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Biomedical subjects

S R Shaw

Publications and source records attributed to S R Shaw.

At least 19 recordsLinked to original sources
PubMed1994

Re-evaluation of the absolute threshold and response mode of the most sensitive known "vibration" detector, the cockroach's subgenual organ: a cochlea-like displacement threshold and a direct response to sound.

Earlier accounts claim from indirect measurements that the subgenual organ (SGO) in the proximal tibia of the cockroach leg can detect vibrational displacements down to 0.002 nm, two orders of magnitude below the threshold for vertebrate hair cells in the cochlea. The SGO vibration threshold is redetermined here more directly by a new method on a cantilever beam, while controlling for particular acoustic and vibrational artifacts that might have compromised earlier efforts. The threshold is revised upwards to about 0.2 nm in the most sensitive preparation, about the same as the cochlea. Recently, it has been determined that the cockroach SGO also has an auditory response, and the data here on subthreshold summation and response-intensity relationships provide further evidence that sound and contact vibration are both sensed by the same receptor neurons. Direct measurements rule out the prevailing hypothesis that sound is detected indirectly as induced vibration of the ground, and also weigh strongly against any significant involvement of generalized leg resonance in acoustic pick-up. The results fit with a recent proposal that the auditory response is direct, and that acoustic fluctuations inside the tracheae may be the primary response mode in the transduction of both vibration and sound.

Acoustic Stimulation↗
PubMed1992

Reactive synaptogenesis following degeneration of photoreceptor terminals in the fly's optic lobe: a quantitative electron microscopic study.

Following photo-ablation of receptor cells in the retina of the housefly's compound eye, their synaptic terminals degenerate with a timecourse which we have followed over 8 d. Degeneration deprives the monopolar interneurons in the first optic neuropile, the lamina, of their main synaptic input. Simultaneously it deprives one monopolar interneuron (L2) of one of its synaptic targets, as L2 makes numerous feedback synaptic contacts at which it is pre-synaptic upon receptor terminals. Because the feedback synapses are dyadic, input still remains available to the second element post-synaptic at the dyad, which does not degenerate. This element is T1, a higher-order interneuron from the next most proximal neuropile (the medulla). Some of the original feedback synaptic sites soon disappear as a consequence of the photo-ablation, but their loss is partly offset by the production of new synaptic contacts. The new pre-synaptic ribbons resemble those at the original sites except for being smaller. The sites are, moreover, monadic, with T1 now the sole post-synaptic partner. These results show that interneurons in the fly's lamina retain a dynamic capacity for synaptogenesis throughout much of adult life, normally a few weeks in Musca, and that during this synaptogenesis they re-enact the same cell preferences expressed earlier in development.

Animals↗
PubMed1991

Invagination of presynaptic ribbons in the fly's optic lobe following loss of their target neuron.

In the first optic neuropile of the housefly Musca, photoreceptor terminals innervate fixed clusters of interneurons, one of which is the monopolar cell L2; L2's synapses in turn feed back upon the terminals. We examined the ultrastructure of these feedback synapses following degeneration of their normal targets, the receptor terminals; this was accomplished by photo-ablating the receptor cells after intraretinal injections of sulforhodamine. Even when all the terminals degenerated, their deafferentated target cells, including L2, remained structurally intact for at least 14 d. Despite this lack of obvious trans-synaptic degeneration, L2's synaptic connections did alter. Presynaptic organelles of the feedback synapses, synaptic ribbons and associated synaptic vesicles, soon appeared in L2's cytoplasm, separating from their site of attachment at the presynaptic membrane by invagination. Similar free-floating organelles and vesicles also occurred in another monopolar cell, L4. They were also occasionally encountered in L2, in normal, newly emerged flies at a time when a naturally occurring loss of feedback synapses is greatest. We interpret the process of internalization that forms these floating ribbons to be the first step in synaptic loss which occurs spontaneously, and that the rate is enhanced in L2 when its main synaptic targets, the receptor terminals, degenerate.

Animals↗
PubMed1991

Terminal degeneration and synaptic disassembly following receptor photoablation in the retina of the fly's compound eye.

A long-term objective of our studies on the first optic neuropil (or lamina) underlying the fly's compound eye is to explore how afferent photoreceptor synapses disappear during normal adult experience. To increase the frequency of this loss and the chances for its detection artificially, we have examined in this study the synapses during the degeneration of their presynaptic elements, the synaptic terminals of the receptor cells. This may be reliably procured by illuminating for 12 min with strong green light eyes that have received an injection of the dye sulforhodamine 101 (Picaud et al., 1988). The lesion is local and develops rapidly. Degeneration among terminals is progressive but asynchronous. There are several different types of degeneration, most interpretable as stages in a temporal progression after illumination-induced injury. Degenerative changes include shrinkage and darkening of terminals and mitochondrial swelling. Synaptic sites are lost in a defined sequence: (1) the T-shaped presynaptic ribbon disappears first; (2) the members of what is normally a tetrad of postsynaptic elements withdraw as an ensemble from the receptor terminal's membrane, and the surrounding epithelial glial cells extend between former pre- and postsynaptic partners; and (3) the postsynaptic elements then separate from each other. In the most rapidly affected terminals, the frequencies for those synaptic sites at which both presynaptic ribbons and postsynaptic elements remain intact decline by 85%, even in the first 8 hr postillumination.

Animals↗
PubMed1990

The photoreceptor axon projection and its evolution in the neural superposition eyes of some primitive brachyceran diptera.

All known brachyceran flies have inherited the same trapezoidal pattern of rhabdomeres in the ommatidia of the compound eye, different from that of their presumed nematoceran ancestors. The projection pattern of axons to lamina cartridges needs to match this in order to generate efficient neural superposition vision. This pattern has been traced in serial sections or via stimulated dye uptake in two families that diverged from the dipteran stem line more than 200 million years ago (Rhagionidae, Stratiomyidae) and in one family of intermediate age (Dolichopodidae). All show the same asymmetric projection as modern muscoid families and a concordantly 180 degrees-twisted axon bundle, suggesting that evolution to the 'modern' versions of both optics and projection coincided closely in evolutionary time. The basic developmental plan of the cartridge has also been retained since the origin of the Brachycera, but the synaptic participation of at least one axon (that of photoreceptor R7) varies markedly between species.

Animals↗
PubMed1990

Changes in geometrical and biomechanical properties of immature male and female rat tibia.

Tibial geometry and mechanical properties in male and female rapidly growing rats were measured to provide comparative data for spaceflight, exercise, or disease experiments that use immature rats as an animal model. A total of 68 immature Sprague-Dawley rats (43-58 d of age) were divided into 8 groups according to age and sex. Tibial length and middiaphysial cross-sectional data were analyzed, and tibial mechanical properties were obtained via three-point bending tests to failure. Results indicated that tibial geometry and mechanical properties were changed significantly during this 15-d period of rapid growth, with male rats having greater increases than female rats. Both male and female tibial lengths nevertheless increased significantly during this growth period, as did middiaphysical cross-sectional area. Tibial flexural rigidity increased significantly for both sexes during this growth period. The male tibiae changed primarily in structural characteristics, but some material properties also changed with age. In the female rats, however, the major changes in mechanical properties of the tibia were attributable only to structural changes in the bone.

Age Factors↗
PubMed1989

Direct connections between the R7/8 and R1-6 photoreceptor subsystems in the dipteran visual system.

Musca and related flies have three main photoreceptor subsystems. The R1-6 group has short axons that terminate in the cartridges of the first optic neuropile, the lamina. The cartridges are bypassed by the longer axons of R7 and R8, which run together to terminate at different levels in the underlying medulla neuropile. The present account describes a shallow, previously unidentified zone in the lamina within which R7/8 make glancing contact with R1-6. At the distal border of the cartridge over no more than 3-4 microns depth, the tangentially directed short axon of R6 squeezes between the pair from R7 and R8, forming quite large areas of mutual contact (approximately 7 microns2). Less frequently, R1 is contacted. At least some of these sites contain smaller membrane specialisations indistinguishable from the more numerous gap junctions found more proximally that interconnect the terminals of R1-6. The R7/8 junctions with R6 are of comparable size (0.15 micron 2) and likewise possess symmetrical membrane densities. They provide proposed pathways for direct electrical interaction to account for observed electrical input from R7/8 to the R1-6 subsystem. In two cases R7/8 was possibly postsynaptic to R1-6 at a multiple-contact synapse, but even if functional, these sites were so rare that they are unlikely to have much operational significance.

Animals↗
PubMed1989

Do local analgesics injected at the time of third molar removal under general anaesthesia reduce significantly post operative analgesic requirements? A double-blind controlled trial.

A prospective, controlled, randomised, double-blind trial suggested that: 1. The use of bupivacaine 0.5% plain and prilocaine 3% with felypressin 0.03 i.u./ml. as peroperative local analgesics reduce postoperative pain scores recorded on the first postoperative morning (P less than 0.05). 2. Bupivacaine 0.5% plain reduces postoperative opiate requirements (P less than 0.025). 3. Patients do not find peroperative bilateral inferior alveolar and lingual nerve blocks with buccal infiltrations significantly unpleasant. Thus peri-operative bupivacaine 0.5% is a valuable adjunct to third molar removal under general anaesthesia.

Adolescent↗
PubMed1989

Evolutionary remodeling in a visual system through extensive changes in the synaptic connectivity of homologous neurons.

The cellular mechanisms by which nervous systems evolve to match evolutionary changes occurring in the rest of the body remain largely unexplored. In a distal visual neuropil of a previously unexamined ancient dipteran family, Stratiomyidae, homologues of all of the periodic neurons known already from more recent Diptera can be recognized, occupying the same locations within the unit structure. This points to extreme developmental stasis for more than 200 million years, conserving both cell identity and position. The arborizations that some neurons make also have remained conservative, but others show marked differences between families in both size and branching patterns. At the electron-microscopical level, extensive differences in synaptic connectivity are found, some sufficient to radically redefine the systems roles of particular neurons. The findings bear out an earlier prediction that changes in the connectivity matrix linking conserved neurons may have been a major factor in implementing evolutionary change in the nervous system.

Animals↗
PubMed1988

Effects of a 1-wk spaceflight on morphological and mechanical properties of growing bone.

The morphological and mechanical responses of tibia and humerus were assessed in growing male rats after a 1-wk spaceflight aboard NASA Spacelab 3. In contrast to flights of longer duration, changes in middiaphysial cross-sectional morphology were minimal. Inhibition of longitudinal growth was not found in the tibia but was apparent in the humerus. The normal age-related increase in tibial middiaphysial density was not observed in the flight animals. Three-point bending tests indicated that a 1-wk spaceflight impeded the maturation of bone strength and stiffness, with the effects more pronounced in the tibia than in the humerus. Material property alterations in bone thus overshadowed morphological factors in determining the bone's mechanical response. It is likely that deprivation of normal weight-bearing loads was a major factor contributing to the observed changes, but endocrine and other local factors must also be considered.

Animals↗
PubMed1987

Mechanical, morphological and biochemical adaptations of bone and muscle to hindlimb suspension and exercise.

The influences of weightbearing forces on the structural remodeling, matrix biochemistry, and mechanical characteristics of the rat tibia and femur and surrounding musculature were examined by means of a hindlimb suspension protocol and highly intensive treadmill running. Female, young adult, Sprague-Dawley rats were designated as either normal control, sedentary suspended, or exercise suspended rats. For 4 weeks, sedentary suspended rats were deprived of hindlimb-to-ground contact forces, while the exercise suspended rats experienced hindlimb ground reaction forces only during daily intensive treadmill training sessions. The suspension produced generalized atrophy of hindlimb skeletal muscles, with greater atrophy occurring in predominantly slow-twitch extensors and adductors, as compared with the mixed fiber-type extensors and flexors. Region-specific cortical thinning and endosteal resorption in tibial and femoral diaphyses occurred in conjunction with decrements in bone mechanical properties. Tibial and femoral regional remodeling was related to both the absence of cyclic bending strains due to normal weightbearing forces and the decrease in forces applied to bone by antigravity muscles. To a moderate extent, the superimposed strenuous running counteracted muscular atrophy during the suspension, particularly in the predominantly slow-twitch extensor and adductor muscles. The exercise did not, however, mitigate changes in bone mechanical properties and cross-sectional morphologies, and in some cases exacerbated the changes. Suspension with or without exercise did not alter the normal concentrations of collagen, phosphorus, and calcium in either tibia or femur.

Animals↗
PubMed1986

Anti-adhesive coating for glass microelectrodes.

A simple procedure is described for coating the tips of glass micropipette electrodes with a durable thin film of a commercially available anti-adhesive agent (Antispread). This lowers the surface tension of the electrode glass, and helps reduce damage and distortion to tissue, by preventing the microelectrode from sticking permanently to cell membranes. In controlled trials, the coating actually increased the probability that nearby cells could be recorded from successively by about 30%. Electrode resistance increased only very slightly (on average by 8%) as a result of the coating. Coated electrodes yielded excellent long-term penetrations of small visual cells, and in addition allowed these to be dye-filled in the normal way. The coating may have applications in all situations where prevention of wetting by tissue is desirable.

Animals↗
PubMed1986

Evolutionary progression at synaptic connections made by identified homologous neurones.

A comparative ultrastructural study of photoreceptor synapses formed upon homologous postsynaptic neurones in insects has been made by using serial-section electron microscopy in representative Diptera from a monophyletic series of 14 families. At all of the synaptic contacts there is a presynaptic dense bar, surmounted in phylogenetically more recent families by a presynaptic platform. Opposite the bar lies a pair of postsynaptic elements that invariably originate one each from two unique monopolar neurones L1 and L2. Both elements contain increasingly elaborate cisternae in more recent flies. Within the phylogenetic series, the postsynaptic ensemble itself changes from the original dyad to a tetradic configuration in more recent Muscomorpha by the addition of two new postsynaptic elements from an amacrine cell. This transition occurs once only in the series, which, gauged by the fossil record, covers divergences from the stem line extending back greater than 200 million years.

Animals↗
PubMed1986

Heterogeneous mechanical response of rat knee menisci to thermomechanical stress.

Thermodilatometric, dynamic thermomechanical, and light-microscopic analyses were done on the anterior and posterior regions of the rat knee menisci to correlate regional differences in morphology and extracellular matrix composition with regional mechanical behavior. Following the administration of a general anesthetic, menisci were excised from 12 young female Sprague-Dawley rats. During thermodilatometric and thermomechanical testing, tissue temperature was increased at a constant rate of 3.5 degrees C/min from 30 to 100 degrees C. Light microscopy revealed regional differences in cell density and proteoglycan content. The anterior horn was significantly heavier (greater than 87%) and thicker (greater than 60%) than the posterior region. During thermal analyses, both the anterior and posterior horns decreased in tissue thickness as the temperature increased from 30 to 73 degrees C. After 73 degrees C, however, the posterior horn expanded significantly, whereas the anterior remained in a comparatively contracted state. The rates of linear contraction and expansion of the posterior horn were seven times those of the anterior horn, and the stiffness of the anterior horn was significantly greater than the posterior horn.

Analysis of Variance↗
PubMed1984

Asymmetric distribution of gap junctions amongst identified photoreceptor axons of Lucilia cuprina (Diptera).

The relative number of gap junctions within the ring of axon terminals of identified adult photoreceptors has been counted, in the lamina cartridges of Lucilia cuprina. Terminals are similar in size, but some pairs of terminals make up to four times as many junctions with each other as do other pairs; the frequency of connection varies markedly along the length of the cartridge. Junctions across the cartridge occasionally occur out of the normal sequence, frequently in the upper cartridge between one pair of axons. The distributions suggest that mechanical factors transmitted down from the upper cartridge are responsible for the non-uniformities. There may be indiscriminate competition between terminals at some stage of development for a fixed quota of junctions, the groupings of which have to reach a minimum size to remain viable. Amongst the neurones of the cartridge, great specificity is shown in which cells form gap junctions: only receptors R1-6 are involved, making junctions only with each other. Within this class R1-6, however, there is no evidence from the distribution of gap junctions for unique cell labelling, that distinguishes one photoreceptor from another. Connections appear to form indiscriminately, on the basis of proximity. This same generalization holds true for a particular class of chemical feedback synapse. The identity of tight-junction-like structures on the photoreceptor axon membrane is also discussed.

Animals↗