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PubMed1979

Transventricular blood vessels in the third ventricle of the armadillo brain.

Transventricular filaments were observed with scanning electron microscopy on the dorsolateral wall of the infundibular recess of the third ventricle of the armadillo brain. Two to seven transventricular filaments per animal were present in 6 of 18 animals. There were two types of transventricular filaments, ciliated and bare. Using transmission electron microscopy, we determined that these filaments consisted of a single, central capillary surrounded by ciliated ependymal cells and a small accumulation of axons in five animals. In one animal, a bare filament had a central capillary surrounded by a large accumulation of axons with no surrounding ependyma. The consistent location and structure of these filaments indicate a possible function for a small vascular network, as well as a possible commissural network connecting right and left hypothalami in the region of infundibular nuclei.

Animals↗
PubMed2006

Small-world network organization of functional connectivity of EEG slow-wave activity during sleep.

OBJECTIVE: To analyze the functional connectivity patterns of the EEG slow-wave activity during the different sleep stages and Cyclic Alternating Pattern (CAP) conditions, using concepts derived from Graph Theory. METHODS: We evaluated spatial patterns of EEG slow-wave synchronization between all possible pairs of electrodes (19) placed over the scalp of 10 sleeping healthy young normal subjects using two graph theoretical measures: the clustering coefficient (Cp) and the characteristic path length (Lp). The measures were obtained during the different sleep stages and CAP conditions from the real EEG connectivity networks and randomized control (surrogate) networks (Cp-s and Lp-s). RESULTS: Cp and Cp/Cp-s increased significantly from wakefulness to sleep while Lp and Lp/Lp-s did not show changes. Cp/Cp-s was higher for A1 phases, compared to B phases of CAP. CONCLUSIONS: The network organization of the EEG slow-wave synchronization during sleep shows features characteristic of small-world networks (high Cp combined with low Lp); this type of organization is slightly but significantly more evident during the CAP A1 subtypes. SIGNIFICANCE: Our results show feasibility of using graph theoretical measures to characterize the complexity of brain networks during sleep and might indicate sleep, and the A1 phases of CAP in particular, as a period during which slow-wave synchronization shows optimal network organization for information processing.

Adult↗
PubMed2007

Realistically coupled neural mass models can generate EEG rhythms.

We study the generation of EEG rhythms by means of realistically coupled neural mass models. Previous neural mass models were used to model cortical voxels and the thalamus. Interactions between voxels of the same and other cortical areas and with the thalamus were taken into account. Voxels within the same cortical area were coupled (short-range connections) with both excitatory and inhibitory connections, while coupling between areas (long-range connections) was considered to be excitatory only. Short-range connection strengths were modeled by using a connectivity function depending on the distance between voxels. Coupling strength parameters between areas were defined from empirical anatomical data employing the information obtained from probabilistic paths, which were tracked by water diffusion imaging techniques and used to quantify white matter tracts in the brain. Each cortical voxel was then described by a set of 16 random differential equations, while the thalamus was described by a set of 12 random differential equations. Thus, for analyzing the neuronal dynamics emerging from the interaction of several areas, a large system of differential equations needs to be solved. The sparseness of the estimated anatomical connectivity matrix reduces the number of connection parameters substantially, making the solution of this system faster. Simulations of human brain rhythms were carried out in order to test the model. Physiologically plausible results were obtained based on this anatomically constrained neural mass model.

Brain Mapping↗
PubMed2002

The cell assembly, Mark III: transitions between brain states and the localization and generalization of function.

A connectionist model of the cerebral cortex as a Markov chain is presented. The model reconciles localization of cortical functions, apparent in neuropsychological data, with the equipotentiality and diffusion of function that was noted in classical studies of learning. In addition, the model suggests that the specificity of some cortical activity can develop quickly from initial connections that are predominantly chaotic. It is not necessary to assume a high degree of "prewired" connections in the cortex of newborns. While the model does not require initial settings to be chaotic, neither does it require initial ordering stronger than chaos. Specifically, a large and initially randomly connected network of neurons rapidly develops several ergodically distinct subnetworks that operate independently. While the ergodic units operate independently and distinctly, they exhibit the property of equipotentially in that the specific events caused by an initial event are equally likely to be at any point in the ergodic set after very few synaptic events.

Animals↗
PubMed2000

Immunocytochemical localization of pedal peptide in the central nervous system of the gastropod mollusc Tritonia diomedea.

Tritonia pedal ganglion peptides (TPeps) are a trio of pentadecapeptides isolated from the brain of the nudibranch Tritonia diomedea. TPeps have been shown both to increase the beating rate of ciliated cells of Tritonia and to accelerate heart contractions in the mollusc Clione limacina. Here we examine the immunocytochemical distribution of TPeps in the Tritonia central nervous system. We found the brain and buccal ganglia to be rich sources of TPep immunoreactivity. Specific cells in both structures, some of them previously identified, were immunoreactive. Moreover, immunoreactive fibers were seen connecting ganglia and exiting almost all the major nerves. In the brain, we found that the paired, ciliated statocysts apparently receive TPep innervation. In addition, we observed unstained cell bodies in each buccal ganglion with extensive TPep immunoreactive projections surrounding their somata and primary neurites. Similar projections were not observed in the brain. We also compared the TPep immunoreactivity with that of SCP(b) in the buccal ganglia. We observed many neurons and processes that were immunoreactive to both peptides. One neuron that contains both TPep- and SCP(b)-like peptides (B12) has an identified role in the Tritonia feeding network. Together, these findings suggest that TPeps may play an active role in the central nervous system of Tritonia as neurotransmitters modulating orientation, swimming, and feeding.

Animals↗
PubMedDate not supplied

Pharmacological testing of intracortical interneuronal connections.

The present study comprises an attempt to investigate the influence of acetylcholine on the functional connections of the cortical cells and their frequency characteristics. The multineuronal activity was recorded in the sensorimotor cortex of immobilized and freely moving rats, and was subsequently analyzed using the method of cross-correlational analysis. In the first series of experiments, the influence of the neuromediator, acetylcholine (ACh), and calcium chelation, ethyleneglycol tetraacetate (EGTA), on the functional characteristics of adjacent neurons during the iontophoretic application of these substances to cells of the sensorimotor cortex of unanesthetized immobilized rats was investigated. In this case, the iontophoretic application of ACh led to a change in the frequency characteristics of individual neurons, and in the majority of cases did not influence the character of the interneuronal interactions. The application of EGTA led to a decrease in the average frequency of impulse activity for the majority of the neurons, as well as to the disappearance, independent of the running frequency, of excitatory connections in the cross-correlograms, with their subsequent recovery following the termination of exposure to the EGTA. In the second series of experiments, carried out in freely moving rats with a chronically implanted recording electrode, the systemic administration of the acetylcholinesterase blocker, galantamine, led to a substantial intensification of the impulse activity of the cortical neurons, and at the same time did not exert a substantial influence on the network activity of the same neurons. The hypothesis is advanced that the intracortical neuronal interactions may be accomplished independently of extracortical influences that are expressed in the fluctuations of the baseline impulse activity of individual cells. The qualitative assessment that was carried out of the influence of ACh on the functional characteristics of the cortical neurons does not make it possible to identify an influence of ACh (including a modulatory influence) on the formation of intracortical connections. The methods used in this investigation may be used for a further study of the influence of various mediator systems of the brain on the functioning of intracortical interneuronal connections.

Acetylcholine↗
PubMed1979

Structure and dynamics of neural network oscillators.

Techniques are given to represent oscillating neural networks by asynchronous logical switching networks, and to analyze the oscillating networks using a directed graph called a state transition diagram. Consideration is restricted to network oscillators containing no rhythm determining pacemaker neurons, and no neurons exhibiting self-limiting properties such as post-inhibitory rebound or accumulating refractoriness. In the state transition diagrams, stable oscillations are associated with a particular geometric configuration called a cyclic attractor (the heavy cycle in Fig. 2). We show that given the network connectivity it is possible to predict autonomous dynamic behaviour, as well as behaviour following hyperpolarizing or depolarizing inputs to neurons of the network. Conversely, given information about patterns of firing activity during cycles and transients of neural networks, the network connectivity can be predicted. The theoretical techniques can be used to generate a census of network structures capable of generating stable oscillations. Several representative network oscillators are discussed in the context of previous theoretical and experimental studies of the structure of neural network oscillators. Although the number of theoretically possile network oscillators capable of generating sustained oscillations is very large, the techniques which are given should be useful in the design of experiments capable of distinguishing between equally plausible hypotheses.

Action Potentials↗
PubMed1994

Tectal activation of premotor and motor networks during feeding in salamanders.

In salamanders, three separate pathways, a crossed and two uncrossed ones, extend from the tectum mesencephali to the brain stem and spinal cord. These pathways arise from different types of tectal neurons; their dendrites arborize in different layers of retinal afferents and, thus, receive different types of retinal information. Collaterals of descending axons extend in regions, where motoneurons and interneurons related to prey capture are situated. The response properties of tectal neurons, interneurons and motoneurons related to prey capture were revealed by intracellular recording with subsequent dye (biocytin) injection. Most tectal neurons exhibit long latencies after optic-nerve stimulation, which indicates a complex processing of visual information inside the tectum. Our data show that no one-way connection exists between the tectum and motor nuclei; rather, these centres, together with a number of interneurons, exhibit a complex interaction during feeding.

Animals↗
PubMed1991

Structural features of neurons in whole grafts of the rat inferior colliculus.

The inferior colliculus (IC) is a midbrain structure that receives ascending auditory input from brainstem nuclei via the lateral lemniscus, sends efferent fibers to the medial geniculate body of thalamus and receives descending projections from auditory cortex. In the rat, the IC consists of dorsal and external cortices surrounding the central nucleus of IC (CNIC) which is populated by discoid and stellate neurons; the CNIC has a laminar appearance arising from organization of lemniscal fibers and processes of discoid cells. The IC of adult rats was chosen for implantation of whole grafts of E16-17 caudal tectum into unilateral lesion sites. Dendritic and somal architecture of graft neurons was examined 1 to 4.5 months following implantation using rapid Golgi, HRP and Nissl methods. The CNIC of rat is dominated by principal neurons with relatively flattened dendritic fields. In grafts of caudal tectum the most common neuron class observed possesses flattened dendritic arbors which often parallel one another. These neurons also resemble CNIC neurons of host tissue adjacent to the graft border. Spine formations appear on both proximal and distal dendrites of this neural type in both normal and implanted tissues. In addition, comparable somal features of graft neurons include ovoid or fusiform shapes with regular nuclear membranes as found in the normal colliculus. In Golgi stained material fewer stellate class neurons appear as in the normal CNIC, although stellate cell classes are more abundant in the pericentral areas of normal tissue. Both neuron populations are retrogradely labelled in graft and normal IC after HRP injection into the medial geniculate body. These features suggest that the graft core typically consists of prototypic CNIC cells. Other features of neuron and glial cell density vary in graft material which also shows a complex network of vasculature. These results demonstrate that whole grafts of caudal tectum placed into the inferior colliculus can form organized neural architecture similar to the normal CNIC. The somal, dendritic and spine features of these neurons form a potential substrate for connectional and functional properties which establish this preparation as suitable for further investigation as a model for development and recovery of function in the central auditory system.

Animals↗
PubMed2005

Image segmentation by networks of spiking neurons.

A network of leaky integrate-and-fire (IAF) neurons is proposed to segment gray-scale images. The network architecture with local competition between neurons that encode segment assignments of image blocks is motivated by a histogram clustering approach to image segmentation. Lateral excitatory connections between neighboring image sites yield a local smoothing of segments. The mean firing rate of class membership neurons encodes the image segmentation. A weight modification scheme is proposed that estimates segment-specific prototypical histograms. The robustness properties of the network implementation make it amenable to an analog VLSI realization. Results on synthetic and real-world images demonstrate the effectiveness of the architecture.

Action Potentials↗
PubMed2005

Functional dysconnectivity in schizophrenia associated with attentional modulation of motor function.

It is not known whether there is a core abnormality that occurs in all cases of schizophrenia. The cognitive dysmetria hypothesis proposes that there is such an abnormality which is characterized cognitively by a disruption in control and coordination processes, and functionally by abnormal inter-regional connectivity within the cortico-cerebellar-thalamo-cortical circuit (CCTCC). In the current study, we used functional MRI (fMRI) to investigate these two key aspects of the hypothesis. Since patients with schizophrenia show deficits in attention which have been characterized extensively using the continuous performance task (CPT) and since functional imaging studies have also demonstrated that this task engages the CCTCC, we used this task to investigate whether two patient groups with distinct symptom profiles would show functional dysconnectivity within this network. Three groups of subjects participated in the study: healthy volunteers (n = 12), schizophrenia patients with both negative and positive symptoms (n = 11) and schizophrenia patients with primarily positive symptoms (n = 11). Patient groups were matched for age of illness onset and medication, and to the control group for age, gender and handedness. Subjects were scanned using fMRI whilst they performed a modified version of the CPT, involving both degraded and non-degraded stimuli. Stimulus degradation has been shown to produce decrements in sensitivity, which is thought to reflect increased demands on the limited capacity of visual attention. Between-group comparisons revealed that patients with schizophrenia, irrespective of symptomatology, showed attenuation of the anterior cingulate and cerebellar response to stimulus degradation in comparison with control subjects. We also observed disruptions of inter-regional brain integration in schizophrenia. A task-specific relationship between the medial superior frontal gyrus and both anterior cingulate and the cerebellum was disrupted in both patient groups in comparison with controls. In addition, patients with negative symptoms showed impaired behavioural performance, and abnormal task-related connectivity between anterior cingulate and supplementary motor area. These findings are consistent with theoretical accounts of schizophrenia as a disorder of functional integration, and with the cognitive dysmetria hypothesis, which posits a disconnection within the CCTCC as a fundamental abnormality in schizophrenia, independent of diagnostic subtype. Furthermore, these data show evidence of additional functional deficits in patients with negative symptoms, deficits which may explain the accompanying attentional impairment.

Adult↗
PubMed2002

Few intrinsic connections cross the hand-face border of area 3b of New World monkeys.

After long-standing loss of afferents from the hand, the hand representation in area 3b of the somatosensory cortex of monkeys becomes responsive to touch on the face. Because the reactivation of deprived hand cortex by the face inputs could depend on axonal connections across the hand-face border, we determined the extent of such connections in New World marmosets, owl monkeys, and squirrel monkeys. Small injections of anatomic tracers were placed in the hand or the face representations after these representations were identified by microelectrode recordings. The positions of retrogradely labeled neurons were plotted in processed brain sections cut parallel to the brain surface, and their locations were related to anatomic isomorphs of the hand and face representations revealed in adjacent brain sections stained for myelin. In these sections, the hand-face border was clearly visualized as a myelin-poor septum. The intrinsic connections of area 3b labeled by injections in either the hand or face representations were almost completely confined to their respective representation, and very few neurons projected across the border. In addition, neurons in the somatosensory thalamus labeled by injections in either face or hand representations were confined to either VPM, representing the face, or the hand subnucleus of VPL. Thus the reactivation of hand cortex by face stimulation does not depend on a previously existing network of intrinsic cortical connections across the hand-face border, or mismatched thalamocortical projections.

Afferent Pathways↗
PubMed2004

A protein interaction network links GIT1, an enhancer of huntingtin aggregation, to Huntington's disease.

Analysis of protein-protein interactions (PPIs) is a valuable approach for characterizing proteins of unknown function. Here, we have developed a strategy combining library and matrix yeast two-hybrid screens to generate a highly connected PPI network for Huntington's disease (HD). The network contains 186 PPIs among 35 bait and 51 prey proteins. It revealed 165 new potential interactions, 32 of which were confirmed by independent binding experiments. The network also permitted the functional annotation of 16 uncharacterized proteins and facilitated the discovery of GIT1, a G protein-coupled receptor kinase-interacting protein, which enhances huntingtin aggregation by recruitment of the protein into membranous vesicles. Coimmunoprecipitations and immunofluorescence studies revealed that GIT1 and huntingtin associate in mammalian cells under physiological conditions. Moreover, GIT1 localizes to neuronal inclusions, and is selectively cleaved in HD brains, indicating that its distribution and function is altered during disease pathogenesis.

Adaptor Proteins, Signal Transducing↗
PubMed2004

Transmission electron microscopy of cortical dendritic spines in the human oedematous cerebral cortex.

The cortical biopsies of 31 patients with clinical diagnosis of congenital hydrocephalus, vascular anomalies, brain tumours and brain trauma were examined with the transmission electron microscope. A variety of swollen spine shapes were found: mushroom shaped, filopodic, lanceolated spines, and megaspines. The spines appeared axonless or making asymmetric synaptic contacts with swollen presynaptic axons. They exhibited a disrupted actin-like network, dilated endoplasmic reticulum profiles, oedematous clear or dense mitochondria, and clusters of free ribosomes. Some spines contained a hypertrophic cytoskeleton. The spine apparatus appeared generally swollen with clear and dilated cisterns. In severe brain trauma and tumours some spine apparatus appeared disorganised or atrophic. In complicated brain trauma with subdural haematoma or hygroma some degenerated spines displayed a high electron density. The ultrastructural findings suggest that alterations of the spines are responsible for the neurological symptoms exhibited by some patients, which imply a disturbance of cortical nerve circuits. The presence of axonless spines is symptomatic of a loss of nerve connectivity, and are expected to have a significant input on neurological and mental functions.

Adolescent↗
PubMed2006

Characterization of displaced white matter by brain tumors using combined DTI and fMRI.

In vivo white matter tractography by diffusion tensor imaging (DTI) has become a popular tool for investigation of white matter architecture in the normal brain. Despite some unresolved issues regarding the accuracy of DTI, recent studies applied DTI for delineating white matter organization in the vicinity of brain lesions and especially brain tumors. Apart from the intrinsic limitations of DTI, the tracking of fibers in the vicinity or within lesions is further complicated due to changes in diseased tissue such as elevated water content (edema), tissue compression and degeneration. These changes deform the architecture of the white matter and in some cases prevent definite selection of the seed region of interest (ROI) from which fiber tracking begins. We show here that for displaced fiber systems, the use of anatomical approach for seed ROI selection yields insufficient results. Alternatively, we propose to select the seed points based on functional MRI activations which constrain the subjective seed ROI selection. The results are demonstrated on two major fiber systems: the pyramidal tract and the superior longitudinal fasciculus that connect critical motor and language areas, respectively. The fMRI based seed ROI selection approach enabled a more comprehensive mapping of these fiber systems. Furthermore, this procedure enabled the characterization of displaced white matter using the eigenvalue decomposition of DTI. We show that along the compressed fiber system, the diffusivity parallel to the fiber increases, while that perpendicular to the fibers decreases, leading to an overall increase in the fractional anisotropy index reflecting the compression of the fiber bundle. We conclude that definition of the functional network of a subject with deformed white matter should be done carefully. With fMRI, one can more accurately define the seed ROI for DTI based tractography and to provide a more comprehensive, functionally related, white matter mapping, a very important tool used in pre-surgical mapping.

Anisotropy↗
PubMed2005

Neural modeling of an internal clock.

We studied a simple random recurrent inhibitory network. Despite its simplicity, the dynamics was so rich that activity patterns of neurons evolved with time without recurrence due to random recurrent connections among neurons. The sequence of activity patterns was generated by the trigger of an external signal, and the generation was stable against noise. Moreover, the same sequence was reproducible using a strong transient signal, that is, the sequence generation could be reset. Therefore, a time passage from the trigger of an external signal could be represented by the sequence of activity patterns, suggesting that this model could work as an internal clock. The model could generate different sequences of activity patterns by providing different external signals; thus, spatiotemporal information could be represented by this model. Moreover, it was possible to speed up and slow down the sequence generation.

Action Potentials↗
PubMed1995

Three-dimensional morphological study of microvascular regeneration in cavity wall of the rat cerebral cortex using the scanning electron microscope: implications for delayed neural grafting into brain cavities.

The present study was carried out to quantify the subsequent vascular regeneration around a lesion cavity made in the rat cerebral cortex and to decide the origin of the regenerated microvessels. A quantitative study utilizing computerized image analysis after microvascular perfusion with India ink indicated approximately 25 and 160% increase of the vascular density adjacent to the cavity compared to the contralateral cortex at 4 and 21 days, respectively, after lesioning. The microvasculature around the cavity was also evaluated by scanning electron microscopy of vascular corrosion casts. Newly formed leptomeningeal vessels began to grow down toward the floor of the cavity 4 days after lesioning and nearly covered the walls of the cavity 21 days after lesioning. A neovascular network of leptomeninges and connective tissue (e.g., dura) was formed as a roof over the cavity. Numerous branches of these newly formed vessels and prominent anastomoses with the capillary network in the walls and floor of the cavity were observed. Newly formed vessels also originated from the choroid plexus in cases where the lateral ventricle had been opened at the time of lesioning. These results document the plasticity of the vascular system in the cerebral cortex after a mechanical injury. The regenerated vascular network may offer a suitable condition for survival of transplanted tissues.

Animals↗
PubMedDate not supplied

The predictive brain: temporal coincidence and temporal order in synaptic learning mechanisms.

Some forms of synaptic plasticity depend on the temporal coincidence of presynaptic activity and postsynaptic response. This requirement is consistent with the Hebbian, or correlational, type of learning rule used in many neural network models. Recent evidence suggests that synaptic plasticity may depend in part on the production of a membrane permeant-diffusible signal so that spatial volume may also be involved in correlational learning rules. This latter form of synaptic change has been called volume learning. In both Hebbian and volume learning rules, interaction among synaptic inputs depends on the degree of coincidence of the inputs and is otherwise insensitive to their exact temporal order. Conditioning experiments and psychophysical studies have shown, however, that most animals are highly sensitive to the temporal order of the sensory inputs. Although these experiments assay the behavior of the entire animal or perceptual system, they raise the possibility that nervous systems may be sensitive to temporally ordered events at many spatial and temporal scales. We suggest here the existence of a new class of learning rule, called a predictive Hebbian learning rule, that is sensitive to the temporal ordering of synaptic inputs. We show how this predictive learning rule could act at single synaptic connections and through diffuse neuromodulatory systems.

Animals↗