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