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Biomedical subjects

G Allison

Publications and source records attributed to G Allison.

At least 19 recordsLinked to original sources
PubMed2006

Strong effect of resonant impurities on Landau-level quantization.

We investigate experimentally the effect of a random distribution of nitrogen (N) impurities on the Landau-level spectrum of a GaAs quantum well. Our magnetotunneling study reveals complex and nonequally spaced Landau levels and a quenching of the Landau states at a well-defined bias and electron energy which is resonant with that of the N atoms. Analysis of the magnetic field dependence of the tunnel current into the Landau levels of the well also provides quantitative information about the nonresonant component of the N-related scattering potential.

Journal Article↗
PubMed2006

Thermodynamic density of states of two-dimensional GaAs systems near the apparent metal-insulator transition.

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition--a crossover in the sign of deltaR/deltaT with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in r(s), the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.

Journal Article↗
PubMed2002

Evaluation of triceps surae muscle length and resistance to passive lengthening in patients with acquired brain injury.

OBJECTIVE: To examine changes in muscle length and resistance to passive lengthening in the triceps surae muscles in patients with recently acquired brain injury. BACKGROUND: Increased passive resistance in the triceps surae muscles is common following acquired brain injury. Adaptive shortening secondary to relative immobility, and increased stiffness due to rheologic changes within the musculo-tendinous unit, may be exacerbated by plantarflexor muscle overactivity related to the brain injury itself. DESIGN: Three variables representing resistance to passive lengthening and soleus muscle length were compared between subjects with recent brain injury and age matched normal controls. Comparison between limbs was made for subjects with unilateral neurological impairment. METHODS: Slow passive dorsiflexion stretches were performed using a computer controlled dynamometer. Muscle stiffness in the initial and latter portion of the range, and the angles achieved at torques of 5 and 10 N m were determined from torque-angle curves. Maximal ankle dorsiflexion with the knee flexed was considered to reflect soleus muscle length. RESULTS: Significant differences were demonstrated for all variables, except passive stiffness near the end of available range. The limb ipsilateral to unilateral brain injury differed from control limbs in that significantly less passive range of dorsiflexion was available and initial resistance to passive stretch was significantly less. CONCLUSIONS: The reduction in soleus muscle length evident in subjects with recent acquired brain injury, even in neurologically unaffected limbs, may reflect the influence of relative immobility. Although plantarflexor muscle overactivity was found to be associated with increased resistance to slow passive stretch, the mechanism was unable to be elucidated from these data. The limb ipsilateral to unilateral neurological impairment cannot be considered to be a 'normal' control for comparative purposes. RELEVANCE: Adaptive shortening and increased resistance to passive lengthening limit active ankle dorsiflexion, and alter ankle biomechanics. Tonic muscle overactivity has the potential to exacerbate these changes. Prophylactic management of inappropriate muscle activity and maintenance of muscle length may facilitate the achievement of rehabilitation goals and reduce subsequent disability following acquired brain injury.

Adolescent↗
PubMed2001

Serial plaster casting to correct equino-varus deformity of the ankle following acquired brain injury in adults.

PURPOSE: Proposed mechanisms via which serial casting might effect increased joint range, muscle extensibility and reduced reflex excitability are outlined in this review. Support for these mechanisms stems largely from animal experimental studies. The applicability of these data to human muscle is unknown. ISSUES: Equino-varus deformity of the ankle is a common secondary complication of acquired brain injury. It results from a combination of sequelae of the brain injury and subsequent immobility, including hypertonia, reduced muscle length and increased stiffness. Some evidence exists for the efficacy of serial plaster casts in the treatment of equino-varus deformity, although most reported studies are uncontrolled and involve small numbers of subjects. Serial casting has been shown to result in decreased resistance to passive lengthening and a reduction in dynamic reflex excitability within the lengthened muscles in children with cerebral palsy. Currently documented effects of serial casting in brain injured adults are limited to changes in range of maximal passive dorsiflexion. CONCLUSION: Serial casting should be considered as an adjunct to therapy aimed at improving functional mobility. A variety of therapeutic interventions have been used to augment the effect of the casting regime. Factors that have been demonstrated to be associated with a favourable outcome from serial casting, and recommendations for future research are also discussed in this review.

Brain Injuries↗
PubMed2001

Reflex and non-reflex elements of hypertonia in triceps surae muscles following acquired brain injury: implications for rehabilitation.

BACKGROUND: Following adult onset acquired brain injury, the triceps surae muscles tend to become shortened and exhibit increased resistance to passive lengthening; a phenomenon that has been termed 'hypertonia'. Spasticity (velocity dependent tonic reflex hyper-excitability) has traditionally been considered a major component of hypertonia. In addition, unmodulated descending excitatory influences on the alpha motorneurone pool may result in inappropriate or excessive muscle activity (dystonia). Non-reflex changes, secondary to the brain injury, and as a consequence of subsequent immobility, also take place in the passive and active elements of the muscle. These non-reflex changes affect the stiffness and extensibility of the musculo-tendinous unit. Atrophy of muscle fibres combines with collagen proliferation to produce increased muscle stiffness. This may be compounded by increased actin-myosin cross-bridge linkages, which are thought to be associated with reduced rates of cross-bridge detachment. Prolonged immobilization in a shortened position results in a loss of sarcomeres in series. Arthrogenic changes associated with disuse include remodelling of dense connective tissue and intra-articular adhesions. CONCLUSION: Decreased muscle extensibility may be exacerbated by muscle overactivity. Consideration of all of the potential factors contributing to hypertonia of the triceps surae muscle will assist clinicians to identify appropriate intervention strategies, which may facilitate better treatment outcomes.

Brain Injuries↗
PubMed2001

Influence of standardized mobilization on the posteroanterior stiffness of the lumbar spine in asymptomatic subjects.

BACKGROUND AND PURPOSE: Spinal mobilization is commonly used to relieve pain and assist recovery of mobility in individuals with low back pain. Fundamental to this concept is the belief that spinal mobilization will influence the mechanical properties of the symptomatic motion segment. The objective of the present study was to examine the segmental effects of a standardized mobilization procedure on the posteroanterior (PA) stiffness of the lumbar spine. METHOD: Audio and visual feedback was used to train a physiotherapist to perform PA mobilization at a consistent load and frequency. After training, twenty-four subjects without low back pain were recruited for the intervention phase of the study. The spinal posteroanterior mobilization (SPAM) apparatus was used to measure the PA stiffness of the lumbar spine at three measurement sites (L1, L3 and L5). The trained physiotherapist then applied the standardized PA mobilization technique via the L3 spinous process for two minutes. Following mobilization, PA stiffness was measured three times at the three locations. RESULTS: The physiotherapist was able to apply a standardized mobilization with a mean force of 146 N (standard deviation (SD) 8 N) at a frequency of 1.5 Hz. The first trial on each assessment demonstrated a pre-condition effect. Two minutes' PA mobilization resulted in no significant change in the PA stiffness of the lumbar spine at the level to which the mobilization was applied, or at the L1 and L5 segments. The 95% confidence intervals (CI) of the difference in PA stiffness before and after testing included zero at each measurement site. CONCLUSIONS: Clinicians should pre-condition the spine when assessing PA stiffness both before and after interventions. A standardized mobilization of 150 N at 1.5 Hz for two minutes had no segmental effect on spinal PA stiffness. Subsequent studies need to consider other mechanisms that may contribute to the changes that occur after PA spinal mobilization.

Adult↗
PubMed2001

A large arteriovenous malformation of the external ear in an adult: report of a case and approach to management.

OBJECTIVES/HYPOTHESIS: Arteriovenous malformations involving the external ear are relatively uncommon in adults. Arteriovenous malformations in the head and neck can poses difficult therapeutic challenges. STUDY DESIGN: Case report. METHODS: We report a case of a large arteriovenous malformation in the external ear in a 41-year-old man. RESULTS: Extensive enlargement of the previously latent and asymptomatic arteriovenous malformation was triggered by blunt trauma to the ear. CONCLUSION: We discuss the definition, clinical findings, diagnostic approaches, and therapeutic management of arteriovenous malformations.

Adult↗
PubMed2000

Transgenic plants as factories for biopharmaceuticals.

Plants have considerable potential for the production of biopharmaceutical proteins and peptides because they are easily transformed and provide a cheap source of protein. Several biotechnology companies are now actively developing, field testing, and patenting plant expression systems, while clinical trials are proceeding on the first biopharmaceuticals derived from them. One transgenic plant-derived biopharmaceutical, hirudin, is now being commercially produced in Canada for the first time. Product purification is potentially an expensive process, and various methods are currently being developed to overcome this problem, including oleosin-fusion technology, which allows extraction with oil bodies. In some cases, delivery of a biopharmaceutical product by direct ingestion of the modified plant potentially removes the need for purification. Such biopharmaceuticals and edible vaccines can be stored and distributed as seeds, tubers, or fruits, making immunization programs in developing countries cheaper and potentially easier to administer. Some of the most expensive biopharmaceuticals of restricted availability, such as glucocerebrosidase, could become much cheaper and more plentiful through production in transgenic plants.

Antibodies↗
PubMed1999

Immune response to rotavirus VP4 expressed in an attenuated strain of Shigella flexneri.

An attenuated strain of Shigella flexneri was utilised to express viral protein (VP) 4 of rotavirus and the immunogenicity of the recombinant constructs was studied in BALB/c mice. VP4 was expressed as a fusion with maltose binding protein (MBP) in both the cytoplasm and periplasm, with a much higher level of expression occurring in the former. While all constructs induced a Shigella-specific response in mice, only the construct expressing MBP-VP4 in the cytoplasm of Shigella stimulated an immune response specific to rotavirus. This study demonstrates that Shigella can be used to deliver rotavirus antigens and induces an immune response directed towards both rotavirus and Shigella.

Animals↗
PubMed1999

Serotype 1a O-antigen modification: molecular characterization of the genes involved and their novel organization in the Shigella flexneri chromosome.

The factors responsible for serotype 1a O-antigen modification in Shigella flexneri were localized to a 5.8-kb chromosomal HindIII fragment of serotype 1a strain Y53. The entire 5.8-kb fragment and regions up- and downstream of it (10.6-kb total) were sequenced. A putative three-gene operon, which showed homology with other serotype conversion genes, was identified and shown to confer serotype 1a O-antigen modification. The serotype conversion genes were flanked on either side by phage DNA. Multiple insertion sequence (IS) elements were located within and upstream of the phage DNA in a composite transposon-like structure. Host DNA homologous to the dsdC and the thrW proA genes was located upstream of the IS elements and downstream of the phage DNA, respectively. The sequence analysis indicates that the organization of the 10.6-kb region of the Y53 chromosome is unique and suggests that the serotype conversion genes were originally brought into the host by a bacteriophage. Several features of this region are also characteristic of pathogenicity islands.

Chromosomes, Bacterial↗
PubMed1998

Kimura's disease: two case reports and a literature review.

Kimura's disease (KD) is an allergic, inflammatory disorder of unknown cause. The typical presentation is in a young Oriental male with nontender subcutaneous swellings in the head and neck region, lymphadenopathy, peripheral eosinophilia, and elevated serum IgE. Many patients with KD also develop renal involvement. Treatment options range from conservative observation for asymptomatic patients to surgical excision, steroid therapy, and radiotherapy for symptomatic patients. Two cases of KD are presented. These emphasize not only the variability of disease presentation, but also the pitfalls of therapy leading to frequent recurrences. The literature is reviewed, summarizing patient presentation, differential diagnosis, current theories on causation, and therapy. A treatment protocol is also proposed.

Adolescent↗
PubMed1998

Reliability of the peroneal latency in normal ankles.

The aim of this study was to determine the reliability of peroneal latency between trials and to compare the right and left sides in healthy subjects. A dual platform trapdoor was constructed to tilt the right or the left ankle by 30 degrees in the frontal plane. Electromyography recorded the onset of peroneal activity in response to this perturbation, and an accelerometer detected the onset of trapdoor movement. Peroneal latency was determined algorithmically as the time difference between the onset of trapdoor movement and the onset of peroneal activity. Twenty trials were recorded from 31 right leg dominant, healthy subjects. Peroneal latency was measured successfully for 613 trials (99% success rate). A repeated measures analysis of variance showed a main effect for side with the right, dominant side recording a longer latency. Intertrial reliability of the peroneal latency was high (intraclass correlation coefficients for the right and left legs were .91 and .82, respectively). Thus, peroneal latency is a reliable measure of the polysynaptic reflex ares involved in the response of these muscles to a sudden inversion stress.

Adult↗
PubMed1997

Activation of HER4 by heparin-binding EGF-like growth factor stimulates chemotaxis but not proliferation.

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a potent mitogen and chemotactic factor for fibroblasts, smooth muscle cells and keratinocytes. It is demonstrated that HB-EGF is not only a ligand for HER1, as previously reported, but for HER4 as well. HB-EGF binds to NIH 3T3 cells overexpressing either HER1 or HER4 alone, but not HER2 or HER3 alone. Binding to HER4 is independent of HER1. The ability of HB-EGF to bind to two different receptors is in contrast to EGF which binds to HER1, but not to HER4, and heregulin-beta1 which binds to HER4, but not to HER1. Besides binding, HB-EGF activates HER4. For example (i) it induces tyrosine phosphorylation of HER4 in cells overexpressing this receptor and of endogenous HER4 in MDA-MB-453 cells and astrocytes; (ii) it induces association of phosphatidylinositol 3-kinase (PI3-K) activity with HER4; and (iii) it is a potent chemotactic factor for cells overexpressing HER4. Chemotaxis is inhibited by wortmannin, a PI3-K inhibitor, suggesting a possible role for PI3-K in mediating HB-EGF-stimulated chemotaxis. On the other hand, HB-EGF is not a mitogen for cells expressing HER4, in contrast to its ability to stimulate both chemotaxis and proliferation in cells expressing HER1. It was concluded that HER4 is a newly described receptor for HB-EGF and that HB-EGF can activate two EGF receptor subtypes, HER1 and HER4, but with different biological responses.

3T3 Cells↗
PubMed1997

A suicide vector for allelic recombination involving the gene for glutamate 1-semialdehyde aminotransferase in the cyanobacterium Synechococcus PCC 7942.

Gabaculine (2,3-dihydro 3-amino benzoic acid) is a potent inhibitor of tetrapyrrole biosynthesis in organisms that use the C5 pathway for the synthesis of delta-aminolaevulinic acid. Glutamate semialdehyde aminotransferase (GSA-AT), the enzyme catalysing the formation of this key precursor of tetrapyrroles, is normally inhibited by concentrations of gabaculine in the order of 5 microM. However, in Synechococcus 6301 strain GR6, a cyanobacterium that is resistant to 100 microM gabaculine, this enzyme has undergone two changes in structure: a deletion of three amino acids from positions 5 to 7 and the substitution of isoleucine for methionine at position 248. To establish the effect in vivo of these specific changes in the gene for GSA-AT (hemL), a suicide vector (pHS7) containing an antibiotic cassette was constructed to achieve the replacement, by homologous recombination, of the wild-type hemL gene in the chromosome by a modified form of the gene. Recombinant strains of Synechococcus 7942 obtained using pHS7-hemLGR6 were indistinguishable from Synechococcus 6301 GR6 in terms of the resistance of growth and of chlorophyll accumulation to high concentrations of gabaculine, while a wild-type recombinant produced using pHS7-hemLWT had retained its sensitivity. Southern hybridisation using gene probes for hemL, ampr and cmr confirmed that chromosomal integration of the plasmids had occurred in both WT and GR6 recombinants. Growth and chlorophyll accumulation in equivalent strains with the hemL gene containing either the deletion or the transition characteristic of Synechococcus 6301 GR6 were inhibited by 10 microM gabaculine. Consequently, resistance in vivo to high concentrations of this compound is dependent on both the changes in gene/enzyme structure. This investigation has established the effectiveness of the suicide vector pHS7 for studying the effect in vivo of specific changes in the hemL gene. It has also demonstrated that replacement of the wild-type gene by that from Synechococcus 6301 GR6 is sufficient to confer resistance in vivo to high concentrations of gabaculine.

Alleles↗