How human sleep in space--investigations during space flights.

Sleep problems have been observed during many of the space flights. The existence of poor quality of sleep, fatigue, insomnia or different alterations in sleep structure, organization and sleep cyclicity have been established. Nevertheless results obtained from investigations of human sleep on board manned space vehicles show that it is possible to keep sleep patterns related to the restorative and adaptive processes. For the first time in the frame of the "Intercosmos" program a multi-channel system for recording and analysis of sleep in space was constructed by scientists of the Bulgarian Academy of Sciences and was installed on board the manned Mir orbiting station. In 1988 during the joint Bulgarian-Russian space flight continues recording of electro-physiological parameters necessary to estimate the sleep stages and sleep organization was made. These investigations were continued in next space flights of different prolongation. The results were compared with the findings obtained under the conditions during the pre- and post-flight periods.

Estimation through experimental data of model parameters of the anterior pituitary LH response.

A mathematical model of the luteinizing hormone (LH) response of the anterior pituitary to the gonadotropin releasing hormone (GnRH) stimulation has been proposed by Yanev et al., 1989. The experimental data available are poor. Usually the output relation LH (GNRH) and only a few of the values of the model parameters are experimentally known while the values of the resting parameters are chosen in the course of computer simulations so that a satisfactory agreement between the experimental and the simulation LH outputs is obtained. Such an agreement is an indicator that the ratios between the simulation values are possibly properly chosen but it does not means categorically that their absolute simulation values are the proper ones. In this work it has been proved that once this agreement is attained and enough experimental values of the model parameters are known then the values of the resting model parameters can be calculated (scaled) making use of their simulation values. With the scaled values introduced in the model the agreement obtained between the model and the experimental LH output functions is reserved. The scaled values can be interpreted as estimations of the native values of the respective model parameters.

A probability concept about size distributions of sonicated lipid vesicles.

The sonication procedure of preparation of small unilamellar vesicles is modelled as a process of uniform random fragmentation of the lipid aggregates. The vesicle size distribution evolving in this process is shown to be identical with the Weibull extremal probability distribution. Size histograms of sonicated small vesicles of various phospholipid composition were obtained by using electron microscopy (negative staining). Their successful simulation with Weibull curves shows that theory agrees with experiment. A similarly good agreement is found also with size histograms obtained by freeze-fracture of phosphatidylcholine-cholesterol vesicles (Van Venetië, R., Leunissen-Bijvelt, J., Verkleij, A.J. and Ververgaert, P.H.J.T. (1980) J. Microsc. 118, 401-408). This analysis allows a refinement of some earlier conclusions about the effect of cholesterol on the size of the sonicated vesicles. It follows from the theoretical model that the only intrinsic characteristic of the sonicated vesicles is the lower limit of their size. The other characteristics of the size distribution such as expectancy, dispersion, position and height of the maximum depend on the intensity of fragmentation. It is concluded that the size distribution of sonicated small vesicles is completely determined by the procedure of their preparation and, therefore, the condition of thermodynamic equilibrium between aggregated and monomeric lipid is irrelevant in this case.