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The lateral diffusion and trapping of neurotransmitter receptors within the postsynaptic membrane of a neuron play a key role in determining synaptic strength and plasticity. Trapping is mediated by the reversible binding of receptors to scaffolding proteins (slots) within a synapse. In this paper we introduce a method for analyzing the transient dynamics of proximal axodendritic synapses in a diffusion-trapping model of receptor trafficking. Given a population of spatially distributed synapses, each of which has a fixed number of slots, we calculate the rate of relaxation to the steady-state distribution of bound slots (synaptic weights) in terms of a set of local accumulation times. Assuming that the rates of exocytosis and endocytosis are sufficiently slow, we show that the steady-state synaptic weights are independent of each other (purely local). On the other hand, the local accumulation time of a given synapse depends on the number of slots and the spatial location of all the synapses, indicating a form of transient heterosynaptic plasticity. This suggests that local accumulation time measurements could provide useful information regarding the distribution of synaptic weights within a dendrite.

We consider a particle undergoing diffusion with stochastic resetting in a bounded domain $\calU\subset \R^d$ for $d=2,3$. The domain is perforated by a set of partially absorbing targets within which the particle may be absorbed at a rate $\kappa$. Each target is assumed to be much smaller than $|\calU|$, which allows us to use asymptotic and Green's function methods to solve the diffusion equation in Laplace space. In particular, we construct an inner solution within the interior and local exterior of each target, and match it with an outer solution in the bulk of $\calU$. This yields an asymptotic expansion of the Laplace transformed flux into each target in powers of $\nu=-1/\ln \epsilon$ ($d=2$) and $\epsilon$ ($d=3$), respectively, where $\epsilon$ is the non-dimensionalized target size. The fluxes determine how the mean first-passage time to absorption depends on the reaction rate $\kappa$ and the resetting rate $r$. For a range of parameter values, the MFPT is a unimodal function of $r$, with a minimum at an optimal resetting rate $r_{\rm opt}$ that depends on $\kappa$ and the target configuration., Comment: 21 pages, 7 figures

We extend the theoretical framework used to study search processes with stochastic resetting to the case of partially absorbing targets. Instead of an absorption event occurring when the search particle reaches the boundary of a target, the particle can diffuse freely in and out of the target region and is absorbed at a rate $\kappa$ when inside the target. In the context of cell biology, the target could represent a chemically reactive substrate within a cell or a region where a particle can be offloaded onto a nearby compartment. We apply this framework to a partially absorbing interval and to spherically symmetric targets in $\R^d$. In each case, we determine how the mean first passage time (MFPT) for absorption depends on $\kappa$, the resetting rate $r$, and the target geometry. For the given examples, we find that the MFPT is a monotonically decreasing function of $\kappa$, whereas it is a unimodal function of $r$ with a unique minimum at an optimal resetting rate $r_{\rm opt}$. The variation of $r_{\rm opt}$ with $\kappa$ depends on the spatial dimension $d$, decreasing in sensitivity as $d$ increases. For finite $\kappa$, $ r_{\rm opt}$ is a non-trivial function of the target size and distance between the target and the reset point. We also show how our results converge to those obtained previously for problems with totally absorbing targets and similar geometries when the absorption rate becomes infinite. Finally, we generalize the theory to take into account an extended chemical reaction scheme within a target., Comment: 24 pages, 11 figures

The arteriovenous graft (AVG) is most often used in hemodialysis patients when an autogenous fistula is not feasible. The optimal location (forearm or upper arm) and configuration (loop or straight) of AVGs are not known. To evaluate relationships of AVG location and configuration with patency, we conducted a secondary analysis using data from a randomized, placebo-controlled trial of dipyridamole plus aspirin for newly placed AVG.Participants of the Dialysis Access Consortium (DAC) Graft Study with newly placed upper extremity prosthetic grafts involving the brachial artery were studied. Multivariable analyses adjusting for trial treatment group, center, gender, race, body mass index, diabetes, current treatment with chronic dialysis, and prior arteriovenous vascular access or central venous catheter were performed to compare outcomes of forearm (fAVG) and upper arm (uAVG) grafts, including loss of primary unassisted patency (LPUP) and cumulative primary graft failure (CGF). Subgroup analyses of graft configuration and outflow vein used were also conducted.A total of 508 of the 649 participants (78%) enrolled in the trial had an upper extremity brachial artery graft placed, 255 with fAVG and 253 with uAVG. Participants with fAVG were less often male (33% vs 43%; P = .03), African American (62% vs 78%; P .001), and receiving dialysis at the time of surgery (62% vs 80%; P .001). Participants with fAVG had a higher mean body mass index (33 vs 29; P .001). The LPUP (fAVG 70% vs uAVG 78%; P = .07) and CGF (33% vs 36%; P = .91) were similar between fAVG and uAVG at 1-year follow-up. In multivariable analysis, AVG location (uAVG vs fAVG) was not associated with LPUP (hazard ratio, 1.21; 95% confidence interval, 0.90-1.63; P = .20) or CGF (hazard ratio, 1.36; 95% confidence interval, 0.94-1.97; P = .10). LPUP did not differ significantly between fAVG and uAVG among subgroups based on AVG configuration (P = 1.00) or outflow vein used (P = .16).Patency was comparable between fAVG and uAVG despite the larger caliber veins often encountered in the upper arm in carefully selected patients. Our findings support the traditional view that, in order to preserve a maximal number of access sites, the forearm location should be considered first before resorting to an upper arm graft.

Domain wall dynamics have been investigated in a variety of ferromagnetic nanostructures for potential applications in logic, sensing, and recording. We present a combination of analytic and simulated results describing the reliable field driven motion of a domain wall through the arms of a ferromagnetic spiral nanowire. The spiral geometry is capable of taking advantage of the benefits of both straight and circular wires. Measurements of the in-plane components of the spirals' magnetization can be used to determine the angular location of the domain wall, impacting the magnetoresistive applications dependent on the domain wall location. The spirals' magnetization components are found to depend on the spiral parameters: the initial radius and spacing between spiral arms, along with the domain wall location. The magnetization is independent of the parameters of the rotating field used to move the domain wall, and therefore the model is valid for current induced domain wall motion as well. The speed of the ...

Coronary artery disease is a frequent complication of end-stage renal disease (ESRD). ST segment depression on ambulatory electrocardiography without patient awareness is a marker of what has been termed "silent ischemia". It has been suggested that in patients with coronary artery disease these transient ST segment depressions are associated with increased cardiovascular mortality. Up to 30% of patients with ESRD may display transient ST segment depression, however the significance of this finding in these group of patients who frequently have associated LV hypertrophy, and rapid electrolyte changes has not been clear. We therefore set out to determine the incidence of transient ST segment depression during ambulatory Holter monitoring in 70 consecutively studied hemodialysis patients. Sixty-seven patients wore the monitor for at least 12 hours and 16 patients (23%) demonstrated at least one minute of 1 mm ST segment depression. The presence of clinically apparent coronary artery disease, diabetes, left ventricular hypertrophy, sex or race were not significantly associated with the probability of demonstrating transient ST segment depression. The survival of patients with or without transient ST segment depression was the same at 2 years with 70% of patients remaining free of death, nonfatal myocardial infarction, or coronary bypass grafting. We conclude that patients with ESRD frequently demonstrate transient ST segment depression, however, the presence of these findings on ambulatory Holter monitoring does not appear to be associated with increased long-term mortality.