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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.
January 12, 2000 -- Vol. 5 Number 2
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. BIOSYSTEMATICS: ON LINNAEUS
Carl Linnaeus (1707-1778) was a botanist and explorer who is
generally considered the first to frame principles for defining
genera and species and the first to create a uniform system for
naming such genera and species. The revolutionary advance of
Linnaeus was in nomenclature, the introduction of a Latin
binomial system. Linnaeus identified species on the basis of
idealized morphology. In the 20th century, a "new systematics"
was introduced, a new classification scheme designed to reflect
evolutionary history. Current systematics in biology tends to
classify living systems in terms of evolutionary history
(phylogeny), and modern taxonomists (systematists) are among the
foremost students of evolution. (Nature 2 Nov 00 408:33)
2. NEUROBIOLOGY: LEARNING AND MEMORY
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.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12403)
3. MEDICAL BIOLOGY: ALZHEIMER'S DISEASE
Alzheimer's disease is now tabulated as the 12th leading cause of
death in the US, with a 1997 death-rate per 100,000 population of
8.4, higher than homicide, AIDS, and atherosclerosis. A directly
observable hallmark of Alzheimer's disease is the prevalence of
clusters of specific proteins in the brain. These accumulations
occur in two forms: those found inside nerve cells (so-called
"neurofibrillary tangles") and those found outside nerve cells
(so-called "amyloid plaques"). Investigation of amyloid plaques
has revealed that a principal component is a peptide ("beta-
amyloid peptide) of 40 to 42 amino acids, this peptide derived
from a longer protein (beta-amyloid precursor protein [betaAPP])
whose gene has been sequenced.
(Scientific American December 2000)
4. COSMOLOGY: ON THE EVIDENCE FOR THE BIG BANG MODEL
In cosmology, what is called the Big Bang model is at present the
most widely accepted theory of the origin and evolution of the
Universe. The question is posed: Should we believe in the Big
Bang scenario? It is suggested that the extrapolation by
astrophysicists and cosmologists back to a stage when the
Universe had been expanding for a few seconds deserves to be
taken as seriously as, for example, what geologists or
paleontologists tell us about the early history of the Earth.
Their inferences are just as indirect and generally less
quantitative. Moreover, there are several discoveries that might
have been made over the last 30 years that would have invalidated
the Big Bang hypothesis and which have not been made.
(Science 8 Dec 00 290:1919)
5. COSMOLOGY: COSMOLOGICAL THEORIES
Cosmologists have firmly established the foundations of their
field, gathering over the past 70 years abundant evidence that
the Universe is expanding and cooling. James Peebles, a leading
senior cosmologist, evaluates current theory in cosmology, and
points out that we do not know what the Universe was doing before
it was expanding. A leading theory, "inflation", is an attractive
addition to the framework of cosmology, but it lacks support from
various other parts of the framework, and such support is
precisely what cosmologists are now seeking. If measurements in
progress agree with the unique signatures of inflation, then such
measurements will be counted as a persuasive argument for
inflation theory. (Scientific American January 2001)
6. MAGNETOELECTRONICS:
CONTROL OF SEMICONDUCTOR MAGNETISM BY EXTERNAL ELECTRIC FIELDS
The ability to externally control the properties of magnetic
materials would be highly desirable from fundamental and
technological perspectives, particularly in view of recent
developments in magnetoelectronics and spintronics. In
semiconductors, the conductivity can be varied by applying an
electric field, but the electrical manipulation of magnetism in
such materials has proved elusive. A new experiment demonstrates
that the magnetic properties of ferromagnetic semiconductors can
be controlled using standard electronic techniques. This finding,
along with the discovery of new ways to control electronic spin,
paves the way for practical spintronics.
(Nature 21/28 Dec 00 408:944)
7. IN FOCUS: ON THERMODYNAMICS
8. FROM THE SCIENCEWEEK ARCHIVE:
MECHANISMS OF CHEMOTAXIS IN BIOLOGICAL CELLS
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Section 2
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1. BIOSYSTEMATICS: ON LINNAEUS
In science, assigning names to various objects is essential
to communication between researchers, and often important in
recognizing natural orders of systems. In the physical sciences,
the atmosphere of the Earth contains what is considered a high
order of variety of components, with 17 different known gases
varying in chemical and physical properties, distribution, and
interactions. In the biological sciences, the biosphere of the
Earth contains 2 million existing species of plants and animals,
with an estimated 10 to 30 million more species still to be
identified. And within each species, there is enough individual
diversity to sustain microclassifications. Confronted with such
an enormous diversity of components of the biosphere, biologists
are forced to focus on classification and nomenclature schemes in
order to avoid intellectual chaos.
Carolus Linnaeus (1707-1778) (Carl Linnaeus; Carl von Linn‚)
was a botanist and explorer who is generally considered the first
to frame principles for defining genera and species and the first
to create a uniform system for naming such genera and species.
The revolutionary advance of Linnaeus was in nomenclature, the
introduction of a Latin binomial system: each species received a
Latin name, which was not influenced by local names and which
invoked the authority of Latin as a language common to the
educated people of that time. The Latin name has two parts: the
first word is capitalized and indicates the genus; the second
word is not capitalized and is the name of the species within
that genus. In addition to species and genera, Linnaeus also
recognized other classifications or "taxa" (singular, taxon)
which are still used: order, class, and kingdom. To these taxa
have been added "family" (between genus and order), and "phylum"
(between class and kingdom). Each taxon can be divided further by
an appropriate prefix sub- or super-. The monumental monograph by
Linnaeus, _Systema naturae_ (1735), published when Linnaeus was
all of 28 years old, went through 12 editions during his
lifetime, the 13th and final edition appearing posthumously.
Linnaeus identified species on the basis of idealized
morphology. In the 20th century, a "new systematics" was
introduced, a new classification scheme designed to reflect
evolutionary history. The basic unit of classification, the
species, is also the basic unit of evolution, i.e., a population
of actually or potentially interbreeding individuals, with such a
population sharing, via interbreeding, its genetic resources.
This creates the population "gene pool", the total genetic
material of the population, the gene pool determining the
biological resources of the species, the resources upon which
natural selection continuously operates. Thus, current
systematics in biology tends to classify living systems in terms
of evolutionary history (phylogeny), and modern taxonomists
(systematists) are among the foremost students of evolution.
... ... Sandra Knapp (Natural History Museum London, UK) presents
an essay on Linnaeus, the author making the following points:
1) The author points out that biological nomenclature is
often dismissed as the arcane and sterile province of academic
taxonomists, but without a name, we find it impossible to
communicate about even common objects. Our ability to call a
rose, not by any name, but by a precise name such as Rosa canina
or Rosa multiflora, allows us to communicate in a common
language. By linking a name with a single specimen, which can be
examined again and again, biology becomes a repeatable science.
Scientific names, the genus and species, provide reference points
in biological space to which we may compare future discoveries
and information.
2) The author points out, however, that although names imply
an underlying order, it is folly to equate them with "biological
reality". Our concepts of how organisms are related differ
radically from that of Linnaeus or that of Charles Darwin (1809-
1882), and these concepts continue to improve. Concepts of what a
species actually is are also changing as knowledge increases --
not only knowledge as to the extent of diversity, but also that
of the genetic structure inherent in all living things.
3) The author concludes: "By inventing a simple, concise
method of naming organisms by genus and species, Linnaeus
revolutionized biology. How much more useful to know that the
creature from whom we obtain a DNA sequence is Drosophila
melanogaster [a species of fruit fly] -- comparable to other such
flies, its identity verifiable and our results repeatable --
rather than one of Jorge Luis Borges' creatures "that from a long
way off look like flies."
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[Editor's note: Jorge Luis Borges (1899-1986) was a writer
acclaimed for his poems, essays, and short stories. The flies in
poetry and fiction may lack precise nomenclature, but literature
is another country, a place with its own realities.]
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Sandra Knapp: What's in a name?
(Nature 2 Nov 00 408:33)
QY: Sandra Knapp: Dept. of Botany, Natural History Museum, UK.
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Summary by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
"It is impossible to dissociate language from science or science
from language, because every natural science always involves
three things: the sequence of phenomena on which the science is
based; the abstract concepts which call these phenomena to mind;
and the words in which the concepts are expressed. To call forth
a concept, a word is needed; to portray a phenomenon, a concept
is needed. All three mirror one and same reality. Words are thus
required to preserve and transmit ideas, so that it is clear that
the advancement of a science and the improvement of its technical
vocabulary go hand in hand. No matter how certain we are of the
phenomena, no matter how adequately our concepts reflect them, we
cannot help perpetuating wrong ideas unless we have a precise
terminology in which to express ourselves.
-- Antoine Laurent Lavoisier (1743-1794)
[From the Preface to _A General Introduction to Chemistry_ (1789)
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2. 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.
-----------
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
-----------
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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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
NEUROBIOLOGY: ON THE BIOLOGICAL BASIS OF MEMORY
Exactly 100 years ago, two psychologists, G.E. Mueller and
A. Pilzecker, proposed what came to be called the perseveration-
consolidation hypothesis of memory. In studies with human
subjects, Mueller and Pilzecker found that memory of newly
learned information was disrupted by the learning of other
information shortly after the original learning, and they
suggested that processes underlying new memories initially
persist in a fragile state and then consolidate over time. This
consolidation hypothesis still guides research, particularly
research in neurobiology on the time-dependent involvement of
neural systems and cellular processes enabling lasting 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.
... ... James L. McGaugh (University of California Irvine, US)
presents a review of current research on memory, the author
suggesting the following caveats concerning the present state of
the field:
1) The author points out that the idea that synaptic
mechanisms of long-term potentiation and long-term facilitation
underlie memory remains a hypothesis.
2) The author points out that although studies of long-term
potentiation and memory have focused on the involvement of the
hippocampus, much evidence indicates that the hippocampus has
only a time-limited role in the consolidation and/or
stabilization of lasting memory.
3) The author points out that there are forms of memory that
apparently do not involve the hippocampus and that may not use
any known mechanisms of synaptic plasticity.
4) The author points out that despite theoretical
conjectures, little is known about system and cellular processes
mediating consolidation that continues for several hours or
longer after learning, consolidation that creates lifelong
memories.
Concerning the above caveats, the author concludes: "These
issues remain to be addressed in this new century of research on
memory consolidation."
-----------
James L. McGaugh: Memory -- a century of consolidation.
(Science 14 Jan 00 287:248)
QY: James L. McGaugh [jlmcgaug@uci.edu]
-----------
Text Notes:
... ... *neurotransmitter substance: Neurotransmitters are
chemical substances released at the terminals of nerve axons in
response to the propagation of an impulse to the end of that
axon. The neurotransmitter substance diffuses into the synapse,
the junction between the presynaptic nerve ending and the
postsynaptic neuron, and at the membrane of the postsynaptic
neuron the transmitter substance interacts with a receptor.
Depending on the type of receptor, the result may be an
excitatory or an inhibitory effect on the postsynaptic nerve
cell.
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE BIOLOGICAL SUBSTRATES OF MEMORY FORMATION
The capacity of the nervous system to change (often referred to
as neural or brain "plasticity") is particularly prominent during
development, but the ability to learn new skills and establish
new memories clearly continues throughout life. The central
question is simply stated: How does the adult nervous system
mediate such changes? An understanding of the mechanisms
responsible for learning and other plastic changes in the adult
brain continues to be one of the most important challenges of
neuroscience, with a great deal of devoted research effort in
many laboratories in a number of associated disciplines. At the
present time, after nearly a century of research, there is a
consensus among investigators that the mechanisms of memory
formation involve carefully regulated changes in the strength of
existing connections between nerve cells (*synapses). Experiments
carried out in a variety of animals have demonstrated that
synaptic strength can be altered over periods ranging from
milliseconds to months, and that the cellular mechanisms
underlying these changes are transient modifications of the
transmission of activity from one nerve cell to another
(*neurotransmission) and, in the case of longer-lasting
alterations, changes in *gene expression.
... ... Jerry Chi-Ping Yin (Cold Spring Harbor Laboratories, US)
presents a review of current research on the localization of
memory functions, the author making the following points:
1) Concerning investigations of memory localization at the
anatomical/systems level, Karl Lashley (1890-1958) used
anatomical lesions in the rat brain to search for the memory
"engram", the physical locus of long-term memory. Over a 30-year
period, Lashley performed surgical removal ("ablations") of
various regions in the rat *cerebral cortex, and came to the
disappointing view that no single well-defined lesion could
totally disrupt learning and memory formation [*Note #1]. This
resulted in the general hypothesis that memories are distributed
throughout the brain. Beginning in the late 1930s, however,
Wilder Penfield (1891-1976), in the course of neurosurgical
procedures for the treatment of human epilepsy, electrically
stimulated the temporal cortex of patients and caused them to
experience extremely vivid "memories" [*Note #2]. These
observations led Penfield to conclude that memories are
localized. Recent studies using non-invasive brain imaging,
coupled with refined animal ablation studies, have led to the
contemporary view that interacting widely distributed networks of
neurons participate in memory formation. A complication is the
apparent existence of functional redundancy ("backup" circuits)
and the possibility that different anatomical regions may be used
at different times after memory formation.
2) Concerning investigations of memory localization at the
cellular level, neurophysiologists during the second half of this
century have developed a conceptual framework involving activity
dependent strengthening of neuronal connections. The search for
the loci of memory formation has become reduced to a search for
mechanisms that strengthen synaptic connectivity. The current
favorite cellular model for learning and memory formation is
"long-term potentiation", a physiological description of
increased synaptic efficacy following high-frequency stimulation.
A continuing controversy is whether the primary locus of changes
is on the pre- or post-synaptic side of the synapse. Proponents
of presynaptic change suggest that potentiation results from
changes in the amount of *transmitter release through one of many
possible mechanisms. Advocates of postsynaptic change propose
alterations in the efficiency of *receptor activation, perhaps
modulated through the "unmasking" of silent synapses.
3) Concerning investigations of memory localization at the
molecular level, recent insights have been made into key
molecules whose activity apparently affects the process of memory
formation. These studies highlight two different uses of the term
"location": a) the various subcellular compartments where
important molecular entities are located; b) the changes in
specific protein amino-acid-residues produced by *post-
translational modification. In both cases, the activity and
interactions of important proteins are involved. At the present
time, there are at least 4 major *kinase systems believed to be
involved in memory formation: a) the *cyclic-AMP (cAMP)-dependent
protein kinase (protein kinase A); b) the *calcium-calmodulin
kinases; c) the *protein kinase C family; and d) the *mitogen-
activated protein (MAP) kinase pathway. The subcellular
localization of these kinases are apparently all regulated
through interactions with other proteins.
4) The author suggests that the experimental results
gathered from different levels of analysis of learning and memory
formation can be integrated through the use of molecular
"reporters" -- molecular tags that allow monitoring of the
activity of important proteins and other molecular entities. For
example, a large amount of data collected from *Drosophila,
*Aplysia, the mouse, and the rat, have demonstrated the
importance of the *transcription factor cAMP-response-element-
binding protein (CREB) (and possibly its related family members)
in the process of consolidating long-lasting plastic changes.
CREB acts by binding to DNA sequences. This body of data,
collected from experiments involving a variety of behavioral
tasks and models of plasticity, supports the hypothesis that
learning-induced changes in gene transcription, at least
partially initiated through the activation of CREB family
members, are critical in the process of long-lasting changes in
plasticity.
-----------
Jerry Chi-Ping Yin: Location, location, location: The many
addresses of memory formation.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9985)
QY: Jerry Chi-Ping Yin, Cold Spring Harbor Laboratory, Cold
Spring Harbor, NY 11724-2213.
-----------
Text Notes:
... ... *synapses: 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.
... ... *neurotransmission: The term "neurotransmission" refers
to all the events at a synapse, particularly the release of
neurotransmitters and their action on the postsynaptic neuron.
(See main report notes for "neurotransmitters".)
... ... *gene expression: In general, the term "gene expression"
includes any gene activity, but particularly an activity that
produces the synthesis or activation of a specific protein.
... ... *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.
... ... *Note #1: Lashley's failure to localize memory in a
specific region of the mammalian cerebral cortex was one of the
great puzzles of the middle part of the 20th century. In 1950,
Lashley wrote: "This series of experiments... has discovered
nothing directly of the real nature of the engram. I sometimes
feel, in reviewing the evidence on the localization of the memory
trace, that the necessary conclusion is that learning just is not
possible."
... ... *Note #2: The observations that came out of Penfield's
surgery and laboratory at McGill University had dramatic
theoretical consequences in psychology and neurobiology.
Penfield, a neurosurgeon with training in physiology who
specialized in therapeutic surgery in the treatment of certain
forms of epilepsy, used electrical currents to stimulate the
surface of the brain. The therapeutic objective was to make a
brain map for that particular patient prior to deciding exactly
which damaged parts of the brain could be safely removed without
producing problems more severe than the epileptic condition. The
technique had been developed in 1909 by the neurosurgeon Harvey
Cushing. Penfield's research on the neurological basis of
language and long-term memory, much of it in collaboration with
Herbert Jasper and Lamar Roberts, revolutionized the concepts of
brain maps that existed in the 1950s.
... ... *transmitter release: (neurotransmitter release) See
above: "neurotransmission".
... ... *receptor activation: In this context, the term
"receptor" refers to postsynaptic membrane receptors.
... ... *post-translational modification: In this context,
translation is protein synthesis, the process during which
polypeptides are synthesized on ribosomes in accordance with RNA
code. The term "post-translational modification" refers to a
modification of protein that occurs after synthesis of that
protein, i.e., the modification is not a result of changes in the
DNA or RNA coding for that protein.
... ... *kinase: In general, a "kinase" is any enzyme involved in
the transfer of a phosphate group.
... ... *cyclic-AMP (cAMP): Cyclic adenosine monophosphate (cAMP)
is an important postsynaptic intracellular substance activated by
incoming synaptic activity, a "messenger" involved in various
aspects of cell regulation and protein synthesis.
... ... *calcium-calmodulin kinases: Calmodulin is a calcium-ion-
binding protein that mediates many of the regulatory effects of
calcium ions in eukaryotic cells (cells with organelles such as
nuclei). A "calcium-calmodulin kinase" is a kinase enzyme whose
activity is dependent on the presence of calcium-calmodulin.
... ... *protein kinase C family: (PKC family) Any of a family of
protein kinase enzymes that require anionic phospholipid for
activity and are regulated by diacylglycerol and calcium ion.
These enzymes phosphorylate hydroxyl groups in substrate serine
and threonine residues.
... ... *mitogen-activated protein (MAP) kinase: A family of
protein kinases that perform a crucial step in relaying signals
from the plasma membrane to the cell nucleus. They are activated
by a wide range of proliferation- or differentiation-inducing
signals. (A "mitogen" is any compound that stimulates mitotic
cell division.)
... ... *Drosophila: A fruit fly genus.
... ... *Aplysia: A large gastropod mollusk with readily
identifiable individual nerve cells in its central nervous
system. It has been used extensively in neurobiological research
since the 1960s. (The class "gastropods" contains the snails,
slugs, limpets, and conchs.)
... ... *transcription factor: "Transcription" is the process by
which the genetic information in DNA is converted into RNA, and
transcription factors are a class of DNA-binding proteins that
regulate RNA transcription.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 1Oct99
For more information: http://scienceweek.com/swfr.htm
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3. MEDICAL BIOLOGY: ALZHEIMER'S DISEASE
Alzheimer's disease is now tabulated as the 12th leading cause of
death in the US, with a 1997 death-rate per 100,000 population of
8.4, higher than homicide and legal intervention (7.0), AIDS
(6.2), and atherosclerosis (6.2). It is estimated that by 2025
more than 20 million people worldwide will be afflicted with the
disease. In general, Alzheimer's disease is a degenerative brain
disorder that develops in mid- to late-adult life, the disease
resulting in a progressive and irreversible decline in memory
coupled with a decline in various other cognitive functions. In
terms of general pathology, the disease is characterized by the
destruction of nerve cells and neural connections in the cerebral
cortex of the brain and by a visible and significant loss of
brain mass.
... ... Peter H. St. George-Hyslop (University of Toronto, CA)
presents a review of current research on the biology of
Alzheimer's disease, the author making the following points:
1) The author points out that post-mortem histological
analysis of the brains of patients with Alzheimer's disease
reveals loss of nerve cells in specific regions of the brain,
such as the hippocampus, a center for memory, and the cerebral
cortex in general, the brain structure most involved in
reasoning, memory, language, and other important cognitive
processes.
2) The second directly observable hallmark of Alzheimer's
disease is the prevalence of clusters of specific proteins in the
brain. These accumulations occur in two forms: those found inside
nerve cells (so-called "neurofibrillary tangles") and those found
outside nerve cells (so-called "amyloid plaques").
3) Investigation of amyloid plaques has revealed that a
principal component is a peptide ("beta-amyloid peptide) of 40 to
42 amino acids, this peptide derived from a longer protein (beta-
amyloid precursor protein [betaAPP]) whose gene has been
sequenced. The isolation of the beta-amyloid peptide and the
isolation of the gene for beta-amyloid precursor protein were
quickly followed by the discovery that the precursor protein gene
is located on chromosome 21, the same chromosome involved in Down
syndrome, another neurodegenerative disorder.
4) Although the precise biological role of normal beta-
amyloid precursor protein remains obscure, it is known that many
kinds of cells and tissues produce this protein, and that the
protein ranges from 695 to 770 amino acids in length. The protein
apparently spans the cell membrane, with a short segment jutting
into the cell interior and a longer segment jutting into the
extracellular space. The beta-amyloid peptide is apparently
snipped out of the section of the precursor protein that spans
the cell membrane. Research has revealed there are two possible
snipping processes: a) the precursor protein is first cleaved by
an apparent enzyme (alpha-secretase), then cut by another
apparent enzyme (gamma-secretase), and together these cuts
produce a harmless peptide fragment called "p3"; b) in a second
possible process, an enzyme that has now been isolated (beta-
secretase) clips the precursor protein, one of the resulting
pieces is then snipped by gamma-secretase, and the result is the
beta-amyloid peptide. Under ordinary conditions, most beta-
amyloid strings contain 40 amino acids. But a small number (less
than 10 percent) have two extra amino acids, and it has been
demonstrated that this slightly longer form is the form that
gives rise to plaques and the form that has a direct toxic effect
on neurons.
5) The idea of changes in beta-amyloid precursor protein as
central to Alzheimer's disease gained further support with the
discovery of mutations in a set of genes that interfere with the
cutting of the precursor protein. Disruptions in the genes
presenilin-1 and presenilin-2, genes that are located on
chromosome 14 and chromosome 1, respectively, cause a very
aggressive form of early-onset Alzheimer's disease. (Early-onset
forms are generally seen in approximately 10 to 60 percent of
patients with familial Alzheimer's disease.) Both genes encode
proteins that span the cell membrane several times, and these
proteins apparently undergo a complicated maturation process,
during which they are cut into two pieces that are in turn
incorporated into a protein complex involved in cutting other
membrane-bound proteins, such as beta-amyloid precursor protein.
6) The author concludes: "The biochemical, molecular,
genetic, epidemiological, and clinical discoveries of the past 10
years or so have significantly advanced our understanding of the
mechanisms underlying Alzheimer's disease and make it
increasingly likely that, in the years to come, useful treatments
will be generated."
-----------
Peter H. St. George-Hyslop: Piecing together Alzheimer's.
(Scientific American December 2000)
QY: Peter H. St. George-Hyslop: University of Toronto, CA.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. COSMOLOGY: ON THE EVIDENCE FOR THE BIG BANG MODEL
In cosmology, what is called the Big Bang model is at present the
most widely accepted theory of the origin and evolution of the
Universe. The essential feature of the model is the emergence of
the Universe from a state of extremely high temperature and
density that occurred approximately 15 billion years ago. Such a
model was proposed by Aleksandr Friedmann (1888-1925) in 1922,
and by Georges Lemaitre (1894-1966) in 1927, but the modern
version was developed by George Gamow (1904-1968) and others
beginning in 1946. The model essentially depends on two
assumptions: a) the general theory of relativity correctly
describes the gravitational interaction of all matter; b) the
"cosmological principle" obtains, i.e., an observer's view of the
Universe in the large depends neither on the direction of
observation nor on the location of the observer. The Big Bang
model accounts for the expansion of the Universe; the existence
of the *cosmic background radiation; and the abundance of low-
mass nuclei such as helium, helium-3, deuterium, and lithium-7,
which are predicted to have been formed approximately 1 second
after the big bang when the temperature was 10^(10) degrees
kelvin.
... ... Martin J. Rees (Cambridge University, UK), in a review of
contemporary cosmology, poses the question: Should we believe in
the Big Bang scenario? The author makes the following points:
1) The author suggests that the extrapolation by
astrophysicists and cosmologists back to a stage when the
Universe had been expanding for a few seconds deserves to be
taken as seriously as, for example, what geologists or
paleontologists tell us about the early history of the Earth.
Their inferences are just as indirect and generally less
quantitative. Moreover, there are several discoveries that might
have been made over the last 30 years that would have invalidated
the Big Bang hypothesis and which have not been made:
... ... a) Astronomers might have found an object whose helium
abundance was far below the amount predicted by the Big Bang
model -- 23 percent. This would have been fatal to the model,
because extra helium made in stars can readily boost helium above
its pregalactic abundance, but there seems no way of converting
all the helium back to hydrogen.
... ... b) The background radiation measured so accurately by the
Cosmic Background Explorer (COBE) satellite might have turned out
to have a spectrum that differed from the expected *blackbody or
thermal form. Furthermore, the radiation temperature could have
been so smooth over the whole sky that it would be incompatible
with the fluctuations needed to give rise to present-day
structures like the clusters of galaxies.
... ... c) A stable *neutrino might have been discovered with a
mass in the range of 100 to 10^(6) electron volts. This would
have been fatal to the Big Bang model, because the hot early
Universe, according to the theory, should have contained almost
as many neutrinos as photons. If each neutrino weighed even a
millionth as much as an atom, the neutrinos would in toto
contribute too much mass to the present Universe -- more, even,
than could be hidden in *dark matter.
... ... d) The observed deuterium abundance could have been so
high that it was inconsistent with big bang *nucleosynthesis (or
so high that it implied an unacceptable low *baryon density).
The author states: "The Big Bang theory's survival gives us
confidence in extrapolating right back to the first few seconds
of Cosmic history and assuming that the laws of microphysics were
the same then as now."
-----------
Martin J. Rees: Piecing together the biggest puzzle of all.
(Science 8 Dec 00 290:1919)
QY: Martin J. Rees: Cambridge University, UK.
-----------
Text Notes:
... ... *cosmic background radiation: What is known as the
"cosmic microwave background radiation" was discovered
accidentally in 1964, when A.A. Penzius and R. Wilson, measuring
noise that might interfere with satellite communications, noted a
mysterious signal that was soon interpreted to be the microwave
background radiation originating in the Big Bang. In 1978,
Penzius and Wilson received the Nobel Prize in Physics for this
discovery. The cosmic microwave background is black-body
radiation (the emission radiation of a perfect absorber of
radiation) at a present temperature of 2.73 degrees Kelvin, and
has an almost equal intensity in all directions in space. The
deviations from isotropic intensity, however, are of extreme
importance in theoretical cosmology.
... ... *blackbody: In physics, a black-body is an ideal
body that absorbs all radiation and reflects none of it, and
black-body radiation is the emission of radiant energy that would
occur from a black-body at a fixed temperature and with a
spectral energy distribution described by Planck's black-body
radiation equation.
... ... *neutrino: Neutrinos are fundamental particles with zero
charge, possibly zero mass, and an angular momentum factor (spin)
of 1/2. Various natural processes produce neutrinos: stellar
nuclear reactions, reactions occurring during supernova
explosions, cosmic ray collisions with matter, etc.
... ... *dark matter: In general, in this context, the term "dark
matter" refers to material whose presence can be inferred from
its effects on the motions of stars and galaxies, but which
cannot be seen directly because it emits little or no radiation.
It is believed that at least 90 percent of the mass in the
Universe exists as some form or dark matter.
... ... *nucleosynthesis: In this context, the fusion reaction in
stars that produce the various elements.
... ... *baryon: A baryon is a nuclear particle, e.g., a proton,
built from 3 quarks (fundamental particles that combine to make
up protons, neutrons, and mesons).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
COSMOLOGY: EXPECTATIONS IN THE NEXT CENTURY OF RESEARCH
Cosmology is one of the grand sciences, a domain of research
whose results have enormous intellectual consequences, at least
for people who care about what they are and where they are.
Martin Rees (Cambridge University, UK) presents an essay on the
near-future research expectations of cosmologists, the author
making the following points:
1) Astronomers still do not know what the Universe is made
of. Observable radiation-emitting objects -- such as stars,
*quasars, and galaxies -- apparently constitute only a small
fraction of the matter in the Universe. The vast bulk of matter
is dark and unaccounted for, and most cosmologists believe this
dark matter is composed of weakly interacting particles left over
from the *Big Bang. But dark matter could be something more
exotic. "Whatever the case, it is clear that galaxies, stars and
planets are a mere afterthought in a Cosmos dominated by quite
different stuff." The author suggests that intensive searches for
dark matter, mainly via sensitive underground experiments
designed to detect elusive subatomic particles, will continue in
the coming decade, and that within the next decade both the
amount and nature of dark matter will be clarified.
2) The author suggests that research in the near-future is
also likely to focus on the evolution of the large-scale
structure of the Universe. The current view is that ever since
the Big Bang, gravity has been amplifying inhomogeneities,
building up structures and enhancing temperature contrasts -- "a
prerequisite for the emergence of the complexity that lies around
us now and of which we're a part." The author suggests that
astronomers are now learning more about the 10 billion year
process of Cosmic evolution by creating virtual universes on
computers, and that in the coming years researchers will be able
to simulate the history of the Universe with ever improving
realism and then compare the results with astronomical
observations.
3) The author suggests that the great mystery for
cosmologists is the series of events that occurred less than 1
millisecond after the Big Bang, when the Universe was
extraordinarily small, hot, and dense. "The laws of physics with
which we are familiar offer little firm guidance for explaining
what happened during this critical period." To solve this
problem, it will necessary to improve and refine current
observations in order to understand the characteristics of the
Universe when it was only one second old: its expansion rate, the
size of its density fluctuations, and its proportions of ordinary
atoms, dark matter, and radiation.
4) The author suggests the following Cosmic timeline for the
evolution of the Universe from the Big Bang to the present:
... ... a) 10^(-43) seconds after the Big Bang: the *Quantum
Gravity Era.
... ... b) 10^(-36) seconds after the Big Bang: Probable *Era of
Inflation.
... ... c) 10^(-5) seconds after the Big Bang: Formation of
protons and neutrons from *quarks.
... ... d) 3 minutes after the Big Bang: Synthesis of atomic
nuclei.
... ... e) 300,000 years after the Big Bang: First atoms form.
... ... f) 1 billion years after the Big Bang: Appearance of
first stars, galaxies, and quasars.
... ... g) 10 to 15 billion years after the Big Bang: Appearance
of modern galaxies.
5) The author concludes: "How did a hot amorphous fireball
evolve, over 10 to 15 billion years, into our complex Cosmos of
galaxies, stars, and planets? How did atoms assemble -- here on
Earth and perhaps on other worlds -- into living beings intricate
enough to ponder their own origins? These questions are a
challenge for the new millennium. Answering them may well be an
unending quest."
-----------
Martin Rees: Exploring our Universe and others.
(Scientific American December 1999)
QY: Martin Rees, Cambridge University, UK.
-----------
Text Notes:
... ... *quasars: (quasi-stellar objects). Extremely luminous
sources radiating energy over the entire spectrum from x-rays to
radio waves, and which are apparently the oldest and most distant
objects in the universe. They are believed to involve massive
black holes.
... ... *Big Bang: The Big Bang theory is the general
cosmological model that proposes that all matter and radiation in
the universe originated in an explosion at a finite time in the
past.
... ... *Quantum Gravity Era: Quantum field theory is the
mathematical fusion of quantum mechanics with special relativity
theory, and the term "quantum gravity" refers to the fusion of
quantum mechanics with general relativity theory. The essential
basis for these fusions is the so-called "equivalence principle",
which identifies the mass involved in the gravitational force
equation with the inertial mass in the equation that relates any
force to the product of inertial mass and acceleration. The
"quantum gravity era" is the time-frame during which both quantum
effects and gravity determined the behavior of particles.
... ... *Era of Inflation: The inflationary model, first
proposed by Alan Guth in 1980, proposes that quantum
fluctuations in the time period 10^(-35) to 10^(-32) seconds
after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame.
... ... *quarks: A quark is a hypothetical fundamental particle,
having charges whose magnitudes are one-third or two-thirds of
the electron charge, and from which the elementary particles may
in theory be constructed.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 28Jan00
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. COSMOLOGY: COSMOLOGICAL THEORIES
James Peebles (P. James E. Peebles) (1935- ), is a distinguished
senior cosmologist whose work has been of great influence in the
field for many decades. Among other contributions, in 1965,
Peebles, in collaboration with Robert Dicke, predicted that a
background radiation should be detectable as a remnant of the
*Big Bang. Peebles also calculated that the amount of helium
present in the Universe as a consequence of the Big Bang should
be 25 to 30 percent, a figure that agrees with current
observations. Peebles is also the author of several advanced
texts that defined the subject of cosmology for a generation of
astronomers. In a recent essay, Peebles confronts current
theories in cosmology and makes the following points:
1) Peebles suggests that cosmologists have firmly
established the foundations of their field, gathering over the
past 70 years abundant evidence that the Universe is expanding
and cooling:
... ... a) The light from distant galaxies is shifted toward the
red, as it should be if space is expanding and galaxies are
pulled away from one another.
... ... b) A sea of thermal radiation fills space, as it should
if space was previously denser and hotter.
... ... c) The Universe contains large amounts of deuterium and
helium, as it should if temperatures were once much higher.
... ... d) Galaxies billions of years ago look distinctly
younger, as they should if they are close to the time when no
galaxies existed.
... ... e) The curvature of space-time seems to be related to the
material content of the Universe, as it should be if the Universe
is expanding according to the predictions of Einstein's gravity
theory -- the general theory of relativity.
The author points out that the idea that the Universe is
expanding and cooling is the essence of the Big Bang theory, and
the author states: "You will notice I have said nothing about an
'explosion' -- the Big Bang theory describes how our Universe is
evolving, not how it began."
2) Peebles points out that we do not know what the Universe
was doing before it was expanding. A leading theory,
"*inflation", is an attractive addition to the framework of
cosmology, but it lacks support from various other parts of the
framework, and such support is precisely what cosmologists are
now seeking. If measurements in progress agree with the unique
signatures of inflation, then such measurements will be counted
as a persuasive argument for inflation theory. "But until that
time, I would not settle any bets on whether inflation really
happened. I am not criticizing the theory; I simply mean that
this is brave, pioneering work still to be tested."
3) Concerning the current use of the "*cosmological
constant" as an explanatory concept in understanding the evidence
for an accelerating Universe, Peebles states: "The evidence is
impressive, but I am still uneasy about details of the case for
the cosmological constant, including possible contradictions with
the evolution of galaxies and their spatial distribution. The
theory of the accelerating Universe is a work in progress. I
admire the architecture, but I would not want to move in just
yet."
4) Peebles concludes: "Over time, inflation, *quintessence,
and other concepts now under debate either will be solidly
integrated into the central framework or will be abandoned and
replaced by something better. In a sense, we are working
ourselves out of a job. But the Universe is a complicated place,
to put it mildly, and it is silly to think we will run out of
productive lines of research anytime soon. Confusion is a sign
that we are doing something right: it is the fertile commotion of
a construction site."
-----------
P. James E. Peebles: Making sense of modern cosmology.
(Scientific American January 2001)
QY: P. James E. Peebles, Princeton University 609-258-3000.
-----------
Text Notes:
... ... *Big Bang: See previous report.
... ... *inflation: The inflationary model, first
proposed by Alan Guth in 1980, proposes that quantum
fluctuations in the time period 10^(-35) to 10^(-32) seconds
after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame.
... ... *cosmological constant: See the following report.
... ... *quintessence: See following report.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON QUINTESSENCE AND THE EVOLUTION OF THE COSMOLOGICAL CONSTANT
In cosmology, the "cosmological constant" is a mathematical term
introduced by Einstein into the equations of general relativity,
the purpose to obtain a solution of the equations corresponding
to a "static universe". The term describes a pressure (if
positive) or a tension (if negative) which can cause the Universe
to expand or contract even in the absence of any matter. In other
words, the cosmological constant represents an effective "vacuum
energy". When the expansion of the Universe was discovered,
Einstein apparently began to regard the introduction of this term
as a mistake, and he described the cosmological constant as the
"greatest mistake of my life". But the term has reappeared as the
proposed source of apparent accelerated cosmic expansion.
... ... P.J.E. Peebles (Princeton University, US) presents a
short review of current ideas concerning the cosmological
constant, the author making the following points: 1) Contrary to
expectations, the evidence is that the Universe is expanding at
approximately twice the velocity required to overcome the
gravitational pull of all the matter the Universe contains. The
implication of this is that in the past the greater density of
mass in the Universe gravitationally slowed the expansion, while
in the future the expansion rate will be close to constant or
perhaps increasing under the influence of a new type of matter
that some call "quintessence". 2) Quintessence began as
Einstein's cosmological constant, Lambda. It has negative
gravitational mass: its gravity pushes things apart. 3) Particle
physicists later adopted Einstein's Lambda as a good model for
the gravitational effect of the *active vacuum of quantum
physics, although the idea is at odds with the small value of
Lambda indicated by cosmology. 4) Theoretical cosmologists have
noted that as the Universe expands and cools, Lambda tends to
decrease. As the Universe cools, *symmetries among forces are
broken, particles acquire masses, and these processes tend to
release an analogue of *latent heat. The vacuum energy density
accordingly decreases, and with it the value of Lambda. Perhaps
an enormous Lambda drove an early rapid expansion that smoothed
the primeval chaos to make the near uniform Universe we see
today, with a decrease in Lambda over time to its current value.
This is the cosmological *inflation concept. 5) The author
suggests that the recent great advances in detectors, telescopes,
and observatories on the ground and in space have given us a
rough picture of what happened as our Universe evolved from a
dense, hot, and perhaps quite simple early state to its present
complexity. Observations in progress are filling in the details,
and that in turn is driving intense debate on how the behavior of
our Universe can be understood within fundamental physics.
----------
P.J.E. Peebles: Evolution of the cosmological constant.
(Nature 4 Mar 99 398:25)
QY: P.J.E. Peebles [pjep@pupgg.princeton.edu]
-----------
Text Notes:
... ... *active vacuum of quantum physics: This refers to the
idea that the vacuum state in quantum mechanics has a zero-point
energy (minimum energy) which gives rise to vacuum fluctuations,
so the vacuum state does not mean a state of nothing, but is
instead an active state.
... ... *symmetries among forces are broken: If a theory or
process does not change when certain operations are performed on
it, the theory or process is said to possess a symmetry with
respect to those operations. For example, a circle remains
unchanged under rotation or reflection, and a circle therefore
has rotational and reflection symmetry. The term "symmetry
breaking" refers to the deviation from exact symmetry exhibited
by many physical systems, and in general, symmetry breaking
encompasses both "explicit" symmetry breaking and "spontaneous"
symmetry breaking. Explicit symmetry breaking is a phenomenon in
which a system is not quite, but almost, the same for two
configurations related by exact symmetry. Spontaneous symmetry
breaking refers to a situation in which the solution of a set of
physical equations fails to exhibit a symmetry possessed by the
equations themselves.
... ... *latent heat: In general, this is the quantity of heat
absorbed or released when a substance changes its physical phase
(e.g., solid to liquid) at constant temperature.
... ... *inflation concept: The inflationary model, first
proposed by Alan *Guth in 1980, proposes that quantum
fluctuations in the time period 10^(-35) to 10^(-32) seconds
after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 30Apr99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. MAGNETOELECTRONICS:
CONTROL OF SEMICONDUCTOR MAGNETISM BY EXTERNAL ELECTRIC FIELDS
In general, "ferromagnetism" is a property of certain
materials subjected to a magnetic field, the magnetic field
causing induced magnetism which combines with the applied field
to increase the local field. Ferromagnetic materials are strongly
attracted to a magnetic pole and have high effective magnetic
permeabilities that are greatly dependent on the applied
magnetizing field. Iron, cobalt, nickel, and certain alloys are
typical examples of ferromagnetic materials.
During the past five decades, several ionically bound
compounds have been discovered to be ferromagnetic. Some of these
compounds are electrical insulators, but others have the
conductivity of semiconductors.
Above its Curie point (Curie temperature), the spontaneous
magnetization of a ferromagnetic material vanishes and the
material becomes "paramagnetic", i.e., it remains only weakly
magnetic. This evidently occurs because the thermal energy
becomes sufficient to overcome the internal aligning forces of
the material.
The term "spintronics" refers to a relatively new field that
aims to combine ferromagnets with semiconductors to develop
electronic devices that exploit the quantum mechanical "*spin" of
electrons as well as their charge. One aim is to integrate
information storage with information processing, but a broader
goal is to develop new functionality that does not exist
separately in a ferromagnet or in a semiconductor. To this end,
investigators are searching for "emergent behavior" in combined
ferromagnetic semiconductor structures.
... ... H. Ohno et al (8 authors at Tohoku University, JP) now
report experiments demonstrating electric-field control of
ferromagnetism, the authors making the following points:
1) The authors point out that it is often assumed that it is
not possible to alter the properties of magnetic materials once
they have been prepared and put into use. For example, although
magnetic materials are used in information technology to store
trillions of bits in the form of magnetization directions
established by applying external magnetic fields, the properties
of the magnetic medium itself remain unchanged on magnetization
reversal. The ability to externally control the properties of
magnetic materials would be highly desirable from fundamental and
technological perspectives, particularly in view of recent
developments in *magnetoelectronics and spintronics. In
semiconductors, the conductivity can be varied by applying an
electric field, but the electrical manipulation of magnetism in
such materials has proved elusive.
2) The authors report experiments that demonstrate electric-
field control of ferromagnetism in a thin-film semiconduction
alloy [(In,Mn)As], using an *insulating-gate field-effect
transistor structure. By applying electric fields, the authors
were able to vary isothermally and reversibly the transition
temperature of *hole-induced ferromagnetism.
... ... In a commentary on this work, D.D. Awschalom and R.K
Kawakami (University of California Santa Barbara, US) state:
"This experiment is a 'proof of concept' for the idea that the
magnetic properties of ferromagnetic semiconductors can be
controlled using standard electronic techniques. This finding,
along with the discovery of new ways to control electronic
spin... paves the way for practical spintronics."
-----------
H. Ohno et al: Electric-field control of ferromagnetism.
(Nature 21/28 Dec 00 408:944)
QY: H. Ohno: ohno@riec.tohoku.ac.jp
-----------
D.D. Awschalom and R.K. Kawakami: Teaching magnets new tricks.
(Nature 21/28 Dec 00 408:923)
QY: David D. Awschalom: awsch@physics.ucsb.edu
-----------
Text Notes:
... ... *spin: See related background material below.
... ... *magnetoelectronics: See related background material
below.
... ... *insulating-gate field-effect transistor: The "field
effect transistor" (FET) is a transistor consisting essentially
of a channel of semiconductor material, the resistance of which
can be controlled by the voltage applied to one or more input
terminals (gates). It is a 3-terminal device in which current
flow through one pair of terminals, the "source" and the "drain",
is controlled or modulated by an electric field that penetrates
the semiconductor, with this field introduced by the voltage
applied at the third terminal, the "gate". The controlling field
applied to the gate must be isolated somehow from the current
flow in the channel, and there are two general methods of
accomplishing this isolation: a) in the "junction field-effect
transistor" (JFET), invented by Shockley, the isolation is
provided by a special junction barrier across which current flow
from gate to channel is very small; in the "insulated gate
field-effect transistor" (IGFET), first proposed in the 1930s but
not realized until 1960, an insulating layer is placed between
the gate electrode and the conducting channel, preventing any
current flow between them. The insulated-gate field-effect
transistor is sometimes called a "surface field- effect
transistor", since the effective conducting channel is the
semiconductor surface. (In contrast, the JFET, in which the bulk
of the semiconductor is the current carrier, is sometimes called
a "bulk field-effect transistor".)
... ... *hole-induced ferromagnetism: In this context, a "hole"
is an independently translocatable positively charged virtual
particle produced by a translocated electron in a crystal
semiconductor lattice, and the conductivity of the semiconductor
is based on the mobility of both electrons and holes. In the
alloy used in the Ohno et al experiments, manganese substitutes
for indium at a number of loci in the alloy and simultaneously
provides a localized magnetic moment and a hole, owing to its
electron-acceptor nature. These holes apparently mediate magnetic
interaction, resulting in so-called "hole-induced
ferromagnetism".
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON MAGNETOELECTRONICS
There is an emerging apparently important approach to electronics
based on the up or down "*spin" of the carriers rather than on
electrons or holes as in traditional semiconductor electronics.
The physical basis for the observed effects is called "giant
magnetoresistance" (GMR). ... ... Gary A. Prinz presents a review
of GMR and its applications, the author making the following
points: 1) GMR is a quantum mechanical effect observed in
magnetic thin-film structures composed of alternating
*ferromagnetic and nonmagnetic layers. When the *magnetic moments
of the ferromagnetic layers are parallel, the *spin-dependent
scattering of the carriers is minimized, and the material has its
lowest resistance. When the ferromagnetic layers are anti-
aligned, the spin-dependent scattering of the carriers is
maximized, and the material has its highest resistance. The
directions of the magnetic moments are manipulated by external
magnetic fields applied to the materials. These materials can now
be fabricated to produce significant changes in resistance in
response to relatively small magnetic fields, and to operate at
room temperature. 2) The first report of the discovery of GMR
appeared in 1988 [M. Baibich et al, Phys. Rev. Lett. 61:2472].
The first commercial product using GMR (a magnetic field sensor)
became available in 1994. The first products involving GMR to
have major economic impact are "read" heads for magnetic hard
disk drives, these devices announced by IBM in November 1997. The
next major economic impact from the discovery of GMR is expected
to come from nonvolatile magnetic computer memory. The Honeywell
Corporation announced the demonstration of GMR random access
memory (RAM) in January 1997. 3) The exploitation of *spin
polarization of carriers represents not only a departure for the
field of magnetism and magnetic materials, but also a new
direction for the field of electronics. Technological advances in
the ability to make increasingly smaller electronic devices, and
in the ability to combine dissimilar materials within a device,
both serve to increase the potential importance of spin-polarized
effects.
-----------
Gary A. Prinz: Magnetoelectronics.
(Science 27 Nov 98 282:1660)
QY: Gary A. Prinz, Naval Res. Lab., Washington, DC 20375 US.
-----------
Text Notes:
... ... *spin: 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. The idea of electron spin was first proposed by
Goudsmit and Uhlenbeck in 1925 to explain the splitting of atomic
spectroscopic emission lines in the presence of a magnetic field.
Elementary particle spin involves a virtual rotation about the
axis of the particle, which means only two spin states are
possible, one clockwise and one counterclockwise.
... ... *ferromagnetic: A ferromagnet is a material (such as
iron) in which there may be a permanent *magnetic moment, and in
which the spins of the atoms are aligned parallel to each other.
... ... *magnetic moments: (magnetic dipole moment) 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.
... ... *spin-dependent scattering: In this context, the term
"scattering" refers to the change in direction of a particle
because of a collision with another particle or system.
... ... *spin polarization: In a spin-polarized system, the
majority of spin-particles have spin-components pointing in one
direction rather than at random.
-------------------
Summary & Notes by SCIENCE-WEEK 15Jan99
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7. IN FOCUS: ON THERMODYNAMICS
"Thermodynamics is mainly concerned with the transformations of
heat into mechanical work and the opposite transformations of
mechanical work into heat. Only in comparatively recent times
have physicists recognized that heat is a form of energy that can
be changed into other forms of energy. Formerly, scientists had
thought that heat was some sort of fluid whose total amount was
invariable, and had simply interpreted the heating of a body and
analogous processes as consisting of the transfer of this fluid
from one body to another. It is therefore noteworthy that on the
basis of the heat-fluid theory [Nicolas Sadi] Carnot [1796-1832]
was able, in the year 1824, to arrive at a comparatively clear
understanding of the limitations involved in the transformations
of heat into work, that is, of essentially what is now called the
second law of thermodynamics. In 1842, only eighteen years later,
J[ulius] R. Mayer [1814-1878] discovered the equivalence of heat
and mechanical work, and made the first announcement of the
principle of the conservation of energy (the first law of
thermodynamics). We know today that the actual basis for the
equivalence of heat and dynamical energy is to be sought in the
kinetic interpretation, which reduces all thermal phenomena to
the disordered motions of atoms and molecules. From this point of
view, the study of heat must be considered as a special branch of
mechanics: the mechanics of an ensemble of such an enormous
number of particles (atoms or molecules) that the detailed
description of the state and the motion loses importance and only
average properties of large numbers of particles are to be
considered. This branch of mechanics, called 'statistical
mechanics', which has been developed mainly through the work of
[James Clerk] Maxwell [1831-1879], [Ludwig] Boltzmann [1844-
1906], and [Josiah Willard] Gibbs [1839-1903], has led to a very
satisfactory understanding of the fundamental thermodynamical
laws. But the approach in pure thermodynamics is different. Here
the fundamental laws are assumed as postulates based on
experimental evidence, and conclusions are drawn from them
without entering into the kinetic mechanism of the phenomena.
This procedure has the advantage of being independent, to a great
extent, of the simplifying assumptions that are often made in
statistical mechanical considerations. Thus, thermodynamical
results are generally highly accurate. On the other hand, it is
sometimes rather unsatisfactory to obtain results without being
able to see in detail how things really work, so that in many
respects it is very often convenient to complete a
thermodynamical result with at least a rough kinetic
interpretation. The first and second laws of thermodynamics have
their statistical foundation in classical mechanics. In recent
years, [Walther] Nernst (1864-1941] has added a third law which
can be interpreted statistically only in terms of quantum
mechanical concepts."
-----------
Enrico Fermi: _Thermodynamics_
(Lectures at Columbia University [US] 1937, Dover Publ. 1956.
From the introduction, p.ix.)
-----------
[Editor's note: Fermi gives Mayer's name as R.J. Mayer, which
reverses the initials and is an error. It is interesting that
the discoverer of the first law of thermodynamics and the law of
conservation of energy was not a physicist but a physician, a
medical doctor without professional training in physics. When
Mayer submitted a paper on the subject to the _Annalen der
Physik_, the submission was not even acknowledged and was thrown
away. Finally, in 1842, the noted chemist Justus von Leibig
accepted the paper for his journal _Annalen der Chemie_ und
Pharmazie. But once published, the paper was totally ignored,
Mayer became depressed, and in 1849 he attempted suicide by
jumping from a 3rd-story window. The suicide attempt failed and
Mayer became permanently lame as the result of leg injuries. By
1851, Mayer was in a mental institution, and he suffered greatly
under the primitive and cruel methods of treatment of the
mentally ill. He was eventually released, but he never fully
recovered, and he languished in obscurity: when, in 1858, Leibig
lectured on Mayer's views, Leibig referred to Mayer as deceased.
Then, in the 1860s, recognition finally arrived for Mayer's work,
and he received many honors, including the Copley Medal of the UK
Royal Society in 1871. Mayer clearly anticipated James Joule
(1818-1889) and Hermann von Helmholtz (1821-1894) in the
discovery of the law of conservation of energy.]
-------------------
Notes by SCIENCE-WEEK http://scienceweek.com 12Jan01
For more information: http://scienceweek.com/swfr.htm
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8. FROM THE SCIENCEWEEK ARCHIVE:
MECHANISMS OF CHEMOTAXIS IN BIOLOGICAL CELLS
All biological cells possess mechanisms that effectively enable
them to sense their environment. The term "directional sensing"
refers to the ability of a cell to determine the direction and
proximity of an extracellular stimulus. Direction sensing is
needed to detect *morphogens that control *differentiation, and
to detect attractants that direct cell migration (chemotaxis).
The chemotaxis phenomenon is critical in immunity, *angiogenesis,
wound healing, *embryogenesis, and *neuronal patterning. A
striking example of chemotaxis is exhibited during the life cycle
of the "social" amoeba *Dictyostelium discoideum. During growth,
these cells behave as essentially individual entities, tracking
down and eating bacteria (*phagocytosis). When the individual
entities of D. discoideum are starved, they move toward secreted
*adenosine 3',5'-monophosphate (cAMP) signals, form an aggregate,
and differentiate into spore and stalk cells, with a ball of
spore cells perched above the substrate on the stalk. The
fundamental role of chemotaxis in this simple and well-
characterized *eukaryote has provided a powerful system for
genetic analysis of chemotaxis, and recent observations in D.
discoideum, as well as in yeast and mammalian *leukocytes, have
clarified views of directional sensing. ... ... C.A. Parent and
P.N. Devreotes (Johns Hopkins University, US) present an
extensive review of recent research on cellular directional
sensing, with a focus on the signal transduction events involved
in gradient detection. The authors make the following points:
1) In eukaryotic cells, directional sensing is mediated by
signal pathways involving heterodimeric guanine nucleotide-
binding protein (*G protein).
2) In D. discoideum amoebae and mammalian leukocytes, the
receptors and G protein subunits are uniformly distributed around
the cell perimeter. Chemoattractants induce the transient
appearance of binding sites for several *pleckstrin homology
domain-containing proteins on the inner surface of the cell
membrane. In gradients of attractant, these sites are
persistently present on the side of the cell facing the higher
concentration of attractant, even in the absence of a functional
*actin cytoskeleton or cell movement.
3) Thus, the biological cell senses direction by spatially
regulating the activity of a signal transduction pathway.
-----------
[Editor's note: Although the terminology in this report is indeed
characteristic of current biology, to say that a cell "senses"
direction perhaps invites an awkward anthropomorphic connotation.
The "sensing" by biological cells in this context is no different
in character from the "sensing" by chemical reactant molecules of
each other in a non-biological solution. Certainly, the responses
of cells to chemical attractants are complicated, involving a
sequence of chemical reactions, but they are indeed chemical
reactions, and biologists consider them exactly that. In a
broader intellectual context, since all human "sensations" can
also be reduced to specific chemical reactions (or specific
chemical reactions following specific physical events), the
chemotaxis events considered here at the level of a single cell
are ultimately joined to the classical "mind vs. body"
philosophical problem.]
-----------
C.A. Parent and P.N. Devreotes: A cell's sense of direction.
(Science 30 Apr 99 284:765)
QY: Peter N. Devreotes [pnd@welchlink.welch.jhu.edu]
-----------
Text Notes:
... ... *morphogens: In general, a "morphogen" is any substance
responsible for some aspect of morphogenesis (the generation of
form and structure during development of an individual organism).
... ... *differentiation: In general, in this context, the term
"differentiation" refers to the structural and functional
specialization of cells, developmental cell specialization
(morphology and biochemistry) resulting from activation of
specific parts of the cell genome.
... ... *angiogenesis: The origin and development of blood
vessels. Angiogenesis is an important consideration in the growth
of cancerous tumors, since the tumor provokes directed
angiogenesis into itself with the end result that the tumor is
supplied with oxygen and nutrients. Without angiogenesis, tumors
can attain only a small size before becoming self-inhibiting.
... ... *embryogenesis: In general, the formation and development
of an embryo.
... ... *neuronal patterning: The "patterning" here refers to the
patterns of connections between nerve cells, i.e., the
"circuitry".
... ... *Dictyostelium discoideum: Although often called a
"cellular slime mold", D. discoideum is not a mold, nor is it
consistently slimy. The term "social amoeba" is more accurate.
When the organism is individualized, the entities are called
"myxamoebae". When they aggregate into a slug, the organism is
called a "pseudoplasmodium" or termed the "grex". The aggregation
into a unitary grex may involve tens of thousands of individual
amoebae. (Cf. the background report that follows.)
... ... *phagocytosis: Literally, "cell eating". A cell capable
of phagocytosis (e.g., an amoeba) has a mobile boundary which can
engulf particles or smaller cells, followed by incorporation of
the particles or smaller cells into the engulfing cell interior.
... ... *adenosine 3',5'-monophosphate (cAMP; cyclic AMP): ATP
(adenosine triphosphate) is the most important chemical energy
source in all living cells, intimately involved in various cell
functions and cell metabolism, and an entity in numerous cyclic
chemical pathways involved in the synthesis of components. One of
the reaction products of ATP is cyclic AMP, which acts as an
intracellular hormone (i.e., a chemical messenger). Cyclic AMP is
derived from ATP in a reaction catalyzed by the enzyme adenylyl
cyclase (also called adenyl cyclase and adenylate cyclase).
Cyclic AMP is called the second messenger; the first messenger is
the hormone that interacts with the receptor for that hormone on
the cell surface.
... ... *eukaryote: In general, any biological cell containing
internal membrane-bound organelles such as a nucleus.
... ... *leukocytes: "White" blood cells, some types of which are
amoeba-like, exhibiting phagocytosis and pronounced chemotaxis.
... ... *G protein: G-proteins are a family of signal-coupling
proteins that act as intermediaries between activated cell
receptors and effectors, for example, the transduction of
hormonal signals from the cell surface to the cell interior. The
G-protein is apparently embedded in the cell membrane with parts
exposed on the outside surface and inside surface. The outside
moiety is activated by the first messenger, and the inside moiety
activates the second messenger, the G-protein thus acting as a
trans-membrane signal transducer.
... ... *pleckstrin homology domain-containing proteins:
Pleckstrin is a protein found in certain blood components, and a
"pleckstrin homology domain-containing protein" is a protein
containing a domain consisting of approximately 100 amino acid
residues found in pleckstrin and that has also been found in more
than 60 different proteins, particularly in those proteins
associated with intracellular signal transduction.
... ... *actin cytoskeleton: Actin is a family of ubiquitous
structural proteins present in all eukaryote cells, and the term
"cytoskeleton" refers to the quasi-rigid matrix that among other
things determines cell shape.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 16Jul99
-------------------
Related Background:
A NEW METHOD FOR FOLLOWING INDIVIDUAL CELLS IN SLIME MOLD
Dictyostelium discoideum is an organism that has been intriguing
biologists for most of this century. Although this organism is
often called a "cellular slime mold", it is not a mold and it is
not consistently slimy. A better common name for it is a "social
amoeba". What is most remarkable about the organism is its life
cycle. In one part of it life cycle, the "organism" consists of
individual dispersed amoebas living on decaying logs, eating
bacteria and reproducing by binary fission like most other
protozoans. Then, when the local food supply becomes exhausted,
a rather astounding event occurs: tens of thousands of these
amoeba join together to form moving streams of cells that
converge at a central point, and there they aggregate to produce
a slug (grex) 2 to 4 millimeters long. The slug migrates as a
single body towards light, and when it reaches an illuminated
area, migration ceases, and the slug differentiates into a
fruiting body composed of spore cells and a stalk, the stalk
rising approximately 1 centimeter high above the plane of the
surface on which the slug has migrated. Inside the globular end
of the fruiting body, each spore cell is cellulose encapsulated.
In the denouement, the stalk cells die and the spore cells are
widely dispersed to become new amoeba, each of which will begin a
separate new population of cells both individual and social.
Thus, in this organism, initially identical cells are
differentiated into one of two alternative cell types, spore
cells and stalk cells. It is an organism where individual cells
come together to form a cohesive structure, aggregating into a
single organism, a quite remarkable feat of organization that
challenges biologists, chemists, and physicists. Much has been
learned about this organism in the past few decades, in
particular the apparent important role of release of cyclic
adenosine monophosphate (cAMP) in the initial aggregation that
produces the slug. ... ... J.T. Bonner (who has spent more than
50 years studying the social amoeba) points out that one of the
obstacles in studying D. discoideum is that it has been difficult
to follow the movements of individual cells within the slug. The
author now describes a new method for studying D. discoideum, the
method producing flat slugs one cell thick at a mineral oil-water
interface where one can follow the movement of all the cells. The
author reports that observations of time-lapse videos reveal the
following about slug migration: 1) While the posterior cells move
straight forward, the anterior cells swirl about rapidly in a
chaotic fashion. 2) Turning of the slug involves shifting the
high point of these hyperactive cells. 3) Both the anterior and
posterior cells move forward on their own power as the slug moves
forward. 4) There are no visible regular oscillations within the
slug. 5) The number of prestalk and prespore cells is
proportionate for a range of sizes of the mini-slugs involved in
these experiments (approximately 300 to 400 cells in each of
these mini-slugs). The author suggests that all of the
observations on thin slugs are consistent with observations of
normal 3-dimensional slugs, and that experiments with 2-
dimensional slugs may provide new insights into differentiation
and movements in this organism.
-----------
J.T. Bonner (Princeton University, US): A way of following
individual cells in the migrating slugs of Dictyostelium
discoideum. (Proc. Natl. Acad. Sci. US 4 Aug 98)
QY: J.T. Bonner, Princeton University 609-258-3000
-------------------
Summary by SCIENCE-WEEK 18Sep98
ON THE MOLECULAR MECHANISMS OF SWARMING IN BACTERIA
Flagella are whiplike extensions from certain types of cells, and
in bacteria that have them, they are responsible for locomotion
of the organism. The term "chemotaxis" refers to the movement of
cells in response to chemical stimuli, and in the context of this
report, "chemoreceptors" are the receptors on the surfaces of
mobile cells that initiate the chemotactic response. Substances
that produce chemotaxis are called "chemoeffectors". In bacteria,
"swarming" is an organized surface translocation on solid media
that depends on extensive flagellar activity and cell-cell
contact. Previous studies of bacteria that swarm have indicated
the chemotaxis system is involved, but the mechanism is not
known. ... ... Now Burkhart et al (University of Texas Austin,
US) report that two of the four chemoreceptors in E. coli can
support swarming individually, but sensing their most powerful
chemoattractants (serine, or aspartate, or maltose) is not
necessary for swarming. The authors suggest that during swarming,
the chemoreceptors signal through the chemotaxis pathways and
induce swarmer cell differentiation (e.g., increased number of
flagella) in response to still unknown signals other than their
known chemoeffectors.
QY: Rasika M. Harshey (rasika@uts.cc.utexas.edu)
(Proc. Natl. Acad. Sci. US 3 Mar 98) (Science-Week 10 Apr 98)
-------------------
Related Background:
CHEMOTAXIS OF NEURONS: VARIABLE RESPONSE MAY ASSIST TARGETING
In those animals that have nervous systems, one task of embryo-
logical development is to ensure the proper functional connect-
ions between nerve cells and other nerve cells, and between nerve
cells and muscle cells. The innervation must be exact, in the
sense that the growing nerve cell extension (the axon), which
will ultimately serve to propagate information, must reach a
specific and often distant target. In humans, for example, there
are nerve cells whose growing axons reach specific targets as
much as a meter distant from the cell body. The term chemotaxis
refers to movement of an organism in response to chemical
concentration gradients, and it is such gradients that in one way
or another apparently guide nerve cells during their growth
phase. The mesencephalon (midbrain) is located in the brainstem,
the region between the brain itself and the spinal cord, and a
commissure, of which there are many, is a band of nerve fibers
that cross from one side of the body to the other. Shirasaki et
al (3 authors at Osaka Univ., JP), in a study of growing cultured
rat embryo mesencephalon commissural axons, report evidence of a
change in the chemoattractant responsiveness of growing axons
during growth across an intermediate target. The authors suggest
such changes in responsiveness to chemoattractants may enable
developing axons to continue to navigate toward their final
destinations, and that encounters with the intermediate targets
might cause sensitization of growing axons to the next set of
cues necessary for guidance to the final target.
QY: Ryuichi Shirasaki
(Science 2 Jan 98) (Science-Week 16 Jan 98)
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