Anatomy and segmentation of the posterior cerebral artery
The posterior cerebral artery (arteria cerebri posterior) plays a crucial role in the blood supply to the structures of the posterior cranial fossa and the occipital regions of the brain. Blood flow into it comes from the basilar artery bifurcation, as well as from the internal carotid artery (ICA) territory via the posterior communicating artery. Interestingly, in early ontogenesis, this vessel is a direct branch of the ICA.
According to Fisher's classification, the anatomical landmark for dividing the vessel into segments is the point of entry of the posterior communicating artery (approximately 10 mm from the basilar artery bifurcation):
- P1 segment (precommunicating part) — the proximal section located prior to the entry point of the anastomosis.
- P2 segment (postcommunicating part) — the distal section beginning after the junction of the posterior communicating artery.
Topographically, the trunk of the artery curves around the midbrain and emerges into the ambient cistern near the edge of the tentorium cerebelli. Penetrating vessels supplying the midbrain and thalamic structures originate from both arteries and their connecting branches.
Blood supply to the thalamus and hippocampus
Thalamic perfusion is provided by a complex network of perforating and choroidal arteries belonging to the carotid and vertebrobasilar systems. The main sources include:
- Anterior thalamoperforating artery — arises from the posterior communicating artery, supplying the rostral regions.
- Posterior thalamoperforating artery — a branch of the P1 segment, supplying the medial and basal parts, as well as the pulvinar of the thalamus.
- Thalamosgeniculate artery — a branch of the P2 segment, responsible for the lateral part.
- Choroidal arteries (lateral and medial posterior choroidal arteries) — supply the geniculate bodies, choroid plexuses, and thalamic nuclei.
Of particular clinical significance is the artery of Percheron (arteria Percheron), an anatomical variant in which the perforating arteries supplying the thalamus arise from a single common trunk. This is often accompanied by unilateral hypoplasia of the P1 segment.
Blood supply to the hippocampus (Ammon's horn) is provided by branches running in the region of the transverse fissure of the brain (foramen of Bichat). These include the temporal branch, the artery of Ammon's horn, the long branch of Uchimura, and the lateral posterior choroidal artery.
Cortical branches and visual structures
The main cortical branches are formed within the ambient cistern: the temporal, occipitotemporal, and calcarine arteries. Their perfusion zone covers the occipital lobe and the medial surface of the temporal lobe.
It is worth noting the significant variability of the vascular territories. In some cases, the Sylvian fissure serves as the boundary between the middle and posterior cerebral artery territories. In other situations, the middle cerebral artery may provide perfusion to the entire convexital surface of the hemispheres up to the occipital pole.
The anatomy of the visual pathway blood supply is critically important for clinical neurology:
- Visual cortex (calcarine sulcus area) is always supplied by the posterior cerebral artery.
- Optic radiation often receives blood from the middle cerebral artery.
Therefore, the presence of homonymous hemianopia in a patient is not a 100% sign of infarction in the posterior cerebral artery territory.
Collateral circulation of the brain
In cases of stenosis or occlusion of major vessels, compensatory mechanisms—blood flow through anastomoses—become critically important. This helps maintain brain tissue perfusion.
Extracranial pathways and connections with the internal carotid artery: In ICA stenosis, blood can flow retrogradely from the external carotid artery (ECA) system. The main pathway passes through the ophthalmic artery: blood from the facial or superficial temporal artery enters the angular artery, then the ophthalmic artery, from which it fills the ICA siphon. Another pathway is via the meningeal branches of the ascending pharyngeal artery (collectively referred to on angiograms as the inferolateral trunk).
Connections with the vertebrobasilar system: There is an extracranial anastomosis between the branches of the external carotid artery (via the occipital artery) and the vertebral artery.
Intracranial collaterals: These include the Circle of Willis, which serves as the main crossroads at the base of the skull, and leptomeningeal collaterals. The latter are located on the convexital surface of the brain and connect the distal cortical branches of the anterior, middle, and posterior cerebral arteries, providing local compensatory circulation.
Briefly: answers to topical questions
Which arteries form the Circle of Willis?
The Circle of Willis is formed by arteries connecting the carotid and vertebrobasilar systems:
- Anterior communicating artery (a. communicans anterior)
- Proximal segments of the anterior cerebral arteries (aa. cerebri anteriores)
- Internal carotid arteries (aa. carotides internae)
- Posterior communicating arteries (aa. communicantes posteriores)
- Posterior cerebral arteries (aa. cerebri posteriores)
- Basilar artery (a. basilaris)
What segments are distinguished in the posterior cerebral artery besides P1 and P2?
The source indicates that each posterior cerebral artery is divided into 4 segments. Besides P1 and P2, the remaining segments are not named or separately described in the provided source excerpt.
- P1 — precommunicating segment
- P2 — postcommunicating segment
According to Fisher's classification, the source highlights P1 and P2.
Which specific thalamic nuclei are supplied by the thalamosgeniculate artery?
The thalamosgeniculate artery supplies the lateral part of the thalamus. Specific thalamic nuclei are not specified in the provided sources.
- It originates from the P2 segment of the posterior cerebral artery, distal to the posterior communicating artery.
- Its involvement is characterized by impaired proprioceptive sensitivity, spontaneous pain, mild hemiataxia, astereognosis, and contralateral choreoathetoid hyperkinesia.
What branches arise from the basilar artery?
Paired cerebellar arteries, posterior cerebral arteries, and penetrating branches to the brainstem branch off from the basilar artery.
- Anterior inferior cerebellar artery (a. cerebelli inferior anterior) — flocculus and anterior part of the cerebellar hemisphere, gives off the labyrinthine artery
- Superior cerebellar artery (a. cerebelli superior) — rostral part of the cerebellar hemisphere and superior part of the vermis
- Posterior cerebral arteries (a. cerebri posterior) — terminal branches formed at the bifurcation
- Paramedian branches (rami paramediani)
- Short and long circumferential branches (rami circumferentes breves) — to the brainstem
What clinical symptoms are characteristic of an infarction in the artery of Percheron territory?
Infarction in the artery of Percheron territory manifests as bilateral thalamic involvement with severe impaired consciousness and cognitive disorders.
- Severe impairment of consciousness — due to bilateral infarctions of the intralaminar thalamic nuclei
- Unusual drowsiness, apathy — up to sudden falling asleep during the examination
- Gross disorientation in time and space
- Pronounced amnesia for events
- Abulia, lack of initiative, negativism — difficulty following instructions
The course may be favorable with rapid regression of the deficit.
To remember the connection between the external and internal carotid arteries: the anastomosis runs "along the face to the eye" — from the facial artery via the angular artery to the ophthalmic artery.
Class and exam questions
What divides the posterior cerebral artery into the P1 and P2 segments?
The anatomical boundary between these segments (according to Fischer) is the site where the posterior communicating artery joins.
What is the clinical significance of Percheron's artery?
This is a common trunk from which perforating arteries arise supplying both halves of the thalamus. Its occlusion results in bilateral thalamic infarction.
Does homonymous hemianopsia necessarily indicate a lesion in the PCA territory?
No. Although the visual cortex itself is always supplied by the PCA, the optic radiation is often supplied by the middle cerebral artery territory.
How is collateral blood flow maintained in internal carotid artery stenosis?
The main pathway is retrograde blood flow from the branches of the external carotid artery (facial, temporal) via the angular and ophthalmic arteries back into the ICA siphon.
What else to study on this topic
- Detailed topography of the Circle of Willis and its anatomical variations
- Characteristics of leptomeningeal collaterals on the convexital surface
- Blood supply to the choroid plexuses of the brain ventricles
- Specifics of the blood supply to the optic radiation
- Structural variants of the inferior surface of the temporal lobe and its arterial network
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