ScienceWeek

SCIENCEWEEK

ScienceWeek - August 23, 2002 Vol. 6 Number 34

An Online Research Digest Published Weekly Since 1997

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A man ceases to be a beginner in any given science and becomes a
master in that science when he has learned that he is going to
be a beginner all his life.
-- R.G. Collingwood (1889-1943)

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Section 1

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1. Spotlight: Neutrino Oscillations

2. On Reconstructing Climate History

3. On Pluto and the Kuiper Belt

4. Molecular Biology: On Motor Proteins

5. Neurobiology: On Cortical Plasticity

6. On the Genetics and Evolution of Maternal Care

7. Spotlight: Spin Electronics

8. On Global Optimization

9. On Optical Networks and Molecular Switches

10. On Silicon Nitride Ceramics

11. On Genome Variation of Indigenous Human Microbes

12. Geographic Variation of Mortality from Stroke in the US

13. In Focus: On Domains in Biological Membranes

14. ScienceWeek Notices and Subscription Information

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Section 2

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1. SPOTLIGHT: NEUTRINO OSCILLATIONS

The history of particle physics during the first 30 years of the
20th century is an excellent example of the intimate interplay
between theory and experiment. One of the central problems in
the physics of matter during this period was to understand the
emissions of radioactive substances first discovered in 1896 by
Henri Becquerel (1852-1908). Spontaneous radioactive decay is
essentially a spontaneous transmutation of an unstable atomic
nucleus (nuclide) A into nuclide B, with nuclide A initially in
a higher energy state and losing energy to transmute into the
"daughter" nuclide B. During the early years of particle
physics, the energy loss was considered to be accomplished by
emission of one of three types, depending on the nature of
nuclide A: positively charged alpha particles (helium nuclei),
negatively charged beta particles (electrons), or neutral gamma
rays (high energy electromagnetic radiation). Since the energies
of decaying nuclides and daughter nuclides are fixed according
to nuclide identity, one would expect the observed energies of
the 3 types of particles to also be fixed for each species of
decaying nuclide. During the period before 1927, this was known
to be true for alpha particles and gamma rays, but there was
intense controversy about whether it was true for beta
particles. Indeed, some early experiments indicated that it was
not true for beta particles, and this posed a problem, since
conservation laws require an accounting for all the energy and
the numbers for beta decay did not add up. The controversy
continued for nearly 30 years, particularly among
experimentalists who disagreed concerning experimental methods
and interpretations of experimental results, until finally in
the late 1920s it was conclusively demonstrated by experiment
that during the beta-decay process high-speed electrons of
various energies are emitted with a continuous beta-emission
energy distribution spectrum (i.e., a plot of the number of
electrons vs. energy of these electrons) over the range of
energies.

Given the experimental evidence of a continuous beta-decay
spectrum, theoreticians tackled the problem of accounting for
beta decay without violating conservation laws. In 1930,
Wolfgang Pauli (1900-1958) proposed that when a beta particle
was emitted, another particle, without charge, and perhaps
without mass, was also emitted, and that this second particle
carried off the missing energy. Enrico Fermi (1901-1954)
suggested the particle carrying the missing energy be called
"neutrino", which is Italian for "little neutral one", and in
1934 Fermi incorporated the neutrino into his theory of beta
decay.

Most theoretical and experimental physicists immediately
accepted the proposed existence of the neutrino as the best
solution to an important puzzle, but it was not until 1956 that
Frederick Reines (1918-1998) and Clyde Cowan (1919-1974) managed
to finally obtain experimental evidence for the existence of the
elusive neutrino by means of experiments involving emission
beams from a fission reactor. Enrico Fermi received the Nobel
Prize in Physics in 1938; Wolfgang Pauli received the Nobel
Prize in Physics in 1945; and Frederick Reines received the
Nobel Prize in Physics in 1995. (Clyde Cowan was not eligible
for the Nobel Prize at the time it was awarded to Reines, since
the Nobel Prize is not awarded posthumously.)

In general, beta-decay is a type of interaction in which an
unstable atomic nucleus changes into a nucleus of the same mass
number but different proton number. The change involves the
conversion of a neutron into a proton with the emission of an
electron and an electron *antineutrino, or of a proton into a
neutron with the emission of a positron and an electron
neutrino. The electrons or positrons emitted are called "beta
particles". (Positrons are electron antiparticles. See
*antineutrino.)

An "antiparticle" (antimatter) is a subatomic particle that has
the same mass as another particle and equal but opposite values
of some other property or properties. For example, the
antiparticle of the electron is the positron. An antineutrino,
the antiparticle to the neutrino, has zero mass, *spin 1/2, and
positive helicity. There are 2 antineutrinos, one associated
with the electron and one associated with the *muon.

"Leptons" are a class of elementary particles. Although they are
affected by electromagnetic and gravitational forces, apart from
that they are involved only with weak interactions, acted upon
by weak forces but not by strong forces, as opposed to quarks,
which are acted upon by strong forces but not by weak forces.
One further difference between leptons and quarks is that
leptons can be isolated as single particles, whereas quarks
apparently cannot. The leptons include the electron, the muon,
the tau, and their associated neutrinos. The mass of the tau is
approximately 3484 times the mass of the electron; the mass of
the muon is intermediate.

In quantum mechanics, "spin" is the intrinsic angular momentum
of a subatomic particle. Spin states are quantized, multiples of
h/2(pi), where h = Planck's constant, and each particle is
characterized by a quantum spin number which is the multiple
factor. All subatomic particles can be classified as one of two
types: "fermions" are particles with half-integer values of
spin, and "bosons" are particles with integer values of spin.
Leptons are fermions.

The "Z boson" is an electrically neutral elementary particle,
discovered in 1983, which is believed to mediate weak
interactions. The "W boson", also discovered in 1983, is of two
types, positively charged or negatively charged, and like the Z
boson it mediates weak interactions.     The particle now called
the "muon" was discovered in 1936 by Carl Anderson (1905-1991),
who discovered the particle while studying cosmic rays. The muon
is a negatively-charged lepton similar to the electron except
that its mass is approximately 207 times the electron mass. The
muon has a average lifetime of 2.2 microseconds, and decays into
an electron, a neutrino, and an antineutrino. "Mesons" are a
class of elementary particles involved in strong interactions in
the atomic nucleus. Muons were originally called "mu-mesons"
because they were thought to be mesons, but that idea was
eventually abandoned, and muons are now recognized as leptons.
Similar to the electron, which has an associated positive
particle called the "positron", there exists a positively
charged muon.

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.

In particle physics, the "Standard Model" is a theoretical
framework whose basic idea is that all the visible matter in the
universe can be described in terms of the elementary particles
leptons and quarks and the forces acting between them. The
fundamental forces comprise the gravitational force, the
electromagnetic force, the nuclear strong force, and the nuclear
weak force. The "electroweak interactions" are a unification of
the electromagnetic and nuclear weak interactions, and are
described by the Weinberg-Salam theory (sometimes called
"quantum flavordynamics"; also called the Glashow-Weinberg-Salam
theory).

The 3 leptons (electron, muon, tau) differ from each other only
in mass. The muon is 200 times more massive than the electron.
In this context, the term "flavor" is a label used to
distinguish different types of leptons.

"Cosmic rays" are highly energetic particles moving at close to
the speed of light and continuously bombarding the Earth's
atmosphere from all directions. The energies of the particles
are enormous and range from 10^(8) to over 10^(19) electronvolts.

Discovered in 1934 by Cerenkov (1904-1990), Cerenkov (Cherenkov)
radiation is electromagnetic radiation, usually bluish light,
emitted by a beam of high-energy charged particles passing
through a transparent medium at a speed greater than the speed
of light in that medium. The radiation is essentially a shock
wave, the effect analogous to that of a sonic boom.

C. Bemporad et al (University of Pisa, IT) discuss neutrino
oscillations, the authors making the following points:

1) Neutrinos have the distinction of being the first elementary
particle whose existence was predicted by a theorist in order to
explain seemingly unrelated phenomena (1). Wolfgang Pauli
(1900-1958) made this prediction in 1930 in his famous letter
attempting to  explain the continuous electron energy
distribution in nuclear beta decay. It became immediately clear
that neutrinos would be difficult to observe, because the
corresponding cross sections are so tiny. But in a series of
experiments from 1953 to 1959, Reines and Cowan (2) were able to
prove convincingly that electron antineutrinos are emitted by
nuclear reactors and hence that they are real particles. Shortly
afterwards, in 1962, the separate identity of muon neutrinos was
demonstrated (3). Another decade later, in 1975, the tau lepton
was discovered (4) and the observation of its decay properties
implied the existence of a third neutrino that was directly
observed only very recently (5). Precise measurements of the
decay width of the Z boson have shown that just three neutrino
flavors participate in weak interactions, at least for neutrinos
with masses less than M(subZ)/2.

2) Phenomenologically, it is obvious that neutrinos of each
flavor are either massless or at least many orders of magnitude
lighter than the corresponding charged leptons with which they
form weak-interaction doublets. Based on these empirical facts,
the standard model of electroweak interactions postulates that
all neutrinos are massless and consequently have conserved
helicity (which is the same as chirality in this case), and that
the separate lepton numbers for electron, muon, and tau flavors
are conserved. Challenging this postulate of the vanishing
neutrino mass has recently become a central issue in many
disciplines of fundamental science, including particle and
nuclear physics, cosmology, and astrophysics.

3) Ironically, while our knowledge of intrinsic neutrino
properties remains quite poor, these particles have been used as
tools to understand other phenomena. The tradition of
underground neutrino detectors began 30 years ago, when Davis
and his collaborators were first able to detect neutrinos from
the Sun. (For a description of the history of solar neutrino
research, see Bahcall (6)) Together with all the other
experimental observations of solar neutrinos, this was, and
still is, the only clear proof that the basic energy generation
in stars is understood.

4) In summary: The technique of neutrino searches using nuclear
reactors as sources, a direct continuation of the experiments
that proved the existence of neutrinos, is today an essential
tool in investigating the indications of oscillations found in
studying neutrinos produced in the Sun and in the Earth's
atmosphere.

References (abridged):

1) Winter, K., 1991, Ed., Neutrino Physics (Cambridge
University,   Cambridge, England).

2) Reines, F, and C. L. Cowan, Jr., 1953, Phys. Rev 92, 830;
1959, Phys. Rev. 113, 273.

3) Danby, G., J.-M. Gaillard, K. Goulianos, L. M. Lederman, N. 
Mistry, M. Schwatz, and J. Steinberger, 1962, Phys. Rev Lett. 
9,36.

4) Peri, M. L., et al., 1975, Phys. Rev. Lett. 35, 1489.

5) Kodama, K., et al., 2001, Phys. Lett. B 504, 218.

6) Bahcall, J. N., 1989, Neutrino Astrophysics (Cambridge
University, Cambridge, England).

Rev. Mod. Phys. 2002 74:297

Web Links: neutrino neutrino oscillations     Wolfgang Pauli

Related Background:

HISTORY OF PHYSICS: WOLFGANG PAULI

Consider an old black-and-white faded photograph: A soccer ball
is flying directly at the camera, almost arrived and large in
view, the soccer ball made fuzzy by its movement, and in the
background, obviously the kicker of the soccer ball, his kicking
foot still raised, is a rather pudgy middle-aged fellow in baggy
trousers, white shirt and tie, grinning with malicious delight
at his aimed kick (the ball actually slammed into the lens of
the expensive Speed Graphic camera), a pudgy fellow with a round
face who might be your uncle, the one who sells real estate, or
the local butcher who chuckles as he chops meat on the chopping
block. But a physiognomy is merely a mask, and the kicker of the
soccer ball, this grinning round-faced fellow facing you with
his foot raised and his back to the shore of Lake Lucerne
(Vierwaldstaetter See) in Switzerland in 1950, the kicker of the
ball is neither your uncle nor the local butcher, but one of
those ephemeral sparks of great genius thrown up by the grinding
gears of history -- the theoretical physicist Wolfgang Pauli
(1900-1958) [*Note #1].

Wolfgang Pauli (Nobel Prize in Physics 1945) [*Note #2] never
published much, certainly not as much as his more competitive
contemporary physicists, but his influence on modern physics was
profound and enduring. He is most remembered for two major
contributions: the *exclusion principle and his prediction of
the existence of the *neutrino, but there were many other
contributions of lasting significance. Pauli exemplifies what
might be called "collegial science": his influence on his
contemporary physicists derived primarily from conversations and
letters.

K. von Meyenn and E. Schucking (2 installations, DE US) present
a biographical essay on Wolfgang Pauli, the authors making the
following points:

1) The authors point out that Pauli established himself in the
world of physics at the age of 20. When Pauli, 19 years old and
a student of Arnold Sommerfeld (1868-1951), was assigned the
task of writing a report on Einstein's special and general
theories of relativity, Pauli produced a 237 page monograph with
394 footnotes, a monograph soon published in the _Encyclopedia
of the Mathematical Sciences_, and later as a book. This
monograph is still considered one of the best treatments of the
relativity theories, and Albert Einstein (1879-1955), in a
review of this monograph in 1922, wrote as follows: "No one
studying this mature, grandly conceived work would believe that
the author is a man of twenty-one. One wonders what to admire
most, the psychological understanding for the development of the
ideas, the sureness of mathematical deduction, the profound
physical insight, the capacity for lucid, systematic
presentation, the knowledge of the literature, the complete
treatment of the subject matter, or the sureness of critical
appraisal."

2) Sommerfeld introduced Pauli (and Pauli's fellow student
Werner Heisenberg [1901-1976]) to quantum theory, and after the
publication of Pauli's review of relativity theory, Pauli's main
interest shifted to quantum physics. Pauli soon proposed the
atomic "*magneton" and named it after Niels Bohr (1885-1962).
Pauli worked on the *anomalous Zeeman effect and he discovered
nuclear magnetism. In 1925, before the formulations of the new
quantum mechanics by Heisenberg and Erwin Schroedinger
(1887-1961), Pauli proposed his famous exclusion principle,
which explained the structure of atoms in conformity with the
periodic table. By 1929, Pauli had become the world's foremost
expert on the *old Bohr-Sommerfeld quantum theory.

3) Much of Pauli's influential work remains unpublished. His
proof of the equivalence of matrix and wave mechanics appears in
a letter to Pascual Jordan (1902-1980), and he wrote down the
uncertainty relation for time and energy in a letter to
Heisenberg. Pauli almost never cared about recognition for his
work, although he took great care in giving credit to other
authors. Unlike Heisenberg and many other physicists, Pauli was
not ambitious or competitive. His principle concern was always
to clarify the greater picture for himself, to obtain a
consistent and coherent description of the totality of the
phenomena. In this lifelong endeavor, he wrote thousands of
letters analyzing details and attempting to get things right,
and in the 1920s, Pauli's letters were passed around, copied,
and studied by many physicists. "His contribution of key ideas,
and his trenchant impartial analyses, should have earned him a
place as co-author of many papers on quantum mechanics. Instead,
he insisted on the idea that authorship was unimportant in this
collective attempt to decipher the book of nature."

4) Pauli and Heisenberg was close friends. The authors (von
Meyenn and Schucking) state: "What clearly emerges from reading
the letters and papers from the incubation period of quantum
mechanics is that, among the score of people creating the new
picture of physics, two protagonists stand out, combining
awesome mathematical power with a global awareness of the
experimental data. These two [were] Pauli and Heisenberg... The
main act in the drama of the new physics is not, as Michael
Frayn imagines in his play _Copenhagen_, the discourse between
Bohr and Heisenberg, but rather the Heisenberg-Pauli dialogue.
Bohr, the revered father figure, no longer had the leading role
he played before 1925."

5) The authors state: "Perhaps we will never know the true
extent of Pauli's contribution to the creation of quantum
mechanics. From the crucial years 1925-1927, we have 34 letters
from Heisenberg to Pauli, but only three of the dozens that
Pauli wrote to Heisenberg have survived. The fate of the others
is in doubt. It was claimed they had been destroyed in a fire.
But, according to another version, they were taken from
Heisenberg when he was arrested by the British in 1945 at the
end of the war in Europe." [Editor's note: Pauli spent the war
years in the US at the Institute for Advanced Study Princeton,
and he became a US citizen in 1946. He later returned to his
professorship at the Federal Institute of Technology Zurich
(CH).]

Physics Today 2001 February

Text Notes:

... ... *Note #1: The described photograph can be found in a
remembrance essay by Roy Glauber (Harvard University, US)
(Physics Today February 2001, p.49). It was Roy Glauber who took
the photograph immediately before his camera was hit by the
soccer ball.

... ... *Note #2: There are in physics a Nobel Laureate Wolfgang
Pauli (1900-1958) and a Nobel Laureate Wolfgang Paul
(1913-1993). Wolfgang Paul developed the so-called "Paul trap"
for confining and studying electrons, and for this work he
received the Nobel Prize in Physics in 1989.

... ... *exclusion principle: According to the exclusion
principle as proposed by Pauli, no two electrons in the same
system can share the same quantum numbers, and therefore no two
electrons can share the same quantum state. It is this quantum
exclusion that requires the electrons in an atom to occupy
different energy levels, instead of all electrons congregating
at the lowest energy level. The exclusion principle was later
generalized to constrain "fermions": fermions (electrons,
protons, neutrons) are particles that obey the Pauli exclusion
principle: i.e., no two fermions of the same kind in a closed
system can occupy the same quantum state.

... ... *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.

... ... *magneton: In general, the "magneton" is a unit for
measuring magnetic moments of nuclear, atomic, or molecular
magnets. The "Bohr magneton", introduced by Pauli, has the value
of the classical magnetic moment of an electron.

... ... *anomalous Zeeman effect: (anomalous Zeeman splitting)
The Zeeman effect involves the splitting of a spectral line due
to a magnetic field. Named after Peter Zeeman (1865-1943). In
general, the Zeeman effect occurs when atoms emit or absorb
radiation in the presence of a magnetic field: the field
modifies the energy configuration of the atom with the result
that a spectral line is split into components, with the spacing
of the components a measure of the magnetic field strength. When
the splitting is in accordance with classical theory, the effect
is called the "normal Zeeman effect" (as predicted by Hendrick
Lorentz [1853-1928]); when the splitting is complex, requiring
quantum theory for explanation, it is called the "anomalous
Zeeman effect".

... ... *old Bohr-Sommerfeld quantum theory: The Bohr model of
the atom, first proposed by Bohr in 1913, was the first atomic
model to involve quantum theory. The model essentially involved
the orbiting of electrons in discrete circular orbits around the
atomic nucleus, with a finite number of allowed energy and
occupancy states. The model had great success in predicting the
spectral lines of hydrogen. Sommerfeld, who encouraged Bohr in
his development of the model, introduced the possibility of
elliptical electron orbits. Bohr received the Nobel Prize in
Physics for his work in 1922. Within a few years, the
Bohr-Sommerfeld model of the atom was swept away by the new
quantum mechanics.

Related Background:

ON NEUTRINO OSCILLATIONS

The fundamental particles of 20th century physics came into
existence as theoretical constructions designed to explain
certain specific experimental observations. In some cases, the
existence of a particular particle has been verified by direct
experiment; in other cases, the required verification
experiments are extremely difficult to accomplish, and the
particles related to these experiments have remained theoretical
constructions. The neutrino was first theoretically postulated
by Wolfgang Pauli (1900-1958) in 1930 in order to maintain the
conservation of energy principle in the analysis of the results
of certain *beta-decay experiments. The Pauli neutrino was a
particle with no charge and zero rest mass. Experimentally, the
particle was tentatively identified by F. Reines and C. Cowan in
1953 and more definitely in 1956. Neutrinos are "leptons", which
are a group of point-like particles with *spin of 1/2 that are
not affected by so-called "*strong interactions" and that are
not constructed of *quarks. In the *Standard Model in particle
physics, there are 6 particle types categorized as leptons: the
electron, the *muon, the massive *tau lepton, and a neutrino
associated with each of these (denoted as 3 neutrino "flavors"
or "generations").  Neutrinos are produced in great numbers by
the Sun, but they almost never interact with atoms, and an
estimated 10^(12) solar neutrinos flow through our bodies each
second without any consequence. Measurements of solar neutrinos,
however, have produced a mystery: the neutrino density measured
by detectors is approximately one-third that expected from
theoretical calculations of solar neutrino emission. Two kinds
of solutions have been proposed to resolve this mystery, one
solution involving revisions to the theory of stellar structure,
and the other solution involving revisions to nuclear particle
theory. In the latter case, the proposal is that the neutrino
may oscillate among the 3 different flavors (states), with the
result that neutrino detectors detect only one flavor or
one-third of the solar emission. The existence of such neutrino
oscillation would have important implications, since it has been
believed that neutrinos, like photons, have zero mass. But
theory indicates that if neutrinos oscillate they must have
mass, and neutrinos are so numerous that even an extremely small
mass would theoretically be sufficient to affect the future of
the Universe as a whole. The question of neutrino oscillation,
therefore, is a critical problem affecting a good deal of
fundamental physics and cosmology, and there is recent evidence
interpreted to indicate that such oscillation does indeed occur
and that neutrinos do indeed have nonzero mass.

K. Kaneyuki and K. Scholberg (2 installations, JP US) present a
detailed review of current research concerning neutrino
oscillations, the authors making the following points:

1) The basic strategy for measuring neutrino oscillations is
simple. Given a source of neutrinos, either natural or
artificial, one allows the neutrinos to propagate for a known
distance, and then one obtains as much quantitative information
as possible concerning their energy and flavor. If the amount of
a given flavor, as a function of energy and distance, is that
expected from the quantum mechanical predictions arising from
the oscillation hypothesis, then neutrino oscillation has been
discovered.

2) Three neutrino sources are currently used in research: The
Sun, atmospheric *cosmic-ray showers, and particle accelerators.
At present, the clearest neutrino oscillation evidence from
atmospheric neutrinos comes from the "Super-Kamiokande"
experiment, which observes neutrino interactions by detecting
*Cherenkov (Cerenkov) radiation. The Super-Kamiokande experiment
has been built and operated by a collaboration of approximately
130 scientists from Japan and the US, the project headed by Y.
Totsuka (University of Tokyo, JP). The apparatus consists of 50
kilotons of ultrapure water housed approximately one kilometer
underground in the Kamioka mine in Japan. The detector consists
of 2 concentric cylinders 40 meters high and with an outer
radius of 20 meters. The inner cylinder contains 11,146
inward-facing photomultiplier tubes, each 50 centimeters in
diameter. These photomultiplier tubes detect Cherenkov radiation
from particle interactions inside the inner cylinder (which
contains the ultrapure water). The outer cylinder has 1885
20-centimeter-diameter photomultiplier tubes facing outward to
check for non-neutrino related Cherenkov radiation from entering
charged particles (cosmic-ray muons and radioactivity). Super-
Kamiokande began operation on April 1, 1996.

3) The essential basis of the Super-Kamiokande experiment is as
follows: When a high-energy cosmic-ray particle (e.g., a proton)
hits an atomic nucleus in the upper atmosphere, the collision
produces a shower of secondary particles. Some of these
particles decay to other particles, some of which are neutrinos.
Most of the charged particles produced in the shower lose energy
as they move through the atmosphere and into the Earth's
surface. Neutrinos, however, because of their extremely small
rate of interaction, pass through the atmosphere and the ground,
the vast majority penetrating to the other side of the Earth.
But a few neutrinos do interact (e.g., with the ultrapure water
in the Super-Kamiokande reservoir), and the Super-Kamiokande
apparatus can detect the interaction of approximately 8
neutrinos per day in its inner volume.

4) The authors conclude: "These are exciting times for neutrino
physics, and for elementary particle physics as a whole. The
atmospheric neutrino data fit the neutrino-oscillation
hypothesis beautifully, and this verification that at least some
neutrinos have mass is an enormous step forward: It is the first
clear indication of physics beyond the Standard Model."

American Scientist 1999 87:222

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2. ON RECONSTRUCTING CLIMATE HISTORY

Hugo Beltrami (St. Francis Xavier University, CA) discusses
Earth's climate history, the author making the following points:

1) For climate predictions from general circulation models to be
interpreted with confidence, a robust record of past climatic
changes is required. Without such a record, natural variability
of the climate system cannot be separated from the possible
changes induced by human activity. Resolving this issue is
essential for addressing future climate change.

2) Two different approaches are widely used to reconstruct
Northern Hemisphere climatic change during the last 500 to 1000
years. Both show a warming in the 20th century, but for earlier
centuries they observe different patterns of climate change. Do
these disagreements reflect only differences in the spatial
distribution of sites, or are they due to intrinsic limitations
of the methods?

3) The first method uses large data sets of various temperature
"proxies", such as tree rings and oxygen isotopes in ice cores,
to construct a model of past temperature change (1). The second
method relies on geothermal data from boreholes worldwide to
model ground temperature changes and the energy balance at
Earth's continental surface (2-4).

4) Comparison of these multiproxy and geothermal paleoclimatic
models is difficult because of differences in the spatial
distribution of data. But preliminary comparison (5) yields some
important differences. In particular, the models disagree over
the existence of a cold period between 1500 and 1800 A.D. Such a
cold spell is documented in all geothermal models but does not
appear as a strong signal in the multiproxy reconstructions (1).
In general, because of the way surface temperatures are
reconstructed in the borehole method, direct comparison with
multiproxy data is not possible.

References (abridged):

1. M. E. Mann et al., Geophys. Res. Lett. 26, 759 (1999)

2. S. Huang et al., Nature 403, 756 (2000)

3. R. N. Harris, D. S. Chapman, Geophys. Res. Lett. 28, 747
(2001)

4. H. Beltrami et al., Geophys. Res. Lett. 29,
10.1029/2001GL014310 (2002)

5. K. R. Briffa, T. J. Osborn, Science 295, 2227 (2002)

Science 2002 297:206

Web Links: paleoclimatology

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3. ON PLUTO AND THE KUIPER BELT

In 1951 the astronomer Gerard P. Kuiper (1905-1973) postulated
the existence of a belt of objects beyond the orbit of Pluto.
Both the existence and nature of the objects were matters of
speculation for decades, until finally in 1992 Jewitt and Luu
identified the first Kuiper object. The current estimate is that
as many as 10^(8) objects larger than 10 kilometers in diameter
may exist in what is called the "Kuiper belt", a disc that hugs
the plane of the planetary system and lies between 35 and 1000
AU from the Sun. Observations to date have yielded some 55
trans-Neptune bodies with radii on the order of 100 km or
larger, and Pluto is considered by some astronomers to be a
member of this population.

The moon Charon, discovered by Walter Christy in 1978, is the
only satellite of Pluto, approximately one-tenth Pluto's mass,
so that the pair are considered an instance of a double-planet.
Charon is 1186 km in diameter, and in the Solar System, is the
largest satellite compared to its primary. Its axial rotation
period is the same as the rotation period of Pluto itself, so
that Charon keeps one face permanently turned towards Pluto and
hangs stationary over one point on Pluto's surface.

William B. McKinnon (Washington University St. Louis, US)
discusses Pluto, the author making the following points:

1) The discovery of the Kuiper Belt in the far regions of the
Solar System is one of the great achievements of the space age.
In addition to the small planet Pluto and its large moon Charon,
the belt contains approximately 100,000 worlds greater than 100
km in diameter, as well as a vast number of smaller, cometary
bodies(1). Unlike the domains of the terrestrial planets
(Mercury to Mars) or the gas giants (Jupiter to Neptune), the
Kuiper Belt has never been explored by spacecraft -- although
one mission, "New Horizons", has been competitively selected and
is in its final design phase(2).

2) In astronomy, there is no substitute for resolution and Pluto
and Charon are, to put it mildly, poorly resolved from Earth.
One clear signature, however, is Pluto's rotational lightcurve
-- the variation of the planet's apparent brightness with time.
Pluto's lightcurve is quite pronounced, both in terms of
brightness and spectral features, and implies at least three
separate types of surface terrain: a bright, nitrogen-ice-rich
terrain containing dissolved methane and carbon monoxide;
another bright, reddish terrain dominated by methane ice; and a
third dark, volatile-depleted terrain betraying only the
slightest hint of the broad infrared absorptions of water
ice(4,5). Such a complex, variegated surface goes a long way
towards explaining the peculiarities of Pluto's heat signature:
the planet simultaneously exhibits a nitrogen-dominated
atmosphere in vapor-pressure equilibrium with nitrogen-frosted
terrain at a temperature of 40 K, and warmer regions where
volatile ices have burned off. In this regard, Pluto is
Mars-like in its surface– atmosphere interaction.

3) The lightcurve data only hint at the complexity of Pluto's
surface. A higher-resolution map -- derived from the mutual
eclipses and transits of Pluto and its moon in the 1980s --
shows that even within the dark, volatile-depleted regions there
exist significant visual color differences, although these
differences are not as extreme as those seen in the Kuiper Belt
population as a whole.

4) The fate of the New Horizons mission presently rests with the
US Congress, but if it goes ahead it will provide the best
answers by far to our fundamental questions about Pluto–Charon
and the Kuiper Belt.

References (abridged):

1. Committee on Planetary and Lunar Exploration Exploring the
Trans-Neptunian Solar System (National Academy Press, Washington
DC, 1998).

2. Stern, S. A. Sci. Am. 286, 56-63 (2002).

3. Stern, S. A. & Tholen, D. J. (eds) Pluto and Charon (Univ.
Arizona Press, Tucson, 1997).

4. Douté, S. et al. Icarus 142, 421-444 (1999).

5. Grundy, W. M. & Buie, M. W. Icarus 157, 128-138 (2002)

Nature 2002 418:135

Web Links: Pluto Charon Kuiper Belt

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4. MOLECULAR BIOLOGY: ON MOTOR PROTEINS

Thomas Duke (Cavendish Laboratory Cambridge, UK) discusses motor
proteins, the author making the following points:

1) Motor proteins use energy derived from the hydrolysis of ATP
to move unidirectionally along microtubules and actin filaments.
They play multifunctional roles in the cell, being intimately
involved in transport processes, cell motility, and the
organization and maintenance of cytoskeletal structures. During
mitosis, the proper arrangement of chromosomes before cell
division involves movement both toward and away from spindle
poles, which is thought to be mediated by both plus- and
minus-directed microtubule motors. In neurons, vesicles are
transported along axonal microtubules both toward and away from
the axon tip, carried by motors of opposite polarity. Given the
functional importance of opposing motion, just what determines
motor directionality has become a pressing question (1).

2) The question has been addressed most effectively by comparing
two molecules from the kinesin family that move in opposite
directions. Conventional kinesin and Ncd have similar dimeric
structures, composed of a coiled-coil stalk attached to a pair
of motor domains, which are closely homologous between the two
species. Yet kinesin moves toward the plus end of a microtubule,
whereas Ncd is a minus-directed motor. Establishing what makes
these two molecules move in different directions might shed
light on how motor proteins work.

3) The first intriguing result was obtained by using a chimera
composed of the Ncd motor domain fused to the kinesin stalk
region (2,3). Unlike kinesin, Ncd is nonprocessive; an
individual molecule is incapable of tracking a microtubule. So
to test motor directionality, a gliding motility assay was used.
Motors were adsorbed on a surface at sufficient density to
enable dozens of molecules to interact with a single
microtubule. Observing the motion of end-labeled microtubules
under a microscope, it was found that the chimera propelled them
across the surface in the opposite direction to the native Ncd
protein. Clearly, the motor domain is not the sole determinant
of directionality. An even more startling result was obtained
recently by Endow and Higuchi (4), who made a mutant of Ncd with
a single amino acid substitution in the neck region, which joins
the motor domain to the stalk. In the gliding assay, the mutant
drove microtubules in both directions. Typically, an individual
microtubule traveled for several micrometers with its plus end
leading, then abruptly reversed direction and traveled for a
similar distance in the opposite sense. The speed was
approximately the same in each direction and the reversals
appeared to occur quite randomly. Badoual et al. (5) present a
theoretical model that suggests that directionality in a gliding
assay is a team property and cannot entirely be reduced to the
characteristics of an individual motor molecule. They ascribe
the ability of the mutant Ncd to push the microtubule both ways
to an instability in the collective dynamics that arises when
many motors work together.

References (abridged):

1.  Endow, S. A. (1999) Nat. Cell Biol. 1, 163-167

2.  Henningsen, U. & Schliwa, M. (1997) Nature (London) 389,
93-96

3.  Case, R. B. , Pierce, D. W. , Hom-Booher, N. , Hart, C. L. &
Vale, R. D. (1997) Cell 90, 959-966

4.  Endow, S. A. & Higuchi, H. (2000) Nature (London) 406,
913-916

5.  Badoual, M. , Jülicher, F. & Prost, J. (2002) Proc. Natl.
Acad. Sci. USA 99, 6696-6701

Proc. Nat. Acad. Sci. 2002 99:6521

Web Links: motor proteins     kinesin microtubules

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5. NEUROBIOLOGY: ON CORTICAL PLASTICITY

J.W. Schnupp and O. Kacelnik (Oxford University, UK) discuss
cortical plasticity, the authors making the following points:

1) Mammals are distinguished from other vertebrate species by
their more highly developed cerebral cortex and by their larger,
more complex and more versatile behavioral repertoire. The
primary function of the cortex may be to mediate complex
behaviors, and to facilitate adaptive changes in behavior
brought about by changes in the circumstances in which an animal
lives. This notion gains support from the observation -- made
about a decade ago -- that it is possible to induce cortical
reorganization simply by training an animal on a new task [1–5] .

2) Primary sensory cortical areas are organized in a
"topographic" fashion. For example, neurons in the most lateral
parts of the somatosensory cortex respond to tactile stimuli
applied to the head, while more medial parts respond to
stimulation of progressively more caudal parts of the body.
Similarly, the sound frequency preferences of neurons in the
primary auditory cortex vary systematically along a frequency
axis, the orientation of which varies from species to species.
Studies by Recanzone et al. [1,2]  and by Weinberger and
colleagues [3–5] have demonstrated that the precise topographic
organization of these sensory fields is amenable to modification
when an animal is trained to perform a specific task.

3) These pioneering studies used classical conditioning, with
either reward or punishment, to train animals on a variety of
tasks, including the detection of pure tone sounds of a
particular frequency and the discrimination of tones or
vibrating tactile stimuli of slightly different frequencies.
When the primary somatosensory [1] or auditory [2,3,5] cortex of
the trained animals was subsequently mapped with microelectrode
recordings, it was found that the region of cortex representing
the sound frequency bands stimulated in the auditory tasks, or
the parts of the forepaw stimulated in the tactile task, had
expanded, presumably at the cost of the representation of
adjacent sound frequencies or body parts.

4) At about the same time, researchers became aware that changes
in the topographic organization of cortical areas can also be
achieved by artificial electrical stimulation techniques,
including intracortical microstimulation or stimulation of the
nucleus basalis [5]. These techniques can both lead to changes
in cortical topography, in some respects resembling those seen
after training in specific sensory tasks. Most notably,
intracortical microstimulation and stimulation of the nucleus
basalis can both increase the size of the cortical region that
appears to be responsive to a particular set of stimuli.
Consequently, some authors have proposed that intracortical
microstimulation or nucleus basalis stimulation may serve as a
useful experimental model of "representational plasticity" in
sensory cortex.

References (abridged):

1. Recanzone G.H., Merzenich M.M., Jenkins W.M., Grajski K.A.
and Dinse H.R. (1992) Topographic reorganization of the hand
representation in cortical area 3b owl monkeys trained in a
frequency-discrimination task. J. Neurophysiol., 67:1031-1056

2. Recanzone G.H., Schreiner C.E. and Merzenich M.M. (1993)
Plasticity in the frequency representation of primary auditory
cortex following discrimination training in adult owl monkeys.
J. Neurosci., 13:87-103

3. Weinberger N.M. (1993) Learning-induced changes of auditory
receptive fields. Curr. Opin. Neurobiol., 3:570-577

4. Weinberger N.M., Javid R. and Lepan B. (1993) Long-term
retention of learning-induced receptive-field plasticity in the
auditory cortex. Proc. Natl. Acad. Sci. USA., 90:2394-2398

5. Weinberger N.M. and Bakin J.S. (1998) Learning-induced
physiological memory in adult primary auditory cortex: receptive
field plasticity, model and mechanisms. Audiol. Neurootol.,
3:145-167

Current Biology 2002 12:R144

Web Links: cerebral cortex     cortical plasticity     learning
and the cortex

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6. ON THE GENETICS AND EVOLUTION OF MATERNAL CARE

J. Hunt and L.W. Simmons (University of Western Australia, AU)
discuss the genetics and evolution of maternal care, the authors
making the following points:

1) The evolution of parental care has been the subject of
intense study (1,2). Most empirical research has focused on the
evaluation of costs of care to parents, the benefits accrued by
offspring, and the inevitable conflict between parents and their
offspring over how much care should be provided (1).
Surprisingly little attention has been paid to the underlying
genetic variation in parental care that is required for its
evolution and assumed by theoretical treatments of the subject
(3). Although studies of lactation in agricultural animals (4-5)
and nesting behavior in laboratory mice show that these aspects
of maternal care can respond to artificial selection, few
studies have examined the quantitative genetics of parental care
from an evolutionary perspective.

2) The amount of genetic variation for parental care has
important evolutionary implications. In many organisms, the
environment provided by parents determines the environmental
conditions experienced by progeny, thus altering the traditional
genotype-phenotype relationship. When there is variation in the
quality of the environment being provided by parents in the form
of parental care, and this variation reflects genetic
differences among parents, indirect genetic effects can exist.
Thus, environmental effects derived from parental variation
should be viewed as "inherited environments" because, while they
constitute environmental effects in the offspring generation,
the phenotypes in the parental generation producing these
environmental effects could be heritable. Both theoretical
models and empirical studies  have suggested that indirect
genetic effects can have far-reaching evolutionary consequences.

3) In the dung beetle, Onthophagus taurus, females provide care
to offspring by provisioning a brood mass. The size of the brood
mass has pronounced effects on offspring phenotype. Using a
half-sib breeding design, the authors demonstrate that the
weight of the brood mass that females produce exhibits
significant levels of additive genetic variance due to sires.
However, variance caused by dams is considerably larger,
demonstrating that maternal effects are also important. Body
size exhibited low additive genetic variance. However, body size
exerts a strong maternal influence on the weight of brood masses
produced, accounting for 22% of the nongenetic variance in
offspring body size. Maternal body size also influenced the
number of offspring produced but there was no genetic variance
for this trait. Offspring body size and brood mass weight
exhibited positive genetic and phenotypic correlations. The
authors conclude that both indirect genetic effects, via
maternal care, and nongenetic maternal effects, via female size,
play important roles in the evolution of phenotype in this
species.

References (abridged):

1.  Clutton-Brock, T. H. (1991) The Evolution of Parental Care
(Princeton Univ. Press, Princeton)

2.  Rosenblatt, J. S. & Snowdon, C. T. (1996) Parental Care:
Evolution, Mechanisms, and Adaptive Significance (Academic,
London)

3.  Mock, D. W. & Parker, G. A. (1997) The Evolution of Sibling
Rivalry (Oxford Univ. Press, Oxford)

4.  Barker, J. S. F. & Robertson, A. (1966) J. Anim. Sci. 35,
221-240

5.  Mavrogenis, A. P. & Papachristoforou, C. (2001) Liv. Prod.
Sci. 67, 81-87

Proc. Nat. Acad. Sci. 2002 99:6828

Web Links: parental care genetics     parental care evolution

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7. SPOTLIGHT: SPIN ELECTRONICS

Researchers in microcircuitry have been eager to exploit a
property of the electron known as "spin". Spin is a purely
quantum phenomenon approximately akin to the spinning of a top.
A top can spin in a clockwise or counterclockwise direction, and
spin therefore lends itself to a new kind of binary logic of
ones and zeros. The movement of spin, like the flow of
electronic charge, can also carry information among devices. One
advantage of spin over charge is that spin can be easily
manipulated by externally applied magnetic fields, a property
already in use in magnetic storage technology. Another and more
subtle (but potentially significant) property of spin is its
relatively long coherence, or relaxation time, unlike charge
states, which are easily destroyed by scattering collisions with
defects, impurities, or other charges. These characteristics of
electron spin open the possibility of developing devices that
could be much smaller, consume less electricity, and be more
powerful for certain types of computations than is possible with
systems based on electron charge. Researchers in the spintronics
community hope that by understanding the behavior of electron
spin in materials they can learn something fundamentally new
about solid state physics that will lead to a new generation of
electronic devices based on the flow of spin in addition to the
flow of charge. In fact, the spintronics dream is a seamless
integration of electronic, optoelectronic, and magnetoelectronic
multifunctionality on a single device that can perform much more
than is possible with today's microelectronic devices. (S.D.
Sarma:  American Scientist 2001 89:516)

In this context, the term "tunneling" refers to a quantum
mechanical phenomenon involving an effective penetration of an
energy barrier by a particle, the penetration resulting from the
width of the barrier being less than the wavelength of the
particle.

S. Yuasa et al (National Institute of Advanced Industrial
Science and Technology Tsukuba, JP) discuss spin electronics,
the authors making the following points:

1) A new field of electronics called "spin electronics" (1,2),
which makes use of both the electric charge and the spin of
conduction electrons, has been developing rapidly in systems
such as metallic magnetic multilayers, magnetic semiconductors,
and strongly correlated electron systems. Among these systems, a
magnetic tunnel junction, which consists of two ferromagnetic
metal layers (electrodes) separated by a thin insulating layer
(tunnel barrier) and shows the tunnel magnetoresistance effect
(3-5), is especially important for application to
magnetoresistive random-access memory devices.

2) Highly functional spin-electronic devices such as spin
transistors cannot be realized without a better understanding of
the mechanism of spin-polarized electron transport, because it
is still unclear how the coherence of the wave functions and
spins of the conduction electrons are conserved in the transport
process. For example, the resonant-tunneling effect (i.e.,
coherent tunneling of electrons from one electrode to the other
through quantum well states formed between the two electrodes),
in which the coherence of electron wave functions is essential,
has never been well controlled in spin-polarized systems such as
magnetic tunnel junctions and magnetic semiconductors.
Spin-polarized resonant tunneling is crucial for the development
of highly functional devices, such as a resonant-tunneling spin
transistor and quantum information devices, because the
coherency of both the wave functions and the spins of conduction
electrons should be conserved in those devices.

3) One of the simplest ways to realize spin-polarized resonant
tunneling is to insert a nonmagnetic metal layer between the
insulating tunnel barrier and one of the two ferromagnetic
electrodes in a magnetic tunnel junction. Because spin-dependent
reflections of the conduction electrons take place at the
ferromagnetic-nonmagnetic interface, spin-polarized quantum well
states are created in the nonmagnetic layer and spin-polarized
tunneling electrons will resonantly pass through the nonmagnetic
layer. Theories predict an oscillation of the tunnel
magnetoresistance effect as a function of the nonmagnetic layer
thickness because the spin polarization of the tunneling
electrons oscillates as a result of resonant tunneling.

4) In summary: The authors report that insertion of a thin
nonmagnetic copper Cu(001) layer between the tunnel barrier and
the ferromagnetic electrode of a magnetic tunnel junction
results in the oscillation of the tunnel magnetoresistance as a
function of the Cu layer thickness. The effect is interpreted in
terms of the formation of spin-polarized resonant tunneling. The
amplitude of the oscillation is so large that even the sign of
the tunnel magnetoresistance alternates. The oscillation period
depends on the applied bias voltage, reflecting the energy band
structure of Cu. The authors suggest the results are encouraging
for the development of spin-dependent resonant tunneling devices.

References (abridged):

1. G. A. Prinz, Science 282, 1660 (1998)

2. S. A. Wolf, et al., Science 294, 1488 (2001)

3. J. S. Moodera, L. R. Kinder, T. M. Wong, R. Meservey, Phys.
Rev. Lett. 74, 3273 (1995)

4. T. Miyazaki and N. Tezuka, J. Magn. Magn. Mater. 139, L231
(1995)

5. J. M. de Teresa, et al., Science 286, 507 (1999)

Science 2002 297:234

Web Links: spin electronics     spintronics magnetoresistance

Related Background:

MAGNETIC MATERIALS: WHAT PROSPECTS FOR SPINTRONICS?

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  of
electrons as well as their charge.

In this context, the term "spin" refers to the part of the total
angular momentum of a particle (electron, atom, etc.) that is
distinct from its orbital angular momentum -- in other words,
spin is essentially a rotation momentum. According to quantum
mechanics, spin is quantized and restricted to a particular set
of values for each type of particle. (For more details
concerning spin, see the notes below.)

One aim of this new field 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.

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.

Of importance in magnetoelectronics is the phenomenon of "giant
magnetoresistance" (GMR). 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 charge carriers is minimized, and
the material has its lowest electrical resistance. When the
ferromagnetic layers are anti-aligned, the spin-dependent
scattering of charge 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.

The first report of the discovery of GMR appeared in 1988.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,
i.e., computer memory that remains intact when the computer is
switched off. The Honeywell Corporation announced the
demonstration of GMR random access memory (RAM) in January 1997.

Reviewing the field of spintronics in the May issue of _Physics
Today_, Peter Gruenberg (Juelich Research Center, DE) points out
that the study of layered magnetic structures is one of the hot
areas in current magnetism research, the interest due largely to
growing applications in magnetic sensors and magnetic storage
media such as computer disks and random access memories.
Gruenberg suggests that magnetic random-access memories (MRAMs)
based on structures of magnetic metallic films interspersed with
nonmagnetic metallic or insulating interlayers could be the next
generation in magnetic-storage technology, replacing the
semiconductor-based dynamic random-access memories (DRAMs) that
are the current standard. The advantages of MRAMs include
retention of information when the computer is switched off, high
storage density, and low energy consumption. Gruenberg states:
"The field of layered magnetic structures is broad and still
expanding, with many different phenomena of interest. It remains
a fascinating field, rich with opportunities both in basic
research and in potential applications."

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.

Related Background:

MAGNETOELECTRONICS: CONTROL OF SEMICONDUCTOR MAGNETISM BY
EXTERNAL ELECTRIC FIELDS

H. Ohno et al (8 authors at Tohoku University, JP) 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."

Nature 2000 408:923,944

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".

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.

Science 1998 282:1660

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.

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8. ON GLOBAL OPTIMIZATION

In this context, "tabu search" refers to a mathematical
optimization method. The overall approach of the tabu search
method is to avoid entrainment in cycles by forbidding or
penalizing moves which take the solution, in the next iteration,
to points in the solution space previously visited ( hence
"tabu"). The tabu search method was apparently originated in
1977 by F. Glover, the method partly motivated by the
observation that human behavior appears to operate with a random
element that leads to inconsistent behavior given similar
circumstances.

U.H. Hansmann and L.T. Wille (Michigan Technological University,
US) discuss global optimization, the authors making the
following points:

1) Global optimization is one of the key issues in modern
science, technology, and economy. Typical examples are the
problem of optimal transportation routes [1], finding molecular
conformations [2-4], or fitting experimental spectra [5].
Consequently, much effort has been spent on designing methods to
find global optima. For this purpose, the system has to be
described by an objective function, and optimality is achieved
when this function reaches its global minimum. If the objective
function is viewed as an "energy", the optimal solution
corresponds to the deepest minimum in the energy landscape. For
most applications of practical interest, competing interactions
and frustration in the system lead to an energy landscape with
many local minima separated by high barriers. Since conventional
minimization techniques tend to get trapped in whichever local
minimum they encounter first, it turns out to be extremely
difficult to find the global minimum in such cases.

2) A general characteristic of successful optimization
techniques is that they avoid entrapment in local minima and
continue to explore the energy landscape for further solutions.
For instance, in "tabu search", the search is guided away from
areas that have already been explored in an effort to cover all
important regions of the solution space. The danger with such an
approach is that it may result in slow convergence since it does
not distinguish between important and less important regions of
the landscape.

3) Entrapment in local minima can also be avoided if the search
is performed in a deformed or smoothed energy landscape, for
example, by lowering diffusion barriers, in stochastic
tunneling, or the various generalized ensemble approaches. In
the optimal case, the original energy landscape is transformed
in a funnel landscape and convergence toward the global minimum
is fast. Although they have been very successful, most of these
methods require a considerable amount of fine-tuning or a priori
information. Moreover, problems may exist when connecting back
to the original landscape since minima on the deformed surface
may have been displaced or merged.

4) In summary: The authors introduce a new heuristic global
optimization method, energy landscape paving (ELP), which
combines core ideas from energy surface deformation and tabu
search. In appropriate limits, ELP reduces to existing
techniques. The approach is very general and flexible and is
illustrated by application to two protein folding problems. For
these examples, the technique gives faster convergence to the
global minimum than previous  approaches.

References (abridged):

1. Mathematical Methods on Optimization in Transportation     
Systems, edited by M. Pursula and J. Niittymaki (Kluwer     
Academic, Dordrecht, 2001)

2. D. J. Wales and H. A. Scheraga, Science 285, 1368 (1999)

3. U. H. E. Hansmann and Y. Okamoto, in Annual Reviews of
Computational Physics, edited by D. Stauffer (World Scientific,
Singapore, 1999), Vol. VI, p. 129

4. L. T. Wille, in Annual Reviews of Computational Physics,
edited by D. Stauffer (World Scientific, Singapore, 2000),     
Vol. VII, p. 25

5. J. Karle and H. Hauptmann, Acta Crystallogr. 17, 392 (1964)

Phys. Rev. Lett. 2002 88:068105

Web Links: global optimization     protein folding

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9. ON OPTICAL NETWORKS AND MOLECULAR SWITCHES

F.M. Raymo and S. Giordani (University of Miami, US) discuss
optical networks, the authors making the following points:

1) The growing demand for telecommunication and Internet
applications continues to stimulate the development of optical
networks for the faster transmission of larger volumes of
data.(1) In these networks, bundles of optical fibers coupled to
optoelectronic devices ensure the communication of optical
signals over long distances. Present optical transport
technology permits the transmission of hundreds of gigabits per
second over hundreds of kilometers. The routes of the optical
signals traveling through these networks are determined by
switching devices in response to electrical stimulations.
Unfortunately, the interplay of optical and electrical signals
in these hybrid devices limits dramatically the transmission
capacity and speed.(1,2) Only a tiny fraction of the huge
bandwidth of optical fibers can be used.

2) In principle, hundreds of optical signals with closely spaced
wavelengths can be transmitted through a single waveguide
relying on current optical fiber technology.(1c) The propagating
light beams do not interfere with each other and can be
transported in parallel. The noninteracting properties of
optical signals, however, do not apply to electrical signals.
Only one electrical signal can be transported through a single
electrical wire. Thus, even although optical fibers can transmit
multiple data in parallel, optoelectronic switches process them
more or less sequentially.(1b) The electronic portion of these
devices simply cannot handle the immense parallelism potentially
offered by optical signals. These intrinsic limitations will
soon limit the rapid escalation of photonic traffic supported in
optical networks(2), and it therefore is necessary to develop
practical strategies to switch propagating optical signals with
optical, rather than electrical stimulations.(3) Ultimately, the
electronic element of present communication networks must be
eliminated completely; all intervening signals must be optical
and only optical effects must be exploited.

3) Molecular switches4 are promising candidates for the
realization of signal processing networks. Simple logic
operations5 have been implemented already at the molecular level
by using chemical, electrical, and/or optical signals. 
Furthermore, the miniaturized dimensions of these chemical
systems have encouraged the design of prototypical devices
incorporating molecular components. However, the lack of
reliable strategies to communicate signals between individual
molecules has so far prevented the integration of molecular
switches into functioning circuits.

4) In summary: The authors report a demonstration that molecular
switches can be used to gate optical signals in response to
optical signals. The authors report the realization of a simple
optical network consisting of three light sources, one cell
containing a solution of three fluorescent molecules, one cell
containing a solution of a three-state molecular switch and a
detector. The light emitted by the three fluorophores is
absorbed by the three states of the molecular switch. Using this
simple operating principle, the authors demonstrate that
multichannel digital transmission can be implemented on an
ensemble of communicating molecules relying exclusively on the
interplay of optical inputs and optical outputs.

References (abridged):

1. (a) Special issue on Optical Networking. Bell Labs Technol.
J. 1999, 4 (1), 3-322. (b) Franz, J. H.; Jain, V. K. Optical
Communications: Components and Systems; CRC Press: Boca Raton,
FL, 2000. (c) Mynbaev, D. K.; Scheiner, L. L. Fiber Optic
Communications Technology; Prentice Hall: Upper Saddle River,
NJ, 2001

2. (a) Kahn, J. M.; Ho, K.-P. Nature 2001, 411, 1027-1030. (b)
Mitra, P. P.; Stark, J. B. Nature 2001, 411, 1027-1030

3. (a) Thylen, L.; Karlsson, G.; Nilsson, O. IEEE Commun. Mag.
1996, 34 (2), 106-113. (b) Nolte, D. D. J. Appl. Phys. 1999, 85,
6259-6289. (c) Jackman, N. A.; Patel, S. H.; Mikkelsen, B. P.;
Korotky, S. K. Bell Labs Technol. J. 1999, 4 (1), 262-281. (d)
McCarthy, D. C. Photonics Spectra 2001, 35 (3), 140-150. (e)
Veeraraghavan, M.; Karri, R.; Moors, T.; Karol, M.; Grobler, R.
IEEE Commun. Magn. 2001, 39 (3), 118-127

4. (a) Special issue on Photochromism: Memories and Switches.
Chem. Rev. 2000, 100, 1683-1890. (b) Molecular Switches;
Feringa, B. L., Ed.; Wiley-VCH: Weinheim, Germany, 2001

5. Mitchell, R.J. Microprocessor Systems: An Introduction;
MacMillan; Houndsmill, UK, 1995

J. Am. Chem. Soc. 2002 124:2004

Web Links: optoelectronics optoelectronic switches

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10. ON SILICON NITRIDE CERAMICS

D.P. Thompson (University of Newcastle-upon-Tyne, UK) discuss
silicon nitride ceramics, the author making the following points:

1) Microstructural control is at the heart of materials science,
especially when a tailored combination of mechanical properties
is required. In the field of nitrogen ceramics, where the final
microstructure is produced by "liquid-phase sintering", it is
difficult to control the morphology of the final product; the
little that can be done is achieved by controlling the starting
composition and the processing parameters -- principally, time
and sintering temperature.

2) Silicon-nitride-based ceramics tend to have the edge over
their competitors in engineering applications, thanks to their
low density and toughness. When silicon nitride, aluminum oxide
and aluminum nitride are reacted together, they form ceramics
called "sialons", after their constituent elements Si, Al, O and
N. There are two common forms of sialons, alpha-sialons and
beta-sialons, and in their atomic arrangements they are like
alpha- and beta-silicon nitride. Since the early 1970s(2) it has
been known that beta-sialons readily form with a needle-like (or
"acicular") morphology. The sialon grains may be elongated (with
aspect ratios as high as ten), resulting in a tough final
product.

3) Similar acicular morphologies have been produced for
alpha-sialon ceramics by using unusual starting materials(3), an
excess of liquid phase or an oxygen-rich starting
composition(4). But 30 years of research have achieved only
limited control of the aspect ratio for either alpha- or
beta-sialons prepared by conventional sintering procedures.
During liquid-phase sintering of sialons, a chemical force
drives the formation of the final phases from the mixture of
oxides and nitrides that comprise the starting material. The
phases at the final stage of the procedure are essentially the
matrix sialon phase (possibly with small amounts of other
crystalline phases) and liquid, and these two phases are
generally close to thermodynamic equilibrium at the temperature
concerned. Subsequent grain growth occurs by "Ostwald ripening",
whereby the small crystals that are initially present dissolve
in the liquid and re-precipitate onto larger crystals, as these
are thermodynamically more stable. Shen et al.(1) have recently
introduced a technique that opens the way towards significantly
greater microstructural control.

References (abridged):

1. Shen, Z., Zhao, Z., Peng, H. & Nygren, M. Nature 417, 266-269
(2002)

2. Drew, P. & Lewis, M. H. J. Mater. Sci. 9, 261-269 (1974)

3. Chen, I.-W & Rosenflanz, A. Nature 389, 701-704 (1997)

4. Zhao, H., Swenser, S. P. & Cheng, Y.-B J. Eur. Ceram. Soc.
18, 1053-1057 (1997)

Nature 2002 417:237

Web Links: silicon nitride ceramics     liquid phase sintering  
  sialons

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11. ON GENOME VARIATION OF INDIGENOUS HUMAN MICROBES

Martin J. Blaser (New York University, US) discuss microbial
genome variation, the author making the following points:

1) Bacteria indigenous to the human body are numerous and varied
and dwell in multiple niches in skin and on mucosal surfaces;
many niches are persistently colonized by one or more species,
often for the host's life span. Such persistence implies an
equilibrium in which the biologic cost of the colonization to
the host is low.(1) The colonizing organisms occupy all
available niches, compete for nutrients and favored locations,
and participate in communal activities, such as waste management
and genetic exchange.

2) Colonization of the stomach by Helicobacter pylori is an
exception to these generalities because it can last for decades
in the absence of antibiotic treatment and because of the
absence of competing microbes. Strains of H. pylori are diverse,
and over a period of years, the populations colonizing a given
host experience genetic variation as a result of point mutations
or recombination, which can involve exchanges across loci within
a genome (intragenomic) or between differing organisms
(intergenomic). Two recent articles have extended our
understanding of these changes as they occur in the course of
the colonization of a single host.(2,3)

3) Israel et al sought to define the genes in H. pylori that
vary over time within a given host.(3) In the 1990s, H. pylori
strain J99, isolated from a gastric-biopsy specimen from a
patient with a duodenal ulcer, was used to identify the entire
genomic sequence of the bacteria.(4) Comparison with strain
26695, for which the genomic sequence was also known, showed
that both strains shared most genes but that about 6 percent of
the genes were unique to each strain. The patient refused
antibiotic treatment and, 6 years later, again underwent
endoscopy. Multiple individual isolates of H. pylori were
obtained from the resulting biopsy specimens. DNA was extracted
from them and hybridized with a microarray chip that included
all the genes from the original J99 and 26695 sequences. The
later isolates were closely related but not identical to either
the earlier isolate or each other; individual and groups of
genes were missing from the new isolates. In total, each of 13
new isolates tested had a unique complement of genes.
Considering that a single gastric-biopsy specimen represents
0.001 percent of gastric mucosal cells, by extrapolation, the
amount of variation within the total population of H. pylori
must be enormous. Furthermore, the simultaneous isolation of
these variants suggests that each one occupies a specific
microniche and competes with the others. Since H. pylori is
capable of DNA uptake, genes or segments lost from some
organisms can be taken up by others,(5) implying that the entire
population of H. pylori within a host's stomach -- or that has
passed through the stomach -- represents the H. pylori gene pool
for that person. This great diversity implies that no single
strain of H. pylori can dominate all niches indefinitely.

References (abridged):

1. Blaser MJ, Kirschner D. Dynamics of Helicobacter pylori
colonization in relation to the host response. Proc Natl Acad
Sci U S A 1999;96:8359-8364

2. Bjorkholm B, Sjolund M, Falk PG, Berg OG, Engstrand L,
Andersson DI. Mutation frequency and biological cost of
antibiotic resistance in Helicobacter pylori. Proc Natl Acad Sci
U S A 2001;98:14607-14612

3. Israel DA, Salama N, Krishna U, et al. Helicobacter pylori
genetic diversity within the gastric niche of a single human
host. Proc Natl Acad Sci U S A 2001;98:14625-14630

4. Alm RA, Ling LS, Moir DT, et al. Genomic-sequence comparison
of two unrelated isolates of the human gastric pathogen
Helicobacter pylori. Nature 1999;397:176-180. [Erratum, Nature
1999;397:719

5. Aras RA, Takata T, Ando T, van der Ende A, Blaser MJ.
Regulation of the HpyII restriction-modification system of
Helicobacter pylori by gene deletion and horizontal
reconstitution. Mol Microbiol 2001;42:369-382

New Engl. J. Med. 2002 346:2083

Web Links: Helicobacter pylori     indigenous human bacteria

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12. GEOGRAPHIC VARIATION OF MORTALITY FROM STROKE IN THE US

J.E. Williams et al (Centers for Disease Control and Prevention,
US) discuss mortality from stroke in the US, the authors making
the following points:

1) In the United States, stroke is the third leading cause of
death and one of the major causes of serious, long-term
disability among adults. Each year, approximately 500,000
persons suffer a first-time stroke, and approximately 167,000
deaths are stroke-related.

2) Among U.S. residents, 167,366 stroke-related deaths occurred
in 1999, with an age-adjusted rate of 63.4 per 100,000
population. The greatest proportion of deaths occurred among
persons aged 85 years (40.1%) followed by those aged 75-84 years
(34.3%), those aged 65-74 years (14.4%), and those aged <65
years (11.2%). Age-specific death rates increased for successive
age groups. By race/ethnicity, the highest age-adjusted death
rates for stroke occurred among blacks followed by whites (225.2
and 166.7 per 100,000 population, respectively). Age-adjusted
death rates for stroke were slightly higher among men (62.4)
than among women (60.5). Ischemic strokes accounted for 68.3% of
all stroke-related deaths; age-adjusted death rates were higher
for ischemic stroke than for all other stroke subtypes.

3) In 1999, a total of 79,663 (47.6%) stroke-related deaths
occurred pretransport, 926 (0.7%) occurred as dead on arrival
(DOA), 5,519 (3.3%) occurred in the emergency department (ED),
and 80,369 (48.0%) occurred after admission to the hospital; for
889 (0.5%) deaths, place-of-death data were not available. The
proportion of pretransport deaths increased with age, and the
proportion of deaths that occurred as DOA or in the ED decreased
with age. The proportion of pretransport deaths was higher among
women (52.2%) than among men (40.3%) and higher among whites
(50.1%) than among other racial/ethnic populations. Conversely,
the proportion of stroke-related deaths that occurred in the ED
was higher among blacks (5.8%) than among other racial/ethnic
populations, and higher among Hispanics (4.8%) than among
non-Hispanics (3.2%). Compared with other stroke subtypes, the
highest proportion of pretransport deaths was among persons who
died of sequelae of stroke or other cerebrovascular disease
(69.1%), followed by ischemic stroke (23.3%), subarachnoid
hemorrhagic stroke (13.7%), and intracerebral hemorrhagic stroke
(12.6%). Persons who died of subarachnoid hemorrhagic stroke
accounted for the highest proportion of deaths that occurred as
DOA or in the ED (1.1% and 7.8%, respectively).

4) The state-specific, age-adjusted death rates for stroke
ranged from 33.0 per 100,000 population in New Hampshire to 83.8
in South Carolina. The proportion of pretransport deaths ranged
from 23.3% in the District of Columbia to 67.3% in Oregon.
States with 60% of stroke deaths reported as occurring
pretransport were Colorado (60.0%), Wisconsin (60.7%), Utah
(60.7%), Minnesota (62.1%), Idaho (64.0%), Washington (64.4%),
Vermont (67.2%), and Oregon (67.3%). The proportion of
stroke-related deaths reported as DOA ranged from zero to 4.6%;
those having occurred in the ED ranged from 0.8% to 8.3%. The
proportion of stroke-related deaths for which place-of-death
data were missing ranged from zero to 11.2%.

5) In summary: The findings in this report indicate that
ischemic strokes account for most stroke-related deaths and that
state-by-state variations exist in the proportion of
stroke-related deaths that occur pretransport. These findings
are consistent with other evidence that many acute ischemic
stroke patients cannot benefit from thrombolytic therapy because
they do not reach medical treatment in time. Thrombolytic
therapy is a time-dependent therapy with a window of efficacy of
3 hours after the onset of symptoms.

Morbidity and Mortality Weekly Report 2002 51:429

Web Links: stroke USA disease mortality geographic variation

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13. IN FOCUS: ON DOMAINS IN BIOLOGICAL MEMBRANES

R.G. Anderson and K. Jacobson (University of Texas, US) discuss
biological membranes, the authors making the following points:

1) A "domain" is a region distinctively marked by some physical
feature that distinguishes it from the surrounding landscape. A
domain in a cellular membrane, therefore, is a region with
physical features that differentiate it from the contiguous
membrane -- for example, the clathrin-coated pit. Coated pits
can be identified in thin-section electron micrographs by the
presence of a cytoplasmic fuzzy coat, and so are easily
distinguished from the surrounding membrane. Their other
properties offer important clues about the rules that govern the
construction and maintenance of all membrane domains (1). One
obvious rule is that the molecular composition of a membrane
domain differs from that of the surrounding membrane. The domain
can be enriched in both peripheral and integral molecules. One
set of enriched molecules has a structural function. The
structural molecules of a coated pit, for example, are the
peripherally associated clathrins and clathrin adaptors that
form the polygonal coat structure. These molecules are recruited
to the plasma membrane in a stepwise process that depends on
interactions between the adaptor proteins and both membrane
phosphoinositides (2,3) and integral membrane proteins (4). The
presence of the lattice on the membrane locally organizes lipids
like cholesterol (5) and serves as a platform that attracts a
variety of integral and peripheral membrane proteins. Therefore,
the molecular composition of this domain is determined by both
the coat structure and the molecules it attracts.

2) Molecules that collect in membrane domains do so because they
contain a specific molecular address for that domain. Most of
what is known about the addresses for coated pits comes from the
study of transmembrane receptors that mediate the uptake of
molecules. Addresses come in two basic forms. Either the
cytoplasmic tail of the protein contains a binding site that
recognizes one or more coat proteins or it recognizes an adaptor
protein that in turn has a binding site for a coat protein. The
address, therefore, is encoded by an amino acid sequence in the
cytoplasmic tail of the protein. Elimination or modification of
the address motif prevents the molecule from accumulating in
coated pits. This raises the possibility that molecular
addresses can be dynamically regulated so that a molecule with
the proper address spends only a portion of its functional life
in its target domain.

3) Coated pits have taught us that membrane domains are dynamic
structures with molecules entering and leaving according to
specific rules. They are constructed with specific cellular
machinery, and without constant maintenance, coated pits would
rapidly dissipate into the surrounding membrane. On the basis of
these considerations, cholesterol-sphingolipid-rich lipid
domains must have unique physical features, upper and lower size
limits, functionality, and a system for removing and adding
specific molecules.

4) In summary: The surface membrane of cells is studded with
morphologically distinct regions, or domains, such as
microvilli, cell-cell junctions, and coated pits. Each of these
domains is specialized for a particular function, such as
nutrient absorption, cell-cell communication, and endocytosis.
Lipid domains, which include caveolae and rafts, are one of the
least understood membrane domains. These domains are high in
cholesterol and sphingolipids, have a light buoyant density, and
function in both endocytosis and cell signaling. A major
mystery, however, is how resident molecules are targeted to
lipid domains. The authors propose that the molecular address
for proteins targeted to lipid domains is a lipid shell.

References (abridged):

1. T. Kirchhausen, Annu. Rev. Cell Dev. Biol. 15, 705 (1999)

2. I. Gaidarov, Q. Chen, J. R. Falck, K. K. Reddy, J. H. Keen,
J. Biol. Chem. 271, 20922 (1996)

3. I. Gaidarov, M. E. Smith, J. Domin, J. H. Keen, Mol. Cell 7,
443 (2001)

4. C. von Poser et al., J. Biol. Chem. 275, 30916 (2000)

5. D. J. McGookey, K. Fagerberg, R. G. W. Anderson, J. Cell
Biol. 96, 1273 (1983)

Science 2002 296:1821

Web Links: cell membrane domains     lipid rafts     biological
membranes

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