Biomedical subjects

H S Green

Publications and source records attributed to H S Green.

11 recordsLinked to original sources

PubMed1989

Gompertzian mortality derived from competition between cell-types: congenital, toxicologic and biometric determinants of longevity.

Gompertz-Makeham kinetics of population mortality is derived in terms of competition between hypothetical life-prolonging and life-shortening regulatory elements (cells) interacting in each organism by generalized Volterra-type competitive exclusion. The model is developed on two levels, the first applicable to homogeneous populations, and the second, a statistical generalization, applicable to inhomogeneous populations. It offers a natural classification of effects of exogeneous agents on longevity, including hormetic and paradoxical effects of toxic substances, thus relating to problems of risk assessment by extrapolation from high to low doses. Two applications, concerning the effects of radiation on mice and Drosophila imagos, respectively, are used to illustrate the flexibility of the model in the analysis and interpretation of observational data.

Animals↗

PubMed1989

A zonal model of cortical functions.

A model of cortical functions is developed with the object of simulating the observed behavior of individual neurons organized in unit circuits and functional systems of the cerebellum, the cerebrum and the hippocampal formation. The neuronal model is capable of representing refractory and potentiated states, as well as the firing and lowest resting states. The unit circuits of each system consist of all common types of cells with known synaptic connections. In the cerebral system these unit circuits are interconnected to form columns as well as zones. A new discrete neural network equation, which takes account of interactions with the extracellular field, is proposed to simulate electrical activity in these circuits. A coherent theory of cortical activity and functions is derived that accounts for many of the observed phenomena, including those associated with the development of long-term potentiation and sequential memory. Three appendices are devoted to the theory of extracellular interactions, the derivation of non-linear network equations, and a computer program to simulate learning in the cortex.

Animals↗

PubMed1988

Information transfer by electromagnetic waves in cortex layers.

Coupling coefficients are derived for the transfer of energy from an electromagnetic wave propagating in the extracellular fluid of a cortex layer, using an ion dynamical model developed earlier (Green & Triffet, 1985). With the assistance of a simple computational algorithm, these are used to document the performance of a basic neural circuit (Eccles, 1979) embedded in a columnar structure of the type described by Mountcastle (1979). Results point to a holographic model of memory, in which calcium configurations in the dendrites of cerebellar granular cells, fixed by modulated alpha-waves, constitute the basic information storage mechanism. Event-related potential waves, known to sweep over selective regions of the cortex in advance of any muscular act, are explained as a logical consequence of circuit function.

Animals↗

PubMed1985

Extracellular fields within the cortex.

A physically based theory of ionic currents associated with nervous activity is extended to provide a model of electrical activity in the cortex and its immediate neighbourhood. It is shown that the most significant variations of potential in the extracellular fluid are associated with fluctuations in density of calcium and potassium ions. Nervous activity may activate a calcium resonance at the external surface of a neural membrane, which in turn excites a potassium resonance. The resulting variations of electrical potential are sufficient to account for the event related potentials observed in the extracellular fluid. In addition, it is found that periodic variations of potential associated with the potassium resonance may be initiated by metabolic changes in ion concentrations and are in close correspondence with those identified experimentally with the alpha- and beta-rhythms. It is shown how to determine the effective conductance of the neural membrane from ionic theory, with a result comparable to that assumed by Hodgkin & Huxley, under the conditions of the voltage clamp.

Alpha Rhythm↗

PubMed1983

Calcium dynamics at a plastic synapse in Aplysia.

Because certain primitive behavioral responses in the large sea snail Aplysia have recently been linked to neurophysiological events at a synaptic level, special interest attaches to the role played by calcium ions at such synapses. Using an extended version of the model applied earlier to trace the flow of energy and information through a ganglion of the medicinal leech (Triffet & Green, 1980), the authors investigate the electropotential effects of small transient localized changes in the calcium concentration near the inner membrane surface of a neuron in the resting state. When this state is well below the firing threshold, changes in Ca2+ concentration less than 10(-8)M are shown to result only in low-level harmonic background oscillations. When the potential of the neurons is closer to threshold, however, and/or the Ca2+ concentration is of the order of 10(-8)M, easily recognizable graded potentials appear, and these grow into firing peaks when the calcium concentration is increased still further. Though no attempt is made to deal with the amplification effects dependent on calcium-vesicle interactions and the related release of transmitter molecules, a unified mechanism for the underlying calcium ion dynamics is proposed. Graded potentials of increasing size are associated with a progressive localized thickening of the inner and outer Debye layers. Moreover, the transverse and longitudinal calcium currents set up in such regions prove adequate to account for both the depletion of Ca2+ ions necessary to achieve habituation, and the increase in their concentration required for sensitization.

Action Potentials↗

PubMed1983

The dexamethasone suppression test in depression.

The DST does appear to be abnormal in a sizable subgroup of patients with major depressive disorder, particularly those characterized as "endogenomorphic" or "melancholic." At present the available data seem clear in indicating this abnormality is significantly less common in normal controls and patients without affective illness. The test holds considerable promise in helping to define new subgroups of depressed individuals for further study. In terms of its ability to predict treatment response to an adequate course of somatic therapy, the test does not appear to be of value, and the clinician should still be guided by the clinical presentation and history in initiating or choosing between various somatic treatments. With further attention to issues of diagnosis and DST methodology this strategy could help in addressing questions of differential diagnosis in potential "variants" of affective illness and providing a better understanding of the pathophysiology of depression.

Depression, Chemical↗

PubMed1982