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Results for “Mechanical Phenomena”

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At least 19 recordsLinked to original sources
PubMedDate not supplied

Potential for multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses.

Recent experimental evidence indicates that in the neocortex, the manner in which each synapse releases neurotransmitter in response to trains of presynaptic action potentials is potentially unique. These unique transmission characteristics arise because of a large heterogeneity in various synaptic properties that determine frequency dependence of transmission such as those governing the rates of synaptic depression and facilitation. A theoretical analysis was therefore undertaken to explore the phenomenologies of changes in the values of these synaptic parameters. The results illustrate how the change in any one of several synaptic parameters produces a distinctive effect on synaptic transmission and how these distinctive effects can point to the most likely biophysical mechanisms. These results could therefore be useful in studies of synaptic plasticity in order to obtain a full characterization of the phenomenologies of synaptic modifications and to isolate potential biophysical mechanisms. Based on this theoretical analysis and experimental data, it is proposed that there exists multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses. Finally, it is shown that the impact of changing the values of synaptic parameters depends on the values of the other parameters. This may indicate that the various mechanisms, phenomena and algorithms are interlinked in a 'synaptic plasticity code'.

Algorithms↗
PubMed2000

Ultrafast dynamics of many-body processes and fundamental quantum mechanical phenomena in semiconductors.

The large dielectric constant and small effective mass in a semiconductor allows a description of its electronic states in terms of envelope wavefunctions whose energy, time, and length scales are mesoscopic, i.e., halfway between those of atomic and those of condensed matter systems. This property makes it possible to demonstrate and investigate many quantum mechanical, many-body, and quantum kinetic phenomena with tabletop experiments that would be nearly impossible in other systems. This, along with the ability to custom-design semiconductor nanostructures, makes semiconductors an ideal laboratory for experimental investigations. We present an overview of some of the most exciting results obtained in semiconductors in recent years using the technique of ultrafast nonlinear optical spectrocopy. These results show that Coulomb correlation plays a major role in semiconductors and makes them behave more like a strongly interacting system than like an atomic system. The results provide insights into the physics of strongly interacting systems that are relevant to other condensed matter systems, but not easily accessible in other materials.

Journal Article↗
PubMed1987

Inhibition phenomena mechanisms in experimental obeliscoidosis in rabbits. II. Local and systemic antibody responses.

A series of experiments was carried out using adult outbred Polish race rabbits of both sexes infected, during spring or autumn, with 10,000 larvae of Obeliscoides cuniculi, either fresh or stored at 4 degrees C. Extracts of mucosal proteins and bile were collected at postmortem 6 or 12 weeks after infection. Antibody levels were determined in antisera, bile and stomach mucosa by haemagglutination and precipitation tests. Local antibody responses were demonstrated in the stomach and bile, and reactions were obtained with the tissue fluids by haemagglutination and precipitation tests with worm antigens and ES products. Additionally, some specific immunological response was observed in the circulation during the primary infection. These results suggest a clear-cut relationship between increased levels of these antibodies and either larval inhibition or worm expulsion during O. cuniculi infections.

Animals↗
PubMed1982

Metoprolol withdrawal phenomena: mechanism and prevention.

Eight patients taking metoprolol (300 mg/day) for essential hypertension were studied after abrupt withdrawal and placebo replacement of the drug. A 52% average rebound increase in cardiac chronotropic sensitivity to isoproterenol and 15% rebound rise in resting heart rate occurred in all patients between 2 to 8 days after metoprolol withdrawal (P less than 0.05). Holter monitoring showed no associated arrhythmia. A transient increase in blood pressure occurred in one patient and withdrawal-like symptoms were noted in three patients. There were no meaningful changes in plasma norepinephrine, epinephrine, thyroxine, or triiodothyronine. Seven of the eight patients were again studied serially after the same metoprolol dosing, during a prolonged low-dose withdrawal schedule (50 mg/day for 10 days) and during placebo. Prolonged low dose before complete metoprolol withdrawal decreased but did not completely prevent the changes observed after abrupt withdrawal. The observed rebound of cardiac beta-adrenergic sensitivity may have application to the mechanism and prevention of the beta-blocker syndrome in patients with angina.

Adult↗
PubMed1990

Calcium oscillations: phenomena, mechanisms and significance.

Many hormones that mobilise intracellular calcium via inositol 1,4,5 trisphosphate induce oscillations in cytoplasmic free Ca. Two basic oscillatory patterns occur: quasi-sinusoidal oscillations and repetitive free Ca transients. The mechanisms responsible for generating these oscillations are not clear; calcium-induced calcium release, interplay between two intracellular calcium pools and repetitive generation of InsP3 are discussed. The significance of different oscillatory patterns induced by different agonists in the same cell is emphasised, and mechanisms by which the oscillators may retain-receptor specific information are proposed, such as negative feedback onto receptors or G-proteins by protein kinase C. Reasons why cells generate free Ca oscillations and possible consequences such as oscillations in downstream pathways are explored. The possibility that pathological conditions such as aluminium toxicity are exerted through distortion of oscillatory free Ca signalling is raised.

Animals↗
PubMed1988

A macromolecular approach to excitation phenomena: mechanical and thermal changes in nerve during excitation.

(1) Nerve fibers and cells were shown to swell when they develop action potentials. Both in the squid giant axon and in the garfish olfactory nerve, the peak of swelling coincides with the peak of the action potential recorded from the site of mechanical recording. Substitution of univalent cations for the bivalent cations (Ca) bound to multi-anion sites in the membrane macromolecules is considered to be at the base of the phenomenon of swelling. (2) Measurements of heat production by the garfish olfactory nerve indicate that the rate of temperature rise is maximal at the peak of the action potential. Since heat production lasts for the entire depolarizing phase of the action potential, the "condenser theory" of heat production is abandoned. Again, substitution of the bound bivalent cations in the membrane macromolecules with univalent cations is regarded as the origin of nerve heat. (3) The two stable states of the nerve membrane, which are readily demonstrable in TEA-treated or internally perfused squid giant axons, are shown to represent bivalent cation-rich and univalent cation-rich states of the nerve membrane. (4) A physicochemical theory is described that explains macromolecular transitions between two stable states. The mechanism of nerve excitation is explained on a physicochemical basis. (5) The importance of the role played by water molecules in the process of nerve excitation is emphasized.

Action Potentials↗