Download Anatomy 4- CNS Vasculature Brain The constant neural activity of

Anatomy 4- CNS Vasculature
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Brain
The constant neural activity of the brain requires a constant supply of fuel and oxygen
– The brain represents ~2.5% of body weight, but receives about 15% of cardiac output and accounts for
~25% of our O2 consumption
• 10 s of brain ischemia = loss of consciousness
• 20 s = loss of electrical activity
• A few minutes = irreversible damage
Thus, the brain is highly vascularized
Blood supply to the brain is derived from two sets of arteries
– Internal carotid arteries
• Supply ~80% of blood to brain (anterior and middle fossae)
– Vertebral arteries
• Supply ~20% of blood to brain (posterior fossa)
Venous drainage occurs via cerebral veins and the dural venous sinuses
Internal Carotid Arteries
• The internal carotid arteries can be divided into four parts
1. Cervical
The internal carotid arteries arise in the neck from the bifurcation of the common carotid arteries
The cervical part of the internal carotid artery ascends in the neck without branching
The cervical part of the artery enters the skull at the opening of the carotid canal, in the petrous part of the
temporal bone
2. Petrous
The petrous part of the internal carotid artery runs in the carotid canal, which is located entirely in the petrous
part of the temporal bone
• Within the canal, the carotid artery—which is initially running straight vertically—turns anteriorly and
medially
It then turns upward, to resume a straight vertical course
3. Cavernous
The cavernous part of the internal carotid artery traverses the cavernous sinus
Within the sinus the artery climbs along the carotid groove
Just inferior and medial to the anterior clinoid process, the artery turns upward and emerges through the dural
roof of the cavernous sinus
4. Cerebral
In order to remove the brain, the internal carotid arteries—and other neurovasculature—must be severed
With the brain removed, the cerebral parts of the internal carotid arteries appear as stubs protruding through
the dura mater
– They are adjacent to the large optic nerves
At this point the internal carotid arteries turn posteriorly
Anatomy 4- CNS Vasculature
Vertebral Arteries
• The paired vertebral arteries ascend in the neck through the transverse foramina of the vertebrae
• They enter the skull through the foramen magnum
• The two vertebral arteries unite along the caudal border of the pons to form the basilar artery
– Basilar = base of the skull
Arteries of the Brain
• The principal arteries supplying the brain can be grouped for didactic purposes
– Cerebral arteries (3)
– Cerebellar arteries (3)
– Two communicating arteries and the anterior choroidal arteries
– Arteries leading to the brain
• Internal carotid, vertebral, and basilar arteries
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All the arteries around the base of the brain give rise to small branches known as perforating arteries
– These arteries supply deep cerebral structures
• Though small, these arteries often supply key brain regions
– Damage to the perforating arteries can cause neurological deficit out of proportion to their size
Cerebral Arteries
• The anterior cerebral arteries are one terminal branch of the internal carotid arteries
They run medially, then anteriorly in the longitudinal fissure
– The branches supply most of the medial and superior surfaces of the brain, and the frontal poles
• The large middle cerebral arteries represent the other terminal branches of the internal carotid arteries
They proceed laterally into the lateral sulcus
– The branches of the middle cerebral arteries supply the lateral surface of the cerebral hemispheres, and
the temporal poles
• Each of the two posterior cerebral arteries represents a terminal branch of the basilar artery
The posterior cerebral arteries curve around the midbrain
– Their branches supply the inferior surface of the brain and the occipital poles
Cerebellar Arteries
• There are three cerebellar arteries, each named for the part of the cerebellum they supply
– Posterior inferior cerebellar artery (PICA)
– Anterior inferior cerebellar artery (AICA)
– Superior cerebellar artery
• The posterior inferior cerebellar artery (PICA) is the largest branch of the vertebral artery
• The anterior inferior cerebellar artery (AICA) is the first major branch of the basilar artery
– Note labyrinthine artery (branch) which supplies the inner ear
• The superior cerebellar artery is the last major branch of the basilar artery
Anatomy 4- CNS Vasculature
Cerebral Arterial Circle
• The cerebral arterial circle (of Willis) is a pentagon-shaped circle of vessels on the inferior surface of the brain
• It consists of the following vessels
– Anterior and posterior cerebral arteries
– Anterior and posterior communicating arteries
– Internal carotid arteries
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Blood flow through the arterial circle is minimal due to insufficient pressure differentials
However, the circle represents an important anastomosis between anterior and posterior blood supplies to the
brain
– It is theoretically possible for the entire brain to be perfused by just one of the four major supply
arteries
• Fewer than half of all circles exhibit a “normal” pattern
– In the other half, segments are too small to be functional (hypoplastic) or they are missing
• The variants displayed next do not generally affect the function of the ring significantly, as collateral routes of
circulation are available
– However, blood supply to the brain may be asymmetrical
Anterior Choroidal Arteries
• The long, thin, anterior choroidal artery is the last branch given off by the ICA before bifurcating into ACA and
MCA
• The anterior choroidal artery supplies many key structures and is thus frequently involved in cerebrovascular
accidents
– Optic tract, choroid plexus of lateral ventricle, and deep brain structures
Anatomy 4- CNS Vasculature
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The system of vertebral arteries, which merge to form the basilar artery, represents the arterial pathway of
blood to the posterior brain
As well, perforating branches from these three arteries themselves supply much of the brainstem
Watershed Zones
• Watershed zones are areas of the brain located between the terminal distributions of adjacent arteries
• Under normal conditions, these areas are supplied by “end arteries” functioning under low, marginally adequate
arterial pressure
• In situations in which cerebral perfusion pressure drops, as happens with falling blood pressure, blood flow to
these watershed zones is diminished despite patent blood vessels
Venous Drainage
• The general pattern of venous drainage in the brain is as follows
– Cerebral veins drain into the dural venous sinuses
– The sinuses ultimately drain into two locations
• The internal jugular veins
• The basilar plexus of veins
– Joins epidural (internal) venous plexus of the spinal cord
• Two general types of valveless veins drain into the dural sinuses
– Cerebral veins
• Consist of superficial and deep veins
Superficial veins lie on the surface of the cerebral hemispheres
These veins are highly variable in their course and location
Only three veins are reasonably constant from brain to brain
Superficial middle cerebral vein
Superior anastomotic vein (vein of Trolard)
Inferior anastomotic vein (vein of Labbé)
Deep veins—located in deep parts of the brain where arteries are tiny—serve as useful
radiological landmarks
The major deep veins—the paired internal cerebral veins—merge to form the great
cerebral vein (vein of Galen)
The great vein merges with the inferior sagittal sinus to form the straight sinus
– Emissary veins
• These connect extracranial veins with the sinuses
• They play a minor role in circulation but have clinical implications (spread of infection into
cranial cavity)
Anatomy 4- CNS Vasculature
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Blood-Brain Barrier
Blood can be problematic to the brain for two reasons
– It is a source of harmful agents
• Pathogens, antibodies, macrophages
– Its composition is quite variable, which is highly disruptive to proper neuron function
• The quantity of amino acids and ions, for instance, must be tightly regulated in the neuronal
microenvironment
Therefore, most nervous tissue of the brain is shielded from the blood
There are two general areas within the brain that must be protected from blood
– Brain extracellular fluid—the microenvironment within which the neurons function
• Provided by a true blood-brain barrier
– Cerebrospinal fluid
• Provided by a blood-CSF barrier
The blood-brain barrier is formed by regular blood capillaries throughout the brain
The barrier consists mainly of tight junctions sealing adjacent capillary endothelial cells, and a thick basement
membrane
– The formation of tight junctions is thought to be stimulated by astrocytes, whose endfeet provide a
nearly continuous covering of the capillaries
The blood-brain barrier is both anatomical and physiological
– Anatomically, it is an inert barrier that separates most nervous tissue of the brain from blood capillaries
• Proteins cannot pass through, but lipid-soluble substances (fats, gases, alcohol) may diffuse
through cells of the barrier
– Physiologically, cells of the barrier actively regulate passage of substances from blood to tissue fluid (e.g.
glucose and ions)
Blood-CSF Barrier
There are two general areas where CSF must be protected from blood
– All along the arachnoid mater
• This is provided by tight junctions in cells of the arachnoid
– At the choroid plexuses and the circumventricular organs
At the choroid plexuses, the (true) blood-brain barrier is absent—in other words the blood
capillaries are very permeable
At the plexuses, the composition of brain extracellular fluid is very similar to plasma
However, the ependymal cells of the choroid plexuses form a barrier between blood and CSF
Mainly tight junctions
Anatomy 4- CNS Vasculature
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In addition to the choroid plexuses, the circumventricular organs lack a true blood-brain barrier
– The term “circumventricular” is collectively applied to a series of organs surrounding the third and
fourth ventricles
However, like the choroid plexuses, these organs feature specialized ependymal cells separating blood from CSF
The absence of the blood-brain barrier in circumventricular organs is thought to be necessary in order for these
areas to “sample” blood
– Requirement for performing controlling functions
• The hypothalamus monitors blood plasma concentrations
• The posterior pituitary releases hormones
Spinal Cord
Arterial Supply
• Blood supply to the spinal cord derives from two sets of arteries
– Longitudinal
• One anterior longitudinal artery
• Two posterior longitudinal arteries
– Segmental
• Segmental medullary arteries
• Radicular arteries
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The longitudinal, segmental, and radicular arteries all give off extensive tiny branches which directly supply the
spinal cord
The most prominent of these are the sulcal arteries
– They enter the spinal cord through the anterior median fissure
– They supply about two-thirds of the cross-sectional area of the cord
The anterior and posterior longitudinal arteries are cranial branches of the vertebral arteries
Anatomy 4- CNS Vasculature
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By themselves, the longitudinal arteries could only supply the upper portions of the spinal cord
– Necessity for segmental arteries
• The anterior and posterior segmental medullary arteries derive from spinal branches of a number of arteries
– Ascending and deep cervical, vertebral, posterior intercostal, and lumbar arteries
• The radicular arteries appear at most segmental levels, but do not reach the longitudinal arteries
– Occasionally a radicular artery is long enough to reach the longitudinal arteries and is termed a
segmental artery
• The largest of the segmental medullary arteries—the artery of Adamkiewicz, which usually arises from a
posterior intercostal or lumbar artery—reinforces circulation to two-thirds of the spinal cord (located on left
side in 65% of population)
Venous Drainage
• In general, venous drainage of the spinal cord is analogous to the arterial supply
– The vertical system consists of six anastamotic veins (three anteriorly and three posteriorly)
– The horizontal system consists of larger medullary segmental veins, and smaller radicular veins
• The spinal veins drain superiorly into cerebellar veins and dural sinuses
• Segmentally, the spinal veins are drained by the segmental medullary veins into the internal vertebral venous
plexus
• From the internal plexus, blood drains into the external vertebral venous plexus—and eventually the caval and
azygous systems—via intervertebral and basivertebral veins