We studied the biomechanical properties of the sarcolemma and its links through costameres to the contractile apparatus in single mammalian myofibers of Extensor digitorum longus muscles isolated from wild (WT) and dystrophin-null (mdx) mice. Suction pressures (P) applied through a pipette to the sarcolemma generated a bleb, the height of which increased with increasing P. Larger increases in P broke the connections between the sarcolemma and myofibrils and eventually caused the sarcolemma to burst. We used the values of P at which these changes occurred to estimate the tensions and stiffness of the system and its individual elements. Tensions of the whole system and the sarcolemma, as well as the maximal tension sustained by the costameres, were all significantly lower (1.8–3.3 fold) in muscles of mdx mice compared to WT. Values of P at which separation and bursting occurred, as well as the stiffness of the whole system and of the isolated sarcolemma, were ~2-fold lower in mdx than in WT. Our results indicate that the absence of dystrophin reduces muscle stiffness, increases sarcolemmal deformability, and compromises the mechanical stability of costameres and their connections to nearby myofibrils.

The effect of protein deprivation on rapid Golgi impregnated pyramidal neurons in layers II/III and V of the rat visual cortex was studied at 30, 90, and 220 days of age using morphometric methods. In order to mimic human under-nutrition female rats were adapted to either an 8% or control 25% casein diet 5 weeks prior to conception and maintained on these diets during gestation and lactation. The pups were then weaned and maintained on their respective diets. The undernourished rats showed a significant decrease in brain weight only at 90 days, indicating that the protein deprivation had a mild effect on brain development. Correspondingly, the number of significant histological differences between the two diet groups were least at 30 and 220 days of age. The effect of the diet was greater on layer V than on layer II/III pyramids. At 30 days of age the effect of the diet was different on the pyramids of these two cell layers, at 90 days there was a mixture of similar and dissimilar effects, and at 220 days the pyramids of these two cell layers showed only minor differences between the two diet groups. Analysis of age-related changes indicated that the effect of the diet was different on layer II/III pyramids compared to layer V pyramidal cells. These different effects apparently accounted for the progression from a dissimilar effect of the diet at 30 days on the pyramids of the two cell layers to only minor differences between them at 220 days. Further analysis of these age-related changes shows that two prominent effects of protein deprivation are for age-related changes to occur in undernourished rats but not in controls and for age-related changes to be out-of-phase with each other in the two diet groups. From these findings, and a review of similar studies in the literature, we propose that these mechanisms are a prominent effect of undernutrition in the post-weaning period and help account for the unexpected increases in morphometric measurements noted in undernourished rats in this and other studies.

Hollow cathode plasma discharges are a fundamental part of a large variety of applications in industry, academia, and space. From surface processing and coatings deposition to plasma–surface interaction research and electric propulsion, advances in hollow cathode modeling and performance are critically important to the progress and evolution of these and other areas of technology. This paper describes perspectives on the progress that has been made in recent years in the capabilities and modeling of hollow cathodes used in plasma discharges. While many of the developments have been driven by the demanding requirements of electric propulsion applications, the information provided applies to all thermionic hollow cathodes and their applications. In the paper, we describe the status of 2D global simulations of hollow cathode plasmas, hollow cathode plume instabilities, and the development of higher current cathodes and low-current heaterless cathode technologies. Advances in our understanding and technology in these areas and some of the challenges that still need to be addressed and solved are discussed.

We present results from 2D axisymmetric fluid simulations of the partially-ionized Xe gas in a 25-A cathode discharge, for a range of flow rates (5-20 sccm) in which transition between the so-called spot and plume modes has been observed in laboratory experiments. The simulations for the first time capture very well the characteristic rise of the peak-to-peak amplitude in the keeper voltage oscillations with decreasing flow rate-the so-called 'plume margin'-thereby allowing for a closer interrogation of the processes that drive them. The oscillations are found to be due to plasma dynamics in the near-plume region that increase in amplitude as the flow rate is reduced. The cathode interior remains largely quiescent and changes in the emitter peak temperature do not exceed 2% across the entire range of flow rates examined. Along the cathode centerline the amplitude of the electron density oscillations peak at a location where the number density of the Xe atoms has diminished to its minimum value and the degree of ionization is close to one. The computed spectra reveal that most of the oscillation power resides in the frequency range of 100-300 kHz. These frequencies are much greater than the local ionization collision frequency (n eA I < 10 kHz), and are associated with wavelengths of several centimeters. Such wavelengths are many orders of magnitude greater than λ D . The waves are largely longitudinal with wave vector in the axial direction, and phase velocity ω/k>10 km s −1 , which is more than 6×greater than the ion acoustic speed. Though the computed and measured plume margins are in good agreement the measured spectra show that most of the power resides between ∼60 and 90 kHz, lower than the computed range (100-300 kHz), and exhibits a sharper distribution. It is argued this is due to the presence of a rotational mode in the experiment that is coupled with the longitudinal mode, yielding a complex 3D plasma motion that cannot be captured by the simulations due to the assumption of azimuthal axisymmetry, but that it is indeed the longitudinal plasma motion that drives the transition from spot to plume modes. An anomalous resistivity has been invoked in the simulations that is based on the formulations of Sagdeev and Galeev for the saturation of IAT which is long known to exist in these discharges. Though the simulations do not account for possible anomalous heating of the ions, the idealized model for the electrons is found to be sufficient in capturing the abovementioned dynamics, yielding a multiplication coefficient α of the anomalous collision frequency ν α =αω pe (T e /T i )(u e /u Te ) that is within a factor of two of the estimated theoretical value.

Since the inception of the Hall-effect thruster more than five decades ago, many theories have been proposed about the source(s) of the anomalous cross-field transport that is known to occur in these devices. Yet, none of these theories has been implemented in two-dimensional (2D) (r-z) simulations and successfully predicted the behavior of the discharge and the erosion of a thruster over different operating conditions and geometries. We present results from numerical experiments with a 2D (r-z) axisymmetric hydrodynamics code that show the plasma solution is relatively insensitive to very large variations in the anomalous transport in some regions of the discharge channel and near-plume. Because the changes of the plasma properties in these regions can be too small or impossible to detect in the laboratory by conventional diagnostics, the verification of a transport model becomes very challenging. Hence, without more advanced diagnostics and improved numerical models, comparisons between plasma measurements and simulation results can lead to wide-ranging theories and scalings of the anomalous resistivity.