Aspects of the synaptic organization of intrinsic neurons in the dorsal lateral geniculate nucleus. An ultrastructural study of the normal and of the experimentally deafferented nucleus in the rat

On the excitatory post-synaptic potential evoked by stimulation of the optic tract in the rat lateral geniculate nucleus

1. The electrophysiological and pharmacological properties of the excitatory post-synaptic potentials (e.p.s.p.) evoked by electrical stimulation of the optic tract were studied in projection neurones of the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat in vitro. 2. No difference was found in the rise time of e.p.s.p.s. recorded in the dorsal and ventral l.g.n. and in their threshold for action potentials. At membrane potentials more negative than -60 mV, e.p.s.p.s. in the dorsal l.g.n. were always followed by a Ca2+-dependent potential. Its amplitude could easily reach threshold for generating an action potential and thus evoke firing from an e.p.s.p. that was subthreshold at resting potential. No Ca2+ potential was observed to follow e.p.s.p.s. recorded in the ventral l.g.n. 3. At resting potential the excitability of dorsal and ventral cells was unaffected following an initial shock to the optic tract. However, in dorsal neurones, at potentials more negative than -60 mV, the presence of Ca2+ potentials evoked by the e.p.s.p.s. resulted in a period of decreased excitability. 4. Using intrasomatic injection of Cs+ the reversal potential (E) of the e.p.s.p. and of the depolarization produced by glutamate could be measured in the same l.g.n. neurone. They were: Eepsp, -0.9 mV; and Eglut, -3.9 mV. 5. gamma-D-glutamylglycine (DGG), an excitatory amino acid antagonist, reversibly inhibited the e.p.s.p. and depolarization produced by quisqualate and glutamate by a competitive action. The concentration of DGG that produced 50% inhibition (IC50) was 2.7 mM. 6. D-2-amino-5-phosphonovalerate (APV), the potent and selective N-methyl-D-aspartate (NMDA) antagonist, had no effect on the e.p.s.p. both in the presence and absence of Mg2+. The isomers of 2-amino-4-phosphonobutyrate (APB) were inactive or had a non-specific action on the e.p.s.p. 7. No difference could be detected in either the reversal potential or the action of the antagonists between neurones of the dorsal and the ventral l.g.n. 8. These results suggest that Ca2+-dependent potentials play an important role in modulating synaptic efficacy in principal neurones of the dorsal l.g.n. The quisqualate/kainate nature of the optic nerve receptors and the similarity of Eepsp and Eglut constitute strong support in favour of a glutamate-like substance as the transmitter of the optic nerve.

The ventral and dorsal lateral geniculate nucleus of the rat: intracellular recordings in vitro

1. The membrane properties and the electrotonic structure of neurones in the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat were studied using an in vitro slice preparation. 2. Following electrophysiological characterization, horseradish peroxidase (HRP) was injected intrasomatically and the morphological features of impaled cells were characteristic of principal neurones of the rat ventral and dorsal l.g.n. 3. Neurones in the ventral l.g.n. had a higher input resistance but similar membrane time constants (tau o) and resting potentials than cells in the dorsal l.g.n. 4. Using a simple neuronal model, the electrotonic length (L) and the dendritic to somatic conductance ratio (rho) were calculated and found to be similar for cells in both divisions of the l.g.n. The mean value of L (0.7) and rho (1.5) suggest that both groups of neurones are electrotonically compact. 5. The width and after-hyperpolarization of directly evoked action potentials, but not their threshold or their amplitude, were different between cells of the ventral and dorsal l.g.n. 6. At potentials more negative than -55 mV, a slow rising and falling potential could be evoked in each neurone (n = 310) of the dorsal l.g.n. but only in three cells of the ventral l.g.n. (n = 94). The electrophysiological and pharmacological properties of this potential were identical with those of the low-threshold Ca2+-dependent potential observed in other thalamic nuclei. 7. These results indicate that some of the passive and active membrane properties of ventral and dorsal l.g.n. neurones are different. The implications of these findings for the control of the integrative capability and the response of l.g.n. neurones to visual stimulation are discussed.

Ultrastructural organisation of the projection from the superior colliculus to the ventral lateral geniculate nucleus of the rat

Retinorecipient regions of the ventral lateral geniculate nucleus of the thalamus and the superior colliculus of the midbrain are linked by reciprocal axonal projections. In this study we have investigated the ultrastructural characteristics, the distribution, and the postsynaptic targets of the terminals of axons projecting to the ventral lateral geniculate nucleus from the superior colliculus. Horseradish peroxidase was injected into the superior colliculi of adult albino rats, and the Hanker-Yates method was used to visualize anterogradely and retrogradely transported peroxidase in the ventral lateral geniculate nuclei 24 hours following the injection. Labelled terminals were found in the lateral and ventrolateral parts of the external division of the ipsilateral ventral lateral geniculate nucleus. The labelled terminals were confined to areas of simple, nonglomerular neuropil. They were 0.45-1.5 micron in diameter; contained small, dark mitochondria and spherical synaptic vesicles; and established Gray type I (asymmetrical) synaptic contacts with the dendritic shafts, dendritic spines, and occasionally cell bodies of cells with the ultrastructural characteristics of projection cells. A few labelled terminals established synaptic contact with retrogradely labelled cells. Thus, in the rat, the projection from the superior colliculus gives rise to a uniform population of axon terminals in the nonglomerular neuropil of the lateral portion of the ventral lateral geniculate nucleus, which synapse with, and are probably excitatory to, geniculocollicular and other projection cells.

Synaptic organization of the nucleus dorsolateralis anterior thalami in the Japanese quail (Coturnix coturnix japonica)

The nucleus dorsolateralis anterior thalami (DLA) of birds is the homologue of the mammalian dorsal lateral geniculate nucleus. The positions of terminals from the retina and visual Wulst upon identified relay neurons in the DLA were examined in Japanese quail with both light and electron microscopic techniques. Injection of horseradish peroxidase (HRP) into the visual Wulst showed that relay neurons projecting ipsilaterally or contralaterally were located in a rostrolateral subdivision (DLAlr) and in Zones A and B of a lateral subdivision (DLL) of the DLA. Removal of the contralateral eye resulted in dense terminal degeneration in the DLAlr and moderate terminal degeneration in Zones A and B. By contrast, lesions in the visual Wulst produced dense degenerating terminals in Zones A and B of the DLL. The somata and proximal dendrites of relay neurons or terminals from the retina in the DLA were identified electron microscopically following HRP injection into the visual Wulst or optic nerve, respectively. Terminals from the retina contained spherical vesicles, glycogen granules, and mitochondria with widely spaced cristae. Terminals from the retina made synaptic contact with proximal dendrites and somata of HRP-labeled relay neurons. Presynaptic dendrites formed symmetric synaptic contact with dendrites of relay neurons. Synaptic glomeruli were observed in the DLAlr that involved dendrites of relay neurons, terminals from the retina and presynaptic dendrites. Lesions of the visual Wulst resulted in degeneration of small terminals with spherical vesicles. These terminals were not involved in the synaptic glomeruli of the DLA, but made asymmetric contacts with spines of unidentified neurons and with terminals of presynaptic dendrites.

Effects of intraocular tetrodotoxin on the postnatal development of the dorsal lateral geniculate nucleus of the rat: a Golgi analysis

The postnatal development of the rat dorsal lateral geniculate nucleus (dLGn) during tetrodotoxin (TTX)-induced monocular impulse blockade was investigated by quantitative Golgi techniques. Beyond 14 days postnatal (dpn), the effectiveness of TTX was monitored by loss of the pupillary light reflex. By 21 dpn, Golgi analysis indicated that TTX had no effect on the pattern of dendritic branching of class A or class B neurons, although the number of dendritic spinous protrusions was reduced. No evidence of any TTX-induced loss of optic axons or neuronal degeneration in the dLGn was found, despite a 16% decrease in the size of the nucleus, suggesting a reduction in the growth of neuropil. These data indicate that optic impulses are important in mediating the proper growth of postsynaptic specializations in the dLGn during ontogenesis, but that the postnatal development of the dendritic arbor of neurons in the dLGn appears to be independent of retinal impulse activity.

Vesicle-containing dendrites in the nucleus raphe dorsalis of the cat. A serial section electron microscopic analysis

The nucleus raphe dorsalis of the cat was examined by serial section electron microscopy and the presence of vesicle-containing dendrites is reported. Such dendrites were divided into two classes according to their synaptic contact. Dendrites containing round and/or pleomorphic vesicles associated with a clear synaptic specialization which was generally intermediate between a Gray's type I or II were classified as presynaptic dendrites. These presynaptic dendrites were presynaptic to conventional dendrites and dendritic spines. In addition, some profiles containing a sparse population of vesicles which may be dendritic in nature were observed involved in serial synaptic arrangements. A second class of dendrites were characterized by the presence of vesicles which were never found associated with any synaptic membrane specialization. Commonly, the vesicles were densely packed and associated with unusual densities. Serial section analysis of these densities showed that they were not presynaptic dense projections. We suggest that the existence of vesicle-containing dendrites in the nucleus raphe dorsalis of the cat constitute the morphological support for the dendritic release of neurotransmitters.

Morphology of physiologically identified thalamocortical relay neurons in the rat ventrobasal thalamus

The anatomical structure of physiologically identified neurons of the rat ventrobasal thalamus was studied in order to determine if there are morphologically distinct subsets of neurons that correlate with the somatosensory submodalities processed by these cells. Intracellular recordings were used to determine the modality and receptive field of a neuron, after which horseradish peroxidase was iontophoretically injected into the cell, allowing it to be histologically visualized. Computer-assisted measurements of the labeled cells were made to quantitatively analyze the dendritic structure. Cells were divided into physiological groups stimulated by whiskers, glabrous skin, furry skin, noxious stimulation, or joint rotation. Qualitatively, all cells appeared similar, with the same types of branching patterns. Dendritic spines and long, sinuous appendages were found on all distal dendrites. Quantitatively, no statistically significant differences in dendritic structure were found between functionally defined groups with the aid of a number of parameters, including a fitted dendritic ellipse. There was a weak correlation between somal cross-sectional area and receptive field size, suggesting larger cells processed larger receptive fields. In summary, the ventrobasal thalamus of the rat, in contrast to that of higher mammals, appears to contain only one major cell type and to have a very simple neuronal circuitry.

A quantitative investigation of the neuronal composition of the rat dorsal lateral geniculate nucleus using GABA-immunocytochemistry

The proportion of neurons immunoreactive for gamma-aminobutyric acid (GABA), and their rostrocaudal distribution in the dorsal lateral geniculate nucleus of the rat, were determined quantitatively using post-embedding GABA-immunochemistry on semithin resin embedded coronal sections followed by stereological analysis. The mean total volume numerical density of neurons (total number of neurons per mm3) in the dLGN was 67,077 +/- 4412 mm-3 (mean +/- SEM; n = 5), comprising a mean volume numerical density for GABA-immunopositive neurons of 14,584 +/- 1324 mm-3, and a mean volume numerical density of GABA-immunonegative neurons of 52,493 +/- 3419 mm-3, GABA-immunopositive neurons constituted 21.7 +/- 0.5% of the total neuronal composition of the rat dorsal lateral geniculate nucleus. Although no rostrocaudal variation was detected in the total volume numerical density of neurons, the relative proportion of GABA-immunopositive neurons was significantly lower in the caudal segment (18.1 +/- 0.6%) compared with the middle (24.9 +/- 0.9%) and the rostral segments (22.1%). Furthermore, on the basis of somatic size distributions, GABA-immunonegative neurons were seen to be significantly smaller in the caudal segment than in the more anterior two segments. The somatic size of GABA-immunopositive neurons showed no rostrocaudal variation through the dorsal lateral geniculate nucleus. These data provide a morphological correlate for the structural and functional subdivision of the dorsal lateral geniculate nucleus described previously in electrophysiological and morphological studies.

A newly recognized element in the monkey dorsal lateral geniculate nucleus exhibiting both presynaptic and postsynaptic sites

The dorsal lateral geniculate nucleus (LGNd) of four normal monkeys (Macaca mulatta) and of two other such animals with total unilateral ablation of the visual cortices (4-6 days survival) were examined in serial thin sections with the electron microscope. In these materials we have observed a new neuropil component which has the cytologic characteristics of principal cell (P-cell) dendrites, i.e. large and dark mitochondria, smooth endoplasmic cisterns and filamentous, non-synaptic contacts with retinal terminals. In addition, these elements contain large round synaptic vesicles and can be seen forming asymmetric synapses exclusively with presynaptic dendrites belonging to interneurons (I-cells). Occasionally, a reciprocal synapse is formed between the two profiles. The novel elements are postsynaptic to various vesicle-containing profiles, i.e. axonal boutons of presumably retinal and cortical origin, and I-cell presynaptic dendrites. They are found more frequently in the specimens with cortical ablations, although their number is still much lower than that of the other classic components of the neuropil. Measurements made on X 80 000 electron micrographs of spheroid vesicles within presumptive retinal terminals, cortical endings and the new profile described in this report, result in mean diameters of 38.6 nm, 33.3 nm and 44.3 nm, respectively. The differences between the means are statistically significant. Although the profile with large dark mitochondria and large round vesicles may represent a dendrite of a different I-cell type, or a recurrent axon collateral of a P-cell, it appears more probable that it is a presynaptic dendrite of a P-cell. The infrequent but consistent occurrence of these elements suggests that at least some P-cells can develop presynaptic sites on their dendrites, a property which contributes to the synaptic complexity of the LGNd.

The postnatal development of the rat primary visual cortex during optic nerve impulse blockade by intraocular tetrodotoxin: a quantitative electron microscopic analysis

The effect of tetrodotoxin (TTX)-induced monocular impulse blockade on various parameters of synaptogenesis during the first 3 postnatal weeks of the developing rat visual cortex was investigated by quantitative electron microscopy. During the injection period, beyond 14 days postnatal (dpn), the effectiveness of TTX in blocking optic nerve impulses was monitored by loss of the pupillary light reflex. Between 5 and 21 dpn, TTX treatment reduced the number of type I axodendritic synapses by approximately 23%, when compared to sham-injected controls. These reductions were found in layers III, IV, and the superficial region of layer V. Layer IV exhibited the greatest decrease (24%) while layers III and V showed reductions of 20% and 18%, respectively. At 21 dpn, the number of type II axodendritic synapses decreased by 19% in the same layers, but no reductions were found at earlier ages. TTX also reduced the mean number of synaptic vesicles within type I and type II terminals by 27% and 15%, respectively. At 9 dpn, reductions were first found in layers IV and V, but by 21 dpn significant decreases were found in layers II/III, IV and V. TTX had no effect on the length of the postsynaptic density of both synaptic types or on cortical thickness at any age. These data indicate that optic impluses are important mediators of synaptogenesis in the developing visual cortex, the loss of which induces localized and specific synaptic alterations, possibly due to a change in cortical circuitry.

Degeneration of thalamic neurons in "Purkinje cell degeneration" mutant mice. II. Cytology of neuron loss

The cytology of thalamocortical relay neuron degeneration in the ventral medial geniculate nucleus (vMG) of mice homozygous for the autosomal recessive Purkinje cell degeneration (pcd) mutation has been studied by light and electron microscopy. More limited sampling of the submedial and mediodorsal nuclei suggested that cytological alterations in the vMG were typical of all degenerating thalamic nuclei. The number of vMG neurons in pcd mutants was comparable to controls at and prior to postnatal day 40 (P40). By P60 seventy percent, and by P90 approximately 90%, of the original complement of vMG neurons had degenerated in mutant mice. At P30, the general cytological organization of vMG neurons closely resembled that of neurons in littermate (+/+ or +/pcd) controls, but neurons in mutants were distinguished by the presence of small aggregates of fine granules (approximately 9 nm in diameter) that were commonly associated with otherwise normal cisternae of rough endoplasmic reticulum; neither the number nor the size of these granular aggregates increased in older mutants. By P50 cytoplasmic organelles were curiously distributed in more severely affected neurons: large areas of cytoplasm were occupied exclusively by polysomes, while profiles of endoplasmic reticulum and the Golgi apparatus appeared to be reduced. Before frank degenerative changes were apparent (at P50), all classes of synaptic terminals identified in normal mice were found to have made morphologically normal synaptic contacts on mutant vMG neuron dendrites. In contrast to the homologous nuclear complex in the cat, presynaptic dendrites were not apparent in synaptic glomeruli in wild-type or mutant murine vMG. Cytopathological alterations in the neuropil of P50 and older mutants were dominated by degenerating dendritic profiles; there was no evidence that the loss of thalamic neurons in pcd mutants was associated with synaptic agenesis or dysgenesis or the prior or concurrent degeneration of afferent synaptic terminals.

Effects of intraocular tetrodotoxin on dendritic spines in the developing rat visual cortex: a Golgi analysis

The effect of tetrodotoxin (TTX)-induced monocular impulse blockade on the growth of dendritic spines in the developing rat primary visual cortex was analysed by quantitative Golgi techniques. Between 5 and 21 days postnatal (dpn), rats were injected with TTX every 2 days into the right eye to chronically eliminate optic impulses. Effectiveness of TTX was monitored by loss of the pupillary light reflex. At 21 dpn, the number of spines located on the portion of the apical dendrite within layers III, IV and the superficial region of layer V was reduced by approximately 26%. These decreases were found on the apical dendrites of both large and medium sized pyramidal cells. TTX also reduced the number of spines on the proximal portion of oblique dendrites in layer IV by 16%, yet did not change the number of spines on basilar dendrites. No evidence of transneuronal degeneration was seen following long-term TTX treatment. These data indicate that dendritic spine development in the visual cortex is sensitive to the loss of optic impulses and that the decrease in spine population is principally due to a reduction in spine growth.

Synaptic organization of the dorsal lateral geniculate nucleus in the adult hamster. An electron microscope study using degeneration and horseradish peroxidase tracing techniques

The synaptic organization of the alpha sector of the dorsal lateral geniculate nucleus has been examined by electron microscopy in normal adult hamsters and in adult hamsters subjected to unilateral eye enucleation or intravitreal injection of horseradish peroxidase. Two types of neuropil are apparent. Islands of complex neuropil partially enclosed by astrocyte processes (synaptic glomeruli) are surrounded by a sea of simpler non-glomerular neuropil. The latter is dominated by small axon terminals with spherical synaptic vesicles and Gray type 1 axodendritic contacts (SR-boutons) and also contains axon terminals with flattened synaptic vesicles (F-boutons). The glomerular neuropil contains exclusively postsynaptic dendrites and dendritic protrusions of presumptive projection cells; pre- and postsynaptic pleomorphic-vesicle-containing P-boutons (interpreted as appendages of the dendrites of interneurons); large axon terminals containing spherical synaptic vesicles and large pale mitochondria (R-boutons) which were experimentally identified as retinal terminals and which are presynaptic to both projection cell dendrites and P-boutons at Gray type 1 contacts; F-boutons (minority component). F-boutons and P-boutons are presynaptic to both projection cell dendrites and P-boutons and P-boutons are the intermediate elements of various serial synapses including triplet (triadic) synapses. Medium-large terminals with spherical synaptic vesicles and dark mitochondria (RLD-boutons) which were commonly invaginated by dendritic spines of projection cells in small glomerulus-like formations were also identified. The origin of RLD-boutons is unknown but SR-boutons probably derive chiefly from ipsilateral visual cortex and possibly also from superior colliculus, and non-glomerular F-boutons probably originate in the ipsilateral thalamic reticular nucleus. No differences in synaptic organization were found between the part of the nucleus which receives uncrossed retinal input and the part which receives crossed input, nor were differences seen in the size, fine structure or relationships between the terminals of identified crossed and uncrossed retinal axons.

Ascending, descending and local control of neuronal activity in the rat lateral geniculate nucleus

Mechanisms of control for activities of relay neurons (P-cells) in the rat dorsal lateral geniculate nucleus (LGNd) are surveyed with special reference to ascending projection arising from the locus coeruleus (LC), the local projection from the visual portion of the thalamic reticular nucleus (vTRN) and the descending projection from the visual cortex (VC). Noradrenaline released from terminals of LC neurons exerts a facilitatory influence on P-cell activity via alpha-receptors. A recurrent projection of vTRN neurons on P-cells is inhibitory, utilizing GABA as a transmitter. P-cells receive an excitatory input from corticothalamic neurons of VC. However, in many P-cells the corticofugal excitation is counterbalanced by inhibition arising in vTRN neurons which are invariably exited by the collateral branches of the corticogeniculate axons. Thus, LGNd is not a simple relay station, but various modifications of visual information are made in this nucleus.

Normal postnatal development of retinogeniculate axons and terminals and identification of inappropriately-located transient synapses: electron microscope studies of horseradish peroxidase-labelled retinal axons in the hamster

Axons from the eyes reach the dorsal lateral geniculate nucleus of the hamster at birth and both crossed and uncrossed axons spread throughout the nucleus within which they overlap extensively between postnatal days 2-6, before segregating to terminate in different parts of the nucleus by days 8-10 [So, Schneider and Frost (1978) Brain Res. 142, 343-352]. We have labelled retinal axons and their terminations between the day of birth (day 0) and day 6 by injecting one eye with horseradish peroxidase a few hours prior to sacrifice. Labelled profiles were then systematically sought, identified and their position determined, by electron microscope study of large frontal thin sections of both dorsal lateral geniculate nuclei. Labelled crossed and a few labelled uncrossed axons were present at day 0 and became progressively more common over the following few days; appropriately-located labelled uncrossed axons and terminals in the centromedial part of the nucleus (future ipsilateral sector) were relatively less common than labelled crossed axons in the ventrolateral part of the nucleus (part of the future contralateral sector), particularly between days 0 and 3. Synaptic contacts established by such labelled axons were characterized by predominantly electron-lucent spherical presynaptic vesicles and a prominent postsynaptic density. At day 4, labelled uncrossed axons made synaptic contact in the future contralateral sector (which is devoid of uncrossed input after days 8-10) and a few crossed axons made synaptic contacts in the future ipsilateral sector (devoid of crossed input after days 8-10). Such terminals and their synaptic contacts, were identical to appropriately-located ones in the same material. Inappropriately-located terminals were not found in the future contralateral sector at day 6, or in adults. No specialized contacts were observed between inappropriately-located axons or terminals and either other axon terminals or glial cell processes. Thus, during the development of the hamster retinogeniculate projection, inappropriately-located axons establish transient synaptic contacts with geniculate cells, and these contacts are lost as the segregated adult pattern of projections is established.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology of identified relay cells and interneurons in the dorsal lateral geniculate nucleus of the rat

The morphology of neurons in the lateral geniculate nucleus of the rat has been examined in both Golgi impregnated and in horseradish peroxidase (HRP) filled material. Two major classes of neurons are seen in Golgi material which encompass the variety of cells described in previous reports. Cells of one group (class A) are routinely labelled by injections of HRP into the visual cortex or optic radiations. This group also displays some morphological variation which may be related to the presence of parallel information channels in the retino-fugal pathway, but clear subgroups cannot be identified on the basis of morphological criteria alone. Cells of the other group (class B) are not labelled by HRP injections into visual cortex or the optic radiations, and are probably local circuit interneurons.

Neuronal types in the lateral geniculate nucleus of man. A Golgi-pigment study

Nerve cell types of the lateral geniculate body of man were investigated with the use of a transparent Golgi technique that allows study of not only the cell processes but also the pigment deposits. Three types of neurons have been distinguished: Type-I neurons are medium- to large-sized multipolar nerve cells with radiating dendrites. Dendritic excrescences can often be encountered close to the main branching points. Type-I neurons comprise a variety of forms and have a wide range of dendritic features. Since all intermediate forms can be encountered as well, it appears inadequate to subdivide this neuronal type. One pole of the cell body contains numerous large vacuolated lipofuscin granules, which stain weakly with aldehyde fuchsin. Type-II and type-III neurons are small cells with few, sparsely branching and extended dendrites devoid of spines. In Golgi preparations they cannot be distinguished from each other. Pigment preparations reveal that the majority of these cells contains small and intensely stained lipofuscin granules within their cell bodies (type II), whereas a small number of them remains devoid of any pigment (type III). Intermediate forms do not occur.

Fine structural morphology of identified X- and Y-cells in the cat's lateral geniculate nucleus

Four physiologically identified neurons in the A laminae of the cat's dorsal lateral geniculate nucleus were filled with horseradish peroxidase and studied using the electron microscope. Two were X-cells and two were Y-cells. Each had electrophysiological properties appropriate for its X- Or Y-cell class, and each also had an axon that projected into the optic radiation, indicative of a geniculocortical relay cell. Representative samples from about 10% of each neuron's entire dendritic arbor (proximal and distal) were taken to obtain an estimate of the types and distributions of synapses contacting these arbors. One X-cell had a cytoplasmic laminar body, but there were no other significant cytological differences seen among the neurons. Common to each of the neurons were the following synaptic features: (i) retinal terminals (r.l.p.) were mostly or entirely restricted to proximal dendrites or dendritic appendages (less than 100 microns from the soma). These terminals constituted about 15-25% of the synapses on the proximal dendrites. (ii) Terminals with flattened or pleomorphic synaptic vesicles (f. terminals) were predominant on the proximal dendrites (30-55% of the total synapses for that region) and were mainly located near the retinal terminals. A smaller percentage (10-20%) were also distributed onto the distal dendrites. (iii) Small terminals with round synaptic vesicles (r.s.d.), many presumably having a cortical origin, predominated (60-80%) on distal dendrites (greater than 100 microns), but also formed a large proportion (40-70%) of the synapses on the intermediate (50-150 microns) dendrites. Total synaptic contacts for one X-cell and one Y-cell were estimated at about 4000 and 5000, respectively. The major fine structural differences observed between X- and Y-cells were almost entirely related to the retinal afferents. First, the retinal synapses for X-cells were mostly made on to dendritic appendages (spines, etc.), whereas Y-cells had most of their retinal synapses onto the shafts of primary and proximal secondary dendrites (that is, near branch points. Second, the retinal terminals that contacted X-cell dendrites nearly always formed triadic arrangements that included nearby f. terminals, but those on Y-cells rarely did so. Finally, the main type of f. terminals associated with X-cells were morphologically different from most of those associated with the Y-cells, and this also related directly to the triadic arrangements; that is, f. terminals in the triadic arrangements were morphologically distinguishable from f. terminals that did not participate in triadic arrangements.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptology of retinal terminals in the dorsal lateral geniculate nucleus of the cat

We have made a fine structural investigation of the synaptic patterns made by axon terminals of retinal ganglion cells in the dorsal lateral geniculate nucleus of the cat. We compared the retinal input to dendritic processes that bear clusters of large appendages with the retinal input to relatively smooth dendritic segments that have only a few isolated spines. The study was restricted to the portion of laminae A and A1 that receive central visual field input. We were able to completely reconstruct 33 individual terminal boutons from long series of consecutive thin sections. Retinal terminals that were presynaptic to dendritic appendages tended to occupy the central position in the complex synaptic zones of geniculate fine structure called glomeruli. These terminals were surrounded by significantly more profiles than retinal terminals that were presynaptic to dendritic stems and averaged twice as many synaptic contacts per terminal bouton. The retinal input to dentritic appendages was heavily involved in a specific synaptic pattern called the triadic arrangement while retinal input to dendritic stems was only lightly involved in triads. Dendritic appendages in triads received greater synaptic input from profiles with flattened vesicles than did the dendritic stems that were found in triads.

Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. II. Methionine-enkephalin immunoreactivity

Enkephalinergic axons and terminals were identified by the PAP immunohistochemical method in lamina I (marginal zone) and lamina IIO (outer substantia gelatinosa) in the dorsal horn of the monkey spinal cord. Synaptic profiles with enkephalin-like immunoreactivity (MELI) contained clear, round, vesicles, sometimes a few large granular vesicles, and usually formed asymmetrical contacts. MELI terminals forming synaptic contacts with various sizes of dendrites and with dendritic spines were the most common type of relationship found; axosomatic contacts were few. Additionally, two types of complexes were observed in which an MELI terminal formed a specialized apposition with an unlabelled terminal. The contact often resembled a synapse and in most cases the MELI terminal was suspected to be presynaptic. One complex consisted of a MELI terminal apposing the LGV type terminal (containing large granular vesicles), which in turn was presynaptic to a dendrite. (The identity of the LGV terminal could not be determined, but it had some characteristics similar to those described for substance P terminals and for a class of primary afferents in the monkey dorsal horn). The other type of complex consisted of a MELI terminal apposing an R-type terminal (containing small, round, clear vesicles) which was in turn presynaptic to a dendrite. Often, the MELI terminal also formed a synapse onto the same dendrite. The axodendritic, axospinous and axosomatic contacts of MELI terminals in the superficial dorsal horn may produce some of the depressive postsynaptic-like effects of enkephalin iontophoresis onto dorsal horn neurons. In these cases the responses of dorsal horn neurons to both low threshold and nociceptive primary afferents is suppressed. However, the opiate receptor-dependent PAD of C-fibers observed in the dorsal horn may be mediated by the MELI complexes formed with LGV and R terminals found in lamina I.

Kainic acid derivatives as excitants of lateral geniculate relay neurons

The activities of kainic acid and its derivatives alpha-ketokainic acid and dihydrokainic acid have been compared with L-glutamic acid as excitants of identified central neurons. The results are consistent with the view that the high activity of kainate is associated with unsaturation in the isopropylene side-chain. It is suggested that a reduction in the lipophilic nature of the alkene bond may cause a concomitant reduction in excitatory potency. This is discussed in relation to the mechanism of receptor activation by kainic acid.

Neural elements containing glutamic acid decarboxylase (GAD) in the dorsal lateral geniculate nucleus of the rat; immunohistochemical studies by light and electron microscopy

Immunoreactive constituents of the dorsal lateral geniculate nucleus of adult albino rats were examined by light- and electron-microscopy, using the unlabelled antibody enzyme method, following treatment of brain slices with a purified antibody to glutamic acid decarboxylase. The neuropil of the dorsal lateral geniculate nucleus displayed a conspicuous granular immunoreactivity. In addition, the antibody was bound to a class of small neurons of characteristic morphology. These cells possessed few (commonly 2-4) sparsely branched, long dendrites from some of which immunoreactive appendages were traced. Many cells were bipolar in form, and the dendrites of some appeared to be preferentially orientated. The immunoreactive cells closely resembled intrinsic interneurons characterized in previous Golgi studies of this nucleus. By electron-microscopy, immunoreactive presynaptic elements were present both in the extraglomerular neuropil and in the synaptic glomeruli. The former were axon terminals containing flattened synaptic vesicles and making Gray type II axo-dendritic synaptic contact; they appeared to correspond to axon terminals whose origin in the thalamic reticular nucleus has been established in previous studies, but it is possible that some were axon terminals of intrinsic interneurons. The immunoreactive glomerular components also contained flattened vesicles, were presynaptic to presumptive projection cell dendrites, postsynaptic to retinal axon terminals, and participated in triplet (triadic) and other complex synaptic arrangements. They corresponded in all respects to the synaptic portions of the complex dendritic appendages of intrinsic interneurons, identified and characterized in previous studies. The finding that there are high levels of glutamic acid decarboxylase in the cell bodies, dendritic shafts and dendritic appendages of intrinsic interneurons in the dorsal lateral geniculate nucleus of the rat, and in the axon terminals of fibres projecting to this site from the thalamic reticular nucleus, allows us to conclude that the inhibitory inputs to the geniculo-cortical projection cells from both of these sources are probably mediated by gamma-aminobutyric acid.

An electron microscopic comparison of the medial interlaminar nucleus and the A laminae in the dorsal lateral geniculate nucleus of the cat

The medial interlaminar nucleus (MIN) of the cat's dorsal lateral geniculate nucleus was studied under the light and electron microscopes and compared with the A laminae. At the light level, MIN has more axons and a lesser cell packing density than the A laminae. Examined at the electron microscopic level, MIN could not qualitatively be distinguished from the A laminae. When quantitative counts of the profiles containing synaptic vesicles were made, MIN had less F profiles and more RSD profiles per unit area than the A laminae. Structure/function correlations suggest that additional F terminals may mediate nondominant eye inhibition and/or a greater amount of inhibition on X-cells versus Y-cells.

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. II. Ultrastructural analyses

Nucleus rotundus in a large, tectorecipient nucleus in the dorsal thalamus of the pond turtles Pseudemys scripta and Chrysemys picta. Rotundal neurons form a single, morphologically homogeneous population (Rainey, '79) that projects to the dorsal ventricular ridge in the telencephalon. The present paper examines the morphology of and the distribution of synapses upon rotundal neurons. Astrocytes, oligodendrocytes, and neurons can be identified in both 1-micrometer sections stained with toluidine blue and electron micrographs of nucleus rotundus. Rotundal neurons contain euchromatic nuclei and the usual complement of mitochondria, rough endoplasmic reticulum, and free ribosomes in their cytoplasm. They are morphologically homogeneous. Two types of terminal boutons can be defined in rotundus. RA boutons contain round synaptic vesicles and form asymmetric synaptic junctions with rotundal dendrites. FS boutons contain small, flattened or pleomorphic vesicles and form nearly symmetric synaptic junctions with rotundal dendrites and somata. RA boutons occasionally form clusters of contiguous boutons that are presynaptic to one or more thin, central profiles. These profiles are probably the dendritic appendages observed on peripheral dendrites in Golgi material. The distribution of RA and FS boutons along dendrites was investigated by a two-step procedure. First, rotundal neurons were retrogradely solid-filled with horseradish peroxidase reaction product. Dendritic diameters were measured at 20 micrometer intervals along dendritic shafts to produce a plot of dendritic diameter as a function of distance from the soma. Second, the percentage of membrane on dendritic profiles of different diameters that was contacted by RA and FS terminals was determined from electron micrographs. Comparison of the two plots indicates that both bouton types are distributed along the full extent of the dendritic tree, but RA boutons are much more common on the distal two-thirds of rotundal dendrites. This analysis suggests that rotundal neurons form a single population of cells that are morphologically homogeneous and project to the forebrain. There is no indication of interaction between neurons in nucleus rotundus, either via axonal collaterals or presynaptic dendrites. Boutons are distributed on rotundal neurons such that FS boutons are prevalent on the somata and most proximal segments of the dendritic shafts, while RA boutons are most common on the more distal dendritic shafts. RA boutons also contribute to synaptic clusters that may center around complex dendritic appendages.

Effects of locus coeruleus stimulation on neuronal activities of dorsal lateral geniculate nucleus and perigeniculate reticular nucleus of the rat

In rats anesthetized with urethane, a stimulating electrode was introduced to the locus coeruleus by observing the antidromic field response to single shock stimulation of the dorsal pathway of noradrenergic axons. Effects of locus coeruleus stimulation were studied on activities of relay neurons and intrinsic interneurons of the dorsal lateral geniculate nucleus and on those of neurons in the perigeniculate reticular nucleus. The intrinsic interneurons and the perigeniculate reticular neurons are believed to exert inhibition upon the relay neurons. The relay neurons were activated by repetitive stimulation of locus coeruleus; spontaneous discharges were increased in rate and the threshold of response to single shock stimulation of the optic nerve was lowered. The activation was rarely seen in rats pretreated with alpha-methyl-p-tyrosine. Iontophoretic application of phentolamine, an alpha-blocker, effectively antagonized the activation, whereas an iontophoretic beta-blocker and cholinergic blockers were virtually ineffective. The activation of the relay neurons was suggested to be due to a direct action of noradrenaline, released by locus coeruleus stimulation. Locus coeruleus stimulation inhibited the interneurons and activated the perigeniculate reticular neurons; spontaneous or light-evoked discharges were suppressed in the interneurons and tonic discharges were elicited in the perigeniculate reticular neurons. These effects of locus coeruleus stimulation were mimicked by noradrenaline applied iontophoretically. Activation of the perigeniculate reticular neurons was antagonized by an iontophoretic alpha-blocker but not by a beta-blocker. Two special features emerge from the present results: (1) the locus coeruleus exerts different effects upon the two neuronal constituents of the dorsal lateral geniculate nucleus, excitation of the relay neurons and inhibition of the intrinsic interneurons; (2) a suggestion previously advocated that locus-coeruleus-induced excitation of the lateral geniculate relay neurons would be due to inhibition of inhibitory neurons (disinhibition) does not hold true, at least with respect to the perigeniculate reticular neurons; the latter neurons have been proved to exert a powerful inhibition upon the geniculate relay neurons and they are excited by stimulation of the locus coeruleus.

Inhibition of axoplasmic transport in the developing visual system of the rat-III. Electron microscopy and Golgi studies of retino-fugal synapses and post-synaptic neurons in the dorsal lateral geniculate nucleus

Quantitative light and electron microscopy, together with Golgi methodology, were used to study alterations in retino-fugal terminals and postsynaptic neurons within the dorsal lateral geniculate nucleus of the rat at various intervals following inhibition of axoplasmic transport in the optic nerve induced by intraocular injections of colchicine from 1-20 days postnatal. Colchicine concentrations used in this study ranged from 10-5 M-10-2 M. These were selected on the basis of our measurements of axon transport suppression described in the preceding article. 61. The volume of the nucleus was determined by section planimetry and reconstruction. Growth of the contralateral dorsal lateral geniculate nucleus was significantly retarded by intraocular colchicine at 1 and 5 days of age, only achieving a volume of between 61-75% of the normal or the control (ipsilateral) nucleus depending upon dosage. Application of colchicine at 10 days of age resulted in minimal stunting of nuclear growth, (79-93% of normal). Mean numbers of neurons in the contralateral and ipsilateral nucleus remained stable throughout all postnatal ages examined suggesting that nuclear volume loss was caused principally by a reduction in the amount of neuropil. Golgi impregnation displayed dendritic stunting in relay neurons characterized by narrowing of the portion of the shaft between 40-80 micrometer from the soma and a reduced incidence of spinous protrusions, particularly those shown by other studies to engage the retino-fungal terminal. 27,69 A concomitant Sholl76 analysis of dendritic branching in relay neurons demonstrated no significant differences in the number of intersections between normal and experimental nuclei. No alterations were observed in intrinsic neurons. Electron microscopy of postsynaptic neurons following concentrations ranging from 10-4 M at birth revealed altered patterns of granular endoplasmic reticulum in many cells characterized by reduced numbers of cisternae and scattered instances of cisternal dilation, together with enhanced infolding of the nuclear membrane at 20 days postnatal. Those animals which were given 5 X 10-3 M-10-2 M colchicine demonstrated an increased incidence of cisternal dilation, loss of ribosomes, disruption of the nuclear membrane and occasionally, complete degeneration. A similar array of alterations took place following intraocular injection at 5 days of age; however, animals receiving colchicine at 10 days postnatal displayed minimal alterations in relay neurons. Synaptic glomeruli, which contain the retinofugal terminal, displayed dose and age-dependent reduction in the size of the presynaptic element of the complex following intraocular colchicine, together with fewer post-synaptic spinous protrusions. Synaptic vesicles remained normal in appearance and distribution and our quantitative analysis demonstrated no loss of such terminals in accordance with colchicine concentrations which were previously found not to be lethal to retinal ganglion cells and optic axons.

High affinity GABA receptors-autoradiographic localization

The distribution of the high affinity gamma-aminobutyric acid (GABA) receptor labeled by [3H]muscimol, has been studied in the rat brain by light microscopic autoradiography. Receptors in slide-mounted tissue sections were labeled in vitro with [3H]muscimol. Most of the gray matter areas presented grain densities significantly higher than background or white matter areas. Wide variations in receptor densities were found between different brain areas and nuclei. Areas with very high grain densities are the granule cell layer of the cerebellum, external plexiform layer of the olfactory bulb and nuclei of the thalamus, such as the ventral nucleus, lateral nucleus and dorsal geniculate body. The molecular layer of the hippocampus and the external (I-IV) layers of the cortex are also rich in GABA receptors. The basal ganglia have moderate concentrations of receptors, while the pons, medulla and brainstem have only low concentrations of autoradiographic grains. These distributions are discussed in correlation with the known distribution of GABAergic terminals and the presence of inhibitory GABAergic mechanisms.

A study of Golgi preparations from the human lateral geniculate nucleus

The Golgi method was used to study nerve cells and their processes in the human lateral geniculate nucleus. Only a few axons were successfully impregnated. These enter the nucleus from the optic radiations and were tentatively identified as corticogeniculate axons. In one brain from a 2-week-old infant these axons showed unusually dense terminal arbors, suggesting that significant developmental changes occur in this fiber system postnatally. Neurons in the magnocellular layers show a great variety of dendritic patterns, and are not readily grouped into well-defined morphological classes. At one extreme are cells with a restricted dendritic arbor, having dendrites that run a sinuous course close to the perikaryon. At the other extreme, cells have a more extended dendritic arbor; their dendrites are long, relatively straight in some cells, markedly tortuous in others. There is a continuous range of intermediate cell types between the "restricted" and the "extended" cells. Many magnocellular dendrites cross freely into adjacent layers; whereas the short dendrites of restricted cells rarely cross from one layer to another, the dendrites of extended cells may either be confined to a single layer (intralaminar) or may have a translaminar distribution across two or even three layers. Small cells, with dendrites entirely confined to an interlaminar region, have been seen in the magnocellular division of the nucleus on either side of layers 2. Parvocellular elements also show a wide range of dendritic patterns, but the extreme forms of restricted and extended cells are not seen. Parvocellular dendrites commonly are oriented perpendicular to the layers and generally show an intralaminar distribution; translaminar dendrites are extremely rare. A minority of parvocellular neurons have most of their dendrites confined to an interlaminar zone. These "interlaminar" cells have large perikarya close to or in the interlaminar zones, and show a predominant dendritic orientation parallel to the layers. A few parvocellular elements with complex axoniform dendrites were seen, and these dendrites, too, tend to run parallel rather than perpendicular to the layers.

Fine structure of the superficial layers of the viper optic tectum. A Golgi and electron-microscopic study

The superficial layers of the viper optic tectum, which receive fibers from he retina, were studied using both light and electron microscopes. The optic fibers layer, or stratum opticum, is composed of 200 to 250 tight fascicles containing thin fibers, nearly all of which are myelinated. The main optic terminal layers, the stratum griseum et fibrosum superficiale, the greatest part of the cellular population is composed of small vertically oriented neurons and horizontal nerve cells, many of which are probably local circuit neurons. The neuropil of the stratum griseum et fibrosum superficiale is made up of small nerve elements, including three types of profiles containing synaptic vesicles; 1) boutons with pleiomorphic synaptic vesicles (P), representing over 47% of the total population of profiles containing synaptic vesicles and comprising three subgroups (P1, P2, and P3); 2) boutons with spheroidal synaptic vesicles (S), forming more than 29% of the total populations of profiles containing synaptic vesicles and comprising two categories, S1 and S2 (S2, the more numerous, represents the optic boutons, which make up 22% of the total populations of profiles containing synaptic vesicles); and 3) dendrites with pleiomorphic vesicles, accounting for approximately 23% of the total populations of profiles containing synaptic vesicles. A study of synaptic patterns revealed a large number of serial synapses and a lesser number of triplets or triadic synapses. The presynaptic components are boutons containing spheroidal (S1, S2) or pleiomorphic (P1, P2, P3) synaptic vesicles. The intermediate profile was always a dendrite with synaptic vesicles which frequently belonged to the small neurons of the stratum griseum et fibrosum superficiale. Comparison of the present results with other recent data shows that the synaptic circuitry in the optic tectum of Vipera aspis closely resembles the pattern observed in the optic tectum of other vertebrates, ranging form fish to mammals. However, quantitative differences exist, especially with regard to the proportion of dendrites containing synaptic vesicles. Their number seems to be higher in sauropsidians than in mammals, particularly in primates.

Electron microscopic identification of axon terminals of retinopretectal fibers in the cat by a combined horseradish peroxidase and tritiated amino acids tracing method

The terminal nucleus of the optic tract in the pretectum was observed electron microscopically in the cat after intravitreous injection of a mixture of HRP and tritriated amino acids. Autoradiographic silver grains, and often also HRP granules, were seen in large axon terminals containing round synaptic vesicles and pale mitochondria. These terminals occasionally made synaptic contracts with the presynaptic dendrites and were involved in the formation of the synaptic triad.

Thalamic reticular nucleus: anatomical evidence that cortico-reticular axons establish monosynaptic contact with reticulo-geniculate projection cells

Cell bodies and dendrites of neurones in the thalamic reticular nucleus projecting to the dorsal lateral geniculate nucleus in adult albino rats, were labelled by retrograde transport of horseradish peroxidase (HRP). In the same animals the terminals of cortico-reticular axons were made identifiable by ipsilateral visual cortex ablation. Degenerating axon terminals established Gray type I synaptic contact with HRP-filled dendrites, indicating a monosynaptic connection between cortico-reticular and reticulo-geniculate projection cells.

Identification and morphometric evaluation of the synapses of optic nerve afferents in the optic tectum of the axolotl (Ambystoma mexicanum)

Ther terminals of retinal afferents in the tectum of the axolotl have been identified ultrastructurally using techniques of horseradish peroxidase-filling and degeneration. The mitochondria in filled structures show a characteristic electron-lucent matrix. After both eyes have been removed, terminals with light mitochondria disappear from the area known to receive an optic input. In this area the presence of light mitochondria is almost always diagnostic of the retinal origin of a bouton. The synapses are similar to those assumed to be of retinal origin in other vertebrates. Detailed morphometric analysis has been carried out on identified optic synapses in the optic tectum of the axolotl.

Ontophyletics of the nervous system: eyeless mutants illustrate how ontogenetic buffer mechanisms channel evolution

Genetics and molecular biology have shown the mechanisms that allow the genome to provide both the continuity and the variation from generation to generation within a phylogeny. Embryology and developmental biology show the mechanisms that turn the genome into an organism. Mutations, the basis for evolutionary change, cannot in themselves ensure concordance between their products and the products of unchanged genes. Thus, mutations will not necessarily produce a viable organism. On the other hand, ontogenetic buffer mechanisms normally maintain concordance in the developing organism. In addition, ontogenetic buffer mechanisms can integrate discordant mutations into viable organisms that can then be perpetuated during evolution. The evolutionary role of one ontogenetic buffer mechanism, compensatory innervation, is well illustrated in the anopthalmic mutant mouse. In the anopthalmic mouse, a single gene mutation removes afferent axons of the dorsal lateral geniculate nucleus, and compensatory innervation by another population of axons ensures that the dorsal lateral geniculate remains integrated into the central nervous system. Within each organism's ontogeny is a hierarchy of sources of compensatory innervation, and this hierarchy will determine how any particular deafferentating mutation will be buffered. In this way, an ontogeny can channel the phylogeny of which it is a member.

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.

An ultrastructural study of the normal synaptic organization of the optic tectum and the degenerating tectal afferents from retina, telencephalon, and contralateral tectum in a teleost, Holocentrus rufus

The ultrastructure of the optic tectum in the squirrel fish, Holocentrus rufus, has been studied and the normal synaptic organization is described. Synaptic terminals were classified into eight types (S1-S6, F1, F2) by their morphology and synaptic relations. The distribution pattern for each type of terminal was determined by counting the relative number of terminals in each layer. Most S1 terminals are localized in Stratum marginale (SM), whereas S2 terminals are most common in Stratum fibrtosum et griseum superficiale (SFGS). S3, S4, S5, F1, and F2 terminals are limited mainly to SFGS and Stratum griseum centrale (SGC). S6 terminals are most frequently seen in SGC and Stratum griseum periventriculare (SPV). In order to determine the origins of the various types of synaptic terminals in the optic tectum, the telencephalon, eye, and optic tectum were removed unilaterally and areas of resultant degeneration examined. Electron microscopic observations show that nearly all S2 terminals in SFGS of the contralateral optic tectum degenerate after eye enucleation, whereas some S4 terminals in SFGS and SGC exhibit degenerative changes after removal of the ipsilateral telencephalon. Unilateral ablation of the optic tectum was associated with degenerative changes in occasional S5 terminals within SGC of contralateral optic tectum. All experiments resulted in some increased electron density of S3 and F terminals, some of which were identified as F2 terminals. The possible origins of S1 and S6 terminals, which were not altered in the present experiments, are discussed.

Neurons and synaptic patterns in the deep layers of the superior colliculus of the cat. A Golgi and electron microscopic study

Golgi and electron microscopic observations were made on the neurons in the deep layers (below the stratum opticum) of the cat superior colliculus. Large neurons, 35-60 micrometers in somal diameter, occur mainly in the lateral two-thirds of the colliculus. They have numerous somatic and dendritic spines and receive a large number of axon terminals (bouton covering ratio: more than 70%). The medium-sized neurons (20-30 micrometer), with a moderate number of dendritic spines, show a lower bouton covering ratio (25-30%). The ratio for small neurons (8-15 micrometers), with very few dendritic spines, is less than 10%. The medium-sized and small neurons are distributed throughout the colliculus and show marked variability in the dendritic arrangement. Seven different types of axon terminals were distinguished: types I, II, V, and VII form asymmetrical and types III, IV, and VI symmetrical synapses. Type I terminals represent small boutons containing predominantly spherical vesicles, and are in contact mainly with small dendritic profiles. Type II terminals are medium-sized and slender, contain a mixture of spherical and slightly oval vesicles, and make synaptic contacts with small to medium-sized dendrites and somatic spines. This type of terminal is occasionally presynaptic to vesicle-containing dendrites (type VIII). Type III terminals are small, contain flattened vesicles predominantly, and are presynaptic to a wide variety of neuronal elements in the deep layers of the superior colliculus. Type IV terminals are represented by medium to large-sized boutons that contain pleomorphic vesicles and make synaptic contacts chiefly with the large neurons. Type V and VI terminals exhibit a quite dense axoplasmic matrix and mainly contact the large neurons. Type VII terminals are often in the form of boutons en passant and contain numerous large granular vesicles. Pleomorphic vesicle-containing dendrites (type VIII terminals) are also observed to participate in the axodendrodendritic serial synapses.

The structural organization of the inferior and lateral subdivisions of the Macaca monkey pulvinar

Previous light microscopic studies of Macaca pulvinar have demonstrated that both the inferior and adjacent portion of the lateral pulvinar subdivisions are reciprocally connected to the entire occipital lobe, including striate cortex. They differ in that inferior but not lateral pulvinar receives a projection from the superficial layers of the superior colliculus. In this study, the internal organization of these two subdivisions in compared by relating light microscopic Golgi morphology to the synaptic organization observed by electron microscopy. The Golgi impregnated neurons in inferior and lateral pulvinar are typical of other thalamic nuclei and are not qualitatively different in the two subdivisions. Projections neurons (PN) vary in cell body (15--40 micrometers) and dendritic tree (150--600 micrometers) diameters but bear the same varieties of dendritic appendages; spine-like, hair-like, and knot-like. Local circuit neurons (LCN) have smaller cell body diameters (10--20 micrometers) but can have very large dendritic field diameters (150--600 micrometers). They are best distinguished from PNs by their elaborate dendritic appendages, which have been identified as pre-synaptic dendrites in the EM. LCN axons are infrequently seen. In the EM both subdivisions contain four types of synaptic terminals. RS and RL terminal both contain round symaptic vesicles and make asymmetric synaptic contacts, but are subdivided on the basis of small (RS = 0.09 micrometers) versus large (RL = 2.2 micrometers) cross sectional diameters and organelle content. RLs contact larger caliber dendrites and frequently form synaptic complexes with presynaptic dendrites of LCNs, while RSs contact fine caliber dendrites and rarely take part in synaptic complexes. F terminal and P boutons both contain flat and pleomorphis vesicles and make symmetric synaptic contacts. They are characterized by vesicle number and cytoplasmic density. Fs are infrequently observed in pulvinar compared to P boutons and are of uncertain origin. P boutons can be equated with LCN dendritic appendages and have been identified as pre-synaptic dendrites. The quantitative distribution of each type is very similar in both subdivisions, avveraging RS 85%, RL 5%, F 0.3%, P 8% and unidentified 2%.

The morphology and distribution of striate cortex terminals in the inferior and lateral subdivisions of the Macaca monkey pulvinar

The origin of the various types of axon terminals in Macaca pulvinar remains uncertain because of the contradictory results obtained in EM degeneration studies. We have used EM-autoradiography to determine the morphology of terminals in the inferior and lateral pulvinar which originate from neurons in visual cortex. After injections of H3 proline into area 17, both the small diameter (RS) and the large diameter (RL) terminals containing round vesicles and making asymmetric contacts are labeled in the two pulvinar subdivisions. Labeled and unlabeled terminals are intermixed within the pulvinar focus which suggests that the dendrites of the same pulvinar neuron receive overlapping inputs from several cortical areas. Because only 5% of the pulvinar terminals are RLs (Ogren and Hendrickson, '79), and this small number of RLs originates from at least two visual cortical areas plus the superior colliculus (Partlow et al., '77), superior colliculus input to inferior pulvinar is small compared to the combined RS and RL cortical input. Together the findings from this study and the preceding paper (Ogren and Henderickson, '79), show that while pulvinar is typical of other thalamic nuclei in the structure of its neurons and synapses, it differs in that the input from subcortical structures is minimal. It is suggested that inferior and lateral pulvinar function principally as integrators of visula cortical information.