Number Of Ribbons Research Articles

Diffraction of waveguide waves by a finite system of strips in a layered dielectric

Microwave engineering uses leaky-wave antennas, which are planar dielectric waveguides with metal strips deposited on their surface. The analysis of such antennas in the literature known to us [i, 2] is made in the approximation of an infinite periodic grating without variation of the field along the strips; this can result in a large error in the calculation of real antennas with a limited number of strips and strip sizes. Nonperiodic gratings, which are the basis for the fabrication of antennas with enhanced characteristics, cannot be studied in this approximation [3]. In our study we obtained the solution of the boundary problem for the diffraction of waveguide waves by a finite number of metal ribbons without any limitation on their size. The solution also makes it possible to study the properties of higher leaky waves of microstrip lines and to make an exact calculation of waveguide filters formed by metal strips deposited on a dielectric substrate in the E plane of a rectangular waveguide. Such filters have found application in the millimeter range [4, 5]. In contrast to those studies, we take the effect of the dielectric substrate into account. Let us consider a two-dimensional layered structure (the boundaries of the layers are parallel to the xz plane), in which N infinitesimally thin ideal conducting strips lie on one boundary (Fig. I). Boundary conditions of the "electric wall" or "magnetic wall" type are satisfied on the horizontal screens. One or both screens may be absent, in which even the conditions for emission are assumed to be satisfied.

A possible role for cyclic guanosine monophosphate in the rat pineal gland

Adrenergic stimulation of pinealocytes induces an increase of both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). However, for cGMP no biological effects have been demonstrated so far. Therefore we tested the effects of the analog 8-bromo-cGMP on synaptic ribbon numbers and on melatonin synthesis as reflected by N-acetyltransferase (NAT) activity in the rat pineal gland in vitro. Incubation for 6 h with 8-bromo-cGMP did not change the activity of serotonin NAT but in increased the number of synaptic ribbons. These results indicate that cGMP is involved as a second messenger in the regulation of synaptic ribbon numbers in the rat pineal gland.

Constancy of synaptic ribbon numbers in the retina of the arctic chary, Salvelinus alpinus

At high latitudes, such as in Iceland, the daily photoperiod varies from almost continuous darkness in winter to virtually constant light in summer. Previous studies of detailed retinal structure in vertebrates have shown significant daily and annual effects of photoperiod. We sampled arctic charr in Iceland during the summer, including fish that were both light- and dark-adapted, during both day and night. We observed retinomotor responses characteristic of light- and dark-adaptation, but found no difference in the number of synaptic ribbons in the retina. The morpho-physiological changes, appearing as retinomotor responses, are thus not expressed at the synaptic level.

Effects of superior cervical preganglionectomy on nerve vesicles and synaptic ribbons in the cat pineal gland.

Previous studies have shown that bilateral preganglionectomy of the superior cervical ganglia (SCGp) modifies the nerve vesicles and synaptic ribbons in the cat pineal gland. In the present study the nerve vesicles (NV) and synaptic ribbons (SR) were analyzed morphometrically after chronic preganglionectomy of the pineal sympathetic nerve fibers. It was shown that bilateral sympathectomy of the preganglionic fibers innervating the superior cervical ganglia (SCG) markedly reduces the number of dense cores of small dark vesicles (SDV) and, on the other hand, modifies the number and shape of the large dark vesicles (LDV). An increase in the number of the synaptic ribbons after chronic preganglionectomy of the SCG supports the hypothesis that the numerical reduction of dense cores of nerve vesicles in the cat pineal gland influences the level of synaptic ribbon formation.

Morphometric analysis of the synaptic ribbons and nerve vesicles of the cat pineal gland after electrical stimulation of the superior cervical ganglia.

The nerve vesicles and synaptic ribbons were quantified in the cat pineal gland after electrical stimulation of the pineal sympathetic nerve fibers. It has been shown that the bilateral electrical stimulation of the preganglionic fibers innervating the superior cervical ganglia (SCG) markedly reduces the number of dense cores of small dark vesicles (SDV) and, on the other hand, modifies the number and shape of the large dark vesicles (LDV). An increase in the number of the synaptic ribbons after stimulation of the SCG supports the hypothesis that the numerical reduction of dense cores of nerve vesicles in the cat pineal gland influences the level of synaptic ribbon formation.

Flare morphologies and coronal field configurations

Chromospheric flares are the footpoints of closed coronal field lines. In this paper we present different flare morphologies from observations and examine the implied coronal field configurations above the flaring region. Flares are grouped according to the number of ribbons, from unresolved compact point-like flare to four-ribbon flares. Quiet region flares having characteristics all their own are also presented here. We find that compact, unresolved point-like flares have two distinct footpoints when viewed in offband Hα. The footpoints of some of the compact flares also show increased separation as a function of time. Unlike large two-ribbon flares, the ribbons of many small and/or short-lived two-ribbon flares usually have no measurable separation of ribbons. Multiple-ribbon (three or more ribbon) flares consist of two or more pairs of two-ribbons, or two or more sets of field lines. Parity of the ribbons in multiple-ribbon flares, or the lack of it, depends on the magnetic makeup of the locale of the ribbons. Flares in old quiet regions resulting from sudden filament eruptions show discrete small patches of emissions reflecting the ‘spottiness’ of decayed and dispersed field of quiet region.

Photoreceptor synaptic ribbons: Three-dimensional shape, orientation and diurnal (non) variation

A variety of different lighting regimes have been reported to cause alterations in the length and number of synaptic ribbons within photoreceptor synaptic terminals. Whether these alterations occur in mammalian visual systems during a diurnal lighting cycle was tested using computer-assisted morphometrics. Adult guinea pigs were entrained to a 12:12/ligh:dark regime and subsequently killed at various times throughout the cycle. Representative samples from all quadrants of the eyes were processed for electron microscopy and the synaptic ribbons within the synaptic terminals of the three different types of photoreceptors in this retina (alpha and paranuclear rods, and cones) were analyzed with the aid of an image analysis computer (Zeiss IBAS) for their length and absolute number per terminal. Contrary to previously published reports on other species, the synaptic ribbons in all three terminal types exhibited no statistically significant change in either number per terminal or in length, throughout the 24 hr cycle. Computer-based three-dimensional reconstructions from serial thin sections revealed horseshoe-shaped synaptic ribbons with pleats in numerous planes conforming to the contours of the invaginating postsynaptic elements.

Correlation of the number of pineal "synaptic" ribbons and spherules with the level of serum melatonin over a 24-hour period in male rabbits.

Previous studies have shown that pineal "synaptic" ribbons and spherules may respond differently under normal and experimental conditions. It has been suggested that the increase in the number of ribbons may be a prerequisite for enhanced melatonin formation. In the present study, the number of ribbons and spherules as well as the level of serum melatonin were monitored over a 24-h period in the male rabbit, the pineal gland of which is known to contain many spherules. It was found that both the number of ribbons and the levels of serum melatonin show the typical nocturnal increase, exhibiting peaks at 02:00 and 06:00 h, respectively. There is a good correlation (R = 0.8) of the two parameters. The spherules, in contrast, show no statistically significant circadian changes in number and cannot be correlated with the levels of serum melatonin. It is concluded that ribbons and spherules may differ in function and that the ribbons may be somehow involved in the regulation of melatonin formation.

Circadian variations in pinealocytes of the Chinese hamster, Cricetulus griseus. A quantitative electron-microscopic study.

Circadian morphological variations of pinealocytes in the superficial pineal of the Chinese hamster (Cricetulus griseus) were studied using quantitative electron-microscopic techniques. The volume of the nucleus and cytoplasm of pinealocytes exhibited similar circadian variations, with the maximum around the middle of the light period and the minimum during the first half of the dark period. Synaptic ribbons in pinealocytes were classified into three groups, type-1, -2 and -3 synaptic ribbons, which appeared as rods, round or irregular bodies and ring-shaped structures, respectively; a synaptic ribbon index was determined for the respective types. The synaptic ribbon index was expressed as the number of synaptic ribbons in the pinealocyte profile representing the cell size. The type-1 synaptic ribbon index, which was smallest during the second half of the light period, was increased during the dark period. The length of straight or slightly curved rods showed a 24-h change similar to that of the type-1 synaptic ribbon index; the length of the rods was maximal during the first half of the dark period and minimal at the end of the light period. There was no apparent circadian variation in the type-2 synaptic ribbon index. The type-3 synaptic ribbon index was higher during the light period than during the dark period; the index attained zero 3 h after the onset of darkness and, thereafter, increased gradually.

Synaptic ribbons in pinealocytes of castrated rats and rats treated with estradiol

Both orchidectomy and estradiol administration caused an increase in number of synaptic ribbons in rat pinealocytes. A cytoplasmic area occupied by microtubular sheaves, numerous microtubules, groups of microfilaments and Golgi complex is described. These elements experiment a larger development after estradiol administration and castration. The origin and functional significance of synaptic ribbons are discussed.

Effect of hypophysectomy on the synaptic ribbons in the pineal organ of the killifish Fundulus heteroclitus.

The Pineal organ of the killifish was investigated by electron microscopy under experimental light and dark conditions and after hypophysectomy. Significant changes were detected in the number and length of the synaptic ribbons in the pineal organ. The pineal synaptic ribbons in-creased in number and length during darkness at night, but decreased during the day in light. Hypophysectomy showed a prominent effect on the decrease in number and lengh of the synaptic ribbons. Tish result suggests that the pineal synaptic ribbons function as receptors for the infor-mation from the pituitary. Meanwhile, the intact fish kept in continuous darkness showed a con-spicuous decrease in number and length of the synaptic ribbons during day time. This means that changes of synaptic ribbons (number and length) might be involved in circadian rhythm rather than light and dark conditions.

Pineal synaptic ribbons in blinded rats.

To study the effects of bilateral ophthalmectomy on the circadian rhythm of the pineal gland, the number of the synaptic ribbons and ribbon fields in the pineal gland of female rats were determined by electron microscopy. In a preliminary experiment, the pineal gland was taken from rats at 15 days, and 1, 2, 3, 4, and 6 months after bilateral ophthalmectomy. It was found that the numbers of both synaptic ribbons and ribbon fields decreased markedly and reached a minimum at 1 month postoperatively, but recovered completely after 4 months. In the main experiment, the number of these intracellular elements was counted in both control and enucleated animals killed at 4 h intervals between 02:00 h and 22:00 h at either 1 month or 6 months after operation. All animals were kept under a lighting regimen of 12 h of illumination (06:00 to 18:00) and 12 h of darkness (18:00 to 06:00). After 1 month, a remarkable decrease in the number of both the synaptic ribbons and ribbon fields was again noted but the circadian rhythm still remained. Complete quantitative and qualitative recovery in the circadian rhythm was obtained 6 months later.

Synaptic ribbons in the pineal system of normal and light deprived golden hamsters.

Synaptic ribbons were studied in the pineal system of the golden hamster (Mesocricetus auratus) under various conditions of light deprivation. Five weeks of constant darkness led to an increase in the number of synaptic ribbons in both the superficial and the deep part of the organ, and after ten weeks a further increase was observed. In animals returned to normal lighting conditions for one week, after ten weeks of constant darkness, a marked decrease was recorded and five weeks after termination of light deprivation the number of synaptic ribbons was slightly below the normal level. In addition to long rod-shaped ribbons, atypically formed aggregations of electron dense material appear in the synaptic ribbon fields of light-deprived hamsters; dense core vesicles were observed in the vicinity of the ribbons with particular frequency. It is assumed that the topographical relationship between dense core vesicles and synaptic ribbons is of functional significance in relation to the regulation of pineal antigonadal activity.

Synaptic ribbons during postnatal development of the pineal gland in the golden hamster (Mesocricetus auratus).

Synaptic ribbons, functionally enigmatic structures of mammalian pinealocytes, were studied during the postnatal development of the pineal gland in the golden hamster (Mesocricetus auratus). On day 4 post partum, synaptic ribbons appear in cells that have already started to differentiate into pinealocytes. Between days 4 and 9, an increase in the number of synaptic ribbons occurs, concomitant with the continuing differentiation of the pineal tissue. Between days 9 and 16, when differentiation of this tissue is almost completed, the number of synaptic ribbons decreases and approaches that characteristic of the adult pineal gland. During development, the synaptic ribbons increase in length, and dense core vesicles are frequently found in the vicinity of these structures. It is assumed that a functional relationship exists between dense core vesicles and the synaptic ribbons, which are considered to be engaged in a certain form of secretory activity of the mammalian pineal gland.

A morphological study of the circadian cycle of the pineal gland of the rat.

To seek a morphological expression of circadian rhythm, we investigated cytologically pineal glands taken from rats every 2 to 4 h under a lighting regime of 12 h of illumination (6:00 to 18:00) and 12h of darkness. The changes in the number of synaptic ribbons and ribbon fields was observed by electron microscopy. The number of these intracellular elements was greatest at 2:00 and lowest at 14:00, the difference being statistically significant. There were no significant local differences in numbers with respect to the part of the pineal gland examined. The data are similar to those of Vollrath from the pineal gland of a guinea pig, and seem to confirm a circadian function in the pineal gland in mammals.

Synaptic arrays of the inner plexiform layer in the developing retina of Xenopus

During embryonic and larval development in Xenopus laevis, arrays of synapses made by amacrine cells form in two phases: an initial phase of rapid synaptic addition and a second phase of slower addition. In the region near the optic nerve, at which all measurements were made, these synapses first appear at stage 40 (approx 66 hr postfertilization). Connectivity increases at a rate of 8.6 synapses per hr per inner nuclear layer (INL) nucleus until stage 47 (132 hr postfertilization). After this phase the rate of formation decreases to 1.19 synapses per hr per INL nucleus. Synaptic arrays made by bipolar cells have only one phase of addition. A synapse made by a bipolar cell may be identified by its presynaptic ribbon, the first of which are seen at stage 40. Ribbons are added to the IPL neuropil at a rate of 4.6 ribbons per hr per INL nucleus until stage 47. After this the number of ribbons per INL nucleus in the area near the optic nerve remains constant. Although they may be found, amacrine to amacrine synapses (serial conventional) remain at low numbers throughout larval and early postmetamorphic life. This is unlike the condition found in Rana pipiens where a dramatic increase in amacrine to amacrine connectivity occurs at metamorphosis.

Darkness-induced reduction of the number of synaptic ribbons in fish retina.

It has long been known that the retinomotor response of the pigment epithelium and of the visual cells in lower vertebrates reflects changes in the activity of rods and cones that depend on the extent of light or dark adaptation. Although there have been several attempts to correlate changes in the structure of retinal photoreceptors and neurones with changes in their activity, no such unequivocal changes have been obtained on the ultrastructural level. The number of presynaptic clear vesicles in the photoreceptor terminal does not vary significantly under light and dark conditions1, 2. A recent paper on light-dependent changes of synaptic ribbons in the pineal gland3 suggested that the analogous structures in the vertebrate retina might show differences correlated with different degrees of light and dark adaptation.

High Energy Photoelectron Spectroscopic Study of Carbon Fibre Surfaces

KNOWLEDGE of the internal structure of carbon fibres, which are derived from polyacrylonitrile and rayon by carbonization in special conditions1–3, has increased significantly in the past few years. Investigations using small-angle and conventional X-ray diffraction4–9, electron diffraction6,10,11, intercalation with caesium12, helium density determinations6, polarized light microscopy11, and high-resolution electron microscopy6,13, have established that the carbon layers form long (greater than 100 nm) ribbons with preferred orientation of the layers parallel to the fibre axis. A number of these parallel ribbons form a so-called microfibril6, the average thickness (or stack size) of which, in fibres that have been heat treated to 1,300 K or beyond, is in the range 3–10 nm. The microfibrils are believed to be wrinkled and bent6,9 and they may even be twisted to form rather inaccessible voids. Within the layers, the C–C bond distance9 is 1.416±0.007 A, and for this, and other reasons6, the bonding is believed to involve only sp2 hybridized carbon atoms. There is no evidence of cross-linking between the layers (which contain a few per cent of structural defects9) perpendicular to the axis of the microfibrils.

Stress analysis of ribbon reinforced composites : Polymer Engineering and Science, Vol 10, No 1, p 43 (January 1970)

In this paper the results of both the theoretical and experimental stress analyses of composite materials reinforced with ribbons are presented. The reinforcing materials for such composites are characterized as two-dimensional elements which are isotropic in planes parallel to the faces. The theoretical work is based on the finite element method. Experimentally, the photoelastic technique is employed to determine the stresses around the glass ribbons embedded in an epoxy matrix. The specimens, containing a number of aligned ribbons, were loaded in tension parallel to the ribbon width. The variation of shearing stress at the ribbon-matrix interface as well as the pertubation effect on both the ribbon normal stress and interfacial shear due to the discontinuity of the neighboring ribbons are discussed.

Stress analysis of ribbon reinforced composites

AbstractIn this paper the results of both the theoretical and experimental stress analyses of composite materials reinforced with ribbons are presented. The reinforcing materials for such composites are characterized as two‐dimensional elements which are isotropic in planes parallel to the faces. The theoretical work is based on the finite element method. Experimentally, the photoelastic technique is employed to determine the stresses around the glass ribbons embedded in an epoxy matrix. The specimens, containing a number of aligned ribbons, were loaded in tension parallel to the ribbon width. The variation of shearing stress at the ribbon‐matrix interface as well as the pertubation effect on both the ribbon normal stress and interfacial shear due to the discontinuity of the neighboring ribbons are discussed.