整合发放神经元模型突触输入参数估计

文章针对突触输入和噪声共同作用下的整合发放神经元模型,在不考虑放电阈值前提下,采用最小二乘法估计突触输入参数;当考虑神经元放电阈值特性时,将放电阈值看成一个吸收边界,导出膜电位转移概率密度函数,再利用极大似然法估计突触输入参数。结果表明:最小二乘估计仅适合阈下活动的参数估计,而对阈上活动无效;极大似然估计适用于神经元放电的阈值行为;无论是从适用范围还是估计精度来说,极大似然估计都要优于最小二乘估计。     

Functional specificity of local synaptic connections in neocortical networks

Neuronal connectivity is fundamental to information processing in the brain. Therefore, understanding the mechanisms of sensory processing requires uncovering how connection patterns between neurons relate to their function. On a coarse scale, long-range projections can preferentially link cortical regions with similar responses to sensory stimuli. But on the local scale, where dendrites and axons overlap substantially, the functional specificity of connections remains unknown. Here we determine synaptic connectivity between nearby layer 2/3 pyramidal neurons in vitro, the response properties of which were first characterized in mouse visual cortex in vivo. We found that connection probability was related to the similarity of visually driven neuronal activity. Neurons with the same preference for oriented stimuli connected at twice the rate of neurons with orthogonal orientation preferences. Neurons responding similarly to naturalistic stimuli formed connections at much higher rates than those with uncorrelated responses. Bidirectional synaptic connections were found more frequently between neuronal pairs with strongly correlated visual responses. Our results reveal the degree of functional specificity of local synaptic connections in the visual cortex, and point to the existence of fine-scale subnetworks dedicated to processing related sensory information.     

Mutations in the Caenorhabditis elegans unc-4 gene alter the synaptic input to ventral cord motor neurons.

Identification of the genes orchestrating neurogenesis would greatly enhance our understanding of this process. Genes have been identified that specify neuron type (for example cut and numb in Drosophila and mec-3 in Caenorhabditis elegans) and process guidance (for example, unc-5, unc-6 and unc-40 in C. elegans and the fas-1 gene of Drosophila). We sought genes defining synaptic specificity by identifying mutations that alter synaptic connectivity in the motor circuitry in the nematode C. elegans. We used electron microscopy of serial sections to reconstruct the ventral nerve-cords of uncoordinated (unc) mutants that have distinctive locomotory choreographies. Here we describe the phenotype of mutations in the unc-4 gene in which a locomotory defect is correlated with specific changes in synaptic input to a subset of the excitatory VA motor neurons, normally used in reverse locomotion. The circuitry alterations do not arise because of the inaccessibility of the appropriate synaptic partners, but are a consequence of changes in synaptic specificity. The VA motor neurons with altered synaptic inputs are all lineal sisters of VB motor neurons; the VA motor neurons without VB sisters have essentially the same synaptic inputs as in wild-type animals. The normal function of the wild-type allele of unc-4 may thus be to invoke the appropriate synaptic specificities to VA motor neurons produced in particular developmental contexts.     

Self-organization model on receptive field of neuron with asymmetric time window of synaptic modification

The spatial-temporal response properties of some simple neurons in visual pathway arise basically prior to birth. In the absence of visual experience, how do these neurons develop in visual system? Based on Wimbauer network with delay, a four-layer feed-forward network model is proposed, which is characterized by modifying the Hebb learning rule through introducing the asymmetric time window of synaptic modification found recently in neurobiology. The model can not only generate by self-organization more diversified spatial-temporal response characteristics of neuronal receptive field than earlier models but also provide some explanations for the possible mechanism underlying the development of receptive fields of contrast polarity sensitive neurons found in visual system of vertebrate. Thus the proposed model may be more widely applicable than Linsker model and Wimbauer model.     

On the spontaneous emergence of cell polarity

Diverse cell polarity networks require positive feedback for locally amplifying distributions of signalling molecules at the plasma membrane. Additional mechanisms, such as directed transport or coupled inhibitors, have been proposed to be required for reinforcing a unique axis of polarity. Here we analyse a simple model of positive feedback, with strong analogy to the 'stepping stone' model of population genetics, in which a single species of diffusible, membrane-bound signalling molecules can self-recruit from a cytoplasmic pool. We identify an intrinsic stochastic mechanism through which positive feedback alone is sufficient to account for the spontaneous establishment of a single site of polarity. We find that the polarization frequency has an inverse dependence on the number of signalling molecules: the frequency of polarization decreases as the number of molecules becomes large. Experimental observation of polarizing Cdc42 in budding yeast is consistent with this prediction. Our work suggests that positive feedback can work alone or with additional mechanisms to create robust cell polarity.     

用毛蛤壳制备柠檬酸钙、丙酸钙的研究

以海产贝壳（毛蛤壳）作为钙源,制备柠檬酸钙、丙酸钙。考察了燃烧条件对中间产物氧化钙的色泽、收率、纯度的影响,以及反应物浓度、原料配比、反应温度等因素对柠檬酸钙的收率的影响。柠檬酸钙的收率最高可达到91％以上。还考察了丙酸钙作为食品防腐剂在防腐方面的效果。     

Genetic evidence that relative synaptic efficacy biases the outcome of synaptic competition

Synaptic activity drives synaptic rearrangement in the vertebrate nervous system; indeed, this appears to be a main way in which experience shapes neural connectivity. One rearrangement that occurs in many parts of the nervous system during early postnatal life is a competitive process called 'synapse elimination'. At the neuromuscular junction, where synapse elimination has been analysed in detail, muscle fibres are initially innervated by multiple axons, then all but one are withdrawn and the 'winner' enlarges. In support of the idea that synapse elimination is activity dependent, it is slowed or speeded when total neuromuscular activity is decreased or increased, respectively. However, most hypotheses about synaptic rearrangement postulate that change depends less on total activity than on the relative activity of the competitors. Intuitively, it seems that the input best able to excite its postsynaptic target would be most likely to win the competition, but some theories and results make other predictions. Here we use a genetic method to selectively inhibit neurotransmission from one of two inputs to a single target cell. We show that more powerful inputs are strongly favoured competitors during synapse elimination.     

Conservation of total synaptic weight through balanced synaptic depression and potentiation

Memory is believed to depend on activity-dependent changes in the strength of synapses. In part, this view is based on evidence that the efficacy of synapses can be enhanced or depressed depending on the timing of pre- and postsynaptic activity. However, when such plastic synapses are incorporated into neural network models, stability problems may develop because the potentiation or depression of synapses increases the likelihood that they will be further strengthened or weakened. Here we report biological evidence for a homeostatic mechanism that reconciles the apparently opposite requirements of plasticity and stability. We show that, in intercalated neurons of the amygdala, activity-dependent potentiation or depression of particular glutamatergic inputs leads to opposite changes in the strength of inputs ending at other dendritic sites. As a result, little change in total synaptic weight occurs, even though the relative strength of inputs is modified. Furthermore, hetero- but not homosynaptic alterations are blocked by intracellular dialysis of drugs that prevent Ca2+ release from intracellular stores. Thus, in intercalated neurons at least, inverse heterosynaptic plasticity tends to compensate for homosynaptic long-term potentiation and depression, thus stabilizing total synaptic weight.     

Link between the geomagnetic polarity reversal and global-geology events

The origin of the earth’s magnetic field is one of the five most important unsolved problems in physics, especially geomagnetic polarity reversal remains one of Nature’s most enigmatic phenomena. It is proposed that the geomagnetic polarity reversal links with some global-geology events, although it is difficult to clarify the mechanism of this correlation. The history of continental structures in eastern China would be also linked with some global-geology events.     

Experimentally induced alteration in the polarity of developing neurons

Despite the great diversity of shapes exhibited by different classes of nerve cells, nearly all neurons share one feature in that they have a single axon and several dendrites. The two types of processes differ in their morphology, in their rate of growth, in the macromolecular composition of their cytoskeletons and surface membranes, and in their synaptic polarity. When hippocampal neurons are dissociated from the embryonic brain and cultured, they reproducibly establish this basic form with a single axon and several dendrites, despite the absence of any spatially organized environmental cues, and without the need for cell to cell contact. We have cut the axons of young hippocampal neurons within a day of their development: in some cases the initial axon regenerated, but more frequently one of the other processes, which if undisturbed would have become a dendrite, instead became the axon. Frequently the stump of the original axon persisted following the transection and subsequently became a dendrite. Evidently the neuronal processes that first develop in culture have the capacity to form either axons or dendrites. The acquisition of axonal characteristics by one neuronal process apparently inhibits the others from becoming axons, so they subsequently become dendrites.     

钙释放激活钙离子通道的发现及研究现状

Ca2+释放激活Ca2+(CRAC)通道是位于非兴奋性细胞质膜上的慢Ca2+通道,是非兴奋性细胞(尤其T淋巴细胞和HEK 293细胞)中胞外Ca2+进入细胞内的主要途径.Ca2+内流是T淋巴细胞激活的最重要的生理生化特征之一.Orai1蛋白单体是组成CRAC通道的亚基,4个Orai1蛋白亚基构成一个四聚体CRAC通道.内质网Ca2+浓度的降低使得STIM1发生定向运动并产生聚集,从而激活了CRAC通道.STIM1蛋白把内质网Ca2+的损耗与CRAC通道上的Ca2+内流联系起来,行使了Ca2+浓度感受器的功能.     

氟化钙和氧化钙在冰晶石熔体中的结构

研究了氟化钙在熔融氟化钠和冰晶石中的4种不同的结构模型。将所计算的二元系液相线与所实测的液相线进行比较。结果表明,当氟化钙加到熔融的氟化钠中时生成一个钙三个氟的阴离子。而在冰晶石-氟化钙熔体中存在着一个钙三个氟的阴离子和一个钙一个氟的阳离子。     

Estimation of the polarity of the protein interior by optical spectroscopy

Crystallographic studies of myoglobin have shown that the haem pocket is lined with nonpolar amino-acid residues. In order to estimate the true polarity of the interior of proteins, certain studies have used bound fluorophores, the spectroscopic properties of which reflect the polarity of their environment. These studies have most often used 1-amino-8-naphthalene sulphonate (ANS) as a probe, but a more suitable probe, in principle, is 6-propionyl 2-(N,N-dimethyl)aminonaphthalene (PRODAN). We have synthesized a molecule with the advantageous spectroscopic properties of PRODAN but with a higher affinity for apomyogloblin: 2'-(N,N-dimethyl)amino-6-naphthopyl-4-trans-cyclohexanoic acid (DANCA), and report here its use to determine the polarity of the myoglobin haem pocket. Our results show that the pocket is actually a polar environment, and the polarity can be accounted for by peptide amide dipoles.