Properties of galaxies reproduced by a hydrodynamic simulation.

Previous simulations of the growth of cosmic structures have broadly reproduced the 'cosmic web' of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies, because of numerical inaccuracies and incomplete physical models. Moreover, they were unable to track the small-scale evolution of gas and stars to the present epoch within a representative portion of the Universe. Here we report a simulation that starts 12 million years after the Big Bang, and traces 13 billion years of cosmic evolution with 12 billion resolution elements in a cube of 106.5 megaparsecs a side. It yields a reasonable population of ellipticals and spirals, reproduces the observed distribution of galaxies in clusters and characteristics of hydrogen on large scales, and at the same time matches the 'metal' and hydrogen content of galaxies on small scales.

Afferent new fiber activity responding to temperature changes of scrotal skin of the rat.

The discharge patterns of single afferent fibers from rat pudendal nerve were studied as a function in temperature of the scrotal skin, an area known to function in temperature regulation. In a number of respects the population of temperature-sensitive afferents here differ from most previously described; 75% of temperature-sensitive afferents were also sensitive to mechanical stimulation. Of the 25% nonmechanosensitive units, half showed dynamic and static responses to cooling, while most of the remaining fibers gave only static discharges to warming. The most frequent thermal reaction of the mechanosensitive units was a dynamic-static cold response or a pure static warm response. However, fibers were also present with only dynamic or only static cold responses. Of the bimodal units, 20% had a dynamic cold response, but showed a minimal static discharge at intermediate temperatures (about 35 degrees C) and an increased discharge on both warming and cooling from that temperature. One unit had static and dynamic warm responses. Whereas a bursting discharge in the cold has previously been considered to be a distinguishing characteristic of specific cold receptors, in the pudendal afferents a bursting discharge on cooling or at low temperatures is common both in mechanosensitive and specific cold fibers. This observation and the identical discharge patterns and mechanisms underlying the thermosensitivity argue for the view that the mechanosensitive afferents participate in thermal sensation and/or regulation along with the specific temperature receptors. Hellon and Misra (7) have concluded that there is processing of thermal information from the scrotal skin at the level of the first synapse in the cord. However, in this study, we have found peripheral afferents which have most of the discharge properties that led Hellon and Misra to conclude that processing had occurred. The unusual characteristics of thermosensitive afferents of the pudendal nerve suggest that there is probably organ specificity of neuronal discharge properties.

Effect of ouabain and potassium-free solution on mammalian thermosensitive afferents in vitro.

Studies have been performed on the afferent fibers innervating the scrotal skin of the rat to test the hypothesis that the generator potential mechanism underlying cold thermosensitivity of these afferent fibers is an electrogenic sodium pump. In these experiments a pudendal nerve-scrotal skin preparation was isolated from the animal and maintained in oxygenated mammalian Ringer's where composition could be varied and drugs added. Application of ouabain resulted in an increae in the discharge of cold sensitive afferent fibers which was more pronounced at 38--41 degrees C than at temperature below 30 degrees C. In most of the cases transient accelerations on cooling were reduced, but often a transient response to warming appeared. The effects of ouabain administration were reversible. Removal of extracellular K at 35 degrees C resulted in an increased discharge of cold sensitive afferents. This observation is consistent with the effects of ouabain, since removal of extracellular K also block Na transport. The generator potential mechanism underlying cold sensitivity of these afferent fibers appears to be an electrogenic Na pump.