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U N I T 1 0
Nervous System
Cerebrovascular Disease
Cerebrovascular disease encompasses a number of dis-
orders involving vessels in the cerebral circulation. As
elsewhere in the circulation, cerebrovascular disor-
ders can involve vessel occlusion or rupture that leads
to either focal or localized brain damage or to global
hypoxia-ischemia that causes widespread brain injury.
The Cerebral Circulation
The blood flow to the brain is supplied by the two inter-
nal carotid arteries anteriorly and the vertebral arter-
ies posteriorly
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(Fig. 37-12A). The internal carotid
artery, a terminal branch of the common carotid artery,
branches into several arteries—the ophthalmic, poste-
rior communicating, anterior choroidal, anterior cere-
bral, and middle cerebral arteries. Most of the arterial
blood in the internal carotid arteries is distributed
through the anterior and middle cerebral arteries (see
Fig. 37-12B). The anterior cerebral arteries supply the
medial surface of the frontal and parietal lobes and the
anterior half of the thalamus, the corpus striatum, part
of the corpus callosum, and the anterior limb of the
internal capsule. The genu and posterior limb of the
internal capsule and medial globus pallidus are fed by
the anterior choroidal branch of the internal carotid
artery.
The middle cerebral artery passes laterally, supply-
ing the lateral basal ganglia and the insula, and then
emerges on the lateral cortical surface, supplying the
inferior frontal gyrus, the motor and premotor frontal
cortex concerned with delicate face and hand control
(Fig. 37-13). It is the major vascular source for the lan-
guage cortices, the primary and association auditory
cortices, and the primary and association somatosen-
sory cortices for the face and hand. Because the middle
cerebral artery is a continuation of the internal carotid,
emboli arising in the internal carotid most frequently
become lodged in branches of the middle cerebral
artery. The consequences of ischemia of these areas may
be the most devastating, resulting in damage to the fine
manipulative skills of the face and upper limbs and to
receptive and expressive communication functions (e.g.,
aphasia).
The two vertebral arteries arise from the subclavian
artery and enter the foramina (opening) in the trans-
verse spinal processes at the level of the sixth cervical
vertebra and continue upward through the foramina of
the upper six vertebrae. They wind behind the atlas and
enter the skull through the foramen magnum and unite
to form the basilar artery, which then diverges to termi-
nate in the posterior cerebral arteries. Branches of the
basilar and vertebral arteries supply the medulla, pons,
cerebellum, midbrain, and caudal part of the diencepha-
lon. The posterior cerebral arteries supply the remaining
occipital and inferior regions of the temporal lobes and
the thalamus.
The distal branches of the internal carotid and ver-
tebral arteries communicate at the base of the brain
through the circle of Willis; this anastomosis of arteries
can provide continued circulation if blood flow through
one of the main vessels is disrupted (see Fig. 37-12B).
For instance, occlusion of one middle cerebral artery
may have limited consequences if the anterior and pos-
terior communicating arteries are patent, allowing col-
lateral flow from the ipsilateral posterior cerebral and
opposite carotid arteries. There are many normal vari-
ants across individuals in the completeness of the circle
of Willis, such that collateral supply may be limited.
Without collateral input, disruption of blood flow in
a cerebral artery results in ischemic neural damage as
metabolic needs of electrically active cells exceed nutri-
ent supply.
The cerebral circulation is drained by two sets of
veins that empty into the dural venous sinuses: the deep
(great) cerebral venous system and the superficial venous
system. In contrast to the superficial cerebral veins that
travel through the pia mater on the surface of the cere-
bral cortex, the deep venous system is well protected.
These vessels are directly connected to the sagittal
sinuses in the falx cerebri by bridging veins. They travel
through the CSF-filled subarachnoid space and pene-
trate the arachnoid and then the dura to reach the dural
venous sinuses. This system of sinuses returns blood to
the heart primarily through the internal jugular veins.
The intracranial veins do not have valves. The direction
of flow depends on gravity and pressures in the venous
sinuses as compared with those of the extracranial
veins. Increases in intrathoracic pressure, as can occur
with coughing or performance of the Valsalva maneuver
(i.e., exhaling against a closed glottis), produce a rise in
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Consciousness is a state of awareness of self
and environment. It exists on a continuum from
normal wakefulness and sleep to the pathologic
continuum of stupor and coma. Any deficit in
level of consciousness, from mild confusion to
stupor or coma, indicates injury to either the
ascending reticular activating (RAS) system
or to both cerebral hemispheres concurrently.
Consciousness may decline due to severe
systemic metabolic derangements that affect
both hemispheres, or from head trauma causing
shear injuries to white matter of both the RAS
and the cerebral hemispheres.
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Deterioration of brain function is manifested
by alterations in sensory and motor function
and changes in the level of consciousness. In
progressive brain injury, coma usually follows a
rostral-to-caudal progression with characteristic
changes in levels of consciousness, the pupillary
light response and eye movements, posturing,
and respiratory responses.
SUMMARY CONCEPTS
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