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BLOOD SUPPLY TO THE CNS Objectives:

• Understand the importance of the cerebral circulation. • Understand stroke, ischemic penumbra, and the types of vascular problems that cause it • Describe the origin and distribution of the blood supply to the cerebral hemispheres

including the internal carotid, anterior cerebral, middle cerebral, posterior cerebral, and anterior choroidal arteries and the territories they supply. Describe the venous drainage.

• Understand regulation of cerebral blood flow. • Describe the Circle of Willis. • Be able to read cerebral angiograms. • Describe the origin and distribution of the blood supply to the brainstem including he

territories supplied as shown in cross sections of brainstem. • Be able to draw the diagram of blood vessels (vertebral arteries to the circle of Willis) on a

figure of the brainstem. • Describe the origin and distribution of blood supply to the spinal cord.

1. Why Cerebral Circulation is Important

• The brain is very active metabolically: It comprises 2% of body weight (1500 g) but uses 15% of cardiac output and consumes 20% of total available oxygen to body.

• brain perfusion is rapid: total cerebral blood flow averages 750 ml/min; total cerebral blood volume is about 100 ml; thus circulating blood to the brain turns over about 7x/ min

•consequences of interruption of blood supply are severe: •loss of sensory, motor, and cognitive functions • brain deprived of blood for 20 seconds leads to unconsciousness; after 4-5 min –

irreversible unconsciousness or death occurs. • stroke is the 3rd most common cause of death in US; about 500,000 strokes/year and

150,000 deaths occur. It is the most common neurological disorder. Heart disease – 720,000 deaths Cancer – 505,000 Stroke – 150,000 Accidents – 92,000

•localization of symptoms to an area of the CNS will be associated with the blood vessel that supplies that area, with important consequences for possible interventions and prognosis.

2. Types of Vascular Problems: • Stroke – a syndrome caused by a disruption of brain circulation and characterized by acute

onset of neurological deficits that last at least 24 hours and involving focal symptoms that correlate with a brain area supplied by the affected vessel.

A) Ischemia (67%)– loss of oxygen and glucose due to interruption of blood supply

impairs neuronal function; may be reversible or irreversible. • Infarction– irreversible ischemic injury with lasting neurological deficits. • Transient ischemic attack (TIA) – transient episode of neurological dysfunction

caused by focal brain, spinal cord, or retinal ischemia without acute infarction. 1/3 of TIA patients have stroke within 5 yrs.

Chris Cohan, Ph.D. Dept. of Pathology/Anat. Sci. Univ. at Buffalo

Chris Cohan, Ph.D. Dept. of Pathology/Anat. Sci. Univ. at Buffalo

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1) Thrombosis– development of a blood clot that occludes a vessel (artery, vein, sinus); symptoms develop over minutes to hours.

2) Embolus– a circulating mass from a distant location that occludes a vessel; symptoms are sudden and maximal.

3) Other contributing factors: atherosclerosis, increased or decreased blood pressure, and other vascular disorders.

B) Hemorrhage (33%)– bleeding from an artery or vein causing destruction or

compression of brain tissue. • Aneurysm– a weakening in the wall of arteries usually at branch points, which causes the

vessel wall to progressively balloon. • Infarcts – their effects are restricted to the territories supplied by a blood vessel:

• Decreased blood supply to an area of the brain, results in an ischemic cascade involving:

release of excitatory transmitters (especially glutamate) from impaired neurons, depolarization, activation of NMDA receptors, influx of large amounts of calcium, enzymatic activation, cytotoxic edema, and ultimately irreversible cell death. These changes occur within minutes to hours in the core of tissue directly supplied by the involved vessels. However, tissue surrounding the core region of infarction is also ischemic, but it is reversibly impaired and is known as the ischemic penumbra. This surrounding, peripheral tissue may remain viable due to collateral circulation, with survival time dependent on the level of cerebral blood flow to the area. The penumbra can be visualized with CT/MRI and represents an area of impaired function. It will eventually infarct if blood supply is not restored over a period of hours to days. Strategies to limit the damage from infarction focus on ways to save the penumbra. Fever and hyperglycemia significantly worsen the effects of ischemia, so those conditions must be minimized therapeutically.

Anterior Spinal Artery occlusion in the Medulla

Occlusion of Middle Cerebral Artery

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• Regulation of Cerebral Blood Flow • Increases in neuronal activity cause a compensatory increase in

blood flow to the active area so that the metabolic demands of neurons can be met. This effect is mediated by pericytes, which are contractile cells that sit on the outside of brain capillaries. Glutamate released at synapses near blood vessels binds to NMDA receptors on pericytes, causing their relaxtion and consequent dilation of capillaries. This provides increased amounts of glucose and oxygen for uptake by astroctyes and neurons for energy utilization. This is the basis for most functional imaging with fMRI and PET, which creates images based on changes in cerebral blood flow.

• Pericytes are very sensitive to ischemia. Ischemic conditions like stroke result in pericyte

constriction and death. This leads to a long lasting increase in the resistance of brain capillaries in the surrounding area (penumbra), eventually causing further neuronal loss.

• Another mechanism for altering cerebral blood flow depends on CO2 levels in the blood.

Hyperventilation causes vasoconstriction, which decreases cerebral blood flow to the brain. This method can be used to reduce increases in intracranial pressure that result from intracranial hemorrhages and other pathological conditions.

3. General Plan of Blood Supply

•Main sources of blood supplying the cerebral hemispheres, brainstem, and spinal cord are from the internal carotid arteries and vertebral arteries.

• The internal carotid arises from the common carotid and gives rise to the anterior, middle, and ophthalmic arteries in the skull. It supplies the cerebral hemispheres and eye. • The vertebral artery arises from the subclavian artery and gives rise to numerous vessels that supply the brainstem, cerebral hemispheres, cerebellum, and spinal cord.

• The internal carotid and vertebral systems anastomoze in a structure called the Circle of Willis.

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4. Supply to Hemispheres/Circle of Willis

• Normally, in the Circle of Willis blood does not flow from one side to the other. However, if occlusion occurs, the opposite side can provide important collateral supply of blood.

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5. Blood Supply to the Cerebral Hemisphere • Arises from two primary sources: internal carotid and vertebral arteries

• Superficial branches supply the surface and penetrating branches supply deep structures. • Internal carotid supplies the dura, optic nerve and chiasm, internal capsule, eye (ophthalmic

artery). Its terminal branches form the anterior and middle cerebral arteries. • Anterior cerebral artery supplies the medial surface of the hemispheres. It also gives branches

to the corpus callosum, caudate, and internal capsule. • Posterior cerebral artery supplies the inferior surface of the temporal and the medial surface

of the occipital lobes. Also supplies thalamus and choroid plexus of lateral ventricle.

•Middle cerebral artery supplies the majority of the lateral surface of the hemispheres. Also supplies the putamen/globus pallidus (via Lenticulostriate arteries) and internal capsule.

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Lenticulostriate arteries are small, penetrating branches of the MCA that supply deep structures including the Putamen and Globus Pallidus. Infarcts can cause focal deficits. •Circle of Willis supplies deep penetrating branches to the diencephalon. • Additional branches of the internal carotid include:

•Anterior choroidal artery supplies the choroid plexus of the temporal horn of lateral ventricle, hippocampus, amygdala, optic tract, lateral geniculate nucleus, Meyer's loop, globus pallidus, and internal capsule.

•Posterior communicating artery supplies optic chiasm/tract, diencephalon, internal capsule • Internal capsule is supplied by: IC, ACA, MCA, Post. Comm A, Ant Chor A. • Thalamus/Diencephalon is supplied by: PCA, Post Comm. A.

Int.Cap

IC, ACA, MCA, Ant Choroidal, Post Comm

Caudate

MCA, (ACA)

Putamen Globus Pallidus

MCA via lenticulostriate, Ant Choroidal

Thalamus

PCA

Hypothalamus

PCA, Post Comm, ACA

• Application to Cortical Symptoms Caused by Stroke

• The different functional areas of the cortex lie within different territories for the blood vessels that supply the hemispheres. Loss of blood supply to the entire area of a vessel will cause symptoms of all the functional systems in that area. However, it is more likely that a branch of a major vessel may be involved in which case a subset of symptoms for that vessel’s territory would be observed. The determination of the specific vessel responsible for symptoms observed in a patient requires that you know the relationship between each vessel and the functional area that lies within it.

Loyola School of Medicine http://www.meddean.luc.edu/lumen/MedEd/Neuro/index.htm

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6. Blood Supply to the Brainstem 2 Vertebral arteries 1 Basilar artery 2 Posterior Cerebral arteries. •Supplies the medulla, pons, midbrain, cerebellum, spinal cord

•PICA and AICA – supply the inferior surface of the cerebellum in addition to brainstem. •Superior cerebellar artery – supplies superior surface of cerebellum and its deep nuclei.

anterior cerebral anterior communicating

posterior cerebral

middle cerebral

internal carotid

anterior choroidal

superior cerebellar

anterior inferior cerebellar

labyrinthine

anterior spinal

posterior spinal

posterior inferior cerebellar

vertebral

basilar

posterior communicating

Occlusion of branches from major arteries (eg basilar) limit damage to smaller areas

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•Medulla

•Pons

BASILAR: many branches supply left or right sides. Overlapping supply: caudal pons – AICA/long circumferentials; rostral pons – SCA/ long circumferentials

•Midbrain

Overlapping supply: long circumferentials/SCA • Application to Brainstem Symptoms Caused by Stroke

• Similar to the cerebral cortex, the different sensory and motor pathways in the brainstem lie within different territories of the blood vessels that supply the brainstem. Loss of blood supply from a major vessel or its branches will cause symptoms corresponding to the pathways lesioned in that area. The determination of the specific vessel responsible for symptoms observed in a patient requires that you first localize the symptoms to a specific level and area of the brainstem and then choose the blood vessel that supplies that area.

Basilar: paramedian, short and long circumferential

Posterior cerebral: paramedian, short and long circumferential

vertebral

anterior spinal anterior spinal vertebral

posterior spinal PICA

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• Alternating hemiplegia, the symptom of paralysis of the body on one side and a cranial nerve deficit on the other side, is a localizing lesion for the brainstem. It is due to the involvement of both the corticospinal tract and a cranial nerve nucleus or its fibers in a lesion. Each level of the brainstem has branches of vessels that supply territories containing the corticospinal tract and cranial nerves. By completing the information below, you will make it much easier to understand and remember these important relationships.

LEVEL

Color-in CN nucleus and fibers CRANIAL NERVE BLOOD VESSEL

State if CN/CST likely to be in same territory

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7. Blood Supply to the Spinal Cord •Derived from the Vertebral arteries that give rise to one anterior and two posterior spinal arteries. Reinforced by anastomotic branches from segmental (radicular) arteries. These arise from intercostal, lumbar, and sacral arteries and travel along the dorsal and ventral spinal roots toward the spinal cord.

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• Territories supplied:

8. Venous Drainage of Brain • Superficial/deep cerebral veins Venous Sinuses (formed by dura) Internal Jugular Vein

Anterior spinal

Posterior spinal

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ICA

ACA

MCA

MCA

ACA

Lateral projection

Anterior-posterior projection

Anterior-posterior projection

Vert

PCA SCA

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