Circle of willis

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Circle of Willis

Overview

The Circle of Willis is an anastomotic arterial ring located at the base of the brain in the subarachnoid space. It connects the anterior circulation (from the internal carotid arteries) with the posterior circulation (from the vertebrobasilar system), forming a collateral network that can redistribute blood if one vessel is occluded. A complete, full-caliber ring is present in only ~34% of individuals; anatomical variants and hypoplasia are very common. - Neuroanatomy through Clinical Cases, 3rd ed.

Diagrams

Circle of Willis and its main branches (view from below the brain):

Full arterial supply from aorta to circle (schematic):

Circle of Willis - full arterial supply schematic

Arterial Inputs

The circle receives blood from two sources:

Source	Arteries	Origin
Anterior circulation	Bilateral internal carotid arteries (ICA)	Common carotid arteries (from aorta/brachiocephalic trunk)
Posterior circulation	Bilateral vertebral arteries	Subclavian arteries - ascend through cervical vertebral foramina transversaria - join at pontomedullary junction to form the basilar artery

Components of the Ring

The circle itself is formed by 7 vessels (bilaterally paired where applicable):

Vessel	Abbreviation	Part of Ring
Anterior communicating artery	AComm	Anterior - connects the two ACAs
Anterior cerebral artery (x2)	ACA	Anterolateral
Internal carotid artery (x2)	ICA	Lateral inflow
Posterior communicating artery (x2)	PComm	Connects ICA to PCA - links anterior and posterior circulations
Posterior cerebral artery (x2)	PCA	Posterior - arise from the top of the basilar artery

Major Branches Arising from the Ring

From the circle and its immediate feeders, the three main cerebral arteries supply the hemispheres:

1. Anterior Cerebral Artery (ACA)

Passes anteriorly into the interhemispheric fissure, sweeps back over the corpus callosum
Supplies: medial frontal and parietal cortex, medial sensorimotor cortex (legs/feet representation)
Deep branches: medial lenticulostriate arteries (basal ganglia)

2. Middle Cerebral Artery (MCA)

Largest branch of the ICA; turns laterally into the Sylvian fissure
Bifurcates into superior and inferior divisions
Supplies: lateral frontal, parietal, temporal cortex - includes primary motor/sensory cortex for the face and arm, Broca's area, Wernicke's area
Deep branches: lateral lenticulostriate arteries (internal capsule, putamen, caudate)

3. Posterior Cerebral Artery (PCA)

Arises from the top of the basilar artery; also receives contribution via PComm
Supplies: occipital lobe (primary visual cortex), medial temporal lobe (hippocampus), thalamus
Deep branches: thalamoperforating arteries

Other branches from the ICA (mnemonic OPAAM):

Ophthalmic artery - retinal supply
Posterior communicating artery (PComm)
Anterior choroidal artery - supplies posterior limb of internal capsule, optic tract, choroid plexus
Anterior cerebral artery (ACA)
Middle cerebral artery (MCA) - Neuroanatomy through Clinical Cases, 3rd ed.

Vascular Territories at a Glance

Artery	Cortical territory	Deep territory
ACA	Medial frontal/parietal (leg area)	Medial basal ganglia
MCA	Lateral frontal, parietal, temporal	Internal capsule, lateral BG
PCA	Occipital lobe, medial temporal	Thalamus

Collateral Function

The circle provides collateral flow in three ways:

Circle of Willis itself - connects carotid and vertebrobasilar systems
Extracranial-intracranial anastomoses - e.g., ophthalmic artery collaterals
Leptomeningeal anastomoses - between terminal branches of ACA, MCA, and PCA at watershed zones

Patients with complete proximal carotid occlusion can sometimes tolerate it if the circle is intact, as blood reaches the hemisphere via the contralateral ICA through the AComm, or from the vertebrobasilar system through the PComm. - Schwartz's Principles of Surgery, 11th ed. | Localization in Clinical Neurology, 8th ed.

Clinical Correlations

Condition	Mechanism
Berry (saccular) aneurysm	Most common at arterial junctions of the circle - AComm is the single most frequent site, followed by PComm-ICA junction. Rupture causes subarachnoid hemorrhage.
MCA stroke	Contralateral face/arm weakness, aphasia (dominant hemisphere), neglect (non-dominant)
ACA stroke	Contralateral leg weakness, abulia, incontinence
PCA stroke	Contralateral homonymous hemianopia, thalamic pain, memory deficits
PComm aneurysm	Compresses CN III (runs adjacent) - ipsilateral pupil dilation, ptosis, "down and out" eye
Carotid endarterectomy	Brain depends on collateral flow from an intact circle when the ICA is cross-clamped. Autopsy studies show majority of specimens have anatomical anomalies in the circle. - Clinical Anesthesia, 9th ed.

Imaging

The circle is best visualized with MR angiography (MRA) or CT angiography (CTA). Both reliably demonstrate the ring and any anatomical variants such as hypoplastic vessels or absent segments. - Grainger & Allison's Diagnostic Radiology

Images of all transverse section at different level in neuroanatomy 1st year

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Transverse Sections of the CNS - Neuroanatomy (1st Year)

All sections below are from Gray's Anatomy for Students and Neuroanatomy through Clinical Cases, 3rd ed. (Crossman/Nolte), arranged rostral to caudal.

Overview: Brainstem Compartments

Every brainstem level is divided into three zones:

Zone	Location	Contents
Tectum ("roof")	Dorsal - midbrain only	Superior/inferior colliculi
Tegmentum ("covering")	Dorsal - all levels	Cranial nerve nuclei, reticular formation, tracts
Basis	Ventral	Descending motor fibers (corticospinal, corticobulbar)

Brainstem three-zone overview: rostral medulla (A), rostral pons (B), rostral midbrain (C)

A = rostral medulla, B = rostral pons, C = rostral midbrain. Tectum (blue) only in midbrain.

1. Rostral Midbrain (Level of Superior Colliculus)

Rostral midbrain - transverse section at superior colliculus

Key structures to identify:

Structure	Notes
Superior colliculus	Tectum - visual reflex
Cerebral aqueduct	CSF passage
Periaqueductal grey (PAG)	Pain modulation
Oculomotor nucleus (CN III)	Near midline, ventral to PAG
Edinger-Westphal nucleus	Parasympathetic to pupil
Red nucleus	Rubrospinal tract origin
Substantia nigra	Dopaminergic; basal ganglia circuit
Crus cerebri (cerebral peduncle)	Corticospinal + corticobulbar fibers
Medial lemniscus	Fine touch/proprioception ascending
MLF (medial longitudinal fasciculus)	Coordinates eye movements

2. Pontine-Mesencephalic Junction (Level of Inferior Colliculus)

Pontine-mesencephalic junction transverse section

Key structures:

Inferior colliculus (auditory reflex)
Decussation of superior cerebellar peduncle (dentatorubrothalamic fibers crossing)
Lateral lemniscus (auditory pathway)
Cerebral aqueduct + periaqueductal grey
Medial lemniscus and spinal lemniscus
Pontocerebellar fibers

3. Rostral Pons

Key structures:

Structure	Notes
Central grey	Around 4th ventricle
Locus coeruleus	Noradrenergic nucleus
Superior cerebellar peduncle	Lateral wall of 4th ventricle
MLF	Near midline
Medial lemniscus	Horizontal slab between tegmentum and basis
Spinothalamic tract	Lateral to medial lemniscus
Pontine nuclei	Relay cortex → cerebellum
Corticospinal fibers	Scattered in basal pons

4. Mid-Pons (Level of CN V - Trigeminal)

Mid-pons transverse section at the level of the trigeminal nerve

Key structures:

Fourth ventricle expanding
Middle cerebellar peduncle (largest peduncle - bridges to cerebellum)
Motor nucleus of CN V + Principal sensory nucleus of CN V
Trigeminal nerve exiting laterally
MLF near midline
Medial lemniscus
Pontine nuclei + Pontocerebellar fibers
Trapezoid body (auditory)
Reticular formation

5. Caudal Pons (Level of CN VI - Abducens)

Key structures:

Fourth ventricle with cerebellum and choroid plexus dorsal
Abducens nucleus (CN VI) - just lateral to MLF
Middle cerebellar peduncle
Dentate nucleus of cerebellum visible dorsally
Medial vestibular nucleus
MLF and Medial lemniscus
Corticospinal fibers + Pontocerebellar fibers

6. Rostral Medulla (Level of Inferior Olivary Nucleus)

Rostral medulla transverse section - inferior olivary nucleus level

Key structures:

Structure	Notes
Inferior olivary nucleus	Large, convoluted - landmark of rostral medulla
Pyramid	Corticospinal fibers anteriorly
Hypoglossal nucleus (CN XII)	Near midline
Dorsal motor nucleus of vagus (CN X)	Lateral to CN XII nucleus
Solitary tract + nucleus solitarius	Taste and visceral afferents
Vestibular nuclei (medial, inferior, lateral)	CN VIII
Inferior cerebellar peduncle	Lateral
Medial lemniscus	Vertical, near midline
MLF
Nucleus raphé magnus	Serotonergic pain modulation
Reticular formation

7. Mid-Medulla (Level of Internal Arcuate Fibers / Sensory Decussation)

Mid-medulla transverse section - internal arcuate fiber decussation

Key structures:

Nucleus gracilis (medial) and Nucleus cuneatus (lateral) - 2nd-order neurons of posterior column pathway
Fasciculus gracilis and Fasciculus cuneatus - ascending dorsal column fibers
Internal arcuate fibers - axons of gracile/cuneate nuclei decussating to form the medial lemniscus
Decussation of internal arcuate fibers - sensory decussation (fine touch crosses here)
Spinal tract and nucleus of trigeminal (CN V)
Spinocerebellar tracts laterally
Pyramid anteriorly

8. Spinal Cord - Cross Section (C8 Cervical Segment)

Key structures:

Structure	Abbreviation	Location
Posterior funiculus	PF	Dorsal white matter (fine touch, proprioception)
Posterior horn	PH	Dorsal grey - receives sensory input
Intermediate grey	IG	Contains lateral horn at T1-L2 (sympathetic)
Anterior horn	AH	Motor neurons
Anterior funiculus	AF	Ventral white matter
Lateral funiculus	LF	Mixed motor (corticospinal) + sensory (spinothalamic)
Anterior white commissure	-	Crossing of spinothalamic fibers
Anterior median fissure	-	Deep midline cleft anteriorly
Lissauer's tract	-	Posterolateral; pain fibers ascend 1-2 levels before synapsing
Substantia gelatinosa (*)	-	Apex of posterior horn; pain gate

The internal grey matter follows an H-shape, further subdivided into Rexed's laminae I-X (I = superficial dorsal horn, IX = motor neurons, X = central canal zone).

Quick Summary: What to Look for at Each Level

Level	Landmark Structure	CN present
Rostral midbrain	Superior colliculus, Red nucleus, Substantia nigra	CN III
Caudal midbrain	Inferior colliculus, SCP decussation	CN IV (exits dorsally)
Rostral pons	Locus coeruleus, SCP	-
Mid-pons	CN V nuclei, MCP	CN V
Caudal pons	Abducens nucleus, Dentate	CN VI, VII
Rostral medulla	Inferior olivary nucleus	CN VIII-XII
Mid-medulla	Sensory decussation, Gracile/Cuneate nuclei	-
Caudal medulla	Pyramidal decussation (motor cross here)	-
Spinal cord	H-shaped grey, Rexed laminae	-

Source: Gray's Anatomy for Students (Crossman), Neuroanatomy through Clinical Cases 3rd ed. (Blumenfeld)

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