Microm Radius Research Articles

Local lateral connectivity of inhibitory clutch cells in layer 4 of cat visual cortex (area 17).

To characterise spatially a major component of the anatomical basis of local lateral inhibition in layer 4 of cat visual cortex (area 17), we analysed the lateral distribution of neuronal somata postsynaptic to electrophysiologically characterised GABAergic clutch (basket) cell axons (CC1 and CC2). We report two main results. First, the clutch cell axons appear to show isotropic lateral connectivity near their cell body (less than 50 microm radius), but beyond this core region they show anisotropic lateral connectivity, preferring particular angular sectors around their cell body. Second, we estimated the probability of lateral connection for each axon arbor as a function of radial distance from the parent soma. We found that this radial function has a brief rising phase, to a peak at 30-45 microm, and a longer, exponential decaying phase, with a space constant of around 50 microm. The shape of the radial connection probability function suggests that most lateral inhibitory connections of clutch cells are formed with neurons in nearest-neighbour cortical columns. Taken together, the results suggest that these individual layer-4 clutch cell axons may inhibit all (isotropic) nearest-neighbour cortical columns with a relatively high probability of connection, but outside this core region may provide a type of anisotropic lateral inhibition of cortical columns with a radially decreasing probability of connection.

Noninvasive picoliter volume thermometry based on backscatter interferometry.

Using the on-chip refractive index (RI) detector based on backscatter interferometry, sensitive, small volume, noninvasive thermometry can be performed. The current optical configuration for the on-chip interferometric backscatter detector (OCIBD) is quite simple and consists of an unfocused laser, an unaltered chip with a hemispherical channel and a photodetector. Alignment is straightforward with the only requirement being that the beam fully fills the channel. The interaction of an unfocused laser beam with the uncoated etched channel with a curvature within the silica plate (chip) produces fringes whose positional changes scale with respect to the refractive index (RI), n, of the fluid in the channel. Due to the inherently high value of dn/dT for most fluids and the high sensitivity of OCIBD to RI changes, the measurement of small temperature variations in sub-nanoliter volumes is possible. Performing OCIBD with a 75 microm diameter laser beam on a silica chip that contains an etched channel with a 40 microm radius facilitates noninvasive thermometry on a N-(2-hydroxyethyl)piperazine-(2-ethanesulfonic acid) (HEPES) solution in a 188 x 10(-12) L probe volume with a temperature resolution of 9.9 x 10(-4) degrees C, at the 99% confidence level.

Finite element analysis of the current-density and electric field generated by metal microelectrodes.

Electrical stimulation via implanted microelectrodes permits excitation of small, highly localized populations of neurons, and allows access to features of neuronal organization that are not accessible with larger electrodes implanted on the surface of the brain or spinal cord. As a result there are a wide range of potential applications for the use of microelectrodes in neural engineering. However, little is known about the current-density and electric field generated by microelectrodes. The objectives of this project were to answer three fundamental questions regarding electrical stimulation with metal microelectrodes using geometrically and electrically accurate finite elements models. First, what is the spatial distribution of the current density over the surface of the electrode? Second, how do alterations in the electrode geometry effect neural excitation? Third, under what conditions can an electrode of finite size be modeled as a point source? Analysis of the models showed that the current density was concentrated at the tip of the microelectrode and at the electrode-insulation interface. Changing the surface area of the electrode, radius of curvature of the electrode tip, or applying a resistive coating to the electrode surface altered the current-density distribution on the surface of the electrode. Changes in the electrode geometry had little effect on neural excitation patterns, and modeling the electric field generated by sharply tipped microelectrodes using a theoretical point source was valid for distances > approximately 50 microm from the electrode tip. The results of this study suggest that a nearly uniform current-density distribution along the surface of the electrode can be achieved using a relatively large surface area electrode (500-1000 microm2), with a relatively blunt tip (3-6 microm radius of curvature), in combination with a thin (approximately 1 microm) moderately resistive coating (approximately 50 omega m).

Oxidation-State Speciation of [ReI(DMPE)3]+/[ReII(DMPE)3]2+ by Voltammetry with a Chemically Modified Microelectrode

The analytical utility of chemically modified microelectrodes for oxidation-state speciation of redox couples by cyclic voltammetry has been explored. [Re(I)(DMPE)3]+/[Re(II)(DMPE)3]2+, where DMPE = 1,2-bis(dimethylphosphino)ethane, was studied at carbon-fiber microelectrodes of approximately 5 microm in radius coated with Nafion-entrapped solgel-derived silica (Nafion-silica) composite. The results are compared with cyclic voltammetry of [Fe(CN)6]3-/[Fe(CN)6]4- at bare carbon-fiber microelectrodes. At both microelectrodes, the cathodic and anodic limiting currents are linearly proportional to the concentrations of the reducible and oxidizable species of a redox couple, respectively. The shape of the cyclic voltammogram and the magnitude of the steady-state limiting current are not affected by the potential at which the scan starts. Speciation of both forms of a redox couple could be achieved voltammetrically at the microelectrodes. However, a considerably slower scan rate was required to achieve steady state at the modified electrode because of the smaller diffusion coefficients of [Re(I)(DMPE)3]+ and [Re(II)(DMPE)3]2+ in the Nafion-silica composite. The detection limit at the modified electrode was considerably lower (5 x 10(-9) M for [Re(I)(DMPE)3]+) than at the bare electrode (6 x 10(-5) M for [Fe(CN)6]3- and [Fe(CN)6]4-) because of the substantial preconcentration of [Re(I)(DMPE)3]+ by the Nafion-silica composite.

Neuroinvasiveness of pseudorabies virus injected intracerebrally is dependent on viral concentration and terminal field density.

Pseudorabies virus (PRV), a neurotropic swine alpha herpesvirus, has been used extensively for transneuronal analysis ofmultisynaptic circuitry after peripheral injection. In the present analysis, we examined the influence of viral concentration and neuronal architecture on the invasiveness, replication, and transynaptic passage of an attenuated strain of PRV (PRV-Bartha) injected into rat striatum. Different concentrations of PRV-Bartha were injected into the striatum at a constant rate of infusion (10 nl/minute), and animals were killed 50 hours later. Viral concentration was manipulated by either altering the volume of the inoculum (100, 50, 20 nl) or by diluting the inoculum within a constant volume of 100 nl. Immunohistochemical localization of infected neurons revealed dramatic differences in the progression of infection that were dependent directly on the concentration of injected virus. In every case, the pattern of infection was consistent with preferential uptake of virions by axon terminals and retrograde transynaptic passage of virus from the injection site. The known topographically organized corticostriatal projections permitted a precise definition of the zone of viral uptake. This analysis demonstrated that the "effective zone of viral uptake" (i.e., the zone within which viral uptake led to productive replication of virus) varied in relation to the concentration of injected virus, with the highest concentration of PRV invading terminals within a 500 microm radius of the canula. Concentration-dependent changes in the progression of retrograde transynaptic infection also were observed. The highest concentration of virus produced the most extensive infection. The distribution of infected neurons in these cases included those with known afferent projections to striatum as well as those that became infected by retrograde transynaptic infection. Lesser concentrations of PRV-Bartha produced an increasingly restricted infection of the same circuitry within the same postinoculation interval. It is noteworthy that neurons known to elaborate dense striatal terminal fields were less sensitive to reduction in viral concentration than those giving rise to terminal fields of lesser density. Collectively, the data indicate that the onset of viral replication after intracerebral injection of PRV is directly dependent on virus concentration and terminal field density at the site of virus injection.

Preferential localization of exocytotic active zones in the terminals of neurite-emitting chromaffin cells

Amperometry using 2.5 microm radius carbon fiber electrodes was employed to study exocytotic catecholamine release from individual cultured bovine chromaffin cells. The secretory responses to either direct depolarization or nicotinic receptor stimulation were focal in nature in both round and neurite-emitting cells. In contrast to the random distribution of active sites found in round cells, bipolar and tripolar chromaffin cells had responsive zones preferentially located at neurite terminals as indicated by the lower probability of finding "silent" electrode positions and an increased nicotinic-receptor responsiveness when compared with the cell body. In agreement with these data we have observed a preferential deposition of dopamine-beta-hydroxylase into the neurite terminal plasmalemma after stimulation of intact cells. These observations might be of interest since the differences in the distribution of secretory "spots" between round and neurite-emitting chromaffin cells could be used to study the molecular factors determining active site localization.

Light-scattering Mueller matrix for a rough fiber

The light-scattering Mueller matrix for an r approximately 2.0 microm radius, rough quartz fiber contains phase information different from that of a perfectly cylindrical fiber of the same optical constants and radius. The rough surface creates higher-frequency, smaller-amplitude oscillations that mask the lowerfrequency oscillations indicative of a perfect cylinder. Roughness also causes scatter outside the plane of incidence.

Porous plug made of tungsten wires and its conductance

A porous plug suitable for uhv systems was fabricated and its conductance was measured under various working conditions. The porous plug was made by inserting tungsten wires into a small hole. The porous plug with four wires inserted into a small hole of 138 microm radius and 0.15 cm long was shown to have a conductance of the order of 10 −4 l s −1 for nitrogen, argon and helium, and to be stable for repeated bakeouts up to a temperature as high as 400°C. The relative conductance for a number of gases, nitrogen, argon and helium, was inversely proportional to the square root of the gas molecular weight. The conductance value of the porous plug can easily be varied by changing the number of tungsten wires and dimensions of the hole.

Micron-sized droplets irradiated with a pulsed CO_2 laser: measurement of explosion and breakdown thresholds

Measurements of minimum CO(2) laser fluence required to explode or disintegrate 10-60 microm radius droplets of water, ethanol, diesel (hexadecane), CCl(4), bromoform, and ethyl bromide are reported. Threshold fluences range from 0.4 J cm(-2) for 10-microm radius ethanol drops to 20 J cm(-2) for 30microm bromoform drops. Threshold fluences for water droplets are ~3 J cm(-2) independent of drop size. Comparison of the measurements to calculations of laser fluence required for considered absorbing droplets to reach superheat temperature is in good agreement for cases where liquid material properties are known, suggesting that superheating of droplets is the dominant mechanism causing explosion/disintegration. Measured droplet-induced laser breakdown thresholds are considerably higher than explosion thresholds and have less dependence on droplet size and composition. The highest breakdown threshold values are for water drops, which range from 150 to 280 J cm(-2) (0.9-1.7 x 10(9)W cm(-2)) compared with 670 J cm(-2) (4.0 x 10(9) W cm(-2)) for clean air breakdown for the laser pulse length and spot size.

Evanescent field excitation of fluorescence by epi-illumination microscopy

By simple modification of the pattern of fluorescence excitation light in an epi-illumination inverted microscope, one can achieve conditions that produce total internal reflection fluorescence (TIRF) by evanescent wave excitation. Though traditionally requiring a collimated beam traversing through a special prism, TIRF also can be achieved by epi-illumination through the periphery of a 1.4 numerical aperture objective. An opaque disk of appropriate size is placed in the illumination path external to the microscope so as to cast a sharp, real-image shadow at the objective's back focal plane. This shadow allows a hollow cone of epiillumination rays traveling at only super-critical angles to reach the glass/water interface at the sample plane. Three kinds of TIRF illumination patterns can be produced by variations of this scheme: (1) a small spot of illumination of 1.5 microm radius by use of a laser light source, (2) a large region of illumination by use of a laser-illuminated diffusing screen located upbeam from the opaque disk, and (3) a large region of illumination by use of a conventional mercury arc.

Cirrus cloud transmittance and backscatter in the infrared measured with a CO_2 lidar

Two independent methods of measuring the transmittance of cirrus clouds are compared. Both used a CO(2) pulsed Doppler lidar at a wavelength of 10.59 microm. The first method used backscatter from the calibration target El Chichon stratospheric cloud that was present over Boulder in 1982 and 1983. The second method used conical lidar scans at different zenith angles when uniform cirrus decks were present. Extinction coefficients measured from both methods average 0.1 km(-1) for tenuous cirrus 1.0 km thick to 0.78 km(-1) for cirrus several kilometers thick. There is a wide standard deviation in extinction values. Extinction-tobackscatter ratios S vary from <1000 sr for tenuous clouds to 2600 sr for dense clouds. Mie scattering and extinction calculations for spherical ice particles of 10-50 microm in radius lead to ratios S > 2000 sr, so long as the ice absorption is entered into the calculations. The backscattering ratio for ice cylinders is 1 order of magnitude lower than for spheres. Backscatter in the IR may, therefore, be reasonably well modeled by some combination of spheres and cylinders. Cloud thickness statistics from lidar returns show that cirrus decks average ~500 m thick. Clouds thinner than 300 m were often overlooked by the unaided surface-based observer. These preliminary results are in rather close agreement with the LOWTRAN 6 cirrus cloud model predictions.

Light scattering device for sizing and velocimetry of large droplets utilizing a ring-shaped laser beam

A newly developed light scattering device for sizing and velocimetry of large droplets is described. A novel beam shaping technique is employed in conjunction with a laser operating in the circular TEM(01) (doughnut) mode as a light source to achieve a ring-shaped source beam. This particular beam geometry allows definition of a sheetlike scattering volume with an approximately constant power density across it resulting in improved sizing and sampling accuracy. The size resolution of the instrument is better than 7%, and the overall sizing accuracy for an input size range of 30 dB (droplets of ~45-1500 microm in radius) is ~10%. While the instrument reported in this paper was designed to analyze water droplets entrained in air, it should also be useful in other sizing applications if appropriate modifications are made.

Coronagraphic technique to infer the nature of the Skylab particulate environment

Photographs taken with the High Altitude Observatory's White Light Coronagraph (Skylab experiment SO52) are shown to contain information on the sizes and velocities of contaminant particulates around Skylab. Sizes as small as 5 microm (radius) are derived for particles as far away as 200 m from the spacecraft. The random error in the size derivation is about 30%, and no particle larger than 120 microm was observed. Transverse velocities are determined to within 0.08 m sec(-1) and radial velocities to within 9 m sec(-1). The S052 data bank contains about 3500 contaminated frames from which the nature of the Skylab environment can be inferred.

Dynamics and Energetics of the Explosive Vaporization of Fog Droplets by a 106-μm Laser Pulse

The explosive vaporization of individual water droplets (5 microm to 25 microm radius) by a 10.6-microm laser pulse has been observed with a high speed schlieren photography system. The hot vapor and the shock wave produced by the explosive vaporization can be clearly seen in the schlieren photographs. The expansion rate of the heated air mass has been measured. Factors affecting the shape of the volume of heated air are discussed, and the energy balance of the process is considered.