Computed myocardial PO2 histograms: effects of various geometrical and functional conditions.

A model of myocardial oxygenation was developed that allows calculation of Po2 histograms under varying conditions. The model consists of parallel tissue cylinders with varying radii, simulating the heterogeneity of capillary spacing, in agreement with our previous experimental results. The facilitated diffusion of O2 by myoglobin, an additional resistance to diffusion at the capillary level, and the Michaelis-Menten type of O2 consumption were also incorporated. The shape of the histograms depends on input data. When no additional barrier to O2 transport is included, the histograms resemble those obtained with Po2 surface electrodes, and they are strongly dependent on heterogeneity in capillary spacing and capillary blood flow. On the other hand, an inclusion of an additional capillary barrier combined with the Michaelis-Menten type of O2 consumption can generate Po2 histograms similar to those derived from myoglobin cryospectroscopy. In this case, the Po2 histograms are relatively independent of heterogeneity of capillary spacing and blood flow. The facilitation of O2 diffusion by myoglobin has only a modest effect on the form of the histograms in all situations considered.

Thermal ecology of the smooth snake, Coronella austriaca Laurenti, during spring.

Thermo sensitive transmitters were used in a study on free-ranging smooth snakes (Coronella austriaca). Except on cold cloudy days, this snake shows a daily tripartite pattern of body temperatures. The pattern consists of a quick rise in body temperature in the morning, a subsequent stable phase during which body temperatures vary between 29 and 33° C, and finally a drop of body temperatures in the evening. The, consistency of the stable phase over the study period indicates that this range represents the thermal preferendum of this species. During the succesive study months a time shift in daily activity pattern was observed.

Spectro-temporal receptive fields of auditory neurons in the grassfrog. III. Analysis of the stimulus-event relation for natural stimuli.

The stimulus-event relation of single units in the auditory midbrain area, the torus semicircularis, of the anaesthetized grassfrog (Rana temporaria L.) during stimulation with a wide ensemble of natural stimuli, was analysed using first and second order statistical analysis techniques. The average stimulus preceding the occurrence of action potentials, in general, did not prove to give very informative results. The second order procedure consisted in the determination of the average dynamic power spectrum of the pre-event stimuli, following procedures as described elsewhere (Aertsen and Johannesma, 1980' Aertsen et al., 1980). The outcome of this analysis was filtered with the overall power spectrum of the complete stimulus ensemble in order to correct for its non-uniform spectral composition. The "stimulus-filtered" average pre-event dynamic spectrum gives a first indication of the "spectro-temporal receptive field" of a neuron under natural stimulus conditions. Results for a limited number of recordings are presented and, globally, compared to the outcome of an analogous analysis of experiments with tonal stimuli.