📚 Central Nervous System Special Senses Module L16 Blood Supply Of The Brain

📚 Lecture Overview

This lecture covers the arterial and venous systems supplying the central nervous system (CNS), detailing the anterior (carotid) and posterior (vertebrobasilar) circulations. It explains how these systems anastomose to form the Circle of Willis and details the clinical consequences of vascular occlusions (strokes). Mastering this material is essential for localizing neurological deficits based on vascular anatomy.

🎯 Key Concepts & Definitions

• Anterior Circulation: The arterial system formed by the internal carotid arteries that supplies supratentorial structures, including the cerebral cortex and diencephalon.
• Posterior Circulation: The arterial system formed by the vertebrobasilar system that supplies the structures within the posterior cranial fossa, including the brainstem and cerebellum.
• Circle of Willis (Circulus Arteriosus): An anastomotic vascular ring at the base of the brain that equalizes blood pressure and provides collateral blood flow.
• Charcot's Artery: A critical central branch of the cerebral circulation (artery of cerebral hemorrhage) that supplies the internal capsule.
• Cerebrovascular Accident (CVA / Stroke): An interruption of blood flow to the brain or brainstem resulting in impaired neurological function lasting more than 24 hours.

📖 Main Content

1. Anterior Circulation: The Carotid System

The anterior circulation is supplied by the internal carotid artery (ICA) and its branches.

• Origin and Course:
  • Arises as one of the two terminal branches of the common carotid artery.
  • Ascends to the base of the skull and passes through the carotid canal.
  • Enters the cranial cavity via the foramen lacerum.
  • Traverses the cavernous sinus and emerges from its roof, lateral to the pituitary gland.
  • Divides below the anterior perforated space into its terminal branches.
• Branches of the ICA:
  1. Ophthalmic artery
  2. Hypophyseal arteries
  3. Anterior choroidal artery
  4. Posterior communicating artery
  5. Anterior cerebral artery (ACA) (Terminal)
  6. Middle cerebral artery (MCA) (Terminal)

Middle Cerebral Artery (MCA)

• The largest and most clinically significant branch of the ICA.
• Central (Deep) Branches: The lateral and medial striate arteries supply the striatum and the internal capsule.
• Cortical Branches: Travel through the lateral fissure to supply most of the lateral (superolateral) surface of the cerebral hemisphere.
• Clinical Note: Occlusion of the deep striate arteries is the primary cause of classic stroke (contralateral hemiplegia and aphasia).

Anterior Cerebral Artery (ACA)

• Winds around the genu of the corpus callosum before dividing.
• Cortical Branches: Supply the medial surface of the frontal and parietal lobes (as far back as the parieto-occipital sulcus) and overlap onto the orbital and lateral surfaces.
• Central Branches: Supply deep structures.

2. Posterior Circulation: The Vertebrobasilar System

The posterior circulation is supplied by the vertebral arteries and the basilar artery.

Vertebral Artery

• Origin: Arises from the first part of the subclavian artery in the neck.
• Course: Passes through the transverse foramina of the C1–C6 vertebrae, enters the skull through the foramen magnum, and runs along the ventral surface of the medulla.
• Termination: The right and left vertebral arteries unite at the lower border of the pons to form the single basilar artery.
• Branches:
  • Anterior spinal artery: Two branches unite to form a single trunk running in the anterior median fissure of the spinal cord.
  • Posterior spinal arteries: Two longitudinal branches descend on each side of the posterior roots of the spinal nerves.
  • Posterior inferior cerebellar artery (PICA): The largest and most tortuous branch of the vertebral artery.
  • Medullary branches: Supply the medulla oblongata.

Basilar Artery

• Origin: Formed by the union of the two vertebral arteries at the lower border of the pons.
• Course: Runs superiorly in the pontine sulcus (basilar groove) on the ventral surface of the pons.
• Termination: Bifurcates at the upper border of the pons into the two posterior cerebral arteries (PCA).
• Branches:
  1. Anterior inferior cerebellar artery (AICA) (supplies the cerebellar tonsil)
  2. Superior cerebellar artery (SCA)
  3. Pontine branches
  4. Labyrinthine (internal auditory) artery
  5. Posterior cerebral artery (PCA) (Terminal)

Posterior Cerebral Artery (PCA)

• Course: Winds around the midbrain to reach the occipital pole, running inside the calcarine sulcus.
• Central Branches: Supply the thalamus, hypothalamus, globus pallidus, midbrain, and geniculate bodies.
• Cortical Branches: Supply the occipital lobe, the medial surface posterior to the calcarine sulcus, and the inferior (tentorial) surface of the cerebrum (except the temporal pole).

3. Summary of Cortical Arterial Territories

4. Circle of Willis (Circulus Arteriosus)

An anastomotic ring located at the base of the brain that connects the carotid and vertebrobasilar systems.

• Anastomotic Components:
  • Anterior communicating artery: Connects the right and left anterior cerebral arteries (ACA).
  • Posterior communicating arteries: Connect the internal carotid artery (ICA) to the posterior cerebral artery (PCA) on each side.
• Key Fact: The Middle Cerebral Arteries (MCA) do NOT form part of the Circle of Willis.
• Function: Equalizes blood pressure between the left and right sides of the brain and provides collateral circulation pathways in case of arterial occlusion.

5. Venous Drainage of the Cerebrum

Venous blood drains from small internal networks into larger cerebral, cerebellar, and brainstem veins, which empty into the dural venous sinuses.

• External Cerebral Veins:
  • Superior cerebral veins: Drain the upper parts of the hemisphere and empty into the superior sagittal sinus.
  • Superficial middle cerebral vein: Runs along the lateral fissure, drains the lateral surface, and empties into the cavernous sinus.
  • Basal (inferior cerebral) vein: Joins the great cerebral vein to drain into the straight sinus.
• Internal Cerebral Veins:
  • Two internal cerebral veins run posteriorly and unite beneath the splenium of the corpus callosum to form the Great Cerebral Vein of Galen, which drains into the straight sinus.

📊 Visual Learning

Diagram 1: Overview of Brain Circulations

Diagram 2: Cerebral Arterial Territories

Diagram 3: Venous Drainage Pathways

💡 Important Points to Remember

• MCA Exclusion: Remember that the Middle Cerebral Artery (MCA) is not a component of the Circle of Willis, even though it is a terminal branch of the internal carotid artery.
• Stroke Cause: The classic stroke presentation (contralateral hemiplegia and aphasia) is most commonly caused by occlusion of the deep, central branches (lateral and medial striate arteries) of the MCA.
• Spinal Artery Origins: The single anterior spinal artery is formed by the union of two branches arising from the vertebral arteries. The posterior spinal arteries remain paired.
• PICA: The Posterior Inferior Cerebellar Artery is a direct branch of the vertebral artery, not the basilar artery.
• AICA and SCA: The Anterior Inferior Cerebellar Artery and Superior Cerebellar Artery arise directly from the basilar artery.
• The Great Vein of Galen: Formed by the union of the two internal cerebral veins beneath the splenium of the corpus callosum.
• Foramen Landmarks:
  • Internal Carotid Artery enters via the foramen lacerum.
  • Vertebral Arteries enter via the foramen magnum.

⚠️ Common Exam Questions

Common Exam Traps & MCQ Tricks

1. The "Which artery is NOT in the Circle of Willis" Trap: Examiners frequently list the Middle Cerebral Artery (MCA) as an option. Students often select a different artery because the MCA is so large and important, but the MCA is excluded from the circle.
2. Functional Localization Questions:
   • Leg/Foot motor and sensory deficits point to an Anterior Cerebral Artery (ACA) stroke (supplying the paracentral lobule on the medial surface).
   • Face/Arm motor and sensory deficits + Aphasia point to a Middle Cerebral Artery (MCA) stroke (supplying the lateral surface/Broca's area).
   • Visual deficits (e.g., affecting the cuneus/calcarine sulcus) point to a Posterior Cerebral Artery (PCA) stroke.
3. Origin Confusions: Examiners often try to trick students by stating that PICA arises from the basilar artery. It does not; PICA arises from the vertebral artery. Conversely, AICA and SCA arise from the basilar artery.

Practice Questions (Based on Lecture Slides)

• Which area is supplied by the anterior cerebral artery? Paracentral lobule (Medial surface).
• Which area is supplied by the middle cerebral artery? Broca's area (Superolateral surface).
• Which area is supplied by the posterior cerebral artery? Cuneus (Occipital lobe, medial surface).

📝 Quick Review Checklist

• [ ] I can trace the path of the internal carotid artery from its origin to its terminal division.
• [ ] I know all the arterial components that form the Circle of Willis.
• [ ] I can identify which cerebral artery supplies the paracentral lobule, Broca's area, and the cuneus.
• [ ] I understand the difference in origin between PICA, AICA, and SCA.
• [ ] I can explain how the Great Cerebral Vein of Galen is formed and where it drains.
• [ ] I can correlate an occlusion of the deep striate branches of the MCA with its clinical presentation (classic stroke).