感觉神经放电中的随机共振现象和噪声的作用

随机共振现象是非线性系统中普遍存在的自然现象 ,其中 ,噪声可以帮助检测弱信号而不是淹没弱信号。本文介绍了感觉神经放电活动中的随机共振现象和产生的机制 ,揭示了神经系统利用噪声检测弱信号的机制 ,并提出了随机共振在神经系统信息处理中的可能作用。     

Spatiotemporal pattern formation in two-dimensional neural circuits: roles of refractoriness and noise

Refractoriness is one of the most fundamental states of neural firing activity, in which neurons that have just fired are unable to produce another spike, regardless of the strength of afferent stimuli. Another essential and unavoidable feature of neural systems is the existence of noise. To study the role of these essential factors in spatiotemporal pattern formation in neural systems, a spatially expended neural network model is constructed, with the dynamics of its individual neurons capturing the three most essential states of the neural firing behavior: firing, refractory and resting, and the network topology consistent with the widely observed center-surround coupling manner in the real brain. By changing the refractory period with and without noise in a systematic way in the network, it is shown numerically and analytically that without refractoriness, or when the refractory period is smaller than a certain value, the collective activity pattern of the system consists of localized, oscillating patterns. However, when the refractory period is greater than a certain value, crescent-shaped, localized propagating patterns emerge in the presence of noise. It is further illustrated that the formation of the dynamical spiking patterns is due to a symmetry breaking mechanism, refractoriness-induced symmetry breaking; that is generated by the interplay of noise and refractoriness in the network model. This refractoriness-induced symmetry breaking provides a novel perspective on the emergence of localized, spiking wave patterns or spike timing sequences as ubiquitously observed in real neural systems; it therefore suggests that refractoriness may benefit neural systems in their temporal information processing, rather than limiting the performance of neurons, as has been conventionally thought. Our results also highlight the importance of considering noise in studying spatially extended neural systems, where it may facilitate the formation of spatiotemporal order.     

Degenerate coding in neural systems

When the dimensionality of a neural circuit is substantially larger than the dimensionality of the variable it encodes, many different degenerate network states can produce the same output. In this review I will discuss three different neural systems that are linked by this theme. The pyloric network of the lobster, the song control system of the zebra finch, and the odor encoding system of the locust, while different in design, all contain degeneracies between their internal parameters and the outputs they encode. Indeed, although the dynamics of song generation and odor identification are quite different, computationally, odor recognition can be thought of as running the song generation circuitry backwards. In both of these systems, degeneracy plays a vital role in mapping a sparse neural representation devoid of correlations onto external stimuli (odors or song structure) that are strongly correlated. I argue that degeneracy between input and output states is an inherent feature of many neural systems, which can be exploited as a fault-tolerant method of reliably learning, generating, and discriminating closely related patterns.     

Global rhythmic activities in hippocampal neural fields and neural coding

Global oscillations of the neural field represent some of the most interesting expressions of the hippocampal activity, being related also to learning and memory. To study oscillatory activities of the CA3 field in theta range, a model of this sub-field of Hippocampus has been formulated. The model describes the firing activity of CA3 neuronal populations within the frame of a kinetic theory of neural systems and it has been used for computer simulations. The results show that the propagation of activities induced in the neural field by hippocampal afferents occurs only in narrow time windows confined by inhibitory barrages, whose time-course follows the theta rhythm. Moreover, during each period of a theta wave, the entire CA3 field bears a firing activity with peculiar space-time patterns, a sort of specific imprint, which can induce effects with similar patterns on brain regions driven by the hippocampal formation. The simulation has also demonstrated the ability of medial septum to influence the global activity of the CA3 pyramidal population through the control of the population of inhibitory interneurons. At last, the possible involvement of global population oscillations in neural coding has been discussed.     

Learning and coding of concepts in neural networks

Our environment consists of virtually an infinite number of scenarios in which we have to function. In order to respond properly to an incoming stimulus, the brain has first to analyze it, and to find out the basic familiar elements that are part of it. In other words, by using a library which contains a relatively small number of basic concepts, the brain analyzes the multitude of incoming events. Some of those basic concepts are innate, but many of them must be learned, in order to accommodate for the arbitrary environment around us. A classifying box is defined as the neural network that finds out the familiar concepts that are present in an incoming stimulus. Models for classifying boxes are introduced, and possible mechanisms by which they may establish their libraries of concepts are suggested, and then compared and evaluated by computer simulations.     

Energy coding in biological neural networks

According to the experimental result of signal transmission and neuronal energetic demands being tightly coupled to information coding in the cerebral cortex, we present a brand new scientific theory that offers an unique mechanism for brain information processing. We demonstrate that the neural coding produced by the activity of the brain is well described by our theory of energy coding. Due to the energy coding model’s ability to reveal mechanisms of brain information processing based upon known biophysical properties, we can not only reproduce various experimental results of neuro-electrophysiology, but also quantitatively explain the recent experimental results from neuroscientists at Yale University by means of the principle of energy coding. Due to the theory of energy coding to bridge the gap between functional connections within a biological neural network and energetic consumption, we estimate that the theory has very important consequences for quantitative research of cognitive function.     

Essential roles of S-nitrosothiols in vascular homeostasis and endotoxic shock

The current perspective of NO biology is formulated predominantly from studies of NO synthesis. The role of S-nitrosothiol (SNO) formation and turnover in governing NO-related bioactivity remains uncertain. We generated mice with a targeted gene deletion of S-nitrosoglutathione reductase (GSNOR), and show that they exhibit substantial increases in whole-cell S-nitrosylation, tissue damage, and mortality following endotoxic or bacterial challenge. Further, GSNOR(-/-) mice have increased basal levels of SNOs in red blood cells and are hypotensive under anesthesia. Thus, SNOs regulate innate immune and vascular function, and are cleared actively to ameliorate nitrosative stress. Nitrosylation of cysteine thiols is a critical mechanism of NO function in both health and disease.     

The state of gustatory neural coding

Debates on gustatory neural coding have been dominated by asmall number of fundamental issues since the inception of thefield in 1941. Three of these are discussed in this review:(1) are there basic tastes? (2) are there gustatory neuron types?(3) is the code for a taste read simultaneously across all participatingneurons (across-fiber patterning), or is it confined to a selectivechannel composed of cells of one type (labeled-line or channeling)?No conclusions are drawn regarding (1), primarily because auniversal definition of ‘basic tastes’ is lacking.It is concluded that gustatory neuron types are likely to existafter reviewing the issue from multiple perspectives and discoveringrecurring indications of neuron types from several. A firm conclusion,however, also awaits a widely accepted definition of what constitutesa neuron type. Issue (3) cannot yet be resolved for lack ofdefinitive data, specifically whether the discharges of inhibited,unresponsive, or weakly responsive cells add to (signal) ordetract from (noise) the neural code for a tastant.     

microRNA与神经系统发育

microRNA(miRNA)介导的基因沉默是生物体内普遍存在的重要基因表达调控方式,其调控失常与很多人类疾病相关.miRNA在神经组织表达丰富.神经系统miRNA的功能研究是近年非常活跃的新领域.基于近期的研究进展,本文重点讨论了miRNA在神经轴模式化、神经元命运决定、神经细胞发生、神经元突触形成及成熟神经元突触重塑中的重要作用.     

Intrinsic gain modulation and adaptive neural coding

In many cases, the computation of a neural system can be reduced to a receptive field, or a set of linear filters, and a thresholding function, or gain curve, which determines the firing probability; this is known as a linear/nonlinear model. In some forms of sensory adaptation, these linear filters and gain curve adjust very rapidly to changes in the variance of a randomly varying driving input. An apparently similar but previously unrelated issue is the observation of gain control by background noise in cortical neurons: the slope of the firing rate versus current (f-I) curve changes with the variance of background random input. Here, we show a direct correspondence between these two observations by relating variance-dependent changes in the gain of f-I curves to characteristics of the changing empirical linear/nonlinear model obtained by sampling. In the case that the underlying system is fixed, we derive relationships relating the change of the gain with respect to both mean and variance with the receptive fields derived from reverse correlation on a white noise stimulus. Using two conductance-based model neurons that display distinct gain modulation properties through a simple change in parameters, we show that coding properties of both these models quantitatively satisfy the predicted relationships. Our results describe how both variance-dependent gain modulation and adaptive neural computation result from intrinsic nonlinearity.     

Central neural coding of sky polarization in insects

Many animals rely on a sun compass for spatial orientation and long-range navigation. In addition to the Sun, insects also exploit the polarization pattern and chromatic gradient of the sky for estimating navigational directions. Analysis of polarization-vision pathways in locusts and crickets has shed first light on brain areas involved in sky compass orientation. Detection of sky polarization relies on specialized photoreceptor cells in a small dorsal rim area of the compound eye. Brain areas involved in polarization processing include parts of the lamina, medulla and lobula of the optic lobe and, in the central brain, the anterior optic tubercle, the lateral accessory lobe and the central complex. In the optic lobe, polarization sensitivity and contrast are enhanced through convergence and opponency. In the anterior optic tubercle, polarized-light signals are integrated with information on the chromatic contrast of the sky. Tubercle neurons combine responses to the UV/green contrast and e-vector orientation of the sky and compensate for diurnal changes of the celestial polarization pattern associated with changes in solar elevation. In the central complex, a topographic representation of e-vector tunings underlies the columnar organization and suggests that this brain area serves as an internal compass coding for spatial directions.     

Signal stabilization and noise suppression in neural systems

Summary Neural activity is viewed as a stochastic point process, in which information resides in the modulation of a background of spontaneous activity. Characteristic features of the spatial and temporal mapping of sensory signals are discussed. One of the puzzling aspects of neural functioning is the integrity of the signal in its passage toward higher brain centers, in view of the fundamentally noisy response of the individual neuron. It is shown, that a process, we call image stabilization, is a direct consequence of the particular mapping function exemplified by lateral inhibition and adaptation.The research reported in this paper was sponsored by the Aerospace Medical Research Laboratories, Aerospace Medical Division, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, under contract No. F 33615-67-C-1413 and the Office of Naval Research, contract No. ONR N0001467-A-0378-0001, with Syracuse University Research Institute. Further reproduction is authorized to satisfy needs of the U.S. Government.     

Essential roles of snap-29 in C. elegans

SNARE domain proteins are key molecules mediating intracellular fusion events. SNAP25 family proteins are unique target-SNAREs possessing two SNARE domains. Here we report the genetic, molecular, and cell biological characterization of C. elegans SNAP-29. We found that snap-29 is an essential gene required throughout the life-cycle. Depletion of snap-29 by RNAi in adults results in sterility associated with endomitotic oocytes and pre-meiotic maturation of the oocytes. Many of the embryos that are produced are multinucleated, indicating a defect in embryonic cytokinesis. A profound defect in secretion by oocytes and early embryos in animals lacking SNAP-29 appears to be the underlying defect connecting these phenotypes. Further analysis revealed defects in basolateral and apical secretion by intestinal epithelial cells in animals lacking SNAP-29, indicating a broad requirement for this protein in the secretory pathway. A SNAP-29-GFP fusion protein was enriched on recycling endosomes, and loss of SNAP-29 disrupted recycling endosome morphology. Taken together these results suggest a requirement for SNAP-29 in the fusion of post-Golgi vesicles with the recycling endosome for cargo to reach the cell surface.     

Essential pro-Bmp roles of crossveinless 2 in mouse organogenesis

We here report essential roles of the Bmp-binding protein crossveinless 2 (Cv2; Bmper) in mouse organogenesis. In the null Cv2 mutant mouse, gastrulation occurs normally, but a number of defects are found in Cv2-expressing tissues such as the skeleton. Cartilage differentiation by Bmp4 treatment is reduced in cultured Cv2(-/-) fibroblasts. Moreover, the defects in the vertebral column and eyes of the Cv2(-/-) mouse are substantially enhanced by deleting one copy of the Bmp4 gene, suggesting a pro-Bmp role of Cv2 in the development of these organs. In addition, the Cv2(-/-) mutant exhibits substantial defects in Bmp-dependent processes of internal organ formation, such as nephron generation in the kidney. This kidney hypoplasia is synergistically enhanced by the additional deletion of Kcp (Crim2) which encodes a pro-Bmp protein structurally related to Cv2. This study demonstrates essential pro-Bmp functions of Cv2 for locally restricted signal enhancement in multiple aspects of mammalian organogenesis.     

Essential roles of caspases and their upstream regulators in rotenone-induced apoptosis

In the present study, we examined whether caspases and their upstream regulators are involved in rotenone-induced cytotoxicity. Rotenone significantly inhibited the proliferation of oral cancer cell lines in a dose-dependent manner compared to normal oral mucosal fibroblasts. Flow cytometric analysis of DNA content showed that rotenone treatment induced apoptosis following G2/M arrest. Western blotting showed activation of both the caspase-8 and caspase-9 pathways, which differed from previous studies conducted in other cell types. Furthermore, p53 protein and its downstream pro-apoptotic target, Bax, were induced in SAS cells after treatment with rotenone. Rotenone-induced apoptosis was inhibited by antioxidants (glutathione, N-acetylcysteine, and tiron). In conclusion, our results demonstrate significant involvement of caspases and their upstream regulators in rotenone-induced cytotoxicity.     

Sox2 roles in neural stem cells

Throughout vertebrate evolution, Sox2 marks the developing nervous system from its earliest developmental stages and, therein, the most undifferentiated precursor cells, including stem cells. Recent gene targeting studies investigated the function of Sox2 in two neuronal systems: the developing eye and brain. These studies uncovered a requirement for Sox2 in the maintenance of neural stem cells, as well as a downstream role in the differentiation of specific neuron sub-types. In both systems, Sox2 action is markedly dose-dependent, and downstream-target gene studies are beginning to reveal the mechanisms of Sox2 function.