There are several branches of the PCA. The posterior communicating artery connects the PCA and therefore the posterior circulation to the ICA and the anterior circulation.
Other important branches of the PCA include the following: anterior and posterior temporal arteries supply the temporal lobe , medial and lateral occipital arteries supply the occipital lobe , calcarine artery supplies the visual cortex , and medial and lateral posterior choroidal arteries supply the choroid plexus.
The PCA can also be divided into 4 segments P1—P4, which are useful for describing the precise localization of a vascular lesion. The anterior and posterior circulations are connected at the base of the skull via the paired anterior communicating arteries and posterior communicating arteries. Therefore, if one part of the cerebral circulation is occluded, the circle of Willis allows collateral supply via an alternate vascular route. The anterior and posterior communicating arteries are also clinically important because they are a frequent site for intracranial aneurysms pathologic vascular dilatations , which can result in a subarachnoid hemorrhage if the aneurysm ruptures.
It is important to note that there are many normal anatomical variations to the circle of Willis. Image : Circle of Willis. The occipital lobe is supplied by the PCA and includes the visual cortex.
The frontal lobes, the medial portion of the hemispheres, and the superior portion of the parietal lobes are supplied primarily by the ACA. For a detailed view of the vascular supply and corresponding anatomy, please review the image. Stroke occurs when there is a disruption of cerebral blood flow and is associated with ischemia in the associated brain tissue. This can occur due to occlusion of a cerebral vessel ischemic stroke or due to the rupture of a cerebral blood vessel hemorrhagic stroke.
The clinical symptoms and signs of a stroke depend on which blood vessel and corresponding brain tissue are affected. Image : Sensory Homunculus. By: OpenStax College. License: CC BY 3.
The classical clinical presentation of stroke in the ACA vascular territory includes hemiparesis i. This is because the anterior cerebral artery supplies the frontal, prefrontal, and supplementary motor cortexes as well as part of the primary motor and sensory cortex.
Additionally, occlusion of the ACA can lead to bladder dysfunction. Review of the homunculus may help clarify these details please see image. When the MCA territory is affected by as stroke, the classical clinical presentation includes contralateral hemiparesis weakness , as well as sensory deficits. Aphasia the inability to speak , may also occur when the dominant hemisphere for language is affected which is the left hemisphere for most right-handed people and many left-handed people.
Since the PCA supplies the visual cortex, strokes in this vascular territory will typically result in visual disorders, e. If the vascular supply to the thalamus is also affected, clinical findings can include sensory deficits numbness in the contralateral face, arm, and leg. The brain stem and cerebellum are supplied by the basilar artery and cerebellar arteries or branches of these vessels.
The branches are differentiated into medial, mediolateral, and lateral portions, based on their localization and which anatomical territory they supply. The medulla, which follows the brain stem, is supplied, among others, by the anterior spinal artery , a branch of the vertebral artery.
The AICA is the first branch of the basilar artery. The PICA is a branch of the vertebral artery. The venous drainage of the brain occurs via the superficial and deep venous systems, both of which drain via the dural venous sinuses. Both systems are connected via anastomoses. At the transition of both drainage areas, a reversal of the venous flow is possible.
Unlike all other veins of the body, cerebral veins do not have valves. The superficial venous system includes the cortical veins and the sagittal sinus and drains the cerebral cortex. The major cortical veins are named based on their location and include a superior, middle, and inferior group.
The superior cerebral veins drain primarily into the superior sagittal sinus and the superficial middle cerebral veins into the cavernous sinus. Based on the direction of the drainage, superficial veins that are located laterally can be further divided into ascending and descending veins.
Ascending veins primarily drain into the superior sagittal sinus , while descending veins prefer to drain into the transverse sinus. Veins that are located between the arachnoid membrane and the dura mater connect the superficial veins with the sinus system, and are also referred to as bridging veins. If these bridging veins rupture, a subdural hemorrhage may occur.
The major cause of the rupture of bridging veins is trauma. The deep venous system includes the deep cerebral veins , straight sinus , sigmoid sinuses, and lateral sinuses. The paired deep cerebral veins and the paired basal veins are part of the deep venous system. The basal vein is formed by the fusion of anterior cerebral vein and deep middle cerebral vein. Similar to the arterial circulation of the Circle of Willis, there is a venous anastomosis at the cranial base formed by a connection of both basal veins basal vein of Rosenthal.
The anterior communicating vein connects both anterior cerebral veins, which leads to the formation of a closed circulation. Further venous drainage takes place via the paired deep cerebral veins , as well as the paired basal veins into the vein of Galen great cerebral vein. The site where both veins fuse together is also referred to as the confluence of sinuses Torcula.
The great cerebral vein originates at this site. The great cerebral vein vein of Galen is an unpaired vein and discharges into the straight sinus. The drainage area of the internal cerebral vein includes the thalamus, striatum, choroid plexus, and septum pellucidum.
The basal vein drains blood from the medial and basal parts of the frontal and temporal lobes, insular cortex, and hypothalamus, as well as from the mesencephalon. The dural venous sinuses are small venous structures that drain blood from the cerebral veins, orbits, and skull into the internal jugular veins. Image : Posterior and Lateral Views of the Neck. The dural venous sinuses are located between the periosteal and meningeal layers of the dura mater , which consists of firm collagenous connective tissue.
Besides the connective tissue of the dura mater, the wall of the dural venous sinuses is also made of the endothelium. Like all other cerebral veins, the dural venous sinuses do not have valves. The dural venous sinuses show extensions at some sites, also referred to as lateral venous lacunes. CSF reabsorption, via the arachnoid villi, takes place in the area of the lateral venous lacunes. The superficial cerebral veins, diploic veins of the surrounding periosteum, and the emissary veins discharge into the dural venous sinuses via the bridging veins.
The emissary veins connect the dural venous sinuses to the extracranial veins, which drain the scalp into the diploic veins. Within the system of dural venous sinuses, there is an upper and a lower group.
Both groups are connected to the veins of the vertebral canal, via the marginal sinus and the basilar plexus. The upper group includes the superior and inferior sagittal sinuses, occipital sinus, transverse sinus, straight sinus, sigmoid sinus , and the confluence of sinuses. The lower group includes the cavernous sinus, together with anterior and posterior intercavernous sinus, the sphenoparietal sinus, as well as the superior and inferior petrosal sinuses.
The inlets of the dural venous sinuses are the superficial and deep cerebral veins. There are numerous anastomoses, which make it possible for larger occlusions in the area of the dural venous sinuses to remain clinically asymptomatic. The main drainage pathway for the dural venous sinuses is the internal jugular veins. There are also accessory drainage pathways, which include the emissary veins, superior ophthalmic vein, marginal sinus, and basilar plexus, as well as the venous plexus of foramen ovale.
Since the cerebral veins and the dural venous sinuses have no valves, the blood flow of the venous system can occur in either direction. This can lead to the spread of extracranial infections into the sinus system. Infection can further result in an occlusion of a segment of the dural venous sinuses , also referred to as venous sinus thrombosis. Clinical symptoms of a venous sinus thrombosis include headache, nausea, vomiting, and even altered consciousness.
These symptoms occur due to increased intracranial pressure resulting from reduced venous drainage. Additionally, infections can spread via the emissary veins, which form the connection between dural venous sinuses and the extracranial veins, from the scalp to the dura mater, and could lead to meningitis.
The veins in the region of the medulla and brain stem are connected to the basal cerebral veins. The veins of the brain stem are connected to each other via a longitudinal and a transverse network.
Furthermore, there is an infratentorial and a supratentorial venous system. Veins arising from the medulla oblongata , pons , and cerebellum belong to the infratentorial system.
The supratentorial system starts at the transition to the mesencephalon. There are numerous anastomoses between the 2 systems. The venous drainage of the cerebellum is differentiated roughly into a medial and a lateral part, with numerous anastomoses between the 2 parts.
The drainage of the cerebellar vermis , cerebellar hemispheres , and medial part of the superior and inferior cerebellar veins takes place via the medial system.
The remaining parts of the cerebellar hemispheres are drained via the lateral system. Bhuiyan, P. After passing through the transverse foramen of C1 , the arteries traverse the foramen magnum. Once inside the cranial vault , the vertebral arteries give off the following branches:. The vertebral arteries then converge to form the basilar artery at the base of the pons , inside the cranium. The vertebral artery is generally divided into four segments : 6.
The basilar artery runs superiorly within the central groove of the pons, giving off a number of branches including the pontine arteries , which supply the pons. The basilar artery eventually anastomoses with the circle of Willis via the posterior cerebral arteries and posterior communicating arteries. Pontine infarcts cause an interruption in the myriad of neuronal pathways enabling communication between the cerebrum, cerebellum and spinal cord.
This can result in complete paralysis of all voluntary muscle groups , sparing those controlling the eyes. Individuals suffering from damage to the pons are fully conscious and cognitively intact. As outlined above, the terminal branches of the anterior and posterior circulation form an anastomosis to create a ring-like vascular structure known as the circle o f Willis , within the base of the cranium highlighted in pink below.
The left and right internal carotid arteries continue as the middle cerebral arteries MCA , after each giving off a branch to supply the anterior cerebral arteries ACA.
The anterior communicating artery links the two anterior cerebral arteries together. The internal carotid arteries also give off the posterior communicating arteries PCoA , linking the middle cerebral arteries MCA with the posterior cerebral arteries PCA. Treatment involves urgent neurosurgical referral and subsequent endovascular coiling or inserting a surgical clip to occlude flow to an aneurysm. The third cranial nerve is commonly affected by aneurysms in the circle of Willis , particularly those involving the posterior communicating artery PoCA due to its close anatomical relationship.
External compression of the third nerve affects parasympathetic fibres surrounding the outermost region of the third nerve. This results in pupillary involvement arising much later. The anterior cerebral arteries , middle cerebral arteries and posterior cerebral arteries each supply a territory of the brain see the image below. Each region of the brain has specific associated functions, enabling clinicians to discern the site of pathology through history and neurological examination of the patient.
As discussed earlier, the brain is extremely vulnerable to stress in the form of depleted blood supply. A cerebrovascular event stroke is a clinical syndrome caused by disruption of blood supply to the brain, characterised by rapidly developing signs of focal or global disturbance of cerebral functions, lasting for more than 24 hours or leading to death. Ischaemic strokes occur when the blood supply to an area of the brain is reduced, resulting in tissue hypoperfusion. Ischaemic strokes are classified using the Bamford Classification System, which uses clinical signs to determine the territory of the infarct.
You can read our guide to stroke classification here. Haemorrhagic strokes occur when there is a rupture of a blood vessel or abnormal vascular structure within the brain. Clinical Examination.
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