Blood
Supply To The Central Nervous System
Blood supply to the brain
The arterial blood supply to the brain comes from four vessels: both the
right and left internal carotid as well as the vertebral arteries.
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The vertebral arteries enter the skull through
the foramen magnum and unite to supply blood to the brainstem (basilar
artery) and posterior parts of the cerebral hemisphere (posterior
cerebral arteries) – the whole network constituting the posterior
circulation.
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The internal carotid arteries (ICAs)
traverse the skull in the carotid canal and the cavernous sinus before piercing
the dura and entering the middle cranial fossa just lateral to the optic
chiasm. They then divide and supply blood to the anterior and middle parts of
the cerebral hemispheres (anterior [ACA] and middle [MCA]
cerebral arteries). In addition, the posterior and anterior cerebral
circulations anastomose at the base of the brain in the circle of Willis,
with the anterior and posterior communicating arteries offering
the potential to maintain cerebral circulation in the event of a major arterial
occlusion. The ICA prior to their terminal bifurcation supply branches
to the pituitary (hypophysial arteries), the eye (ophthalmic artery), parts of
the basal ganglia (globus pallidus) and limbic system (anterior choroidal
artery) as well as providing the posterior communicating artery.
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The MCA forms one of the two terminal
branches of the ICA and supplies the sensorimotor strip surrounding the central
sulcus (with the exception of its medial extension which is supplied by the
ACA) as well as the auditory and language cortical areas in the dominant
(usually left) hemisphere. Therefore, occlusion of the MCA causes
a contralateral paralysis that affects the lower part of the face and arm especially,
with contralateral sensory loss or inattention and a loss of language if the
dominant hemisphere is involved (see Chapters 28, 31, 34, 35, 38, and 39). In
addition, there are a number of small penetrating branches of the MCA that
supply subcortical structures such as the basal ganglia and internal capsule
(see below).
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The two ACAs, which form the other major
terminal vessels of the ICAs, are connected via the anterior communicating
artery and supply blood to the medial portions of the frontal and parietal
lobes as well as the corpus callosum. Occlusion of an ACA characteristically
gives paresis of the contralateral leg with sensory loss, and on occasions
deficits in gait and micturition accompanied with mental impairment and dyspraxia
(see Chapter 34).
•
The vertebral arteries, which arise from
the subclavian artery, ascend to the brainstem via foramina in the transverse
processes of the upper cervical vertebrae. At the level of the lower part of
the pons the vertebral arteries unite to form the basilar artery, which
then ascends before dividing into the two posterior cerebral arteries (PCAs)
at the superior border of the pons. Each vertebral artery en route to forming
the basilar artery gives off a number of branches including the posterior
spinal artery, the posterior inferior cerebellar artery (PICA) and the anterior
spinal artery. These spinal arteries supply the upper cervical cord (see
below), whereas the PICA supplies the lateral part of the medulla and
cerebellum. Occlusion of this vessel gives rise to the lateral medullary
syndrome of Wallenberg.
•
The PCAs supply blood to the posterior
parietal cortex, the occipital lobe and inferior parts of the temporal lobe.
Occlusion of these vessels causes a visual field defect (usually a homonymous
hemianopia with macular sparing, as this cortical area receives some
supply from the MCA; see Chapter 25), amnesic syndromes (see Chapters 45 and
46), disorders of language (see Chapter 26) and, occasionally, complex visual
perceptual abnormalities (see Chapter 28). The PCA has a number of central
perforating or penetrating branches that supply the midbrain, thalamus,
subthalamus, poste- rior internal capsule, optic radiation and cerebral
peduncle, and these are commonly affected in hypertension, when occlusion of
the PCA produces small lacunar infarcts.
Apart from occlusion, haemorrhage from cerebral vessels may
involve the brain substance (intracerebral), the subarachnoid space or both.
Such haemorrhages usually occur in the context of either trauma, hypertension
or rupture of congenital aneurysms in the circle of Willis (berry
aneurysms) (see Chapter 64).
Venous drainage of the brain
The brainstem and cerebellum directly drains into the dural venous
sinuses adjacent to the posterior cranial fossa. The cerebral hemi- spheres, in
contrast, have internal and external veins – the external cerebral veins drain
the cortex and empty into the superior sagittal sinus (see Chapter 5). This
sinus drains into the transverse sinus, then the lateral sinus, before emptying
into the internal jugular vein. The internal cerebral veins drain the deep
structures of the cerebral hemisphere to the great vein of Galen and thence
into the straight sinus. Occlusion of either of these venous systems can occur,
causing raised intracranial pressure with or without focal deficits.
Blood supply to the spinal cord
The blood supply to the spinal cord comes in the form of a single
anterior spinal artery and paired posterior spinal arteries. The anterior
spinal artery arises from the vertebral arteries and extends from the
level of the lower brainstem to the tip of the conus med- ullaris. The
posterior spinal arteries take their origin from the vertebral arteries. At
certain sites along the spinal cord there are a number of reinforcing inputs
from other arteries (see figure).
Vascular insults to the spinal cord occur most commonly at the watershed
areas in the cord, namely the lower cervical and lower thoracic cord. Occlusion
of the anterior spinal artery produces a loss of power and spinothalamic
sensory deficit with preservation of the dorsal column sensory modalities
(joint position sense and vibration perception; see Chapters 9 and 54).
