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ScienceWeek
>SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
September 29, 2000 -- Vol. 4 Number 39
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What would have happened if Darwin and Einstein
as young men had needed to apply for government
support? Their probability of getting past the
grant reviewers would be similar to a snowball
surviving in Hell.
-- Craig Loehle
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. SCIENCE POLICY:
RACIAL AND ETHNIC DISPARITIES IN FACULTY PROMOTION
IN ACADEMIC MEDICINE
Several studies have demonstrated that female faculty in US
medical schools are less likely than men to be promoted to senior
rank, and now a new study of 50,145 full-time US medical school
faculty who became assistant professors or associate professors
between 1980 and 1989 confirms that racial and ethnic minority
faculty in US medical schools are promoted at lower rates
compared with white faculty.
(J. Amer. Med. Assoc. 6 Sep 00 284:1085)
2. MEDICAL BIOLOGY:
EVIDENCE FOR AN ABSENT SPECIES BARRIER IN PRION DISEASE
Transmission of prions between mammalian species is thought to be
limited by a "species barrier", which depends on differences in
the primary structure of prion proteins in the infecting inoculum
and in the host. Now there is new evidence that a strain of
hamster prions thought to be nonpathogenic for conventional mice
leads to prion replication to high levels in such mice -- but
without causing clinical disease in the mice. The authors suggest
their results demonstrate the existence of subclinical forms of
prion infection with important public health implications, and
that current definitions of the "species barrier", which have
been based on clinical end-points, need to be fundamentally
reassessed. (Proc. Natl. Acad. Sci. US 29 Aug 00 97:10248)
3. PHYSIOLOGY:
ON THE PHYSIOLOGY OF EXERCISE
Underlying the various beneficial effects of physical exercise on
the health of the human body are a constellation of physical,
biochemical, and physiological factors that have been intensively
studied for more than a century. At present, maximal oxygen
consumption is the primary measure of exercise capacity, and
mechanisms related to the delivery of oxygen to the muscles are
considered to be the main factors determining exercise capacity.
(New England J. Med. 31 Aug 00 343:633)
4. ASTROPHYSICS:
ON SUPERMASSIVE BLACK HOLES
In recent years, astrophysicists have come to distinguish
ordinary black holes and supermassive black holes, and these
latter objects now reign as the supreme cosmic exotica, with
masses 1 million to nearly 10 billion times the mass of the Sun,
and an origin completely unknown. The center of our own Galaxy is
apparently a supermassive black hole, and observations indicate
that supermassive black hole growth and galaxy formation in
general are closely linked. Supermassive black holes are becoming
an integral part of our understanding of galaxy formation.
(Science 1 Sep 00 289:1484)
5. MATERIALS SCIENCE:
REVERSIBLE CRYSTAL SEQUESTRATION OF GASES
Porous materials such as zeolites and clays have been widely used
by the petrochemicals industry to bind various kinds of small
molecules, and the binding ability of robust inorganic zeolites
has resulted in a repertoire of designed porous structures
potentially strong enough to withstand the formation of holes in
their structure as molecular "guests" enter and leave the "host"
framework. Now new studies demonstrate that solid materials that
can reversibly bind small molecular guests do not need to be
porous. (Nature 31 Aug 00 406:970)
6. MATERIALS SCIENCE:
GEL CATALYSTS THAT SWITCH ON AN OFF
Natural enzymes catalyze chemical reactions and regulate such
reactions by reversibly and repeatedly switching their catalytic
activities on and off. A research group now reports the
development of a polymer gel with a catalytic activity that can
be switched on and off when the solvent composition is changed.
The gel consists of two species of monomers. The major component,
N-isopropylacrylamide, makes the gel swell and shrink in response
to a change in composition of ethanol/water mixtures. The minor
component, vinylimidazole, which is capable of catalysis, is
copolymerized into the gel network.
(Proc. Natl. Acad. Sci. US 29 Aug 00 97:9861)
7. IN FOCUS: ON THE EVOLUTION OF HUMAN BEHAVIOR
8. FROM THE SCIENCEWEEK ARCHIVE:
ON SCIENCE, POLITICS, AND ASTEROIDS
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Section 2
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1. SCIENCE POLICY:
RACIAL AND ETHNIC DISPARITIES IN FACULTY PROMOTION
IN ACADEMIC MEDICINE
From a purely rational perspective, the last place one would
expect to find racial, ethnic, and gender discrimination is in
professional academia, an arena where merit is assumed (and
touted) as the basis for advancement. Unfortunately, in the US,
this assumption concerning merit is an apparent myth. Several
studies have demonstrated that female faculty in US medical
schools are less likely than men to be promoted to senior rank,
and now a new study confirms that racial and ethnic minority
faculty in US medical schools are promoted at lower rates
compared with white faculty.
... ... D. Fang et al (3 authors (Association of American Medical
Colleges, US) present a study designed to compare promotion rates
of minority and white medical school faculty in the US. The study
involved a total of 50,145 full-time US medical school faculty
who became assistant professors or associate professors between
1980 and 1989. Faculty of historically black and Puerto Rican
medical schools were excluded from the study. The authors
tabulated attainment of associate or full professorships among
assistant professors and full professorships among associate
professors by 1997, among white, Asian or Pacific Islanders,
"underrepresented minority" (including black, Mexican American,
Puerto Rican, Native American, and Native Alaskan), and other
Hispanic faculty.
1) The authors report the following results:
Assistant Professors Promoted by 1997:
--------------------------------------
White 46 percent
Asian or Pacific Islanders 37
Underrepresented Minorities 30
Other Hispanic 43
Associate Professors Promoted by 1997:
--------------------------------------
White 50 percent
Asian and Pacific Islanders 44
Underrepresented Minorities 36
Other Hispanic 43
2) The authors report that racial/ethnic disparities in
promotion were evident among tenure and nontenure faculty and
among faculty who received and did not receive National
Institutes of Health research awards. After adjusting for cohort,
sex, tenure status, degree, department, medical school type, and
receipt of NIH awards, underrepresented minority faculty remained
less likely to be promoted compared with white faculty (relative
risk 0.68 for assistant professors and 0.81 for associated
professors). Asian and Pacific Islander assistant professors also
were less likely to be promoted (relative risk 0.91), whereas
Asian and Pacific Islander associate professors and other
Hispanic assistant and associate professors were promoted at
comparable rates.
3) The authors state: "The major finding of this study is
that racial/ethnic minority faculty, at both the assistant and
associate professor rank, are lagging in rates of promotion
compared with white faculty, even though their representation in
academic medicine has steadily increased over time."
-----------
D. Fang et al: Racial and ethnic disparities in faculty promotion
in academic medicine.
(J. Amer. Med. Assoc. 6 Sep 00 284:1085)
QY: Ernest Moy emoy@aamc.org
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Summary by SCIENCE-WEEK http://scienceweek.com 29Sep00
For more information: http://scienceweek.com/swfr.htm
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Related Background:
MINORITY FACULTY AND ACADEMIC RANK IN MEDICINE
Recent efforts to improve the representation of minority faculty
in US academic medicine have focused on increasing the number of
minority physicians who pursue academic careers. However, the
number of minority students entering US medical schools has
plateaued, despite efforts to achieve racial and ethnic diversity
in these schools. At the present time, only 3.9 percent of all US
medical faculty identify themselves as black, Native American,
Mexican American, or Puerto Rican, and these groups have been
classified as underrepresented in medicine compared to their
representation in the general population. Asian Americans are
currently not classified as underrepresented. ... ... A. Palepu
et al now report a study to determine whether minority faculty
were as likely as majority faculty to have attained senior rank
(associate professor or full professor) after adjusting for other
factors that typically influence promotion. The study consisted
of a self-administered mailed survey of US medical school faculty
using the Association of American Medical Colleges database, the
sample stratified by department, graduation cohort, and sex. Of
1807 respondents, 54 percent had attained senior academic rank.
The authors report that after adjusting for the medical school,
department, years as medical school faculty, number of peer-
reviewed publications, receipt of research grant funding,
proportion of time in clinical activities, sex, and tenure
status, the odds ratios relative to white faculty of holding
senior rank were 0.33 for black faculty, 0.36 for Hispanic
faculty, and 0.58 for Asian faculty. The authors conclude that
minority faculty are less likely than white faculty to hold
senior academic rank, and they suggest this finding is not
explained by potential confounders such as years as a faculty
member or measures of academic productivity. The authors further
suggest that "medical school deans and department heads need to
foster and provided greater support for the careers of minority
faculty to ensure their equitable representation at all levels in
academic medicine."
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A. Palepu et al (6 authors at 2 installations, US): Minority
faculty and academic rank in medicine.
(J. Amer. Med. Assoc. 2 Sep 98 280:767)
QY: Anita Palepu anita@hivnet.ubc.ca
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Summary by SCIENCE-WEEK http://scienceweek.com 25Sep98
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Related Background:
US WOMEN IN SCIENCE LESS ACCEPTED THAN WOMEN IN BUSINESS
A meeting last month at the New York Academy of Sciences (US)
focused on the past 25 years of progress for women scientists and
engineers in the US. The apparent consensus at the conference was
that the climate for women in both industry and government has
improved much faster than in academia, with the changes in
industry mostly due to a drive by industry for diversity. The
Motorola Corporation (US) now has 43 women vice-presidents. In
1996, women were 51% of the US population, 46% of the labor
force, but only 22% of scientists and engineers. Nobel Laureate
Gertrude Elion advised women in science to follow Farragut's
order: "Damn the torpedoes -- full speed ahead."
(Chem. & Eng. News 6 Apr 98) (Science-Week 17 Apr 98)
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Related Background:
SWEDISH STUDY SHOWS SEX BIAS AFFECTS SCIENCE EMPLOYMENT
Two female Swedish scientists have published a study which
indicates that women must publish more often than men to compete
successfully for scientific jobs in Sweden. This is the first
scientific study of sex discrimination in the awarding of a
large number of research positions, and was financed by a
Swedish government grant. The researchers, Christine Wenneras
and Agnes Wold, are both from the University of Goteborg. The
journal Nature, which published the study, states that
the "results severely undermine the credibility of the
peer-review system, not just in Sweden but elsewhere in the
world. (Nature 22 May 97) (Science-Week 22 May 97)
For more information: http://scienceweek.com/swfr.htm
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2. MEDICAL BIOLOGY:
EVIDENCE FOR AN ABSENT SPECIES BARRIER IN PRION DISEASE
The prion diseases include *Creutzfeldt-Jakob disease and *kuru
in humans, and *scrapie and *bovine spongiform encephalopathy in
animals. All these diseases are transmissible to the same species
by inoculation with, or dietary exposure to, infected tissues.
According to the protein-only hypothesis, prions are composed
principally or entirely of abnormal isoforms of a host-encoded
glycoprotein, known as "prion protein" (PrP). The disease-related
isoform [PrP(supSc)] is derived from a normal cellular precursor
by a process subsequent to protein biosynthesis (i.e., a post-
translational process) that involves a conformational change, and
is distinguished biochemically by its partial *protease
resistance and detergent insolubility. The disease-related
isoform is hypothesized to act as a conformational template that
promotes the conversion of normal protein to disease-related
protein. Transmission of prions between mammalian species is
thought to be limited by a "species barrier", which depends on
differences in the primary structure of prion proteins in the
infecting inoculum and in the host.
... ... A.F. Hill et al (6 authors 2 installations, UK) now
report a study of inter-species prion transmission, the authors
making the following points:
1) The authors report a demonstration that a strain of
hamster prions thought to be nonpathogenic for conventional mice
leads to prion replication to high levels in such mice -- but
without causing clinical disease in the mice. Prions subsequently
pathogenic in both mice and hamsters are produced.
2) The authors suggest their results demonstrate the
existence of subclinical forms of prion infection with important
public health implications, both with respect to *iatrogenic
transmission from apparently healthy humans and dietary exposure
to cattle and other species exposed to bovine spongiform
encephalopathy prions. The authors suggest current definitions of
the "species barrier", which have been based on clinical end-
points, need to be fundamentally reassessed.
-----------
A.F. Hill et al: Species-barrier-independent prion replication in
apparently resistant species.
(Proc. Natl. Acad. Sci. US 29 Aug 00 97:10248)
QY: John Collinge: j.collinge@ic.ac.uk
-----------
Text Notes:
... ... *Creutzfeldt-Jakob disease: Until 30 years ago,
Creutzfeldt-Jakob disease was an obscure form of dementia unknown
to most physicians. The name is now familiar to the medical
community as the major *transmissible spongiform encephalopathy
(or prion disease) in humans, and familiar to research scientists
because of its strange causative agent (prions) that exhibit
apparently novel modes of replication and transmission.
... ... *kuru: This disease is similar to Creutzfeldt-Jakob
disease, and is a human spongiform encephalopathy. (The term
"spongiform" refers to the sponge-like appearance of the infected
brain.) Kuru occurs only in the easter highlands of New Guinea,
occurs more frequently in women than in men, which apparently
coincides with the customs surrounding cannibalism in a society
where the remains of dead relatives are handled and eaten
primarily by children and women. After cannibalism was outlawed,
the incidence of the disease decreased, and the current consensus
is that cannibalism was the primary mode of transmission of the
pathological agent.
... ... *scrapie: This is one animal form of Creutzfeldt-Jakob
disease. Susceptibility to scrapie varies among different breeds
of sheep, with goats 100 percent susceptible. The disease is
transmissible to laboratory monkeys, mice, and hamsters.
... ... *bovine spongiform encephalopathy: "Mad cow disease" is
similar to scrapie. In 1996, the new variant of Creutzfeldt-Jakob
disease (new-variant CJD) was recognized in the UK population,
primarily in younger people, the new disease with distinctive
pathological characteristics similar to those seen in macaque
monkeys infected with the agent of bovine spongiform
encephalopathy. The major present concern is that the pathogen of
bovine spongiform encephalopathy may have spread to humans in the
UK and continental Europe through ingestion of infected cattle
meat.
... ... *protease: In general, any enzyme that cleaves proteins,
usually by hydrolysis.
... ... *iatrogenic: In general, this denotes any result produced
by surgery or other treatment. In this context, and in the usual
usage, the term is used for a result that is unwanted and
injurious to the patient (e.g., an infection due to contaminated
surgical instruments).
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Sep00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
EVIDENCE FOR TRANSMISSION OF ANIMAL PRION DISEASE TO HUMANS
Prions are a class of poorly understood proteins implicated in a
number of exotic human neurological diseases and in some common
animal diseases such as sheep scrapie and bovine spongiform
encephalopathy in cattle ("mad cow disease"). Spongiform
encephalopathies are a type of brain disease found in humans and
animals and are characterized by macroscopic vacancies produced
by the disease process (the brain has a sponge-like appearance).
What is remarkable about prions is that they behave as infectious
agents, but they are 100 times smaller than viruses and their
mechanism of replication is unclear. One human disease in which
prions have been strongly implicated is Creutzfeldt-Jakob disease
(CJD), which appears to have a genetic basis in about 15% of the
cases. All the prion diseases are apparently associated with the
accumulation in the brain of an abnormal *protease-resistant
isoform of the prion protein PrP. In other words, an abnormal
variant of the normal PrP is somehow copied or produced by the
disease process, which can be initiated by introducing infectious
prion into the system.
... ... M.R. Scott et al (7 authors at 2 installations, US UK)
present transgenetic evidence for transmission of bovine
spongiform encephalopathy prions to humans, the authors making
the following points:
1) There is concern that bovine spongiform encephalopathy
may have been recently passed from cattle to humans, resulting in
approximately 50 cases of an atypical new variant of Creutzfeldt-
Jakob disease in teenagers and young adults. Epidemiological
findings, *gel electrophoresis of the prion protein, and
transmission to inbred mice and primates have each raised the
possibility of a link between bovine spongiform encephalopathy
and the new variant of Creutzfeldt-Jakob disease.
2) More than 175,000 cattle, primarily dairy cows, have died
of bovine spongiform encephalopathy over the past decade. How
many more cattle were exposed to BSE prions but slaughtered
before developing clinical signs is uncertain. Given the enormity
of the affected cattle population in the UK, a means of assessing
risks to the human population is paramount, and more sensitive
methods for the detection of prions are urgently needed. The
magnitude of the potential risk to the human population is still
speculative, but the death rates from new-variant-Creutzfeldt-
Jakob disease per year had remained approximately constant until
recently, when a disturbing high number of deaths from the
disease, a total of 9 new cases, was reported in the last quarter
of 1998. Although it is not yet known whether this trend will
continue, the possibility that a large section of the population
is at high risk must be seriously entertained.
3) The authors report their new studies indicate that
*transgenic mice expressing bovine prion protein *serially
propagate bovine spongiform encephalopathy prions, and that there
is no species barrier for transmission from cattle to mice. These
same mice were also highly susceptible to a new variant of
Creutzfeldt-Jakob disease and natural sheep scrapie. The
incubation times (approximately 250 days), neuropathology, and
disease-causing prion protein isoforms in the experimental mice
inoculated with human new-variant Creutzfeldt-Jakob disease and
bovine spongiform encephalopathy brain extracts were
indistinguishable and differed dramatically from those seen in
these mice with natural scrapie prions.
4) The authors suggest their findings provide the most
compelling evidence to date that prions from cattle with bovine
spongiform encephalopathy have infected humans and caused fatal
neurodegeneration.
[Editor's note: Although we do not disagree with the
interpretations and conclusions of the authors, it should be
pointed out that despite the title of their paper, the authors
provide direct evidence for transmission of new-variant
Creutzfeldt-Jakob disease from humans to mice, but evidence only
by implication for transmission from mice (or any other species)
to humans. The import of the work is the apparent experimental
absence of a significant species barrier to transmission of this
disease, at least in experiments involving human to animal
transmission in the reported population of transgenic mice.]
-----------
M.R. Scott et al: Compelling transgenetic evidence for
transmission of bovine spongiform encephalopathy prions to
humans.
(Proc. Natl. Acad. Sci. US 21 Dec 99 96:15137)
QY: Michael R. Scott: abbott@itsa.ucsf.edu
-----------
Text Notes:
... ... *protease-resistant isoform: A "protease" is any enzyme
that breaks down proteins by hydrolysis. In this context, an
"isoform" is any one of multiple forms of a functional protein
that differ in amino acid sequence and *electrophoretic mobility.
... ... *electrophoretic mobility: In general, electrophoresis is
a laboratory technique used to separate macromolecules on the
basis of electric charge and size, the technique involving
application of an electric field to a population of
macromolecules dispersing according to their electric mobilities.
... ... *gel electrophoresis: Gel electrophoresis is a type of
"zone electrophoresis" in which the supporting medium is a gel of
uniform concentration. In zone electrophoresis, a solution of
protein (or other molecules) is placed at the starting position
as a thin band or spot in an inert supporting medium (paper,
starch gel, polyacrylamide gel, etc.) containing buffer solution.
An electric potential is then applied to the supporting medium,
causing the proteins (or other substances) to migrate to give
distinct bands or zones which can be located in situ by staining,
light absorption, etc., or by analysis after elution of discrete
pieces of the supporting medium.
... ... *transgenic mice: A "transgenic mouse" is a mouse into
which genetic material from another organism has been
transferred, the transferred and incorporated new genes then
being expressed with the resultant production of specific
proteins.
... ... *serially propagate: The general paradigm for "serial
propagation" of a disease is as follows: After infection of
individual (or group) A by the disease, following the incubation
period, infectious material is removed from A and used to infect
B; then, after the incubation period, infectious material is
removed from B and used to infect C; and so on.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 21Jan00
For more information: http://scienceweek.com/search/search.htm
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3. PHYSIOLOGY:
ON THE PHYSIOLOGY OF EXERCISE
Underlying the various beneficial effects of physical exercise on
the health of the human body are a constellation of physical,
biochemical, and physiological factors that have been intensively
studied for more than a century. At present, maximal oxygen
consumption is the primary measure of exercise capacity, and
mechanisms related to the delivery of oxygen to the muscles are
considered to be the main factors determining exercise capacity.
... ... N.L. Jones and K.J. Killian (McMaster University, CA)
present a review of exercise limitation in health and disease,
the authors making the following points concerning the physiology
of exercise:
1) A fit 25-year-old man can generate 650 watts while
bicycling for a few seconds and can maintain a power of 400 watts
while bicycling for 1 minute, 230 watts while bicycling for 10
minutes, and 175 watts while bicycling for 30 minutes. He is able
to reach 275 watts in a progressive incremental (increasing at
16.7 watts per minute) test to capacity; this power represents
peak exercise and is the power at which maximal oxygen
consumption (3.3 liters per minute) is measured. To put these
figures in perspective, brisk walking represents an output of
approximately 50 watts of power and an oxygen intake of 0.8
liters per minute.
2) Exercise depends on the oxidation of carbohydrate and fat
for the regeneration of adenosine triphosphate required to
sustain muscular contraction. Ventilation, gas exchange, and the
circulation are adjusted to meet the requirements for delivery of
oxygen and removal of carbon dioxide. As the intensity of
exercise increases, the concentration of intramuscular creatine
phosphate decreases, and the concentrations of intramuscular
adenosine diphosphate, adenosine monophosphate, and inorganic
phosphate increase. Increases in the intramuscular lactate
concentration and decreases in the intramuscular potassium
concentration contribute to a marked decline in muscle pH to
below 6.5.
3) With prolonged submaximal exercise, the changes in
intramuscular metabolite concentrations are less marked, but
intramuscular glycogen is progressively depleted. The ability to
sustain exercise depends on the initial intramuscular glycogen
concentration. Fat stores represent a huge reservoir of potential
energy, but the rate at which fat can be oxidized is limited to
approximately one-fourth of the rate at which glycogen can be
oxidized. Thus, even with maximal utilization of fat, the ability
to maintain exercise is dependent on the oxidation of glycogen,
which eventually becomes depleted, leading to muscle fatigue.
4) In exercise lasting longer than a minute or two, the
cardiac output and heart rate increase linearly with peripheral
oxygen uptake. The mean systemic arterial pressure and the
vascular resistance in active muscle falls, leading to a large
increase in blood flow to the muscles. Blood is pumped back to
the heart by muscular contraction, and the cardiac output is
determined by the venous return.
5) Muscle contraction during exercise is initiated by a
central command from the *motor cortex of the brain that leads to
activation of *motor neurons, depolarization of *motor end
plates, propagation of *muscle action potentials, calcium
release, formation of *cross-bridges, and shortening of
*myofibrils. The magnitude of the central motor command increases
in parallel with the power output, but it also increases if the
responsiveness of the motor neurons or muscles decreases during
fatigue. A maximum voluntary command is capable of activating
virtually 100 percent of the *motor units in a fresh muscle
(i.e., a muscle that has not been exercised). The responsiveness
of motor neurons may be decreased by central and peripheral
factors acting through reflexes in the spinal cord and by
stimulation of receptors in the muscles.
6) Among the changes that accompany increasing intensity of
exercise is a large increase in the intramuscular hydrogen ion
concentration from 100 nanomoles per liter (pH = 7.0) at rest to
400 nanomoles per liter (pH = 6.4) or more at exhaustion, leading
to inhibition of *excitation-contraction coupling and thus
reducing the responsiveness of the muscle to stimulation of motor
units.
-----------
N.L. Jones and K.J. Killian: Exercise limitation in health and
disease.
(New England J. Med. 31 Aug 00 343:633)
QY: Norman L. Jones jonesn@fhs.mcmaster.ca
-----------
Text Notes:
... ... *motor cortex of the brain: (cortex) The cerebral cortex
is a thin surface layering of nerve cells of the brain, the
region only several millimeters thick but covering all of the
brain surface. This is the part of the central nervous system
most intimately involved with the so-called "higher faculties",
although the cortex operates in concert with other parts of the
brain. The structure is primitive in lower mammals, and is found
progressively more pronounced and with greater surface area in
primates and man. The motor cortex is the region of the cortex
involved in voluntary muscle movements.
... ... *motor neurons: In this context, motor neurons are
neurons with cell bodies in the spinal cord and extensions that
leave the spinal cord to terminate on muscle fibers. In this
report, the general paradigm for activation of voluntary muscles
is neurons in brain (motor cortex) to neurons in spinal cord
(motor neurons) to peripheral muscle fibers.
... ... *motor end plates: The junctions between nerve fiber
(axon) terminations and muscle fibers.
... ... *muscle action potentials: The muscle "action potential"
is completely analogous to the nerve action potential and
consists of a brief (approximately 1 millisecond) reversal of
polarization that travels along the fiber. This electrical change
initiates calcium movements that begin the contraction process.
... ... *cross-bridges: In general, connections between
contractile elements of muscle fibers.
... ... *myofibrils: In general, any of the long cylindrical
contractile elements, 1 to 2 microns in diameter, that constitute
the major component of muscle fibers.
... ... *motor units: In general, a single motor neuron and all
the muscle fibers that are innervated by it. (The axons of motor
neurons branch to make contact with a number of muscle fibers.)
... ... *excitation-contraction coupling: In general, the
coupling of an excitatory stimulus to the contraction of muscle;
at the cellular level, the process by which muscle fibers are
caused to contract by the stimulation of a neuron.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Sep00
For more information: http://scienceweek.com/swfr.htm
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4. ASTROPHYSICS:
ON SUPERMASSIVE BLACK HOLES
The death of supermassive stars must result in collapse,
since no known force can resist gravity in such stars once their
nuclear energy sources are exhausted. For the most massive stars,
the result of this inevitable collapse is the "*black hole", at
the present time the most exotic astronomical object in the
Universe. Although existence of black holes is predictable from
Einstein's theory of general relativity, the possibility of
something like a black hole exists even within the Newtonian
theory of gravity, if one considers the Newtonian escape velocity
for supermassive objects: when the mass of an object is so great
that the velocity required to escape the object exceeds the speed
of light, the object turns "black", since no light can escape.
In recent years, astrophysicists have come to distinguish
ordinary black holes and supermassive black holes, and these
latter objects, of unknown origin, now reign as the supreme
cosmic exotica.
... ... John Kormandy (University of Texas Austin, US) presents
an account of supermassive black hole research presented at the 6
June 2000 meeting of the American Astronomical Society, the
author making the following points:
1) Black holes 10^(6) to 10^(9.5) times as massive as the
Sun were first invoked in the 1960s to explain the enormous
energy output of *active galactic nuclei (AGN) such as *quasars.
These supermassive black holes are in contrast to ordinary black
holes, which have masses of only a few solar masses and which are
well known to form when massive stars die.
2) The origin of supermassive black holes is unknown, and
their existence long remained a hypothesis. By the mid-1980s,
however, black hole "engines" had become part of the theoretical
framework for understanding the activity of active galactic
nuclei, but evidence for their existence was still lacking. In
the 1990s, much effort was made to look for dynamical evidence of
dark objects in galactic nuclei, and that evidence is now strong.
In two objects, our Galaxy and the galaxy NGC 4258, the evident
dark mass must live inside such a small radius that plausible
alternatives to a supermassive black hole can be excluded.
3) The author points out that although until recently
supermassive black holes were studied to understand the
spectacular but restricted phenomena of active galactic nuclei,
the situation is changing rapidly. Surveys with the Hubble Space
Telescope are finding evidence of supermassive black holes in
every galaxy that has an *elliptical-galaxy-like central "bulge"
component. These observations strengthen hints from ground-based
spectroscopy that supermassive black holes are standard features
of galaxy bulges. The observations indicate that supermassive
black hole growth and galaxy formation are closely linked, and
these results have profoundly changed the view of supermassive
black holes: they are more than just exotica needed to explain
rare active galactic nuclei: "Supermassive black holes are
becoming an integral part of our understanding of galaxy
formation."
-----------
John Kormandy: Monsters at the heart of galaxy formation.
(Science 1 Sep 00 289:1484)
QY: John Kormandy: kormandy@astro.as.utexas.edu
-----------
Text Notes:
... ... *black hole: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable.
... ... *active galactic nuclei (AGN): Some galaxies are known to
have very "active" central regions from which enormous amounts of
energy are emitted each second. These "active galactic nuclei"
are probably powered by accretion of matter into a supermassive
black hole of 10^(6) to 10^(9) solar-masses.
... ... *quasars: (quasi-stellar object). An extremely luminous
source radiating energy over the entire spectrum from x-rays to
radio waves. Quasars are apparently the oldest and most distant
objects in the universe.
... ... *elliptical-galaxy: These are galaxies that have no
disc component, the shape varying from almost circular to narrow
ellipses. The stars within elliptical galaxies are predominantly
old stars. Elliptical galaxies display the greatest variation in
mass, ranging down to extreme dwarfs (approximately 10^(6) solar-
masses.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Sep00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY
Recent observations have led to the conclusion that at the center
of many galaxies there is an object producing effects
characteristic of a supermassive *black hole. Alexei V.
Filippenko (University of California Berkeley, US) reviews
current research on black holes, the author making the following
points concerning the apparent massive black hole at the center
of our own Galaxy:
1) Some galaxies are known to have very "active" central
regions from which enormous amounts of energy are emitted each
second. These "active galactic nuclei" are probably powered by
accretion of matter into a supermassive black hole of 10^(6) to
10^(9) solar-masses. The center of our own Galaxy exhibits mild
activity, especially at radio wavelengths: so-called "nonthermal
radiation" characteristic of high-energy electrons spiraling in
magnetic fields is emitted by a compact object at the Galactic
center known as *Sagittarius A*. Does the center harbor a
supermassive black hole? One approach is to determine whether
stars in the central region are moving very rapidly, as would be
expected if a large mass were present. During the past 5 years,
two teams have obtained high-resolution images of our Galactic
center, each team on several occasions, so that temporal changes
in the positions of stars could be detected. The observations
were conducted at infrared wavelengths, which penetrate the gas
and dust between Earth and the Galactic center (a distance of
approximately 25,000 light years) much more readily than optical
light. In summary, the data are in excellent agreement with the
conclusion that the gravitational potential of the central region
of our Galaxy is dominated by a single object. The derived mass
of this object is (2.6 +- 0.2) x 10^(6) solar-masses, and the
mass density within a radius of 0.05 light-years is at least 6 x
10^(9) solar-masses per cubic light-year, effectively eliminating
all possibilities other than a black hole.
2) Although our Galaxy provides the most convincing case for
the existence of supermassive black holes, observations of the
centers of a few other galaxies bolster the conclusion. For
example, very precise measurements of the galaxy NGC 4258 reveal
a central compact object with a derived mass 3.6 x 10^(7) solar-
masses. On somewhat larger scales, spectra obtained with the
Hubble Space Telescope show gas and stars rapidly moving in a
manner consistent with the presence of a supermassive black hole.
The most massive existing case, that of the giant elliptical
galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover,
x-ray observations of some active galactic nuclei reveal emission
from a hot disk of gas apparently very close to a black hole,
since extreme relativistic effects are detected. In general, it
now seems that a supermassive black hole is found in nearly every
large galaxy amenable to such observations.
3) The author concludes: "In the last decade of the 20th
century, black holes have moved firmly from the arena of science
fiction to that of science fact. Their existence in some *binary
star systems, and at the centers of massive galaxies, is nearly
irrefutable. They provide marvelous laboratories in which the
strong-field predictions of Einstein's general theory of
relativity can be tested."
-----------
Alexei V. Filippenko: Black holes in the Milky Way galaxy.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9993)
QY: Alexei V. Filippenko [alex@astro.berkeley.edu]
-----------
Text Notes:
... ... *black hole: See notes in main report.
... ... *Sagittarius A*: Sagittarius A is a prominent radio
source in the constellation Sagittarius, coincident with or close
to the center of our Galaxy. It is a highly complex region
consisting of a central core approximately 50 light-years in
diameter. Sagittarius A* is a compact component at the heart of
the central core of Sagittarius A. Sagittarius A* is an intense
source of radio waves, and is apparently unique in our Galaxy:
while everything else in our Galaxy is on the move as they follow
their orbits, Sagittarius A* is absolutely stationary and must
therefore lie exactly at the Galaxy's center. Many astronomers,
in fact, use Sagittarius A* as the "Greenwich Meridian" of the
Galaxy.
... ... *binary star systems: Binary stars are a pair of stars
revolving around a common center of mass under the influence of
their mutual gravitational attraction, and apparently the
majority of stars in the Universe are binaries and not singlets.
In some cases the binary system is resolvable into two
components, and in other cases the presence of a second star is
inferred by perturbations in the motion or emitted radiation of
the first star. If the binaries are close enough, they may share
stellar material, and this results in a particular kind of
stellar evolution. In the black hole-binary systems mentioned in
this report, matter transfers from a relatively normal star
(known as the "secondary star") to a dark compact object (the
"primary"). Recent comparisons of x-ray and optical brightness
suggest that in many cases the dark primary in such a binary
system is a black hole.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Oct99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANALYSIS OF A SUPERMASSIVE BLACK HOLE ACCRETION DISK
Matter with high angular momentum attracted to a black hole does
not fall directly into the black hole but forms a rapidly
spinning "accretion disk" around the black hole, and this can
produce considerable energy, particularly at x-ray wavelengths,
as the accretion disk loses angular momentum and spirals inward.
The dynamical evolution and fate of such accretion disks has been
the subject of much theoretical analysis and model simulations.
Supermassive black holes are black holes with masses of the order
of 10^(6) to 10^(9) solar masses and are believed to occupy the
centers of some galaxies. The term "iron-line emission" refers to
emission at the frequency characteristic (the "line") of iron
atoms in transit from excited states to lower energy states.
Because of the nature of nucleosynthesis -- the fusion reactions
in stars -- the cores of many stars consist of iron. Bromley et
al (3 authors at 3 installations, US RU) present an analysis of
the iron-line emission of galaxy MCG-6-30-15 that is independent
of parametric details of the disk model used, and they deduce
that from this galaxy there are being observed emissions from
gravitationally bound material in the strong-field region of a
supermassive black hole.
-----------
(Nature 1 Jan 98) (Science-Week 16 Jan 98)
QY: B.C. Bromley: bbromley@cfa.harvard.edu
For more information: http://scienceweek.com/swfr.htm
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5. MATERIALS SCIENCE:
REVERSIBLE CRYSTAL SEQUESTRATION OF GASES
Zeolites are members of a group of hydrated aluminosilicate
minerals occurring in the cavities of igneous rocks (rocks
produced by solidification from a molten state, e.g., granite)
and various volcanogenic sediments. They are also manufactured
for their ion exchange properties (the exchange of zeolite ions
with solute ions) and selective adsorption properties (specific
adsorption of solutes). They have an open crystal structure and
can also be used as molecular sieves for partitioning and
sequestration of solutes according to the dimensions of the
zeolite pores and the dimensions of the solute molecules.
Porous materials such as zeolites and clays have been widely
used by the petrochemicals industry to bind various kinds of
small molecules, and the binding ability of robust inorganic
zeolites has resulted in a repertoire of designed porous
structures potentially strong enough to withstand the formation
of holes in their structure as molecular "guests" enter and leave
the "host" framework. Now new studies demonstrate that solid
materials that can reversibly bind small molecular guests do not
need to be porous.
... ... M. Albrecht et al (4 authors at Utrecht University, NL)
report the controlled and fully reversible crystalline-state
reaction of gaseous SO(sub2) with non-porous crystalline
materials consisting of organoplatinum molecules. The authors
make the following points. The authors report this process,
including repetitive expansion-reduction sequences (on gas uptake
and release) of the crystal lattice, modifies the structures of
these molecules without affecting their crystallinity. The
process is based on the incorporation of SO(sub2) into the
colorless crystals and the subsequent liberation of the gas from
the orange adducts by reversible bond formation and cleavage. The
authors suggest such crystalline materials will find applications
for gas storage devices and as opto-electronic switches.
... ... In a commentary on this work, Jonathan W. Steed (King's
College London, UK) points out that non-porous crystals normally
cannot bind gas molecules without crumbling. But in the work of
Albrecht et al, when the colorless organoplatinum crystal absorbs
SO(sub2), an orange color spreads through the structure as the
square organoplatinum molecules transform into pyramids. With
SO(sub2) present, the total volume of the crystal increases by 25
percent, but when the gas is "exhaled" the crystal returns to its
former size and colorless state. The crystalline structure
remains intact throughout. Steed states: "The work of [Albrecht
et al]... offers hope of a new 'synthesizing up' approach to
nanotechnology... It is the 3-dimensional ordering of the
process, bringing together nanotechnology and the macroscopic
world, that is the remarkable feature of this work."
-----------
M. Albrecht et al: Organoplatinum crystals for gas-triggered
switches.
(Nature 31 Aug 00 406:970)
QY: Gerard van Koten: g.vankoten@chem.uu.nl
-----------
Jonathan W. Steed: Crystals that breathe.
(Nature 31 Aug 00 406:943)
QY: Jonathan W. Steed: jon.steed@kcl.ac.uk
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 29Sep00
For more information: http://scienceweek.com/swfr.htm
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6. MATERIALS SCIENCE:
GEL CATALYSTS THAT SWITCH ON AN OFF
Our understanding of dynamic biochemical processes is greatly
improved when we can mimic these processes in detail in the
laboratory, and this is particularly true for understanding
enzymes -- the biological protein catalysts. Natural enzymes
catalyze chemical reactions and regulate such reactions by
reversibly and repeatedly switching their catalytic activities on
and off. What are the possible mechanisms for these reversible
switching actions?
... ... G. Wang et al (8 authors at Massachusetts Institute of
Technology, US) now report the development of a polymer gel with
a catalytic activity that can be switched on and off when the
solvent composition is changed. The authors make the following
points:
1) The gel consists of two species of monomers. The major
component, N-isopropylacrylamide, makes the gel swell and shrink
in response to a change in composition of ethanol/water mixtures.
The minor component, vinylimidazole, which is capable of
catalysis, is copolymerized into the gel network. The reaction
rate for catalytic hydrolysis of p-nitrophenyl caprylate was
small when the gel was swollen; in contrast, when the gel was
shrunken, the reaction rate increased 5 times. The authors report
the activity changes discontinuously as a function of solvent
composition: thus, the catalysis can be switched on and off by an
infinitesimal change in solvent composition.
2) The authors report the kinetics of catalysis by the gel
in the shrunken state is well described by the *Michaelis-Menten
formula, indicating that the absorption of the substrate by the
hydrophobic environment created by the N-isopropylacrylamide
polymer in the shrunken gel is responsible for enhancement of
catalytic activity. In the swollen state, the rate vs. active
site concentration is linear, indicating that the substrate
absorption is not a primary factor determining the kinetics. The
authors report that catalytic activity of the gel was studied for
substrates with various alkyl chain lengths, and that of those
studied, the switching effect is most pronounced for the
substrate p-nitrophenyl caprylate.
3) The authors report the following interpretation of the
dynamics in this system:
The major component (N-isopropylacrylamide) allows the gel
to reversibly swell and shrink in response to changes in
environmental parameters such as temperature and solvent. On
swelling and shrinking, the local density of the hydrophobic
moiety changes, and the affinity of the substrate molecules is
altered accordingly. The minor component (vinylimidazole),
capable of catalysis of substrate decomposition, is copolymerized
with the responsive monomers into the gel network. The catalysis
activity of the gel is thus expected to be switched on and off as
the substrate is reversibly bound and released during the cycle
of gel swelling and shrinking.
4) The authors conclude: "The design idea shown in this
paper can be applied to development of gels that can be used for
many other purposes were enzymes have been used. The gel system,
once its catalytic rate is well improved, will have advantages
over enzymes in some important aspects -- e.g., it can be applied
where enzymatic action is needed but enzymes cannot be used."
-----------
G. Wang et al: Gel catalysts that switch on and off.
(Proc. Natl. Acad. Sci. US 29 Aug 00 97:9861)
QY: Kenichi Kuroda: kkuroda@mit.edu
-----------
Text Notes:
... ... *Michaelis-Menten formula: One form of the Michaelis-
Menten equation is as follows: v = V/(k + S) where (v) is the
initial velocity of the reaction, (V) is the maximum reaction
rate (i.e., when the enzyme is saturated with substrate), (k) is
the Michaelis constant, which approximates to the enzyme-
substrate dissociation constant, and (S) is substrate
concentration. In biochemistry, a system described by such an
equation is said to obey "Michaelis kinetics".
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Sep00
For more information: http://scienceweek.com/swfr.htm
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7. IN FOCUS: ON THE EVOLUTION OF HUMAN BEHAVIOR
"The uniqueness of Man is not a matter of his structure. His body
and its functions are in general very similar to those of other
mammals -- that is why medicine can study in animals the
functions of, say, kidneys, of the heart, of eye and ear, even
the basic functions of the nerve cells, and extrapolate with
confidence. The uniqueness of Man is agreed to be a matter of
behavior. If it is to be reduced to structural characters, it is
the brain that is unique, that functions in a unique way. There
are of course links with other structural characteristics, but
attempts (made in the past) to reduce Man's behavioral uniqueness
to, say, the possession of hands, or to his upright posture, have
not been very convincing -- they have been too simplistic, too
one-sided. The way the biologist approaches this problem is based
on his knowledge of the fact of evolution. No informed person can
doubt any more that Man has evolved, slowly and very gradually,
from ancestors which were far more similar to other mammals than
Man is now. This means that everything Man is and does now must
have evolved, through a long series of minute evolutionary steps,
from what his animal ancestors were and did. Man has diverged
very gradually from monkey or ape-like stock to what he is now,
just as modern closely related animal species have diverged from
common stock... It has often been pointed out that Man, himself a
product of evolution of a type similar to that which has created
all other animal forms, namely adaptive hereditary change, has
now embarked on a new type of evolution, which Huxley calls
'psycho-social evolution'. I prefer the term 'cultural
evolution'. It is based on _accumulated_ transfer, by tradition,
from one generation to the next, of knowledge (or _phenotypic_)
behavior changes, i.e., changes acquired through individual
experience. Our culture is very different from that of Cro-Magnon
Man, but generally we may not have changed much -- most of our
modern attributes are due to the accumulation of transferred
knowledge. We differ from animals not merely in the extent of
what we can ourselves learn, but in the progressive (and steadily
accelerating) accumulation of experience through the
generations."
-----------
Niko Tinbergen: _The Animal in its World_
(George Allen & Unwin, London 1972)
-----------
[Editor's note: Nikolaas Tinbergen (1907-1988) was awarded the
Nobel Prize in Physiology and Medicine in 1973, and is considered
one of the founders of ethology, the objective study of animal
behavior. Although researchers who approach human behavior in
terms of the evolution of genes (e.g., sociobiologists) often
cite Tinbergen as one of their intellectual sources, the above
quoted passage illustrates that Tinbergen's view of human
behavior was otherwise: Tinbergen approached human behavior
primarily in terms of the evolution of culture, rather than in
terms of the evolution of genes. (The quoted passage is from a
lecture given by Tinbergen at Oxford University, 27 October
1964.)]
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8. FROM THE SCIENCEWEEK ARCHIVE:
ON SCIENCE, POLITICS, AND ASTEROIDS
It is not often that a leading science journal invites a leading
science-fiction writer to present a lead essay in its pages. But
there are many scientists at work today who made their first
youthful contact with the adventurous aspects of science in the
novels of Arthur C. Clarke. Now 81 years old and Chancellor of
the University of Moratuwa (LK), Clarke was trained as a
scientist and originated the idea of satellite communication in a
scientific article in 1945. In a recent essay in the journal
*Science*, Clarke makes the following points: 1) For more than a
century, science and its occasionally ugly sister technology have
been the chief driving forces shaping our world. They decide the
kinds of futures that are possible. Human wisdom must decide
which are desirable. Clarke says it is "truly appalling" that so
few of our politicians have any scientific or engineering
background. 2) Clarke says, "I have encountered a few
'creationists' and because they were usually nice, intelligent
people, I have been unable to decide whether they were _really_
mad or only pretending to be mad. If I was a religious person, I
would consider creationism nothing less than blasphemy. Do its
adherents imagine that God is a cosmic hoaxer who has created the
whole vast fossil record for the sole purpose of misleading
humankind?" 3) Clarke says the scientific establishment has only
slowly understood that the history of this planet, and perhaps of
civilization itself, has been modified in important ways by
physical impacts from space, and he proposes that we embark on a
serious study of the probability of comet or asteroid impactors
on the planet Earth. 4) Concerning energy production from new
sources, Clarke says his guess is that large scale industrial
application will begin around the turn of the century -- "at
which point one can imagine the end of the fossil-fuel-nuclear
age, making concerns about global warming irrelevant, as oil-and-
coal-burning systems are phased out."
QY: Arthur C. Clarke, 25 Barnes Place, Colombo 7, LK.
(Science 5 Jun 98 280:1532) (Science-Week 26 Jun 98)
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