Transient cellular structures in developing corpus callosum of the human brain

The corpus callosum connects two cerebral hemispheres as the most voluminous fiber system in the human brain. The developing callosal fibers originate from immature pyramidal neurons, grow through complex pathways and cross the midline using different substrates in transient fetal structures. We analyzed cellular structures in the human corpus callosum on postmortem brains from the age of 18 weeks post conception to adult, using glial fibrillary acidic protein, neuron-specific nuclear protein, and chondroitin sulphate immunocytochemistry. We found the presence of transient cellular structures, callosal septa, which divide major fiber bundles and ventrally merge with subcallosal zone forming grooves for callosal axons. The callosal septa are composed of glial fibrillary acidic protein reactive meshwork, neurones and the chondroitin sulphate immunoreactive extracellular matrix. The developmental window of prominence of the callosal septa is between 18-34 weeks post conception which corresponds to the period of most intensive growth of callosal axons in human. During the early postnatal period the callosal septa become thinner and shorter, lose their neuronal and chondroitin sulphate content. In conclusion, transient expression of neuronal, glial and extracellular, growing substrate in the callosal septa, as septa itself, indicates their role in guidance during intensive growth of callosal fibers in the human brain. These findings shed some light on the complex morphogenetic events during the growth of the corpus callosum and represent normative parameters necessary for studies of structural plasticity after perinatal lesions.     

Interaction of reticular structures in the brain of the cat

In acute and chronic experiments on 35 cats an inhibitory influence was found of the caudal reticular nucleus of pons Varolii on unit activity of the sensorimotor cortex and dorsal part of the midbrain reticular formation. The influence of this structure on unit activity of the ventral part of the midbrain reticular formation was mainly of a facilitatory character. Activation of the ventral part inhibited the unit activity of the dorsal part of the same structure. Consequently, the caudal reticular nucleus of pons Varolii elicits inhibition at the level not only of the cerebral cortex but also of the midbrain reticular formation (of its dorsal part). The character of these influences coincides with that of unit activity changes of these two areas of the midbrain reticular formation during the development of the paradoxical phase of sleep. The obtained facts must underlie the stopping of convulsive activity in this phase of sleep.     

Distribution of directionally selective neurons within the thalamic nuclei and striopallidal complex of the human brain

Spike activity was analyzed in the course of visual testing for directional sensitivity in 177 neuronal populations in different thalamic nuclei and the striopallidal complex in the brain of nine parkinsonian patients, diagnosed and treated using implanted intracerebral electrodes. Directionally selective neurons were discovered in the centrum medianum, the thalamic zona incerta and reticular nucleus, the caudate nucleus, and the central area of the globus pallidus. Proportions and distribution of neurons with different properties were investigated in the thalamic nuclei and striopallidal complex.Institute of Experimental Medicine, Academy of Medical Sciences of the USSR, Leningrad. Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 21, No. 5, pp. 652–660, September–October, 1989.     

Absence of Cajal-Retzius cells and subplate neurons associated with defects of tangential cell migration from ganglionic eminence in Emx1/2 double mutant cerebral cortex

Emx1 and Emx2, mouse orthologs of the Drosophila head gap gene, ems, are expressed during corticogenesis. Emx2 null mutants exhibit mild defects in cortical lamination. Segregation of differentiating neurons from proliferative cells is normal for the most part, however, reelin-positive Cajal-Retzius cells are lost by the late embryonic period. Additionally, late-born cortical plate neurons display abnormal position. These types of lamination defects are subtle in the Emx1 mutant cortex. In the present study we show that Emx1 and Emx2 double mutant neocortex is much more severely affected. Thickness of the cerebral wall was diminished with the decrease in cell number. Bromodeoxyuridine uptake in the germinal zone was nearly normal; moreover, no apparent increase in cell death or tetraploid cell number was observed. However, tangential migration of cells from the ganglionic eminence into the neocortex was greatly inhibited. The wild-type ganglionic eminence cells transplanted into Emx1/2-double mutant telencephalon did not move to the cortex. MAP2-positive neuronal bodies and RC2-positive radial glial cells emerged normally, but the laminar structure subsequently formed was completely abnormal. Furthermore, both corticofugal and corticopetal fibers were predominantly absent in the cortex. Most importantly, neither Cajal-Retzius cells nor subplate neurons were found throughout E11.5-E18.5. Thus, this investigation suggests that laminar organization in the cortex or the production of Cajal-Retzius cells and subplate neurons is interrelated to the tangential movement of cells from the ganglionic eminence under the control of Emx1 and Emx2.     

Depression of evoked potentials in rat thalamic ventro-basal complex and somatosensory cortex after reticular stimulation

Experiments on unanesthetized rats immobilized with D-tubocurarine showed that electrical stimulation (100/sec) of the central gray matter and the mesencephalic and medullary reticular formation considerably depressed potentials in the somatic thalamic relay nucleus and somatosensory cortex evoked by stimulation of the forelimb or medial lemniscus. The mean threshold values of the current used for electrical stimulation of these structures did not differ significantly and were 70 (20–100), 100 (20–120), and 120 (50–200) µA, respectively. On comparison of the amplitude-temporal characteristics of inhibition of evoked potentials during electrical stimulation of the above-mentioned structures by a current of twice the threshold strength, no significant differences were found. Immediately after the end of electrical stimulation the amplitude of the cortical evolved potential and the post-synaptic components of the thalamic evoked potential was 50–60% (P<0.01) below the control values. The duration of this depression varied from 0.5 to 1 sec. An increase in the intensity of electrical stimulation of brain-stem structures to between three and five times the threshold led to depression of the presynaptic component of the thalamic evoked potential also. Depression of the evoked potential as described above was found with various ratios between the intensities of conditioning and testing stimuli.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 467–475, September–October, 1976.     

A spatial bias for the origins of interneuron subgroups within the medial ganglionic eminence

Although it is well established that the ventral telencephalon is the primary source of GABAergic cortical interneurons in rodents, little is known about the specification of specific interneuron subtypes. It is also unclear whether the potential to achieve a given fate is established at their place of origin or by signals received during their migration to or during their maturation within the cerebral cortex. Using both in vivo and in vitro transplantation techniques, we find that two major interneuron subgroups have largely distinct origins within the MGE. Somatostatin (SST)-expressing interneurons are primarily generated within the dorsal MGE, while parvalbumin (PV)-expressing interneurons primarily originate from the ventral MGE. In addition, we show that significant heterogeneity exists between gene expression patterns in the dorsal and ventral MGE. These results suggest that, like the spinal cord, neuronal fate determination in the ventral telencephalon is largely the result of spatially segregated, molecularly distinct microdomains arranged on the dorsal-ventral axis.     

Whole brain segmentation: automated labeling of neuroanatomical structures in the human brain

We present a technique for automatically assigning a neuroanatomical label to each voxel in an MRI volume based on probabilistic information automatically estimated from a manually labeled training set. In contrast to existing segmentation procedures that only label a small number of tissue classes, the current method assigns one of 37 labels to each voxel, including left and right caudate, putamen, pallidum, thalamus, lateral ventricles, hippocampus, and amygdala. The classification technique employs a registration procedure that is robust to anatomical variability, including the ventricular enlargement typically associated with neurological diseases and aging. The technique is shown to be comparable in accuracy to manual labeling, and of sufficient sensitivity to robustly detect changes in the volume of noncortical structures that presage the onset of probable Alzheimer's disease.     

Neuronal responses of the reticular and anterior ventral thalamic nuclei to stimulation of the thalamic ventrolateral nucleus and motor cortex

Responses of 137 neurons of the rostral pole of the reticular and anterior ventral thalamic nuclei to electrical stimulation of the ventrolateral nucleus and motor cortex were studied in 17 cats immobilized with D-tubocurarine. The number of neurons responding antidromically to stimulation of the ventrolateral nucleus was 10.5% of all cells tested (latent period of response 0.7–3.0 msec), whereas to stimulation of the motor cortex it was 11.0% (latent period of response 0.4–4.0 msec). Neurons with a dividing axon, one branch of which terminated in the thalamic ventrolateral nuclei, the other in the motor cortex, were found. Orthodromic excitation was observed in 78.9% of neurons tested during stimulation of the ventrolateral nucleus and in 52.5% of neurons during stimulation of the motor cortex. Altogether 55.6% of cells responded to stimulation of the ventrolateral nucleus with a discharge of 3 to 20 action potentials with a frequency of 130–350 Hz. Similar discharges in response to stimulation of the motor cortex were observed in 30.5% of neurons tested. An inhibitory response was recorded in only 6.8% of cells. Convergence of influences from the thalamic ventrolateral nucleus and motor cortex was observed in 55.7% of neurons. The corticofugal influence of the motor cortex on responses arising in these cells to testing stimulation of the ventrolateral nucleus could be either inhibitory or facilitatory.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 5, pp. 460–468, September–October, 1978.     

Effect of novocaine block of limbic system formations of the brain, hypothalamus and reticular formation on evoked potentials in the ventrobasal thalamic nucleus of the rat

The effect of novocain on evoked potentials (EP) recorded from the ventrobasal complex (VBC) of rats thalamus has been studied during a single pulse stimulation of the contralateral hind paw immobilized with curare. It has been established that significant sinchronized decrease of amplitudes of all the three early components of EP has been observed during the novocain administration in the reticular thalamic nucleus, lateral amygdaloid area and septum. During the blocking of the anterior and posterior hypothalamus the amplitude of the second negative component of EP in VBC decreases, while during the blocking of the dorsal hypothalamus an increase of the amplitudes of all the three early components of EP is mainly observed.     

Ultrastructural characteristics of the reticular thalamic nucleus neurons in WAG/Rij rats

The aim of this study was to investigate the ultrastructure of the reticular thalamic nucleus (RTN) in rats of WAG/Rij strain, an established model for human absence epilepsy. Most RTN neurons are medium-to large-sized and have either dark or light appearance, depending on their functional state. Moreover, small-sized neurons with short axons are present, their characteristics being described for the first time.     

Dynamics of action potential initiation in the GABAergic thalamic reticular nucleus in vivo

Understanding the neural mechanisms of action potential generation is critical to establish the way neural circuits generate and coordinate activity. Accordingly, we investigated the dynamics of action potential initiation in the GABAergic thalamic reticular nucleus (TRN) using in vivo intracellular recordings in cats in order to preserve anatomically-intact axo-dendritic distributions and naturally-occurring spatiotemporal patterns of synaptic activity in this structure that regulates the thalamic relay to neocortex. We found a wide operational range of voltage thresholds for action potentials, mostly due to intrinsic voltage-gated conductances and not synaptic activity driven by network oscillations. Varying levels of synchronous synaptic inputs produced fast rates of membrane potential depolarization preceding the action potential onset that were associated with lower thresholds and increased excitability, consistent with TRN neurons performing as coincidence detectors. On the other hand the presence of action potentials preceding any given spike was associated with more depolarized thresholds. The phase-plane trajectory of the action potential showed somato-dendritic propagation, but no obvious axon initial segment component, prominent in other neuronal classes and allegedly responsible for the high onset speed. Overall, our results suggest that TRN neurons could flexibly integrate synaptic inputs to discharge action potentials over wide voltage ranges, and perform as coincidence detectors and temporal integrators, supported by a dynamic action potential threshold.     

Immobilization stress-induced lesions of structures of the midbrain reticular formation

Emotional stress in rats resulted in blood-brain barrier increased permeability and brain parenchymal vessel disruptions. Stress induced microvascular damages were mainly observed in midbrain reticular formation. In this article the components of midbrain reticular formation were studied 1, 2, 4 and 6 weeks after immobilization stress. The destructive changes in some neurons, glia cells and myelin fibers were shown up to 6 weeks after immobilization. The signs of the recovery were also observed. It was supposed that the brain parenchymal vessel damages under emotional stress were due to the stress induced locus coeruleus dysfunctions.