Blood Supply of the Brain

The Internal Carotid Arteries Supply Most of the Cerebrum

An internal carotid artery ascends through each side of the neck, traverses the petrous temporal bone, passes through the cavernous sinus, and finally reaches the subarachnoid space at the base of the brain (see Fig. 6.15 ). Just as it leaves the cavernous sinus, it gives rise to the ophthalmic artery, which travels along the optic nerve to the orbit, where it supplies the eye, other orbital contents, and some nearby structures. ^a

a The ophthalmic artery, being one of the first divisions of the internal carotid arteries, is vulnerable to atherosclerotic plaques causing a transient ischemic attack (TIA), referred to as amaurosis fugax (see later).

The internal carotid artery then proceeds superiorly alongside the optic chiasm ( Fig. 6.3 ) and bifurcates into the middle and anterior cerebral arteries. Before bifurcating, the internal carotid artery gives rise to two smaller branches, the anterior choroidal artery and the posterior communicating artery. The anterior choroidal artery is a long, thin artery that can be significant clinically in that it supplies a number of different structures and is frequently involved in cerebrovascular accidents. Along its course (see Fig. 6.6 ) it supplies the optic tract; the choroid plexus of the inferior horn of the lateral ventricle; and some deep brain structures such as the amygdala and portions of the internal capsule, thalamus, and hippocampus (see Fig. 6.23 ). The posterior communicating artery passes posteriorly, inferior to the optic tract and toward the cerebral peduncle, and joins the posterior cerebral artery (part of the vertebral artery system). Small perforating branches from the posterior communicating artery supply parts of the thalamus and hypothalamus (see Table 6.1 ).

The anterior cerebral artery runs medially, superior to the optic nerve, and enters the longitudinal fissure (see Fig. 6.3 ). The two anterior cerebral arteries, near their entrance into the longitudinal fissure, are connected by the anterior communicating artery (see Figs. 6.6 and 6.11 ). Small penetrating arteries branch off from the anterior communicating artery; these supply anterior portions of the hypothalamus (see Table 6.1 ). The anterior cerebral arteries then arch posteriorly, following the corpus callosum, to supply medial parts of the frontal and parietal lobes ( Fig. 6.4A ). Some of the smaller branches extend onto the posterolateral surface of the hemisphere (see Fig. 6.4B ). Distal to the anterior communicating artery, the anterior cerebral artery continues as the pericallosal artery, which stays immediately adjacent to the corpus callosum. Near the genu of the corpus callosum, the callosomarginal artery typically branches off from the pericallosal artery and follows the cingulate sulcus (see Fig. 6.4A ). Parts of the precentral and postcentral gyri extend superiorly onto the medial surface of the frontal and parietal lobes, so occlusion of an anterior cerebral artery causes restricted contralateral motor and somatosensory deficits (affecting the leg more than other parts of the body, because of the somatotopic arrangement shown in Fig. 3.30 ).

The large middle cerebral artery proceeds laterally into the lateral sulcus (see Fig. 6.3 ). It divides into a number of branches that supply the insula, emerge from the lateral sulcus, and spread out to supply most of the lateral surface of the cerebral hemisphere (see Figs. 6.5 and 6.4B ). Most of the precentral and postcentral gyri are within this area of supply, so occlusion of a middle cerebral artery causes major motor and somatosensory deficits with the exceptions of the leg and foot, as noted earlier. In addition, if the left hemisphere is the one involved, language deficits are almost invariably found.

Small Perforating Arteries Supply Deep Cerebral Structures

Along its course toward the lateral sulcus, the middle cerebral artery gives rise to as many as a dozen very small branches that penetrate the brain near their origin and supply deep structures of the diencephalon and telencephalon ( Figs. 6.6 to 6.8 ). These particular arteries are called the lenticulostriate arteries, but similar small branches, referred to collectively as perforating (or ganglionic ) arteries, arise from all the arteries around the base of the brain. Perforating arteries are particularly numerous in the area adjacent to the optic chiasm and in the area between the cerebral peduncles; for this reason they are called the anterior and posterior perforated substances, respectively. The narrow, thin-walled vessels of the anterior perforated substance are involved frequently in strokes ( Fig. 6.9 ). They supply such deep cerebral structures that damage to these small vessels can cause neurological deficits out of proportion to their size. For example, the somatosensory projection from the thalamus to the postcentral gyrus must pass through the internal capsule; damage to a small part of the internal capsule from rupture or occlusion of a perforating artery can cause deficits similar to those resulting from damage to a large expanse of cortex.

The Vertebral-Basilar System Supplies the Brainstem and Parts of the Cerebrum and Spinal Cord

The two vertebral arteries run rostrally alongside the medulla and fuse at the junction between the medulla and pons to form the midline basilar artery, which proceeds rostrally along the anterior surface of the pons (see Fig. 6.3 ).

Before joining the basilar artery, each vertebral artery gives rise to three branches: the posterior spinal artery, anterior spinal artery, and posterior inferior cerebellar artery. Each of the posterior spinal arteries runs caudally along the posterolateral aspect of the spinal cord, and together they supply the posterior third of the cord. The anterior spinal arteries join together soon after branching from each of the vertebral arteries, forming a single anterior spinal artery that runs caudally along the anterior midline of the spinal cord, supplying the anterior two-thirds of the cord (see Figs. 10.29 and 10.30 ). These small spinal arteries cannot carry enough blood from the vertebral arteries to supply more than the cervical segments of the spinal cord and must be refilled at various points caudal to this (discussed in Chapter 10 ). The posterior inferior cerebellar artery (often referred to by the acronym PICA ), as its name implies, supplies much of the inferior surface of the cerebellar hemisphere ( Fig. 6.10 ); however, it sends branches to other structures on its way to the cerebellum. As it curves around the brainstem, the PICA supplies much of the lateral medulla, as well as the choroid plexus of the fourth ventricle. This is a uniform occurrence in the large named branches of the vertebral-basilar system, directly comparable to the perforating branches from vessels such as the middle cerebral artery: on their way to their major area of supply, arteries in the vertebral-basilar system send branches to brainstem structures. Knowing the brainstem level at which these large named branches emerge facilitates reasonably accurate inferences about the blood supply of any given region of the brainstem (see Figs. 11.29 and 11.30 ). The basilar artery proceeds rostrally and, at the level of the midbrain, bifurcates into the two posterior cerebral arteries. Before this bifurcation, it gives rise to numerous unnamed branches and two named branches, the anterior inferior cerebellar artery and the superior cerebellar artery.

The anterior inferior cerebellar artery (or AICA ) arises just rostral to the origin of the basilar artery and supplies the more anterior portions of the inferior surface of the cerebellum (e.g., the flocculus), as well as parts of the caudal pons. The superior cerebellar artery arises just caudal to the bifurcation of the basilar artery and supplies the superior surface of the cerebellum and much of the caudal midbrain and rostral pons (see Fig. 11.30 ). The many smaller branches of the basilar artery, collectively called pontine arteries, supply the remainder of the pons. One of these, the internal auditory or labyrinthine artery (which is often a branch of the AICA), although hard to distinguish from the others by appearance, is functionally important because it also supplies the inner ear. Its occlusion can lead to vertigo and ipsilateral deafness.

The posterior cerebral artery curves around the midbrain and passes through the superior cistern; its branches spread out to supply the medial and inferior surfaces of the occipital and temporal lobes (see Figs. 6.3, 6.4A, and 6.10 ). Along the way, it sends branches to the rostral midbrain and posterior parts of the diencephalon, including the thalamus and the posterior parts of the corpus callosum known as the splenium. It also gives rise to several posterior choroidal arteries, which supply the choroid plexus of the third ventricle and the body of the lateral ventricle. The anterior and posterior choroidal arteries form anastomoses ^b

b Anastomosis comes from the Greek meaning to provide with an outlet or to connect—described as the reconnection of two streams that previously branched out.

in the vicinity of the glomus (an enlarged area of choroid plexus). The primary visual cortex is located in the occipital lobe, so occlusion of a posterior cerebral artery at its origin leads to visual field losses in addition to other deficits referable to the midbrain and diencephalon.

The Cerebral Arterial Circle (Circle of Willis) Interconnects the Internal Carotid and Vertebral-Basilar Systems

The posterior cerebral artery is connected to the internal carotid artery by the posterior communicating artery. This completes an arterial polygon called the cerebral arterial circle (circle of Willis) ( Fig. 6.11 and see Fig. 6.3 ), through which the anterior cerebral, internal carotid, and posterior cerebral arteries of both sides are interconnected. Normally, because of pressure differentials very little blood flows around this circle: the arterial pressure in the internal carotid arteries is about the same as that in the posterior cerebral arteries, so little blood flows through the posterior communicating arteries. However, if one major vessel becomes occluded, either within the cerebral arterial circle or proximal to it, the communicating arteries may allow critically important anastomotic flow and prevent neurological damage. By such a mechanism it would be theoretically possible (although highly unlikely) for the entire brain to be perfused by just one of the four major arteries that normally supply it. The anterior and posterior communicating arteries vary in size (see later), so the establishment of effective anastomotic flow in the event of an arterial occlusion may also depend on the time course of the occlusion. A small communicating artery can enlarge slowly to compensate for a slowly developing occlusion, but in such a case an abrupt blockage may cause serious damage.

Designating the cerebral arterial circle shown in Fig. 6.11 as “normal” is largely a nod to an aesthetic need, because fewer than half the circles have this appearance. Some frequently seen “abnormalities” are indicated in Fig. 6.12 . Asymmetries are common (as in the brain shown in Fig. 6.3A ): one or more of the communicating arteries may be very small (hypoplastic) or absent ^c

c Although parts of the circle are seldom missing entirely, vessels smaller than 0.5 to 1 mm in diameter carry so little blood that they are functionally ineffective and are considered “missing.”

; one anterior cerebral artery may be much smaller at its origin than the other; one posterior cerebral artery may retain its embryological origin from the internal carotid and be connected to the basilar artery through a posterior communicating artery. These asymmetries in the cerebral arterial circle lead to corresponding asymmetries in the way the brain is supplied ( Fig. 6.13 ).

Other routes of collateral circulation are available, although the cerebral arterial circle is likely to be the most important. There are anastomoses at the arteriolar and capillary levels between terminal branches of the cerebral arteries (e.g., anastomoses between the middle cerebral artery [MCA] and the anterior cerebral artery [ACA] territory at the dorsal lateral surface along the midline). These are usually inadequate in adults for maintaining the entire territory of a major cerebral artery if it becomes occluded, but occasionally they may be sufficient for maintaining a large part of this territory. In addition, well-defined arterial anastomoses may enlarge to a remarkable degree to compensate for slowly developing occlusions. For example, there have been documented cases in which the territory of one posterior cerebral artery was supplied by the internal carotid artery on that side by means of flow through the anterior choroidal artery and, from there, through a posterior choroidal artery and into the posterior cerebral artery.

Finally, there are a limited number of intracranial-extracranial anastomoses that can enlarge to a functional degree. The most important are anastomoses in the orbit between the ophthalmic artery and branches of the external carotid artery. If an internal carotid artery becomes occluded, it is possible for blood from the external carotid artery to flow backward through the ophthalmic artery to reach internal carotid territory.