BLOOD SUPPLY OF THE BRAIN AND SPINAL CORD (BOOK SUMMARY)

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I. MEDICAL IMPORTANCE

Cerebrovascular accidents (stroke) – remain the third leading cause of

morbidity and death in the United States

Internal capsule

- contains the major ascending and descending pathways to

the cerebral cortex

- Commonly disrupted by arterial hemorrhage or thrombosis

FIG 1. INTERNAL CAPSULE OF THE BRAIN

II. BLOOD SUPPLY OF THE BRAIN

A. ARTERIES OF THE BRAIN

The brain is supplied by two internal carotid arteries and two

vertebral arteries

Subarachnoid space – where the arteries lie

Circle of Willis – formed by the anastomoses of these four arteries on

the interior surface of the brain

FIG 2. ORIGIN AND COURSES OF THE ICA AND VERTEBRAL ARTERY AS

THEY ASCEND THE NECK TO ENTER THE SKULL

I. INTERNAL CAROTID ARTERIES

• Begins at the bifurcation of the common carotid artery

• Carotid sinus – localized dilation at the bifurcation

FIG 3. BIFURCATION OF THE ICA

ICA Track

1. Passes through the carotid canal of the temporal bone

2. Runs horizontally forward through the cavernous sinus

3. Perforates the dura mater and emerges on the medial side

of the anterior clinoid process

4. Pierces the arachnoid mater and enters the subarachnoid

space

5. Turns posteriorly to the region of the medial end of the

lateral cerebral sulcus

6. Divides into the anterior and middle cerebral arteries

Branches of the Cerebral Portion

1. Ophthalmic Artery

• Arises as the ICA emerges from the carotid sinus

• Enters the orbit through the optic canal below

• Lateral to the optic nerve

• Supplies: eye, other orbital structures

• Terminal branches supply:

o the frontal area of the scalp

o ethmoid and frontal sinus

o dorsum of the nose

2. Posterior Communicating Artery

• Emerges close to the terminal bifurcation of the ICA

• Runs posteriorly to the oculomotor nerve to join the

posterior cerebral artery

• Forms a part of the Circle of Willis

3. Choroidal Artery

• Also emerges close to the terminal bifurcation of the ICA

• Passes posteriorly close to the optic tract

• Enters the inferior horn of the lateral ventricle, ends in the

choroid plexus

• Gives off branches to:

o Crus cerebri

o Lateral geniculate body

o Optic Tract

o Internal Capsule

4. Anterior Cerebral Artery

• Smaller terminal branch of the ICA

• Runs forward and medially, superior to the optic nerve and

enters the longitudinal fissure of the cerebrum

• Joined to the anterior cerebral artery of the opposite side by

the anterior communicating artery

• Curves backward over the corpus callosum, finally

anastomoses with the posterior cerebral artery

• Supplies the “leg” area of the precentral gyrus (see cortical

branches below)

FIG 4. “LEG AREA” OF THE PRECENTRAL GYRUS (10 MOTOR CORTEX)

A. Cortical branches

• Supply the medial surface of the cerebral cortex until

the parietooccipital sulcus

• Also supplies a strip of the cortex 2.5 cm wide on the

adjoining lateral surface

B. Central branches

• A group pierces the anterior perforated substance

• Helps supply parts of the lentiform and caudate nuclei

and the internal capsule

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I. MEDICAL IMPORTANCE

Cerebrovascular accidents (stroke) – remain the third leading cause of morbidity and death in the United States Internal capsule

contains the major ascending and descending pathways to the cerebral cortex
Commonly disrupted by arterial hemorrhage or thrombosis FIG 1. INTERNAL CAPSULE OF THE BRAIN II. BLOOD SUPPLY OF THE BRAIN A. ARTERIES OF THE BRAIN The brain is supplied by two internal carotid arteries and two vertebral arteries Subarachnoid space – where the arteries lie Circle of Willis – formed by the anastomoses of these four arteries on the interior surface of the brain FIG 2. ORIGIN AND COURSES OF THE ICA AND VERTEBRAL ARTERY AS THEY ASCEND THE NECK TO ENTER THE SKULL I. INTERNAL CAROTID ARTERIES
Begins at the bifurcation of the common carotid artery
Carotid sinus – localized dilation at the bifurcation FIG 3. BIFURCATION OF THE ICA ICA Track 1. Passes through the carotid canal of the temporal bone 2. Runs horizontally forward through the cavernous sinus 3. Perforates the dura mater and emerges on the medial side of the anterior clinoid process 4. Pierces the arachnoid mater and enters the subarachnoid space 5. Turns posteriorly to the region of the medial end of the lateral cerebral sulcus 6. Divides into the anterior and middle cerebral arteries Branches of the Cerebral Portion 1. Ophthalmic Artery - Arises as the ICA emerges from the carotid sinus - Enters the orbit through the optic canal below - Lateral to the optic nerve - Supplies: eye, other orbital structures - Terminal branches supply: o the frontal area of the scalp o ethmoid and frontal sinus o dorsum of the nose 2. Posterior Communicating Artery - Emerges close to the terminal bifurcation of the ICA - Runs posteriorly to the oculomotor nerve to join the posterior cerebral artery - Forms a part of the Circle of Willis 3. Choroidal Artery - Also emerges close to the terminal bifurcation of the ICA - Passes posteriorly close to the optic tract - Enters the inferior horn of the lateral ventricle, ends in the choroid plexus - Gives off branches to: o Crus cerebri o Lateral geniculate body o Optic Tract o Internal Capsule 4. Anterior Cerebral Artery - Smaller terminal branch of the ICA - Runs forward and medially, superior to the optic nerve and enters the longitudinal fissure of the cerebrum - Joined to the anterior cerebral artery of the opposite side by the anterior communicating artery - Curves backward over the corpus callosum, finally anastomoses with the posterior cerebral artery - Supplies the “leg” area of the precentral gyrus (see cortical branches below) FIG 4. “LEG AREA” OF THE PRECENTRAL GYRUS (1^0 MOTOR CORTEX) A. Cortical branches
Supply the medial surface of the cerebral cortex until the parietooccipital sulcus
Also supplies a strip of the cortex 2.5 cm wide on the adjoining lateral surface B. Central branches
A group pierces the anterior perforated substance
Helps supply parts of the lentiform and caudate nuclei and the internal capsule

The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. FIG 5. INTERNAL CAPSULE ANATOMY The lentiform nucleus lies beneath the insula. It is said to resemble a lens (Latin: lentiform, lens-shaped) but is best regarded as a cone with the base underlying the insula and a blunt apex pointing towards the midline. FIG 6. LENTIFORM NUCLEUS AND RELATED STRUCTURES

5. Middle Cerebral Artery - Largest branch of the ICA - Runs laterally in the lateral cerebral sulcus - Supplies all the motor area except the “leg area” (see cortical branches below) FIG 7. CORONAL SECTION OF THE CEREBRAL HEMISPHERES SHOWING THE ARTERIAL SUPPLY TO THE DEEP CEREBRAL STRUCTURES FROM THE MIDDLE CEREBRAL ARTERY A. Cortical branches

Supply the entire lateral surface of the hemisphere except for: o the narrow strip supplied by the anterior cerebral artery o occipital pole o inferolateral surface of the hemisphere (supplied by the posterior cerebral artery) B. Central branches
Enter the anterior perforated substance
Supply the lentiform and caudate nuclei and the internal capsule (yes, too) II. VERTEBRAL ARTERY FIG 7. SUBCLAVIAN ARTERIES AND THEIR BRANCHES
Branch of the first part of the subclavian artery
Passes through the foramina in the transverse processes of C1-C
Enters the skull through the foramen magnum
Pierces the dura and arachnoid mater to enter the subarachnoid space
Passes through the medulla oblongata
Joins the vessel of the opposite side to form the basilar artery at the lower border of the pons FIG 8. CIRCLE OF WILLIS

FIG 12. AREAS SUPPLIED BY CEREBRAL ARTERIES; BLUE: ANT CEREBRAL ARTERY; PINK: MIDDLE CEREBRAL ARTERY; BROWN: POSTERIOR CEREBRAL ARTERY IV. CIRCLE OF WILLIS

Lies in the interpeduncular fossa at the base of the brain
Formed from the anastomosis between 2 ICAs and 2 vertebral arteries
Arteries contributing to the circle: o Anterior communicating artery o Anterior cerebral artery o ICA o Posterior communicating artery o Posterior cerebral artery o Basilar artery
Allows blood that enters either thru ICA or vertebral arteries to be distributed to any part of both cortical hemispheres
Has cortical and central branches that supply the brain substance
Arteries making up this circle vary in size (common) o Absence of one or both posterior communicating arteries have been reported **V. ARTERIES TO SPECIFIC BRAIN AREAS

Corpus Striatum and Internal Capsule**

Middle cerebral artery – medial and lateral striate central branches are the main supply
Anterior cerebral artery – supplies the rest 2. Thalamus
Posterior communicating artery
Basilar artery
Posterior cerebral artery 3. Midbrain
Basilar artery
Posterior cerebral artery
Superior cerebellar artery 4. Pons
Basilar artery
Anterior, Inferior, Superior cerebellar arteries 5. Medulla Oblongata
Vertebral arteries
Anterior and posterior spinal arteries
Posterior inferior cerebellar arteries
Basilar arteries 6. Cerebellum
Superior cerebellar artery
Anterior inferior cerebellar artery
Posterior inferior cerebellar artery

V. NERVE SUPPLY OF CEREBRAL ARTERIES

Rich supply of sympathetic, postganglionic nerve fibers
From superior cervical sympathetic ganglion
Stimulation causes vasoconstriction of the cerebral arteries o Note that under normal conditions, local blood flow is mainly controlled by carbon dioxide, hydrogen ion, oxygen concentrations present in the nervous tissue o Vasodilation from: ▪ ↑ [CO 2 ], [H+] ▪ ↓ Oxygen tension B. VEINS OF THE BRAIN
Have no muscular tissue in their very thin walls
No valves
Emerge from the brain and lie in the subarachnoid space
Pierce the arachnoid mater and meningeal layer of the dura and drain into the cranial venous sinuses **FIG 13. VEINS OF THE BRAIN I. EXTERNAL CEREBRAL VEINS

Superior Cerebral Veins**

Pass upward over the lateral surface of the cerebral hemisphere
Empty into the superior sagittal sinus 2. Superficial Middle Cerebral Vein
Drains the lateral surface of the cerebral hemisphere
Runs inferiorly in the lateral sulcus
Empties into the cavernous sinus 3. Deep Middle Cerebral Vein
Drains the insula
Joined by the anterior cerebral and striate veins to form the basal vein o Basal vein ultimately joins the great cerebral vein which then drains into the straight sinus II. INTERNAL CEREBRAL VEINS
There are two internal cerebral veins which are formed by the union of the thalamostriate vein and the choroid vein at the interventricular foramen
Run posteriorly in the tela choroidea of the third ventricle and unite beneath the splenium of the corpus callosum to form the great cerebral vein which empties into the straight sinus as well **III. VEINS OF SPECIFIC BRAIN AREAS

Midbrain**

Drained by veins that open into the basal or great cerebral veins 2. Pons
Drained by veins that open into the basal vein, cerebellar veins, or neighboring venous sinuses

3. Medulla Oblongata - Drained by veins that open into the spinal veins and neighboring venous sinuses 4. Cerebellum - Drained by veins that empty into the great cerebral vein or adjacent venous sinuses FIG 14. MORE DETAILED VENOUS SYSTEM OF THE BRAIN C. BRAIN CAPILLARIES - Capillary blood supply to the brain is greater in the gray matter than in the white matter o Metabolic activity is greater in the neuronal cell bodies in the gray matter is much greater than the nerve processes in the white matter - Blood-brain barrier o Isolates brain tissue from the rest of the body o Formed by tight junctions that exist between endothelial cells in the capillary beds D. CEREBRAL CIRCULATION - Brain supplied from 2 ICA and 2 vertebral arteries - Blood supply to half of the brain provided by ICA and vertebral arteries on that side and their respective streams come together through the posterior communicating artery o Point where pressure is equal and they do not mix o Two streams of blood from the vertebral arteries remain separate and on the same side of the lumen of the basilar artery and do not mix FIG 15. CIRCLE OF WILLIS SHOWING DISTRIBUTION OF BLOOD FROM THE 4 MAIN ARTERIES Occlusion - Blood passes forward or backward at that point to compensate for the reduction of blood flow - Blood flow can also pass through the midline thru the arterial circle - Once the arteries enter the brain substance, no further anastomoses occur o Circle of Willis anastomose on the surface of the cerebral hemispheres Arterial blood pressure - most important factor in forcing blood through the brain - opposed by: o ↑ intracranial pressure o ↑ blood viscosity o ↓ vascular diameter Cerebral blood flow (50-60 mL/100 g brain/ min) - remains constant despite changes in general blood pressure - autoregulation accomplished by a compensatory: o lowering of the cerebral vascular resistance when arterial pressure is decreased o raising of the vascular resistance when the arterial pressure is increased - Measured by intracarotid injection or inhalation of radioactive Xe or Kr NOTE: AUTOREGULATION DOES NOT MAINTAIN ADEQUATE BLOOD FLOW WHEN BLOOD PRESSURE FALLS TO A VERY LOW LEVEL Diameter of cerebral blood vessels - main factor contributing to cerebral vascular resistance - innervated by cervical postganglionic sympathetic fibers and respond to norepi o play little to no part in control of cerebral vascular resistance in normal human beings - most powerful vasodilator influence is an increase in CO 2 or H+^ conc, ↓ O 2 conc also causes vasodilation III. BLOOD SUPPLY OF THE SPINAL CORD A. ARTERIES OF THE SPINAL CORD - From 3 small arteries: o 2 posterior spinal arteries o Anterior spinal artery - Longitudinally running arteries reinforced by small segmentally arranged arteries that arise from arteries outside the vertebral canal and entre the vertebral canal thru the intravertebral foramina - Anastomose on the surface of the cord - Has variations I. POSTERIOR SPINAL ARTERIES - Arise either directly from vertebral arteries inside the skull or indirectly from the posterior cerebellar arteries - Descends on the posterior surface of the spinal cord close to the posterior nerve roots - Branches enter the substance of the cord - Supply the posterior 1/3 of the spinal cord - Small in the upper thoracic region o First three especially vulnerable to ischemia (due to occlusion) FIG 16. ARTERIAL SUPPLY OF THE SPINAL CORD; B. TRANSVERSE SECTION OF THE SPINAL CORD SHOWING SEGMENTAL SPINAL ARTERIES II. ANTERIOR SPINAL ARTERIES - Formed by union of two arteries - Arises from vertebral arteries from the inside the skull - Descends on the anterior surface of the spinal cord within the anterior median fissure - Branches enter the substance of the cord - Supply the anterior 2/3 of the spinal cord - Upper and lower thoracic segments of the spinal cord o Anterior spinal artery may be extremely small

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I. MEDICAL IMPORTANCE

V. NERVE SUPPLY OF CEREBRAL ARTERIES

V. APPENDIX