Electrophysiological investigation of tecto-thalamo-telencephalic relations in frogs

Projections of the tectum mesencephali to the thalamus and telencephalon were investigated inRana temporaria. Individual cells in the optic neuropil of the lateral zone of the thalamus (corpus geniculatum thalami, n. Bellonci) and n. geniculatus lateralis respond to stimulation of the tectum mesencephali and to flashes but not to somatic stimuli. Many of the tectally reactive neurons in the medio-central zone of the thalamus, including n.postero-centralis, n.postero-lateralis, and n.rotundus, and in the telencephalon (the primordium of the hippocampus and septum) are convergent for somatic and visual impulses. The character of the macroresponses and spike responses to stimulation of the tectum mesencephali is the same for both zones of the thalamus. Tetanization within the lateral zone of the thalamus inhibits the conduction of visual and tectal impulses to the telencephalon, whereas during tetanization of the medio-central zone only the later components of visual and tectal evoked potentials of the telencephalon are suppressed. Responses with shorter latency than to stimulation of the medio-central zone arise in the telencephalon (primordium of the hippocampus) in response to electrical stimulation of the lateral zone by single pulses. In frogs the two divisions of the visual system — thalamo-telencephalic and tecto-thalamo-telencephalic — thus overlap considerably at the level of the thalamus and completely at the telencephalic level. In vertebrate phylogeny there is a progressive demarcation and specialization of these two visual channels.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 147–155, March–April, 1973.     

Comparative electrophysiological characteristics of afferent representation in the cortical and striatal divisions of the turtle forebrain

In experiments on immobilized, lightly anesthetized turtles the presence of visual and somatic representation was established in the subcortical striatal division of the forebrain — the pallial thickening, the dorsal ventricular ridge, and the putamen. In their physiological characteristics they are similar to the corresponding representation in the general cortex. The absence of significant differences between the latent periods of cortical and striatal evoked potentials to flashes and to stimulation of the dorsal thalamus indicates that visual projection fibers (from the lateral geniculate body) terminate at both cortical and striatal levels. Differences in the distribution of latent periods of unit responses in the cortex to visual and thalamic stimulation are due to the presence of a rotundo-telencephalic visual channel, with direct connections with the striatal and polysynaptic connections with the general cortex, as well as the geniculo-telencephalic tract. Considerable differences between the latent periods of the evoked potentials and also between unit responses to electrodermal stimulation in the cortical and striatal structures indicate that somatic projection fibers relay in the striatum on their path to the general cortex. Consequently, the somatosensory system of turtles is less corticalized than the visual system. Comparison of the results described with those obtained by workers studying other vertebrates suggests that the afferent supply of the striatum may be reorganized in the transition from premammals to mammals.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 7, No. 2, pp. 184–193, March–April, 1973.     

Transthalamic conduction of visual impulses to the cortex and subcortical forebrain structures in turtles

The thalamic relays for the conduction of impulses arising during photic stimulation of the eyes and electrical stimulation of the tectum in the general cortex, hyperstriatum (the dorsal ventricular ridge), and the striatum proper were studied in the turtleEmys orbicularis. Acute experiments on immobilized animals showed that anodal polarization temporarily and destruction of n. rotundus irreversibly suppress the main negative wave of the responses to tectal stimulation and to flashes in the hyperstriatum, whereas the corresponding responses in the general cortex still persist. Polarization and destruction of the lateral thalamic region, including the lateral geniculate body, have the opposite effect: responses in the hyperstriatum to photic and tectal stimulation are virtually unchanged whereas those in the general cortex disappear, except their late components. Preceding single stimulation of the tectum or n. rotundus depresses responses in the hyperstriatum evoked by flashes. However, during stimulation of the lateral thalamic region, combined potentials and single unit responses appear in the hyperstriatum and interact with responses evoked by tectal stimulation. It is concluded that the main pathways in turtles which supply visual information to the general cortex and hyperstriatum differ: the former relay in the lateral thalamic region, the latter in n. rotundus, although some overlapping of their projections in the hyperstriatum and striatum is possible.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 486–494, September–October, 1977.     

THE CENTRAL NERVOUS SYSTEM OF CHAMELEON VULGARIS

An analysis of the brain of Chameleon has given evidence of a number of modifications in the general plan of the reptilian nervous system. The telencephalon shows a reduced olfactory mechanism and suggests the presence of the true motor cortex. The ventral peduncle of the lateral forebrain bundle is unusually large with the majority of its fibres terminating in two new nuclear masses in the hypothalamus, the nucleus entopeduncularis, and a few in the nucleus ruber. A high degree of effective response to optic stimuli is indicated by the extensive correlation between the optic tectum and the nuclear masses of the midbrain and the diencephalon. The wide range of eye movements is probably correlated with the development of the basal optic root ganglion and its numerous secondary connections and the elaborate relationship of the geniculate complex. The usually highly developed tecto-bulbar system is partly replaced by a large tegmento-bulbar system that relays the tectal impulses caudally. An interesting mesencephalic nucleus is a large nucleus profundus mesencephali that receives a great variety of impulses and appears to be an important correlation center. Ascending visceral mechanisms were described bringing the hypothalamus into relationship with the lower centers. The great excursion of the tongue is associated with a highly differentiated hypoglossal nucleus. Components of the medial lemniscus from the spinal cord and the medulla to the vicinity of the thalamus were demonstrated.     

Evolutionary evaluation of reciprocity of connections in the turtle tectofugal visual system

In two turtle species—Emys orbicularis and Testudo horsfieldi—by the method of anterograde and retrograde traicing at the light and electron microscopy level, the existence is proven of direct descending projections from the thalamic nucleus of the tectofugal visual system n. rotunds (Rot) to the optic tectum. After injection of tracers into Rot alone and into Rot with involvement of the tectothalamic tract (Trtth), occasional labeled fibers with varicosities and terminals are revealed predominantly in the deep sublayers of SGFS of the rostral optic tectum, while in the lower amount—in other tectal layers. After the tracer injections into the optic tectum, a few retrogradely labeled neurons were found mainly in the Rot ventral parts and within Trtth. Their localization coincides with that of GABA-immunoreactive cells. Electron microscopy showed the existence of many retrogradely labeled dendrites throughout the whole Rot; a few labeled cell bodies were also present there, some of them being also GABA-immunoreactive. These results allow us to conclude about the existence of reciprocal connections between the optic tectum and Rot in turtles, these connections being able to affect processing of visual information in tectum. We suggest that reciprocity of tectothalamic connections might be the ancestral feature of the vertebrate brain; in the course of amniote evolution the functional significance of this feature can be decreased and even lost in parallel with a rise of the role of direct corticotectal projections.     

Dorsal thalamic connections of the ventral lateral geniculate nucleus of rats

In order to understand better the organisation of the ventral lateral geniculate nucleus of the ventral thalamus, this paper has examined the patterns of connections that this nucleus has with various nuclei of the dorsal thalamus in rats. Injections of biotinylated dextran or cholera toxin subunit B were made into the parafascicular, central lateral, posterior thalamic, medial dorsal, lateral dorsal, lateral posterior, dorsal lateral geniculate, anterior, ventral lateral, ventrobasal and medial geniculate nuclei of Sprague-Dawley rats and their brains were processed using standard tracer detection methods. Three general patterns of ventral lateral geniculate connectivity were seen. First, the parafascicular, central lateral, medial dorsal, posterior thalamic and lateral dorsal nuclei had heavy connections with the parvocellular (internal) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown previously to receive heavy inputs from many functionally diverse brainstem nuclei. Second, the visually related dorsal lateral geniculate and lateral posterior nuclei had heavy connections with the magnocellular (external) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown by previous studies to receive heavy inputs from the visual cortex and the retina. Finally, the anterior, ventral lateral, ventrobasal and medial geniculate nuclei had very sparse, if any, connections with the ventral lateral geniculate nucleus. Overall, our results strengthen the notion that one can package the ventral lateral geniculate nucleus into distinct visual (magnocellular) and non-visual (parvocellular) components.     

Geniculate relay of interhemispherical transmission of visual information

Electrophysiological experiments were made on conscious cats with different degree of dissection of the classical and commissural optic tracts to the lateral geniculate body. The data obtained indicated that dissection of the direct retinal tracts to the lateral geniculate body (unilateral dissection of the optic tract) leads merely to the reduction in the amplitude of evoked potentials in response to light flashes. However, the latent period of the first component of the response in this structure appeared the same as in intact animals. Comparison of these data with those obtained in analogous experiments made on the visual cortex allows the conclusion that the geniculate body is the main subcortical relay of the visual commissural effects which are transmitted from the retina to the cortical regions of the brain.     

Level of metabolic activity (cytochrome oxidase) as an index of functional significance of tectofugal and thalamofugal channels of the reptilian visual system

Using histochemical determination of activity of the mitochondrial oxidative enzyme cytochrome oxidase (CO) the metabolic activity in structures of brain of turtles and lizards was shown to be higher in centers of the tectofugal channel (tectal SGC, pretectal Ptv, thalamic Rot, telencephalic visual area of Advr of) than in the centers of the thalamofugal channel (thalamic GLd, Cxdl and Path of telencephalon) of the visual system. Some interspecies differences in distribution of CO activity in the tectal, thalamic, and telencephalic visual centers were revealed between terrestrial and pond turtles and lizards. The obtained data confirm the idea of the dominating role of the tectofugal visual pathway in information processing and organization of the usual behavior of reptiles.     

Changes in visual evoked potentials after injection of tetanus toxin into the lateral geniculate body

Changes in visual evoked potentials were studied in rats at different stages of formation of experimental photogenic epilepsy induced by injection of tetanus toxin into the lateral geniculate body. The greatest change in evoked potentials in the lateral geniculate body consisted of the appearance of an aditional component in the series of waves of the primary response. Meanwhile in the ipsilateral visual cortex the amplitude of the first negative component of the evoked potential was considerably increased. Correlation was found between the changes in the amplitude of this component in the visual cortex and the change in steepness of the additional component of the evoked potential in the geniculate body, reflecting functional reorganization of that nucleus. The results are evidence of significant disturbances of the relay function of the lateral geniculate body when a generator of pathologically enhanced excitation is formed in it.Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 142–149, March–April, 1978.     

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.     

The tecto-thalamo-telencephalic system of the tortoise (electrophysiological investigation)

Using tortoises immobilized with diplacin to which chloralose had been added, or under chloralose — nembutal narcosis, we investigated the distribution of evoked potentials and neuronal responses in the thalamus and forebrain induced by electrical excitation of the optic tectum and by flashes of light. In the thalamus the main mass of cells that reacted to these stimuli was concentrated in the nucleus rotundus and the tecto-thalamic tract; in the forebrain it was concentrated in the general cortex, the pallial thickening, and the neostriatum. In the two latter structures responses with shorter latent periods than those in the general cortex predominated. Visual and tectal neuronal responses, especially those of convergent cells, displayed correlation in their latent periods and types of response, which was more clearly shown in the thalamus. Submaximal tetanization of the optic tectum had a facilitating effect on cortical responses produced by light flashes and excitation of the nucleus rotundus. Complete blocking of transmission of tectal impulsation to the forebrain was observed on destruction of the tecto-thalamic tract region bounded by the lateral bundle of the forebrain, the lateral geniculate body, and the nucleus rotundus. High-frequency excitation of the nucleus rotundus produced only partial blocking of transmission (of the late components). It is concluded that there are various pathways of tectal impulsation through the thalamus to the forebrain.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 296–306, May–June, 1970.     

Evoked potentials to electrical stimulation of the facial nerve in the carp tectum mesencephali

Tectal evoked potentials to stimulation of the facial nerve, containing afferent fibers of nonolfactory chemoreception, in the carp are positive evoked potentials with a latent period of 5 to 25 msec which show no phase shift as the microelectrode is advanced to a depth of 600 µ. Depending on the amplitude and latency of evoked potentials seven active zones differing in one or both parameters were distinguished in the ipsilateral tectum mesencephali. The role of impulses from the medulla in the mechanism of tectal evoked potentials to facial nerve stimulation is proved by differences in latent periods and disappearance of the tectal response (although it is preserved in the primary center) after severance of connections between the two parts of the brain. Descending influences from the tectum on the primary center were found: its extirpation disturbs evoked potential generation in several parts of the medullla, so that they either disappear completely or their parameters are modified.A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 39–46, January–February, 1976.     

The role of nAChR-mediated spontaneous retinal activity in visual system development

In the developing vertebrate retina, nAChR synapses are among the first to appear. This early cholinergic circuitry plays a key role in generating "retinal waves," spontaneously generated waves of action potentials that sweep across the ganglion cell layer. These retinal waves exist for a short period of time during development when several circuits within the visual system are being established. Here I review the cholinergic circuitry of the developing retina and the role these early circuits play in the development of the retina itself and of retinal projections to the lateral geniculate nucleus of the thalamus.     

The level of metabolic activity (cytochrome oxidase) as an index of functional significance of tectofugal and thalamofugal channels of the reptilian visual system

Using histochemical determination of activity of the mitochondrial oxidative enzyme cytochrome oxidase (CO) in brain structures, metabolic activity both in turtles and in lizards has been shown to be higher in centers of the tectofugal channel (the tectal stratum griseum centrale, SGC; nucleus pretectalis ventralis, Ptv; thalamic nucleus rotundus, Rot; telencephalic visual area of the anterior dorsal ventricular ridge, Advr) than in the thalamofugal channel centers (the thalamic nucleus geniculatus lateralis pars dorsalis, GLd; cortex dorsolateralis, Cxdl; and pallial thickening, Path) of the visual system. Some interspecies differences in distribution of the CO activity in the tectal, thalamic, and telencephalic visual centers between terrestrial and pond turtles and lizards were revealed. The obtained data confirm the idea on the dominating role of the tectofugal channel over the thalamofugal channel of the visual system in information processing and organization of the day-to-day behavior of reptiles.     

Differentiation of efferent projections of the medial (cuneiform nucleus) and lateral regions of the reticular formation in cat midbrain

Efferent connections of medial (nucleus cuneiformis) and lateral regions of the midbrain reticular formation (MRF) were investigated using an anterograde autoradiographic technique in cats. Efferent fibers from the MRF ascend to the globus pallidus, substantia innominata, hypothalamus, subthalamus, and nonspecific associative and relay nuclei of the thalamus. Descending pathways to the conclusion that the cuneiform nucleus is more of a nonspecific structure than an association auditory center. The lateral reticular region had numerous projections to the lateral geniculate body and, together with the parabigeminal nucleus, forms the midbrain visual complex.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 17, No. 5, pp. 646–652, September–October, 1985.     

The classification of neurons in the corpus geniculatum laterale pars ventralis of the albino rat: a Golgi-Kopsch study

Neurons in the ventral lateral geniculate nucleus of 120 rats of a wistar strain were investigated with the Golgi Kopsch technique. Various features, such as size of neurons, their dendritic fine structure and their localization inside the nucleus were used to find new aspects for the classification of vLGN neurons except geniculo tectal relay neurons (GTR neurons). The results of our detailed investigation are in accordance with the classification given by Brauer et al. (1984). The dendrites show different morphological surface specializations, as there are smooth and varicose dendrites besides the spiny ones. Medium-sized neurons are most variable in their dendritic structure. It can be assumed that they represent a heterogenous group. The density of spines in the neurons investigated is much lower than in GTR neurons.     

The role of the thalamus in the flow of information to the cortex

The lateral geniculate nucleus is the best understood thalamic relay and serves as a model for all thalamic relays. Only 5-10% of the input to geniculate relay cells derives from the retina, which is the driving input. The rest is modulatory and derives from local inhibitory inputs, descending inputs from layer 6 of the visual cortex, and ascending inputs from the brainstem. These modulatory inputs control many features of retinogeniculate transmission. One such feature is the response mode, burst or tonic, of relay cells, which relates to the attentional demands at the moment. This response mode depends on membrane potential, which is controlled effectively by the modulator inputs. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to the cortex for the first time. By contrast, the other main thalamic relay of visual information, the pulvinar region, is largely a higher-order relay, since much of it relays information from layer 5 of one cortical area to another. All thalamic relays receive a layer-6 modulatory input from cortex, but higher-order relays in addition receive a layer-5 driver input. Corticocortical processing may involve these corticothalamocortical 're-entry' routes to a far greater extent than previously appreciated. If so, the thalamus sits at an indispensable position for the modulation of messages involved in corticocortical processing.     

Effect of injury to the lateral reticular nucleus and nuclei of the inferior olive on electrical responses of the cerebellar cortex evoked by vagus nerve stimulation in cats

The role of the lateral reticular nucleus and nuclei of the inferior olive in the formation of cerebellar cortical evoked potentials in response to vagus nerve stimulation was determined in experiments on 28 cats anesthetized with chloralose and pentobarbital. After electrolytic destruction of the lateral reticular nucleus, in response to vagus nerve stimulation, especially ipsilateral, lengthening of the latent period and a decrease in amplitude of evoked potentials were observed; after bilateral destruction of this nucleus, evoked potentials could be completely suppressed. It is concluded that the lateral reticular nucleus relays interoceptive impulses in the vagus nerve system on to the cerebellar cortex. Additional evidence was given by the appearance of spike responses of Purkinje cells, in the form of mainly simple discharges, to stimulation of the vagus nerve. After destruction of the nuclei of the inferior olive, the latent period and the number of components of evoked potentials in response to vagus nerve stimulation remained unchanged but their amplitude was reduced. The role of the nuclei of the inferior olive as a regulator of the intensity of the flow of interoceptive impulses to the cerebellum is discussed.N. I. Pirogov Medical Institute, Vinnitsa. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 290–299, May–June, 1977.     

Predictions about chromatic receptive fields assuming random cone connections

We hypothesize that color vision depends on random connections between cones containing different pigments and neurons at higher levels in the macaque visual system. This hypothesis predicts the same types and proportions of chromatic receptive fields reported in the physiological literature at least up through the lateral geniculate nucleus. The results suggest that the specificity of connections demanded by the labelled-line model of color coding are unnecessary to account for current physiological data.     

Developmental changes in intestinal globule leukocytes of normal rats

Among vertebrates, telencephalo-pontine systems exist only in birds and mammals. However, three nuclei in the diencephalon and mesencephalon of teleost fishes have been indicated — analogous to the pons — to represent relay stations between telencephalon and cerebellum. Since two of these nuclei (dorsal preglomerular nucleus, dorsal tegmental nucleus) have only been described in the highly derived, electrosensory mormyrids, we investigated telencephalic connections in two nonelectrosensory teleosts, the goldfish Carassius auratus and the freshwater butterflyfish Pantodon buchholzi, and cerebellar connections only in the latter species, since for C. auratus these connections are already established. Horseradish peroxidase tracing reveals that C. auratus has a dorsal tegmental nucleus and a paracommissural nucleus both of which are telencephalo-recipient and project to the cerebellum, and that P. buchholzi has a dorsal preglomerular nucleus with such connections. These results extend our knowlegde of the distribution and, therefore, the phylogeny of telencephalo-cerebellar systems in teleosts. Similar to tetrapods, teleosts appear to have developed telencephalo-cerebellar systems several times independently.