Serial electron microscopy and 3-D reconstructions of dendritic spines from hippocampal area CA 1 dendrites were obtained to evaluate 2 questions about relationships between spine geometry and synaptic efficacy. First, under what biophysical conditions are the spine necks likely to reduce the magnitude of charge transferred from the synapses on the spine heads to the recipient dendrite? Simulation software provided by Charles Wilson (1984) was used to determine that if synaptic conductance is 1 nS or less, only 1% of the hippocampal spine necks are sufficiently thin and long to reduce charge transfer by more than 10%. If synaptic conductance approaches 5 nS, however, 33% of the hippocampal spine necks are sufficiently thin and long to reduce charge transfer by more than 10%. Second, is spine geometry associated with other anatomical indicators of synaptic efficacy, including the area of the postsynaptic density and the number of vesicles in the presynaptic axon? Reconstructed spines were graphically edited into head and neck compartments, and their dimensions were measured, the areas of the postsynaptic densities (PSD) were measured, and all of the vesicles in the presynaptic axonal varicosities were counted. The dimensions of the spine head were well correlated with the area of PSD and the number of vesicles in the presynaptic axonal varicosity. Spine neck diameter and length were not correlated with PSD area, head volume, or the number of vesicles. These results suggest that the dimensions of the spine head, but not the spine neck, reflect differences in synaptic efficacy. We suggest that the constricted necks of hippocampal dendritic spines might reduce diffusion of activated molecules to neighboring synapses, thereby attributing specificity to activated or potentiated synapses.

We report the first measurements of the glass transition temperature T g for thin freely standing polystyrene (PS) films. We have used Brillouin light scattering to measure T g for freely standing films of different thicknesses. We find that T g decreases linearly with film thickness h for h # 700 Å, with a reduction of 70 K for a film with h 290 Å. These measurements characterize unambiguously the effects of the free surface on T g of thin polymer films. Results are compared to similar results for supported PS films [Keddie et al., Europhys. Lett. 27, 59 (1994)], and we find that their measured values are influenced strongly by the substrate. [S0031-9007(96)01093-9] PACS numbers: 64.70. Pf, 68.60.Bs, 78.35.+c From organic liquids to metals to polymers, almost any substance can be transformed into a glassy state. One of the fundamental parameters describing a glass is the glass transition temperature T g . For temperatures greater than T g , the material is a viscous liquid. As the liquid is cooled below this temperature, the material forms an amorphous solid. Despite the technological significance of glass forming materials, the glass transition itself is poorly understood. In particular, it is not clear whether the glass transition is a thermodynamic transition [1] or a purely kinetic phenomenon [2]. Adam and Gibbs [3] introduced the concept of cooperative rearrangement in an attempt to unify these two views of the glass transition by demonstrating that such cooperativity, coupled with a thermodynamic glass transition, resulted naturally in system dynamics such as those described by the WLF equation [4] for temperatures near T g . Donth [5] has estimated the size of such cooperatively rearranging regions (CRR) to be of the order of 10 Å for a number of glass forming materials. The introduction of such a length scale suggests that studying samples with dimensions comparable to the CRR may lead to the observation of finite size effects.An attractive choice for studies of finite size effects on the glass transition is the use of polymer molecules. For polymers the molecule can be characterized by the endto-end distance R EE ϳ 2R g , where R g is the radius of gyration of the molecule. R EE is typically much larger than the size of the CRR, and R EE can be adjusted by changing the molecular weight M w of the molecules. Polymer molecules can be confined by preparing samples in a thin film geometry. The relevant length scale is the film thickness h which can be adjusted to be comparable to, much larger than or smaller than R EE so that the effects of chain confinement can be investigated. Glassy polymer films can be cast onto any substrate, even those that the polymer melt itself does not wet.The first direct measurements of the glass transition temperature in thin polymer films were performed recently by Keddie, Jones, and Cory [6]. For polystyrene (PS) films on hydrogen-passivated Si(111), the measured T g values were lower than the bulk value T g ͑bulk͒ for films with thicknesses h # 400 Å. Because the changes ...

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The effect of salt concentration on the molal conductivity of various composite electrolytes was studied. Conductivity enhancement is achieved for composite electrolytes over the basic poly(ethylene oxide)-LiClO 4 system in the salt concentration range where the filler concentration corresponds to that of LiClO 4 . On the basis of impedance spectroscopy, DSC, and FT-IR studies, it is concluded that changes in the conductivity result from acid-base type interactions involving polyether oxygens, filler acid or base centers, and alkali metal cations. The effect of a filler is to change the fraction of available oxygen sites which in turn results in changes in the formation of ionic aggregates. The region in which the enhancement of ionic conductivity is observed corresponds to a decrease in the fraction of contact-ion pairs and higher aggregates; this is due to the location of filler molecules in the vicinity of the coordination sphere of Li + cations.