Blood supply of the CNS

Comprehensive Notes on the Blood Supply of the Central Nervous System

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1. Arterial Inflow to the Brain & The Circle of Willis

The brain is a highly metabolically active organ. Despite accounting for only about 2% of total body weight, it receives 15-20% of the body's cardiac output and consumes 20% of its oxygen. This massive demand is met by two major arterial systems that converge at the base of the brain.

A. The Anterior Circulation (Internal Carotid System)

The anterior circulation supplies the majority of the cerebral hemispheres, specifically the frontal, parietal, and lateral temporal lobes, as well as deep structures like the basal ganglia.

• Pathway from the Heart: Aortic Arch → Brachiocephalic trunk (on the Right) / Left Common Carotid (on the Left) → Common Carotid Artery (CCA).
• Bifurcation: The CCA bifurcates at the C3-C4 vertebral level into the External Carotid (supplying the face/neck) and the Internal Carotid Artery (ICA).
• Distribution: The ICA enters the skull and ultimately bifurcates into the Anterior Cerebral Artery (ACA) (supplying the medial hemisphere) and the Middle Cerebral Artery (MCA) (supplying the lateral hemisphere).

The Four Segments of the Internal Carotid Artery (ICA)

B. The Posterior Circulation (Vertebrobasilar System)

The posterior circulation supplies the brainstem, cerebellum, occipital lobes, and inferior temporal lobes.

• Pathway: Subclavian Artery → Vertebral Artery → Ascends through the transverse foramina of cervical vertebrae (C6-C1) → Enters the skull via the Foramen Magnum.
• The Basilar Artery: The two vertebral arteries merge at the pontomedullary junction to form the single Basilar Artery, which runs up the front of the pons.
• Distribution: The basilar artery bifurcates at the midbrain into the two Posterior Cerebral Arteries (PCA). These join the Circle of Willis via the Posterior Communicating Arteries (PCoA).

C. The Circle of Willis

The Circle of Willis is a ring of vessels at the base of the brain that connects the anterior and posterior circulations, as well as the left and right sides of the brain.

Components of the Circle:

• Anterior Communicating Artery (AComm): Connects the left and right ACAs. This is the most common site for berry aneurysms.
• Anterior Cerebral Artery (ACA): Supplies the medial cerebral hemispheres.
• Internal Carotid Artery (ICA): The main input for the anterior system.
• Posterior Communicating Artery (PComm): The vital bridge connecting the ICA (anterior) to the PCA (posterior).
• Posterior Cerebral Artery (PCA): Formed from the basilar artery; supplies the occipital lobe.
• Basilar Artery: (While technically the input, it is the foundation of the posterior portion of the circle).

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2. Cerebral Cortical Territories & Watershed Zones

Understanding which artery supplies which part of the brain cortex is essential for diagnosing strokes based on a patient's physical symptoms. This relies heavily on the "motor and sensory homunculus"—the map of the body on the brain's surface.

Arterial Territories & Functional Correlates

Watershed Zones (Border Zones)

Watershed zones are the regions at the very periphery of arterial territories where the most distal branches of two different major arteries meet.

• Why are they vulnerable? Because they represent the absolute end of the line for blood flow, they have the lowest perfusion pressure. During states of severe systemic hypotension (e.g., massive blood loss, cardiac arrest, or severe shock), the brain shunts blood to core territories first to survive. The distal watershed areas dry up first, leading to ischemic infarcts.
• ACA-MCA Watershed: Located high on the lateral convexity. Infarct causes proximal arm and shoulder weakness (classic "man in a barrel" syndrome where the trunk and proximal limbs are paralyzed but hands and face are spared).
• MCA-PCA Watershed: Located in the posterior temporal-parietal-occipital junction. Infarct causes complex higher-order visual and language processing deficits.
• Internal Border Zones: Deep white matter zones situated between the deep penetrating lenticulostriate arteries and the superficial cortical branches of the MCA.

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3. Deep Structures, Ventricles & Brainstem Perfusion

While the large cortical branches supply the surface, tiny, fragile penetrating arteries dive straight into the brain tissue to supply the critical relay stations (basal ganglia, thalamus, internal capsule) and the life-sustaining brainstem.

A. Blood Supply to the Ventricles (Choroid Plexus)

To address the specific vascularization of the ventricular system (which produces Cerebrospinal Fluid via the choroid plexus):

• Anterior Choroidal Artery (AChA): A direct branch of the distal ICA. It supplies the choroid plexus of the lateral ventricles, as well as the optic tract, hippocampus, and posterior limb of the internal capsule.
• Posterior Choroidal Arteries (Medial and Lateral): Branches of the PCA. They supply the choroid plexus of the third and lateral ventricles, and parts of the thalamus.

B. Lenticulostriate Arteries

• Origin: Arise at sharp 90-degree angles directly from the M1 segment of the MCA. There are typically 6-12 small vessels.
• Structures Perfused: Caudate nucleus, putamen, globus pallidus, and the critical internal capsule (posterior limb and genu).
• Vulnerability: Because these are "end-arteries" with NO collateral circulation, and because they branch off a high-pressure main pipe at a sharp angle, they are extremely susceptible to hypertensive damage (lipohyalinosis and microatheroma), leading to Lacunar Infarcts.
• Note: The Recurrent Artery of Heubner is an analogous deep branch originating from the ACA, supplying the anteromedial caudate and anterior limb of the internal capsule.

C. Thalamic Blood Supply

The thalamus is the brain's central sensory relay station, supplied almost entirely by deep branches of the Posterior Cerebral Artery (PCA):

• Thalamoperforating arteries (P1 segment): Supply the medial thalamus.
• Thalamogeniculate arteries (P2 segment): Supply the lateral thalamus (including the VPL/VPM nuclei for bodily and facial sensation).

D. Brainstem Blood Supply Zones

The brainstem (midbrain, pons, medulla) is supplied by the vertebrobasilar system. Each level is divided into Medial and Lateral vascular zones.

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4. Spinal Cord Blood Supply

The spinal cord relies on a delicate balance of longitudinal arteries running its entire length, reinforced by horizontal arteries feeding in from the body wall.

A. Longitudinal Arteries

• Anterior Spinal Artery (ASA):
  • Origin: Formed by the fusion of branches from both vertebral arteries near the foramen magnum.
  • Course: A single, continuous artery running down the anterior median fissure.
  • Territory: Supplies the Anterior 2/3 of the cord. This includes the motor tracts (corticospinal), pain/temperature tracts (spinothalamic), and anterior horn motor cells.
• Posterior Spinal Arteries (PSA):
  • Origin: Arise from the vertebral arteries or PICA.
  • Course: Paired arteries running down the posterolateral sulci.
  • Territory: Supply the Posterior 1/3 of the cord. This specifically includes the dorsal columns (responsible for fine touch, vibration, and proprioception) and the dorsal grey matter.

B. Segmental Medullary Arteries

The ASA and PSAs alone are not powerful enough to supply the entire length of the spinal cord. They must be reinforced at irregular intervals by radicular and medullary arteries arising from the aorta (via intercostal and lumbar arteries). The most dominant of these exist to keep the lower cord alive.

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5. Venous Drainage & Dural Sinuses

Unlike veins in the rest of the body, cerebral veins lack valves and have extremely thin walls without a muscular layer. They drain into large, endothelium-lined channels formed between layers of the dura mater called Dural Venous Sinuses.

A. Major Cerebral Veins & Sinus Flow Pathway

• Superior Cerebral Veins: Drain the lateral/superior cortical surfaces directly into the Superior Sagittal Sinus (located along the upper margin of the falx cerebri).
• Great Vein of Galen: Drains deep structures (thalamus, basal ganglia). It joins with the Inferior Sagittal Sinus to form the Straight Sinus.
• The Flow Pathway: Superior Sagittal Sinus + Straight Sinus meet at the back of the skull at the Confluence of Sinuses (Torcular Herophili).
  From there, blood flows outward laterally into the Transverse Sinuses.
  The Transverse Sinuses curve downward to become the S-shaped Sigmoid Sinuses.
  The Sigmoid Sinuses exit the skull via the jugular foramen to become the Internal Jugular Veins (IJV), returning blood to the heart.

B. The Cavernous Sinus

Located on either side of the sella turcica (which houses the pituitary gland), this sinus is unique because multiple vital structures pass directly through it. It receives blood from the ophthalmic veins and superficial MCA veins, eventually draining into the petrosal sinuses.

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6. Clinical Anatomy & Pathophysiology

Combining the anatomical pathways above allows us to quickly pinpoint the location of vascular lesions based purely on clinical presentation.

A. Lacunar Infarcts

Small (<15mm) ischemic strokes occurring in deep, penetrating artery territories. Because these vessels (like the lenticulostriate arteries) have no collateral supply, occlusion leads to highly specific, localized tissue death.

• Pure Motor Stroke: Caused by a lacunar infarct in the posterior limb of the internal capsule or the basis pontis. The internal capsule packs motor fibers tightly together. A tiny stroke here wipes out motor control to the contralateral face, arm, and leg equally, with absolutely NO sensory or visual deficits. This accounts for ~45% of all lacunar strokes.
• Pure Sensory Stroke: Caused by a lacunar infarct in the ventral posterolateral (VPL) nucleus of the thalamus (supplied by thalamogeniculate arteries). Presents as contralateral sensory loss (pain, temperature, touch) across the face, arm, and leg, with no motor weakness. Can eventually lead to Dejerine-Roussy syndrome (severe post-stroke thalamic pain).

B. Intracranial Hemorrhages: Anatomical Contrast

Understanding exactly which vessel breaks defines the type of hematoma and the clinical presentation.

Quick Reference Summary & Takeaways

• Anterior Circulation: ICA → ACA (Leg) + MCA (Face/Arm).
• Posterior Circulation: Vertebral → Basilar → PCA (Vision).
• Collateral Safety: Circle of Willis routes blood via AComm and PComm.
• Watershed Vulnerability: Border zones (ACA-MCA, MCA-PCA) suffer first during massive blood pressure drops.
• Spinal Cord Safety: Artery of Adamkiewicz is the massive backup generator for the lower spine's ASA.
• Cavernous Sinus Danger: CN VI is free-floating next to the ICA; highly susceptible to pressure/thrombosis.
• Bleed Types: Epidural = Arterial/Lens/Lucid; Subdural = Venous/Crescent/Elderly; SAH = Aneurysm/Thunderclap.

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List of References:

• Core Vascular Anatomy & Pathophysiology: Complete Illustrated Notes. (Generated 2026-06-30).
• Core Vascular Neuroanatomy: Comprehensive Illustrated Learning Notes. Professional Medical Education Resource.
• Content synthesized and structured based on standard medical neuroanatomy and vascular pathophysiology curricula regarding cerebrovascular arterial inflow, cortical territories, brainstem perfusion, spinal cord supply, and associated clinical syndromes.