Cerebral microcirculation: heterogeneity of pial arterial network controlling microcirculation of cerebral cortex.

Analysis of the functional behaviour of pial arterial ramifications feeding small areas of the cerebral cortex of rabbits uncovered well-defined heterogeneity of vascular responses during development of functional hyperemia. In the network of the smaller pial arteries, under 100 microns in diameter, the most active segments, the sphincters of offshoots of smaller arterial branches from larger trunks and the precortical arteries just before their penetration as the radial arteries into the cerebral cortex, have been discovered. The frequency of their dilatation was found to be higher, the latent periods of the vascular responses significantly shorter, and the degree of vasodilatation greater than of the adjacent arterial segments. An abundant amount of cholinergic nerve plexuses (containing the cholinesterase) was found in the walls of these active vascular segments, and microapplication of atropine resulted in a considerable decrease of their dilatation. These experimental results indicate the involvement of the cholinergic neurogenic mechanism in the functional vasodilatation, as distinct from the largely accepted effect of the humoral mechanism accomplished by diffusion of active vasodilatory substances from tissue elements to the walls of the feeding arterial branches.

[The function of the precortical arteries]. / Funktsiia prekortikal'nykh arterii.

In anesthetized adult rabbits, the responses of pial and precortical arteries to retinal photostimulation and the application of 0.5% strychnine solution to the brain surface were studied, as well as the distribution of cholinergic and adrenergic nerve fibres in these microvessels. The precortical arteries showed the highest degree of dilatation and frequency of responses. Dense innervation was mostly in evidence in the precortical arteries. Upon atropin application, the response suppression was also more obvious in the precortical arteries. The data suggest that the precortical arteries play an important role in regulation of the adequate blood supply to the brain tissue.

[The variously directed reactions of the pial and cortical arterial branches in postischemic hyperemia in the cerebral cortex]. / Raznonapravlennye reaktsii pial'nykh i korkovykh arterial'nykh vetvlenii pri postishemicheskoi giperemii v kore golovnogo mozga.

In adult rabbits, functional behaviour of pial and cortical (radial) arteries was rather different: the pial arteries showed regular dilatation while the lumen of the cortical arteries underwent regular reduction. Despite the different responses of the arterial segments, the vascular resistance diminished entailing an increase of local blood flow in the cerebral cortex.

[Response of active microvascular segments in the pial artery system to photostimulation of the ey in the rabbit]. / Reaktsiia aktivnykh mikrososudistykh segmentov v sisteme pial'nykh arterii pri fotostimuliatsii glaza u krolikov.

Photomicrography revealed the dynamics of pial ertarial responses, their active segments in particular: the sphincters at offshoots of minor arteries from the larger vessels, and the precortical arteries. On retinal photostimulation, the precortical arteries were found to dilate quite often (72%); the sphincters at offshoots in 50%, and the minor arteries in 38% of cases, while the larger pial arteries showed no dilatation at all. The dilatation was most pronounced in the precortical arteries and in the sphincter areas (25% of the control diameter), and less significant in minor pial arteries (10%). The data obtained indicate that the precortical arteries are the basic vascular effectors of regulation of adequate microcirculation in the cerebral cortex.

[Participation of the cholinergic nervous mechanism in regulating adequate blood supply to the rabbit cerebral cortex]. / Uchastie kholinergicheskogo nervnogo mekhanizma v regulirovanii adekvatnogo krovosnabzheniia kory golovnogo mozga krolika.

Dilatation of the pial arteries and their active segments (sphincters of the offshots and precortical arteries) was studied in rabbits under the conditions of enhanced neuronal activity of the brain cortex, induced by application of 0.5% strychnine to its surface. The blockade of the cholinergic transmission by microapplication of atropine to vessel walls caused a significant inhibition of the dilatatory responses of the study microvessels. Reduction of functional dilatation was most demonstrable in the precortical arteries, less marked in the sphincters of the offshots and still less marked in the small pial arteries. No differences in the responses of the large pial arteries were discovered either before or after atropine microapplications. The author suggests that the cholinergic mechanism plays an important part in regulation of adequate brain blood supply and that such a regulation may be performed locally within the area of a single radial artery occupying ca. 1/5 mm2 of the brain surface in rabbits.

[Spread of the dilatational reactions in the pial artery system to the microapplication of strychnine to the surface of the brain]. / Rasprostranenie dilatatornykh reaktsii po sisteme pial'nykh arterii pri mikroapplikatsii strikhnina k poverkhnosti golovnogo mozga.

In anesthetized adult rabbits, organization of pial arterial bed and responses of its elements to microapplication of strychnine, were studied. The pial arterial responses were primarily related to the vessels' structural and functional features. The most considerable dilatation was characteristic of the precortical arteries. The reactions of sphincters of offshoots and smaller pial arteries were almost similar. The least dilatation occurred in larger pial arteries (over 100 mu in diameter). The limits of the vasodilatation were the microanastomoses which separated individual "terminal microvascular units" in the pial arterial bed. The data obtained show that the anatomy of the pial arterial bed provides a possibility for relatively independent regulation of microcirculation in rather small areas of cerebral neocortex.

[Effect of sympathectomy on responses of pial arteries during acute deficiency of cerebral blood supply]. / Vliianie simpatéktomii na reaktsii pial'nykh arterii pri ostrom defitsite krovosnabzheniia golovnogo mozga.

The dilatatory responses of small pial arteries including their active segments (sphincters at off-shoots, precortical arteries), to blood supply deficiency in the cerebral cortex were compared in control and sympathectomized rabbits. The sympathectomy resulted in complete disappearance of histochemically detected adrenergic and a considerable decrease of cholinergic nerve fibers in the pial arterial walls. The vasodilatation was much less obvious in sympathectomized than in control animals. This was associated with (and probably caused by) a considerable rise in histochemically detected serotonin activity of the pial arteries walls. After recovery of blood supply to the brain the constriction of the pial arterial active segments restricting the excessive cerebral blood flow, was significantly reduced due, probably, to the sympathetic deprivation. Therefore, the sympathetic control plays an important part in pial arterial responses regulating the adequate blood supply of the cerebral cortex.

[Reactions of pial microvascular effectors in regulating adequate cortical blood supply to neuronally isolated cerebral cortex]. / Reaktsii pial'nykh mikrovaskuliarnykh éffektorov regulirovaniia adekvatnogo krovosnabzheniia kory mozga v usloviiakh ee neíronal'noi izoliatsii.

Responses of pial arteries were studied in experiments on adult rabbits with the use of serial photomicrography of the cerebral surface under conditions of the enhanced activity of the brain cortex, induced by application of 0.5% strychinine. No significant differences in the responses of smaller and larger pial arteries as well in those of the sphincters at their off-shots and precortical arteries were found after dissection of all neural pathways of the vascular responses are neurogenic, the feed-back should operate locally, inside the appropriate cortical areas.

On the control of microcirculation in the cerebral cortex during and following ischemia.

The aim of the present study was the elucidation of the functional behaviour of the pial arteries and their microvascular effectors which are responsible for the microcirculation in the cerebral cortex - large and small pial arteries (PA), sphincters at the offshoots at the pial arteries (SOPA), precortical arteries (PCA) and pial arterial microanastomoses (PAMA) - both under ischemic and postischemic conditions. During ischemia the majority of the studied microvessels underwent dilatation which under conditions of decreased intravascular pressure seems to be active and should be aimed at compensating for the defficiency of the blood supply to the cerebral tissue. Besides, a constriction of some microvessels, especially of SOPA and PCA, was also observed, the amount of such responses of SOPA being increased in the postischemic period. It might be conjectured that the vasoconstrictor responses may be responsible for the ischemic damage of some cortical areas, but it may be also directed to a redistribution of blood to compensate for ischemic changes in individual areas of the cerebral tissue. The active constriction of SOPA in the postischemic period might be also considered as a compensatory microvascular reaction which is directed to restrict the excessive blood supply to the brain tissue and to prevent edema development in it.