Dr. Amjad PT

The consequences of stroke depend upon which areas of the brain are affected by ischemia, hemorrhage, hematoma or aneurysm. Therefore, it is important to understand the organization of the cerebral blood supply system.

Two arterial systems supply the brain with blood: the internal carotid and vertebral. The internal carotids and vertebro-basilar arteries are connected via the circle of Willis, which allows blood to pass from one system to another in the event of blockage.

C1

C2

C3

C4

C5C6

C7

ant. clinoid proc.

cavernous sinus

foramen lacerum

carotid canal

The Internal Carotid Arteries

There are two internal carotid arteries. They ascend from

the aortic arch toward the brain along each side of the

neck. They pass behind the ear into the temporal

lobe and enter the subarachnoid space. They then go to

the circle of Willis where each bifurcates to form two

main branches: the anterior cerebral artery and

the middle cerebral artery. The course of the internal carotid arteries and their branches is tortuous. Because of their many twists and turns these arteries are all subject to blockages caused by cholesterol build-up. For this reason, the internal carotids, and the middle and anterior cerebral arteries are more vulnerable to ischemic rather than hemorrhagic stroke.

The contralateral internal carotid may use the circle of Willis/anterior

communicating artery to supply blood to both pairs of anterior and

middle cerebral arteries. Complete interruption of blood flow in the

anterior cerebral artery is not very likely because the opposite anterior

cerebral can use the anterior communicating artery to make up for the

deficiency.

A patient who has suffered such a blockage might have a paralyzed leg

or be incontinent of bowel and/or bladder. Since the parts of the motor

and sensory strips connected to the arm receive some blood from the

anterior cerebral artery, the arm may also be somewhat affected by this

kind of blockage. Finally, apraxia of gait may also occur if an anterior

cerebral artery fails to supply an adequate amount of blood to the

medial part of the motor strip.

The anterior cerebral arteries also supply blood to the anterior aspects

of the frontal lobes. As these areas are involved in higher level cognition

such as reasoning and judgment, a condition called cerebral dementia

may result from anterior cerebral artery blockages. Confused

language..

Anterior cerebral arteries smaller terminal branch of the ICA enters longitudinal fissure connected to the opposite anterior cerebral artery by

anterior communicating artery (part of the CoW) curves over corpus callosum– central branch supply deep masses of gray matter in the

cerebral hemisphere– Supply:

• cortical branches supply all medial surface of cerebral cortex up to parieto-occipital sulcus

• corpus callosum• approximately 1 inch of the frontal and parietal cortex on the

superior aspect of their lateral surface (this include the leg area of the precentral gyrus)

• anterior portions of the basal ganglia and internal capsule

Middle Cerebral Arteries

These large arteries have tree-like branches that bring blood to most of the lateral cortex of each cerebral hemisphere. This means that the middle cerebral arteries supply blood to the cortical areas involved in speech, language and swallowing runs laterally in the lateral sulcus cortical branches supply entire lateral surface of the

hemisphere EXCEPT area supplied by anterior cerebral artery inferolateral surface supplied by posterior cerebral artery occipital pole

supply all motor area except “leg area” supply parts of the internal capsule and basal ganglia central branches supply deep masses of gray matter within

the cerebral hemisphere

If the left middle cerebral artery is

blocked, aphasia and apraxia may

occur.

Blockages of the right middle cerebral

artery can cause left side neglect,

which is an impairment in the ability to

recognize and respond to stimuli on the

left side of the body, prosopagnosia or

the inability to recognize faces, and

various cognitive problems,

including agnosia. All of these are part

of the right hemisphere syndrome.

pituitary stalk

pituitary stalk

optic chiasm

oculomotor n.

mamillary bodies

abducens n.

• Ophthalmic artery– enters orbit through optic canal, below and lateral to

optic nerve– supplies the eye, including retina and optic nerve

• Posterior communicating artery – runs backward to join posterior cerebral artery at

interpeduncular fossa• deep depression on inferior of midbrain between cerebral

peduncles• part of the circle of willis

Choroidal artery enter inferior horn of lateral ventricle to supply choroid

plexus branches may help supply the optic tract, LGB, internal

capsule and crus cerebri

branch of the 1st part of the subclavian artery Both of the vertebral arteries ascend through the spinal

column and enter the brain through the magnum foramen. Once in the brain, they continue to ascend, traveling beside the brain stem. At the lower border of the pons the two vertebral arteries join together to form the basilar artery or vertebro-basilar artery.

Cranial branches– meningeal arteries– anterior and posterior spinal arteries – posteroinferior cerebellar artery

• largest branch of the vertebral artery and supplies parts of the cerebellum and the dorsolateral portion of the rostral medulla

– medullary arteries• along with posteroinferior cerebellar artery, supply most of the

medulla

at lower border of pons, vertebral arteries unite to form the BASILAR

ARTERY

ascends along the ventral midline of the pons and terminates near the

rostral border of pons by dividing into 2 posterior cerebral arteries

Vertebrobasilar arterial system

posterior circulation of brain

Branches:

branches to pons, cerebellum, internal ear

labyrinthine artery

follows the course of the CN VIII and supplies inner ear

anterior inferior cerebellar artery

supplies part of the pons and the anterior and inferior regions of

the cerebellum

superior cerebellar artery

supplies part of the rostral pons and superior region of the

cerebellum

pontine branches

supply most of pons

Posterior Cerebral Arteries

formed by the terminal bifurcation of the basilar artery

anastomoses with the posterior communicating artery in the CoW

Supply:

lateral surface of the hemisphere – occipital pole and inferior

temporal lobe

medial surface of the hemisphere – occipital lobe and posterior

2/3 of temporal lobe

At the superior border of the pons, the basilar artery divides to

form the two posterior cerebral arteries. These arteries

supply blood to the part of the brain that lies in the posterior

fossa of the skull, including the medial aspects of the occipital

lobes, the inferior portions of the temporal lobes, the

brainstem, and the cerebellum. They also deliver blood to the

thalamus and some other subcortical structures. A vascular

lesion may result in Thalamic Aphasia or Thalamic Syndrome

The basilar and posterior cerebral arteries may hemorrhage

due to shearing injuries caused by accidents or pressure due to

edema of the brain.

Since they provide blood to

the occipital lobes, damage

to these arteries can cause a

variety of visual problems,

including cortical blindness,

which is an inability to read

and visual agnosia, or the

inability to recognize stimuli

presented visually. Because

these arteries supply blood to

the cerebellum and to the

brain stem, blockages or

hemorrhaging there can

cause

either ataxic (cerebellar) or

flaccid (lower motor neuron)

dysarthria..

• located at base of brain

• anterior part lies in front of optic chiasm

• posterior part situated just below the mamillary bodies

• allows for excellent collateral circulation– occlusion of an individual artery

• often completely compensated by collateral circulation through the CoW

Formed by: terminal part of the ICA proximal parts of the anterior, middle and posterior cerebral

arteries anterior and posterior communicating arteries

1 – Vertebral artery

2 – AICA

3 – Basilar artery

4 – Superior cerebellar artery

5 – Posterior cerebral artery

6 – Posterior communicating artery

7 – Middle cerebral artery

8 – ICA

9 – Opthalmic artery

10 – Anterior cerebral artery

11 – Anterior communicating artery

12 – Hypothalamic artery

13 – Anterior choroidal artery

In the Circle of Willis the two internal carotids are joined together by

the anterior communicating artery while the posterior communicating

artery links the internal carotid system with the basilar artery. These

connections make collateral circulation, which Love and Webb define as

"the flow of blood through an alternate route," possible. This is a safety

mechanism, allowing brain areas to continue receiving adequate blood

supply even when there is a blockage somewhere in an arterial system.

When all arteries are functioning normally, their blood supplies will not

mix where they meet in the Circle because the pressure of their streams

will be equal.

As long as the Circle of Willis can maintain blood pressure at fifty

percent of its normal level, no infarction or death of tissue will occur in

an area where a blockage exists. If collateral circulation is good,

sometimes a blockage will have no permanent effects. Sometimes, an

adjustment time is required before collateral circulation can reach a

level that supports normal functioning; the communicating arteries will

enlarge as blood flow through them increases. In such cases, a transient

ischemic attack may occur, meaning that parts of the brain are

temporarily deprived of oxygen.

• veins of the brain have no muscular tissue in the wall and no valves

• Veins – Located in the SAS

• Venous sinuses – located between 2 layers of the Dura

• superficial and deep veins of the brain drain into the dural venous sinuses (which in turn drain into the internal jugular veins)

• Superficial veins– drain the cortex and the more superficial hemispheric

white mater mainly into the superior sagittal and cavernous sinuses

• Deep or internal veins– drain the deep hemispheric white mater and basal

ganglia into the 2 internal cerebral veins which unite to form the great cerebral vein

• Great cerebral vein– formed by the union of 2 internal cerebral veins and

drains into the straight sinus