Initial Segment (initial + segment)

Distribution by Scientific Domains
Distribution within Life Sciences


Selected Abstracts


Proxy caching algorithms and implementation for time-shifted TV services

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 2 2008
Tim Wauters
The increasing popularity of multimedia streaming applications introduces new challenges in content distribution networks (CDNs). Streaming services such as Video on Demand (VoD) or digital television over the Internet (IPTV) are very bandwidth-intensive and cannot tolerate the high start-up delays and poor loss properties of today's Internet. To solve these problems, caching (the initial segment of) popular streams at proxies could be envisaged. This paper presents a novel caching algorithm and architecture for time-shifted television (tsTV) and its implementation, using the IETF's Real-Time Streaming Protocol (RTSP). The algorithm uses sliding caching windows with sizes depending on content popularity and/or distance metrics. The caches can work in stand-alone mode as well as in co-operative mode. This paper shows that the network load can already be reduced considerably using small diskless caches, especially when using co-operative caching. A prototype implementation is detailed and evaluated through performance measurements. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Validation of the murine aortic arch as a model to study human vascular diseases

JOURNAL OF ANATOMY, Issue 5 2010
Christophe Casteleyn
Abstract Although the murine thoracic aorta and its main branches are widely studied to gain more insight into the pathogenesis of human vascular diseases, detailed anatomical data on the murine aorta are sparse. Moreover, comparative studies between mice and men focusing on the topography and geometry of the heart and aorta are lacking. As this hampers the validation of murine vascular models, the branching pattern of the murine thoracic aorta was examined in 30 vascular corrosion casts. On six casts the intrathoracic position of the heart was compared with that of six younger and six older men of whom contrast-enhanced computer tomography images of the thorax were three-dimensionally reconstructed. In addition, the geometry of the human thoracic aorta was compared with that of the mouse by reconstructing micro-computer tomography images of six murine casts. It was found that the right brachiocephalic trunk, left common carotid artery and left subclavian artery branched subsequently from the aortic arch in both mice and men. The geometry of the branches of the murine aortic arch was quite similar to that of men. In both species the initial segment of the aorta, comprising the ascending aorta, aortic arch and cranial/superior part of the descending aorta, was sigmoidally curved on a cranial/superior view. Although some analogy between the intrathoracic position of the murine and human heart was observed, the murine heart manifestly deviated more ventrally. The major conclusion of this study is that, in both mice and men, the ascending and descending aorta do not lie in a single vertical plane (non-planar aortic geometry). This contrasts clearly with most domestic mammals in which a planar aortic pattern is present. As the vascular branching pattern of the aortic arch is also similar in mice and men, the murine model seems valuable to study human vascular diseases. [source]


Morphology and ultrastructure of the malpighian tubules of the Chilean common tarantula (Araneae: Theraphosidae)

JOURNAL OF MORPHOLOGY, Issue 1 2002
S. Renee Hazelton
Abstract Relatively little is known about the morphology and ultrastructure of the Malpighian tubules of spiders (Arachnida: Araneae). Our study represents the first investigation of the Malpighian tubules of a theraphosid spider and is the only study to examine the living Malpighian tubules using confocal laser scanning microscopy. In theraphosid spiders, the Malpighian tubules originate from the stercoral pocket in the posterior portion of the opisthosoma and extend forward toward the prosoma in a dendritic pattern. There are three distinct segments (initial, main, and terminal), all dark brown in appearance. Each segment has distinctive ultrastructural features. Both the terminal and the main segment appear to be composed of at least two cell types with finger-like cytoplasmic protrusions associated with one of these types. The terminal segment, which is most proximal to the stercoral pocket, is the largest in diameter. It is composed of large, cuboidal cells containing many mitochondria and lipid inclusions. The main segment is intermediate in diameter with many mitochondria and secretory vesicles present. The initial segment is relatively thin in comparison to the other segments and is intimately associated with the digestive gland. The cells of the initial segment contain very little cytoplasm, fewer mitochondria, secretory vesicles, and prominent inclusions. J. Morphol. 251:73,82, 2002. © 2002 Wiley-Liss, Inc. [source]


Dependence of axon initial segment formation on Na+ channel expression

JOURNAL OF NEUROSCIENCE RESEARCH, Issue 4 2005
Xiaorong Xu
Abstract Spinal motor neurons were isolated from embryonic rats, and grown in culture. By 2 days in vitro, the axon initial segment was characterized by colocalization and clustering of Na+ channels and ankyrinG. By 5 days, NrCAM, and neurofascin could also be detected at most initial segments. We sought to determine, as one important aim, whether Na+ channels themselves played an essential role in establishing this specialized axonal region. Small hairpin RNAs (shRNAs) were used to target multiple subtypes of Na+ channels for reduced expression by RNA interference. Transfection resulted in substantial knockdown of these channels within the cell body and also as clusters at initial segments. Furthermore, Na+ currents originating at the initial segment, and recorded under patch clamp, were strongly reduced by shRNA. Control shRNA against a nonmammalian protein was without effect. Most interestingly, targeting Na+ channels also blocked clustering of ankyrinG, NrCAM, and neurofascin at the initial segment, although these proteins were seen in the soma. Thus, both Na+ channels and ankyrinG are required for formation of this essential axonal domain. Knockdown of Na+ channels was somewhat less effective when introduced after the initial segments had formed. Disruption of actin polymerization by cytochalasin D resulted in multiple initial segments, each with clusters of both Na+ channels and ankyrinG. The results indicate that initial segment formation occurs as Na+ channels are transported into the nascent axon membrane, diffuse distally, and link to the cytoskeleton by ankyrinG. Subsequently, other components are added, and stability is increased. A computational model closely reproduced the experimental results. © 2005 Wiley-Liss, Inc. [source]


Expanding the additive reduct of a model of Peano arithmetic

MLQ- MATHEMATICAL LOGIC QUARTERLY, Issue 4 2003
Masahiko Murakami
Abstract Let M be a model of first order Peano arithmetic (PA) and I an initial segment of M that is closed under multiplication. LetM0 be the {0, 1,+}-reduct ofM. We show that there is another model N of PA that is also an expansion of M0 such that a · Ma = a · Na if and only if a , I for all a , M. [source]


Novel epididymis-specific mRNAs downregulated by HE6/Gpr64 receptor gene disruption

MOLECULAR REPRODUCTION & DEVELOPMENT, Issue 5 2007
Ben Davies
Abstract Targeted disruption of the epididymis-specific HE6/Gpr64 receptor gene in mice led to male infertility. In order to characterize the phenotype at a molecular level, we compared the gene expression patterns of wild type (wt) versus knockout (KO) caput epididymides. The caput region of KO males, although morphologically normal, nevertheless showed an aberrant expression pattern. Combining micro array analysis, differential library screening, Northern blot analysis and quantitative RT-PCR, we found that the knockout of the HE6/Gpr64 receptor was mainly associated with the downregulation of genes specific to the initial segment. The list of KO downregulated transcripts comprised Enpp2/autotaxin, the lipocalins 8 and 9, the ,-defensin Defb42, cystatins 8 and 12, as well as the membrane proteins Adam (A Disintegrin And Metalloprotease) 28, claudin-10, EAAC1, and the novel Me9. Clusterin/ApoJ and osteopontin/Spp1 mRNAs, on the other hand, were upregulated in the KO tissues. The Me9 transcript was studied in further detail, and we report here a cluster of related epididymis-specific genes. Me9 is specifically expressed in the initial segment and is representative of a novel and highly conserved mammalian gene family. The family consists of single-exon genes only; intron-containing paralogs have not yet been ascertained. The cloned cDNA sequences predicted hydrophobic polytopic membrane proteins containing the DUF716 motif. Protein expression was shown in the rodent caput epididymidis but remained uncertain in primates. Mol. Reprod. Dev. 74: 539,553, 2007. © 2006 Wiley-Liss, Inc. [source]


A modelling study of locomotion-induced hyperpolarization of voltage threshold in cat lumbar motoneurones

THE JOURNAL OF PHYSIOLOGY, Issue 2 2002
Yue Dai
During fictive locomotion the excitability of adult cat lumbar motoneurones is increased by a reduction (a mean hyperpolarization of ,6.0 mV) of voltage threshold (Vth) for action potential (AP) initiation that is accompanied by only small changes in AP height and width. Further examination of the experimental data in the present study confirms that Vth lowering is present to a similar degree in both the hyperpolarized and depolarized portions of the locomotor step cycle. This indicates that Vth reduction is a modulation of motoneurone membrane currents throughout the locomotor state rather than being related to the phasic synaptic input within the locomotor cycle. Potential ionic mechanisms of this locomotor-state-dependent increase in excitability were examined using three five-compartment models of the motoneurone innervating slow, fast fatigue resistant and fast fatigable muscle fibres. Passive and active membrane conductances were set to produce input resistance, rheobase, afterhyperpolarization (AHP) and membrane time constant values similar to those measured in adult cat motoneurones in non-locomoting conditions. The parameters of 10 membrane conductances were then individually altered in an attempt to replicate the hyperpolarization of Vth that occurs in decerebrate cats during fictive locomotion. The goal was to find conductance changes that could produce a greater than 3 mV hyperpolarization of Vth with only small changes in AP height (< 3 mV) and width (< 1.2 ms). Vth reduction without large changes in AP shape could be produced either by increasing fast sodium current or by reducing delayed rectifier potassium current. The most effective Vth reductions were achieved by either increasing the conductance of fast sodium channels or by hyperpolarizing the voltage dependency of their activation. These changes were particularly effective when localized to the initial segment. Reducing the conductance of delayed rectifier channels or depolarizing their activation produced similar but smaller changes in Vth. Changes in current underlying the AHP, the persistent Na+ current, three Ca2+ currents, the ,h' mixed cation current, the ,A' potassium current and the leak current were either ineffective in reducing Vth or also produced gross changes in the AP. It is suggested that the increased excitability of motoneurones during locomotion could be readily accomplished by hyperpolarizing the voltage dependency of fast sodium channels in the axon hillock by a hitherto unknown neuromodulatory action. [source]


Heat opens axon initial segment sodium channels: A febrile seizure mechanism?,

ANNALS OF NEUROLOGY, Issue 2 2009
Evan A. Thomas PhD
Objective A number of hypotheses have been put forward as to why humans respond to fever by seizing. The current leading hypotheses are that respiratory alkalosis produces an as yet unidentified change in neural excitability or that inflammatory mediators potentiate excitatory synaptic transmission. However, it is well known that ion channel gating rates increase with increased temperature. Furthermore, skeletal and cardiac sodium channel activation can be temperature sensitive in some situations. We measured the temperature sensitivity of the brain sodium channel, NaV1.2, to determine whether febrile temperatures might produce a direct increase in neuronal excitability. Methods The effect of temperature on NaV1.2 electrophysiological properties was measured in a transfected mammalian cell line. The subcellular location of NaV1.2 in the mouse brain was ascertained using antibodies against NaV1.2 and ankyrin-G. Computer simulation of a hippocampal granule cell model was used to predict the effect of temperature on action potential firing. Results As well as the expected increase in gating rates, the voltage dependence of activation became 7.6mV more negative when the temperature was increased from 37°C to 41°C. NaV1.2 was localized to the axon initial segment in hippocampal and cortical neurons. Computer simulation showed that increased gating rates and the more negative activation dramatically increase neuronal excitability. Interpretation The direct effect of heat on ion channels localized to the site of action potential initiation potentially causes a profound increase in neuronal excitability. This is likely to contribute to febrile seizure genesis. Ann Neurol 2009;66:219,226 [source]


Activity-dependent formation and functions of chondroitin sulfate-rich extracellular matrix of perineuronal nets

DEVELOPMENTAL NEUROBIOLOGY, Issue 5 2007
Alexander Dityatev
Abstract Extracellular matrix molecules,including chondroitin sulfate proteoglycans, hyaluronan, and tenascin-R,are enriched in perineuronal nets (PNs) associated with subsets of neurons in the brain and spinal cord. In the present study, we show that similar cell type-dependent extracellular matrix aggregates are formed in dissociated cell cultures prepared from early postnatal mouse hippocampus. Starting from the 5th day in culture, accumulations of lattice-like extracellular structures labeled with Wisteria floribunda agglutinin were detected at the cell surface of parvalbumin-expressing interneurons, which developed after 2,3 weeks into conspicuous PNs localized around synaptic contacts at somata and proximal dendrites, as well as around axon initial segments. Physiological recording and intracellular labeling of PN-expressing neurons revealed that these are large fast-spiking interneurons with morphological characteristics of basket cells. To study mechanisms of activity-dependent formation of PNs, we performed pharmacological analysis and found that blockade of action potentials, transmitter release, Ca2+ permeable AMPA subtype of glutamate receptors or L-type Ca2+ voltage-gated channels strongly decreased the extracellular accumulation of PN components in cultured neurons. Thus, we suggest that Ca2+ influx via AMPA receptors and L-type channels is necessary for activity-dependent formation of PNs. To study functions of chondroitin sulfate-rich PNs, we treated cultures with chondroitinase ABC that resulted in a prominent reduction of several major PN components. Removal of PNs did not affect the number and distribution of perisomatic GABAergic contacts but increased the excitability of interneurons in cultures, implicating the extracellular matrix of PNs in regulation of interneuronal activity. © 2007 Wiley Periodicals, Inc. Develop Neurobiol, 2007 [source]


Convergence of excitatory and inhibitory inputs onto CCK-containing basket cells in the CA1 area of the rat hippocampus

EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 5 2004
Ferenc Mátyás
Abstract The number and distribution of excitatory and inhibitory inputs affect the integrative properties of neurons. These parameters have been studied recently for several hippocampal neuron populations. Besides parvalbumin- (PV) containing cells that include basket and axo-axonic cells, cholecystokinin (CCK)-containing interneurons also form a basket cell population with several properties distinct from PV cells. Here, at the light microscopic level, we reconstructed the entire dendritic tree of CCK-immunoreactive (IR) basket cells to describe their geometry, the total length and laminar distribution of their dendrites. This was followed by an electron microscopic analysis of serial ultrathin sections immunostained against ,-aminobutyric acid, to estimate the density of excitatory and inhibitory synapses on their somata, axon initial segments and different subclasses of dendrites. The dendritic tree of CCK-IR basket cells has an average length of 6300 µm and penetrates all layers. At the electron microscopic level, CCK basket cells receive dendritic inputs with a density of 80,230 per 100 µm. The ratio of inhibitory inputs is relatively high (35%) and increases towards the soma (83%). The total numbers of excitatory and inhibitory synapses converging onto CCK-IR cells are ,,8200. Comparison of the two, neurochemically distinct basket cells reveals that CCK-containing basket cells receive much less synaptic input than PV cells; however, the relative weight of inhibition is higher on CCK cells. Additional differences in their anatomical and physiological properties predict that CCK basket cells are under a more diverse, elaborate control than PV basket cells, and thus the function of the two populations must be different. [source]


Dependence of axon initial segment formation on Na+ channel expression

JOURNAL OF NEUROSCIENCE RESEARCH, Issue 4 2005
Xiaorong Xu
Abstract Spinal motor neurons were isolated from embryonic rats, and grown in culture. By 2 days in vitro, the axon initial segment was characterized by colocalization and clustering of Na+ channels and ankyrinG. By 5 days, NrCAM, and neurofascin could also be detected at most initial segments. We sought to determine, as one important aim, whether Na+ channels themselves played an essential role in establishing this specialized axonal region. Small hairpin RNAs (shRNAs) were used to target multiple subtypes of Na+ channels for reduced expression by RNA interference. Transfection resulted in substantial knockdown of these channels within the cell body and also as clusters at initial segments. Furthermore, Na+ currents originating at the initial segment, and recorded under patch clamp, were strongly reduced by shRNA. Control shRNA against a nonmammalian protein was without effect. Most interestingly, targeting Na+ channels also blocked clustering of ankyrinG, NrCAM, and neurofascin at the initial segment, although these proteins were seen in the soma. Thus, both Na+ channels and ankyrinG are required for formation of this essential axonal domain. Knockdown of Na+ channels was somewhat less effective when introduced after the initial segments had formed. Disruption of actin polymerization by cytochalasin D resulted in multiple initial segments, each with clusters of both Na+ channels and ankyrinG. The results indicate that initial segment formation occurs as Na+ channels are transported into the nascent axon membrane, diffuse distally, and link to the cytoskeleton by ankyrinG. Subsequently, other components are added, and stability is increased. A computational model closely reproduced the experimental results. © 2005 Wiley-Liss, Inc. [source]


Arithmetic of divisibility in finite models

MLQ- MATHEMATICAL LOGIC QUARTERLY, Issue 2 2004
Marcin Mostowski
Abstract We prove that the finite-model version of arithmetic with the divisibility relation is undecidable (more precisely, it has ,01 -complete set of theorems). Additionally we prove FM-representability theorem for this class of finite models. This means that a relation R on natural numbers can be described correctly on each input on almost all finite divisibility models if and only if R is of degree ,0,. We obtain these results by interpreting addition and multiplication on initial segments of finite models with divisibility only. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Differential dye coupling reveals lateral giant escape circuit in crayfish

THE JOURNAL OF COMPARATIVE NEUROLOGY, Issue 1 2003
Brian L. Antonsen
Abstract The lateral giant (LG) escape circuit of crayfish mediates a coordinated escape triggered by strong attack to the abdomen. The LG circuit is one of the best understood of small systems, but models of the circuit have mostly been limited to simple ball-and-stick representations, which ignore anatomical details of contacts between circuit elements. Many of the these contacts are electrical; here we use differential dye coupling, a technique which could help reveal connection patterns in many neural circuits, to reveal in detail the circuit within the terminal abdominal ganglion. Sensory input from the tailfan forms a somatotopic map on the projecting LG dendrites, which together with interafferent coupling mediates a lateral excitatory network that selectively amplifies strong, phasic, converging input to LG. Mechanosensory interneurons contact LG at sites distinct from the primary afferents and so maximize their summated effect on LG. Motor neurons and premotor interneurons are excited near the initial segments of the LGs and innervate muscles for generating uropod flaring and telson flexion. Previous research has shown that spatial patterns of input are important for signal integration in LG; this map of electrical contact points will help us to understand synaptic processing in this system. J. Comp. Neurol. 466:1,13, 2003. © 2003 Wiley-Liss, Inc. [source]


Differential targeting and functional specialization of sodium channels in cultured cerebellar granule cells

THE JOURNAL OF PHYSIOLOGY, Issue 3 2005
Nancy Osorio
The ion channel dynamics that underlie the complex firing patterns of cerebellar granule (CG) cells are still largely unknown. Here, we have characterized the subcellular localization and functional properties of Na+ channels that regulate the excitability of CG cells in culture. As evidenced by RT-PCR and immunocytochemical analysis, morphologically differentiated CG cells expressed Nav1.2 and Nav1.6, though both subunits appeared to be differentially regulated. Nav1.2 was localized at most axon initial segments (AIS) of CG cells from 8 days in vitro DIV 8 to DIV 15. At DIV 8, Nav1.6 was found uniformly throughout somata, dendrites and axons with occasional clustering in a subset of AIS. Accumulation of Nav1.6 at most AIS was evident by DIV 13,14, suggesting it is developmentally regulated at AIS. The specific contribution of these differentially distributed Na+ channels has been assessed using a combination of methods that allowed discrimination between functionally compartmentalized Na+ currents. In agreement with immunolocalization, we found that fast activating,fully inactivating Na+ currents predominate at the AIS membrane and in the somatic plasma membrane. [source]