Size Of Active Zone Research Articles

A Modified Procedure to Evaluate Active Earth Pressure at High Seismic Loads

ABSTRACT A modified and pseudo-static limit-equilibrium approach to evaluate active earth pressure at high seismic load levels is proposed. Although it is similar to the Mononobe-Okabe method, the proposed method considers the effects of strain localization in the backfill soil and associated post-peak reduction in the shear resistance from peak to residual values along a previously formed failure plane. The proposed method can reflect differences in the peak shear resistance of the backfill soil with different degrees of compaction; yields a realistic active earth pressure coefficient which is smaller than that predicted by the Mononobe-Okabe method using a residual shear resistance; can be adapted to analyses with a large horizontal seismic coefficient where the Mononobe-Okabe method using the residual shear resistance is not applicable; and renders a reduced and more realistic size of active failure zone in the backfill soil at high seismic load levels compared to that predicted by the Mononobe-Okabe method.

Identification of gamma-aminobutyric acid-immunoreactive axon endings associated with mesencephalic periodontal afferent terminals and morphometry of the two types of terminals in the cat supratrigeminal nucleus.

A previous study has shown that mesencephalic periodontal afferent terminals receive contacts more frequently from axonal endings containing pleomorphic, synaptic vesicles (P-endings) in the supratrigeminal nucleus (Vsup) than in the trigeminal motor nucleus, suggesting that interneurons in Vsup play an important role in modulating the jaw-closing reflex. The present study was attempted to identify neurotransmitters in P-endings associated with mesencephalic periodontal afferents in cat Vsup through the use of intracellular staining of horseradish peroxidase combined with the postembedding immunogold methods. A morphometric analysis was carried out to compare the ultrastructural features of these two types of terminals. Serial sections of 31 labeled boutons and of their associated 38 P-endings were examined. They were processed for postembedding immunogold labeling with antibodies to the neurotransmitter gamma-aminobutyric acid (GABA). The 38 P-endings presynaptic to periodontal afferents showed GABA-like immunoreactivity, but the afferent terminals were free from the labeling. The morphometric analysis indicated that bouton volume, apposed surface area, total active zone size, and mitochondrial volume were smaller in GABA-immunoreactive P-endings than in periodontal afferents, but the pooled data of the two types of terminals showed that each synaptic parameter was highly correlated in a positive, linear manner with bouton volume. These observations provide evidence that P-endings presynaptic to mesencephalic periodontal afferents contain the neurotransmitter GABA and that their axoaxonic synapses are organized in accordance with the ultrastructural "size principle" proposed by Pierce and Mendell (Pierce and Mendell [1993] J. Neurosci. 13:4748-4763) on Ia-motoneuron synapses.

Assessing ultrastructure of crustacean and insect neuromuscular junctions

Motor nerve terminals of arthropods provide excellent models for study of synaptic transmission, and their ultrastructure can be investigated in the same endings from which physiological recordings have been obtained. An experimental procedure for marking a recording site for subsequent ultrastructural analysis is described. The most commonly used procedure for ultrastructural analysis has been serial sectioning and three-dimensional reconstruction. This procedure has the advantage of providing information about the entire nerve terminal, including quantitative information on number, sizes, and relative positions of individual synapses and presynaptic ‘active zones’. However, several errors may be generated in the process of viewing the sections and making the reconstruction; these errors can in principle lead to overestimation of synapse and active zone size. The errors become relatively more serious for smaller structures. Procedures for alleviating some of the possible errors are outlined. It is desirable to have additional information from other methods, such as freeze-fracture replication, to guide analysis of reconstructions from serial sections. Combined physiological and ultrastructural analysis of arthropod terminals has shown that each terminal has many small synapses, differing in size and in number of active zones, and that in some terminals, many of the observed synapses have a very low probability of transmission when nerve impulses occur at low frequencies.

Structural features of crayfish phasic and tonic neuromuscular terminals.

We examined the fine structure of terminals of the phasic and tonic excitatory axon to the crayfish limb extensor muscle. The phasic terminals are known to release 50-100 times more transmitter for a small length of terminal for a single impulse. Phasic terminals labeled with horseradish peroxidase (HRP) were relatively thin and contained a single unbranched mitochondrion; tonic terminals were much thicker, and their varicosities contained several multibranched mitochondria. Tonic terminals devoted a larger proportion of their total volume to mitochondria. The percentage volume of clear synaptic vesicles was slightly higher in phasic axon terminals, but as the tonic axon terminals were fivefold larger in volume, the total synaptic volume is much greater in tonic than phasic terminals. The number of synapses per length of terminal, and the total number of active zones per length of terminal, were greater for tonic terminals, and individual synapses were, on average, slightly larger in surface contact area for tonic terminals. In contrast, individual active zones were, on average, longer in phasic synapses. A higher proportion (50%) of phasic synapses had multiple active zones than was the case for tonic synapses (16%), and pairs of closely spaced active zones were more frequently found on phasic synapses. These findings clearly rule out synapse and active zone number as a factor contributing to higher transmitter output, but suggest that active zone size and synaptic complexity, as evidenced by multiple closely spaced active zones in a single synapse, are likely to play a causal role in the greater transmitter release of the phasic terminal. Even synapse complexity would not be enough to account fully for the large difference in terminal transmitter output, and additional factors may include electrical and biochemical differences.

Active zone size versus activity: a study of different seismicity patterns in the context of the prediction algorithm M8 : V. G. Kossobokov & J. M. Carlson, Journal of Geophysical Research, 100(B4), 1995, pp 6431–6441

Active zone size versus activity: a study of different seismicity patterns in the context of the prediction algorithm M8 : V. G. Kossobokov & J. M. Carlson, Journal of Geophysical Research, 100(B4), 1995, pp 6431–6441

Active zone size versus activity: A study of different seismicity patterns in the context of the prediction algorithm M8

A new precursory measure called active zone size (AZS), based on the development of spatial correlations, was recently introduced in the context of predicting large events in dynamical models of faults. The new measure counts the fraction of the total seismic area which has exhibited earthquakes recently. In each of the models tested, AZS was found to be more strongly correlated with an increase in the likelihood of a large event than the more conventional measure activity (N), which is defined to be the total number of earthquakes. In this paper we report the first application of AZS to seismicity catalogs. We compare measurements of AZS and N both individually and in the context of the intermediate‐term earthquake prediction algorithm M8 applied here specifically to data from the western United States. To test the performance we replace functions in M8 which are based on N with similar functions based on AZS. When this is done, at least on this limited data set, the performance of MS is somewhat improved: false alarm time is reduced, and the Landers earthquake is successfully predicted.

On the dependence of the dynamic crack tip temperature fields in metals upon crack tip velocity and material parameters

Although various approximations have been used to analytically predict the temperature rise at a dynamic crack tip and its relation to the crack tip velocity or the material properties, few experimental investigations of these effects exist. Here, the method of using a high speed infrared detector array to measure the temperature distribution at the tip of a dynamically propagating crack tip is outlined, and the results from a number of experiments on different metal alloys are reviewed. First the effect of crack tip velocity in 4340 steel is investigated, and it is seen that the maximum temperature increases with increasing velocity, the maximum plastic work rate density increases with velocity and the active plastic zone size decreases with increasing velocity. Also, it is observed that a significant change in the geometry of the temperature distribution occurs at higher velocities in steel due to the opening of the crack faces behind the crack tip. Next, the effect of thermal properties is examined, and it is seen that, due to adiabatic conditions at the crack tip, changes in thermal conductivity do not significantly affect the temperature field. Changes in density and heat capacity (as well as material dynamic fracture toughness) are more likely to produce significant differences in temperature than changes in thermal conductivity. Finally, the effect of heat upon the crack tip deformation is reviewed, and it is seen that the generation of heat at the crack tip in steel leads to the localization of deformation in the shear lip. The shear lip is actualy an adiabatic shear band formed at 45° to the surface of the specimen. In titanium, no conclusive evidence of shear localization in the shear lip is seen.

Quantitative ultrastructure of Ia boutons in the ventral horn: scaling and positional relationships

Three physiologically characterized spindle (group Ia) afferents were labeled by the intracellular injection of HRP and were processed for light-level reconstruction. Thirty-five boutons in the ventral horn were then selected for analysis. They were serially thin sectioned and characterized in terms of volume, total surface area and the surface area of apposition to postsynaptic neurons (apposed surface area), mitochondrial volume, vesicle and active zone features, relation to presynaptic contacts, postsynaptic profile size, and position within the terminal arbor. Virtually all of these characteristics were widely variable, both within the entire population and in the endings of a single fiber. Apposed surface area, mitochondrial volume, vesicle number, active zone vesicle number, active zone number, and total active zone area were highly correlated in a positive linear manner with bouton volume. This suggests a type of ultrastructural "size principle," in which the morphological features associated with synaptic release scale directly in proportion to bouton size. This pattern also extends to local circuit interactions: the extent of an Ia bouton's input from axoaxonal contacts (86% receive at least one axoaxonal contact) was directly proportional to its size. In addition, the characteristics of an Ia bouton were related to its position on the postsynaptic element and within the terminal arbor. Vesicle density, percentage mitochondrial volume, and active zone size increased as the postsynaptic process decreased in size, while volume, apposed surface area, active zone number and area, and vesicle number all decreased as one moved downstream within a terminal branch, with the exception of the terminal bouton. Vesicle density also decreased as one moved away from the dorsal root entry zone.

Structural plasticity at identified synapses during long-term memory in Aplysia.

We have used the gill- and siphon-withdrawal reflex of Aplysia californica to determine the morphological basis of the prolonged changes in synaptic effectiveness that underlie long-term habituation and sensitization. We have found that clear structural changes accompany behavioral modification and have demonstrated that these can be detected at the level of identified sensory neuron synapses, a critical site of plasticity for the short-term forms of both types of learning. These alterations occur at two different levels of synaptic organization and include (1) changes in focal regions of synaptic membrane specialization--the number, size and vesicle complement of sensory neuron active zones are larger in sensitized animals and smaller in habituated animals compared with controls--and (2) a parallel but more dramatic and global trend involving modulation of the total number of presynaptic varicosities per sensory neuron. Quantitative analysis of the time course over which these structural alterations occur during sensitization has further demonstrated that changes in the number of varicosities and active zones persist in parallel with the behavioral retention of the memory. This increase in the number of sensory neuron synapses during long-term sensitization in Aplysia is similar to changes in the number of synapses in the mammalian brain following various forms of environmental manipulations and learning (Greenough, 1984). Therefore learning may involve a form of neuronal growth across a broad segment of the animal kingdom, thereby suggesting a role for structural synaptic plasticity during long-term behavioral modifications.

Time course of structural changes at identified sensory neuron synapses during long-term sensitization in Aplysia

We have used the gill- and siphon-withdrawal reflex of Aplysia californica to explore the morphological basis of the synaptic plasticity that underlies long-term sensitization. In earlier studies (Bailey and Chen, 1983, 1988a), we described 2 classes of structural changes at identified sensory neuron synapses that occur following long-term sensitization: (1) increases in the number, size, and vesicle complement of active zones and (2) an overall increase in the total number of synaptic varicosities per sensory neuron. In the present study, we have begun to examine which of these anatomical changes might be necessary for the maintenance of long-term sensitization by exploring the time course over which they occur and, in particular, their duration relative to the persistence of the memory assessed behaviorally. Toward this end we have quantitated changes in both the total number of varicosities and their active zone morphology in single HRP-labeled sensory neurons taken from long-term sensitized and control animals at different intervals (1-2 d, 1 week, and 3 weeks) following training. We have found that long-term sensitized animals examined within 48 hr after the completion of training demonstrate an increase in the total number of varicosities per sensory neuron as well as an increase in the incidence, size, and vesicle complement of their synaptic active zones compared with control animals. The increase in the number of varicosities and active zones persists unchanged for at least 1 week, and the increase in active zone number is only partially reversed at the end of 3 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Bouton ultrastructure and synaptic growth in a frog autonomic ganglion.

Postmetamorphic growth in the frog, Xenopus laevis, is accompanied by an increase both in the size of autonomic neurons in the heart and in the number of synaptic boutons that contact their surface. To determine whether the properties of individual boutons change as their number increases, serial-section electron microscopy was used to examine bouton ultrastructure at the end of metamorphosis and in the adult. The area of bouton contact, number of active zones per bouton, active zone size, percent of bouton area occupied by active zone, and vesicle density were examined. No differences were found between the two bouton populations for any of the parameters examined. These results support the hypothesis that boutons are structural units of synaptic growth, whereby the total area of synaptic contact increases through the addition of boutons without a change in their morphological properties.

Morphological basis of short-term habituation in Aplysia

We have explored the morphological basis of the synaptic depression that underlies short-term habituation of the gill-withdrawal reflex in Aplysia by examining the fine structure of the presynaptic terminals of identified sensory neurons--a critical site of plasticity for the biochemical and biophysical changes that underlie this elementary form of learning. The structure of sensory neuron synapses from control (unstimulated) cells was compared with that of sensory neuron synapses from cells in which synaptic transmission had been depressed by repeated activation. We focused our analysis, as we had in an earlier study of long-term memory (Bailey and Chen, 1983), on the morphology of active zones at sensory neuron synapses. We found that both the incidence and size of serially reconstructed active zones were not changed in cells exposed to short-term habituation. This contrasts sharply with the reduction in both the frequency and surface area of sensory neuron active zones that accompanies long-term habituation, and suggests that modulation of active zone number and size may be an anatomical correlate that lies in the long-term domain. A quantitative analysis of the relationship between the active zone and nearby vesicle populations revealed a possible morphological substrate for the homosynaptic depression that underlies short-term habituation. Habituation leads to a depletion of synaptic vesicles immediately adjacent to the active zone. The ratio of this readily releasable pool of vesicles to the total population of vesicles associated with the active zone is 28% for control terminals, but only 11.5% for habituated terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlations between active zone ultrastructure and synaptic function studied with freeze-fracture of physiologically identified neuromuscular junctions

The active zone is a unique presynaptic membrane specialization that is believed to be the site of neurotransmitter release. To examine directly the relationship between active zone ultrastructure and synaptic efficacy, frog neuromuscular junctions were studied with a new technique combining electrophysiology, light microscopy, and freeze-fracture of identified single muscle fibers. This technique allows correlations to be made between quantal content (measured in low Ca2+ and high Mg2+ Ringer solution), endplate size, and active zone structure at the same neuromuscular junctions. By measuring physiological and morphological variables at the same junctions, the validity of structure-function correlations is significantly improved. Synaptic quantal content in 91 physiologically identified muscle fibers varied considerably and was only poorly correlated with endplate size, as shown in previous studies. To measure the total length of endplate branches, either a modified cholinesterase stain or rhodamine-labeled peanut agglutinin stain was used. When the same identified muscle fibers were freeze-fractured, active zones were exposed in 17 junctions. In a replica that contained a large part of one nerve terminal, there was no detectable gradient in active zone structure along the length of 3 different nerve terminal branches identifiable with both light and electron microscopy. The results from these 17 identified junctions indicate that quantal content per unit terminal length is positively correlated with the amount of active zone per unit terminal length. The estimated total active zone length and total number of active zone particles per junction are also positively correlated with the quantal content in these identified junctions. This study suggests that active zone size and spacing are better indicators of transmitter release than is endplate size and that the active zone may play an important role in regulating synaptic efficacy at the neuromuscular junction.

Ultrastructural correlates of experimentally altered transmitter release efficacy in frog motor nerve terminals

After experimentally inducing long term changes in transmitter release, a series of frog neuromuscular junctions were studied with intracellular recording and then semi-serially sectioned and examined in the electron microscope. Transmitter release per unit length of motor nerve terminal was well correlated with several measures of the length of individual presynaptic active zones and with the number of mitochondria per terminal. Total release from each terminal correlated with estimates of the total amount of active zone. This study of neuromuscular junctions in sartorius muscles of the frog Rana pipiens was undertaken to search for ultrastructural correlates of the increase in transmitter release efficacy that follows denervation of the contralateral sartorius. This treatment typically results in greatly enhanced release at some synapses while others appear unaffected. In the present study, nine identified junctions with known physiological properties were sectioned every 6 μm throughout much of their length to yield 40–105 cross-sectional profiles per junction. Overall, these 9 synapses showed a 33-fold range in quantal transmitter release and an 18-fold range in release per unit nerve terminal length. Release correlated with estimates of active zone size. No correlations were found between release and the density of synaptic vesicles adjacent to active zones. Our results suggest that active zones in motor nerve terminals are plastic structures, and that changes in active zone size may be the structural basis of long term changes in transmitter release and synaptic efficacy.

Ultrastructural correlates of naturally occurring differences in transmitter release efficacy in frog motor nerve terminals.

Motor nerve terminals in cutaneous pectoris muscles of the frog Rana pipiens release more transmitter and form synapses with higher levels of effectiveness than do those in sartorius muscles. Neuromuscular junctions from these two muscles were compared in the electron microscope to search for ultrastructural correlates of differences in transmitter release and synaptic effectiveness. The following measurements were made from cross-sections of junctions with known levels of effectiveness: (a) the presence of active zones, the presumed sites of transmitter release, (b) active zone size, (c) the perimeter, cross-sectional area, height and width of nerve terminals, (d) number of mitochondria, (e) vesicle density, and (f) the extent to which Schwann cells wrap terminals. Nerve terminals in the two muscles did not differ in size, shape or vesicle density. The more strongly releasing cutaneous pectoris terminals did, however, have significantly larger active zones due to deeper invagination of the terminal into the postsynaptic gutter and lesser interposition of Schwann cell processes between presynaptic and postsynaptic membranes. Cutaneous pectoris terminals also contained more mitochondria, presumably to supply the greater energy demand imposed by high release levels.

Morphological basis of long-term habituation and sensitization in Aplysia.

The morphological basis of the persistent synaptic plasticity that underlies long-term habituation and sensitization of the gill withdrawal reflex in Aplysia californica was explored by examining the fine structure of sensory neuron presynaptic terminals (the critical site of plasticity for the short-term forms of both types of learning) in control animals and in animals whose behavior had been modified by training. The number, size, and vesicle complement of sensory neuron active zones were larger in animals showing long-term sensitization than in control animals and smaller in animals showing long-term habituation. These changes are likely to represent an anatomical substrate for the memory consolidation of these tasks.