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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 7, 2001 -- Vol. 5 Number 36
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We don't know where the next revolution will be.
Maybe the discontinuous character of space-time.
Then quantum mechanics will be a footnote.
-- Unknown
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. Biology of the Unconscious
2. Defense Mechanisms in Phytoplankton
3. Decline of Blood Lead Levels in Children
4. New Neurons and Memory
5. Longevity in Yeasts and Animals
6. Neuronal Effects of Drug Addiction
7. Solar Forcing, Droughts, and the Collapse of the Maya
8. Chemistry of Cyclic Transmembrane Charge Transport
9. Structure of Earth's Mantle
10. The Puzzle of the Sun's Hot Corona
11. Water and the Martian Landscape
12. Dynamical Tunneling
13. In Focus: On the History of Immunology
14. SW Archive: Condensed-Matter Physics: On the Ising Model
15. Sources
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Section 2
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1. BIOLOGY OF THE UNCONSCIOUS
C. Koch and F. Crick (California Institute of Technology, US)
discuss the neurobiology of consciousness. To what extent are we
conscious of everything occurring in our brains? F. Nietzsche
(1844-1900) and S. Freud (1856-1939) popularized the idea of the
unconscious as a realm of the mind that controls human behavior
but which is not itself accessible to conscious introspection of
knowledge. By "unconscious" we mean any neuronal activity that
does not give rise to conscious sensation, thought, or memory.
Science has provided credible evidence for the existence of
sensorimotor systems in the primate brain that function in the
absence of consciousness. Many mammalian brain systems perform
complex yet routine tasks without direct conscious input. Goodall
and Milner (1995) described the brain parts responsible for this
as "online systems", by analogy with software that processes
information in real time. Such systems can deal with certain
commonly encountered situations automatically, which is why we
call them "zombie" agents. One can become conscious of the
actions of one's own zombie, but usually only in retrospect. The
best evidence comes from studying dissociation of "vision for
perception" and "vision for action" in both healthy humans and
patients: in certain tracking experiments, the subject does not
perceive extra deliberate target motions, even though the
oculomotor system automatically corrects for them.
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NAT 2001 411:893
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Notes:
The F. Crick in the above report is the Nobel Laureate Francis
Crick. Although the report focuses on historical origins such as
Nietzsche and Freud, the extensive work of 19th century
experimental psychologists should not be ignored. The subject of
conscious vs. non-conscious behavior was widely discussed in the
classical literature of experimental psychology, particularly by
Gustav Fechner (1801-1887).
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Related Background:
NEUROBIOLOGY: LEARNING AND MEMORY
Neurobiologists concerned with learning and memory in higher
animals often find themselves with an evolution of attitude: as
young researchers, they are imbued with an enthusiastic optimism
that the grand puzzles of learning and memory will shortly be
solved; later, as senior researchers, many find their optimism
overshadowed by a sense of being humbled by the brain.
... ... H. Okano et al (3 authors at 3 installations, JP US)
present a concise summary of current ideas in the neurobiology of
learning and memory, the authors making the following points:
1) The authors state they define memory as a behavioral
change caused by an experience, and they define learning as a
process for acquiring memory. According to these definitions,
there are different kinds of memory. Some memories, such as those
concerning events and facts, are available to our consciousness;
this type of memory is called "declarative memory". However,
another type of memory, called "procedural memory", is not
available to consciousness. This is the memory that is needed,
for example, to use a previously learned skill. We can improve
our skills through practice: with training, the ability to play
tennis, for example, will improve. Declarative memory and
procedural memory are independent: there are patients with
impaired declarative memory whose procedural memory is completely
normal. Because of this fact, current researchers believe there
must be separate mechanisms for each type of memory, and that
these separate mechanisms probably also require separate brain
areas as well.
2) The *cerebrum and *hippocampus are considered important
for declarative memory, and the *cerebellum is considered
important for procedural memory. The current belief is that
memory requires alterations in the brain. The most popular
candidate site for memory storage is the *synapse, where nerve
cells communicate with each other. A change in the transmission
efficacy at the synapse (called "synaptic plasticity") has been
considered to be the cause of memory, and a particular pattern of
synaptic usage or stimulation (conditioning stimulation) is
believed to induce synaptic plasticity. Many questions remain to
be answered, such as how synaptic plasticity is induced and how
synaptic plasticity is implicated in learning and memory.
3) One current frontier in the study of synaptic plasticity
is the attempt to clarify the role of plasticity in learning and
memory. The strategy has been to examine the correlation between
synaptic plasticity and learning by inhibiting the plasticity in
a living animal. To do this, investigators have used inhibitors
for certain molecules that are apparently required for synaptic
plasticity. Another set of useful tools involves genetically
engineered mutant mice, such as "knockout" and transgenic mice. A
"knockout" mouse is a mutant mouse that is deficient in a
specific native molecule. By using mutant mice, the relationship
between synaptic plasticity and learning ability has been
examined in detail.
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H. Okano et al: Learning and memory.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12403)
QY: Hideyuki Okano: okano@nana.med.osaka-u.ac.jp
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Text Notes:
... ... *cerebrum: What is called the "cerebrum" is the bulk of
brain as seen by the naked eye, the "great ravelled knot" that
sits on top of the phylogenetically older parts (brainstem and
midbrain) of the whole brain. The surface of the cerebrum, an
enormously extended surface because of the many deep folds of the
cerebrum, is a thin sheet called the "cerebral cortex" (cortex =
rind or bark).
... ... *hippocampus: A region of the cerebral cortex in the
*medial part of the temporal lobe. In humans, among other
functions, the hippocampus is apparently involved in short-term
memory, and analysis of the neurological correlates of learning
behavior in animals indicates that the hippocampus is also
involved in memory in other species.
... ... *cerebellum: The human cerebellum is about the size of a
large apple, is placed at the lower back of the head under the
optic lobes of the cerebrum, and is apparently involved in the
input-output control of automatic sensorimotor functions. If you
are sitting at your breakfast table, holding a newspaper in one
hand, and using the other hand to routinely and repetitively dip
a spoon into cold cereal and bring the cold cereal to your mouth
while you read the newspaper, it is the cerebellum which is
governing the automatic feeding movements while your cerebral
cortex processes the information that you read.
... ... *synapse: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. The term "synapse" refers to the
junction between the terminal of a neuron's axon and another
neuron. When studying the synapse, the first neuron is called the
"presynaptic" neuron, and the second neuron is called the
"postsynaptic" neuron.
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SW 2001 12 Jan
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2. DEFENSE MECHANISMS IN PHYTOPLANKTON
Victor Smetacek (Wegener Institute for Polar and Marine Research,
DE) discusses defense mechanisms in *phytoplankton. Forests and
algal *blooms fix approximately the same amount of carbon (a few
grams per square meter per day), because both are based on
essentially the same photosynthetic machinery fuelled by
*chlorophyll-alpha in *chloroplasts, the descendants of free-
living *cyanobacteria that have since evolved into plant
organelles by *endosymbiosis. Chloroplasts provide their host
cells with food in return for resources and protection. The range
of defense systems in phytoplankton is only now coming to light.
The size range of phytoplankton spans 3 orders of magnitude, but
that of its predators spans 5 orders of magnitude, from micron-
scale *flagellates to shrimp-sized krill. Pathogens (viruses and
bacteria) pose a further challenge. Most predators and pathogens
of phytoplankton feed or infect selectively. Smaller predators
hunt individual cells, whereas larger predators use feeding
currents, mucous nets, or elaborate filters to collect
phytoplankton en masse. Captured cells are pierced, ingested,
engulfed, or crushed, but have evolved specific defense measures.
The phytoplankton can escape by swimming or by mechanical
protection; mineral or tough organic cell walls ward off piercers
or crushers. In adapting to the deterrence of predators,
phytoplankton cells have increased in size, formed large chains
and colonies, or grown spines. Noxious chemicals also provide
defense.
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NAT 2001 411:745
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Notes:
... ... *phytoplankton: Small, usually microscopic, aquatic
plants capable of photosynthesis; e.g., unicellular algae.
Phytoplankton and plankton are not equivalent. The term
"plankton" is a general designation for various drifting
microscopic aquatic organisms in the upper regions of the oceans,
both photosynthetic and non-photosynthetic.
... ... *blooms: In this context, the term "bloom" refers to an
explosive increase in the density of phytoplankton within an
area.
... ... *chlorophyll-alpha: (chlorophyll-a) Chlorophylls are
magnesium complexes of various closely related porphyrins or
chlorins. Chlorophylls a and b are dihydroporphyrins.
... ... *chloroplasts: The chloroplasts containing several
photosynthetic pigments (chlorophylls). Chloroplasts are found in
all photosynthetic plant cells, but not in photosynthetic
prokaryotes (i.e., not in cells without membrane-bound
organelles). The typical higher plant chloroplast is lens-shaped,
approximately 5 microns across the larger dimension, and the
number of chloroplasts per cell can vary from 1 to 100 depending
on the type of cell. A mature chloroplast is typically bounded by
two membranes, an inner membrane and an outer membrane, the
membranes possessing significantly different chemical
constituents. In addition to a number of enzymes involved in
photosynthesis, chloroplasts also contain in their interior a
circular DNA molecule and protein synthetic machinery typical of
prokaryotes. The current consensus is that chloroplasts may have
originated from *cyanobacteria that became endosymbionts.
... ... *cyanobacteria: A phylum of bacteria characterized by
blue-green (cyan) photosynthetic pigments, abundant in a variety
of habitats, particularly in fresh water and soil. Cyanobacteria
are responsible for generating a large portion of the free oxygen
in the Earth's atmosphere. They apparently produced stromatolite
limestone deposits, as well as the bulk of modern petroleum
deposits. (Stromatolites are laminated calcareous microbial
fossil deposits formed principally by cyanobacteria and algae.)
... ... *endosymbiosis: An arrangement in which one organism
lives inside another organism, but the term is usually restricted
to arrangements of mutual benefit, thus not including
parasite-host relationships. A number of eukaryotic cell
organelles (including mitochondria) are believed to have
originated from endosymbiotic relationships between eukaryotic
cells and simpler cells.
... ... *flagellates: Organisms possessing one or more flagella.
A flagellum is a long threadlike extension providing locomotion
for a cell.
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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3. DECLINE OF BLOOD LEAD LEVELS IN CHILDREN
The US CDC Morbidity and Mortality Report discusses trends in
blood lead levels among children in Boston (US). Data from a
national survey of 19 states have indicated that average blood
lead levels in young children decreased during the late 1990s. A
similar survey of Boston, where a high proportion of children are
tested each year, found similar results: the prevalence of
elevated blood lead levels declined consistently during 1994 to
1999. Contributions to this decrease may have been building of
new houses and remodeling of older houses with removal of lead-
painted building components such as windows. Although blood lead
levels declined in all Boston neighborhoods, levels remained
higher in 1999 in the areas with the highest levels in 1994.
These high-risk neighborhoods are characterized by higher
proportions of minority children, children living in poverty, and
vacant properties. A high proportion of old housing likely to
have leaded paint is found in all neighborhoods. Low
socioeconomic status and associated deterioration of older
housing are major contributors to lead exposure in Boston.
Approximately 1300 children in Boston are identified annually
with blood lead levels >= 10 micrograms per deciliter, high
enough to adversely affect cognitive development. In 19 states in
1998, the proportion of children tested who showed blood lead
levels equal to or greater than 10 micrograms per deciliter was
7.6 percent, with a higher proportion in certain counties.
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MMWR 2001 50:337
JAMA 2001 285:2575
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Related Background:
CHELATION THERAPY AND CHILDREN EXPOSED TO LEAD
Thousands of children in the US, especially poor children living
in deteriorated urban housing, are exposed to enough lead to
produce neurodevelopmental damage and cognitive impairment. Blood
lead levels as low as 10 micrograms per deciliter are associated
with cognitive deficits. A new study demonstrates that treatment
with succimer, a lead chelator, lowers blood lead levels but does
not improve scores on tests of cognition, behavior, or
neuropsychological function in children with blood lead levels 20
to 44 micrograms per deciliter. Since succimer is as effective as
any lead chelator currently available, chelation therapy is
apparently useless for children with such blood lead levels.
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NEJM 2001 344:1421
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Related Background:
A DANGEROUS NEW SOURCE OF ENVIRONMENTAL LEAD
There is ample evidence that the developing nervous system of a
child is highly sensitive to a number of toxic substances, the
effects of which are various encephalopathies (brain disorders)
[*Note #1]. One environmental toxic substance of considerable
importance in this context is lead, an urban environmental
contaminant that in the past few decades has become a focus of
public awareness. The classical sources of urban environmental
lead contamination are the gasoline exhaust fumes of motor
vehicles and lead-based paints, but recently another important
source of such contamination has become apparent.
... ... Howard W. Mielke (Xavier University New Orleans, US)
presents a review of the problem of lead in inner cities with
emphasis on the newly recognized danger of lead contamination of
inner city soil and dust. The author makes the following points:
1) Since the 1920s, millions of US children have been quietly
poisoned by lead, and thousands of deaths are attributed to this
over the long term. 2) Although childhood lead exposure in the
US has diminished during the past 2 decades, the problem has not
been solved. Instead, the demographics has shifted. 3) Over 50
percent (and perhaps even 70 percent) of children living in the
inner city of New Orleans and Philadelphia have blood lead
levels above the current guideline of 10 micrograms per
deciliter [*Note #2]. In contrast, in the concrete "jungle" of
Manhattan, where very little of the soil is exposed and almost
all apartments and housing contain lead-based paints, only
between 5 and 7 percent of children under the age of 6 have been
reported to have blood-lead levels of 10 micrograms per
deciliter or higher. It is of significance that in Brooklyn,
across the river from Manhattan, where yards containing soil are
common, the percentage of affected children is several times
higher than in Manhattan. 4) The serious of the problem has been
recognized by the US Centers for Disease Control and Prevention
since the early 1990s, which has called pediatric lead poisoning
"entirely preventable". 5) The author suggests that effective
prevention assumes an accurate identification of the
environmental reservoirs of lead, and that current policies to
reduce lead exposure are based on the assumption that the
greatest lead hazard comes from lead-based paints [*Note #3].
Most lead-based have now been removed from the market, and
parents have been instructed to guard their children from eating
paint flakes. However, for children, paint is now neither the
most abundant nor the most accessible source of lead. The common
problem is lead dust in the environment, with the soil a giant
reservoir of tiny particles of lead. The greatest risk for
exposure of inner city children is in the yards around houses
and to a lesser extent in public playgrounds. 6) The author
suggests that an accurate and complete appreciation of the
distribution of lead in the environment can help shape policies
that more effectively protect the health of children. The author
concludes: "It took nearly 10 decades for lead to accumulate to
its current levels in urban areas. With judicious planning, the
problem can be resolved in much less time."
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AS 1999 87:62
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Notes:
... ... *Note #1: There is much data concerning certain
syndromes, e.g., fetal alcohol syndrome, lead poisoning, etc. One
research problem is that effects of low levels of environmental
toxins on the developing nervous system can be subtle and not
detected unless specific rather than general behavioral measures
are applied.
... ... *Note #2: There is hardly a consensus concerning
acceptable levels of lead in the whole blood of children. Some
clinicians consider the danger point to be in the region of 50
micrograms per deciliter whole blood; other clinicians consider
anything above 10 micrograms per deciliter as a cause for alarm.
In terms of low-level effects on the developing central nervous
system, general concentration cut-off points are perhaps
arbitrary, since there is considerable individual variation in
toxic susceptibility.
... ... *Note #3: In the US, lead was used in residential paint
between 1884 and 1978, and leaded paint remains on the walls of
many old buildings.
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SW 1999 15 Jan
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Related Background:
PRENATAL LEAD EXPOSURE AND POSTNATAL LEAD TOXICITY
Lead is a highly toxic metal, especially in children, but
unfortunately the incidence of lead poisoning in metropolitan
slums remains high due to past widespread use of lead based
paints and lead water pipes. There are also other sources that
contribute to dangerous concentrations in the environment. In
children, concentrations of lead in the range 40 micrograms per
deciliter, and probably as low as 10 micrograms per deciliter,
will produce definite serious cognitive deficits. Higher blood
concentrations produce encephalopathies that are both malignant
and difficult to treat. In the past, the focus has been on the
exposure of young children to environmental lead, since children
are the most vulnerable and environmental lead the apparent
primary source. Now William H. Bowen et al (University of
Rochester, US) present evidence that in rats toxic concentrations
of lead can pass from mother to offspring when mother rats are
drinking water that produces blood lead levels of only 40
micrograms per deciliter, with transmission evidently occurring
via blood to the fetus and via milk to the postnatal rat pup.
Evidently one consequence in these lead-exposed rat pups is a
high incidence of dental caries. Bowen's group is a dental
research group, and they apparently became interested in the
problem after considering that although dental caries in children
has shown a marked drop in prevalence in the U.S., about 80 per
cent of the cases that still occur are occurring in only about
20% of the children -- those that live in inner cities, where
lead exposures can still be relatively high. Metropolitan
children are thus faced with another source of lead poisoning --
lead of maternal origin.
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NATM 1997 September
SW 1887 12 Sep
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4. NEW NEURONS AND MEMORY
At the present time, the concept of "synaptic plasticity"
underlies nearly all theories of memories, the term referring to
changes in the behavior of the junction (synapse) between two
nerve cells resulting from past history. Two prominent aspects of
synaptic plasticity considered to be related to memory are
"facilitation" and "potentiation". The term "facilitation" refers
to a progressive increase in the amount of neurotransmitter
substance released at a synapse by successive nerve impulses
(action potentials), the increase occurring during an input
barrage consisting of repetitive stimulation (stimulus train).
The term "potentiation" refers to an increase in neurotransmitter
substance released by an action potential following repetitive
stimulation of a synapse. Both facilitation and potentiation can
be long-lasting, and "long-term potentiation" has been a focus of
much research on the cellular basis of memory, particularly in
the hippocampus, a brain cortex structure in the medial part
of the temporal lobe. In humans, among other functions, the
hippocampus is apparently involved in short-term memory, and
analysis of the neurological correlates of learning behavior in
the rat indicates that the hippocampus of the rat is also
involved in memory.
... ... T.J. Shors et al (Rutgers University, US) discuss the
involvement of neurogenesis in the formation of trace memories.
The vertebrate brain apparently continues to produce new neurons
throughout life. In the rat *hippocampus, several thousand new
neurons are produced each day, many of which die within weeks.
Associative learning can enhance their survival, but it has been
unclear whether new neurons are involved in memory formation. The
authors report experimental evidence that a substantial reduction
in the number of newly generated neurons in the adult rat impairs
hippocampus-dependent trace conditioning, a task in which an
animal must associate stimuli that are separated in time. A
similar reduction in newly generated neurons did not affect
learning when the same stimuli were not separated in time, a task
that is known to be not dependent on the hippocampus. The
reduction in neurogenesis did not induce death of mature
hippocampal neurons or permanently alter neurophysiological
properties of the hippocampus CA1 region such as *long-term
potentiation. Moreover, recovery of nerve cell production was
associated with the ability to acquire new trace memories. The
authors suggest their results indicate that newly generated
neurons in the adult are not only affected by the formation of a
hippocampal-dependent memory, but also participate in it. The
number of newly generated neurons was reduced with a toxin --
subcutaneous injection of the DNA-methylating agent
methylazoxymethanol acetate.
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NAT 2001 410:372
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Related Background:
INDUCTION OF NEUROGENESIS IN THE MAMMALIAN BRAIN
During most of the 20th century, a dogma that pervaded all
teaching and textbooks in neurobiology was that neurons in the
adult mammalian central nervous system (including the adult human
central nervous system) are not replaced after destruction. No
regeneration, no neurogenesis, no repair of damaged central
nervous system tissue. These days we know better; there are
exceptions and the dogma is false. We have evidence of
neurogenesis in limited but important areas of the adult
mammalian brain: the *hippocampus, the *olfactory bulb, and some
regions of the *cerebral cortex of one species of monkey
(Macaque). The continuing refutation of the dogma is based
largely on new techniques for marking the appearance of new
neurons, and many neurobiologists are now hopeful that methods
will eventually be found to induce neurogenesis useful to
clinical neurology.
... ... S.S. Magavi et al (3 authors at Harvard University, US)
now provide evidence that endogenous neural precursors (*stem
cells) can be induced in situ to differentiate into mature
neurons in regions of adult mammalian neocortex that do not
normally undergo any neurogenesis. The authors report the
following:
1) The apparent stem cell differentiation into mature
neurons occurs in a layer- and region-specific manner, and the
new neurons can apparently reconstruct appropriate *cortico-
thalamic connections.
2) In their experiments, the authors induced synchronous
*apoptotic degeneration of corticothalamic neurons in layer VI of
the anterior cortex of adult mice and subsequently examined the
fates of dividing cells within cortex, using markers for DNA
replication (marker: 5-bromodeoxyuridine; BrdU) and progressive
neuronal differentiation. Newly produced BrdU-marker-positive
cells expressed various chemical markers of mature neurons in
regions of cortex undergoing targeted neuronal death, and these
new nerve cells survived for at least 28 weeks. The authors
report that backward labeling from thalamus to cortex (retrograde
labeling) demonstrated that the new neurons can form long-
distance cortico-thalamic connections.
3) The authors suggest their results indicate that neuronal
replacement therapies for neurodegenerative disease and central
nervous system injury may be possible through manipulation of
endogenous neural precursors in situ, so that transplantation of
exogenous cells would not be required.
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NAT 2000 405:951
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Notes:
... ... *hippocampus: The hippocampus (plural: hippocampi) is a
region of the cerebral cortex in the medial part of the temporal
lobe. In humans, among other functions, the hippocampus is
apparently involved in short-term memory, and analysis of the
neurological correlates of learning behavior in animals indicates
that the hippocampus is also involved in memory in other species.
... ... *olfactory bulb: The olfactory relay station that
receives axons from the olfactory cranial nerve and transmits the
information via the olfactory tract to higher centers.
... ... *cerebral cortex: (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.
... ... *stem cells: In general, the term "stem cells"
refers to undifferentiated cells that upon differentiation can
give rise to various specialized cell lines such as blood cells,
skin cells, nerve cells, etc.
... ... *cortico-thalamic connections: Neural connections from
neurons in the cortex to neurons in the *thalamus.
... ... *thalamus: The thalamus is a deep brain structure that
consists of groups of nerve cells that project to various other
regions of the brain (thalamo-cortical pathways). In general,
these groups of nerve cells are specific relay stations for
sensory information (e.g., visual, auditory, pain, temperature,
etc.). Although active as a relay station to the cerebral cortex,
like most central nervous system relay stations, the thalamus
also receives feedback input from the cortex via cortico-thalamic
pathways.
... ... *apoptotic degeneration: In general, programmed cell
death produced by control mechanisms designed to destroy
defective cells.
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SW 2000 4 Aug
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Related Background:
ON NEW NERVE CELLS IN THE ADULT HUMAN BRAIN
During most of this century, one of the dogmas in neurobiology
was that in the adult human brain new connections between neurons
may arise, but never new neurons. The dogma, in other words, was
that in the adult human brain new nerve cells are not produced,
and the neurons present at birth are the neurons present in the
adult, albeit a maximum number of nerve cells at birth, since a)
the number of neurons in the healthy adult human brain apparently
decreases with age beginning at about age 35; and b) various
neurodegenerative diseases can markedly reduce the population of
neurons in either specific regions of the brain or globally
nearly everywhere in the brain. In recent years, this dogma, the
idea that new nerve cells are not produced in the adult human
brain, has effectively crumbled for at least one specific and
important brain locus called the "hippocampus", which is a region
of the cerebral cortex in the *medial part of the temporal lobe.
In humans, among other functions, the hippocampus is apparently
involved in short-term memory, and analysis of the neurological
correlates of learning behavior in animals indicates that the
hippocampus is also involved in memory in other species.
... ... G. Kempermann and F.H. Gage (2 installations, DE US)
present a review of past and current research in adult
neurogenesis in humans, the authors making the following points:
1) In 1965, Altman and Das reported neurogenesis in the
hippocampus of rats, in a subregion of the hippocampus called the
"dentate gyrus". But this data was not viewed as evidence of
significant neurogenesis in adult mammals, primarily because the
methods available then could not accurately estimate the number
of new neurons nor demonstrate definitively that the new cells
were indeed nerve cells. In addition, the concept of *stem cells
in the brain had not yet been introduced, and the belief was that
for new neurons to appear, the only source would be replication
(i.e., mitosis) of adult neurons. There was also no evidence that
neurogenesis occurred in non-human primates, and so the relevance
of the rat data for the human brain seemed remote.
2) In the mid 1980s, Nottebohm discovered that neurogenesis
occurred in adult canaries in brain centers responsible for song
learning, and that the process accelerated during the seasons in
which the adult birds acquired their songs. Nottebohm and his co-
workers then demonstrated that neurogenesis also occurred in the
hippocampus of adult chickadees, particularly during seasons when
the birds had to keep track of dispersed food storage sites.
3) In 1997, Gould and McEwan reported that some neurogenesis
occurs in the hippocampus of the primate-like tree shrew, and in
1998, these authors found the same phenomenon in marmoset
monkeys, which are classified as actual primates.
4) Because of research difficulties, demonstration of
neurogenesis in the adult human brain had to await special
techniques. In 1998, Peter S. Eriksson reported the use of
bromodeoxyruridine as a marker for neurogenesis and the first
evidence for neurogenesis in the hippocampus of adult humans. The
use of this marker depended on its already established use as a
tumor marker in cancer patients. Bromodeoxyuridine is a marker
that becomes integrated only into the DNA of cells preparing to
divide, and the marker was in use with terminally ill patients
with cancer of the tongue or larynx. Eriksson obtained consent
from a number of patients to investigate their brains after
death, and when 5 patients died, all 5 brains displayed new
neurons in the dentate gyrus subregion of the hippocampus. At the
same time as this study was reported, other research groups
reported nerve cell production in the hippocampus of adult rhesus
monkeys, which are primates closer to humans than marmoset
monkeys.
5) In their review, the authors refer to their own work,
noting that beginning in 1997, they have demonstrated that adult
mice given enriched living conditions generate substantial
increases in dentate gyrus hippocampal neurons over that found in
genetically identical control animals.
6) The authors suggest that studies of neurogenesis in the
adult human brain, while difficult, may lead to better treatments
for a variety of neurological diseases. The authors conclude:
"The expected benefits of unlocking the brain's regenerative
potential justify all the effort that will be required."
-----------
SA 1999 May
-----------
Notes:
... ... *medial part of the temporal lobe: The temporal lobes are
roughly the lower sides of the brain, above the ears and behind
the temporal bones of the skull, but when the human brain is
viewed from the side, as it usually is in common gross
depictions, the large and functionally important ventral and
infolded parts of the temporal lobes are not visible. In general,
the larger anatomical regions of the human brain are best
visualized as highly corrugated lobular structures extensively
folded and densely packed to fit inside the volume-limiting
protective skull. Isolated verbal descriptions of the
architecture are of limited use: anatomical graphics are the best
sources for visualization of gross brain structures.
... ... *stem cells: In general, the term "stem" cells
refers to undifferentiated cells that upon differentiation can
give rise to various specialized cell lines such as blood cells,
skin cells, nerve cells, etc. Adult bone marrow, for example,
contains stem cells that are the precursors of the various
specialized types of blood cells.
-----------
SW 1999 18 Jun
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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5. LONGEVITY IN YEASTS AND ANIMALS
David Gems (University College London, UK) discusses the biology
of the ageing process. A continue question concerning studies of
yeast longevity has been whether experimental results with this
simple organism are applicable to considerations of ageing in
animals. Mutations in several yeast genes can increase the
lifespan of yeast, a unicellular fungus, but it is not clear that
that is of any relevance for understanding the ageing process in
multicellular organisms. Now Tissenbaum and Guarente (2001)
report that the gene _sir-2.1_, a relative of a yeast gene that
controls yeast lifespan, also controls longevity in an animal --
the nematode worm C. elegans. In C. elegans, yeast (C.
cerevisiae), the fruitfly Drosophila melanogaster, and even in
mice, there are many genes that upon mutation increase longevity.
In many cases, the proteins encoded by such genes have
equivalents in higher animals. For example, the adult lifespan of
C. elegans can be tripled as a result of reduced activity of a
signaling pathway resembling that which responds to insulin or
insulin-like growth factor-I (IGF-I) in mammals. But the
important question is whether such genes control ageing in all
animals. Has the biology of ageing been conserved throughout
evolution? Although most types of animals grow old and die, it
does not follow that ageing involves the same processes in all
species. It is possible that in different species the underlying
mechanisms of ageing are different.
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NAT 410:154
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6. NEURONAL EFFECTS OF DRUG ADDICTION
Eric J. Nestler (University of Texas, US) discusses the neural
bases of drug addiction. Drugs of abuse cause long-lasting neural
changes in the brain that underlie the behavioral abnormalities
associated with drug addiction, and clues to the nature and site
of these neural changes can be found by studying the cellular and
molecular events that accompany learning and memory. Recent
experiments on the molecular pathways of learning and memory on
the one hand, and of drug addiction on the other hand, have
converged. Both learning and memory and drug addiction are
modulated by the same neurotrophic factors, share certain
intracellular signaling cascades, and depend on activation of the
transcription factor CREB (cyclic AMP response element binding
protein). The two phenomena are associated with similar
adaptations in neuronal morphology, such as the formation or loss
of dendritic spines. Even more compelling, they are accompanied
by alterations in neural plasticity at glutamatergic synapses
(regions where nerve cells containing glutamate meet and
communicate with other types of neurons). For example, long-term
potentiation (LTP) and long-term depression -- which alter the
excitability of postsynaptic neuronal membranes as part of memory
formation -- have been found at the glutamatergic synapses
implicated in drug addiction.
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SCI 2001 292:2266
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7. SOLAR FORCING, DROUGHTS, AND THE COLLAPSE OF THE MAYA
D.A. Hodell et al (University of Florida, US) discuss solar
forcing of drought frequency in the Mayan lowlands. Lake
Chichancanab, on the north-central Yucatan Peninsula (MX),
possesses sediments that preserve a highly sensitive record of
past climate changes. In 1993, the authors retrieved a 4.9-meter
*sediment core from the lake. The 9000-year-long record of oxygen
isotopes and gypsum concentrations from the core was used to
infer relative changes in the ratio of evaporation to
precipitation. The record demonstrated that the period 800 to
1000 AD was the driest of the middle to late *Holocene, and that
it coincided with the collapse of Classic Maya civilization in
the 9th century AD. The cause of this drought remained unclear,
however, although it apparently was widespread in the *Neotropics
and *Sahel. The authors report analyses from new Lake
Chichancanab sediment cores obtained in May 2000, and demonstrate
that the terminal classic drought was only one episode in a
recurrent pattern of dry events that occurred during the past
2600 years. The record demonstrates that the arid events centered
at 485 BC and 285 BC were part of the 208-year cycle. Similarly,
the droughts between 125 and 210 AD (associated with *Preclassic
Abandonment), at approximately 800 AD (associated with terminal
*Classic Collapse), and at approximately 1020 AD also fit the
pattern of 208-year drought recurrence. The Maya were highly
dependent on rainfall and surface reservoirs as their principal
water supply. Consequently, these multidecade- to multicentury-
scale oscillations probably had a detrimental impact on Maya food
production and culture.
-----------
Notes: The Mayas (Mayans) were Meso-American Indians occupying a
nearly continuous territory in southern Mexico, Guatemala, and
northern Belize. Before the Spanish conquest of Mexico and
Central America, the Maya possessed one of the great
civilizations of the Western Hemisphere, practicing agriculture,
building enormous stone buildings and pyramid temples, working
gold and copper, and using a form of hieroglyphic writing. At its
height, Mayan civilization consisted of more than 40 cities, each
with a population of from 5,000 to 50,000. The peak Mayan
population may have reached 2,000,000 people, most of whom were
settled in the lowlands of what is now Guatemala.
... ... *sediment core: A sediment core is a reasonably
undisturbed sediment sample obtained by boring with any one of a
variety of devices.
... ... *Holocene: The time-frame 10,000 years ago to the
present.
... ... *Neotropics: (Neogea) One of the biogeographical realms
into which the Earth is divided according to the distribution of
animal life (fauna). The Neotropic region includes South America,
the West Indies, and Central America from the Mexican plateau
south.
... ... *Sahel: A large region to the south of the Sahara,
extending from Mauritania and Senegal in the west to Ethiopia and
Sudan in the east.
... ... *Preclassic Abandonment: "Preclassic", in this context,
refers to the period approximately prior to 250 AD. The
"abandonment" considered here is that of 125 to 210 AD at El
Mirador in northern Guatemala. The boundaries of historical time-
frames prior to the Classic Period vary according to source.
... ... *Classic Collapse: The Maya "Classic Period" covers the
approximate time-frame 250 to 900 AD. The collapse began around
750 AD.
-----------
SCI 2001 292:1367
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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Related Background:
ARCHAEOLOGY: CLIMATE AND THE COLLAPSE OF SOCIETIES
The archeological and historical records contain much evidence of
the collapse of prehistoric, ancient, and premodern societies,
with these collapses occurring suddenly and frequently involving
abandonment of the region, replacement of one subsistence base by
another (e.g., replacement of agriculture by livestock-raising
[pastoralism]), or conversion to a lower energy sociopolitical
organization (e.g., conversion to local states from an
interregional empire). Although the consensus among archeologists
and historians has been that such collapses result from a
combination of social, political, and economic factors, a new
perspective is that the collapse of societies in history has
often been the result of sudden changes in climate.
... ... H. Weiss and R.S. Bradley (2 installations, US) present a
review of current research on climate forcing of societal
collapse, the authors making the following points:
1) The authors point out that the accumulation of high-
resolution paleoclimatic data that provide an independent measure
of the timing, amplitude, and duration of past climate events
relevant to societal collapse indicates that these climate events
were abrupt, involved new conditions that were unfamiliar to the
inhabitants of the time, and persisted for decades to centuries.
These climate events were therefore highly disruptive, leading to
societal collapse, which can be viewed as an adaptive response to
otherwise insurmountable stresses.
2) Examples of the relationship between paleoclimate and
societal collapse in the Old World suggest that prehistoric and
early historic societies, from villages to states or empires,
were highly vulnerable to climatic disturbances, and many lines
of evidence now point to climate forcing as the primary agent in
repeated social collapse. In the New World, high-resolution
archeological records also point to abrupt climate change as the
proximal cause of repeated social collapse.
3) Climate during the past 11,000 years was long believed to
have been uneventful, but new evidence increasingly demonstrates
climatic instability. Droughts lasting decades to centuries
started abruptly, were unprecedented in the experience of the
existing societies, and were highly disruptive to the
agricultural foundations of these societies because social and
technological innovations were not available to counter the
rapidity, amplitude, and duration of changing climate conditions.
4) The authors point out that these past climate changes
were unrelated to human activities. In contrast, future climate
change will involve both natural and anthropogenic forces and
will be increasingly dominated by the latter. Current estimates
suggest changes will be large and rapid. Global temperature will
rise and atmospheric circulation will change, leading to a
redistribution of rainfall difficult to predict. The authors
point out that in spite of technological changes, most of the
world's people will continue to be subsistence or small-scale
market agriculturalists who are as vulnerable to climate
fluctuations as the late prehistoric and early historic
societies. Furthermore, in an increasingly crowded world, change
of habitat ("habitat tracking") as an adaptive response will not
be an option.
5) In conclusion, the authors suggest we must use current
information "to design strategies that minimize the impact of
climate change on societies that are at greatest risk. This will
require substantial international cooperation, without which the
21st century will likely witness unprecedented social
disruptions."
-----------
SCI 2001 291:609
SW 2001 9 Feb
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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Related Background:
ON GLOBAL CLIMATE CHANGE
Environmental change involves jumps, fluctuations, and trends,
the environment changing through the operation of the internal
machinery of the *ecosphere (biosphere), and through the external
agencies of cosmic and geological forces. Evidence of past
environmental change, almost always incomplete, derives from
geochemical, physical, biological, historical, and instrumental
sources. In recent years, high-speed computers have allowed
researchers to manipulate complicated and reasonably realistic
models of environmental change, with modelling particularly
useful for studying changes in *sedimentary basins,
biogeochemical cycles, and climate. General circulation models,
run with appropriate boundary conditions, predict climates of the
past, and these predicted climates can be compared with
paleoclimatic indicators.
... ... R.B. Alley et al (3 authors 3 installations, US) present
a review of current research on global climate change, the
authors making the following points:
1) Prediction of climate change requires observational
constraints on the current climate state, knowledge of the way
the coupled air-ocean-ice-earth-life system behaves, and
information on changing forcings such as solar variability.
Studies of past climate are also required to focus model-building
efforts on climate components that are likely to change, and to
allow testing of the ability of models to predict time-evolution
of the system.
2) The last few million years have been generally cold and
icy compared with the previous hundred million years but have
alternated between warmer and colder conditions. These
alternations have been linked to changes over tens of thousands
of years in the seasonal and latitudinal distribution of sunlight
on Earth caused by features of Earth's orbit. Globally
synchronous climate change despite some hemispheric asynchrony of
the forcing is explained at least in part by lowering carbon
dioxide during colder times in response to changes in ocean
chemistry. We live in one of the warmer times of these orbital
cycles; the coolest times brought glaciation to nearly one-third
of the modern land area.
3) Studies of past climate changes indicate that the Earth
system has experienced greater and more rapid changes over larger
areas that was generally believed possible, with jumping between
fundamentally different modes of operation in as little as a few
years. Most of the last 100,000 years or longer has been
characterized by large and abrupt regional-to-global climate
changes, and agriculture and industry have developed during
anomalously stable climatic conditions. New high-resolution
analysis of sediment cores indicates these past changes have been
caused by "*band jumps" between modes of operation of the climate
system. Recurrence of such band jumps is possible and might be
affected by human activities.
-----------
PNAS 1999 96:9987
-----------
Text Notes:
... ... *ecosphere (biosphere): In general, the term "biosphere"
refers to the portion of the planet capable of supporting life.
It ranges from elevations of approximately 10,000 meters above
sea level to the deep ocean, and a few hundred meters below the
surface of the soil. The biosphere consists of the hydrosphere,
the lower atmosphere (troposphere), and the surface of the
*lithosphere, all three regions inhabited by metabolically active
organisms.
... ... *lithosphere: In current geology, the lithosphere is the
approximately 100 kilometer rigid upper layer of the crust and
upper mantle of the Earth.
... ... *sedimentary basins: The term "sedimentary basin" refers
to a subsiding area of the Earth's crust, which permits the net
accumulation of sediment, i.e., material derived from pre-
existing rock, from biogenic sources, or precipitated by chemical
processes.
... ... *band jumps: In this context, the term "band jump" refers
to an abrupt change from one range of variation to another.
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SW 1999 1 Oct
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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8. CHEMISTRY OF CYCLIC TRANSMEMBRANE CHARGE TRANSPORT
R.F. Khairutdinov and J.K. Hurst (Washington State University,
US) discuss cyclic transmembrane charge transport. Closed bilayer
membranes have been effectively used as organizing matrices for
controlling reactivities of a wide variety of chemical and
photochemical processes. Many projected applications that rely on
compartmentation of reaction components, e.g., water photolysis
or biomimetic photochemical syntheses, require electro-neutral
carrier-mediated transmembrane redox steps in the overall
reaction cycles. Nature makes widespread use of lipophilic
quinones for this purpose, the quinones cotransporting protons
and electrons as one arm of respiratory and photosynthetic redox
loops that polarize energy-transduction, driving coupled ATP
synthesis and other forms of cellular work. Quinones appear to
suffer some disadvantages for applications outside living cells,
however, including a) the requirement of a relatively large
quinone pool within the membrane to obtain efficient charge
transport; b) chemical instability associated with formation of
one-electron reduced hydroquinone radicals as initial redox
products, and c) relatively slow transmembrane permeation rates.
The authors report a study of the capacity of pyrylium and
thiopyrylium ions to act as combined quenchers of photoexcited
redox dyes and transmembrane charge carriers to generate long-
lived charge separation across bilayer membranes.
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JACS 2001 123:7352
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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9. STRUCTURE OF EARTH'S MANTLE
Seismic studies indicate the interior of the Earth consists
of three parts: a metallic core, a dense rocky mantle, and a thin
low-density crust. The central part of the core is solid, but the
outer part of the core is evidently liquid. The mantle, the layer
of dense rock and metal oxides between the molten part of the
core and the surface, has plastic properties (i.e., the mantle is
apparently a solid capable of flow under pressure).
The term "lithosphere" refers to the outer layer of the
Earth, comprising the crust and upper mantle, and extending to a
depth of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer.
The term "subduction" refers to the process of
underthrusting of the edge of an oceanic plate into the mantle
underlying an adjacent plate. In this context, the term "plate"
derives from "plate tectonics", the current consensus theory that
the Earth's lithosphere is broken into fairly rigid plates, seven
major plates and many smaller plates, and that convection within
the underlying less rigid "asthenosphere" causes the plates (and
the associated continents and crust) to move relative to each
other.
In general, the term "tomography" refers to a representation
in cross-section in which neighboring 2-dimensional cross-
sections are combined to provide a 3-dimensional model. The use
of computer-aided tomography (CAT) in medical diagnosis is well-
known as a non-invasive method of examining internal organs for
abnormal regions. X-rays or ultrasonic waves are absorbed
unequally be different materials, and computer-aided tomography
consists of studying the attenuation of x-rays or ultrasonic
waves that pass through the body in distinctly controlled planar
sections. The technique of "seismic tomography" uses the same
principles, with the difference that the travel-times of the
signals, rather than their attenuation, are observed. Thus, the
technique of seismic tomography may be described as the 3-
dimensional modeling of the velocity distribution of seismic
waves in the Earth. In general, the technique requires powerful
computational facilities and sophisticated programming.
... ... G.R. Helffrich and B.J. Wood (Tokyo Institute of
Technology, JP) discuss current research on the Earth's mantle.
The Earth's mantle comprises 82 percent of its volume and 65
percent of its mass. It constitutes virtually all of the silicate
part of the Earth, extending from the base of the crust (0.6
percent of Earth's silicate mass) to the top of the metallic
core. Seismological images of the Earth's mantle reveal three
distinct changes in velocity structure at depths of 410, 660, and
2700 kilometers. The first two are best explained by mineral
phase transformations, whereas the third (the so-called D" layer)
probably reflects a change in chemical composition and thermal
structure. Tomographic images of cold slabs in the lower mantle,
the displacements of the 410-kilometer and 660-kilometer
discontinuities around subduction zones, and the occurrence of
small-scale heterogeneities in the lower mantle, all indicate
that subducted material penetrates the deep mantle. which implies
whole-mantle convection. In contrast, geochemical analyses of the
basaltic products of mantle melting are frequently used to infer
that mantle convection is layered, with the deeper mantle largely
isolated from the upper mantle. The authors demonstrate that the
geochemical, seismological, and heat-flow data are all consistent
with whole-mantle convection provided that the observed
heterogeneities are remnants of recycled oceanic and continental
crust that make up approximately 16 and 0.3 percent,
respectively, of mantle volume.
-----------
NAT 2001 412:501
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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Related Background:
GEOPHYSICS: ON SEISMIC TOMOGRAPHY AND MANTLE DYNAMICS
... ... T. Tanimoto and T. Ley (2 installations, US) present a
review of current research on mantle dynamics and seismic
tomography, the authors making the following points:
1) The authors point out that the advent of the theory of
plate tectonics approximately 30 years ago established that most
near-surface geological phenomena such as earthquakes, volcanoes,
and mountain belts can be understood in the context of a unifying
model of interacting surface plates. However, our understanding
of this system has largely been limited to detailed kinematics of
plate motions, leaving the nature of the driving motions in the
interior as a puzzle. Questions such as what is the configuration
of convection, and how are surface tectonics controlled by
internal processes, have long been raised, but a lack of tools
and a lack of evidence prevented evaluation of various
hypotheses. Thus, most views regarding mantle dynamics remained
highly speculative until recently. Seismic tomography, which
emerged in the early 1980s, has provided a major probe of the
dynamical system of which plates are just the surface veneer.
2) The primary question concerning mantle dynamics is
whether mantle convection occurs in mantle-wide convective cells
or whether it involves a layered system, with separate flow
regimes in the upper mantle (i.e., above 650 kilometers) and
lower mantle. One of the most exciting results from work during
the last 5 years is the verification of deep penetration of
former oceanic lithosphere into the lower mantle. Tomography
shows thickened tabular extensions of subducted material to
depths as great as 2000 kilometers directly below deep subduction
zones where earthquakes occur in oceanic slabs down to
approximately 650-kilometer depth. Thus, strictly layered mantle
convection can now be ruled out with good confidence.
3) In summary, seismic tomography has resulted in
breakthrough advances in the last two decades, revealing
fundamental geodynamical processes throughout the Earth's mantle
and core. Convective circulation of the entire mantle is taking
place, with subducted oceanic lithosphere sinking into the lower
mantle, overcoming the resistance to penetration provided by the
phase boundary near 650-kilometer depth that separates the upper
and lower mantle. The boundary layer at the base of the mantle
has been revealed to have complex structure, involving local
stratification, extensive structural anisotropy, and massive
regions of partial melt. The Earth's high *Rayleigh number
convective regime is now recognized to be much more interesting
and complex than suggested by textbook cartoons, and continued
advances in seismic tomography, geodynamic modeling, and high-
pressure-high-temperature mineral physics will be needed to fully
quantify the complex dynamics of our planet's interior.
-----------
PNAS.2000 97:12409
-----------
Notes:
... ... *Rayleigh number: The Rayleigh number is a dimensionless
parameter used in the theory of fluid dynamics. In general, the
Rayleigh number provides a determination of when convection is
initiated in a fluid.
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SW 200 24 Nov
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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Related Background:
IN FOCUS: THE LAYERED EARTH
"During the nineteenth century, the nature of the Earth's
interior was a matter of fierce and fascinating debate. All
theories were hampered by a lack of evidence -- the nature of
rocks deep below the surface was unknown. In 1906, Richard D.
Oldham observed that compressional seismic waves (P waves) slow
abruptly deep within the Earth and can penetrate no further. This
was strong evidence in favor of a liquid core. Three years later,
Andrija Mohorovicic noticed that the velocity of seismic waves
leaps from 7.2 to 8.0 km/s at around 60 km deep. He had
discovered the 'Moho' seismic discontinuity that marks the crust-
mantle boundary. In 1926, Beno Gutenberg obtained evidence for a
seismic discontinuity at the core-mantle boundary. This, the
Gutenberg discontinuity, was confirmed during the 1950s when
world-wide records of blasts from underground nuclear detonations
were scrutinized. Subsequent studies of the Earth's seismic
properties, using seismic waves propagated by earthquakes and by
controlled explosions to 'x-ray' the planet (a technique called
seismic tomography), have revealed a series of somewhat distinct
layers or concentric shells in the solid Earth. Each shell has
different chemical and physical properties..."
-----------
Richard John Huggett: _Environmental Change_
(Routledge, London 1997, p.56)
[The author is Senior Lecturer in Geography at the University of
Manchester, UK] (Science-Week 13 Sep 99)
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10. THE PUZZLE OF THE SUN'S HOT CORONA
B.N. Dwivedi and K.J. Phillips (Banaras Hindu University, IN)
discuss current research on the Sun's corona. As might be
expected, the temperature of the Sun drops steadily from the core
temperature of 15 million kelvins to the photosphere at 6000
kelvins, but then the temperature gradient reverses: the
chromosphere temperature steadily rises to 10,000 kelvins, and
going into the corona, the temperature jumps to 1 million
kelvins, with parts of the corona associated with sunspots even
hotter. Considering that the energy must originate beneath the
photosphere, this temperature gradient reversal is a puzzle. The
first hints of this mystery emerged in the 19th century, when
eclipse observers detected spectral emission lines that no known
element could account for. In the 1940s, researchers associated
two of these lines with iron atoms that had lost up to half their
normal retinue of 26 electrons -- a situation that requires
extremely high temperatures. Later, instruments on rockets and
satellites indicated that the Sun emits copious x-rays and
extreme ultraviolet radiation, which can occur only if the
coronal temperature is in the megakelvin range. A solution to the
puzzle now seems at hand: researchers have implicated magnetic
fields in the coronal heating. Where such fields are strongest,
the corona is hottest. Such magnetic fields can transport energy
in a form other than heat, thereby sidestepping the usual
thermodynamic restrictions. The energy must still be converted to
heat, and there are currently two theories: a) small-scale
magnetic field reconnections, and b) magnetic waves.
-----------
SA 2001 June
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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Related Background:
ON THE SOLAR CORONA
The surface of the Sun and the regions immediately exterior to it
constitute a domain in which various forces are played out on an
immense scale. The bright surface layer of the Sun is called the
"photosphere", a region a few hundred kilometers thick at a
temperature that ranges from 5770 degrees kelvin at its innermost
part to 4400 degrees kelvin at its outermost part, the Sun's
temperature minimum. Above this is the "chromosphere",
approximately 9000 kilometers thick, where the temperature ranges
from the minimum at the photosphere-chromosphere interface to
approximately 20,000 degrees kelvin. And above the chromosphere
is the "corona", the Sun's faint outer atmosphere, where the
temperature is 2 million degrees kelvin or more. The corona is a
low-density hot gas that glows with a pale white color, and one
of the major problems of solar astrophysics is to explain the
extreme temperature increase from the surface of the Sun to its
corona region. ... ... B. Haisch and J. Schmitt (2 installations,
US DE) present a review of recent research concerning the solar
corona, the authors making the following points:
1) Solar physics has traditionally focused on short-term
changes in the atmosphere of the Sun. In contrast, stellar
studies have for decades concentrated on static properties in
order to understand the evolution of stars on long time scales.
Now a change in research perspective has occurred as space-based
instruments have made possible observations of stars previously
precluded by the Earth's atmosphere. Astronomers have now shifted
much of their attention from stellar interiors and surfaces to
activity occurring in the outer atmospheres of stars, and these
observations have contributed to our understanding of the outer
atmosphere of the Sun.
2) The corona of the Sun has been observed throughout human
history during total eclipses, which make it possible to see
sunlight scattered by coronal electrons. By the 1930s,
spectroscopic observations of the visible-light corona had
precisely measured a number of prominent spectroscopic emission
lines, but the source of these lines was unknown.
3) In the 1940s, it became clear that highly ionized iron
and calcium atoms were responsible for the puzzling spectral
emission lines. The highly ionized state of these elements
indicated that the tenuous outer region of the Sun was somehow
being heated to temperatures of more than 1 million degrees
kelvin. The problem then became to explain the temperature
gradient rising from approximately 5000 degrees kelvin at the
light-emitting surface -- the photosphere -- to the million-
degree temperature of the corona.
4) The breakthrough occurred in 1968, when the first high-
resolution x-ray pictures of the Sun became available from an x-
ray telescope at an altitude of 250 kilometers aboard a rocket.
This telescope provided x-ray pictures with unprecedented
resolution, and the corona that appeared in these pictures
consisted of giant loops filled with x-ray emitting hot ionized
gas (plasma). These new images provided the first hint that the
outer atmosphere of the Sun is structured by magnetic fields.
Shortly afterward, time-lapse x-ray photographs from *Skylab
indicated these coronal structures constantly change. Both the
inhomogeneity of the solar corona and its variability forced
theoreticians away from the reigning hypothesis, in which sound
waves generated by turbulent gas flows below the surface of the
Sun propagated into the corona and heated it. Theory indicated
the collective action of these sound waves would be uniform and
unchanging, which was opposite to what was observed. Astronomers
then began to realize that shifting and changing magnetic fields
must be heating the corona. From recent studies of the
atmospheres and rotations of stars other than Sun, it is now
clear that the rotation of the Sun powers the dynamic magnetic
fields which are responsible for the extraordinary heating of the
corona region.
-----------
Sky & Telescope October 1999
-----------
Notes:
... ... *Skylab: A NASA space station launched in 1973. The
manned station contained 6 telescopes for observing the Sun's
chromosphere and corona at x-ray, ultraviolet, and visible
wavelengths. Three crews of 3 astronauts each spent a total of
171 days aboard the space station. Skylab was then abandoned, and
the empty station eventually reentered Earth's atmosphere in
1979, with fragments falling in Western Australia.
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SW 1999 17 Sep
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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11. WATER AND THE MARTIAN LANDSCAPE
Victor R. Baker (University of Arizona, US) discusses current
research on the Martian landscape. The surface of Mars is today
extremely cold and dry. The atmosphere at the land surface is
over 100 times less dense than that of Earth, and it holds only
minuscule amounts of water vapor. With the present Mars obliquity
of 25 degrees, the residual north polar ice cap sublimates water
in the northern spring/summer, and the vapor moves to and
condenses at south polar areas, a pattern that may reverse in
northern autumn/winter. On present-day Mars, wind and not water
is the most continuously active surface-modifying process. In the
remote past, however, the situation on Mars was apparently quite
different. During the past 30 years, the water-generated
landforms and landscapes of Mars have been revealed in increasing
detail by a succession of spacecraft missions. Recent data from
the Mars Global Surveyor mission confirm the view that brief
episodes of water-related activity, including glaciation,
punctuated the geological history of Mars. The most recent of
these episodes seems to have occurred within the past 10 million
years. These new results anomalous in regard to the prevailing
view that the Martian surface has been continuously extremely
cold and dry, much as it is today, for the past 3.9 billion
years. Interpretations of the new data are controversial, but
explaining the anomalies in a consistent manner leads to
potentially fruitful hypotheses for understanding the evolution
of Mars in relation to Earth.
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NAT 2001 412:228
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12. DYNAMICAL TUNNELING
Barbara Goss Levi (Physics Today) discusses dynamic tunneling of
atoms. It has been long understood that atoms in a double
potential well can move back and forth between wells, even though
classically an atom with insufficient energy is precluded from
surmounting the energy barrier between the two compartments.
Thus, it not surprising to learn that an atom executing one type
of regular motion can suddenly be found to be moving 180 degrees
out of phase with its initial motion, i.e., moving in the
opposite direction. That is the idea behind "dynamic tunneling",
or the hopping of particles between separate and stable regions
in phase space. Although there has been some evidence for
dynamical tunneling in molecular systems, it has now been
observed very directly in new experiments on ultracold atoms. In
general, dynamical tunneling involves a system phase space in
which two modes are separated not by an energy barrier but by a
barrier in some other variable of the system. The transition from
one mode to another was termed "dynamical tunneling" by Heller
and Davis in 1981. One example of dynamical tunneling is a
rotating molecule like formaldehyde. This molecule can flip
between two states, with the line from the carbon to the oxygen
atom either parallel or antiparallel to the molecule's angular
momentum vector. One cannot directly photograph the molecule
flipping between these two states, but one can infer that such
flipping occurs from the splitting of the rotational energy
levels. Dynamical tunneling splits the energy levels just as
tunneling through an energy barrier splits the energy levels of a
particle in a potential well.
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PT 2001 August
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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13. IN FOCUS: ON THE HISTORY OF IMMUNOLOGY
"If there has been one lesson in the history of immunotherapy, it
is that reductionism, the single most productive approach in 20th
century biology, has brought us full circle, leading to a global,
almost holistic understanding of an incredibly complex system,
namely immune function... Science is not different from any other
field of endeavor when it comes to taking the path of least
resistance, so for nearly half a century after Pasteur, Koch, and
von Behring, the attention of the field that would ultimately be
called immunology concentrated almost exclusively on [the] so-
called serum factors (or antibodies). Indeed, the very name of
the field at that time, serology, suggests just how fixed the
focus was on the serum, or liquid and noncellular fraction of the
blood. Cells by definition were not part of the serum, and by
extension were not part of the science; in fact, the
conscientious immunologist at the turn of the century would take
great pains to eliminate all cells before getting down to the
hard work of studying the serum. With the advantage of hindsight,
the delayed investigation of the cellular side of immunology
might be seen as an early hint that this century's emphasis on
reductionism can have a downside, namely a narrowness of
intellectual vision that creates blind spots that can persist, as
in this case, for decades.
-----------
Stephen S. Hall: _A Commotion in the Blood: Life, Death, and the
Immune System_
(Henry Holt & Co., New York 1997, p.8,49)
http://www.amazon.com/exec/obidos/ASIN/0805037969/scienceweek
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14. SW ARCHIVE:
CONDENSED-MATTER PHYSICS: ON THE ISING MODEL
In theoretical physics, one approach that has proved to be
of great general utility is to begin with an attempt to identify
and understand the simplest model exhibiting the same essential
features as the physical problem in question. In condensed-matter
physics, such a model is the so-called "Ising model", an approach
that has been applied to ferromagnetism, and also to a number of
other systems. In general, the Ising model consists of an array
of entities in one, two, or three dimensions, with each entity
capable of being in one of two possible states, with each entity
interacting only with its nearest neighbors, with a condition
that when two neighboring entities are in the same state the
total energy of the pair is reduced compared to when the same two
neighboring entities are in opposite states. These are the
elements of the model, with other conditions imposed depending on
how the model is used. Various versions of the model have been of
great utility in studies of cooperative phenomena in condensed-
matter systems, and the model itself has an interesting human
story attached to it.
A "ferromagnet" is a material (e.g., iron) in which there
may be a permanent magnetic moment (magnetic dipole moment), and
in which the *spins of the atoms are aligned parallel to each
other. Concerning permanent magnetic moments: In general,
according to theory, the intrinsic spins of the electrons in an
atom, together with the motion of the electrons around the
nucleus, give rise to a magnetic field around the atom, and the
magnitude of this field is related to the magnetic dipole moment
of the atom or ion. The term "Curie point" refers to the
temperature above which ferromagnetic materials lose their
ferromagnetism. The Curie point is thus the critical point for a
phase transition.
... ... Brian Hayes (American Scientist, US) presents an essay on
the Ising model and its application to ferromagnetism, the author
making the following points:
1) The Ising model was invented in 1920 by Wilhelm Lenz, who
proposed it as a simplified version of a ferromagnet (Physik. Z.
1920 21:613). In 1925, a student of Lenz, Ernst Ising, chose the
model as the subject of his doctoral dissertation at the
University of Hamburg (DE), and the model has subsequently borne
Ising's name.
2) Lenz and Ising formulated the original model in terms of
"spins", although the concept of rotation is never used. In the
original model, a spin is merely one of two states, characterized
by an arrow pointing either up or down but in no other direction.
The spins are arranged in a grid or lattice pattern. Spins at
neighboring sites prefer to point the same way: the energy is
lower when adjacent spins are parallel, and the energy is higher
when adjacent spins are antiparallel. Except for these nearest
neighbor preferences, the spins do not interact at all. Thermal
fluctuations tend to randomize the spins. Finally, an external
magnetic field may impose a bias on the spin directions.
3) Hayes points out that the Ising model is indeed a crude
picture of a ferromagnet: a) the Ising spins correspond to
spinning electrons in iron atoms; b) the lattice represents the
crystal structure; c) the nearest neighbor interaction mimics the
overlap of quantum mechanical wave functions in adjacent iron
atoms. The one element in the model that has no obvious
counterpart in real systems is the requirement that spins take on
only two possible orientations.
4) Ising's doctoral dissertation examined whether the
1-dimensional version of the model exhibited a Curie point. The
results were negative: the 1-dimensional Ising model exhibits no
phase transition at any temperature above absolute zero. Ising
apparently believed this negative result would hold in higher
dimensions as well, but in this conjecture he was wrong.
5) Ising's published results (Z. fur Physik 1925 31:253)
were essentially ignored until 1936, when Rudolf Peierls (1907-
1995) showed that a 2-dimensional Ising model might exhibit a
temperature-dependent phase transition . An exact calculation of
such a system, a mathematical tour de force, was made by Lars
Onsager (1903-1976) in 1944. Exact calculations for 3-dimensional
Ising models have remained intractable, but approximations and
computer simulations involving the model have proved extremely
useful, and the value of the model has grown rather than
diminished through the years. An important approximation method
is known as "the renormalization group": the simplest version of
this algorithm gathers sets of spins into blocks, replaces each
block with a single new spin, and finally adjusts the couplings
between spins to compensate for the coarsening of the lattice.
6) Concerning Ernst Ising, there is no record of Ising ever
publishing anything else in physics. After receiving his
doctorate, Ising taught physics in German public high schools,
but as a Jew he was dismissed from his teaching post when Hitler
came to power in 1933. Ising then taught at a Jewish boarding
school in Potsdam (DE), until that school was destroyed in the
Kristallnacht pogrom of 1938. Ising and his wife fled Germany,
but they escaped only as far as Luxembourg before the war
overtook them. They managed to survive the occupation, and they
finally reached the US in 1947. Ising taught physics and
mathematics in Minot, North Dakota (US), and then taught for
almost 30 years more at Bradley University in Peoria, Illinois
(US). In 1998, Ernst Ising died at the age of 98.
-----------
AS 2000 88:384
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Notes:
... ... *spins: In quantum mechanics, electrons, protons, and
neutrons have an intrinsic angular momentum known as "spin", and
a magnetic moment parallel or antiparallel to that angular
momentum. When electrons are combined together to form an atom or
ion, there is a resultant angular momentum which is a combination
of the intrinsic spin of the electrons and the angular momentum
due to their motion about the nucleus, and this is the "spin" of
the atom or ion. Atoms or ions with non-zero spin are magnetic
atoms or ions.
-------------------
SW 2000 15 Sep
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SCIENCE-WEEK 7 Sep 2001 http://scienceweek.com
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15. SOURCES:
AS: Amer. Scientist; CEN: Chem. & Eng. News; GD: Genes & Dev.;
GR: Genome Res.; JACS: J. Amer. Chem. Soc.; JAMA: J. Amer. Med.
Assoc.; JCE: J. Chem. Educ.; MMWR: CDC Morbidity and Mortality
Weekly Report; NAT: Nature; NATM: Nature Medicine; NEJM: New
Engl. J. Med.; NYT: New York Times; NYR: New York Review; PNAS:
Proc. Natl. Acad. Sci.; PRL: Phys. Rev. Lett.; PT: Physics Today;
SA: Scientific American; SCI: Science; SW: ScienceWeek; TS: The
Scientist
In the text, the affiliation following the author's name is the
affiliation of the lead author. The indication (na) signifies no
known research affiliation.
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