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Biomedical subjects

J A Macdonald

Publications and source records attributed to J A Macdonald.

At least 19 recordsLinked to original sources
PubMed2005

Measurement of the Michel parameter rho in muon decay.

The TWIST Collaboration has measured the Michel parameter rho in normal muon decay, mu(+)--> e(+)nu(e)nu (mu). In the standard model, rho = 3/4. Deviations from this value imply mixing of left- and right-handed muon and electron couplings. We find rho=0.750 80+/-0.000 32(stat) +/- 0.000 97(syst) +/- 0.000 23, where the last uncertainty represents the dependence of rho on the Michel parameter eta. This result sets new limits on the W(L)-W(R) mixing angle in left-right symmetric models.

Journal Article↗
PubMed2004

New limit on the T-violating transverse muon polarization in K+-->pi0mu+nu decays.

A search for T-violating transverse muon polarization (P(T)) in the K+-->pi(0)mu(+)nu decay was performed using kaon decays at rest. A new improved value P(T)=-0.0017+/-0.0023(stat)+/-0.0011(syst) was obtained giving an upper limit |P(T)|<0.0050. The T-violation parameter was determined to be Imxi=-0.0053+/-0.0071(stat)+/-0.0036(syst) giving an upper limit |Imxi|<0.016.

Journal Article↗
PubMed2004

Improved measurement of the K+-->pi+nunu; branching ratio.

An additional event near the upper kinematic limit for K+-->pi(+)nunu; has been observed by experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is B(K+-->pi(+)nunu;)=(1.47(+1.30)(-0.89))x10(-10) based on three events observed in the pion momentum region 211<P<229 MeV/c. At the measured central value of the branching ratio, the additional event had a signal-to-background ratio of 0.9.

Journal Article↗
PubMed2002

Further evidence for the decay K+ -->pi+nu(nu).

Additional evidence for the rare kaon decay K+-->pi+nu(nu) has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, 211 pi+nu(nu)) = 1.57(+1.75)(-0.82)x10(-10).

Journal Article↗
PubMed2001

Precise half-life measurement for the superallowed 0(+)-->0(+) beta emitter (74)Rb: first results from the new radioactive beam facility (ISAC) at TRIUMF.

Presently, the world data for superallowed beta decay leads to a result in disagreement (at the 98% confidence level) with the predictions of the minimal standard model for the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. Precise data for the superallowed 0(+)-->0(+) beta decay of (74)Rb would provide a critical test of the nucleus-dependent isospin symmetry-breaking corrections that must be calculated for these superallowed Fermi beta decays. The present work reports the first precise measurement of the half-life for (74)Rb ( t(1/2) = 64.761+/-0.031 ms). The data were obtained at the radioactive beam facility (ISAC) at TRIUMF using a beam of approximately 4000 (74)Rb ions s(-1).

Journal Article↗
PubMed2001

Temperature compensation of postsynaptic currents from the extraocular muscle of temperate teleost fishes.

Miniature end plate currents were recorded from white inferior oblique extraocular muscle fibres of one temperate marine teleost (Aldrichetta forsteri, Family Mugilidae) and two temperate freshwater teleosts (Galaxias fasciatus, Family Galaxiidae and Oncorhynchus mykiss, Family Salmonidae). Miniature end plate currents were digitised and averaged over a temperature range of 5-25 degrees C. For each species, decay of miniature end plate currents was exponential and exhibited a strong temperature dependence. Lower temperatures resulted in prolonged decay phases, which decreased exponentially as a function of absolute temperature. Although values of the exponential time constant tau (tau) obtained for each species at 5 degrees C, 15 degrees C and 25 degrees C were significantly different, at any given temperature, there were no significant differences between tau values for the three species, despite differences in phylogeny (different families) and habitat (marine versus freshwater). At their normal temperature of 15 degrees C, mean values of tau for the three species ranged from 840 micros to 940 micros, and apparent activation energies ranged from -41 kJ mol(-1) deg(-1) to 50 kJ mol(-1) deg(-1). These observations confirm earlier reports that teleost miniature end plate currents are consistently shorter than those of other vertebrates.

Adaptation, Physiological↗
PubMed1996

Mitochondrial phylogeny of trematomid fishes (Nototheniidae, Perciformes) and the evolution of Antarctic fish.

The subfamily of fishes Trematominae is endemic to the subzero waters of antarctica and is part of the longer notothenioid radiation. Partial mitochondrial sequences from the 12S and 16S ribosomal RNA (rRNA) genes and a phylogeny for 10 trematomid species are presented. As has been previously suggested, two taxa, Trematomus scotti and T. newnesi, do not appear to be part of the main trematomid radiation. The genus Pagothenia is nested within the genus Trematomus and has evolved a unique cyropelagic existence, an association with pack ice. Using a mitochondrial rRNA molecular clock rate of 0.14% transversion changes per million years, the average age of the trematomids is estimated at 3.4 million years (MY). If the age of the trematomids is approximately 3.4 MY, this group could have speciated during the period of deglaciation in Antarctica 2.5-4.8 million years ago. This era was marked by significant changes on the Antarctic shores, such as the opening of fjords, which might have provided a stimulus for specification.

Animals↗
PubMed1992

Antarctic fishes survive exposure to carbon monoxide.

The extensive in vivo conversion of haemoglobin to the carbon monoxide derivative has no discernible effect on the survival of the red-blooded Antarctic fish Pagothenia bernacchii. Analysis of caudally sampled blood of cannulated specimens revealed that reconversion of carbon-monoxy haemoglobin to oxyhaemoglobin was complete within 48 hours. Thus, under stress-free conditions, haemoglobin is not necessary for survival of P. bernacchii. Red-blooded Antarctic fishes can carry oxygen necessary for routine delivery dissolved in plasma, in a similar way to the haemoglobinless Channichthyidae, although they lack the morphological and physiological adaptations which allow the latter to prosper without any haemoglobin.

Animals↗
PubMed1991

Adaptation of sleep and circadian rhythms to the Antarctic summer: a question of zeitgeber strength.

Adaptation of sleep and circadian rhythms was examined in three temperate zone dwellers arriving in Antarctica during summer. Rectal temperature, wrist activity, and heart rate were monitored continuously, sleep timing and quality noted on awakening, and mood and fatigue rated every 2 h while awake. Sleep was poorer in 2/3 subjects in Antarctica, where all subjects reported more difficulty rising. Sleep occurred at the same clock times in New Zealand and Antarctica, however, the rhythms of temperature, activity, and heart rate underwent a delay of about 2 h. The subject with the most Antarctic experience had the least difficulty adapting to sleeping during constant daylight. The subject with the most delayed circadian rhythms had the most difficulty. The delay in the circadian system with respect to sleep and clock time is hypothesized to be due to differences in zeitgeber strength and/or zeitgeber exposure between Antarctica and New Zealand.

Activities of Daily Living↗
PubMed1990

Effects of temperature on nervous system: implications for behavioral performance.

Temperature change has a major impact on the function of the nervous system and its components, including altering synaptic gain and changing synaptic and conduction delays. Although many animals are subject to changes in body temperature, the degree of temperature change actually experienced by many poikilotherms is not well documented. The fact that many animals continue to exhibit coordinated sensorimotor function during changes in body temperature indicates that some form of temperature compensation has occurred within the nervous system. Compensations may occur automatically (opposing effects of temperature offsetting each other), they may be an inherent property of closed-loop systems, or they may be effected by more sophisticated control mechanisms such as those of the vertebrate cerebellum.

Action Potentials↗
PubMed1989

Theoretical principles of optical pachometry.

Various configurations have been suggested for the optical measurement of corneal thickness by slit beam doubling, and corresponding ray traces have been provided to relate actual thickness to apparent thickness. In this report, a set of equations is provided which apply to the general situation in which neither the angle of illumination nor the angle of observation are aligned with or symmetrical about the normal to the cornea. An explicit solution to these equations giving true corneal thickness in terms of apparent thickness cannot be established, and an iterative procedure is used to develop a relationship for a number of different pachometer configurations. The near linearity of these relationships suggests the existence of a simple equation to relate these entities. By making appropriate assumptions, a linear approximation relating the inverse of the true thickness to the inverse of the apparent thickness is developed. An equation for determining the location of measurements of peripheral corneal thickness is also derived. The equations presented here may be generally applied to all optical pachometer configurations, can be used to estimate the relative contributions of various sources of error in pachometry and provide a basis for obtaining uniform absolute derivations of corneal thickness.

Cornea↗
PubMed1986

Temperature dependence of neurotransmitter release in the antarctic fish Pagothenia borchgrevinki.

The quantal contents of endplate potentials from extraocular muscles of an antarctic fish Pagothenia borchgrevinki were measured over a range of temperatures. Quantal release was maximal at about 5 degrees C but showed little dependence on temperature between -2 degrees C and 10 degrees C. Above 10 degrees C quantal content declined until release ceased about 18 degrees C. In view of the fact that the ambient temperature at which these fish live is constant at -1.9 degrees C, the results suggest that Pagothenia borchgrevinki is only partially adapted to its environment despite 25 million years acclimatization.

Animals↗
PubMed1985

Oculomotor function at low temperature: antarctic versus temperate fish.

The peripheral oculomotor system can be modelled as a first order linear system (Montgomery, 1983), and hence specified by its characteristic frequency and 'd.c.' gain. These parameters can be determined by recording eye movements produced by stimulation of the abducens nerve with sinusoidally modulated pulse trains, and compare well with those independently derived from the relationship between motoneurone firing and spontaneous eye movement. Characteristic frequency and gain of the peripheral oculomotor system were determined for two species of antarctic teleost and one temperate species, to examine temperature compensation within a complete motor output pathway. Compared with low temperature function in temperate fish, the characteristic frequency is clearly temperature compensated in antarctic fish, which explains in part the observed temperature compensation of their rapid eye movement. The 'd.c.' gain of the peripheral oculomotor system is inversely related to temperature, providing an automatic compensation for possible reductions in central nervous system output and sensory gain at low temperature.

Animals↗