Size effect in Quincke rotation: a numerical study.

This paper deals with the Quincke rotation of small insulating particles. This dc electrorotation of insulating objects immersed in a slightly conducting liquid is usually explained by looking at the action of the free charges present in the liquid. Under the effect of the dc electric field, the charges accumulate at the surface of the insulating particle which, in turn, acquires a dipole moment in the direction opposite to that of the field and begins to rotate in order to flip its dipole moment. In the classical Quincke model, the charge distribution around the rotor is supposed to be purely superficial. A consequence of this assumption is that the angular velocity does not depend on the rotor size. Nevertheless, this hypothesis holds only if the rotor size is much larger than the characteristic ion layer thickness around the particle. In the opposite case, we show thanks to numerical calculations that the bulk charge distribution has to be accounted for to predict the electromechanical behavior of the rotor. We consider the case of an infinite insulating cylinder whose axis is perpendicular to the dc electric field. We use the finite element method to solve the conservation equations for the positive and the negative ions coupled with Navier-Stokes and Poisson equations. Doing so, we compute the bulk charge distribution and the velocity field in the liquid surrounding the cylinder. For sufficiently small cylinders, we show that the smaller the cylinder is, the smaller its angular velocity is when submitted to a dc electric field. This size effect is shown to originate both in ion diffusion and electromigration in the charge layer. At last, we propose a simple analytical model which allows calculating the angular velocity of the rotor when electromigration is present but weak and diffusion can be neglected.

Odorant-evoked potassium changes in the frog olfactory epithelium.

Electroolfactogram (EOG) and extracellular potassium activity (aK) measurements were carried out in frog olfactory epithelia in vivo. Odorant-evoked changes in aK were characterized on the basis of depth profile analysis. Following an olfactory stimulation with butanol vapours, an increase in aK was measured in the mucus and the proximal part of the epithelium; this response started after the beginning of the EOG and was proportional to the amplitude of the latter. In the deeper part of the epithelium, the aK response had complex waveforms showing an initial K decrease which was suppressed by local application of ouabain, suggesting the existence of a pumping mechanism at this level. The results are discussed in terms of extracellular accumulation of K ions following neuroreceptor activation with respect to EOG generation theories.

Influence of GABA, baclofen and bicuculline on the extracellular K+ activity in the olfactory bulb of the developing rat.

gamma-Aminobutyric acid (GABA) and baclofen were iontophoresed into the glomerular and plexiform layers of the olfactory bulb of rats at different ages. Subsequent changes in K+ activity and mitral cell firing rates were monitored to specify GABA receptor activation. Only bicuculline-insensitive receptors are shown to be already present in the glomerular layer of the newborn rat. The possible distribution and maturation of GABA receptors on the olfactory nerve endings or on the dendrites of the mitral or periglomerular cells is discussed.

Electrical stimulation of primary olfactory nerve induces two types of variations in the extracellular potassium activity within the glomerulus of the rat olfactory bulb in vivo.

Primary olfactory nerve stimulation induces a two-step increase in extracellular potassium, as recorded by K+-sensitive microelectrodes inserted into single glomeruli of the rat olfactory bulb. The second rise in K+ (K-peak) occurs in a graded, activity-dependent manner at the basal pole of the glomerular neuropile. This phenomenon appears to be directly related to the rate of extracellular K+ accumulation and is accompanied by a substantial depression of the mitral cell firing rate.

Na+,K(+)-ATPase in the rat olfactory bulb: evidence for a higher enzymatic activity in the glomerular layer.

Protein content and ATPase activities have been determined in the superficial and deep layers of the rat olfactory bulb. Protein levels, Mg2(+)-ATPase and Na+,K(+)-ATPase activities were significantly higher in the whole homogenate for the superficial layers. These differences were amplified when activity was expressed on a wet weight basis in a crude microsomal preparation isolated by differential centrifugation. Specific activities, however, showed similar values in the microsomal fractions from superficial and deep layers. The results are discussed in terms of differences in the density of neuronal processes and efficiency of K+ reuptake mechanisms.

Evidence for an absence of K+ spread in the glomerular layer of the rat olfactory bulb.

Using K+-sensitive microelectrodes inserted into the olfactory bulb, the effects of distant K+ ejection on the extracellular K+ activity (aK), were monitored in the glomerular and plexiform layers. A ouabain-sensitive mechanism, which appeared to be markedly more efficient in the glomerular layer, prevents spread of distal ejected K+. The results are discussed on the basis of Na+,K+-pump activation in both glomerular neuronal networks and the glial capsule enclosing each glomerulus.