An experimental anatomical study of the optic nerve fibers in the rat: Courses of the accessory optic tract

By means of silver impregnation and an HRP method, courses of the accessory optic tract were examined in albino and pigmented rats. The accessory optic tract is composed of 3 fasciculi: anterior, lateral and dorsal. The anterior fasciculus gives off fibers to the subthalamic nucleus and terminates in the medial terminal nucleus. The lateral fasciculus branches from the main optic tract at the level of the ventral nucleus of the lateral geniculate body and descends the lateral surface of the crus cerebri to enter the medial terminal nucleus after contributing a few fibers to the lateral terminal nucleus. The dorsal fasciculus originates from the brachium colliculi superioris and descends the posterior surface of the medial geniculate body and the posterolateral surface of the crus cerebri as an independent fasciculus to enter the medial terminal nucleus. This fasciculus supplies many fibers to the dorsal terminal nucleus.     

Cytochrome oxidase and NADPH-diaphorase on the afferent relay branch of the optokinetic reflex in the opossum

In the present study, histochemical techniques combined with more conventional anatomical methods were used to refine the identification of the nucleus of the optic tract and the nuclei of the accessory optic system in the opossum. The distribution of the enzyme cytochrome oxidase (CO)<0B> <0R>was examined in the cells and the neuropil of the opossum's mesodiencephalic region. Strong CO labeling was present in the nucleus of the optic tract (NOT)-dorsal terminal nucleus (DTN). Alternate sections, taken from animals that had received bilateral injections of horseradish peroxidase centered in the region of the inferior olive, were subjected to assays for CO and horseradish peroxidase. The region occupied by CO-labeled cells in the NOT-DTN superimposed with the one defined by retrogradely labeled cells. Cell counts along the NOT-DTN anteroposterior axis revealed that although the olivary and CO-positive cells were confined within similar boundaries, the latter are up to twofold more numerous than the former. As revealed by cytochrome oxidase histochemistry, the outlines of the NOT-DTN, the other pretectal nuclei and the nuclei belonging to the accessory optic system coincided with those revealed by the histochemistry for nicotinamide dinucleotide phosphate diaphorase (NADPH-d). After an intraocular injection of cholera toxin beta subunit and alternate sections processing for NADPH-d and CO, the distribution of labeled retinal terminal fields in the mesodiencephalic region was shown to be coincident with regions of high levels of histochemical labeling. These results are discussed in the light of previous anatomofunctional assessments of the pretectum and accessory optic system. J. Comp. Neurol. 398:206–224, 1998. © 1998 Wiley-Liss, Inc.     

Dendritic morphology of projection neurons in the cat pretectum

The distribution and dendritic morphology of neurons in the cat pretectal nuclear complex were analyzed with respect to their projection to the ipsilateral dorsal lateral geniculate nucleus (LGNd) and the ipsilateral inferior olive (IO). Single and double retrograde tracing techniques were combined with intracellular injections of either horseradish peroxidase into electrophysiologically identified pretectal neurons or Lucifer Yellow into retrogradely labeled somata. Pretectal cells afferent to the LGNd were located in the nucleus of the optic tract (NOT), adjacent dorsal terminal nucleus of the accessory optic system (DTN), and posterior pretectal nucleus (NPP). Cells projecting to the IO were also distributed throughout the NOT-DTN and dorsal part of the NPP. Separate tracer injections (fluorogold and horseradish peroxidase [HRP] or granular blue) into the LGNd and the IO showed considerable overlap of labeled neurons in the NOT and dorsal NPP. Double-labeled neurons, however, were not observed after double tracer injections into LGNd and IO. Partial topographical segregation of the two populations was observed along the dorsoventral axis because LGNd-projecting neurons exhibited maximum density ventral to that of IO neurons. Pretectal cells to the LGNd had cell body diameters between 16 and 48 μm. Somatic shapes varied between fusiform and multipolar with considerable overlap between these two morphological appearances. Neurons projecting to the IO exhibited similar cell body sizes and their morphology also varied from fusiform to multipolar. Quantitative analysis of dendritic field size and orientation, number and order of dendritic arborizations, and symmetry of the dendritic tree revealed no statistically significant difference between the two neuronal populations. Hence, neurons of the two populations cannot be unequivocally identified just from the dendritic morphology. By contrast, dendritic morphology was correlated with the topographical location of either cell type within the pretectal nuclei rather than projection. Thus, the morphological appearance of neurons located dorsally predominantly was fusiform while neurons located ventrally mostly were multipolar. © 1996 Wiley-Liss, Inc.     

Spatial distribution of evoked potentials in the inferior olivary nucleus by stimulation of the visual afferents in the rat

Visual pathways (optic disc, optic nerve and pretectal regions) were electrically stimulated and evoked potentials were explored throughout the inferior olive in the anesthetized rat. Responsive areas were identified as the caudal half of the dorsal cap, nucleus beta and the most caudal region of subnucleus c of the medial accessory olive. No field potentials were identified in the rostral half of the dorsal cap, its ventrolateral outgrowth or the dorsomedial cell column. Contralateral retinal afferents were only effective all over the responsive areas.     

Medial terminal nucleus terminals in the nucleus of the optic tract contain GABA: an electron microscopical study with immunocytochemical double labeling of GABA and PHA-L.

In this study the medial terminal nucleus (MTN) projection to the nucleus of the optic tract (NOT) was investigated in pigmented rats at the light and electron microscopical levels with a new combination of techniques. MTN terminals were anterogradely labeled with Phaseolus vulgaris-leucoagglutinin (PHA-L). Preembedding immunocytochemistry, followed by gold intensification, was used to visualize PHA-L. Postembedding immunocytochemistry with 15 nm immunogold particles was carried out to demonstrate the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Both PHA-L and GABA labeling can be easily discriminated at the electron microscopical level even when present in the same neuronal profiles. Light microscopically MTN-NOT fibers proved to have several branches with many varicosities. MTN terminals were found concentrated in terminal fields. Electron microscopically, it was shown that MTN boutons display characteristics resembling F-type terminals, i.e., terminals with dark mitochondria, pleomorphic vesicles, and symmetrical synapses. All NOT afferents originating from the MTN contained GABA and made multiple contacts exclusively with GABA negative NOT somata and dendrites. These results indicate the existence of a strong and direct inhibitory input onto GABA negative projection neurons in the NOT. This substantiates earlier physiological and morphological reports. It was further demonstrated that the location and organization of MTN terminals in the neuropil differ from that of the retinal input: MTN terminals are largely separated from retinal terminals. MTN terminal fields contain large amounts of GABA positive F terminals in contrast to retinal terminal areas. MTN terminals take part in irregularly shaped agglomerations of terminals, which contain many F terminals and dendritic processes and are surrounded by a glial sheet. Retinal terminals are found grouped together in small circular arrangements contacting a central dendrite.     

Glutamate-like immunoreactivity in retinal terminals in the nucleus of the optic tract in rabbits.

The ultrastructural organization of retinal terminals within the nucleus of the optic tract of rabbits was investigated with a combination of anterograde tracing and immunocytochemistry. The anterogradely transported WGA-HRP injected in the vitreous of the eye was visualized with the sensitive gold-substituted silver peroxidase (GSSP) method. Glutamate and GABA immunoreactivity were identified with postembedding colloidal gold particles. Retinal ganglion cell terminals (R-terminals) in the nucleus of the optic tract formed asymmetric synapses and contained spherical vesicles and electron lucent mitochondria. R-terminals were observed in large clusters in the neuropil and in synaptic contact with large initial dendrites and somata. Within the clusters of neuropil the R-terminals formed two types of glomeruluslike arrangements: (1) an R-terminal centrally located and surrounded by small dendritic and axonal profiles and (2) several R-terminals surrounding a single dendrite or a group of dendritic profiles, presumably of interneuronal origin. All R-terminals identified with WGA-HRP as well as those exhibiting similar ultrastructural characteristics showed high levels of glutamate immunoreactivity, but no GABA immunoreactivity. The presence of glutamate and the absence of GABA in R-terminals suggest that glutamate is involved in neurotransmission in the pathway from retina to the nucleus of the optic tract of rabbits.     

The dorsal tegmental nucleus: an axoplasmic transport study

The afferent and efferent connections of the dorsal tegmental nucleus (DTN) were studied in the rat using axoplasmic transport techniques. Horseradish peroxidase (HRP) and Fast Blue were injected stereotaxically into either pars centralis or pars ventromedialis of the DTN, two subdivisions of the nucleus with distinctive connected with the ipsilateral lateral mammillary and interpeduncular neclei; these projections constitute the major afferent and efferent systems of the DTN. Commissural fibers from the corresponding pars centralis and intrinsic fibers systems are massive and form a complex fiber meshwork within the subnucleus. The prepositus hypoglossi nuclei (bilateral) also project to the pars centralis. Smaller numbers of afferent fibers arise from the lateral habenular nucleus, the posterior hypothalamus and the brainstem reticular formation.The pars ventromedialis of the DTN receives diverse inputs which include the septal nuclei, diagonal band of Broca, preoptic area, anterior and lateral hypothalamus, lateral and medial habenular nuclei, medial mammillary nucleus and many nuclei of the brainstem reticular formation. Based on the differences of connections and cytoarchitecture between the pars and the pars ventromedialis, the pars ventromedialis may be an entity separate from the dorsal tegmental nucleus.     

Direct projections from the anterior pretectal nucleus to the dorsal accessory olive in the cat: an anterograde and retrograde WGA-HRP study

Direct projections from the anterior pretectal nucleus (APN) to the dorsal accessory olive (DAO) were found in the cat by the anterograde and retrograde WGA-HRP methods. The dorsal or the ventral portions of the rostral half of the APN pars compacta send fibers respectively to the lateral or the medial portions of the whole rostrocaudal extent of the DAO. These APN-DAO fibers can be considered to play roles in some somatomotor mechanisms.     

Visual function in the ventral lateral geniculate nucleus of the cat

The visual receptive fields of 293 single units in the ventral lateral geniculate nucleus of the cat were studied. In addition to the wide variety of types described by others, a group of units responding differentially to color was identified that included units responding particularly to blue and others with opponent color properties. Some units with spontaneous firing and without definite visual receptive fields were inhibited by stimulation of the optic chiasm (OX). A study of latency of firing to OX stimulation suggested that these cells were driven by retinal ganglion cells of the W type. One-third of all units studied were binocularly driven.     

Branched projections from the nucleus prepositus hypoglossi to the oculomotor nucleus and the cerebellum. A retrograde fluorescent double-labeling study in the cat

The retrograde transport of fluorescent substances was used in order to investigate divergent axon collaterals of neurons in the nucleus prepositus hypoglossi (Ph). Fast blue (FB) was injected into the flocculus, paraflocculus and/or the vermis, while nuclear yellow (NY) was injected into the oculomotor nucleus alone or combined with injections in the nucleus of Darkschewitsch, the interstitial nucleus of Cajal and the medial longitudinal fascicle. Within optimal survival time, separate populations of single-labeled neurons of both dyes were found in Ph in all cases. Double-labeled neurons were seen in the rostral Ph following FB injections into the flocculus and the paraflocculus and NY injections restricted to the oculomotor nucleus. The present findings demonstrate that many neurons in the rostral Ph give collateral branches to the cerebellum and to the oculomotor nucleus.     

Anatomical connections of the nucleus prepositus of the cat

The afferent and efferent connections of the nucleus prepositus hypoglossi with brainstem nuclei were studied using anterograde and retrograde axonal transport techniques, and by intracellular recordings and injections of horseradish peroxidase into prepositus hypoglossi neurons. The results of experiments in which horseradish peroxidase was injected into the prepositus hypoglossi suggest that the major inputs to the prepositus hypoglossi arise from the ipsi- and contralateral perihypoglossal nuclei (particularly the prepositus hypoglossi and intercalatus), vestibular nuclei (particularly the medial, inferior, and ventrolateral nuclei), the paramedian medullary and pontine reticular formation, and from the cerebellar cortex (flocculus, paraflocculus, and crus I; the nodulus was not available for study). Regions containing fewer labeled cells included the interstitial n. of Cajal, the rostral interstitial n. of the medial longitudinal fasciculus, the n. of the posterior commissure, the superior colliculus, the n. of the optic tract, the extraocular motor nuclei, the spinal trigeminal n., and the central cervical n. The efferent connections of the prepositus hypoglossi were studied by injecting 3H-leucine into the prepositus hypoglossi, and by following the axons of intracellularly injected prepositus hypoglossi neurons. The results suggest that in addition to the cerebellar cortex, the most important extrinsic targets of prepositus hypoglossi efferents are the vestibular nuclei (particularly the medial, inferior, and ventrolateral nuclei, and the area X), the inferior olive (contralateral dorsal cap of Kooy and ipsilateral subnucleus b of the medial accessory olive), the paramedian medullary and pontine reticular formation, the reticular formation surrounding the parabigeminal n., the contralateral superior colliculus and pretectum, the extraocular motor nuclei (particularly the contralateral abducens nucleus and the ipsilateral medial rectus subdivision of the oculomotor nucleus), the ventral lateral geniculate n., and the central lateral thalamic nucleus. Other areas which were lightly labeled in the autoradiographic experiments were the contralateral spinal trigeminal n., the n. raphe pontis, the Edinger Westphal n., the zona incerta, and the paracentral thalamic n. Many of the efferent connections of the prepositus hypoglossi appear to arise from principal prepositus hypoglossi neurons whose axons collateralize extensively in the brainstem. On the other hand, small prepositus hypoglossi neurons project to the inferior olive, and multidendritic neurons project to the cerebellar flocculus, apparently without collateralizing in the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)     

Abnormal lateral geniculate nucleus and optic chiasm in human albinism

Our objective was to measure how the misrouting of retinal ganglion cell (RGC) fibers affects the organization of the optic chiasm and lateral geniculate nuclei (LGN) in human albinism. We compared the chiasmal structures and the LGN in both pigmented controls and patients with albinism by using high‐resolution structural magnetic resonance imaging (MRI). We studied 12 patients with oculocutaneous albinism and 12 age‐matched pigmented controls. Using a 3T MRI scanner, we acquired a T₁‐weighted three‐dimensional magnetization‐prepared rapid gradient‐echo (MPRAGE) image of the whole brain, oriented so that the optic nerves, chiasm, and tracts were in the same plane. We acquired multiple proton density‐weighted images centered on the thalamus and midbrain, and averaged them to increase the signal, enabling precise manual tracing of the anatomical boundaries of the LGN. Albinism patients exhibited significantly smaller diameters of the optic nerves, chiasm and tracts, and optic chiasm and LGN volume compared with controls (P < 0.001 for all). The reductions in chiasmal diameters in the albinism compared with the control group can be attributed to the abnormal crossing of optic fibers and the reduction of RGCs in the central retina. The volume of the LGN devoted to the center of the visual field may be reduced in albinism due to fewer RGCs representing the area where the fovea would normally lie. Our data may be clinically useful in addressing how genetic deficits compromise proper structural and functional development in the brain. J. Comp. Neurol. 522:2680–2687, 2014. © 2014 Wiley Periodicals, Inc.     

Efferent pathways of the nucleus of the optic tract in monkey and their role in eye movements

To clarify the role of the pretectal nucleus of the optic tract (NOT) in ocular following, we traced NOT efferents with tritiated leucine in the monkey and identified the cell groups they targeted. Strong local projections from the NOT were demonstrated to the superior colliculus and the dorsal terminal nucleus bilaterally and to the contralateral NOT. The contralateral oculomotor complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, also received inputs. NOT efferents terminated in all accessory optic nuclei (AON) ipsilaterally; contralateral AON projections arose from the pretectal olivary nucleus embedded in the NOT. Descending pathways contacted precerebellar nuclei: the dorsolateral and dorsomedial pontine nuclei, the nucleus reticularis tegmenti pontis, and the inferior olive. Direct projections from NOT to the ipsilateral nucleus prepositus hypoglossi (ppH) appeared to be weak, but retrograde tracer injections into rostral ppH verified this projection; furthermore, the injections demonstrated that AON efferents also enter this area. Efferents from the NOT also targeted ascending reticular networks from the pedunculopontine tegmental nucleus and the locus coeruleus. Rostrally, NOT projections included the magnocellular layers of the lateral geniculate nucleus (lgn); the pregeniculate, peripeduncular, and thalamic reticular nuclei; and the pulvinar, the zona incerta, the mesencephalic reticular formation, the intralaminar thalamic nuclei, and the hypothalamus. The NOT could generate optokinetic nystagmus through projections to the AON, the ppH, and the precerebellar nuclei. However, NOT also projects to structures controlling saccades, ocular pursuit, the near response, lgn motion sensitivity, visual attention, vigilance, and gain modification of the vestibulo-ocular reflex. Any hypothesis on the function of NOT must take into account its connectivity to all of these visuomotor structures. © 1996 Wiley-Liss, Inc.     

Relative distribution of synapses in the pulvinar nucleus of the cat: implications regarding the "driver/modulator" theory of thalamic function

To provide a quantitative comparison of the synaptic organization of "first-order" and "higher-order" thalamic nuclei, we followed bias-corrected sampling methods identical to a previous study of the cat dorsal lateral geniculate nucleus (dLGN; Van Horn et al. [2000] J. Comp. Neurol. 416:509-520) to examine the distribution of terminal types within the cat pulvinar nucleus. We observed the following distribution of synaptic contacts: large terminals that contain loosely packed round vesicles (RL profiles), 3.5%; presynaptic profiles that contain densely packed pleomorphic vesicles (F1 profiles), 7.3%; profiles that could be both presynaptic and postsynaptic that contain loosely packed pleomorphic vesicles (F2 profiles), 5.0%; and small terminals that contain densely packed round vesicles (RS profiles), 84.2%. Postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) was used to distinguish the postsynaptic targets as thalamocortical cells or interneurons. The distribution of synaptic contacts on thalamocortical cells was as follows: RL profiles, 2.1%; F1 profiles, 6.9%; F2 profiles, 5.4%; and RS profiles, 85.6%. The distribution of synaptic contacts on interneurons was as follows: RL profiles, 11.8%; F1 profiles, 9.7%; F2 profiles, 2.8%; and RS profiles, 75.6%. These distributions are similar to that found within the dLGN in that the RS inputs (the presumed "modulators") far outnumber the RL inputs (the presumed "drivers"). However, in comparison to the dLGN, the pulvinar nucleus receives significantly fewer numbers of RL, F1, and F2 contacts and significantly higher numbers of RS contacts. Thus, the RS/RL synapse ratio in the pulvinar nucleus is 24:1, in contrast to the 5:1 RS/RL synapse ratio in the dLGN (Van Horn et al., 2000). In first-order nuclei, the lower RS/RL synapse ratio may result in the transfer of visual information that is largely unmodified. In contrast, in higher-order nuclei, the higher RS/RL synapse ratio may allow for a finer modulation of driving inputs.     

Laminar distribution of tectal, parabigeminal and pretectal inputs to the primate dorsal lateral geniculate nucleus: Connectional studies in Galago crassicaudatus

We have studied the distribution of 3 extraretinal, subcortical inputs to the dorsal lateral geniculate nucleus of the prosimian primate Galago. Our connectional findings reveal that axons arising from the superior colliculus and the parabigeminal nucleus influence the W-cell system via their innervation of the two small-celled geniculate laminae (internal and external koniocellular) and the interlaminar zones; parabigeminal axons also innervate each of the 4 non-tectally innervated layers. Pretectal axons, on the other hand, distribute mainly to the parvocellular laminae and thus influence primarily the X-cell system.     

Terminations of individual optic tract fibers in the lateral geniculate nuclei of Galago crassicaudatus and Tupaia belangeri

The morphology and laminar distribution of individual optic fibers projecting to the lateral geniculate nucleus (GL) of Galago and Tupaia were studied following iontophoretic injections of horseradish peroxidase (HRP) into the optic tract. In Galago the GL is composed of three functionally matched pairs of layers, each characterized by cells of a given size, one large, one medium-sized, and one small. The results show that there is a close correspondence between the size of the afferent fibers and the size of the neurons in the target layer: large axons project to the magnocellular layers, medium-sized axons project to the parvicellular layers, and small fibers project to the intercalated layers. In Tupaia the GL is composed of two functionally matched pairs and two unmatched layers. Optic fibers that project to the medial matched pair (1 and 2) are only slightly larger than those that project to the lateral matched pair (4 and 5), but both are larger than those that project to the unmatched layers (3 and 6). In both species terminal arbors and the distribution of terminal boutons within layers corresponded closely with the organization of dendritic processes of cells in the target layer. This correspondence was particularly evident in the parvicellular layers in Galago and in layer 6 in Tupaia: parvicellular terminal arbors, like the dendrites of parvicellular cells, are organized in narrow columns oriented along lines of projection, whereas layer 6 terminal arbors, like the dendrites of layer 6 cells, are oriented in elongated strips perpendicular to lines of projection. In both species there was evidence for sublaminar terminations in some layers. These were restricted to the parvicellular layers in Galago and layers 4 and 5 in Tupaia. With the exception of a small number of fine fibers in the intercalated layers in Galago, optic fibers in both species terminated in one and only one layer in a set. The significance of this result depends on the relation between ganglion cell classes and what is being segregated in different GL layers. Lateral geniculate lamination varies even in closely related species and has evolved independently in such distantly related lines as carnivores and primates. It is not surprising, therefore, that what is being segregated varies from species to species.     

Identification of ventral lateral geniculate nucleus cells projecting to the pretectum and superior colliculus in the cat

Among 235 histologically identified cells of the ventral lateral geniculate nucleus (LGV) in the cat, 66 responded antidromically to electrical stimulation of the pretectum (PT) and/or superior colliculus (SC): 22 projected to PT, 22 to SC and 22 to both sites. The LGV cells were innervated by optic tract fibers corresponding to axons of X- as well as W-type retinal ganglion cells.     

Fibre organization of the monkey's optic tract: II. Noncongruent representation of the two half-retinae

The representations of the two half-retinae were examined in the monkey's optic tract. Intravitreal injections of tritiated amino acids were made to reveal the distributions of the crossed and uncrossed populations of optic axons, while localized implants of horseradish peroxidase (HRP) were made into different regions of the optic tract in order to examine the distributions and morphological types of retrogradely labelled cells at corresponding loci in the two half-retinae. Crossed and uncrossed optic axons are intermingled throughout most of the optic tract, but uncrossed axons are very sparse or absent along both the deep and superficial extremes of the tract. Implants of HRP into the deeper regions of the tract demonstrate that the crossed and uncrossed optic axons of the P beta retinal ganglion cells are slightly out of binocular registration, with the uncrossed map being shifted to a slightly superficial location relative to the crossed map. The optic axons for the remaining cell classes, revealed by implants of HRP into the superficial portion of the tract, are much more conspicuously out of binocular registration (in particular, the P alpha optic axons); but in their cases, the uncrossed optic axons are shifted to deeper locations relative to the crossed optic axons. Further evidence that these optic axon classes are markedly out of binocular registration comes from the two optic tracts of a bilaterally destriated monkey, in which most of the P beta optic axons have undergone a transneuronal retrograde degeneration. Following a uni-ocular injection of tritiated amino acids, the distributions of the remaining crossed and uncrossed axonal labelling occupied different positions within the tract rather than being intermingled, with the uncrossed optic axons situated deep to the majority of crossed optic axons. These results demonstrate that the optic chiasm does not combine binocularly corresponding optic axons of similar type. They also demonstrate that noncongruent field defects should be a common consequence of damage to the optic tract in humans. If the fibre order in the mammalian optic tract arises as a consequence of the sequence of axonal addition during development, then differences in the relative times of genesis for nasal and temporal members of any cell class, and/or differences in the relative pathlengths between the eye and two optic tracts, may produce the fibre ordering described herein.