The control of posture and movements during REM sleep: neurophysiological and neurochemical mechanisms

Mammalian sleep is characterized by synchronized sleep, in which high-amplitude, low-frequency waves appear in the electroencephalogram, and desynchronized sleep, characterized by small-amplitude, high-frequency waves, absence of tonic muscle activity, and rapid eye movements (REM), which are associated in humans with dreams. The postural atonia typical of desynchronized sleep is due to postsynaptic inhibition of spinal motoneurons resulting from tonic activation of a bulbospinal inhibitory system. Superimposed on this background of postural atonia, a motor pattern appears, characterized by rapid contractions of the limb musculature synchronous with the REM bursts. Simultaneously one observes phasic inhibition of transmission of somatic afferent volleys to motoneurons and ascending spinal pathways. The bursts of REM depend upon rhythmic discharges of vestibulo-oculomotor neurons, due to extralabyrinthine volleys originating from the brainstem. Ascending and descending vestibular volleys are also able to excite corticospinal and other supraspinal descending neurons responsible for the motor events synchronous with the bursts of REM. Activation of cholinergic neurons located in the brainstem reticular formation reproduces the postural atonia typical of desynchronized sleep, as well as the phasic events characterized by the REM bursts and the related changes in spinal cord activities. Even in this instance the bursts of REM and the related spinal effects depend upon rhythmic changes in the discharge of vestibular nuclear neurons. Experimental evidence indicates that the cholinergic reticular neurons fire asynchronously, thus being able to trigger the bulbospinal inhibitory system responsible for postural atonia. Even the vestibulo-oculomotor neurons are activated by these cholinergic reticular neurons; however, the continuous stream of these extralabyrinthine impulses is transformed into rhythmic changes of discharge of the vestibular nuclear neurons due to the presence of inhibitory neurons interposed with the vestibulo-oculomotor system. Waxing and waning in the activity of these cholinergic reticular neurons accounts for the regular occurrence of the cholinergically induced bursts of REM.     

Planetary characteristics from radar observations

Conclusions Long wavelength radar observations of Venus yield a surface reflectivity of about 15%. Total power measurements at 12.5 cm and 3.6 cm strongly suggest that significant atmospheric absorption is operative in this wavelength region. If the observed low value of reflectivity at 3.6 cm is attributed to atmospheric absorption alone an opacity of = 1.14 is implied at this wavelength rather independently from assumptions concerning the surface scattering characteristics of Venus. An inverse ² opacity law for the atmosphere is consistent with the reflectivity measurements over the complete range of observations wavelengths.The mathematical characteristics of the Venusian backscatter law are the same as for the moon but wavelength-dependent mean effective slopes indicate that Venus appears smoother than the moon at all radar wavelengths.Considerable progress has been made toward obtaining a precise value for the Venusian axial rotation vector which is found to be oriented to within 10 degrees of the planet's orbital plane. The period of (retrograde) rotation lies within the range 242–250 days with the lower value favored by the statistics of the data. Regions of enhanced radar return fixed to the surface have been found and verified at a later conjunction. Measurements of the surface radar depolarization support the hypothesis that the prominences are due to increased surface roughness as opposed to regional increases of dielectric constant.Observations of Mercury strongly suggest that the rotation period of the planet is about 59 days, a conclusion which has been supported, a posteriori, by theoretical tidal calculations and rediscussions of optical observations of surface markings. Mercury has radar backscatter characteristics more similar to the moon than Venus and exhibits a reflectivity of about 5%.Mars has demonstrated strong variations of radar backscatter characteristics which appear correlated with the Martian longitude and, in turn, with the dark surface markings in its north equatorial zone. Particularly reliable correlations have been discovered with the positions of Trivium Charontis and Syrtis Major. The observed variations appear to be primarily manifested in terms of the Martian radar backscatter law or surface roughness as opposed to variations in the intrinsic surface material reflectivities although the observations are not sufficiently precise to resolve this question. Variations in surface materials are apparently also present but their degree is currently unassayable. The reflectivity of the average surface has been crudely determined to be about 7% which suggests that the surface of Mars is composed of underdense materials. The 7% value is consistent with the values of 7.5% and 5% for the moon and Mercury, respectively, and is significantly different from the 15% value for Venus,No unequivocal radar detection of Jupiter has been made although a statistically weak detection has been reported for a single opposition which could not be verified in succeeding attempts.     

COMPTEL observations of gamma-ray bursts

The COMPTEL experiment on GRO images 0.7 – 30 MeV celestial gamma-radiation that falls within its 1 steradian field of view. During the first fifteen months in orbit, preliminary localizations from BATSE triggers indicated that about 1 in 6 cosmic events could have fallen within COMPTEL's field of view. We summarize work on the brightest of these gamma-ray bursts and present new position constraints for GRB 911118 and GRB 920622.     

A proportional counter/image intensifier system for the detection of low energy X-rays

Proportional counters (PC's) and gas scintillation proportional counters (GSPC's) currently used for detection of low energy X-rays provide information on event position and energy. Although at 1.5 keV PC's have good position resolution (～ 200 μm FWHM) they have relatively poor energy resolution (～ 40% FWHM). Conversely GSPC's have reasonable energy resolution (～ 20% FWHM), but poor spatial resolution (～ 1mm FWHM).We describe a scheme in which a parallel plate PC with a transparent anode deposited on a fibre optic substrate has been used. This allows the light emitted by electron avalanches caused by X-ray events in the PC to be detected by an image intensifier with electronic readout. Using this scheme spatial resolution better than that of conventional PC's should be attainable. In addition avalanches induced by single electrons can be resolved through observation of the time structure of the light flash. Using the ability to count the number of primary electrons created by each X-ray event, it is shown that energy resolution can be achieved which is comparable to that of the GSPC.     

Analysis of solar spike events by means of symbolic dynamics methods

We have searched for interrelations of spikes emitted simultaneously at different frequencies during the impulsive phase of flare events (Fig.1). As the spikes are related to the flare energy release and are interpreted as emissions that originate at different sites having different magnetic field strengths, any relation in frequency is interpretated as a relation in space. Quantities of symbolic dynamics, such as mutual information, Shannon information and algorithmic complexity are appropriate to characterize such spatiotemporal patterns, whereas the popular estimate of fractal dimensions can be applied to low-dimensional systems only.The goal is to decide between two possible types of fragmentation depending on the energy release and emission processes, which we callglobal andlocal organization. In the global organization the whole region becomes supercritical, and the energy is released in independent, small regions. The alternative local scenario requires a trigger that spreads from initial localized events and ignites nearby regions.Mutual information which is a generalization of correlation indicates a relation in frequency beyond the bandwidth of individual spikes. The scans in the spectrograms with large mutual information also show a low level of Shannon information and algorithmic complexity, indicating that the simultaneous appearance of spikes at other frequencies is not a completely stochastic phenomenon (white noise). It may be caused by a nonlinear deterministic system or by a Markov process. By means of mutual information we find a memory over frequency intervals up to 60 MHz (Fig. 2). Shannon information and algorithmic complexity, however, describe spike events as a whole, i.e. a global source region. A global organization is also apparent in quasi-periodic changes of the Shannon information and algorithmic complexity in the range of 2–8 seconds (Fig. 3).This findings is compatible with a scenario of local organization in which the information of one spike event spreads spatially and hence triggers further spike events at different places. The region is not an ensemble of independently flashing sources, each representing a system that cascades in energy after an initial trigger. On the contrary, there is a causal connection between the sources at any time.The analysis of four spike events suggests that the simultaneous appearance of spikes is not stochastically independent but a process in which spikes at nearby locations are simultaneously triggered by a common exciter.We have shown in the case in the case of spikes that quantities from nonlinear dynamics used in this paper are helpful in detecting structural properties of complex spatio-temporal patterns. This approach seems to be promising also for several other astrophysical applications.     

On deterministic chaos,stationarity, periodicity and intermittency in coronal bursts and flares

Solar and stellar flares are highly structured in space and in time, as is indicated for example by their radio signatures: the narrowband spikes, type III, type II and IV, and pulsation events. Structured in time are also the not flare related type I events (noise storms). The nature of this observationally manifest fragmentation is still not clear. Either, it can be due to stochastic boundary or initial conditions of the respective processes, such as inhomogeneities in the coronal plasma. Or else, a deterministic non-linear process is able to cause complicated patterns of these kinds.We investigate the nature of the fragmentation in time. The properties of processes we enquire are stationarity, periodicity, intermittency, and, with dimension estimating methods, we try to discriminate between stochastic and low-dimensional deterministic processes. Since the measured time series are rather short, the dimension estimate methods have to be used with care: we have developed an extended dimension estimate procedure consisting of five steps. Among others, it comprises again the questions of stationarity and intermittency, but also the more technical problems of temporal correlations, judging scaling and convergence, and limited number of data points (statistical limits).We investigate 3 events of narrowband spikes, 13 type III groups, 10 type I storms, 3 type II bursts and 1 type IV event of solar origin, and 3 pulsation-like events of stellar origin. They have in common that all of them have stationary phases, periodicities are rather seldom, and intermittency is quite abundant. However, the burst types turn out to have different characteristics. None of the investigated time series reveals a low-dimensional behaviour. This implies that they originate from complex processes having dimensions (degrees of freedom) larger than about 4 to 6, which includes infinity,i. e. stochasticity. The lower limit of the degrees of freedom is inferred from numerical experiments with known chaotic systems, using time series of similar lengths, and it depends slightly on the burst types.     

Geological interpretation of a segment of Venus from radar images of the U.S.S.R. spacecrafts Venera -15 and -16

A first geological interpretation of a part of the Maxwell Mountains area is given using radar images of the Soviet spacecrafts Venera 15 and 16. The region is characterized by following features: plateaus, ridges and depressions, ring structures, volcanoes, and lineations of different types and, very probably, also of different age. The dominant surface modeling mechanism should be gliding processes connected to faults producing many complicated superpositions of features. To obtain more objective and reproducible, results a method was used for quantitative texture analysis to decipher some of the geological indications.     

A modular 550 watt, 25 watts per cubic inch power supply for nextgeneration aircraft

The development of the HVS-550 high-performance power supply is described. The HVS-550 was designed and built utilizing state-of-the-art surface mount construction, low-profile magnetics, and aggressive thermal management techniques. This combination allowed the module height to be only 0.58 in. The unique requirements of the military system that bounded the power-supply parameters are discussed. Cooling is provided either by liquid circulating through the chassis or by conduction through the card edges. The measured power supply efficiency is in excess of 85%, with 11-V output. The output noise spectrum is excellent because of noise-cancelling techniques utilized to achieve the level of performance required by the system     

NASA's new Mars Exploration Program: the trajectory of knowledge

NASA's newly restructured Mars Exploration Program (MEP) is finally on the way to Mars with the successful April 7 launch of the 2001 Mars Odyssey Orbiter. In addition, the announcement by the Bush Administration that the exploration of Mars will be a priority within NASA's Office of Space Science further cements the first decade of the new millennium as one of the major thrusts to understand the "new" Mars. Over the course of the past year and a half, an integrated team of managers, scientists, and engineers has crafted a revamped MEP to respond to the scientific as well as management and resource challenges associated with deep space exploration of the Red Planet. This article describes the new program from the perspective of its guiding philosophies, major events, and scientific strategy. It is intended to serve as a roadmap to the next 10-15 years of Mars exploration from the NASA viewpoint. [For further details, see the Mars Exploration Program web site (URL): http://mars.jpl.nasa.gov]. The new MEP will certainly evolve in response to discoveries, to successes, and potentially to setbacks as well. However, the design of the restructured strategy is attentive to risks, and a major attempt to instill resiliency in the program has been adopted. Mars beckons, and the next decade of exploration should provide the impetus for a follow-on decade in which multiple sample returns and other major program directions are executed. Ultimately the vision to consider the first human scientific expeditions to the Red Planet will be enabled. By the end of the first decade of this program, we may know where and how to look for the elusive clues associated with a possible martian biological record, if any was every preserved, even if only as "chemical fossils."