Discrete Packets Research Articles

Assembly of presynaptic active zones from cytoplasmic transport packets.

Little is known about presynaptic assembly during central nervous system synaptogenesis. Here we used time-lapse fluorescence imaging, immunocytochemistry and electron microscopy to study hippocampal neuronal cultures transfected with a fusion construct of the presynaptic vesicle protein VAMP and green fluorescent protein. Our results suggest that major cytoplasmic and membrane-associated protein precursors of the presynaptic active zone are transported along developing axons together as discrete packets. Retrospective electron microscopy demonstrated varied vesicular and tubulovesicular membrane structures. Packets containing these heterogeneous structures were stabilized specifically at new sites of dendrite- and axon-initiated cell-cell contact; within less than one hour, evoked vesicle recycling was observed at these putative nascent synapses. These observations suggest that substantial membrane remodeling may be necessary to produce the uniform vesicles typical of the mature active zone, and that many presynaptic proteins may be united early in their biogenesis and sorting pathways.

Morphology of spermathecae in the estuarine crab Chasmagnathus granulata Dana 1851 (Grapsidae, Sesarminae)

The morphology of the spermathecae (seminal receptacles) was studied in the grapsid crab Chasmagnathus granulata, both from the macroscopic and the microscopic points of view. Female adult crabs were collected throughout the year. Once in the laboratory, the right spermatheca was fixed in Bouin solution; histological sections were stained with haematoxilin–eosin. The observations indicated that the spermathecae belonged to the ‘ventral type’ described by Diesel (1991), related to the ‘hard female’ mating system of C. granulata described previously. They presented two distinct zones: the inner one (white-coloured), where the double layer oviduct opened ventrally; and the outer one, of translucent, ‘jelly-like’ appearance, where the communication with the genital duct could be observed. The spermatozoa were only observed in the ‘jelly-like’ zone, but not as discrete packets, suggesting that spermatozoa from different matings could mix in the spermathecae of C. granulata. The spermathecae also showed seasonal variation, with the lowest degree of development observed during autumn. Post-moulted females always showed collapsed spermathecae.

Wavelet analysis for geophysical applications

Wavelet transforms originated in geophysics in the early 1980s for the analysis of seismic signals. Since then, significant mathematical advances in wavelet theory have enabled a suite of applications in diverse fields. In geophysics the power of wavelets for analysis of nonstationary processes that contain multiscale features, detection of singularities, analysis of transient phenomena, fractal and multifractal processes, and signal compression is now being exploited for the study of several processes including space‐time precipitation, remotely sensed hydrologic fluxes, atmospheric turbulence, canopy cover, laud surface topography, seafloor bathymetry, and ocean wind waves. It is anticipated that in the near future, significant further advances in understanding and modeling geophysical processes will result from the use of wavelet analysis. In this paper we review the basic properties of wavelets that make them such an attractive and powerful tool for geophysical applications. We discuss continuous, discrete, orthogonal wavelets and wavelet packets and present applications to geophysical processes.

Radar observations of gravity waves over Jicamarca, Peru, during the CADRE campaign

We present results obtained with the mesosphere‐stratosphere‐troposphere (MST) radar at Jicamarca, Peru, from three 10‐day experiments in January 1993, March 1994, and August 1994. Horizontal and vertical velocities were measured over ranges spanning the lower part of the stratosphere and most of the mesosphere. In the stratosphere, the fluctuating part of the wind field was found to be dominated by inertia‐gravity waves. Sinusoids of different period were fit to the velocity time series using a least squares procedure. The dominant periods of the inertia‐gravity wave motions were found to be 1.5 days for the January 1993 experiment and 2.1 days for the August 1994 experiment. For the January 1993 experiment, the amplitudes and phases of the inertia‐gravity wave oscillations were extracted for the vertical as well as the horizontal components. The vertical amplitude of the 1.5‐day period wave was small (<0.1 ms−1), but measurable with the Jicamarca radar. The intrinsic periods of the inertia‐gravity waves were inferred from the fits using two different methods. The first method predicted the intrinsic period using the orbital ellipses traced out in time by fits to the horizontal winds. The second method used information taken from the vertical as well as the horizontal fits. The values of intrinsic period made using the second method were found to have much less scatter than the values inferred solely from the orbital ellipses. The momentum flux estimates in both the stratosphere and mesosphere were found to depend significantly on the exact methodology used. A technique was adopted whereby estimates were formed only when radial velocities were measured simultaneously on both beams of a coplanar beam pair. Most of the total wave stress was usually contributed by waves at periods ≥4 hours in the stratosphere and ≥1 hour in the mesosphere. In the stratosphere, localized layers of enhanced momentum flux were sometimes observed. The obvious anticorrelation between the shear of the mean wind and the momentum flux in these layers suggests that they were caused by in situ generation of waves by the Kelvin‐Helmholtz instability, rather than gravity waves propagating from lower levels. At short periods, momentum flux spectra in the stratosphere and mesosphere showed numerous positive and negative peaks. A correlation analysis revealed that the high‐frequency peaks were associated with discrete wave packets. The short‐scale waves associated with these packets were fairly isotropic in their direction of propagation, and due to cancellation they contributed little net momentum.

Interductile energy transfer of high-frequency pulse propagation in a two-ducted ocean waveguide in the yellow sea

In a multiducted ocean wave guide, energy may be trapped in each of the ducted regions. For sources suitably placed, energy may be stimulated initially in only one of the ducts when eigenfunctions associated with secondary ducts have negligible overlap with the primary duct. For fixed frequency, no energy is transferred, and although high-angle modes eventually traverse to the secondary ducted region, the energy is not trapped there for monochromatic sources. In reality one must take into consideration a spread of frequencies. Monjo and DeFerarri have noted this effect in collecting and interpreting data. This has led to a very exciting phenomenon, the ‘‘Monjo–DeFerarri effect,’’ which is quite dramatic for the case they studied. The Monjo–DeFerarri effect is a mechanism whereby energy in a lower duct, with the source in the lower duct, may in time transfer energy to an upper ducted region. The energy transport is cyclical, and at suitably high frequencies energy is trapped in the upper channel in discrete time segments. This leads to the arrival of discrete packets of energy in time which are highly localized spatially. These ‘‘precursors’’ arrive in advance of predominant pulse arrivals, and may determine, among other things, the source distance. Here, a study of interductile energy transfer is made for the Yellow Sea. In suitable cases, discrete packets of energy are captured at each high-angle modal contact with the secondary duct. Expressions that predict the arrivals of the upper ducted discrete arrivals, the number in range, the lag time between arrivals, and the lowest frequencies above which this will happen, are presented. [Work sponsored by ONR, NRL, and the University of New Orleans.]

‘SAMP’ model for water and solute movement in unsaturated porous media involving thermodynamic subsystems and moving packets: l. Theory

SAMP (subsystems and moving packets) is a new type of model for simulating non-steady movement of water in unsaturated porous media under equilibrium and non-equilibrium conditions, and the associated movement of a non-sorbing solute. It can be used in many types of simulations, including: solving Richards' equation; advection and dispersion of solute in a medium containing dead-space; preferential water and solute movement in a medium containing macropores and determining the fate of ‘new’ and ‘old’ water and solute when the new enters a region containing the old. The model is approximate in that the modelled region is divided into cells, and discrete packets of water (which may contain solute) are moved within and between these cells, and stochastic in that each move is determined by probability calculations and the use of random numbers. The pore space within each cell is divided into 100 or more distinct independent thermodynamic subsystems, and the internal moisture state of a cell is described by a binary number: bit i of this number is 1 when subsystem i is saturated (i.e. contains a packet) and 0 when it is dry. The basic parameters of the model are those for the matric potential and unsaturated hydraulic conductivity (equilibrium) functions, as used with Richards' equation. Property data for each subsystem are derived from the equilibrium functions and these data are assumed to apply even under non-equilibrium conditions. The only further parameter is the internal scale, λ The necessary new theory is developed here, and the design of a 1-dimensional SAMP model is described in Ewen (1996. J. Hydrol., 182: 195–207), where typical results are also given. These results demonstrate the role of λ in controlling the efficiency with which old water is displaced by new water, and the degree to which there is non-equilibrium movement.

Kinematics of Underthrusting in the Paleozoic Antler Foreland Basin

Field mapping and structural analysis of Paleozoic rocks in the northern Roberts Mountains in central Nevada reveal that fine-grained siliciclastic strata deposited early in the history of the Antler foreland basin underthrust and underplated as two discrete fault-bound packets to the Roberts Mountains allochthon as it encroached upon the North American continental shelf. Underplating also resulted in the intercalation of North American outer shelf/ slope strata between the two foreland basin packets as the basal detachment of the Roberts Mountains allochthon stepped downward in the sedimentary section. Deformation of foreland basin and outer shelf/slope strata was in response to ESE in-layer shortening accompanied by top ESE layer-parallel shear within 10 m of fault surfaces along which underplating and underthrusting occurred. Stratal disruption, fold tightness and asymmetry, and cleavage development near the lower and upper-bounding faults are greater than in the center of underthrust packages, indicat...

Performance of modulation-doped charge-coupled devices (MD-CCD's) in the microwave and millimeter-wave bands

A two-phase, modulation-doped charge coupled device (MD-CCD) has been characterized by both phase shift and charge transfer efficiency (CTE) measurements from 1.25 MHz to 16.4 GHz. Both two-dimensional transient simulations and experimental evidence support the conclusion that the cutoff frequency for transport of discrete charge packets emulates the cutoff frequency of small signal HEMT devices in the short gate length regime. These simulations predict a CTE of almost 0.999 at 40 GHz for an In/sub 0.53/Ga/sub 0.47/As channel device. The device is fabricated using conventional MMIC processing techniques. In addition, measurement methods used for characterization of a prototype 5-stage delay line chip agree well with simulations using a new CCD SPICE model. >

A model of the role of adaptation and disadaptation in olfactory receptor neurons: implications for the coding of temporal and intensity patterns in odor signals.

Natural odors occur as turbulent plumes resulting in spatially and temporally variable odor signals at the chemoreceptor cells. Concentrations can fluctuate widely within discrete packets of odor and individual packets are very intermittent and unpredictable. Chemoreceptor cells display the temporally dynamic properties of adaptation and disadaptation, which serve to alter their responses to these fluctuating odor patterns. A computational model, modified from one previously published, was used to investigate the effect of adaptation and recovery of adaptation (disadaptation) on the spike output of model olfactory receptor cells under natural stimulus conditions. The response characteristics of model cells were based upon empirically determined dose-response, adaptation, disadaptation and flicker fusion properties of peripheral olfactory cells. The physiological properties of the model cell (adaptation and disadaptation rate and the dose-response relationship) could be modified independently, allowing assessment of the role of each in shaping the responses of the model cell. Complete adaptation and disadaptation time courses ranged from 500 ms (rapid cells) to 10 s (slow cells). The stimuli for the model cells were quantified odor plume recordings obtained under a variety of biologically relevant flow conditions. As expected, the rapidly adapting model cells displayed different response characteristics than the slowly adapting model cells to identical temporal odor profiles. Responses of the model cells depended upon their adaptation and disadaptation rates, and the frequency characteristics of the odor presentation. These results indicate that adaptation and disadaptation determine the range of concentration fluctuations over which a particular cell will respond. Thus, these properties function as an olfactory equivalent of a band-pass filter in electronics. This type of filtering has implications for the extraction of information from odor signals, such as the coding of temporal and intensity features.

Discrete wave packets at the proton cyclotron frequency at comet P/Halley

We present in this letter the first experimental evidence for wave packets near the local proton cyclotron frequency in the plasma environment of a comet. The observations have been made by the GIOTTO magnetometer experiment (MAG) around comet P/Halley on both sides of closest approach. The waves are always left‐handed in the spacecraft frame, elliptically or nearly circularly polarized and they propagate at small angles from the ambient magnetic field. Their period in the spacecraft frame always closely fits the local proton cyclotron period. The possible generation mechanisms of these waves are discussed. They can be consistently interpreted as waves generated by a resonant helical beam instability fed by the cometary pick‐up protons. These waves are intrinsically right‐handed waves in the plasma rest frame and are anomalously Doppler‐shifted from the plasma frame to the spacecraft frame because their phase velocity is small compared to the local solar wind speed. A generation by a resonance with heavy (water group) ions introducing a temperature effect is also discussed but is less satisfactory.

Multistation Observations of Pc1-2 ULF Pulsations in the Vicinity of the Polar Cusp.

Geomagnetic pulsations with frequencies in the range 0.1-2.5 Hz have been recorded with a six station Antarctic magnetometer array ranging in latitude from -62.3 to -80.6° invariant. The observed spectral characteristics were compared with the cusp and boundary layer locations for selected days in November and December, 1986, and July, 1990, in order to determine wave source regions. Specific pulsation features have been identified with different magnetospheric regions. The cusp is characterised by intense unstructured 0.15-0.45 Hz noise which diminishes rapidly away from this region. These signals are likely to result from a mixture of ion-cyclotron, Kelvin-Helmholtz and drift-wave instabilities. Narrowband unstructured Pc2 emissions typically around 0.2 Hz and short discrete Pc1 packets or bursts with frequencies below 0.4 Hz, occur in the local morning at boundary layer latitudes, accompanied by generally enhanced power levels between about 0.1 and 0.5 Hz. These emissions are probably due to ion-cyclotron waves generated near or below the He+ gyrofrequency on boundary layer field lines. Propagation of such signals into the polar cap via the ionospheric waveguide is uncommon, perhaps due to the existence of severe ionospheric irregularity features near the dayside auroral oval. Structured and quasi-structured emissions such as hydromagnetic chorus are recorded at the foot of plasmatrough field lines and are consistent with ion-cyclotron wave generation and propagation on closed field lines in the outer magnetosphere.

Wave phenomena in the upstream region of Saturn

The magnetic field data returned from the Voyager 1 and 2 transit through the upstream region of Saturn have been examined from 0.1 mHz to 0.25 Hz to search for the signatures of waves associated with both the electron and ion foreshocks. In contrast to the Earth, in the ion foreshock of Saturn we find two distinct bands of wave activity at frequencies around 0.5 and 2‐mHz in the spacecraft frame. These two waves differ considerably in their properties. The 0.5‐mHz waves are right handed, strongly elliptically polarized, while the 2‐mHz waves are elliptically or circularly polarized in the left‐handed sense. These two different waves may be associated with two different backstreaming particle distributions, possibly those reflected from the shock and those leaking from the hot magnetosheath. We also observe right‐hand elliptically polarized upstream waves propagating obliquely to the magnetic field at frequencies between 90 and 150 mHz, which are above the proton gyrofrequency in the spacecraft frame. The frequency of these waves appears to be dependent on the interplanetary magnetic field orientation. Such dependence is characteristic of the 1‐Hz waves at Earth. Discrete wave packets associated with shocklets are for the first time observed in the foreshock of an outer planet.

Phyllosilicates in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 ? a TEM and AEM study

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or ohvine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from< 100 A to a few hundred A. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystal-linity of saponite. by contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of A thick. The Si/(Si+Al) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe+Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+Al) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite±mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.

A study of ULF wave foreshock morphology—II: spatial variation of ULF waves

Two case studies are used to show the evolution of upstream ULF waves in the Earth's foreshock region. During these two intervals the IMF cone angle was nearly constant over an extended time period and the ISEE spacecraft was in the foreshock region and traveled a large distance through it. Since the solar wind conditions were nearly constant, the change of wave properties in these cases is primarily related to the geometry in the foreshock, e.g. the different depth from the foreshock boundary and different distance from the bow shock. The observations from ISEE magnetic field data show that ULF waves become stronger, more compressional, and more linearly polarized as the spacecraft travels further downstream from the ULF foreshock boundary and closer to the bow shock. The peak of the ULF wave power spectrum becomes broader. Associated with the increasing intensity of the lower frequency waves, or shocklets, the discrete wave packets become more intense and develop more cycles as the spacecraft gets deeper into the foreshock and closer to the bow shock. We can also estimate the growth rate of discrete wave packets for one of the cases.

Cytokines and local factors which affect osteoclast function.

Bone remodeling is a local phenomenon which occurs in discrete packets throughout the skeleton. The cellular events which comprise the remodeling sequence are controlled by cytokines which are generated in the microenvironment of the bone resorbing pockets. These cytokines are derived from marrow mononuclear cells or from bone cells themselves, or they are incorporated into the bone matrix and released in biologically active form as bone resorbs. Evidence is accumulating that some of these cytokines play an important role not just in physiological bone remodeling, but also in common diseases of bone remodeling such as osteoporosis, osteopetrosis, Paget's disease, and malignant diseases which involve bone and chronic inflammatory diseases such as rheumatoid arthritis and periodontal disease. Normal bone remodeling is clearly under local control. It occurs in discrete packets throughout the skeleton, each of which is geographically distinct. Local packets of bone remodeling are also asynchronous with respect to each other. The cellular events which comprise the remodeling sequence are thus regulated primarily by factors which are enriched in that microenvironment. The remodeling sequence, which is continuous, is the same on cancellous bone surfaces as it is within the Haversian systems of cortical bone. Since it is now known that powerful osteoclastotropic factors are produced in the microenvironment of these bone remodelling packets, these local factors or cytokines are the most likely major regulators of osteoclast function.

Inflammatory mediators and the destruction of bone

Bone is remodelled by the coordinated actions of osteoclasts and osteoblasts. Cellular remodelling occurs in discrete packets of bone, and is regulated by local cytokines produced in the environment of the remodelling cells. These cytokines are secreted by immune cells and by bone cells. In addition, some growth regulatory factors are incorporated into the noncollagenous bone matrix and are released in an active form when bone is stimulated to resorb. Complex interactions between these cytokines and their target cells are responsible for the normal delicate balance between bone resorption and bone formation, and disorders of bone loss are due to imbalances between the rates of resorption and formation.

Cellular and Molecular Biology of the Presynaptic Nerve Terminal

Chemical neurotransmission represents the primary form of intercellular communication in the nervous system, yet relatively little is known about the molecular processes involved. The vesicle hypothesis states that spe­ cialized organelles located in the synaptic region are responsible for the accumulation, storage, and release of neurotransmitters in discrete packets called "quanta." These synaptic vesicles, or their components, are thought to be assembled in the cell body and transported to the terminals via fast axonal transport. At the nerve terminal, vesicles interact with the cytoskeleton and with soluble vesicle-binding proteins prior to docking at specialized membrane sites known as active zones. Following voltage­ gated Ca 2+ influx, when the local intracellular Ca 2+ concentrations ([Ca 2+]i) may reach several hundred micromolar, the vesicle and the plasma membrane fuse, releasing the vesicle contents into the synaptic cleft. A typical synaptic vesicle from a motoneuron releases approximately 5000 molecules of acetylcholine, and each nerve impulse releases 100-200 quanta at a representative neuromuscular synapse. Following exocytosis,

Nicotinic receptor-mediated catecholamine secretion from individual chromaffin cells. Chemical evidence for exocytosis.

Nicotinic receptor-mediated secretion of catecholamines from individual cultured bovine adrenal medullary chromaffin cells was measured and characterized with a voltametric microelectrode placed adjacent to the cells. Nicotine-induced secretion is associated with a large increase in chemical spikes that is temporally resolved into the apparent secretion of discrete packets of attomole quantities of easily oxidized molecules. These data are consistent with direct chemical measurement of single exocytotic events.

Matrix mineralogy of the Lancé CO3 carbonaceous chondrite: A transmission electron microscope study

The Lancé CO3 carbonaceous chondrite (CC) is less altered than the CI and CM chondrites and so provides a view of the mineralogy and textures resulting from the earliest stages of aqueous alteration of CCs. Matrix olivine in Lancé has been partly altered to fine-grained, Fe-bearing serpentine and poorly crystalline Fe 3+ oxide, a process that required both hydration and oxidation. Serpentine occurs as discrete packets separated from the olivine surfaces by the Fe 3+ oxide. The Fe released during the dissolution of olivine was partly incorporated into the serpentine; the remainder was oxidized to form Fe 3+ oxide. Matrix metal was also altered to produce Fe oxides, leaving the residual metal enriched in Ni. Olivine grains in Lancé matrix contain channels along their [100]and [001]directions. The formation and convergence of such channels resulted in a grain-size reduction of the olivine. The alteration was pervasive but incomplete, suggesting a limited availability of fluid. A brief study of two other CO chondrites, Kainsaz and Warrenton, shows that these meteorites do not contain phyllosilicates in their matrices, although both contain Fe 3+ oxide between olivine grains. Prior to its alteration, Lancé probably resembled Kainsaz, an unaltered CO3 chondrite. The alteration assemblage in Lancé is only slightly different from that in Mokoia and essentially the same as that in C3 xenoliths from Murchison. Alteration products in Lancé show greater similarities to CI than to CM chondrites.

An Information-Based Model of Market Volatility

A model for volatility, based on the relation between volatility and information flows, leads to the specification of a stochastic process for volatility. This allows one to compute potentially useful properties, such as the probability that volatility will change from one level to another within a specific time period. The model relies on three characteristics of information. (1) Information arrives in discrete packets, and the probability of its arrival is a function of time. (2) Different pieces of information have different degrees of impact on the market, hence on the market's volatility. (3) It takes time for the market to digest information; the greater the impact of the information, the longer its effect on volatility will last. Comparison of actual daily volatility for the Treasury bond yield with the volatility given by the information-based model shows clear similarities between the frequencies of variability, the ranges of volatility, the speeds with which spikes dampen and the longer-term waviness of the processes. The model also did well in explaining equity market volatility following the 1987 crash. The model suggests that volatility is mean-reverting. The tendency is for above-average volatility to decline and for below-average volatility to increase. The nature of the information flows underlying the model suggests that volatility can be expected to be reasonably stable over long time periods; the mean level of volatility may not change dramatically, even over five or 10 years.