Cerebral Vasculature Overview

Overview

This lecture reviews the key segments and branches of the cerebral vasculature, focusing on the middle cerebral, posterior cerebral, and vertebrobasilar arterial systems, their clinical relevance, and anatomical variants.

Middle Cerebral Artery (MCA) Segments

• M2 (insular) segments arise after MCA bifurcation.
• M3 (opercular) branches loop at the Sylvian fissure and course under the opercula.
• M4 are cortical branches exiting the Sylvian fissure onto the lateral hemisphere.
• Medial and lateral lenticulostriate branches arise from ACA and MCA, respectively.

Posterior Cerebral Artery (PCA) Segments and Branches

• P1 segment is pre-communicating, extending from the basilar artery bifurcation to the PCom junction, lying above cranial nerve III.
• P2 (ambient) segment runs from the PCom-P1 junction posterior-laterally around the midbrain.
• Important P2 branches: anterior/posterior temporal arteries, thalamogeniculate arteries, peduncular perforators, medial and lateral posterior choroidal arteries.
• P3 (quadrigeminal) segment lies in the quadrigeminal cistern.
• P4 (calcarine) segment terminates in the calcarine fissure and supplies the occipital lobe.

PCA Variants

• Fetal PCA: PCA arises from ICA via PCom instead of basilar artery in ~10-30% of cases.
• Artery of Percheron: Single dominant thalamoperforator from P1 supplies bilateral medial thalami and upper midbrain; occlusion can cause coma.

Vertebrobasilar System

• Vertebral artery arises from subclavian, enters the transverse foramen at C6, ascends through C6–C1.
• V3 (extraspinal) segment curves around C1, gives posterior meningeal artery.
• V4 (intradural) segment gives rise to anterior/posterior spinal arteries, medullary perforators, PICA.
• PICA supplies lateral medulla and cerebellum; infarct causes Wallenberg syndrome, characterized by ipsilateral facial and contralateral body sensory loss.
• Vertebral arteries unite at the pontomedullary junction to form the basilar artery.

Basilar Artery and Branches

• Basilar artery runs in pre-pontine cistern, ends by dividing into PCAs.
• First major branch: AICA, gives labyrinthine (internal auditory) artery supplying CN VIII and inner ear.
• SCA arises just before the PCA, runs below CN III, supplies parts of the cerebellum.
• Extensive anastomoses exist between external and internal carotid circulations, which can become significant in ICA occlusion.

ICA Segments and Key Branches

• ICA segments: C1 (cervical), C2 (petrous), C3 (lacerum), C4 (cavernous), C5 (clinoidal), C6 (ophthalmic), C7 (communicating).
• Cavernous ICA gives off meningohypophyseal and inferolateral trunks.
• Communicating segment gives rise to PCom and anterior choroidal arteries.
• Circle of Willis provides collateral flow between anterior and posterior circulations.

Key Terms & Definitions

• MCA (Middle Cerebral Artery) — Major brain artery, with M1–M4 segments.
• PCA (Posterior Cerebral Artery) — Supplies posterior cortex, has P1–P4 segments.
• PCom (Posterior Communicating Artery) — Connects anterior and posterior circulation.
• PICA (Posterior Inferior Cerebellar Artery) — Largest branch off vertebral artery, supplies cerebellum and medulla.
• AICA (Anterior Inferior Cerebellar Artery) — First major basilar branch, supplies inner ear and cerebellum.
• SCA (Superior Cerebellar Artery) — Basilar branch just below PCA.
• Fetal PCA — Variant PCA origin from ICA via PCom.
• Artery of Percheron — Single perforator artery variant to both thalami/midbrain.
• Wallenberg Syndrome — Lateral medullary syndrome due to PICA infarct.

Action Items / Next Steps

• Review cerebral vascular anatomy diagrams, especially the Circle of Willis.
• Memorize major arterial segments and their key branches for exams.
• Learn the clinical signs and syndromes associated with vascular territories (e.g., Wallenberg syndrome).
• Read further on arterial variants and their implications in stroke and surgery.