Persistent spatial patterns for semi-discrete models of excitable media

Excitable media have the property that in the spatially homogeneous configuration there is a globally asymptotically stable equilibrium, and no sustained non-decaying oscillations are possible. If diffusion effects are present, however, experiments seem to indicate the possibility of persistent oscillations. In more than one space dimension this may occur, apparently, even if the initial disturbance is restricted to a compact subset of an open domain in the medium. In this paper we discuss a mathematical model of this phenomenon. The model consists of a doubly infinite coupled system of ordinary differential equations, and therefore is intended to represent a spatially discrete network of interconnected cells.     

Drift of large-core spiral waves in inhomogeneous excitable media

Spiral waves in excitable media may drift due to interaction with medium inhomogeneities. We describe this drift asymptotically, within the kinematic (eikonal) approximation.     

Traveling waves and pulses in a one-dimensional network of excitable integrate-and-fire neurons

 We study the existence and stability of traveling waves and pulses in a one-dimensional network of integrate-and-fire neurons with synaptic coupling. This provides a simple model of excitable neural tissue. We first derive a self-consistency condition for the existence of traveling waves, which generates a dispersion relation between velocity and wavelength. We use this to investigate how wave-propagation depends on various parameters that characterize neuronal interactions such as synaptic and axonal delays, and the passive membrane properties of dendritic cables. We also establish that excitable networks support the propagation of solitary pulses in the long-wavelength limit. We then derive a general condition for the (local) asymptotic stability of traveling waves in terms of the characteristic equation of the linearized firing time map, which takes the form of an integro-difference equation of infinite order. We use this to analyze the stability of solitary pulses in the long-wavelength limit. Solitary wave solutions are shown to come in pairs with the faster (slower) solution stable (unstable) in the case of zero axonal delays; for non-zero delays and fast synapses the stable wave can itself destabilize via a Hopf bifurcation. Received: 27 October 1998     

The emergence of wave emitting centres in an excitable medium

The response of an excitable biological medium to a double local stimulus is considered within the context of a mathematical model for a layer of starving cells of Dictyostelium discoideum, with both spatially one- and two-dimensional (1D and 2D) system being investigated. In contrast to the response usually seen in excitable media, whereby each superthreshold stimulus delivered to the relaxed medium results in the initiation of just one travelling wave, a source emitting a sequence of waves can develop in the present excitable medium after the second stimulus. In a 1D system, only transient wave sources forming a limited number of waves are found. In 2D systems, a permanent wave sources consisting in a pair of spirals are observed as well as the transient wave sources forming circular wave patterns. The general features of the medium dynamics that underlie the observed responses to the double stimulus are discussed.     

Continuation and bifurcation analysis of a periodically forced excitable system

The response of an excitable cell to periodic electrical stimulation is modeled using the FitzHugh-Nagumo (FHN) system submitted to a gaussian-shaped pacing, the width of which is small compared with the action potential duration. The influence of the amplitude and the period of the stimulation is studied using numerical continuation and bifurcation techniques (AUTO97 software). Results are discussed in the light of prior experimental and theoretical findings. In particular, agreement with the documented behavior of periodically stimulated cardiac cells and squid axons is discussed. As previously reported, we find many different "M:N" periodic solutions, period-doubling sequences leading to seemingly chaotic regimes, and bistability phenomena. In addition, the use of continuation techniques has allowed us to track unstable solutions of the system and thus to determine how the different stable rhythms are connected with each other in a bifurcation diagram. Depending on the stimulus amplitude, the aspect of the bifurcation diagram with the stimulus period as main varying parameter can vary from very simple to very complex. In its most developed structure, this bifurcation diagram consists of a main "tree" of period-2(P) branches, where the 1:1, 1:0, 2:2, 2:1,... rhythms are located, and of several closed loops made up of period-{N x 2(P)} branches (N>2), isolated from each other and from the main tree. It is mainly on such loops that N:1 rhythms (N>2) on one hand, and N:N-1 or Wenckebach rhythms (N>2) on the other hand, are located. Stable M:N and M:N-1 rhythms (M>or=N) can be found on the same branch of solutions. They are separated by a region of unstable solutions at small stimulus amplitudes, but this region shrinks gradually as the stimulus amplitude is raised, until it finally disappears. We believe that this property is related to the excitability characteristics of the FHN system. It would be interesting to know if it has any correspondence in the behavior of real excitable cells.     

Analysis of an excitable dendritic spine with an activity-dependent stem conductance

 Dendritic spines are the major target for excitatory synaptic inputs in the vertebrate brain. They are tiny evaginations of the dendritic surface consisting of a bulbous head and a tenuous stem. Spines are considered to be an important locus for plastic changes underlying memory and learning processes. The findings that synaptic morphology may be activity-dependent and that spine head membrane may be endowed with voltage-dependent (excitable) channels is the motivation for this study. We first explore the dynamics, when an excitable, yet morphologically fixed spine receives a constant current input. Two parameter Andronov–Hopf bifurcation diagrams are constructed showing stability boundaries between oscillations and steady-states. We show how these boundaries can change as a function of both the spine stem conductance and the conductance load of the attached dendrite. Building on this reference case an idealized model for an activity-dependent spine is formulated and analyzed. Specifically we examine the possibility that the spine stem resistance, the tunable “synaptic weight” parameter identified by Rall and Rinzel, is activity-dependent. In the model the spine stem conductance depends (slowly) on the local electrical interactions between the spine head and the dendritic cable; parameter regimes are found for bursting, steady states, continuous spiking, and more complex oscillatory behavior. We find that conductance load of the dendrite strongly influences the burst pattern as well as other dynamics. When the spine head membrane potential exhibits relaxation oscillations a simple model is formulated that captures the dynamical features of the full model. Received: 10 January 1997/Revised version: 25 March 1997     

食用仙人掌的组织培养

在优良蔬莱品种——米邦塔的推广过程中,采用组织培养的方法,即使在种源较少的情况下在有限的时间内可以获得大量的优质种苗。     

Growth of Bifidobacterium bifidum in whey-based media

Bifidobacteria play an important role in human health including the enhancement of resistance against infection in infants. To develop an inexpensive whey-based medium for Bifidobaterium bifidum, potential growth promoters — yeast extract, casein, bovine casein digest, tryptone, peptone and glucosamine — singly or in combinations, were evaluated for their bifidus growth-promoting activity. The effect of environmental conditions on growth in cheese whey was also evaluated. A whey-based medium for B. bifidum was formulated. Cheese whey supplemented with N-acetylglucosamine (1 mg/ml) and yeast extract (10 mg/ml) in the presence of sodium thioglycolate (0.1%) at pH 6.8 promoted the growth of B. bifidum at 37°C. Journal of Industrial Microbiology & Biotechnology (2000) 25, 177–179. Received 20 May 2000/ Accepted in revised form 20 July 2000     

Effects of Shear Flows on Nonlinear Waves in Excitable Media

If an excitable medium is moving with relative shear, the waves of excitation may be broken by the motion. We consider such breaks for the case of a constant linear shear flow. The mechanisms and conditions for the breaking of solitary waves and wavetrains are essentially different: the solitary waves require the velocity gradient to exceed a certain threshold, whilst the breaking of repetitive wavetrains happens for arbitrarily small velocity gradients. Since broken waves evolve into new spiral wave sources, this leads to spatio-temporal irregularity.     

Influence of cultural and physiochemical factors on ascorbate stability in plant tissue culture media

Ascorbic acid rapidly decays in plant tissue culture media. Within 50 min to 3 h after preparing 100 mM solutions, ascorbic acid was destroyed. Autoclaving, shaking flasks, high light intensity and increasing pH over a range from 4.5–7 accelerated decay. Ascorbic acid was oxidized to dehydroascorbic acid which also underwent decay. Within 11 h and 15 min after adding ascorbic acid both ascorbic acid and its oxidation product, dehydroascorbic acid, disappeared from medium. Since ascorbic acid is rapidly destroyed in plant tissue culture media it may not exert its effect as an intact molecule. Instead its antioxidant/antibrowning role in plant cell, tissue and organ cultures may be mediated by some product of further oxidation.     

Glutathione in conditioned media of tobacco suspension cultures

Conditioned media of suspension cultures of Nicotiana tabacum var. Samsun contain 0.15-0.20 mmol/l glutathione. These concentrations correspond closely to the intracellular content of glutathione and represent about three to four times the amount of glutathione needed to maintain the intracellular level of glutathione. In contrast to the GSH:GSSG ratio inside the cells, the amount of GSSG in the media rises to 20% of the GSH content.     

L-Lysine alpha-oxidase: physicochemical and biological properties

This review summarizes data on the properties of L-lysine -oxidase, an enzyme that belongs to the group of oxidases of L-amino acids. This enzyme acts virtually only on L-lysine with a rather low K _m yielding -keto--aminocaproic acid. The decrease in the level of the essential amino acid L-lysine and the formation of hydrogen peroxide during the reaction possibly provide the basis for the unique properties of L-lysine -oxidase: cytotoxic, antitumor, antimetastatic, antiinvasive, antibacterial, and antiviral activities, as well as an immunomodulating effect. Native L-lysine -oxidase and its immobilized forms are promising tools for determination of concentration of L-lysine in various biological materials.     

Excitability of an age-structured plankton ecosystem

 We adapt a simple two-component model of a plankton ecosystem to account for the life spans of individual predatory organisms. We investigate the system’s short-term dynamics, in particular its excitability, and its long-term dynamics, and show how both can be highly sensitive to initial conditions. We discover that this effect is enhanced by imposing age structure on the system. Received: 21 June 1996 / Revised version: 19 October 1998     

栝楼的组织培养与快速繁殖

栝楼（Trichosanthes kirilowii Maxim．）,也称瓜楼、药瓜、吊瓜等,为葫芦科栝楼属（Trichosanthes L．）多年生攀援植物。栝楼根、茎、叶、瓜皮及种子均具有一定的药用价值,其籽是炒货中的佳品。由于栝楼野生资源有限且遭严重破坏,尽管各地有少量引种栽培,但其产量却远不能满足市场需求,因此,对栝楼进行组织培养与快速繁殖具有重要意义。目前,仅有以栝楼腋芽为材料进行组织培养的研究报道。作者以栝楼的茎尖、茎切段和幼叶为外植体进行组织培养,通过对不同培养基的筛选和优化,初步建立了栝楼组织培养的快速繁殖体系,并获得了大量优良品种的无菌苗,为扩大生产提供了保障。     

Effects of altered gravity on the actin and microtubule cytoskeleton of human SH-SY5Y neuroblastoma cells

Summary. Human SH-SY5Y neuroblastoma cells were used to study the effects of altered gravity on the actin and microtubule cytoskeleton dynamics. A cholinergic stimulation of the cells during a 6 min period of changing gravity (3 parabolas) resulted in an enhanced actin-driven protrusion of evoked lamellipodia. Likewise, the spontaneous protrusive activity of nonactivated cells was promoted during exposure to changing gravity (6 up to 31 parabolas). Ground-based experiments revealed a similar enhancement of the spontaneous and evoked lamellar protrusive activity when the cells were kept at 2 g hypergravity for at least 6 min. This gravity response was independent of the direction of the acceleration vector in respect to the cells. Exposure of the cells to “simulated weightlessness” (clinorotation) had no obvious influence on this type of lamellar actin cytoskeleton dynamics. A 20 min exposure of the cells to simulated weightlessness or to changing gravity (6 to 31 parabolas) – but not to 2 g (hypergravity, centrifugation) – resulted in an altered arrangement of microtubules indicated by bending, turning, and loop formation. A similar altered arrangement was shown by microtubules which had polymerized into lamellipodia after release from a taxol block at simulated weightlessness (clinorotation) or during changing gravity (5 parabolas). Our data suggest that in human SH-SY5Y neuroblastoma cells, microgravity affects the dynamics and spatial arrangement of microtubules but has no influence on the Rac-controlled lamellar actin cytoskeleton dynamics and cell spreading. The latter, however, seems to be promoted at hypergravity. Correspondence and reprints: Cell and Developmental Neurobiology, Institute of Zoology, University of Hohenheim, Garbenstrasse 30, 70593 Stuttgart, Federal Republic of Germany.     

不同营养基质与条件对灰树花生长的影响

探讨了不同营养基质与条件对灰树花[Grifola frondosa(Fr.)S.F.Gray]生长的影响。试验结果表明：菌丝生长的最适pH为5-6;最适氮源为蛋白胨、牛肉膏和谷氨酰胺,葡萄糖和蛋白胨最适浓度分别为100和1g/L;菌丝在黑暗环境下生长良好;使用马铃薯蛋白胨培养基进行振荡培养,菌丝球生长较好;母种采用马铃薯麦麸培养基,栽培种选用棉子壳麦麸石培养料,可缩短制种周期,获得质量较高的菌种;菌丝含有较多的钙、铁和锌等元素。     

Effect of various growth media upon survival during storage of freeze-dried Enterococcus faecalis and Enterococcus durans

AIMS: The effects of three different growth media (MRS, M17 and Lee's) on survival during freeze-drying and subsequent storage of six strains of Enterococcus faecalis and two strains of E. durans were investigated. METHODS AND RESULTS: Distinct Enterococcus spp. strains were grown on M17, MRS and Lee's broth, freeze-dried and stored at 20 degrees C in air under darkness. At regular intervals throughout storage, freeze-dried samples were rehydrated and then plated on M17 agar. CONCLUSIONS: A higher survival rate during storage of dried E. durans was obtained when growth occurred in MRS. The same effect was not observed, however, for the majority of E. faecalis strains, which clearly survived better in the dried state when this organism had been grown in M17 or Lee's medium. SIGNIFICANCE AND IMPACT OF STUDY: The survival of the dried Enterococcus spp. tested during storage was shown to be strain-specific and dependent on the growth medium.     

Halotolerance is enhanced in carrot callus by sensing hypergravity: influence of calcium modulators and cytochalasin D

Carrot callus was centrifuged at 10 g and compared to callus growing at 1 g on agar in the presence of increasing sodium chloride concentrations. Growth after 14 days was enhanced in the centrifuged samples versus samples kept at 1 g. This effect was not found when the samples were grown on potassium chloride. At 50 mM NaCl, the calcium ionophore ionomycin was applied to centrifuged and noncentrifuged callus samples. In both experiments, the growth of callus increased with increasing ionomycin concentrations but under 10 g this increase was more enhanced. As inhibitors of calcium influx, lanthanum and gadolinium chloride were chosen in the presence of 50 mM NaCl. Both inhibitors inhibited growth at 1 g at low concentrations of around 2 microM, whereas the centrifuged samples were not or much less so inhibited. We tested an involvement of actin by application of cytochalasin D to callus grown in the presence of 50 mM NaCl. In both types of samples, growth at 1 g and growth at 10 g, cytochalasin D enhanced growth but the effect was clearly stronger at 10 g than at 1 g. As increased halotolerance was only observed in the presence of increased sodium ions, not potassium ions, and as halotolerance is known to be induced by an influx of calcium, the data suggest that a calcium influx induced by hypergravity and possibly modulated by actin caused the observed increase in halotolerance at 10 g.     

南荻的组织培养与快速繁殖技术

以南荻幼穗为外植体,进行了组织培养与快速繁殖技术的研究。结果表明：最佳诱导培养基：MS＋2.0mg·L-12,4-D＋0.1mg·L-16-BA＋0.5g·L-1水解酪蛋白＋0.5g·L-1脯氨酸;最佳分化培养基：MS＋0.5mg·L-1NAA＋0.5mg·L-1KT＋0.5mg·L-1IAA＋2.0mg·L-16-BA＋0.5g·L-1水解酪蛋白＋0.5g·L-1脯氨酸;最佳生根培养基：MS＋1.0mg·L-1NAA＋0.25mg·L-1Met。炼苗后,移入营养土与珍珠岩（1：1）的基质中,移栽成活率高达100%。该体系的建立为南荻规模化生产及遗传改良提供了前提条件。     

中国水仙(Narcissus tazetta var.chinensis)的组织培养

以栽培的中国水仙Narcissus tazetta var.Chinensis鳞茎为实验材料,采取“无液处理”的简捷方法获取外植体,并筛选出中国水仙离体鳞茎培养的最佳培养基。