Opening of K+ channels by capacitive stimulation from silicon chip

The development of stable neuroelectronic systems requires a stimulation of nerve cells from semiconductor devices without electrochemical effects at the electrolyte/solid interface and without damage of the cell membrane. The interaction must rely on a reversible opening of voltage-gated ion channels by capacitive coupling. In a proof-of-principle experiment, we demonstrate that Kv1.3 potassium channels expressed in HEK293 cells can be opened from an electrolyte/oxide/silicon (EOS) capacitor. A sufficient strength of electrical coupling is achieved by insulating silicon with a thin film of TiO₂ to achieve a high capacitance and by removing NaCl from the electrolyte to enhance the resistance of the cell-chip contact. When a decaying voltage ramp is applied to the EOS capacitor, an outward current through the attached cell membrane is observed that is specific for Kv1.3 channels. An open probability up to fifty percent is estimated by comparison with a numerical simulation of the cell-chip contact. PACS 73.40.Mr; 82.45.Vp; 87.16.Uv; 87.19.Nn     

The Transport Properties of the Cell Membrane Ion Channels in Electric Fields: Bethe Lattice Treatment

The interactive two-state model of cell membrane ion channels in an electric field is formulated on the Bethe lattice by means of the exact recursion relations. The probability of channel opening or maximum fractions of open potassium and sodium channels are obtained by solving a non-linear algebraic equation. Using known parameters for the conventional mean-field theory the model gives a good agreement with the experiment both at low and high trans-membrane potential values. For intermediate voltages, the numerical results imply that collective effects are introduced by trans-membrane voltage.     

A naturally occurring omega current in a Kv3 family potassium channel from a platyhelminth

Background  

Voltage-gated ion channels are membrane proteins containing a selective pore that allows permeable ions to transit the membrane in response to a change in the transmembrane voltage. The typical selectivity filter in potassium channels is formed by a tetrameric arrangement of the carbonyl groups of the conserved amino-acid sequence Gly-Tyr-Gly. This canonical pore is opened or closed by conformational changes that originate in the voltage sensor (S4), a transmembrane helix with a series of positively charged amino acids. This sensor moves through a gating pore formed by elements of the S1, S2 and S3 helices, across the plane of the membrane, without allowing ions to pass through the membrane at that site. Recently, synthetic mutagenesis studies in the Drosophila melanogaster Shaker channel and analysis of human disease-causing mutations in sodium channels have identified amino acid residues that are integral parts of the gating-pore; when these residues are mutated the proteins allow a non-specific cation current, known as the omega current, to pass through the gating-pore with relatively low selectivity.     

Electrical interfacing of neurotransmitter receptor and field effect transistor

The interfacing of a ligand-gated ion channel to a transistor is studied. It relies on the transduction of ion current to a voltage in a cell-transistor junction. For the first time, a genetically modified cell is used without external driving voltage as applied by a patch-pipette. Using a core-coat conductor model, we show that an autonomous dynamics gives rise to a signal if a driving voltage is provided by potassium channels, and if current compensation is avoided by an inhomogeneous activation of channels. In a proof-of-principle experiment, we transfect HEK293 cells with the serotonin receptor 5-HT3A and the potassium channel Kv1.3. The interfacing is characterized under voltage-clamp with a negative transistor signal for activated 5-HT3A and a positive signal for activated Kv1.3. Without patch-pipette, a biphasic transient is induced by serotonin. The positive wave is assigned to 5-HT3A receptors in the free membrane that drive a potassium outward current through the adherent membrane. The negative wave is attributed to 5-HT3A receptors in the adherent membrane that are activated with a delay due to serotonin diffusion. The implementation of a receptor-cell-transistor device is a fundamental step in the development of biosensors that combine high specificity and universal microelectronic readout.     

用晚钠电流终止心脏中的螺旋波和时空混沌

采用人类心脏模型研究了用晚钠电流控制二维心脏组织中的螺旋波和时空混沌,我们提出这样的控制策略来产生晚钠电流:让慢失活门变量j始终等于0.7,同时实时调节钠电流的快失活门变量h的阈值电压V_I,即先让阈值电压V_I经过T_1时间从71.55 mV均匀减少到50.55 mV,然后经过T_2时间再从50.55 mV均匀增加到71.55 mV,当阈值电压V_I回到71.55 mV,钠电流的快、慢失活门变量恢复正常变化.数值模拟结果表明:只要适当选择控制时间,不论心肌细胞是否存在自发的晚钠电流,控制产生的晚钠电流都可以有效抑制螺旋波和时空混沌,而且需要的晚钠电流都很小,且控制时间都很短,因为螺旋波和时空混沌消失主要是通过传导障碍消失,少数情况下时空混沌是通过转变为靶波消失.我们希望这种控制方法能为室颤控制提供新的思路.     

Noisy saltatory spike propagation: The breakdown of signal transmission due to channel noise

Noisy saltatory spike propagation along myelinated axons is studied within a stochastic Hodgkin-Huxley model. The intrinsic noise (whose strength is inversely proportional to the nodal membrane size) arising from fluctuations of the number of open ion channels influences the dynamics of the membrane potential in a node of Ranvier where the sodium ion channels are predominantly localized. The nodes of Ranvier are linearly coupled. As a measure for the signal propagation reliability, we focus on the ratio between the number of initiated spikes and the transmitted spikes. This work supplements our earlier study [A. Ochab-Marcinek, G. Schmid, I. Goychuk and P. Hänggi, Phys. Rev E 79, 011904 (2009)] towards stronger channel noise intensity and supra-threshold coupling. For strong supra-threshold coupling the transmission reliability decreases with increasing channel noise level until the causal relationship is completely lost and a breakdown of the spike propagation due to the intrinsic noise is observed.     

Tuning of electron transport through a moebius strip: Shot noise

We study electron transport through a moebius strip attached to two metallic electrodes by the use of a Green’s function technique. A parametric approach is used based on the tight-binding model to characterize the electron transport through such a bridge system and it is observed that the transport properties are significantly affected by (a) the transverse hopping strength between the two channels and (b) the strip-to-electrodes coupling strength. In this context we also describe the noise power of the current fluctuations, which provide key information about the electron correlation which is obtained by calculating the Fano factor (F). The knowledge of these current fluctuations gives important ideas for the fabrication of efficient molecular devices.     

基于随机拓扑的复杂腔体电磁辐射统计特性

针对电磁波经孔缝耦合进入多端口微波混沌腔体的电磁辐射问题,推导了孔缝随机耦合模型,以预测单腔体各孔缝处耦合电压的统计特性;并将该模型与电磁拓扑理论结合,提出了一种复合计算方法孔缝随机拓扑模型,用于分析级联腔体的孔缝耦合电压和传输系数等电磁量的统计特性,将该算法的计算结果与网络级联理论的计算结果进行对比,验证了算法的正确性。该模型可成为分析复杂不规则混沌腔体的孔缝耦合问题以及混响室研究的有利工具。     

S波段多注相对论速调管的小信号理论

多注相对论速调管利用多注电子注并行工作,各电子注在传输过程中彼此独立,利于提高注波互作用效率,抑制杂模振荡。分析多注相对论速调管的小信号理论,从谐振腔内电场的场形函数出发确定了不同腔体结构的耦合系数的计算方法,得到了不同电子注形状、注数时的基波电流分量轴向变化过程,并进行模拟验证。结果表明:同轴谐振腔的间隙耦合系数要大于圆柱腔的间隙耦合系数,采用同轴谐振腔更有利于注波互作用;电子群聚过程与每注电子注的势能密切相关,提高电子注数目有助于得到更大的基波电流分量。在电子注电压600 kV、电流5 kA、间隙电压30 kV的条件下,输入腔后的基波电流分量达800 A。     

Current-induced coupling between conductance channels in membranes

The driven system of conducting channels in a nerve membrane is investigated. A current flow generates a coupling between the channels: the current through a channel is influenced by the presence of other conducting channels via the deformation of the equipotential surfaces within the media adjacent to the membrane. We derive an integral equation for the membrane voltageV(s) (s in the membrane plane) and solve it for different membrane conductance distributions(s) including models for stochastic distributions of conducting channels.V(s) is a nonlinear functional of(s). The system of coupled channels is compared with an Ising model. The system exhibits a multi-channel interaction which can be characterized by two different rangesd _int andD ₁. For a mean channel distanced ₀d _int interaction effects are negligible, and ford ₀D ₁ all channel-voltages are equal and thus represent a mean-field for the channels. Increasing conductivity of the medium decreasesd _int and increasesD ₁. With experimental data on sodium channels in nerve membranes we find:d _intd ₀, i.e. a 50% decrease of the channel-voltages by the interaction, andD ₁10³10⁴ d ₀, which indicates mean-field behaviour of the channels. In a subsequent paper we shall treat the statistics of channels which open and close stochastically under the influence of the local membrane voltage.     

Tetrodotoxin: a brief history

Tetrodotoxin (TTX), contained in puffer, has become an extremely popular chemical tool in the physiological and pharmacological laboratories since our discovery of its channel blocking action in the early 1960s. This brief review describes the history of discovery of TTX action on sodium channels, and represents a story primarily of my own work. TTX inhibits voltage-gated sodium channels in a highly potent and selective manner without effects on any other receptor and ion channel systems. TTX blocks the sodium channel only from outside of the nerve membrane, and is due to binding to the selectivity filter resulting in prevention of sodium ion flow. It does not impairs the channel gating mechanism. More recently, the TTX-resistant sodium channels have been discovered in the nervous system and received much attention because of their role in pain sensation. TTX is now known to be produced not by puffer but by bacteria, and reaches various species of animals via food chain.(Communicated by Masanori OTSUKA, M.J.A.).     

Effects of fractal gating of potassium channels on neuronal behaviours

The classical model of voltage-gated ion channels assumes that according to a Markov process ion channels switch among a small number of states without memory, but a bunch of experimental papers show that some ion channels exhibit significant memory effects, and this memory effects can take the form of kinetic rate constant that is fractal. Obviously the gating character of ion channels will affect generation and propagation of action potentials, furthermore, affect generation, coding and propagation of neural information. However, there is little previous research on this series of interesting issues. This paper investigates effects of fractal gating of potassium channel subunits switching from closed state to open state on neuronal behaviours. The obtained results show that fractal gating of potassium channel subunits switching from closed state to open state has important effects on neuronal behaviours, increases excitability, rest potential and spiking frequency of the neuronal membrane, and decreases threshold voltage and threshold injected current of the neuronal membrane. So fractal gating of potassium channel subunits switching from closed state to open state can improve the sensitivity of the neuronal membrane, and enlarge the encoded strength of neural information.     

利用短期心脏记忆消除螺旋波和时空混沌

在Luo-Rudy的心脏模型中引入了记忆效应, 该记忆效应表现为膜间电压的延迟耦合. 研究了记忆效应对螺旋波的影响, 数值模拟结果表明:心脏记忆可导致螺旋波无规则漫游;当延迟时间适当选取时, 增加记忆强度会导致螺旋波的频率减小, 如果记忆强度超过临界值, 心脏记忆效应还可以使螺旋波和时空混沌消失, 因为含时外行钾离子电流被心脏记忆过度抑制.     

Ab initio analysis of electron-phonon coupling in molecular devices

We report a first principles analysis of electron-phonon coupling in molecular devices under external bias voltage and during current flow. Our theory and computational framework are based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. Using a molecular tunnel junction of a 1,4-benzenedithiolate molecule contacted by two aluminum leads as an example, we analyze which molecular vibrational modes are most relevant to charge transport under nonequilibrium conditions. We find that the low-lying modes are most important. As a function of bias voltage, the electron-phonon coupling strength can change drastically while the vibrational spectrum changes at a few percent level.     

Rectification effects in coherent transport through single molecules

A minimal model for coherent transport through a donor/acceptor molecular junction is presented. The two donor and acceptor sites are described by single levels energetically separated by an intramolecular tunnel barrier. In the limit of strong coupling to the electrodes a current rectification for different bias voltage polarities occurs. Contacts with recent experiments of molecular rectification are also given.     

Evolution of entanglement between magnetic moments under interaction with tunneling current

We investigate the evolution of entanglement between two magnetic moments which are initially independent and interact with electron current tunneling through them. The magnetic moments may be nanomagnets, magnetic atoms or atom clusters with spins larger than \frac12\frac{1}{2}. The tunneling of electrons through two moments can be realized by equally coupling two leads (electron reservoirs) to them and applying bias voltage to the leads. In the sequential regime the effect of electron current on the entanglement is calculated fully quantum-mechanically by using extended scattering-matrix theory. It is found that under certain conditions the entanglement can be enhanced from zero to unity by applying the current. We investigate the dependence of the entanglement on the interaction strength and the value of the moments. We discuss the favorite conditions for the realization of entangling gates using such setup.     

Exactly solvable toy model that mimics the mode coupling theory of supercooled liquid and glass transition

A toy model is proposed which incorporates the reversible mode coupling mechanism responsible for ergodic-nonergodic transition of the mode coupling theory (MCT) of structural glass transition with trivial Hamiltonian. The model can be analyzed without relying on uncontrolled approximations inevitable in the current MCT. The strength of hopping processes can be easily tuned and the ideal glass transition is reproduced only in a certain range of the strength. On the basis of the analyses of our model, we discuss about a sharp ergodic-nonergodic transition and its smearing out by "hopping."     

Model of surface structure formation by stabilized spinodal decomposition

A one-dimensional model of stabilized spinodal decomposition is proposed and investigated. In the model, a curvature field couples adiabatically to the order parameter field of standard nonlinear Cahn-Hilliard theory for spinodal decomposition on a membrane. A simple geometric interpretation of the specific coupling is given. Alternatively the theory can also be described by an additional nonlocal self coupling introduced into the usual Cahn-Hilliard theory. An exact stationary solution is found. It predicts a sinusoidal decomposition profile and a global curvature of the initially flat membrane which also develops long wavelength ripples. Stability of this solution against small fluctuations is investigated and conditions for stability are derived. The case of maximal effective coupling is studied in detail and the geometrical surface profile is evaluated analytically.     

一种基于传输线方程的埋地电缆电磁脉冲耦合时域分析方法

 埋地电缆和其它金属长导体对核电磁脉冲的耦合分析对于确定合理的工程防护措施具有重要意义。采用Agrawal模型和时域有限差分算法对线缆耦合问题进行了分析，针对该模型中需要计算土壤阻抗与电流项卷积、土壤导纳与电压项卷积的问题，提出了一种土壤阻抗特性的线性拟合处理方法，可避免复杂的卷积计算。介绍了该方法的基本原理和试用结果。这一模型拟合法适用于确定无时域解析表达式的传输线模型参数，与传输线模型的时域有限差分方法相结合，可用于分析非均匀场作用下传输线的耦合。     

Current drag in semiconductor devices

Electrical currents in semiconductor devices are often carried simultaneously by both electrons and holes. The electron and holes. The electron and hole currents have traditionally been treated as independent, except for recombination phenomena. Simple thermodynamic arguments indicate the possibility that the electrical current of one species may exert a dragging effect upon the other, through their mutual Coulomb interaction. The mutual coupling between current components we have called current drag. We calculate this coupling strength, and show its influence on the operation of a pn diode as an example.