Biomedical subjects

P S Ulinski

Publications and source records attributed to P S Ulinski.

At least 19 recordsLinked to original sources

PubMed1991

Excitatory amino acid receptor-mediated transmission in geniculocortical and intracortical pathways within visual cortex.

1. A preparation of turtle (Chrysemys picta and Pseudemys scripta) brain in which the integrity of the intracortical and geniculocortical pathways in visual cortex are maintained in vitro has been used to differentiate the excitatory amino acid (EAA) receptor subtypes involved in geniculocortical and intracortical synapses. 2. Stimulation of the geniculocortical fibers at subcortical loci produces monosynaptic excitatory postsynaptic potentials (EPSPs) in visual cortical neurons. These EPSPs are blocked by the broad-spectrum EAA receptor antagonist kynurenate (1-2 mM) and the non-N-methyl-D-aspartate (NMDA) antagonist 6, 7-dinitroquinoxaline-2,3-dione (DNQX, 10 microM), but not by the NMDA antagonist D,L-2-amino-5-phosphonovalerate (D,L-AP-5, 100 microM). These results indicate that the geniculocortical EPSP is mediated by EAAs that access principally, if not exclusively, EAA receptors of the non-NMDA subtypes. 3. Stimulation of intracortical fibers evokes compound EPSPs that could be resolved into three components differing in latency to peak. The component with the shortest latency was not affected by any of the EAA-receptor antagonists tested. The second component, of intermediate latency, was blocked by kyurenate and DNQX but not by D,L-AP-5. The component of longest latency was blocked by kynurenate and D,L-AP-5, but not by DNQX. These results indicate that the compound intracortical EPSP is comprised of three pharmacologically distinct components that are mediated by an unknown receptor, by quisqualate/kainate, and by NMDA receptors, respectively. 4. Repetitive stimulation of intracortical pathways at 0.33 Hz produces a dramatic potentiation of the late, D,L-AP-5-sensitive component of the intracortical EPSP. 5. These experiments lead to a hypothesis about the subtypes of EAA receptors that are accessed by the geniculocortical and intracortical pathways within visual cortex.

2-Amino-5-phosphonovalerate↗

PubMed1990

Organization of geniculocortical projections in turtles: isoazimuth lamellae in the visual cortex.

The projection from the dorsal lateral geniculate complex to the visual cortex in Pseudemys and Chrysemys turtles was examined by using the anterograde transport of horseradish peroxidase (HRP) in vitro and the retrograde transport of HRP in vivo. In vitro HRP injections into the lateral forebrain bundle were used to fill geniculocortical axons anterogradely, which were then analyzed in cortical wholemount preparations. Geniculocortical axons gain access to the visual cortex along its entire rostral-caudal extent. They course in slightly curved trajectories for up to 2 mm from the lateral edge of the cortex through both the lateral (or pallial thickening) and medial parts of Desan's cortical area D2. Single axons are of fine caliber. They tend to cross each other and sometimes branch in the pallial thickening, but are generally unbranched in the medial part of D2. They bear small, fusiform varicosities at irregular intervals along their lengths. Although axons show small variations in the number of varicosities per 100 microns segment, no consistent variation in varicosity number as a function of distance could be detected. These results indicate that geniculocortical axons project to the visual cortex in an orderly pattern. The retrograde transport experiments provide some clue as to the significance of this pattern. Small, ionotophoretic injections of HRP in the visual cortex retrogradely labeled neurons in the dorsal lateral geniculate complex. Injections in the rostral visual cortex retrogradely labeled neurons in the caudal pole of the geniculate complex. Injections at progressively more caudal loci within the visual cortex labeled neurons at progressively more rostral loci within the geniculate complex. Thus, there is a representation of the rostral-caudal axis of the geniculate complex along the caudal-rostral axis of the visual cortex. Consistent with the anterograde transport experiments that showed individual geniculocortical axons coursing through both lateral and medial parts of the visual cortex, HRP injections restricted to the medial edge of the visual cortex retrogradely labeled neurons along the entire dorsal-ventral axis of the geniculate complex at the appropriate rostral-caudal position. The neurophysiological studies of Mazurskaya ('72: J. Evol. Biochem. Physiol. 8:550-555; respond to a small, moving stimulus anywhere in visual space, implying a convergence of inputs from all points in visual space somewhere along the retinogeniculocortical pathway. The experiments reported here suggest a convergence in the geniculocortical projections of information along the vertical meridians, or azimuth lines, of visual space onto neurons lying along lateral to medial transects through the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1990

Spatial organization of axons in turtle visual cortex: intralamellar and interlamellar projections.

The spatial pattern of projections within turtle visual cortex was studied by using focal injections of horseradish peroxidase into visual cortex in an in vitro wholebrain preparation. Injections anterogradely filled the axons of many layer 2 neurons, which could be followed for 200-500 microns from the injection sites. Axons were typically unbranched, relatively straight, and bore small varicosities at irregular intervals. They radiated from the injection sites in all directions, but showed some preference toward orientations along the lateral-medial axis of the cortex. Earlier work (Mulligan and Ulinski, '90) had demonstrated that turtle visual cortex contains a series of isoazimuth lamellae, each representing an individual azimuth of visual space and oriented perpendicular to the rostral-caudal axis of the cortex. The present study provides evidence for intrinsic projections both along isoazimuth lamellae and between adjacent lamellae. These projections may play roles in the elaboration of wide receptive fields of cortical neurons.

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PubMed1988

Organization of retinogeniculate projections in turtles of the genera Pseudemys and Chrysemys.

Organization of retinal projections to the dorsal lateral geniculate complex in turtles has been studied by means of light and electron microscopic axon tracing techniques. Orthograde degeneration studies with Fink-Heimer methods following restricted retinal lesions show the entire retina has a topologically organized projection to the contralateral dorsal lateral geniculate complex. The nasotemporal axis of the retina projects along the rostrocaudal axis of the geniculate complex; the dorsoventral axis of the retina projects along the dorsoventral axis of the geniculate complex. The projection to the ipsilateral dorsal lateral geniculate complex originates from the ventral, temporal and nasal edges of the retina. The nasotemporal axis of the ipsilateral retina projects along the rostrocaudal axis of the geniculate complex. It was not possible to determine the orientation of the dorsoventral axis of the ipsilateral retina on the geniculate complex. Light microscopic autoradiographic tracing experiments and electron microscopic degeneration experiments show the retinogeniculate projection has a laminar organization. Retinogeniculate terminals are found in both the neuropile and cell plate throughout all three subnuclei of the dorsal lateral geniculate complex but have a distinctive distribution in each subnucleus. In the subnucleus ovalis, they are frequent in both the neuropile and cell plate which forms the rostral pole of the complex. In the dorsal subnucleus, they are most prevalent in the outer part of the neuropile layer, less frequent in the inner part of the neuropile, and rare in the cell plate. In the ventral subnucleus, they are frequent in the outer part of the neuropile but are also common in the inner part of the neuropile and cell plate. These observations point to several principles of geniculate organization in turtles. First, the complex receives projections from the entire contralateral retina and a segment of the ipsilateral retina. It thus has monocular and binocular segments that together receive a topologically organized representation of the binocular visual space and the contralateral monocular visual space. Second, the three geniculate subnuclei receive information from different, specialized regions of the retina and visual space. Subnucleus ovalis receives information from the frontal binocular visual field. The ventral subnucleus receives information from the caudal binocular field. The dorsal subnucleus receives input from the contralateral monocular field. Third, there is a lamination of retinal inputs in the geniculate complex which differs in character within the three subnuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1987

Caudal topographic nucleus isthmi and the rostral nontopographic nucleus isthmi in the turtle, Pseudemys scripta.

Isthmotectal projections in turtles were examined by making serial section reconstructions of axonal and dendritic arborizations that were anterogradely or retrogradely filled with HRP. Two prominent tectal-recipient isthmic nuclei--the caudal magnocellular nucleus isthmi (Imc) and the rostral magnocellular nucleus isthmi (Imr)--exhibited strikingly different patterns of organization. Imc cells have flattened, bipolar dendritic fields that cover a few percent of the area of the cell plate constituting the nucleus and they project topographically to the ipsilateral tectum without local axon branches. The topography was examined explicitly at the single-cell level by using cases with two injections at widely separated tectal loci. Each Imc axon terminates as a compact swarm of several thousand boutons placed mainly in the upper central gray and superficial gray layers. One Imc terminal spans less that 1% of the tectal surface. Imr cells, by contrast, have large, sparsely branched dendritic fields overlapped by local axon collaterals while distally, their axons nontopographically innervate not only the deeper layers of the ipsilateral tectum but also ipsilateral Imc. Imr receives a nontopographic tectal input that contrasts with the topographic tectal input to Imc. Previous work on nucleus isthmi emphasized the role of the contralateral isthmotectal projection (which originates from a third isthmic nucleus in turtles) in mediating binocular interactions in the tectum. The present results on the two different but overlapping ipsilateral tecto-isthmo-tectal circuits set up by Imc and Imr are discussed in the light of physiological evidence for selective attention effects and local-global interactions in the tectum.

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PubMed1987

Morphology of geniculocortical axons in turtles of the genera Pseudemys and Chrysemys.

An analysis has been made of the morphology of axons in the geniculocortical pathway of turtles using the anterograde transport of horseradish peroxidase in both in vivo and in vitro preparations. Following injections of HRP into the dorsolateral thalamus, labeled axons could be traced from the dorsal lateral geniculate complex to the telencephalon. They are unbranched and free of varicosities within the diencephalon. They travel in the dorsal peduncle of the lateral forebrain bundle, through the basal telencephalon and dorsally into the pallial thickening. Many axons are situated deep in the pallial thickening and bear numerous varicosities that often appear apposed to the proximal dendrites or somata of neurons retrogradely labeled by thalamic injections of horseradish peroxidase. Individual axons continue from the pallial thickening into the dorsal cortex where they shift dorsally and bear varicosities as they course from lateral to medial in the superficial third of layer 1. These data indicate that the terminal zone of the dorsal lateral geniculate complex within the telencephalon of turtles is more extensive in the mediolateral direction than previously believed. Geniculate axons bear varicosities both within the pallial thickening as well as the dorsal cortex, but have different relationships to potential postsynaptic elements in the two areas. Geniculocortical axons overlie somata and proximal dendrites of neurons in the pallial thickening, but intersect the distal dendrites of neurons in the dorsal cortex.

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PubMed1986

Organization of corticogeniculate projections in the turtle, Pseudemys scripta.

The efferent pathways from the visual cortex to the dorsal lateral geniculate complex of turtles have been studied by using the orthograde and retrograde transport of horseradish peroxidase (HRP). Injections of HRP in the lateral thalamus retrogradely label neurons throughout the visual cortex. The majority of labeled neurons have somata in layer 2 of the lateral part of dorsal cortex (D2); a minority have somata in layer 3. Labeled neurons in layer 2 tend to have vertically oriented, fusiform somata and dendrites that ascend into layer 1. Labeled neurons in layer 3 have fusiform somata and dendrites, both oriented horizontally. Injections of HRP in visual cortex orthogradely label corticofugal axons. Those projecting to the lateral geniculate complex course laterally from the visual cortex, pass through the striatum (occasionally bearing varicosities), and enter the diencephalon in the ventral peduncle of the lateral forebrain bundle. Individual axons leave the ventral peduncle and run dorsally in the transverse plane, entering the dorsal lateral geniculate complex from its ventral edge. They continue dorsally, principally in the cell plate of the geniculate complex, where they bear varicosities.

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PubMed1986

Morphology of neurons in the dorsal lateral geniculate complex in turtles of the genera Pseudemys and Chrysemys.

The morphology of neurons in the dorsal lateral geniculate complex of pond turtles has been studied by extracellular filling with horseradish peroxidase. The dorsal lateral geniculate complex is a rostrocaudally elongate structure that includes the nucleus ovalis and dorsal lateral geniculate nucleus of Papez. It is divided into three cytoarchitecturally distinct subnuclei: the subnucleus ovalis, the dorsal subnucleus, and the ventral subnucleus. Each subnucleus consists of a neuropile immediately internal to the optic tract and a cell plate of densely packed somata forming the medial face of the complex. The cell plate contains medial (or parvicellular) and lateral (or magnocellular) sublaminae that are separated by a cell-poor zone in subnucleus ovalis and the ventral subnucleus. The density and size of somata vary between subnuclei. Geniculate neurons fall into two distinct groups. Cell plate neurons have somata in the cell plate and vary substantially in morphology. Those in the medial sublamina have fusiform somata and dendrites that extend either mediolaterally (subnucleus ovalis) or run rostrocaudally in the cell plate (dorsal and ventral subnuclei). Those in the lateral sublamina have some dendrites that extend medially within the cell plate and others that extend into the neuropile. The latter dendrites branch and bear arbors of fingerlike, varicose branchlets in the outer half of the neuropile. By contrast, neuropile cells have fusiform somata and dendrites extending concentrically with the optic tract in the neuropile. Both groups of geniculate neurons can be retrogradely labeled by horseradish peroxidase injections in the lateral forebrain bundle. These results lead to the recognition of two principles of geniculate organization in turtles. The first is that the geniculate complex is divided into three subnuclei that vary in size and density of their neurons. The second is that the complex has a form of laminar organization different than that seen in the geniculate complex of mammals.

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PubMed1986

Optic tectum of the eastern garter snake, Thamnophis sirtalis. V. Morphology of brainstem afferents and general discussion.

Brainstem neurons that project to the optic tectum of the eastern garter snake were identified by retrograde transport of horseradish peroxidase. The distribution and morphology of tectal afferent axons from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation were then studied by anterograde transport of horseradish peroxidase. Diencephalic projections to the tectum arise from the ventral lateral geniculate complex ipsilaterally and the ventrolateral nucleus, suprapeduncular nucleus, and nucleus of the ventral supraoptic decussation bilaterally. Three pretectal groups (the lentiform thalamic nucleus, the lentiform mesencephalic-pretectal complex and the geniculate pretectal nucleus) give rise to heavy, bilateral tectal projections. Small neurons in nucleus isthmi and large reticular neurons in nucleus lateralis profundus mesencephali also give rise to bilateral projections. Caudal to the tectum, projections arise bilaterally from the pontine and medullary tegmentum, nuclei of the lateral lemniscus, the posterior colliculus, and the sensory trigeminal nucleus. A small contralateral projection arises from the medial vestibular complex. Tectal afferents from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation had characteristic morphologies and laminar distributions within the tectum. However, these afferents fall into two groups based on their spatial organization. Afferents from the thalamus and nucleus isthmi arise from small neurons with spatially restricted, highly branched dendritic trees. Their axons terminate in single, highly branched and bouton-rich arbors about 100 micron in diameter. By contrast, afferents from the midbrain reticular formation and the pretectum arise from large neurons with long, radiate, and sparsely branched dendritic trees. Their axons course parallel to the tectal surface and emit numerous collateral branches that are distributed widely through the mediolateral and rostrocaudal extent of either the central or superficial gray layers. Each collateral bears several small, spatially disjunct clusters of boutons.

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PubMed1986

Optic tectum of the eastern garter snake, Thamnophis sirtalis. III. Morphology of intrinsic neurons.

Extracellular iontophoretic injections of horseradish peroxidase and Golgi preparations were used to study the distribution and morphology of intrinsic neurons of the garter snake optic tectum. Four morphologically distinct classes of neurons were identified. The type A neuron is found throughout the retinorecipient tectal layers. It has a large, fusiform soma and infrequently branching dendrites that radiate in the horizontal plane and are studded with varicose appendages. An axon arises from the soma or proximal dendrite and gives rise to widely spreading branches that overlap the cell's dendritic field. The type B neuron has a small, spherical soma in sublayer b of the stratum fibrosum et griseum superficiale. Thick, varicose dendrites ascend from the soma and form a bushy arbor in the overlying sublayer a. A thin axon descends vertically from the soma and arborizes in vertical alignment with the cell's dendritic field in sublayer c of the stratum fibrosum et griseum superficiale and the upper third of the stratum griseum centrale. The type C neuron is a bipolar cell with a small, vertically fusiform soma situated at the upper border of the stratum griseum centrale. Thin, sparsely branching dendrites extend vertically into the superficial and central gray layers. An axon arises from the soma and courses ventrally into the stratum griseum centrale where it gives rise to a plexus of widely spreading branches that extend medially from the cell's dendritic field. The type D neuron is a small, stellate cell with a spherical soma and fine, appendage-laden dendrites that are restricted to the stratum griseum centrale. The axon of the type D cell courses in the central gray where it gives rise to widely spreading branches that extend laterally from the cell's dendritic field.

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PubMed1986

Optic tectum of the eastern garter snake, Thamnophis sirtalis. IV. Morphology of afferents from the retina.

The morphology of single retinal terminals in the optic tectum of the eastern garter snake was demonstrated by orthograde filling from extracellular injections of horseradish peroxidase (HRP) into the optic tract. HRP-filled terminals share a characteristic shape and structure. Their parent axons course caudally in the stratum opticum within fascicles of 200-300 fibers of varying diameters. Single axons exit a fascicle and course into either the stratum fibrosum et griseum superficiale, ventrally, or the stratum zonale, dorsally, where they bifurcate successively two or three times into preterminal branches. Each preterminal branch gives rise to many thin, terminal branchlets laden with boutons. The arbors are ellipsoidal with their long axes oriented mediolaterally and their short axes oriented rostrocaudally. Arbors vary in their overall size (from 45 to 150 micron), in the diameters of their parent axons (from less than 0.5 to 3.0 micron), and in the size of their terminal boutons (from 0.5 to 3.5 micron). Bouton size increased with increasing diameter of the parent axon. The great majority of arbors are confined to one of three retinorecipient sublayers in the superficial tectum. However, the full range of arbor sizes and axon diameters is present in each sublayer.

Afferent Pathways↗

PubMed1986

Optic tectum of the eastern garter snake, Thamnophis sirtalis. II. Morphology of efferent cells.

Tectal efferent neurons were retrogradely filled from extracellular injections of horseradish peroxidase (HRP) into pathways efferent from the tectum. Tectorotundal neurons have cylindrical dendritic trees, 80-100 microns in diameter, that extend vertically across the central and superficial tectal layers. Apical and basal dendrites are laden with complex appendages. The axon gives rise to an intratectal, collateral arbor that extends horizontally into the stratum griseum centrale beyond the cell's dendritic tree. The parent axon exits the tectum laterally in the tectothalamic tract. Tectogeniculate neurons also have narrow, radially oriented, and highly branched apical dendrites, but their basal dendrites are infrequently branched and lack appendages. An intratectal axon collateral forms a small, spherical arbor overlapping the apical dendrites in sublayer c of the stratum fibrosum et griseum superficiale. The parent axon ascends vertically and just below the stratum opticum turns rostrad to follow the optic fibers to the diencephalon. Tectoisthmi neurons have small somata and thin, radial dendrites that arborize below the pial surface in the stratum zonale. An intratectal axon collateral forms a spatially restricted arbor ventral to the soma in register with the dendritic tree. Tectoisthmobulbar neurons have dendrites that arborize extensively in sublayer a of the stratum fibrosum et griseum superficiale. The axon exits the tectum without collateralizing and joins a small-caliber component of the ventral tectobulbar tract. Ipsilateral tectobulbar neurons have stellate dendritic fields, 150-250 microns in diameter, that are restricted to the deep layers of the tectum. Sparsely branched dendrites are appendage-free but bear many short, fine spicules. The axon initially ascends from the soma and recurves into the stratum album centrale without collateralizing before joining a medium-caliber component of the ventral tectobulbar tract. Crossed tectobulbar neurons have large, stellate dendritic trees with diameters ranging from 200 to 500 microns. Like ipsilateral tectobulbar neurons, their dendrites are appendage-free but bear spicules. Their thick-caliber axons exit the tectum without collateralizing and course deep in the stratum album centrale to reach the dorsal tectobulbar tract.

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PubMed1986

Optic tectum of the eastern garter snake, Thamnophis sirtalis. I. Efferent pathways.

Extracellular, iontophoretic injections of horseradish peroxidase were used to anterogradely fill axons efferent from the optic tectum in garter snakes. The tectal efferent pathways consist of six axon types with distinct projections and terminal morphologies. Tectogeniculate axons pass into the diencephalon via the optic tract, bearing collaterals that form spatially restricted, rodlike arbors in the pretectum, the ventral lateral geniculate nucleus, and the ventrolateral nucleus. Tectoisthmi axons exit the tectum as a thin-caliber component of the ventral tectobulbar tract. They form spatially restricted, spherical arbors within nucleus isthmi. Tectoisthmobulbar axons also give rise to small, spherical arbors within nucleus isthmi, but the parent axons continue caudally into the pontine and medullary reticular formation issuing many short collateral branches. Tectorotundal axons reach the diencephalon via the tectothalamic tract and give rise to fine terminal collaterals in the nucleus of the tectothalamic tract ipsilaterally and in nucleus rotundus bilaterally. Single axons form sheetlike terminal fields that span the rostrocaudal extent of nucleus rotundus. Ipsilateral tectobulbar axons descend into the midbrain tegmentum where they issue several thick collaterals that terminate widely throughout the nucleus lateralis profundus mesencephali. The parent axon continues caudally giving off several widely spreading collaterals within the pontine and medullary reticular formation. Crossed tectobulbar axons enter the dorsal tectobulbar tract and cross the midline to form the predorsal bundle. Single axons give rise to terminal collaterals in the nucleus lateralis profundus mesencephali bilaterally, the contralateral pontine and medullary reticular formation, and the intermediate gray of the cervical spinal cord.

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PubMed1986

Conduction velocity, size and distribution of optic nerve axons in the turtle, Pseudemys scripta elegans.

Electrophysiological and morphological techniques have been used to characterize optic nerve axons in the red-eared turtle. Three distinct groups of axons are identified on the basis of conduction velocity and axon diameter. The first group (T1) is a small population of axons with large diameters (2.8-4.5 microns) and mean conduction velocities of 13 m/sec. The second group (T2) is a large population of axons with medium diameters (0.4-2.8 microns) and mean conduction velocities of 3 m/sec. The third group (T3) is a medium sized population of small diameter (0.2-0.6 micron), mostly unmyelinated axons with mean conduction velocities of 1 m/sec. There is a significant regional variation in the size, density and myelination of axons in the optic nerve. Large axons are found dorsally and ventrally, while smaller axons and the majority of unmyelinated fibers are found along a dorsotemporal to ventronasal axis through the nerve. Fink-Heimer techniques were used to trace the trajectories of axons of different sizes from the retina to the brain. Large diameter axons can be traced along the dorsal and ventral portions of the optic tract, with a dorsal group leaving the tract in the pretectum and a ventral group entering the basal optic tract. These observations suggest that the distribution of axons within the optic nerve reflects in part the distribution of ganglion cell somata in the retina. However, there is also some segregation of axons of different sizes according to their various central targets.

Action Potentials↗

PubMed1986

Ultrastructure of the dorsal lateral geniculate complex in turtles of the genera Pseudemys and Chrysemys.

The ultrastructure of the dorsal lateral geniculate complex in the turtles Pseudemys scripta elegans and Chrysemys picta belli has been studied. The majority of neurons have somata situated in a cell plate that forms the medial face of the complex. Relatively few synaptic contacts occur on the somata of cell plate neurons, but three types of axon terminals contact their proximal dendrites as they course through the cell plate. The most frequent are terminals with clear, round synaptic vesicles that form asymmetric junctional complexes. A second type has both clear and round, and dense-cored vesicles and forms asymmetric junctional complexes. The third type has clear, pleomorphic vesicles and forms symmetric junctional complexes. The same three types of terminals are presynaptic to the dendrites of cell plate neurons as they proceed through the inner half of the geniculate neuropile that is interposed between the cell plate and optic tract. These dendrites branch into varicose, finger-like distal dendrites within the outer half of the neuropile. The finger-like processes contain clear, pleomorphic vesicles and are linked with each other via dendrodendritic synapses with symmetric junctional complexes. The distal dendrites are postsynaptic to axon terminals with clear, round synaptic vesicles and asymmetric junctional complexes. Cell plate neurons thus comprise three regions, each with a distinct synaptic organization. The first region is formed by the somata of cell plate neurons and receives few synaptic contacts. The second region is formed by the proximal dendrites both in the cell plate and the inner half of the neuropile. It receives a moderate contingent of axondendritic terminals. The last region is formed by the distal dendrites of the cell plate neurons. They participate in dendrodendritic synapses and are postsynaptic to axon terminals with clear, round synaptic vesicles.

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PubMed1985

Morphology of retinogeniculate terminals in the turtle, Pseudemys scripta elegans.

The dorsal lateral geniculate complex in turtles receives a bilateral, topographic projection from the retina and projects to the telencephalon. This study examined the morphology of individual retinogeniculate terminals that were filled with horseradish peroxidase by injections in the optic tract or optic tectum. A large number of retinogeniculate terminals were successfully filled and detailed drawings were prepared of 87 terminals. Terminals were classified into three types based on the size and number of varicosities in the terminal, and (if a terminal formed a spatially restricted arbor) the volume of the arbor. Type I retinogeniculate terminals form spatially restricted, large-volume arbors with a low density of large varicosities. Type II retinogeniculate terminals form small volume arbors with a high density of small varicosities. Type III retinogeniculate terminals, in contrast to types I and II, do not form spatially restricted arbors. Rather, they consist of sparsely branched axons that parallel the optic tract and contain scattered en passant varicosities. Plots of the distribution of different terminal types throughout the geniculate complex show that all three terminal types occur throughout the rostrocaudal and mediolateral extents of the complex. However, each terminal type has a preferential distribution with type II terminals being concentrated in the outer half of the neuropile, type I terminals in the inner half of the neuropile, and type III terminals in the cell plate. All three types can arise from axons that continue caudally to terminate in the tectum. These findings raise the possibility that various classes of retinal ganglion cells differ in their mode of termination within the geniculate complex, but the precise relation between the three types of retinogeniculate terminals and the classes of ganglion cells remains to be determined.

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PubMed1985

Tectoreticular pathways in the turtle, Pseudemys scripta. II. Morphology of tectoreticular cells.

The morphology of tectoreticular neurons in turtles was examined with serial section reconstructions of neurons retrogradely filled with HRP. Six classes of tectal neurons project into the three tectobulbar pathways characterized in the preceding paper (Sereno, '85). (1) Large multipolar neurons with somata in the central gray layers, and with moderately branched dendrites sometimes spanning over a millimeter, project into the dorsal tectobulbar pathway, TBd. Their dendrites are covered with fine spicules and tend to arborize in the lower third of the superficial gray layers. (2) Medium-sized neurons with multiple radial dendrites and somata in the central white and upper periventricular layers probably project into the ipsilateral intermediate tectobulbar pathway, TBi. Their dendrites also bear fine spicules and usually reach the tectal surface. (3) Small radial cells in the periventricular layers, and (4) small bitufted radial cells in the superficial gray project into the small caliber component of the ipsilateral ventral tectobulbar pathway, TBv(sm). (5) Medium-sized central gray neurons with stratified dendrites, and (6) medium-sized central gray neurons with horizontal dendrites probably project into the medium caliber component of the ventral tectobulbar pathway, TBv(med). In contrast to TBd and TBi neurons, these last four classes emit a spray of long, filamentous dendritic appendages in the central gray and have dendritic arbors near the top of the superficial gray. The morphology of the neurons described in this and the preceding paper is briefly discussed in light of current ideas about tectally mediated sensorimotor transformations.

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PubMed1985

Synaptic organization of dorsal area of the turtle, Pseudemys scripta elegans.

The dorsal ventricular ridge is a subcortical structure receiving sensory information from the thalamus in reptiles. In the red-eared turtle, Pseudemys scripta elegans, it contains four cytoarchitectonic areas each characterized by distinct thalamic projections. This is an electron microscopic study of one of these, the dorsal area, which receives its thalamic input from the tectorecipient nucleus rotundus. It contains four concentric zones, internal to the ependymal zone, each of which is distinguished by the distribution of spiny and aspiny neurons. The ependymal zone of dorsal area contains tanycytes whose tails extend into zones 2 and 4. Synapses, usually with asymmetric junctional complexes and round synaptic vesicles, occur on these processes. Zone 1 neurons have fusiform somata and dendrites that parallel the ventricular surface. Their cytoplasm contains rough endoplasmic reticulum located primarily in Nissl bodies, lipofuchsin granules, multivesicular bodies, extensive arrays of Golgi apparatus, and large numbers of mitochondria. Synapses occur mainly on dendritic spines and shafts of zone 1 neurons and less frequently on somata. The majority have round vesicles and asymmetric junctional complexes. In contrast to those in zone 1, neurons in zones 2 and 4 have large amounts of rough endoplasmic reticulum, giving their cytoplasm an electron-dense quality. Synapses occur mainly on spines and shafts of zone 2 and 4 neurons. As in zone 1, the majority have round synaptic vesicles and contain asymmetric junctional complexes. Zones 2 and 4 contain clusters of neurons distributed among isolated neurons. The clusters are larger and less frequent in zone 2. Protoplasmic and fibrous glial processes, axon boutons, dendrites, and axon fascicles surround the neuron clusters. Though less numerous, the same structures also occur inside the clusters. Most synapses inside the clusters have round synaptic vesicles, asymmetric junctional complexes, and occur mainly on spines. Some neurons in clusters have somata whose plasma membranes are in direct apposition. In contrast to dorsal ventricular ridge in snakes, no specialized intercellular contacts were seen between somata in clusters.

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