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The lipids of biological membranes and intact biomembranes display chain melting transitions close to temperatures of physiological interest. During this transition the heat capacity, volume and area compressibilities, and relaxation times all reach maxima. Compressibilities are thus nonlinear functions of temperature and pressure in the vicinity of the melting transition, and we show that this feature leads to the possibility of soliton propagation in such membranes. In particular, if the membrane state is above the melting transition solitons will involve changes in lipid state. We discuss solitons in the context of several striking properties of nerve membranes under the influence of the action potential, including mechanical dislocations and temperature changes.sound ͉ action potential ͉ compressibility ͉ Hodgkin-Huxley theory T he lipid membrane is the major building block of biological membranes, which consist mainly of large numbers of different lipids and proteins with a composition specific to the particular membrane under consideration. The isolated lipids of biomembranes display order-disorder transitions in the temperature regime of about Ϫ20°C to ϩ60°C in which membranes absorb heat (25-40 kJ͞mol), and both the lateral order and chain order of the lipid molecules are lost. This transition is accompanied by an increase in volume of Ϸ4% and an increase in area of Ϸ25%. The low and high temperature phases are called solid-ordered and liquid-disordered, respectively, indicating the simultaneous change in lateral crystalline arrangement and chain order. They are also known as gel and fluid phase, respectively. Mixed systems display a wealth of different phase diagrams. The melting profiles of lipid mixtures are therefore generally more complex than those of single lipids and cover a wider temperature range. Both peripheral and integral proteins change lipid melting caused by molecular interactions that influence the cooperative nature of the membrane fluctuations as a whole (1). Fluctuations in volume and area, and the related fluctuations in curvature, give rise to pronounced changes in elastic constants, e.g. compressibilities, bending elasticity, and relaxation times, all of which have maxima in the region of the chain melting transition. It has been suggested on theoretical and experimental grounds that these response functions are all simple functions of the heat capacity (2-4). The sound velocities of lipid dispersions obtained with ultrasonic measurements and the bending elasticities of giant vesicles are practically identical to the profiles calculated from the heat capacity (5-8). Within certain limits, it is thus possible to calculate the response functions from the heat capacity without detailed knowledge of the composition of the lipid mixture. In the transition region, membranes thus become more compressible and easier to bend. Relaxation times grow and are found to be in the range of 10 Ϫ3 s Ϫ1 ⅐min (4, 9). In unilamellar vesicles of single lipids, these changes can be pronounced. In compar...

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We consider the conditions under which solitary waves can exist in elongated clouds of Bose-Einstein condensed atoms. General expressions are derived for the velocity, characteristic size, and spatial profile of solitary waves, and the low-and high-density limits are examined.PACS numbers: 03.75. Fi, 05.30.Jp, 67.40.Db Clouds of Bose-Einstein condensed atoms in elongated traps provide excellent conditions for investigating the propagation of essentially one-dimensional sound pulses under [1]. In previous work, propagation of pulses in such traps was considered in the Thomas-Fermi approximation, and it was demonstrated that pulses propagate at a speed of (nU 0 /m) 1/2 in the linear regime. Here,2 a sc /m is the effective two-body interaction matrix element,n is the particle density averaged over the cross section of the cloud [2,3], m is the atomic mass, and a sc is the scattering length for atom-atom collisions. Non-linear effects were also investigated in Ref.[3] and were found to be important for conditions of experimental relevance. The effects of dispersion were neglected in this study since the length scales of interest were much larger than the superfluid coherence length, which sets the scale on which dispersive effects become important.

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We formulate a random matrix model which mimics the chiral phase transition in QCD with two light flavors. Two critical exponents are calculated. We obtain the mean field values β = 1 2 and δ = 3. We also find that the chiral phase transition can be characterized by the dynamics of the smallest eigenvalue of the Dirac operator. This suggests an alternative order parameter which may be of relevance for lattice QCD simulations.

It is known that the action of general anesthetics is proportional to their partition coefficient in lipid membranes (Meyer-Overton rule). This solubility is, however, directly related to the depression of the temperature of the melting transition found close to body temperature in biomembranes. We propose a thermodynamic extension of the Meyer-Overton rule, which is based on free energy changes in the system and thus automatically incorporates the effects of melting point depression. This model accounts for the pressure reversal of anesthesia in a quantitative manner. Further, it explains why inflammation and the addition of divalent cations reduce the effectiveness of anesthesia.

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