3,662 research outputs found
A theory for the alignment of cortical feature maps during\ud development
We present a developmental model of ocular dominance column formation that takes into account the existence of an array of intrinsically specified cytochrome oxidase blobs. We assume that there is some molecular substrate for the blobs early in development, which generates a spatially periodic modulation of experience–dependent plasticity. We determine the effects of such a modulation on a competitive Hebbian mechanism for the modification of the feedforward afferents from the left and right eyes. We show how alternating left and right eye dominated columns can develop, in which the blobs are aligned with the centers of the ocular dominance columns and receive a greater density of feedforward connections, thus becoming defined extrinsically. More generally, our results suggest that the presence of periodically distributed anatomical markers early in development could provide a mechanism for the alignment of cortical feature maps
The Limiting Speed of the Bacterial Flagellar Motor
Recent experiments on the bacterial flagellar motor have shown that the
structure of this nanomachine, which drives locomotion in a wide range of
bacterial species, is more dynamic than previously believed. Specifically, the
number of active torque-generating units (stators) was shown to vary across
applied loads. This finding invalidates the experimental evidence reporting
that limiting (zero-torque) speed is independent of the number of active
stators. Here, we propose that, contrary to previous assumptions, the maximum
speed of the motor is not universal, but rather increases as additional
torque-generators are recruited. This result arises from our assumption that
stators disengage from the motor for a significant portion of their
mechanochemical cycles at low loads. We show that this assumption is consistent
with current experimental evidence and consolidate our predictions with
arguments that a processive motor must have a high duty ratio at high loads.Comment: 8 pages, 3 figures (main text); 7 pages, 3 figures (supplementary
Tools or Toys? The Impact of High Technology on Scholarly Productivity
Toys. The impact of computers on productivity has been examined directly on macro data and indirectly (on wages) using microeconomic data. This study examines the direct impact on the productivity of scholarship by considering how high technology might alter patterns of coauthoring of articles in economics and their influence. Using all coauthored articles in three major economics journals from 1970-79 and 1992-96, we find: 1) Sharp growth in the percentage of distant coauthorships (those between authors who were not in the same metropolitan areas in the four years prior to publication), as the theory predicts. Contrary to the theory: 2) Lower productivity (in terms of subsequent citations) of distant than close-coauthored papers; and 3) No decline in their relative disadvantage between the 1970s and 1990s. These findings are reconciled by the argument that high-technology functions as a consumption rather than an investment good. As such, it can be welfare-increasing without increasing productivity.
Paradoxical signaling regulates structural plasticity in dendritic spines
Transient spine enlargement (3-5 min timescale) is an important event
associated with the structural plasticity of dendritic spines. Many of the
molecular mechanisms associated with transient spine enlargement have been
identified experimentally. Here, we use a systems biology approach to construct
a mathematical model of biochemical signaling and actin-mediated transient
spine expansion in response to calcium-influx due to NMDA receptor activation.
We have identified that a key feature of this signaling network is the
paradoxical signaling loop. Paradoxical components act bifunctionally in
signaling networks and their role is to control both the activation and
inhibition of a desired response function (protein activity or spine volume).
Using ordinary differential equation (ODE)-based modeling, we show that the
dynamics of different regulators of transient spine expansion including CaMKII,
RhoA, and Cdc42 and the spine volume can be described using paradoxical
signaling loops. Our model is able to capture the experimentally observed
dynamics of transient spine volume. Furthermore, we show that actin remodeling
events provide a robustness to spine volume dynamics. We also generate
experimentally testable predictions about the role of different components and
parameters of the network on spine dynamics
Mechanics of torque generation in the bacterial flagellar motor
The bacterial flagellar motor (BFM) is responsible for driving bacterial
locomotion and chemotaxis, fundamental processes in pathogenesis and biofilm
formation. In the BFM, torque is generated at the interface between
transmembrane proteins (stators) and a rotor. It is well-established that the
passage of ions down a transmembrane gradient through the stator complex
provides the energy needed for torque generation. However, the physics involved
in this energy conversion remain poorly understood. Here we propose a
mechanically specific model for torque generation in the BFM. In particular, we
identify two fundamental forces involved in torque generation: electrostatic
and steric. We propose that electrostatic forces serve to position the stator,
while steric forces comprise the actual 'power stroke'. Specifically, we
predict that ion-induced conformational changes about a proline 'hinge' residue
in an -helix of the stator are directly responsible for generating the
power stroke. Our model predictions fit well with recent experiments on a
single-stator motor. Furthermore, we propose several experiments to elucidate
the torque-speed relationship in motors where the number of stators may not be
constant. The proposed model provides a mechanical explanation for several
fundamental features of the flagellar motor, including: torque-speed and
speed-ion motive force relationships, backstepping, variation in step sizes,
and the puzzle of swarming experiments
Speleothem Paleoclimatology for the Caribbean, Central America, and North America
Speleothem oxygen isotope records from the Caribbean, Central, and North America reveal climatic controls that include orbital variation, deglacial forcing related to ocean circulation and ice sheet retreat, and the influence of local and remote sea surface temperature variations. Here, we review these records and the global climate teleconnections they suggest following the recent publication of the Speleothem Isotopes Synthesis and Analysis (SISAL) database. We find that low-latitude records generally reflect changes in precipitation, whereas higher latitude records are sensitive to temperature and moisture source variability. Tropical records suggest precipitation variability is forced by orbital precession and North Atlantic Ocean circulation driven changes in atmospheric convection on long timescales, and tropical sea surface temperature variations on short timescales. On millennial timescales, precipitation seasonality in southwestern North America is related to North Atlantic climate variability. Great Basin speleothem records are closely linked with changes in Northern Hemisphere summer insolation. Although speleothems have revealed these critical global climate teleconnections, the paucity of continuous records precludes our ability to investigate climate drivers from the whole of Central and North America for the Pleistocene through modern. This underscores the need to improve spatial and temporal coverage of speleothem records across this climatically variable region
Spontaneous recanalization of the basilar artery with conservative management months after symptom onset
Basilar artery occlusion may be associated with a poor prognosis in the absence of recanalization. Choices in aggressive treatment for this potentially fatal condition vary from intra-arterial or intravenous thrombolysis, endovascular removal, or a combination of the two, with adjunct anti-coagulation therapy. These therapies have proven to be effective in recanalization, whereas conservative management with anti-coagulants alone has had more limited success in the literature. We report a case of basilar artery occlusion managed conservatively with unfractionated heparin, resulting in complete recanalization 3.5 months after symptom onset. Conservative management of basilar artery occlusion with unfractionated heparin was associated with complete recanalization long after symptom onset
Impact of Ranch and Market Factors on an Index of Agricultural Holding Period Returns
This study develops nominal and real holding period return indices for operating ranches. The indices contain two components, monthly ranch operating profit and capital appreciation. A regression model that determines the effect of various attributes on total market value is used to estimate capital appreciation, and a second model determines operating profit. The two different ranch data series are compared with United States Long Term Government Bonds, the S&P 500 Index and Small Capitalization Stocks. Both ranch indices show very low correlation with the S&P 500 Index and very low or negative betas. Further, both ranch series have excess actual nominal and real returns when compared to expected returns determined from the Capital Asset Pricing Model.
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