002 Blood Supply of the Central Nervous System

Question 1

• describe the concept of an end-artery organ, and explain why the brain is an end-artery organ

Answer 1

 Each specific region of the organ gets all of its blood from a single artery
 There is little anastomosis (connecting) of blood vessels within the organ

Question 2

• describe the internal carotid system of blood supply for the brain

Answer 2

o Internal carotid arteries supply most of the forebrain (except the occipital lobes)
o Branches from the common carotid artery at the upper border of the thyroid cartilage
o ICA enters the skull via the carotid canal in the petrous part of the temporal bone
 The portion located here is known as the petrous or intraosseous portion of the ICA
o Once inside the skull it undergoes a series of bends known as the carotid siphon
o The ICA gives off several important arteries before it splits into its terminal branches

Question 3

Branches of the ICA in the brain

Answer 3

 Hypophyseals
• Several small branches that arise from the cavernous portion to supply the pituitary gland
• Help form the hypothalamic-hypophyseal portal system
 Ophthalmic
• Usually arises just after the ICA exits the dura
• Enters the orbit to supply structures of the eye
 Posterior communicating
• Usually arises just before the terminal bifurcation of ICA
• Runs posteriorly to join the posterior cerebral artery to help form the circle of Willis
 Anterior choroidal
• Usually arise just before the terminal bifurcation of ICA
• Supplies
o Choroid plexus of the lateral ventricle (which forms CSF)
o Optic tract
o Hippocampus
o Globus pallidus (of basal ganglia)
o Lateral Geniculate nucleus

Question 4

Terminal Branches of the ICA

Answer 4

 Anterior cerebral (smaller)
• Runs anteriorly and slightly medially above the optic nerve
• Ascends in the longitudinal fissure, bends around the genu of the corpus callosum and travels posteriorly along the upper border of the corpus callosum
• Branches:
o Anterior communicating
o Many unnamed cerebral arteries
• Supplies
o Superolateral and entire medial portions of the frontal lobe
o Superolateral and entire medial portions of the parietal lobe
o Corpus callosum
 Middle cerebral (larger)
• Runs laterally then turns posteriorly to run in the lateral (Sylvian) fissure between the frontal/parietal and temporal lobes
• Branches (many unnamed)
o Frontals
o Parietals
o Temporals
• Supplies
o Inferolateral portion of the frontal lobe
o Inferolateral portion of the parietal lobe
o Superolateral and entire anterior portions of the temporal lobe

Question 5

Carotid Siphon

Answer 5

o First, there is an anterior bend
 Here, the ICA runs inside of the cavernous sinus (sometimes it is known as the cavernous portion)
o Second, there is a superior bend
 Occurs in the medial side of the anterior clinoid process
 Here, the ICA pierces the dura to exit the cavernous sinus
o Third, there is a posterior bend
 Here, the ICA runs beneath the optic nerve
 Sometimes known as the cranial or cerebral portion
o Fourth, there is another superior bend
 Here, the ICA runs lateral to the optic chiasm

Question 6

• describe vertebral (vertebrobasilar) system of blood supply for the brain

Answer 6

o supplies mostly the upper spinal cord, brain stem, cerebellum and occipital lobe
o Arises from the subclavian artery
o Ascends in the neck through the vertebral foramina C6-C1 and enters the skull through the foramen magnum
o Each vertebral artery ascends in the cranial vault on the anterolateral aspect of the medulla
o Each vertebral artery joins to form a single midline structure on the surface of the pons – forms the basilar artery
o Branches of vertebral artery
 Anterior Spinal artery
• Supplies the anterior aspect of spinal cord and runs in the ventral median fissure
 Posterior Spinal artery
• Supplies the posterior lateral aspect of the spinal cord
 Posterior Inferior Cerebellar Artery (PICA)
• Supplies the posterior and inferior portions of the cerebellum and the choroid plexus of the 4th ventricle

Question 7

What does the anterior spinal artery supply?

Answer 7

• Supplies the anterior aspect of spinal cord and runs in the ventral median fissure

Question 8

What does the posterior spinal artery supply?

Answer 8

• Supplies the posterior lateral aspect of the spinal cord

Question 9

What does the posterior inferior cerebellar artery supply

Answer 9

• Supplies the posterior and inferior portions of the cerebellum and the choroid plexus of the 4th ventricle

Question 10

• describe branches of the basilar artery and their targets

Answer 10

 Anterior Inferior Cerebellar Artery (AICA)
• Supplies the anterior and inferior portions of the cerebellum
 Internal Auditory Artery (AKA Labyrinthine A. or Acoustic A.)
• Supplies the inner ear
 Pontine arteries
• Several unnamed branches that supply the pons
 Superior Cerebellar artery
• Supplies the superior portion of the cerebellum
 Posterior Cerebral Artery (Terminal branch)
• Receives the posterior communicating artery
• Arises in the interpeduncular cistern then curves around the midbrain
• Then, it runs posteriorly to run in the longitudinal fissure
• Branches (Unnamed)
o Temporals
o Parieto-occipitals
 Calcarine artery supplies the primary visual cortex in the occipital lobe
• Supplies
o Superolateral, entire medial and entire inferior portions of the occipital lobe
o Medial and inferior portions of the temporal lobe (except the most anterior portions

Question 11

What does AICA supply

Answer 11

• Supplies the anterior and inferior portions of the cerebellum

Question 12

What does Internal Auditory artery supply

Answer 12

inner ear

Question 13

What do the pontine arteries supply

Answer 13

• Several unnamed branches that supply the pons

Question 14

What does the superior cerebellar artery supply?

Answer 14

• Supplies the superior portion of the cerebellum

Question 15

describe the cerebral artery supply?

Answer 15

It is the terminal branch
• Receives the posterior communicating artery
• Arises in the interpeduncular cistern then curves around the midbrain
• Then, it runs posteriorly to run in the longitudinal fissure
• Branches (Unnamed)
o Temporals
o Parieto-occipitals
 Calcarine artery supplies the primary visual cortex in the occipital lobe
• Supplies
o Superolateral, entire medial and entire inferior portions of the occipital lobe
o Medial and inferior portions of the temporal lobe (except the most anterior portions

Question 16

• explain the clinical manifestations of infarcts from each of the main cerebral arteries

Answer 16

o Infarcts of anterior cerebral artery
 Loss of blood flow to
• Superolateral and entire medial portions of the frontal lobe
• Superolateral and entire medial portions of the parietal lobe
• Corpus callosum
o Infarcts of Middle Cerebral Artery
 Loss of blood flow to
• Inferolateral portion of the frontal lobe
• Inferolateral portion of the parietal lobe
• Superolateral and entire anterior portions of the temporal lobe
o Infarcts of Posterior Cerebral Artery
 Loss of blood flow to
• Superolateral, entire medial and entire inferior portions of the occipital lobe
• Medial and inferior portions of the temporal lobe (except the most anterior portions

Question 17

Circle of Willis

Answer 17

o	Large arterial anastomosis found on the inferior aspect of the brain
o	Formed by
	Anterior communicating A
	Anterior cerebral A
	Internal carotid A
	Posterior communicating A
	Posterior cerebral A
o	***It is not uncommon to have an incomplete circle of Willis

Question 18

• describe the venous drainage of the brain

Answer 18

o Two groups of named veins
 External cerebral veins
• Run between the arachnoid and dura mater on the outside of the brain
• Drain mostly the lateral surface of the cerebral hemispheres
• Empty into dural venous sinuses
• Injury to these causes subdural hematoma
 Internal Cerebral Veins
• Form on the lateral aspects of the lateral ventricles
• More on the inside of the brain
• Drain the internal structures of the forebrain
• Unite to form the great cerebral vein of Galen (AKA vena magna cerebri)
• Then dumps into straight sinus

Question 19

Describe the dural venous sinuses

Answer 19

 Formed by the meningeal and periosteal dura that separate to form blood channels
 Line with endothelium like blood vessels, but do not have the tunica media, intima nor adventitia, so they are NOT blood vessels
 All of them are specifically named
• Superior Sagittal Sinus
o Contains arachnoid villi for recycling CSF
o Empties (mostly) into the right transverse sinus
• Inferior Sagittal Sinus
o Empties into the straight sinus
• Straight Sinus (AKA rectus sinus)
o Receives blood from:
 the great cerebral vein of Galen
 inferior sagittal sinus
• Occipital Sinus
o Empties (mostly) into the left transverse sinus
• Confluence of Sinuses (AKA torcular herophili)
o The joining of several sinuses at the internal occipital protruberance
o There is usually a flap of tissue that directs most of the blood in specific directions
• Transverse Sinuses (right and left)
• Sigmoid Sinus
• Superior Petrosal Sinus
• Inferior Petrosal Sinus

• Cavernous Sinus
o Intercavernous channels connect the right and left cavernous sinuses in the region of the diaphragm sellae
 Cavernous sinus + intercavernous channels = circular sinus
o Receive blood from
 Superior ophthalmic vein
 Inferior ophthalmic vein
• Only some, because some drains into the pterygoid venous plexus
 Sometimes the central retinal vein
 Superficial cerebral veins
o Empties into
 Superior petrosal sinus
 Inferior petrosal sinus
• Basilar Sinus
o Network of small venous sinuses
o Located on the basilar portion of the occipital bone just posterior to the cavernous sinus
o Connects the cavernous sinuses and the inferior petrosal sinuses

Question 20

Superior Sagittal sinus

Answer 20

o	Contains arachnoid villi for recycling CSF
o	Empties (mostly) into the right transverse sinus

Question 21

Inferior Sagittal Sinus

Answer 21

empties into the straight sinus

Question 22

Straight Sinus

Answer 22

AKLA rectus sinus
o Receives blood from:
 the great cerebral vein of Galen
inferior sagittal sinus

Question 23

Occipital sinus

Answer 23

o Empties (mostly) into the left transverse sinus

Question 24

Confluence of Sinuses

Answer 24

(AKA torcular herophili)
o The joining of several sinuses at the internal occipital protruberance
o There is usually a flap of tissue that directs most of the blood in specific directions

Question 25

Cavernous Sinus

Answer 25

o Intercavernous channels connect the right and left cavernous sinuses in the region of the diaphragm sellae
 Cavernous sinus + intercavernous channels = circular sinus
o Receive blood from
 Superior ophthalmic vein
 Inferior ophthalmic vein
• Only some, because some drains into the pterygoid venous plexus
 Sometimes the central retinal vein
 Superficial cerebral veins
o Empties into
 Superior petrosal sinus
 Inferior petrosal sinus

Question 26

Basilar sinus

Answer 26

o Network of small venous sinuses
o Located on the basilar portion of the occipital bone just posterior to the cavernous sinus
o Connects the cavernous sinuses and the inferior petrosal sinuses

Question 27

• describe the emissary veins and their clinical importance

Answer 27

o True veins
o Connect venous sinuses with veins outside the cranium
o Have no valves, so blood flow depends on pressure gradients
 Usually blood flows from brain to scalp
o These veins are a possible route that infection to a superficial scalp wound can spread to the cranial vault
 Can cause
• Meningitis – infection of the meninges
• Encephalitis – infection of the brain

Question 28

• describe the blood brain barrier

Answer 28

o the boundary between the CNS parenchyma and blood vessels
o certain substances can enter other tissues from the blood but CANNOT do so in the CNS
o Limits the exchange of material between the blood and the CNS
o Factors that lead to low permeability
 Capillaries in the CNS are continuous
• Have tight junctions between endothelial cells so all material must pass through the cells rather than between
• NO fenestrations so there are no thinner parts of the walls
 Few pinocytotic vesicles in the endothelial cells of capillaries in the CNS
• so they seem to transport less materials
 Astrocytic end feet
• Known as perivascular feet
• Surround the basal lamina/basement membrane of the endothelium
• Add an extra layer of control of diffusion of material
 Pericytes
• Located outside the capillary endothelium but inside the basement membrane
• Play a role in the development of the blood-brain barrier
• Help regulate its permeability
o Molecules that CAN pass through the barrier
 Most lipids
 Glucose (facilitated diffusion)
 Dissolved gasses (O2 and CO2)

Question 29

o Factors that lead to low permeability in the bbb?

Answer 29

 Capillaries in the CNS are continuous
• Have tight junctions between endothelial cells so all material must pass through the cells rather than between
• NO fenestrations so there are no thinner parts of the walls
 Few pinocytotic vesicles in the endothelial cells of capillaries in the CNS
• so they seem to transport less materials
 Astrocytic end feet
• Known as perivascular feet
• Surround the basal lamina/basement membrane of the endothelium
• Add an extra layer of control of diffusion of material
 Pericytes
• Located outside the capillary endothelium but inside the basement membrane
• Play a role in the development of the blood-brain barrier
• Help regulate its permeability

Question 30

Molecules that can pass through the bbb?

Answer 30

 Most lipids
 Glucose (facilitated diffusion)
 Dissolved gasses (O2 and CO2)

Question 31

• list regions in the CNS that lack the blood-brain barrier

Answer 31

o all the regions collectively known as Circumventricular organs
 most have some sort of neuroendocrine function
o Median eminence (hypothalamus)
 Associated with pituitary function
o Pineal gland
o Area postrema
 Area at the obex of the medulla that appears to be the trigger for the vomit reflex

Question 32

• describe the blood supply of the spinal cord

Answer 32

o 3 main sources
 Anterior Spinal Artery
• Unpaired, midline
• Formed by the branches from both vertebral arteries
• Descends in the ventral median fissure
 Posterior Spinal Artery
• Direct branch of the vertebral artery
• Descends in the posterolateral aspect of the spinal cord
 Segmental Arteries
• Provides the majority of blood to the spinal cord
• Enter the spinal canal through each intervertebral foramen throughout the length of the spinal column
• Reinforce the anterior and posterior spinal arteries

Question 33

Veous Drainage of the spinal cord

Answer 33

 Internal vertebral venous plexus
• Group of veins found in the epidural space within the spinal canal
• Drain blood out mostly through segmental veins that run through the intervertebral foramina
• NO valves (route of spread of infections)
 External vertebral venous plexus
• Located on the outside of the vertebrae
• Receives blood from the internal vertebral venous plexus and the vertebrae
• Eventually drains into systemic circulation