VIP-immunoreactive terminals also formed asymmetrical axodendritic contacts. SS- and NT-immunoreactive terminals commonly formed symmetrical contacts on SS- and NT-immunoreactive cell bodies, respectively. VIP-immunoreactive axon terminals were postsynaptic to nonreactive terminals. Type B terminals appeared more frequently on VIP neurons than on NT or SS neurons.

The magnocellular and parvocellular paraventricular nucleus of rat: Intrinsic organization

The magnocellular and paravocellular regions of the rat hypothalamic paraventricular nucleus (PVN) were examined in several hundred brains. Converging qualitative and quantitative anatomical methods, including Golgi impregnations, Nissl stains, silver stains, and immunocytochemistry were used to study the intrinsic organization of the PVN with light, scanning, and transmission electron microscopy. A computer-assisted quantitative analysis of dendritic branching patterns was used to examine total dendritic length, center of mass, orientation of dendritic tree, and several other parameters of dendritic organization and revealed statistically significant differences between cells in the lateral and posterolateral magnocellular and medial parvocellular areas of PVN. Electron microscopy, Golgi impregnation, and neurophysin immunohistochemistry showed that dendrites of posterolateral cells were generally oriented perpendicular to the third ventricle; dendrites of cells in the lateral PVN usually projected medially from the perikaryon. Cells in the medial zone of PVN had dendritic trees which often paralleled the third ventricle. Large numbers of axons entered and left PVN ventrally near the midline and laterally in the area of the posterolateral PVN; axons generally were oriented parallel to the mean major axis of dendritic trees in these areas. Ultrastructural examination of serial thin sections showed a peculiar astroglia multiple lamellar isolation of axodendritic synaptic contacts. Intrinsic axons commonly arose from parvocellular but not from magnocellular neurons and contacted dendrites of both medial parvocellular and more lateral magnocellular neurons. Synapses were found on shafts and spines of dendrites, on perikarya and somatic appendages, and invaginated into the soma. Both dendrites axons with large neurosecretory vesicles and immunostained with neurophysin antiserum were found postsynaptic to other axons. Presynaptic neurosecretory axons were not found within the PVN. A semiquantitative analysis of catecholamine axons identified with the glyoxylic acid method and fibers immunoreactive with ACTH and Substance P antisera indicated that the parvocellular region of PVN received ggreater innervation than the lateral magnocellular area; similarly, a reater density of stained fibers was found in the medial parvocellular PVN region with Golgi impregnations and silver stains. With a stereological analysis of 1-micrometer plastic sections, the parvocellular area had a significantly greater neuropil to cell volume ration, with cells accounting for 48 +/- 9% in the lateral magnocellular zone, but only for 26 +/- 7% in the parvocellular area. A quantitative analysis of vasculature from thin sections showed that the PVN had 3.3 times more blood vessels, and 3.6 times more lumen perimeter than a control area ventrolateral to PVN; an interesting finding here was that the medial parvocellular PVN had a high degree of vascularity, not significantly different from the lateral magnocellular zone...

Electron microscopic observations of axon collateral synaptic endings of cat oculomotor motoneurons stained by intracellular injection of horseradish peroxidase

Motoneurons in the cat oculomotor nucleus have been identified electrophysiologically and stained by intracellular injection of horseradish peroxidase. Axon collateral arborizations with preterminal and terminal boutons identified by light microscopy corresponded to synaptic endings observed by electron microscopy. Despite variations in size and shape, synaptic endings showed similar ultrastructural features and established asymmetrical predominantly axodendritic synaptic contacts usually characterized by the presence of subjunctional dense bodies underlying the postsynaptic membrane densification.

Immunocytochemical localization of vasoactive intestinal polypeptide- containing cells and processes in the suprachiasmatic nucleus of the rat: light and electron microscopic analysis

The distribution of vasoactive intestinal polypeptide-like (VIP) immunoreactivity in neurons and processes within the suprachiasmatic nuclei (SCN) of the rat was investigated with light and electron microscopic immunocytochemical techniques. These studies utilized well characterized antisera directed to synthetic vasoactive intestinal polypeptide. Specificity was established by absorption of the antisera with synthetic vasoactive intestinal polypeptide. Neurons and dendrites exhibiting specific VIP immunoreactivity are concentrated in the ventral half of the nucleus, with the greatest concentration of immunoreactive perikarya occurring in the ventral SCN immediately adjacent to and within the optic chiasm (OC). Thin varicose axons containing VIP immunoreactivity are present throughout the SCN. A large number of immunoreactive axons leave the dorsal aspect of the SCN to reach the periventricular hypothalamic nucleus and continue dorsally to form an extensive plexus along the ventral border of the paraventricular hypothalamic nucleus. Other immunoreactive axons project upon the contralateral SCN via the OC. Ultrastructurally, VIP-containing neurons in the SCN are characterized by a spherical to slightly elongated soma and an invaginated nucleus that fills the majority of the cell body. In the soma, peroxidase reaction product is localized on the outer membrane of all cellular organelles. The reaction product of immunoreactive boutons is related primarily to vesicles, and some of these boutons establish axodendritic synaptic contacts in the SCN. The demonstration of VIP-containing neurons in the SCN provides further evidence that this nucleus is composed of a heterogeneous population of neurons which form distinct subfields within it.

The pallido-subthalamic projection in rat: Anatomical and biochemical studies

Horseradish peroxidase injected into the rat globus pallidus was transported retrogradely to subthalamic nucleus neuronal cell bodies and anterogradely to axon terminals in the subthalamic nucleus. Electron microscopic observations revealed that the labeled axon terminals made symmetrical axosomatic and axo-dendritic synaptic contacts with labeled subthalamic nucleus perikarya and dendrites. Injection of kainic acid in the globus pallidus several days prior to the horseradish peroxidase injection abolished the anterograde but not the retrograde transport of the tracer. This suggested the anterograde labeling observed in the subthalamic nucleus originated from neuronal cell bodies in the globus pallidus. Kainic acid lesions identical to those employed in the above anatomical studies resulted in a loss of neuronal cell bodies throughout the globus pallidus and caused a drop in glutamic acid decar☐ylase and choline acetyltransferase levels in the globus pallidus. Levels of these two enzymes were not changed in the subthalamic nucleus after the globus pallidus kainic acid lesions, but both showed small, statistically significant decreases in the substantia nigra. It was concluded that there is a massive pathway from the globus pallidus to the subthalamic nucleus, which terminates on subthalamic nucleus neurons projecting back to the globus pallidus. Neither γ-aminobutyric acid nor acetylcholine is the major neurotransmitter in the massive pallido-subthalamic pathway.

Mode of termination of afferents from the thalamic reticular nucleus in the dorsal lateral geniculate nucleus of the rat

The terminals of axons projecting to the dorsal lateral geniculate nucleus from the thalamic reticular nucleus were identified by electron microscopy 8–24 h after placing small lesions in the ipsilateral reticular nucleus. The terminals contained flattened synaptic vesicles and made Gray type II axo-dendritic synaptic contacts with geniculate neurons. Their identification as F-axons accords well with physiological evidence for a powerful monosynaptic inhibitory input to geniculocortical projection cells from reticular nucleus neurons.

Ultrastructure of the organum vasculosum of the lamina terminalis of the Japanese quail, Coturnix coturnix japonica.

The structure of the organum vasculosum of the lamina terminalis (OVLT) of the Japanese quail has been studied by light and electron microscopy. The OVLT has a palisade arrangement. It forms a part of the terminal plate extending from the optic chiasma to the anterior commissure and is characterized by a special vascular arrangement. The organ consists of ependymal, internal and external zones. The ventricular surface of the organ is covered by non-ciliated ependymal cells characterized by the presence of raspberry-like ventricular protrusions. The ependymal zone is composed of two types of ependymal cells, one being a large, cuboidal cell with columinous cytoplasmic protrusions suggesting secretion into ventricle, and the other a slender tanycyte with long balar plexus of the external surface. The internal zone contains two types of secretory neurons, parvocellular neurons containing a few dense-cored granules 1,000 A in diameter, and mediocellular neurons containing in their perikarya many larger granules 1,300-1,500 A in diameter and well developed granular endoplasmic reticulum and Golgi apparatus. The granular formations are usually observed in the Golgi area of both types of cells, thus indicating of their secretory activities. There are three types of nerve terminals in the neuropil: (1) nerve endings containing spherical, dense-cored granules 800 A in diameter, which display axosomatic synapses with perikarya of the neurons, (2) nerve endings containing dense-cored granules 1,000 A in diameter and clear vesicles 400 A in diameter, and (3) nerve endings containing dense granules 1,300 A in diameter and clear vesicles. Types 2 and 3 do not display axo-xomatic synapses but often show axo-dendritic synaptic contacts with dendrites in the neuropil; thus they are probably axons originating from the parvocellular and mediocellular neurons of this organ. The functional significance of the neurons and axons of the OVLT is discussed and it is suggested that two kinds of neurohormones may be secreted from the OVLT of the Japanese quail.

Decrease in the number of synapses in the senescent brain: A quantitative electron microscopic analysis of the dentate gyrus molecular layer in the rat

Axo-dendritic synapses were counted in electron micrographs taken from the middle third of the dentate gyrus molecular layer of young adult and senescent Fischer 344 rats. A significant decrease in the number adult synapses was found in senescent animals relative to young ones. This loss of synapses, which involved all the morphological varieties of axo-dendritic synaptic contacts in the dentate gyrus molecylar layer, appeared to be unrelated to changes in dimensions of synapses, tissue volume or number of postsynaptic granule cells. It is proposed that the age-related loss of synaptic contacts might be attributed to a reduced capacity of senescent brains for synaptic regeneration and remodelling.

Synaptic reorganization in the degenerating lateral geniculate nucleus of the rabbit.

AbstractAn investigation of synaptic plasticity in the lateral geniculate nucleus has been carried out in rabbits surviving one day to one year following ipsilateral visual cortex removal. There is extensive retrograde degeneration of LGN neurons, and the altered nucleus thus provides a model for examining reorganization of optic tract axons which have been deprived of their normal postsynaptic membrane.The distribution of synaptic contacts in normal LGN was quantitatively determined. In the animals surviving 1–14 days after cortex ablation, there was extensive cell death and loss of the axodendritic synapses of cortical origin, which constitute the most numerous synaptic type in normal LGN.In animals surviving 4–12 months, there were few remaining neurons and dendrites. The fine structure of the nucleus was characterized by a shift from axodendritic synaptic contacts, as in the normal, to a ten–fold increase in axoaxonal synapses, which became the dominant population. The contribution of optic axons to this synaptic population was determined by contralateral retinal removal. It was found that the synaptic axoaxonal organization in the altered LGN follows the rules of normal organization, in that optic afferents are invariably the presynaptic component in round to flat vesicle axoaxonal synapses.The LGN's undergoing chronic retrograde degeneration shrank to about twothirds the size of the normal nucleus. There was no evidence of reduction in the length of optic tract axons within the nucleus.The evidence indicates that there are new synaptic contacts formed between surviving axons in the degenerating LGN, as a consequence of the loss of postsynaptic dendritic membrane. The new synapse formation is guided by the normal rules of axoaxonal organization, indicating a maintenance of recognition of appropriate membrane for synaptic contact in the reordered LGN. It cannot be stated whether sprouting of axons accompanies the formation of new synapses or whether the new synaptic contacts are exclusively between preexisting synaptic knobs in LGN.

Fine structural aspects of the synaptic organization of spinal trigeminal nucleus (pars interpolaris) of the cat

Pars interpolaris of the spinal trigeminal nuclear complex has been studied electron microscopically in normal adult cats and after total ipsilateral retrogasserian rhizotomy. Small cell body profiles are numerous and have few axo-somatic synapses. Larger cell body profiles are few in number but have several axo-somatic synapses. Axo-dendritic synaptic contacts are numerous and axo-axonic serial synapses are seen at all levels. Presynaptic profiles containing predominantly round vesicles are usually characterized by an asymmetrical postynaptic specialization and those with many flattened vesicles usually have a symmetrical specialization. A small proportion of profiles with flattened vesicles are associated with an asymmetrical specialization. Axo-axonic synapses have flattened vesicle profiles presynaptic to round vesicle ones. Degeneration of axons and presynaptic processes are seen at 30 h survival and become rare after 7 days. Most degenerated processes are electron-dense and associated with an asymmetrical specialization. In serial synapses the round vesicle profile degenerates. After 3 days many degenerated endings are replaced by glial and other processes which are applied to the denervated postsynaptic specialization. After 7 days survival some unaltered flattened vesicle profiles appear to migrate into the denervated postsynaptic sites. Later survivals show such endings applied to both symmetrical and asymmetrical specializations on the same dendritic profile. Other origins for unaltered presynaptic processes are considered. A correlation between denervated postsynaptic sites and neuronal hyperactivity is discussed, as is the apparent synaptic reorganization or re-innervation of these denervated sites.

Degeneration in the parvocellular portion of the lateral geniculate nucleus of the Cebus monkey. A light and electron microscope study.

The synaptology of the Cebus lateral geniculate nucleus (LGN) was studied after varying (3–15 days) periods of survival following unilateral and bilateral eye enucleations. Part of the material was processed with the Glees and Nauta techniques for light microscopy while the rest was processed for electron microscope observation. The study revealed a variety of degenerated terminals in the parvocellular portion of the LGN and allowed the differentiation of the retinal from the extraretinal terminals. The most frequent synaptic type of retinal origin is a glomerular large central terminal (up to 20 μ long) which makes axodendritic and axoaxonic synaptic contacts with geniculate dendrites and peripheral small terminals. Simple axodendritic and axosomatic terminals of retinal and extraretinal origin were also found. The early changes affecting the geniculate neurons and astrocytes during the degenerative process are described.

The location and termination of tectospinal fibers in the cat

The location, caudal extent, sites and mode of termination of tectospinal fibers have been studied with silver impregnation methods in cats following lesions in the superior and inferior colliculi. Only fibers from the former project to the spinal cord. The tectospinal fibers are completely crossed and do not descend below the cervical region, most of them terminating in the upper four segments. The sites of termination have been mapped with reference to the laminae of the spinal gray matter described by Rexed. The tectospinal fibers end mainly in the central and lateral parts of lamina VII, some in the lateral part of lamina VI and in lamina VIII. No fibers have been seen ending on motoneurons. The tectospinal fibers appear to establish axosomatic and axodendritic synaptic contacts with large as well as small neurons within their laminae of termination.