1. General Considerations
Although the brain accounts for just over 2% of the body's total weight, it receives nearly 20% of the cardiac output. Whereas larger cerebral vessels are influenced by a balance of sympathetic and parasympathetic tone, vascular tone in medium and small cerebral vessels is altered primarily by the mechanism of autoregulation. Blood flow is tightly regulated at the arterial level to maintain an average cerebral blood flow (CBF) of 50 ml/100 g/min.
Smooth muscle cells within the cerebral vessels vasoconstrict or vasodilate in response to wall stress and shear to maintain a constant blood flow over a wide range of blood pressures. Within the endothelial cell, a complex balance exists between calcium levels and phosphorylation states of myosin. Central to this balance are endothelium-derived relaxation factors (EDRFs) and endothelial-derived constricting factors (eDCFs). The EDRFs include nitric oxide (NO), prostacyclin, and endothelium-derived hyperpolariza-tion factor; notable EDCFs are endothelin, angiotensin II, prostaglandin F2a, and the thromboxanes.
Through autoregulation, local vasculature can also increase CBF to compensate for increased neuronal activity and metabolism (CBF-metabolism coupling). Metabolites produced in the brain and throughout the body can also cause vessel changes and alter CBF. Carbon dioxide has potent vasoactive characteristics. Increased CO2 causes vasodilatation and a resultant increase in CBF. CBF is less sensitive to arterial oxygen concentrations and increases only at significantly low oxygen levels. Extracellular pH, lactic acid, adenosine, and adenosine triphosphate also have vasoactive properties.
The blood supply to the brain, brain stem, and much of the spinal cord is derived from two paired vessels, the internal carotid and the vertebral arteries. The internal carotid arteries form the anterior circulation and supply most of the telencephalon and much of the diencephalon (Table I). The posterior circulation is a derivative of the vertebrobasilar system and supplies the brain stem and cerebellum as well as parts of the diencephalon, spinal cord, and occipital and temporal lobes (Table II).
2. The Anterior Circulation a. Internal Carotid Artery The anterior circulation is supplied by the two internal carotid arteries (ICAs). The extracranial ICA is divided into two segments. The most proximal portion of the ICA is dilated and is termed the carotid bulb. The remaining distal portion then ascends through the neck to the skull base without branching. This distinguishes the extracranial ICA from the external carotid artery, which has numerous branches to the face and neck. The ICA then enters the skull base at the carotid canal, traverses the petrous portion of the temporal bone, passes through the cavernous sinus, and finally enters the subarachnoid space at the base of the brain. The ICA can therefore be divided into cervical, petrous, cavernous, and cerebral parts (Fig. 1).
The petrous portion of the carotid is composed of a vertical and horizontal segment. It has two possible branches, the caroticotympanic and vidian arteries, but neither is visualized in the majority of angiograms. The cavernous ICA begins at the level of the petrolingual ligament ascending vertically, then horizontally, and finally vertically again before ending at the level of the anterior clinoid. During its course it gives rise to the meningohypophyseal trunk, the inferolateral trunk, and the capsular arteries of McConnell. The meningohypophyseal trunk divides into the inferior hypophyseal artery, tentorial artery, and small clival arteries. The inferolateral trunk branches into arteries supplying several cranial nerves and the cavernous sinus dura. The capsular arteries, when present, supply the pituitary capsule.
After exiting the cavernous sinus, the supraclinoid ICA gives rise to its first intradural branch, the ophthalmic artery. The ophthalmic artery travels along with the optic nerve through the optic canal to the orbit, where its branches supply various orbital and ocular structures. The supraclinoid ICA also gives rise to the superior hypophyseal artery, which supplies the optic chiasm, pituitary stalk, and anterior pituitary gland. The ICA then proceeds superiorly adjacent to the optic chiasm and bifurcates into its terminal branches, the middle and anterior cerebral arteries.
Before bifurcating, it gives rise to two smaller branches, the posterior communicating artery (PCoA) and the anterior choroidal artery (AChA). The PCoA courses posteriorly, superior to the oculomotor nerve, and joins the posterior cerebral artery, thus creating an anastomosis between the anterior and posterior circulation. The AChA supplies important structures, including the optic tract, cerebral peduncle, internal capsule, thalamus, and hippocampus.
b. Anterior Cerebral Artery The anterior cerebral artery (ACA) runs medially, superior to the optic nerve, and enters the longitudinal fissure, where it arches posteriorly, following the corpus callosum, to supply the medial aspects of the frontal and parietal lobes (Fig. 2). Some of the smaller branches extend onto the dorsolateral surface of the hemisphere.
The proximal ACA gives rise to the medial lenticulostriate and perforating arteries, including the recurrent artery of Huebner, that supply the basal forebrain and optic nerves. The proximal ACAs are connected by the anterior communicating artery
Major Branches of the Anterior Circulation Major vessel Branches and the areas supplied
Internal carotid artery Petrous portion
Middle and inner ear Vidian artery
Anastomoses with branches of the external carotid artery Cavernous portion
Meningohypophyseal trunk Inferior hypophyseal artery
Posterior pituitary capsule Marginal tentorial artery
Tentorium Clival arteries Clivus Inferolateral trunk
Capsular arteries of McConnell Anterior and inferior pituitary capsule Supraclinoid portion Ophthalmic artery
Optic nerve, choroid, retina, conjunctivae, lacrimal gland, extraocular muscles, falx cerebri, anastomoses with external carotid artery Superior hypophyseal artery
Pituitary stalk, anterior pituitary, optic chiasm Anterior choroidal artery
Optic chiasm and tract, thalamus, internal capsule, cerebral peduncle, choroid plexus, medial temporal lobe
Posterior communicating artery
Thalamus, hypothalamus, internal capsule Anterior cerebral artery Middle cerebral artery Anterior cerebral artery Medial lenticulostriate arteries
Optic nerve, optic chiasm, hypothalamus, fornix, striatum Perforating arteries Recurrent artery of Heubner
Basal ganglia, internal capsule, portions of frontal lobe Anterior communicating artery
Infundibulum, optic chiasm, hypothalamus Orbitofrontal artery
Ventromedial frontal lobe, olfactory tract Frontopolar artery
Medial frontal lobe, lateral surface of superior frontal gyrus Pericallosal artery Callosomarginal artery
Along with pericallosal artery, their branches supply the anteromedial frontal and parietal cortex
Table I (continued)
Major vessel Branches and the areas supplied
Middle cerebral artery Ml and M2
Anterior temporal artery Anterior temporal lobe Lateral lenticulostriate arteries Basal ganglia, internal capsule M3 and M4
Orbitofrontal and prefrontal arteries
Middle and inferior frontal gyri Precentral, central and postcentral sulcus arteries
Precentral, central, and postcentral gyri, anterior parietal lobe Posterior parietal artery
Parietal lobules, supramarginal gyrus Angular artery
Posterior superior temporal gyrus, supramarginal gyrus, angular gyrus Temporooccipital artery
Posterior portions of temporal lobe Posterior, medial, and anterior temporal arteries Corresponding portions of temporal lobe
(ACoA) near their entrance into the longitudinal fissure. The ACA then gives rise to the orbitofrontal and frontopolar arteries before terminating into the pericallosal and callosomarginal arteries.
c. Middle Cerebral Artery The middle cerebral artery (MCA) begins at the ICA bifurcation and courses into the Sylvian fissure. Before entering the fissure, the MCA bifurcates and these branches ramify over the insula. After emerging from the fissure, the MCA spreads out to supply most of the lateral surface of the cerebral hemisphere (Fig. 2).
This large artery is subdivided into four segments. The first segment, M1, contains the bifurcation and ends at the entrance to the Sylvian fissure. The Ml segment gives rise to the anterior temporal and lateral lenticulostriate arteries supplying the basal ganglia and internal capsule. Within the Sylvian fissure, the M2, or insular, segment ramifies into 6-10 branches that course along the insula. The M3, or opercular, segment begins at the superior portion of the circular sulcus of the insula and ends as the MCA exits the Sylvian fissure to course along the cortical surface. Like the M2 segment, the M3 vessels also ramify and form the end arteries of the M4, or cortical, segment.
These cortical vessels supply much of the lateral surface of the frontal, parietal, and temporal lobes.
3. The Posterior Circulation a. Vertebrobasilar System The posterior circulation denotes the network of the two vertebral arteries (VAs) and the basilar artery (BA). The paired VAs then fuse at the junction of the medulla and pons to form the midline BA, which proceeds rostrally along the anterior surface of the pons (Fig. 3).
b. Vertebral Artery The VA ascends within the transverse foraminae of C6 to Cl. After exiting the transverse foramen of C1 it angles posteromedially along the arch of Cl around its superior facet and then turns abruptly again to course rostrally alongside the medulla through the foramen magnum. Along its extracranial course, the VA gives rise to small unnamed muscular and segmental spinal arteries. The distal extracranial VA also supplies meningeal arteries that serve the dura of the posterior fossa.
Before the intracranial VAs join to form the BA, each vertebral artery gives rise to three branches: the posterior spinal, anterior spinal, and posterior inferior
Major Branches of the Posterior Circulation Major vessel Branches
Vertebral artery Muscular arteries
Deep cervical muscles
Segmental spinal arteries
Vertebral bodies, spinal cord
Falx cerebelli, dura around foramen magnum and occipital bone
Posterior spinal artery
Posterior third of spinal cord Anterior spinal artery
Anterior two-thirds of spinal cord Posterior inferior cerebellar artery Lateral medulla; cranial nerves IX, X, and XI; choroid plexus of fourth ventricle; cerebellum (posteroinferior portions, vermis, tonsils) Basilar artery Pontine perforating arteries
Ventral pons and midbrain Anterior inferior cerebellar artery Cranial nerves VII and VIII, ventrolateral pons, medulla, cerebellum (flocculus, anterolateral portions)
Superior cerebellar artery
Deep cerebellar nuclei, superior portions of cerebellum, midbrain
Posterior cerebral artery Posterior cerebral artery Posterior communicating artery Perforating arteries
Cranial nerves III and IV, thalamus, hypothalamus, midbrain, internal capsule
Posterior choroidal artery
Choroid plexus of third ventricle, thalamus, fornix, tectum, pineal gland Anterior and posterior temporal arteries
Posterior temporal and anterior occipital lobes
Occipital lobe and corpus callosum cerebellar artery (PICA). The posterior spinal artery runs caudally along the dorsolateral aspect of the spinal cord and supplies the posterior third ofthat half of the spinal cord. The anterior spinal artery joins its counterpart from the opposite side, forming a single anterior spinal artery that runs caudally along the ventral midline of the spinal cord, supplying the anterior two-thirds of the spinal cord. PICA arises at the level of the medulla and, as its name implies, supplies much of the inferior surface of the cerebellum. It also supplies the choroid plexus of the fourth ventricle, cranial nerves (CNs) IX, X, and XI, and much of the lateral medulla.
c. Basilar Artery From its origin at the pontome-dullary junction, the BA proceeds rostrally and, at the level of the midbrain, bifurcates into the two posterior cerebral arteries (PCAs). Before this bifurcation, it gives rise to numerous pontine perforating branches, the labyrinthine arteries, and two paired vessels—the anterior inferior cerebellar artery (AICA) and the superior cerebellar artery (SCA). The AICA arises just distal to the basilar origin and supplies the anterolat-eral cerebellum, the pons, and rostral medulla. The SCA arises proximal to the basilar bifurcation, courses inferior to the oculomotor nerve, and supplies the superior cerebellum, its deep nuclei, and much of the caudal midbrain.
The PCA curves over the oculomotor nerve, around the midbrain, and passes through the superior cistern. The proximal PCA sends perforators to the rostral midbrain, caudal diencephalon, and CNs III and IV. It also gives rise to several posterior choroidal arteries, which supply the choroid plexus of the third ventricle, thalamus, and pineal region. The anterior and posterior choroidal arteries form anastomoses in the vicinity of the glomus. The proximal PCA also gives rise to the posterior communicating artery, connecting the anterior and posterior cerebral circulation. The cortical PCA branches include the anterior temporal, posterior temporal, lateral occipital, and medial occipital arteries. These branches spread out to supply the medial and inferior surfaces of the occipital and temporal lobes. In effect, the PCAs supply areas that serve vision, visual memory, and eye motility.
The circle of Willis is a polygonal arcade that connects the two halves of the anterior circulation with the posterior circulation. Its components include both ICAs and ACAs, the ACoAs, PCoAs, PCAs, and the BA (Fig. 4). In cases of major vessel occlusion, either within the circle of Willis or proximal to it, the communicating arteries theoretically permit vital ana-stomotic flow and prevent neurological damage. In
fact, fewer than half the circles have the classical appearance and asymmetries are common. In rare cases, one of the communicating arteries may be missing, resulting in an incomplete circle.
Other routes of collateral circulation are available. Arterial anastomoses between the extracranial and intracranial carotid exist and may enlarge to a remarkable degree to compensate for slowly developing occlusions. The most important are anastomoses in the orbit between the ophthalmic artery and branches of the external carotid artery. In cases of proximal ICA occlusion, retrograde flow from the external carotid artery through the ophthalmic artery can reach the distal internal carotid territories.
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