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Biomedical subjects

B Horwitz

Publications and source records attributed to B Horwitz.

At least 127 records · Page 7Linked to original sources
PubMed1984

Electrophoretic migration due to postsynaptic potential gradients: theory and application to autonomic ganglion neurons and to dendritic spines.

A postsynaptic mechanism for transforming electrical activity at a synapse into structural modifications near the synapse is proposed. The firing of a synapse produces a postsynaptic potential which is attenuated as it spreads electrotonically into other parts of the neuron. The potential difference so generated gives rise to an intraneuronal electric field, and permits the electrophoretic migration of charged metabolites, both along the inner surface of the membrane and in the cytoplasm. Estimates of the sizes of these fields suggest that they can play a significant role in locally organizing the distribution of the charged species, thus allowing differential biochemical activity at different synaptic locations. The mechanism is applied first to the innervation pattern found in ciliary ganglion neurons by Purves and Hume [(1981) J. Neurosci. 1, 441-452]. They showed that although ganglionic neurons are multiply innervated in early postnatal life, adult neurons are innervated such that the number of separate pre-ganglionic axons is approximately proportional to the number of major ganglionic dendrites. It is demonstrated how electrophoretic competition can result in individual dendrites establishing individual domains of innervation. The electrophoretic mechanism is next applied to dendritic spines. Theoretical calculations are presented which show that synaptic activity at a spine head can establish large electric fields along the spine, whereas synaptic activity at the base of the spine generates very small electric fields in the spine. The thinner the spine, the stronger the fields. If one assumes that these electric fields can lead to electrophoretic migration of charged metabolites necessary for either synaptic stabilization or enhancement, significant accumulation should occur within spines.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Intercorrelations of glucose metabolic rates between brain regions: application to healthy males in a state of reduced sensory input.

We use a correlational analysis of regional metabolic rates to characterize relations among different brain regions. Starting with rates of local glucose metabolism (rCMRglc) obtained by positron emission tomography using [18F]fluorodeoxyglucose, we propose that the strength of the association is proportional to the magnitude of the correlation coefficient. Partial correlation coefficients, controlling for whole brain glucose metabolism, are used in the analysis. We also introduce a graphical technique to display simultaneously all the correlations, allowing us to examine patterns of relations among them. The method was applied to 40 very healthy males under conditions of reduced auditory and visual inputs (the "resting state"). Dividing the brain into 59 regions, and keeping only those partial correlation coefficients significant to p less than 0.01, we found the following: (a) All regions were significantly correlated with their contralateral homologues. For the most part, the largest partial correlation coefficients were between homologous brain regions. (b) Generally, the pattern of significant correlations between any two lobes in the left hemisphere did not differ statistically from the corresponding pattern in the right hemisphere. (c) Strong correlations were observed between primary somatosensory areas and premotor association areas. Correlations between these association areas and primary visual and auditory regions were not statistically significant. (d) Significant correlations between inferior occipital and temporal areas were found. Metabolic rates in the superior part of the occipital lobe were not correlated significantly with metabolic rates in regions of the temporal lobe, nor with metabolism in the parietal lobe. (e) As a whole, there were numerous correlations among frontal and parietal lobe regions, on the one hand, and among temporal and occipital lobe regions, on the other, but few statistically significant correlations between these two domains. We relate our results to various aspects of known brain anatomy, physiology, and cognitive functioning.

Acoustic Stimulation↗
PubMed1983

Unequal diameters and their effects on time-varying voltages in branched neurons.

A theoretical method, developed in a previous paper, enables one to calculate analytical expressions for time-varying voltages at specific locations in branching dendritic systems in response to synaptic current inputs at other sites. Exact results were obtained for a number of dendritic trees that possessed certain symmetries: all branch lengths had to be integral multiples of one another, and all branch diameters had to be equal. Because the second of these conditions is unrealistic, the method has been generalized to treat dendritic trees whose branches differ in diameter. The method entails adding onto the symmetric results a sum of correction terms. It is found that the correction terms, as well as the symmetric results, can be expressed as combinations of two families of functions. These functions, generalizations of those found in our earlier paper, provide a precise formalism for analyzing how voltage transients depend on the geometrical structure of the dendritic tree. Examples are given that show how the correction terms affect the value of the voltage, and how variations in branch diameters alter the behavior of the propagated postsynaptic potential. The implications of these results for our understanding of neuronal functioning are discussed.

Animals↗
PubMed1981

Neuronal plasticity: how changes in dendritic architecture can affect the spread of postsynaptic potentials.

Calculations from a theoretical model are presented which illustrate the electrophysiological changes that can result from alterations in dendritic branching. Much evidence has implicated modifications in dendritic architecture with learning, aging and certain neural pathologies. It is found that both the peak amplitude and the half-width of a time-varying postsynaptic potential generated by a previously established synapse can be affected by dendritic changes distal to the input. For the examples considered in this paper peak amplitude is decreased if distal branching occurs, whereas half-width is increased.

Animals↗
PubMed1981

Molecular diffusion cannot account for spreading of isotope distribution peaks during rapid axoplasmic transport.

Diffusion coefficients calculated from spreading peaks of isotope distributions formed during rapid axoplasmic transport are two orders of magnitude larger than coefficients established for proteins in water. For slow transport, however, the coefficients approximate those of diffusion in water. We propose a near-equilibrium partitioning mass transfer process as the major source of peak spreading during rapid transport and suggest the axon can be modeled by a chromatographic column.

Animals↗
PubMed1981

An analytical method for investigating transient potentials in neurons with branching dendritic trees.

An analytical method is developed that allows one to explore the way in which the geometrical structure of a neuron's dendritic tree affects the time-course and amplitude of transient potentials generated at different locations on dendritic branches. The method requires that, for a given dendritic arborization, one associates a symmetric geometry for which exact mathematical expressions for time-varying dendritic potentials can be calculated. The value of the dendritic potential for the asymmetric geometry is evaluated by adding correction terms to the results for the symmetric geometry. Several model trees are examined, and in each case the analytical results are expressed in terms of two closely related families of functions. These functions provide a precise formulation for systematically analyzing the way in which the voltage transient at a given point depends upon the geometrical structure of the dentritic tree. Several numerical examples are presented. A discussion of how to generalize the method and of some potential applications are given.

Animals↗
PubMed1980

Induced drug release from lipid vesicles in serum by pH-change.

Drugs can be released from lipid vesicles by pH-change in calf, horse or human serum when pH-sensitive trigger molecules are incorporated in the vesicle lipid bilayer. The lipid composition is so chosen that the drug release is best performed at 37 degrees C. Specific drug targeting is envisaged to loci of the body with lower than physiological pH, such as primary or metastatic tumors.

Animals↗
PubMed1977

Retrobulbar hemorrhage.

Retrobulbar hemorrhage is a rare complication of orbital trauma or surgery. Although the process is usually self-limiting, infrequently visual impairment may result. Various methods for relieving the associated intraocular hypertension (caused by the extraocular compression posteriorly) have been suggested. The effectiveness of some remains questionable. To reassess the effectiveness of conservative management of this problem, we reviewed the medical records of all 10 patients who had had retrobulbar hemorrhage at our institution during the past 8 years. Additionally, we did an experimental study on rabbits to evaluate the visual effects of a transient but abnormally high intraorbital pressure. The results are reported.

Adult↗
PubMed1975

Norepinephrine-induced depolarization of skeletal muscle cells.

I.v. administration of norepinephrine to anesthetized hamsters was followed by a significant depolarization of the cell membranes of skeletal muscle (gracilis anticus and sartorius). The occurrence of this depolarization in consistent with the suggestion that changes in ionic distribution across the cell membrane are associated with activation of non-shivering thermogenesis in muscle cells as has been proposed for brown adipocytes.

Animals↗
PubMedDate not supplied

Activation of cerebral blood flow during a visuoperceptual task in patients with Alzheimer-type dementia.

Changes in regional cerebral blood flow (rCBF) associated with a face-matching task were examined using positron emission tomography (PET) and H2(15)O in 7 patients with mild-moderate dementia of the Alzheimer type (DAT) and in 8 healthy age-matched controls. rCBF was normalized to whole brain flow and pixel-by-pixel difference images were computed by contrasting flow during a control task to flow during face matching. Both patients and controls showed bilateral rCBF increases in occipitotemporal extrastriate cortex during face matching. The magnitude of these increases was not significantly different between the groups. In addition, the patients showed greater rCBF activation in regions of occipital and frontal cortex. These results show that early in the course of DAT, patients utilize extrastriate cortex to perform a visuoperceptual task, as do control subjects but also show rCBF increases in additional cortical areas. Activation of these additional areas of cortex in the patients may reflect an increased attentional load during face matching due to their reduced cognitive capacity.

Aged↗
PubMedDate not supplied

Age-related differences in visual perception: a PET study.

To assess age-related differences in cortical activation during form perception, two classes of visual textures were shown to young and older subjects undergoing positron emission tomography (PET). Subjects viewed even textures that were rich in rectangular blocks and extended contours and random textures that lacked these organized form elements. Within-group significant increases in regional cerebral blood flow (rCBF) during even stimulation relative to random stimulation in young subjects were seen in occipital, inferior and medial temporal regions, and cerebellum, and in older subjects, in posterior occipital and frontal regions. Group by texture type interactions revealed significantly smaller rCBF increases in older subjects relative to young in occipital and medial temporal regions. These results indicate that young subjects activate the occipitotemporal pathway during form perception, whereas older subjects activate occipital and frontal regions. The between-group differences suggest that age-related reorganization of cortical activation occur during early visual processes in humans.

Adult↗
PubMedDate not supplied

Measurement of disease progression in Alzheimer's disease.

Riege and Metter's comprehensive review deals with many issues pertaining to Alzheimer's disease research. We agree with the authors that early diagnosis and evaluation of disease progression represent key points that must be addressed in an integrated manner. Brain imaging holds great promise for the resolution of both issues.

Alzheimer Disease↗
PubMedDate not supplied

Decomposing memory: functional assignments and brain traffic in paired word associate learning.

The recent covariance structural equation model for word-pair associate encoding and retrieval (Krause, Horwitz, Taylor, Schmidt, Mottaghy, Halsband et al., 1998; Krause, Horwitz, Taylor, Schmidt, Mottaghy, Herzog et al., 1999) is analysed to deduce possible functional assignments of the various brain modules used by subjects in solving the task. Specific processing aspects are considered, in particular, that of long-term working memory sites and how they are coupled to buffer working memory sites to enable deposition and manipulation of remembered associates. The new concept of 'brain traffic' is introduced as an aid to the assessment of how important are various brain modules. A set of functional assignments is produced for the relevant modules.

Association Learning↗
PubMedDate not supplied

Neural modeling and functional brain imaging: an overview.

This article gives an overview of the different functional brain imaging methods, the kinds of questions these methods try to address and some of the questions associated with functional neuroimaging data for which neural modeling must be employed to provide reasonable answers.

Brain↗