Effect of PaCO2 on hyperemia and ischemia in experimental cerebral infarction

To assess the effects of Pa CO CO 2 on cerebral ischemia and reactive hyperemia, the right middle cerebral artery was occluded in 18 cats. Four to six hours later, Pa CO 2 was adjusted by mechanical ventilation, with or without CO 2 , to less than 20 torr in four cats, 31 to 38 torr in six, and 46 to 68 torr in eight. Regional cerebral blood flow (CBF) then was measured at multiple sites in each hemisphere by autoradiography. In regions of brain tissue outside the distribution of the occluded middle cerebral artery, log 10 CBF correlated positively with Pa CO CO 2 . In ischemic regions, CBF was higher in normocapnic cats. Reactive hyperemia occurred in two cats of the hypocapnic group, in four cats of the normocapnic group, but in only one hypercapnic cat (Pa CO CO 2 = 46 torr). Hyperemia also was found outside potentially ischemic regions in five cats. Multiple hyperemic foci developed in six cats. Neither hypocapnia nor hypercapnia was associated with a smaller size or higher CBF of regions of cerebral ischemia produced by occlusion of a middle cerebral artery, although hypercapnia inhibited the development of hyperemia.

Effects of inhalation of oxygen on blood flow and microvasculature of ischemic and nonischemic cerebral cortex

The right middle cerebral artery was occluded in cats, and Pa o o2 was increased by increasing the amount of oxygen inhaled by the animals at atmospheric pressure. Cortical blood flow (CBF) was measured with Krypton-85, and observations and photographs of the superficial cortical microvasculature were made bilaterally. In two of five animals, increasing the Pa o o2 caused constriction of surface arterioles of the nonischemic hemispheres, with an associated decrease of CBF; in the three other animals, there were no circulatory responses to the increased Pa o o2 . In seven animals, increasing the Pa o o2 had no apparent effect on CBF or arteriolar caliber of the ischemic cerebral hemispheres. In four animals, at Pa o o2 greater than 400 torr, reactivity to increases of Pa CO CO2 was preserved in nonischemic cortex but impaired in ischemic cortex. Reddening of venous blood of the microvasculature of ischemic cerebral cortex occurred when Pa o o2 was increased, indicating that more oxygen was made available to the ischemic cerebral tissue. However, no beneficial effects could be demonstrated on the changes in the microvasculature produced by ischemia.

Assessment of cerebral circulation (cortical blood flow) with an infrared microscope

An infrared microscope was used for the remote detection of the surface temperature of the cerebral cortex of experimental cats. With this instrument a qualitative assessment of the circulation of the cerebral cortex can be made without interference with the integrity of the brain or vascular tissues or the reactivity of blood vessels. Although the instrument is sensitive enough to detect minor changes of cortical blood flow, its use appears limited to the detection of rapid or transient changes of cerebral circulation and is not suitable for long-term measurements of surface temperature of the brain.

Barrier properties in electrophotographic zinc oxide

A number of factors are known to influence greatly the photoelectrical and the electrophotographical properties of zinc oxide powder. Some of them, such as the degree of zinc excess and the state-species and amount-of chemisorbed oxygen, are in most cases uncontrollable, in the sense that they are almost determined at the time of manufacturing, unless some drastic treatment is imposed on samples before use. While some other factors such as the surface doping can be used to improve or control behaviors of samples at the time of use, these factors seem to be intimately related to the hard-to-control ones. After a brief review of studies of the oxygen states on zinc oxide surface, a discussion is developed concerning the photoconductive properties and the electrophotographic charge acceptance of zinc oxide bearing various degrees of zinc excess. The potential barrier at the surface of microcrystals, the height of which is estimated from dark conductivities, gives a good measure for the charge acceptance. Further, based upon the conventional theory of barrier development by oxygen chemisorption, the photo-to-dark conductivity ratio is argued in terms of the donor density, the barrier height, and other factors.

Integration of Visual Stimuli by the Crayfish Central Nervous System

1. For the crayfish, properties of visually reacting interneurone types in the optic nerve are described on the basis of single-unit analysis.

2. Sustaining fibres show: (a) ‘surround’ inhibition over the whole retina, including the excitatory field; (b) ‘on’ and ‘off’ effects at field boundaries; (c) a dark discharge in deteriorated preparations; (d) an increase in adapted firing rate and response to flashes in an ‘excited state’ of the preparation; (e) increased overall impulse frequency to fast-moving shadows at frequencies of 2-10/sec. which elicit short bursts.

3. Dimming fibres, having mainly reversed properties, show: (a) bursts followed by adaptation to a lower ferquency level of spikes on light dimming; (b) total inhibition by illumination for times proportional to light intensity; (c) responsiveness to quickly moving shadows.

4. ‘Jittery’ movement fibres lack directional sensitivity. For them: (a) total illumination and contrast have, within a wide range, no effect on discharges to moving targets; (b) large, dark cards moved at constant speed are seen only near their visual field boundary; (c) the less predictable the movement of a small dark object is, the longer its excitatory effect lasts; (d) responsiveness of field parts exposed is decreased for long durations; (e) no firing is caused by active or passive eye motions.

5. Space-constant fibres show changed location and size of their excitatory fields with eye position. Their potential visual fields below the horizontal plane are unresponsive, due to inhibition caused mainly by statocyst input.

6. Two eye muscle motor fibres, also under statocyst control, are influenced by light on the limited sensory fields of two pairs of identifiable sustaining fibres. The two motor fibres, which innervate antagonistic muscles, are reversely excited and inhibited by these fields.