Biomedical subjects

K Stratford

Publications and source records attributed to K Stratford.

7 recordsLinked to original sources

PubMed2006

Nonequilibrium steady states in sheared binary fluids.

We simulate by lattice Boltzmann the steady shearing of a binary fluid mixture undergoing phase separation with full hydrodynamics in two dimensions. Contrary to some theoretical scenarios, a dynamical steady state is attained with finite domain lengths L(x,y) in the directions (x,y) of velocity and velocity gradient. Apparent scaling exponents are estimated as Lx approximately gamma (-2/3) and Ly approximately gamma(-3/4). We discuss the relative roles of diffusivity and hydrodynamics in attaining steady state.

Journal Article↗

PubMed2005

Colloidal jamming at interfaces: a route to fluid-bicontinuous gels.

Colloidal particles or nanoparticles, with equal affinity for two fluids, are known to adsorb irreversibly to the fluid-fluid interface. We present large-scale computer simulations of the demixing of a binary solvent containing such particles. The newly formed interface sequesters the colloidal particles; as the interface coarsens, the particles are forced into close contact by interfacial tension. Coarsening is markedly curtailed, and the jammed colloidal layer seemingly enters a glassy state, creating a multiply connected, solidlike film in three dimensions. The resulting gel contains percolating domains of both fluids, with possible uses as, for example, a microreaction medium.

Journal Article↗

PubMed2005

Physical and computational scaling issues in lattice Boltzmann simulations of binary fluid mixtures.

We describe some scaling issues that arise when using lattice Boltzmann (LB) methods to simulate binary fluid mixtures--both in the presence and absence of colloidal particles. Two types of scaling problem arise: physical and computational. Physical scaling concerns how to relate simulation parameters to those of the real world. To do this effectively requires careful physics, because (in common with other methods) LB cannot fully resolve the hierarchy of length, energy and time-scales that arise in typical flows of complex fluids. Care is needed in deciding what physics to resolve and what to leave unresolved, particularly when colloidal particles are present in one or both of two fluid phases. This influences steering of simulation parameters such as fluid viscosity and interfacial tension. When the physics is anisotropic (for example, in systems under shear) careful adaptation of the geometry of the simulation box may be needed; an example of this, relating to our study of the effect of colloidal particles on the Rayleigh-Plateau instability of a fluid cylinder, is described. The second and closely related set of scaling issues are computational in nature: how do you scale-up simulations to very large lattice sizes? The problem is acute for systems undergoing shear flow. Here one requires a set of blockwise co-moving frames to the fluid, each connected to the next by a Lees-Edwards like boundary condition. These matching planes lead to small numerical errors whose cumulative effects can become severe; strategies for minimizing such effects are discussed.

Complex Mixtures↗

PubMed1993

A common rule governs the synaptic locus of both short-term and long-term potentiation.

BACKGROUND: At synapses between neurons in the brain, transmitter molecules are released from presynaptic terminals in multi-molecular packets called quanta. Excitatory synapses in the CA1 region of the hippocampus show a long-lasting increase in strength known as long-term potentiation (LTP), which may be important for some kinds of learning and memory. LTP can involve an increase in the number of quanta released, or in the size of the response each quantum produces in the postsynaptic cell, or both, depending on the initial condition of the synapse. These synapses also show two forms of brief potentiation: post-tetanic potentiation (PTP), which lasts for a minute or less and involves only modifications at the presynaptic terminal, and short-term potentiation (STP), which lasts rather longer. The significance of STP, the mechanisms whereby it is produced and its relationship to other forms of potentiation are poorly understood. We have studied STP electrophysiologically using slices of the rat hippocampus maintained in vitro. RESULTS: We found that STP, like LTP, can involve increases in either the number of quanta released, or their postsynaptic effect, or both. The rule governing the relative contribution from these two mechanisms appears to be the same as operates during LTP. Both the presynaptic and postsynaptic changes can develop equally rapidly and so must involve fast-acting messenger systems. CONCLUSIONS: STP seems to be a separate phenomenon from PTP, but appears closely related to LTP. The rapidity of its onset may require a reappraisal of current understanding of the messenger systems involved in bringing about changes in synaptic strength.

Journal Article↗

PubMed1992

Presynaptic release probability influences the locus of long-term potentiation.

The quantal hypothesis proposes that chemical synaptic transmission involves the probabilistic release of multimolecular packets of transmitter. Analysis of the resulting trial-to-trial fluctuations in postsynaptic response can provide estimates both of the number of quanta released and of the size of their postsynaptic effect. This in turn permits the quantification of the relative contributions of pre- and postsynaptic factors to the strength of a given synapse. Quantal analysis of excitatory synapses in the hippocampus has proved difficult and has led to contradictory conclusions when applied to long-term potentiation. Here we report the use of a combination of quantal analysis procedures to provide evidence that both pre- and postsynaptic changes can contribute substantially to the maintenance of long-term potentiation in the CA1 region of the hippocampus. The initial setting of the presynaptic release mechanism seems to determine their relative importance.

Animals↗

PubMed1991

Quantal analysis of excitatory synaptic action and depression in hippocampal slices.

Quantal analysis can provide a quantitative description of important aspects of chemical synaptic transmission and its modification. The technique has recently been applied to excitatory synapses within the hippocampus, especially the form of synaptic plasticity known as long-term potentiation. However, these attempts have met with only limited success, in that the individual quantal amplitudes making up the synaptic response generally could not be resolved. Here we have paid attention to the possible instability of the quantal fluctuation pattern over time. We were able to resolve individual quantal component amplitudes for a high proportion of the experiments, and so demonstrate the quantal nature of excitatory transmission in the CA1 region of the hippocampus. Mean quantal amplitudes for individual excitatory postsynaptic potentials were 84-197 microV, with a mean of 131 +/- 29 microV. For periods during which the fluctuation pattern was stable, the variance associated with individual quantal amplitudes was low. We have also used quantal analysis to show that synaptic depression following prolonged stimulation at these synapses is primarily a presynaptic phenomenon.

Analysis of Variance↗

PubMed1991

Synaptic transmission between individual pyramidal neurons of the rat visual cortex in vitro.

Synaptic transmission between pairs of neurons in layer 2/3 of in vitro slices from the rat visual cortex was studied by dual intracellular recording. The intrinsic electrophysiological properties of these neurons suggested that they were pyramidal cells. More than 1/3 of the total number of synaptically connected neurons were stained by intracellular iontophoresis of biocytin, and all had pyramidal morphology. Postsynaptic potentials (PSPs) were evoked by single action potentials (elicited by current injection) in presynaptic cells. Measurements of PSP latency, amplitude, and shape were made on spike-triggered averages. Forty-eight synaptic connections were found out of a possible total of 549, equivalent to a probability of about 0.09. For these 48 connections, the distance between the 2 impalement sites ranged from 50 to 340 microns. All PSPs were depolarizing at rest (-74 +/- 5 mV, mean +/- SD), and all 14 of the PSPs that were also recorded with the postsynaptic cell depolarized to around action potential threshold (about -55 mV) remained depolarizing. PSPs had short latencies (1.2 +/- 0.6 msec, mean +/- SD), suggesting that they were mediated by monosynaptic pathways. Peak amplitudes of the averaged PSPs varied widely (range, 0.05-2.08 mV), but the majority were less than 0.5 mV. PSPs decayed exponentially with time constants that were correlated with, but slightly longer than, the membrane time constants of the postsynaptic cells measured using injected current pulses. Four cell pairs were connected reciprocally, and 6 examples of convergent input were found in which a single cell was postsynaptic to more than 1 presynaptic cell. Trial-to-trial fluctuations in PSP amplitude were analyzed for 16 synaptic connections. The amplitude of the PSP evoked by a presynaptic action potential fluctuated more than could be accounted for by the background noise.

Action Potentials↗