8,458 research outputs found
Spatial and Temporal Sensing Limits of Microtubule Polarization in Neuronal Growth Cones by Intracellular Gradients and Forces
Neuronal growth cones are the most sensitive amongst eukaryotic cells in
responding to directional chemical cues. Although a dynamic microtubule
cytoskeleton has been shown to be essential for growth cone turning, the
precise nature of coupling of the spatial cue with microtubule polarization is
less understood. Here we present a computational model of microtubule
polarization in a turning neuronal growth cone (GC). We explore the limits of
directional cues in modifying the spatial polarization of microtubules by
testing the role of microtubule dynamics, gradients of regulators and
retrograde forces along filopodia. We analyze the steady state and transition
behavior of microtubules on being presented with a directional stimulus. The
model makes novel predictions about the minimal angular spread of the chemical
signal at the growth cone and the fastest polarization times. A regulatory
reaction-diffusion network based on the cyclic
phosphorylation-dephosphorylation of a regulator predicts that the receptor
signal magnitude can generate the maximal polarization of microtubules and not
feedback loops or amplifications in the network. Using both the
phenomenological and network models we have demonstrated some of the physical
limits within which the MT polarization system works in turning neuron.Comment: 7 figures and supplementary materia
Frustration induced disordered magnetism in Ba3RuTi2O9
The title compound Ba3RuTi2O9 crystallizes with a hexagonal unit cell. It
contains layers of edge shared triangular network of Ru4+ (S=1) ions. Magnetic
susceptibility chi(T) and heat capacity data show no long range magnetic
ordering down to 1.8K. A Curie-Weiss (CW) fitting of chi(T) yields a large
antiferromagnetic CW temperature theta_CW=-166K. However, in low field, a
splitting of zero field cooled (ZFC) and field cooled (FC) chi(T) is observed
below ~30K. Our measurements suggest that Ba3RuTi2O9 is a highly frustrated
system but only a small fraction of the spins in this system undergo a
transition to a frozen magnetic state below ~30K.Comment: 5 pages, 6 figures (accepted in EPJB
A S=1/2 vanadium-based geometrically frustrated spinel system Li2ZnV3O8
We report the synthesis and characterization of Li2ZnV3O8, which is a new
Zn-doped LiV2O4 system containing only tetravalent vanadium. A Curie-Weiss
susceptibility with a Curie-Weiss temperature of CW ~214 K suggests the
presence of strong antiferromagnetic correlations in this system. We have
observed a splitting between the zero-field cooled ZFC and field cooled FC
susceptibility curves below 6 K. A peak is present in the ZFC curve around 3.5
K suggestive of spin-freezing . Similarly, a broad hump is also seen in the
inferred magnetic heat capacity around 9 K. The consequent entropy change is
only about 8% of the value expected for an ordered S = 1=2 system. This
reduction indicates continued presence of large disorder in the system in spite
of the large CW, which might result from strong geometric frustration in
the system. We did not find any temperature T dependence in our 7Li nuclear
magnetic resonance NMR shift down to 6 K (an abrupt change in the shift takes
place below 6 K) though considerable T-dependence has been found in literature
for LiV2O4- undoped or with other Zn/Ti contents. Consistent with the above
observation, the 7Li nuclear spin-lattice relaxation rate 1/T1 is relatively
small and nearly T-independent except a small increase close to the freezing
temperature, once again, small compared to undoped or 10% Zn or 20% Ti-doped
LiV2O4.Comment: 7 pages, 8 figures, accepted in JPCM (Journal of Physics condensed
matter
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