Cerebrovascular system

The cerebrovascular system comprises the vessels that transport blood to and from the brain. The brain's arterial supply is provided by a pair of internal carotid arteries and a pair of vertebral arteries, the latter of which unite to form the basilar artery. The anterior cerebral artery, a branch of the internal carotid artery, perfuses the anteromedial cerebral cortex; the middle cerebral artery, a branch of the internal carotid artery, perfuses the lateral cerebral cortex; and the posterior cerebral artery, a branch of the basilar artery, perfuses the medial and lateral portions of the posterior cerebral cortex. The internal carotid arteries, the anterior cerebral arteries, and the posterior cerebral arteries anastomose through the anterior and posterior communicating arteries to form the circle of Willis, a vascular circuit surrounding the optic chiasm and pituitary stalk. The circle of Willis equalizes the blood flow between the cerebral hemispheres and provides anastomotic circulation, connecting the anterior and posterior cerebral circulations and, thereby, permitting continued perfusion of the brain in the event of carotid occlusion. The cerebral hemispheres are drained by superficial cerebral veins (superior cerebral veins, middle cerebral veins, inferior cerebral veins) and deep cerebral veins (great cerebral vein, basal vein), which drain into the dural venous sinuses. Brain perfusion is regulated by the partial pressure of carbon dioxide (PaCO₂). The interruption of perfusion due to occlusion or hemorrhage of the cerebral vessels results in a stroke, which manifests with focal neurologic deficits in the body parts controlled by the affected brain territory.

The arterial supply of the brain is provided by the internal carotid arteries and the vertebral arteries, which derive from branches of the aortic arch.

Internal carotid arteries (ICA)

• Description: terminal branches of the common carotid artery
• Course
  • Neck: lie within the carotid sheath and enter the cranium through the carotid canal
  • Cranium: lie on the roof of the cavernous sinus, in close proximity to CN VI
• Branches: only have intracranial branches
  • Caroticotympanic artery (the first intracranial branch of the internal carotid arteries that enters the middle ear cavity and anastomoses with the inferior tympanic artery)
  • Artery of the pterygoid canal (Vidian artery)
  • Meningohypophyseal trunk (posterior trunk)
  • Inferolateral trunk
  • Ophthalmic artery
  • Superior pituitary artery
  • Posterior communicating artery: anastomosis in Circle of Willis
  • Anterior choroidal artery
  • Terminal branches
    • Middle cerebral artery: supplies lateral cerebrum
    • Anterior cerebral artery: supplies anterior cerebrum

Vertebral arteries

• Description: arise from the subclavian arteries
• Course
  • Neck: The vertebral arteries pass through the foramina in the transverse processes of the cervical vertebrae of C1–C6 (C7 transverse foramina transmit only veins) and enter the cranium through the foramen magnum.
  • Cranium: The right and left vertebral arteries unite at the midline of the anterior surface of the pons to form the basilar artery.
• Branches
  • Anterior spinal artery and posterior spinal arteries: supply spinal cord
  • Posterior inferior cerebellar arteries (PICA): terminal branches that supply inferior cerebellum

Basilar artery

• Description: formed by the union of the two vertebral arteries
• Course: runs cranially in the central pontine groove
• Branches
  • Anterior inferior cerebellar artery (AICA): supplies anteroinferior cerebellum, middle cerebellar peduncle, and inferolateral pons
    • Labyrinthine artery: supplies the inner ear
  • Pontine arteries: supply the pons
  • Superior cerebellar artery (SCA): supplies the superior surface of the cerebellum, superior and middle cerebellar peduncles, and lower midbrain
  • Posterior cerebral artery: supplies occipital cortex, inferior and medial temporal cortices, thalamus, and rostral midbrain

• Definition: a vascular circuit formed by the anastomoses between branches of the internal carotid arteries (anterior circulation) and vertebral arteries (posterior circulation) around the optic chiasm and pituitary stalk
• Consists of paired:
  • Internal carotid arteries (proximal to the origin of the middle cerebral arteries)
  • Anterior cerebral arteries (terminal branches of the internal carotids)
  • Posterior cerebral arteries (terminal branches of the vertebral arteries)
• Two anastomoses
  • Anterior communicating artery: connects the two anterior cerebral arteries
  • Posterior communicating artery (branch of internal carotid artery): connects the ICA to the posterior cerebral artery
• Functional significance
  • Equalizes arterial flow between the cerebral hemispheres
  • Provides anastomotic circulation, connecting the anterior and posterior cerebral circulations and, thus, permitting continued perfusion of the brain in the event of carotid occlusion.
• Clinical significance
  • Berry aneurysms
    • Saccular cerebral aneurysms that most commonly occur in the anterior circulation of the brain, usually at the junction of the anterior cerebral artery and the anterior communicating artery in the circle of Willis.
    • Most common cause of nontraumatic subarachnoid hemorrhage
  • Subclavian steal syndrome
  • Moyamoya disease

Most saccular cerebral aneurysms, also known as berry aneurysms, occur in the anterior circulation of the brain, usually at the junction of the anterior cerebral artery and the anterior communicating artery in the circle of Willis. They are the most common cause of nontraumatic subarachnoid hemorrhage.

The cerebral hemispheres are drained by superficial and deep cerebral veins, which drain into the dural venous sinuses.

Superficial cerebral veins

Deep cerebral veins

Deep cerebral veins drain the cerebral medulla and drain into the straight sinus.

• Medullary veins: drain the gray matter
• Subependymal veins: receive blood from the medullary veins
• Basal vein (vein of Rosenthal): paired paramedian veins that receive blood from the temporal lobe and drain into the great cerebral vein
• Great cerebral vein (vein of Galen): receives blood from the deep veins

Dural venous sinuses

• The dural venous sinuses drain blood from cerebral veins and CSF from the arachnoid granulations into the internal jugular vein.
• The sinuses are located intracranially between the two layers of dura mater (endosteal layer and meningeal layer).

Cerebral veins run in the subarachnoid space, have no valves to allow bidirectional blood flow, and have no muscular layer in the vessel wall.

• Neurometabolic coupling: enables adequate delivery of nutrients or substrates to areas with increased neural activity
• Neurovascular coupling (functional hyperemia): enables adequate delivery of oxygen through constriction or dilation of microvessels to areas with increased neural activity

Therapeutic hyperventilation reduces pCO₂ → ↓ cerebral blood flow → ↓ intracranial pressure (e.g., used when patients with acute cerebral edema are unresponsive to other treatments).

• Arterial conditions
  • Stroke
  • Cerebral aneurysms
  • Subarachnoid hemorrhage
  • Epidural hematoma
  • Intraparenchymal hemorrhage
  • Trigeminal neuralgia (compression of superior cerebellar artery)
  • Hydrostatic cerebral edema (e.g., in severe hypertension)
• Venous conditions
  • Cerebral venous thrombosis
  • Cavernous sinus syndrome
  • Subdural hematoma (caused by ruptured bridging veins)