Anatomy chapters
Blood supply of the brain and spinal cord
The internal carotid and vertebral systems, the circle of Willis, the territories of the cerebral arteries, venous sinuses, the supply of the spinal cord, and the stroke syndromes examiners link to each artery.
- The brain is supplied by the internal carotids (anterior circulation) and the vertebrals and basilar (posterior circulation), linked by the circle of Willis.
- The MCA supplies the lateral surface (face and arm areas, and speech on the dominant side); the ACA supplies the medial surface (leg area).
- The anterior spinal artery supplies the front two thirds of the cord; its blockage spares dorsal column sensation.
On this page
Overview
The brain receives blood from two pairs of arteries:
- Internal carotid arteries: the anterior circulation.
- Vertebral arteries, which join to form the basilar artery: the posterior circulation.
They join at the base of the brain in the circle of Willis. Each artery has a territory, and a block gives a predictable stroke.
Internal carotid artery
It enters the skull through the carotid canal, runs through the cavernous sinus, and ends by dividing into the anterior and middle cerebral arteries.
Its branches inside the skull include the ophthalmic artery, the posterior communicating artery and the anterior choroidal artery.
Vertebral and basilar arteries
Each vertebral artery arises from the subclavian artery, climbs through the foramina transversaria of C6 to C1 and enters the skull through the foramen magnum.
Its branches include the posterior inferior cerebellar artery (PICA) and the anterior and posterior spinal arteries.
The two vertebrals join at the lower border of the pons to form the basilar artery. Its branches are the anterior inferior cerebellar, pontine, labyrinthine, superior cerebellar arteries, and finally the two posterior cerebral arteries.
Circle of Willis
The circle lies in the subarachnoid space around the optic chiasma and pituitary stalk. It is formed by:
- Two anterior cerebral arteries, joined by the anterior communicating artery.
- The two internal carotid arteries.
- Two posterior communicating arteries.
- Two posterior cerebral arteries.
It allows blood to flow from one side to the other or from front to back if a main artery is narrowed slowly. In many people parts of it are small, so it may not protect against a sudden block.
The middle cerebral artery is not part of the circle of Willis.
Territories of the cerebral arteries
| Artery | Supplies | Block causes (opposite side) |
|---|---|---|
| Anterior cerebral | Medial surface of frontal and parietal lobes, corpus callosum | Weakness and numbness of the leg more than the arm; personality change; urinary incontinence |
| Middle cerebral | Most of the lateral surface; deep branches to basal ganglia and internal capsule | Weakness and numbness of the face and arm more than the leg; aphasia if the dominant (usually left) side; neglect if the non-dominant side |
| Posterior cerebral | Occipital lobe, inferior temporal lobe, thalamus | Homonymous hemianopia with macular sparing |
Leg weakness more than arm points to the anterior cerebral artery. Face and arm more than leg, with speech problems, points to the middle cerebral artery.
The macula is often spared in a PCA stroke because the occipital pole also receives some blood from the MCA.
The deep arteries
The lenticulostriate (central) arteries are small branches of the MCA that supply the basal ganglia and internal capsule. They are end arteries and are prone to rupture in high blood pressure.
The lenticulostriate arteries are called the "arteries of stroke". Bleeding or blockage here damages the internal capsule, causing weakness of the whole opposite half of the body.
Arteries of the brainstem and cerebellum
| Artery | Block causes |
|---|---|
| PICA (from vertebral) | Lateral medullary (Wallenberg) syndrome: dizziness, ataxia, hoarseness and difficulty swallowing, loss of pain and temperature on the same side of the face and the opposite side of the body, Horner's syndrome |
| Basilar | Coma, quadriplegia, or "locked-in" syndrome (awake but only eye movements remain) |
| Paramedian branches to the medulla | Medial medullary syndrome: opposite-side weakness and loss of vibration sense, same-side tongue weakness |
Venous drainage
The brain is drained by veins into the dural venous sinuses, which lie between the two layers of dura. They have no valves.
- Superior sagittal sinus: along the upper edge of the falx cerebri; CSF returns to blood here through the arachnoid granulations.
- Inferior sagittal and straight sinuses.
- Transverse and sigmoid sinuses, which drain to the internal jugular vein.
- Cavernous sinuses on each side of the sella turcica.
Blood supply of the spinal cord
- One anterior spinal artery (from the two vertebral arteries) runs down the front of the cord. It supplies the front two thirds.
- Two posterior spinal arteries supply the back third, including the dorsal columns.
- Segmental (radicular) arteries from the vertebral, intercostal and lumbar arteries reinforce them.
The great radicular artery (of Adamkiewicz) is a large segmental artery, usually on the left, in the lower thoracic or upper lumbar region. It supplies much of the lower cord.
Blockage of the anterior spinal artery causes paralysis and loss of pain and temperature below the lesion, but touch, vibration and position sense are spared (dorsal columns).
Surgery on the aorta can block the artery of Adamkiewicz and cause paraplegia.
Clinical relevance
Berry aneurysm and subarachnoid haemorrhage
How it happens: a small aneurysm forms at a branching point on the circle of Willis, most often at the anterior communicating artery, and bursts.
What you see: sudden, very severe headache ("worst ever"), neck stiffness and possible loss of consciousness. A posterior communicating artery aneurysm can press on the oculomotor nerve, causing a dilated pupil and drooping lid.
Haemorrhage in the internal capsule
How it happens: rupture of a lenticulostriate artery in a patient with high blood pressure.
What you see: sudden weakness of the opposite face, arm and leg.
Watershed infarcts
How it happens: severe low blood pressure reduces flow to areas between two arterial territories.
What you see: damage in the border zones, such as between the ACA and MCA areas.
Common angles: the vessels forming the circle of Willis, the territory of each cerebral artery and its stroke pattern, the lenticulostriate arteries and internal capsule, the commonest site of berry aneurysm, lateral medullary syndrome, and anterior spinal artery syndrome.
Quick revision
- Anterior circulation: internal carotids. Posterior: vertebrals and basilar.
- Circle of Willis: ACAs, ACom, ICAs, PComs, PCAs (MCA not part).
- ACA: leg. MCA: face, arm, speech. PCA: vision with macular sparing.
- Lenticulostriate arteries: internal capsule, hypertensive bleeds.
- Commonest berry aneurysm: anterior communicating artery.
- PICA: lateral medullary syndrome.
- Anterior spinal artery: front two thirds of the cord; dorsal columns spared.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
Which arteries form the circle of Willis?
The anterior cerebral, anterior communicating, internal carotid, posterior communicating and posterior cerebral arteries.
Which artery supplies the leg area of the motor cortex?
The anterior cerebral artery.
Where is the commonest site of a berry aneurysm?
At the anterior communicating artery.
What is spared in anterior spinal artery syndrome?
Touch, vibration and position sense, which travel in the dorsal columns supplied by the posterior spinal arteries.