Biophysical models of cardiac electrical activity

A system for 3D simulation of heart electrical activity at different structural levels based on fundamental knowledge on the spatiotemporal organization of extracellular electric fields in the myocardium is being developed at the Institute of Theoretical and Experimental Biophysics of the Russian Academy of Sciences. The system is based on a biophysical model of the genesis of electrocardiosignals (ECSs) in the form of a double electric layer on the surface of the electrically active myocardium, which was proposed earlier and then modified. The system combines a model of the activation and repolarization of the heart ventricles, an advanced model for determining the parameters of the heart electric field, which makes it possible to obtain model ECSs both by direct calculation of the potentials and calculation of ECSs from preliminarily determined components of a multipole equivalent heart generator, a database of model parameters and their combinations in the form of cards of simulated “patients,” and a database of simulated ECSs. This paper (the first in a series of three on the subject) briefly describes simulation methods used in electrocardiology and the biophysical model of heart electrical activity that forms the basis of the system for computer simulation of direct and inverse problems concerning the heart electric field. Electrophysiological, anatomical, and biophysical characteristics of the heart are the parameters of the model.     

Effect of vasoactive intestinal polypeptide on porcine gastrointestinal electrical activity

The influence of intravenous infusion of VIP, 150 and 300 pmol/kg/min, on gastrointestinal electrical activity was studied in conscious piglets with electrodes implanted in the wall of the antrum pylori, duodenum, jejunum, and ileum. Both doses resulted in a decrease in antral electrical activity. In the small intestine, only the lower dose caused a shortening of the irregular spiking activity phase in the jejunum and ileum. In the jejunum this resulted in a reduction of the MMC interval. It may be concluded that the prevailing effect of VIP is an inhibition of gastrointestinal electrical activity in the piglet.     

Mathematical models for fatigue minimization during functional electrical stimulation

We previously reported the development of a force- and fatigue-model system that predicted accurately forces during repetitive fatiguing activation of human skeletal muscles using brief duration (six-pulse) stimulation trains. The model system was tested in the present study using force responses produced by longer duration stimulation trains, containing up to 50 pulses. Our results showed that our model successfully predicted the peak forces produced when the muscle was repetitively activated with stimulation trains of frequencies ranging from 20 to 40 Hz, train durations ranging from 0.5 to 1 s, and varied pulse patterns. The predicted peak forces throughout each protocol matched the experimental peak forces with r² values above 0.9 and predicted successfully the forces at the end of each protocol with <15% error for all protocols tested. The success of our model system further supports its potential use for the design of optimal stimulation patterns for individual users during functional electrical stimulation.     

Role of interneuronal systems in the formation of main patterns of field electrical activity in the hippocampus

Studies on the cellular and subcellular levels promote elucidation of the fundamental principles of formation of effective neuronal systems from cell units. To estimate the interrelations between electrical activity of neuronal networks and processes realized on the cellular level, we need to adequately understand the general patterns of behavior of populations of interneurons, which are components of these networks, under different physiological conditions. In this review, we describe and discuss the relations between the electrical activity of single hippocampal neurons and different components of the field electrical activity, as well as modern concepts on the mode of involvement of the system of hippocampal interneurons in the formation of physiologically important patterns of efferent activity of the above-mentioned structure (in particular in encoding of information on the neuronal level). Neirofiziologiya/Neurophysiology, Vol. 40, No. 1, pp. 58–68, January–February, 2008.     

Computer simulation of biological systems

The current status of mathematical models of biological systems is reviewed. Advances in supercomputer hardware allows more complex models to be constructed. The new generation of microcomputers are quite adequate for many computer simulations of biological systems. A theory of modeling is being developed to improve the relationship between the real biological system and the model. Deterministic models, stochastic models and applications of control theory and optimization methods are discussed. Examples given include models of molecular structure, of experimental techniques, and of biochemical reactions. It is recommended that experimental biologists consider the use of microcomputers to model the system under study as a part of their research program.     

Effect of temperature on the electrical activity of the rat epididymis in vitro

1. 1.|Regional differences in the frequency of electrical activity in rat epididymis were maintained at all temperatures below 39°C.

2. 2.|The change in frequency per deg C increased with temperature and was highest in the temperature region of 34–39°C and the Arrhenius plots of the frequency were linear and parallel in all parts of the epididymis.

3. 3.|The Q₁₀ of the frequency varied between 2.2.–4.3.

4. 4.|The conduction velocity at the cauda epididymis was highest (2.8 mm/s) at 37°C. The Q₁₀ of the conduction velocity was 2.3 in the temperature region of 24–37°C.

Comparison of several mathematical models for describing the joint effect of temperature and ph on glucanex activity

The aim of the present work was to evaluate with different statistical criteria the suitability of nine equations for describing and optimizing the simultaneous effect of temperature and pH on glucanex activity using two characteristic polysaccharides (curdlan and laminarin) as substrates. The most satisfactory solutions were found with an empirical equation constituted with parameters of practical interest (Rosso model), and a hybrid model between the Arrhenius equation and the mathematical expression generated by the protonation-hydroxylation mechanism (Tijskens model). The joint optimal values of pH and temperature calculated with the Rosso model were obtained at 4.64 and 50°C with curdlan and 4.64 and 48°C using laminarin as substrate.     

Linear electrical properties of isolated cardiac cells

Summary A frequency domain equivalent circuit analysis of isolated ventricular cells indicated the presence of an internal membrane structure which has a total capacitance four- to sixfold larger than the surface membrane. The internal membrane was mainly attributed to the sarcoplasmic reticulum since other morphological studies have shown that its area is many-fold larger than that of the surface membrane. Corresponding estimates from the transverse tubular system indicate an area less than that of the surface; thus this structure is not a likely candidate for the observed internal capacitance. Measurements in hypertonic solutions showed that the access resistance to the internal membrane reversibly increased as the tonicity was elevated. Freeze-fractured electron microscopic studies confirmed that hypertonic solutions increased the volume of transverse tubular system, which thus appears to have little relation to the access resistance. The most probable source of the access resistance is the diadic junction to the sarcoplasmic reticulum, which therefore would electrically couple it to the surface membrane.     

辐射传输模型多尺度反演植被理化参数研究进展

植被是生态系统最重要的组成成分之一,许多与植被有关的物质能量交换过程都与植被的理化参数密切相关,因此定量估算植被的理化参数含量对监测植被生长状况、森林火灾预警以及研究全球碳氮循环过程等都具有重要意义.在众多定量反演植被理化参数的方法中,基于数学、物理学以及生物学的基本理论建立起来的辐射传输模型受到越来越多的关注.辐射传输模型描述了植被与入射辐射之间的相互作用过程和特征,相对于传统的经验/半经验方法,辐射传输模型物理意义明确,具有稳定性和可移植性强的特点.在分析国内外最新相关研究的基础上,首先从植被叶片、冠层和像元3个不同的尺度阐述反演植被理化参数的辐射传输模型.叶片尺度上主要介绍PROSPECT模型和LIBERTY模型;冠层尺度上主要介绍SAIL冠层辐射传输模型以及PROSPECT与SAIL耦合的PROSAIL叶片-冠层辐射传输模型;像元尺度的植被理化参数反演目前主要采用冠层尺度的辐射传输模型.其次,分析尺度变化下植被理化参数遥感反演所面临的主要问题,如不同尺度下模型参数敏感性的变化、辐射传输模型的选取以及混合像元的影响等.最后,总结展望植被理化参数反演多模型与多种数据源相互结合的研究趋势,以及将来具有高空间分辨率的高光谱遥感卫星升空后所带来的发展前景.     

A framework for biomechanics simulations using four-chamber cardiac models

Computational cardiac models have been extensively used to study different cardiac biomechanics; specifically, finite-element analysis has been one of the tools used to study the internal stresses and strains in the cardiac wall during the cardiac cycle. Cubic-Hermite finite element meshes have been used for simulating cardiac biomechanics due to their convergence characteristics and their ability to capture smooth geometries compactly–fewer elements are needed to build the cardiac geometry–compared to linear tetrahedral meshes. Such meshes have previously been used only with simple ventricular geometries with non-physiological boundary conditions due to challenges associated with creating cubic-Hermite meshes of the complex heart geometry. However, it is critical to accurately capture the different geometric characteristics of the heart and apply physiologically equivalent boundary conditions to replicate the in vivo heart motion. In this work, we created a four-chamber cardiac model utilizing cubic-Hermite elements and simulated a full cardiac cycle by coupling the 3D finite element model with a lumped circulation model. The myocardial fiber-orientations were interpolated within the mesh using the Log-Euclidean method to overcome the singularity associated with interpolation of orthogonal matrices. Physiologically equivalent rigid body constraints were applied to the nodes along the valve plane and the accuracy of the resulting simulations were validated using open source clinical data. We then simulated a complete cardiac cycle of a healthy heart and a heart with acute myocardial infarction. We compared the pumping functionality of the heart for both cases by calculating the ventricular work. We observed a 20% reduction in acute work done by the heart immediately after myocardial infarction. The myocardial wall displacements obtained from the four-chamber model are comparable to actual patient data, without requiring complicated non-physiological boundary conditions usually required in truncated ventricular heart models.     

The Noble cardiac ventricular electrophysiology models in CellML

We present a review of the cardiac ventricular cell electrophysiology models developed by Prof. Denis Noble and colleagues as an example of how models may be published using a web-based CellML publication framework. The models reviewed have been marked-up in CellML and then used to compute all results presented here. The models are freely available from a website as are the specific numerical experiments discussed in this review and the tools used to perform the simulations.     

Frequent false detection of positive selection by the likelihood method with branch-site models

Positive Darwinian selection promotes fixations of advantageous mutations during gene evolution and is probably responsible for most adaptations. Detecting positive selection at the DNA sequence level is of substantial interest because such information provides significant insights into possible functional alterations during gene evolution as well as important nucleotide substitutions involved in adaptation. Efficient detection of positive selection, however, has been difficult because selection often operates on only a few sites in a short period of evolutionary time. A likelihood-based method with branch-site models was recently introduced to overcome such difficulties. Here I examine the accuracy of the method using computer simulation. I find that the method detects positive selection in 20%-70% of cases when the DNA sequences are generated by computer simulation under no positive selection. Although the frequency of such false detection varies depending on, among other things, the tree topology, branch length, and selection scheme, the branch-site likelihood method generally gives misleading results. Thus, detection of positive selection by this method alone is unreliable. This unreliability may have resulted from its over-sensitivity to violations of assumptions made in the method, such as certain distributions of selective strength among sites and equal transition/transversion ratios for synonymous and nonsynonymous substitutions.     

A biophysical model of Sugarcane growth

Scientists predict that global agricultural lands will expand over the next few decades due to increasing demands for food production and an exponential increase in crop‐based biofuel production. These changes in land use will greatly impact biogeochemical and biogeophysical cycles across the globe. It is therefore important to develop models that can accurately simulate the interactions between the atmosphere and important crops. In this study, we develop and validate a new process‐based sugarcane model (included as a module within the Agro‐IBIS dynamic agro‐ecosystem model) which can be applied at multiple spatial scales. At site level, the model systematically under/overestimated the daily sensible/latent heat flux (by ?10.5% and 14.8%, H and λE, respectively) when compared against the micrometeorological observations from southeast Brazil. The model underestimated ET (relative bias between ?10.1% and –12.5%) when compared against an agro‐meteorological field experiment from northeast Australia. At the regional level, the model accurately simulated average yield for the four largest mesoregions (clusters of municipalities) in the state of São Paulo, Brazil, over a period of 16 years, with a yield relative bias of ?0.68% to 1.08%. Finally, the simulated annual average sugarcane yield over 31 years for the state of Louisiana (US) had a low relative bias (?2.67%), but exhibited a lower interannual variability than the observed yields.     

The effects of sex hormones,prolactin, and chorionic gonadotropin on pineal electrical activity in guinea pigs

Microelectrophoretic application of sex hormones onto pineal cells in guinea pigs has shown different responses in pregnant females as compared to males. In pregnant females estrone caused excitation in 74% of the cells tested, while progesterone and testosterone, prolactin, and HCG were inhibitory in a majority of the cells. In contrast, in males estrone caused excitation of only 19% but inhibition of 37%. A smaller percentage of cells was inhibited by progesterone, while the predominant response to testosterone was excitation. These results suggest that the pineal gland may be under a feedback control.This work was supported by the Volkswagenwerk-Stiftung, Grant I/35472.Visiting scientist supported by grant from the Deutsche Forschungsgemeinschaft and the Royal Society.     

Millimeter wave effects on electrical responses of the sural nerve in vivo

Millimeter wave (MMW, 42.25 GHz)‐induced changes in electrical activity of the murine sural nerve were studied in vivo using external electrode recordings. MMW were applied to the receptive field of the sural nerve in the hind paw. We found two types of responses of the sural nerve to MMW exposure. First, MMW exposure at the incident power density ≥45 mW/cm² inhibited the spontaneous electrical activity. Exposure with lower intensities (10–30 mW/cm²) produced no detectable changes in the firing rate. Second, the nerve responded to the cessation of MMW exposure with a transient increase in the firing rate. The effect lasted 20–40 s. The threshold intensity for this effect was 160 mW/cm². Radiant heat exposure reproduced only the inhibitory effect of MMW but not the transient excitatory response. Depletion of mast cells by compound 48/80 eliminated the transient response of the nerve. It was suggested that the cold sensitive fibers were responsible for the inhibitory effect of MMW and radiant heat exposures. However, the receptors and mechanisms involved in inducing the transient response to MMW exposure are not clear. The hypothesis of mast cell involvement was discussed. Bioelectromagnetics 31:180–190, 2010. © 2009 Wiley‐Liss, Inc.     

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.     

基于棉花功能叶高光谱参数的土壤电导率监测模拟

通过2008-2009年在江苏南京农业大学牌楼试验站的盆栽试验,选择耐盐棉花品种中棉所44和盐敏感性品种苏棉12号为材料,模拟5种不同含盐水平的滨海盐土(0、0.35％、0.60％、0.85％和1.00％),分析了棉花生育期棉田土壤电导率与棉花功能叶光谱反射率和高光谱参数的关系,并建立了棉田土壤电导率(EC)的定量监测模型.结果表明:棉花功能叶光谱反射率在近红外和中红外区域均随土壤盐分水平的升高而升高;以敏感波段1350nm和2307 nm构建的归一化光谱指数NDSI(R1350,R2307)与土壤电导率的决定系数最高,基于此构建了基于NDSI(R1350,R2307)的棉田土壤EC监测模型:EC=-42.899NDSI(R1350,R2307)+27.338;在光谱微分参数中,以TM影像第5个波段的光谱反射率(TM5-SWIR)与棉田土壤EC的决定系数最高,构建了基于TM5-SWIR的棉田土壤EC监测模型:EC=0.0574 TM5-SWIR2-2.5928 TM5-SWIR+ 30.021.以NDSI(R1350,R2307)和TM5-SWIR为自变量的监测模型的预测精度均较高,分别为0.887和0.814,根均方差均较小,分别为1.09和1.29 dS·m-1.利用棉花功能叶NDSI(R1350,R2307)和TM5-SWIR均能较好地监测棉田土壤电导率.     

青藏高原人类活动强度时空变化与影响因素

科学评估人类活动强度对于统筹协调青藏高原生态保护与人类活动具有重要意义。基于多期土地利用现状调查数据和陆地表层人类活动强度算法,测算和分析了青藏高原1984年、1997年、2008年和2018年的人类活动强度及其时空变化特征,使用地理探测器定量解析了影响青藏高原人类活动强度空间分异的驱动因素。结果表明：(1)1984—2018年青藏高原人类活动强度总体处于低水平阶段,大致以2008年为节点,前期缓慢下降,后期快速上升,1984年的人类活动强度为1.44%,2018年上升到1.70%;(2)西藏“一江两河”地区(雅鲁藏布江、拉萨河和年楚河)和青海河湟谷地的人类活动强度最高,沿日喀则-拉萨-那曲-玉树-果洛-西宁形成条带状的相对高值分布区,川藏高山峡谷区、藏北-青南高原区和帕米尔山区的人类活动强度最低;(3)人类活动强度空间分异的主要因素为人口密度、道路密度、经济规模、地表起伏度、城镇化水平、第一产业占比和区域发展导向,且各因子间的交互作用解释力显著高于单因子,表现为非线性增强和双因子增强。     

Electrical activity of rat retinal ganglion cells

The electrical activity of rat retinal ganglion cells is described. It was found that most such cells generate tonic discharges, while cells that demonstrate a phasic type of activity are less numerous. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 382–384, July–October, 2007.