Concept
of border zone versus core territory of the cerebral vessels
We have seen that the cerebral cortex is supplied by the cortical branches
of MCA, ACA, and PCA. These vessels reach the cortex, divide, and
give off branches that penetrate the cortex perpendicularly. Long
arteries and arterioles pass through the gray matter and penetrate the white
matter to a depth of 3-4 cm, where they supply the core of the hemisphere's
white matter. These vessels intercommunicate very little, and thus
constitute many independent small systems. The deeply penetrating
vessels that branch directly from major arterial stems (like the lenticulostriate
branches of MCA) also do not interconnect. Therefore if one of these vesselsis
occluded, the tissue it normally supplies will be infarcted since it has
no other source of blood flow.
By contrast, the surface arteries supplying the cortical gray matter do
anastomose. The anastomoses form a continuous network of tiny arteries
covering the perimeters or border zones between core (central) territories
of the 3 major cerebral arteries. Since these anastomoses are usually tiny,
they carry little blood. Therefore, if one of the major arteries is occluded,
the anastomoses cannot carry enough blood to rescue the core territory of
that artery from infarction. However, they may be able to supply the
border zone, so that the size of the infarction is reduced a little. Conversely,
if there is a dramatic impairment of systemic blood pressure (secondary
to cardiac arrest or massive bleeding), or decreased pressure specifically
in the carotid system (due to an occlusive event), the regions of each vascular
territory farthest from the core will become ischemic first. Thus there
is risk of infarction specifically in the border zones.
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ACA-MCA
border zone may include these key functional areas:
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- Hip, shoulder or less usually
arm region of primary motor or somatosensory cortex
- Areas related to language
production - in the dominant hemisphere
- Frontal eye fields
- Frontal lobe motor planning
area
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ACA/MCA-PCA
border zone may include these key functional areas:
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- Visual radiations
- Foveal region of the striate
cortex in some people (in others it may be in PCA territory)
- Inferior temporal lobe
cortex important for visual recognition
- Regions related to language
comprehension, word finding, and possibly reading - in the dominant hemisphere
- Regions of parietal lobe
important for visuospatial perceptions - in the nondominant hemisphere
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