Cortical control of the afferent flow in the cat lateral geniculate body

Single unit responses in the dorsal lateral geniculate body to stimulation of the optic chiasma (testing) and area 17 (conditioning) of the visual cortex were studied in unanesthetized cats immobilized with tubocurarine. Two types of unit responses were found: P-responses (considered to be of relay, or principal, cells) and I-responses (considered to be of interneurons), whose parameters differed distinctly. Interaction between stimulation of the visual cortex and optic chiasma consisted of depression of the ability of the P cells of the nucleus to respond to testing stimulation. It is suggested that cortical influences on stimulus conduction by P cells of the nucleus is based on postsynaptic inhibition with the participation of interneurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 563–570, November–December, 1981.     

Afferent and efferent connections of lateral geniculate body pars ventralis in albino rats

The afferent and efferent connections of the Corpus geniculatum laterale, pars ventralis (Cglv) of the albino rat were investigated lightmicroscopically by using either silver degeneration methods and the HRP-method as well. In contrast to the dorsal Cgl the results show remarkably different afferent connections in the ventral Cgl. The afferent fibres originate from the following areas: a) Retina: the terminal degeneration area of optic fibres includes the lateral part of the Cglv only. b) Regio praetectalis: degenerating fibres from these region can also be observed in the medial part of the Cglv. c) Colliculus superior: degenerating fibres terminate mainly in the lateral part of the Cglv. In the superior colliculus these fibres originate particularly from the cells of lamina III. d) Visual cortex: neurons of layer V of area 17 project mainly to the lateral half of the Cglv. There was no evidence for a projecting of the parastriate cortex (area 18 a) to the Cglv. The efferent fibres reach the following target areas: a) Nucleus lateralis posterior, Regio praetectalis and Colliculus superior. Evidence for a projection to the dorsal Cgl requires further investigation. b) Zona incerta, Formatio reticularis mesencephali and Nucleus medialis et lateralis pontis. Neurons in the medial half ot eh Cglv project to the pons region. c) Crossing the Commissura posterior and the Commissura suprachiasmatica, efferent fibres reach the contralateral Cglv, the Regio praetectalis and the Colliculus superior. The results obtained from the rat are compared with findings from other mammalian species. The functional importance of the Cglv in the visual processes is discussed taking into consideration the specific connections.     

Cortical modulations of the fine temporal structures of impulse trains in the dorsal lateral geniculate body in cats]

In response to a visual stimulation, "replicated triplets" of impulses may appear in many spike trains recorded from the cat dorsal lateral geniculate nucleus (dLGN). The number and the temporal structure of these triplets depend upon the general organization of the geniculate impulse trains. In this study, we show that a pharmacological blockade of the corticothalamic activity, obtained through microinjection of GABA into area 17, affects the replicated triplet production and leads to an increase in the dispersal of their structure. These results suggest that the corticothalamic pathway closely influences the fine temporal organization of the geniculate messages.     

Peptide immunoreactivity of unmyelinated primary afferent axons in rat lumbar dorsal roots

The present study demonstrates calcitonin gene-related peptide (CGRP), somatostatin (SOM), bombesin (BOM), and substance P (SP) at the electron microscopic level in lumbar dorsal root axons of normal rats. The highest percentages of labeled axons were for CGRP (14%) and then, in descending order, for SP (8.6%), SOM (6.8%), and BOM (3.1%). The labeled axons were exclusively unmyelinated for SP, SOM, and BOM, and predominantly unmyelinated for CGRP. These data are consistent with the data for labeled sensory cell bodies for these same compounds. We emphasize that these peptides were immunocytochemically visualized in the dorsal roots without experimental manipulation, such as colchicine or dorsal root ligation. Quantitative sampling of this type can be used to assay changes in response to physiological stimuli in numbers of sensory axons that contain identifiable concentrations of these peptides.     

Functional heterogeneity of lateral geniculate body neurons in the rabbit]

Background and evoked activity of LGB units was studied on immobilized and anaesthetized rabbits. Two groups of projection units were revealed, differing by the level of background activity, latencies and mean frequency of discharges in responses to single photic flashes and to electrical stimulation of the optic nerve. It is assumed that these groups of units belong to the slowly and rapidly conducting paths of sensory information transmission in the visual projection system.     

Morphometric analysis of the dorsal lateral geniculate body of the laboratory mouse and microphthalmic variants

The dorsal lateral geniculate nucleus (dLGN) of the mouse (strain 944 with microphthalmus syndrome, P. Hertwig 1942) has been morphometrically investigated. The eyes of the gray littermates develop normally (control animals) while the white littermates show the microphthalmus syndrome (microphthalmic animals). Four male twentyone day old animals were examined. Following perfusion with formaldehyde (diluted 1:4 with H2O) Nissl stained frontal sections were used for classification of neurons. we differentiate between projection neurons (PN) (geniculo-cortical relay neurons) and interneurons (IN). We succeeded in identifying these two cell types using deimpregnated Golgi preparations (Fairén et al. 1977), which were counterstained with methylene blue (0.1%). The neuronal packing density and the ratio PN:IN were conventionally determined using 20 micron thick sections from both control and microphthalmic animals. In both cases this ratio was 12:1 (= 8% IN). Measuring fields distributed in a meander-like manner were selected for estimating the neuronal density. Measuring fields giving equal values were connected. The results show that we can distinguish between 3 regions of dLGN: lateral, medial and caudal. They differ with regard to neuronal packing density, size and structure of the PN. Using 3 micron thick Nissl stained sections from both control and microphthalmic animals the size and structure of PN were automatically determined using the picture processing device "MORPHOQUANT" VEB Carl Zeiss JENA). The distribution pattern of IN is apparently uniform. The microphthalmus syndrome produces a diminished number of pathologically changed retinal fibers. The morphometrically registered findings show that within the dLGN, of the microphthalmus mouse the neuronal packing density increased (diminution of neuropil), the size of PN decreased and their structural density increased (transneuronal dystrophy). Compared with the other parts, the medial part of the dLGN is minimally altered.     

Electrophysiologic relations in the lateral geniculate body

Responses were evoked in the lateral geniculate body (L.G.B.) of "encéphale isolé" cats by single-shock stimulation of either the geniculate body or the optic tract of the other side. Responses to optic tract stimulation were modified following excitability changes in the contralateral L.G.B. due to topical application of strychnine and KCl. Laminar stimulation and recording in different layers (A, A1, and B) suggested the existence of a certain homotopic organization of L.G.B. interconnections. The activity evoked in the L.G.B. was found to be abolished by electrocoagulation of the posterior commissure and intermediate gray matter. These results point to the presence of a transthalamic pathway which might mediate L.G.B. activity to the contralateral visual cortex.     

Receptive field types in the lateral geniculate body and their frequency characteristics]

Time amplitude -- frequency characteristics of the I and II types of receptive fields (RF) of lateral geniculate and their dependence on the contrast and spatial parameters of the light stimulus were studied. It is shown that the frequency characteristics of the RF I type depends on the contrast and area of the light stimulus, the higher being the contrast at a small area the smaller are the low frequencies. However at a large area of the stimulus the inhibition of low frequencies is greater at a small contrast. The transmitting band of frequency characteristics of RF II type does not depend on the contrast at a small area of the stimulus, at a large area a fall of low frequencies takes place at high contrasts of the stimulus. Such different behaviour of the receptive fields is explained by the models, which take into account RF spatial characteristics.     

Fine structure of synapses in the dorsal nucleus of the lateral geniculate body of normal and blinded rats

Summary Degenerating boutons, observed from 2 to 60 days after eye enucleation, displayed decreased plasma membrane density, increased axoplasmic density, and enlarged mitochondria with deformed cristae when compared with boutons from normal animals. There was also a loss of synaptic plasma membrane specialization and the boutons abnormally indented contiguous dendrites. The number and appearance of synaptic vesicles in some degenerating boutons were notably altered. Phagocytosis of boutons in most instances appeared to be accomplished by astrocytes. When degeneration was first apparent in some boutons, the subsynaptic organelle in the adjacent dendritic cytoplasm was enlarged, somewhat less dense and was associated with small granular and circular profiles. Subsynaptic organelles in experimental animals were absent from contiguities between dendrites and other cell processes, except in a few instances when only small portions of boutons remained at their synaptic sites, suggesting that the organelles disappeared when boutons had been completely phagocytized.Degenerating myelinated axons, observed from 2 to 300 days after enucleation, exhibited the same triad of features as degenerating boutons. They appeared to be phagocytized in most instances by dense glial processes, presumably oligodendrocytic, which were normally situated between the axon and its myelin sheath and were related to the inner mesaxon.This investigation was supported by U.S.P.H.S. Training Grants Nos. 2 T1 GM 202 T1 CA 505506, and 2RO 1 AM 368806.The author expresses his appreciation to Dr. A. J. Ladman for acquainting him with the techniques used in the study and to Dr. R. J. Barrnett for valuable criticism of this report. Gratitude is also extended to Mr. E. Z. Rutkowski for making the drawing.     

Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey

Summary Synaptic junctions are found in all parts of the nucleus, being almost as densely distributed between cell laminae as within these laminae.In addition to the six classical cell laminae, two thin intercalated laminae have been found which lie on each side of lamina 1. These laminae contain small neurons embedded in a zone of small neural processes and many axo-axonal synapses occur there.Three types of axon form synapses in all cell laminae and have been called RLP, RSD and F axons. RLP axons have large terminals which contain loosely packed round synaptic vesicles, RSD axons have small terminals which contain closely packed round vesicles and F axons have terminals intermediate in size containing many flattened vesicles.RLP axons are identified as retinogeniculate fibers. Their terminals are confined to the cell laminae, where they form filamentous contacts upon large dendrites and asymmetrical regular synaptic contacts (with a thin postsynaptic opacity) upon large dendrites and F axons. RSD axons terminate within the cellular laminae and also between them. They form asymmetrical regular synaptic contacts on small dendrites and on F axons. F axons, which also occur throughout the nucleus, form symmetrical regular contacts upon all portions of the geniculate neurons and with other F axons. At axo-axonal junctions the F axon is always postsynaptic.Supported by Grant R 01 NB 06662 from the USPHS and by funds of the Neurological Sciences Group of the Medical Research Council of Canada. Most of the observations were made while R. W. Guillery was a visiting professor in the Department of Physiology at the University of Montreal. We thank the Department of Physiology for their support and Mr. K. Watkins, Mrs. E. Langer and Mrs. B. Yelk for their skillful technical assistance.     

The beta sector of the rabbit's dorsal lateral geniculate nucleus

The beta sector of the rabbit's dorsal lateral geniculate nucleus is a small region of nerve cells scattered among the fibres of the geniculocortical pathway. In its topographical relations it resembles the perigeniculate nucleus of carnivores, which contains neurons driven by geniculate and visual cortical neurons and which sends inhibitory fibres back into the geniculate relay. We have traced retinogeniculate, geniculocortical and corticogeniculate pathways in rabbits by using horseradish peroxidase or radioactively labelled proline and have found that the beta sector resembles the perigeniculate nucleus in receiving no direct retinal afferents, sending no efferents to the visual cortex (V-I), and receiving afferents from the visual cortex. The corticogeniculate afferents are organized so that the visual field map in the beta sector and the main part of the lateral geniculate relays are aligned, as are the maps in the cat's perigeniculate nucleus and the main part of the geniculate relay of carnivores. Electron microscopical studies show similar types of axon terminals in the rabbit and the cat for the main part of the geniculate relay on the one hand and for the beta sector and the perigeniculate nucleus on the other. Earlier observations that the proportion of putative inhibitory terminals (F-type terminals) is lower in the rabbit's than the cat's geniculate region are confirmed. A major difference between the beta sector and the perigeniculate nucleus has been revealed by immunohistochemical staining for GABA. Whereas almost all of the cat's perigeniculate cells appear to be GABAergic, the proportion in the beta sector is much lower, and not significantly different from that found in the main part of the rabbit's geniculate relay. It is concluded that the beta sector shares many of the organizational features of the perigeniculate nucleus. A common developmental origin seems probable, but the functional differences remain to be explored.     

A multi-compartment model for interneurons in the dorsal lateral geniculate nucleus

GABAergic interneurons (INs) in the dorsal lateral geniculate nucleus (dLGN) shape the information flow from retina to cortex, presumably by controlling the number of visually evoked spikes in geniculate thalamocortical (TC) neurons, and refining their receptive field. The INs exhibit a rich variety of firing patterns: Depolarizing current injections to the soma may induce tonic firing, periodic bursting or an initial burst followed by tonic spiking, sometimes with prominent spike-time adaptation. When released from hyperpolarization, some INs elicit rebound bursts, while others return more passively to the resting potential. A full mechanistic understanding that explains the function of the dLGN on the basis of neuronal morphology, physiology and circuitry is currently lacking. One way to approach such an understanding is by developing a detailed mathematical model of the involved cells and their interactions. Limitations of the previous models for the INs of the dLGN region prevent an accurate representation of the conceptual framework needed to understand the computational properties of this region. We here present a detailed compartmental model of INs using, for the first time, a morphological reconstruction and a set of active dendritic conductances constrained by experimental somatic recordings from INs under several different current-clamp conditions. The model makes a number of experimentally testable predictions about the role of specific mechanisms for the firing properties observed in these neurons. In addition to accounting for the significant features of all experimental traces, it quantitatively reproduces the experimental recordings of the action-potential- firing frequency as a function of injected current. We show how and why relative differences in conductance values, rather than differences in ion channel composition, could account for the distinct differences between the responses observed in two different neurons, suggesting that INs may be individually tuned to optimize network operation under different input conditions.