189 research outputs found
Quantal analysis of long-term potentiation of combined neuronal postsynaptic potentials on hippocampal slices in vitro.
10.1007/BF0105247
Specific Heat Study of 1D and 2D Excitations in the Layered Frustrated Quantum Antiferromagnets CsCuClBr
We report an experimental and theoretical study of the low-temperature
specific heat and magnetic susceptibility of the layered anisotropic
triangular-lattice spin-1/2 Heisenberg antiferromagnets
CsCuClBr with = 0, 1, 2, and 4. We find that the ratio
of the exchange couplings ranges from 0.32 to , implying a
change (crossover or quantum phase transition) in the materials' magnetic
properties from one-dimensional (1D) behavior for to
two-dimensional (2D) behavior for behavior. For , realized for = 0, 1, and 4, we find a magnetic contribution to the
low-temperature specific heat, , consistent with spinon
excitations in 1D spin-1/2 Heisenberg antiferromagnets. Remarkably, for =
2, where implies a 2D magnatic character, we also observe
. This finding, which contrasts the prediction of made by standard spin-wave theories, shows that Fermi-like
statistics also plays a significant role for the magnetic excitations in
frustrated spin-1/2 2D antiferromagnets
FACKEL is a sterol C-14 reductase required for organized cell division and expansion in Arabidopsis embryogenesis
In flowering plants, the developing embryo consists of growing populations of cells whose fates are determined in a position-dependent manner to form the adult organism. Mutations in the FACKEL (FK) gene affect body organization of the Arabidopsis seedling. We report that FK is required for cell division and expansion and is involved in proper organization of the embryo. We isolated FK by positional cloning. Expression analysis in embryos revealed that FK mRNA becomes localized to meristematic zones. FK encodes a predicted integral membrane protein related to the vertebrate lamin B receptor and sterol reductases across species, including yeast sterol C-14 reductase ERG24. We provide functional evidence that FK encodes a sterol C-14 reductase by complementation of erg24. GC/MS analysis confirmed that fk mutations lead to accumulation of intermediates in the biosynthetic pathway preceding the C-14 reductase step. Although fk represents a sterol biosynthetic mutant, the phenotype was not rescued by feeding with brassinosteroids (BRs), the only plant sterol signaling molecules known so far. We propose that synthesis of sterol signals in addition to BRs is important in mediating regulated cell growth and organization during embryonic development. Our results indicate a novel role for sterols in the embryogenesis of plants
Heterosynaptic plasticity in the neocortex
Ongoing learning continuously shapes the distribution of neurons’ synaptic weights in a system with plastic synapses. Plasticity may change the weights of synapses that were active during the induction—homosynaptic changes, but also may change synapses not active during the induction—heterosynaptic changes. Here we will argue, that heterosynaptic and homosynaptic plasticity are complementary processes, and that heterosynaptic plasticity might accompany homosynaptic plasticity induced by typical pairing protocols. Synapses are not uniform in their susceptibility for plastic changes, but have predispositions to undergo potentiation or depression, or not to change. Predisposition is one of the factors determining the direction and magnitude of homo- and heterosynaptic changes. Heterosynaptic changes which take place according to predispositions for plasticity may provide a useful mechanism(s) for homeostasis of neurons’ synaptic weights and extending the lifetime of memory traces during ongoing learning in neuronal networks
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