5 Blood supply of the brain
The brain is absolutely dependent on a continuous supply of oxygenated blood. It controls the delivery of blood by sensing the momentary pressure changes in its main arteries of supply, the internal carotids. It controls the arterial oxygen tension by monitoring respiratory gas levels in the internal carotid artery and in the cerebrospinal fluid (CSF) beside the medulla oblongata. The control systems used by the brain are exquisitely sophisticated, but they can be brought to nothing if a distributing artery ruptures spontaneously or is rammed shut by an embolus.
Figure 5.1 (A) Brain viewed from below, showing background structures related to the circle of Willis. Part of the left temporal lobe (to right of picture) has been removed to show the choroid plexus in the inferior horn of the lateral ventricle. (B) The arteries comprising the circle of Willis. The four groups of central branches are shown; the thalamoperforating artery belongs to the posteromedial group, and the thalamogeniculate artery belongs to the posterolateral group. ACA, MCA, PCA, anterior, middle, posterior cerebral arteries; ICA, internal carotid artery.
Each internal carotid artery enters the subarachnoid space by piercing the roof of the cavernous sinus. In the subarachnoid space, it gives off ophthalmic, posterior communicating, and anterior choroidal arteries before dividing into the anterior and middle cerebral arteries.
The basilar artery divides at the upper border of the pons into the two posterior cerebral arteries. The cerebral arterial circle (circle of Willis) is completed by a linkage of the posterior communicating artery with the posterior cerebral on each side, and by linkage of the two anterior cerebrals by the anterior communicating artery.
Dozens of fine central (perforating) branches are given off by the constituent arteries of the circle of Willis. They enter the brain through the anterior perforated substance beside the optic chiasm and through the posterior perforated substance behind the mammillary bodies. They have been classified in various ways but can be conveniently grouped into short and long branches. Short central branches arise from all the constituent arteries and from the two choroidal arteries. They supply the optic nerve, chiasm, and tract, and the hypothalamus. Long central branches arise from the three cerebral arteries. They supply the thalamus, corpus striatum, and internal capsule. They include the striate branches of the anterior and middle cerebral arteries.
The anterior cerebral artery passes above the optic chiasm to gain the medial surface of the cerebral hemisphere. It forms an arch around the genu of the corpus callosum, making it easy to identify in a carotid angiogram (see later). Close to the anterior communicating artery, it gives off the medial striate artery, also known as the recurrent artery of Heubner (pron.‘Hoibner’), which contributes to the blood supply of the internal capsule. Cortical branches of the anterior cerebral artery supply the medial surface of the hemisphere as far back as the parietooccipital sulcus (Table 5.1). The branches overlap on to the orbital and lateral surfaces of the hemisphere.
|Orbitofrontal||Orbital surface of frontal lobe|
|Polar frontal||Frontal pole|
|Callosomarginal||Cingulate and superior frontal gyri; paracentral lobule|
The middle cerebral artery is the main continuation of the internal carotid, receiving 60–80% of the carotid blood flow. It immediately gives off important central branches, then passes along the depth of the lateral fissure to reach the surface of the insula. There it usually breaks into upper and lower divisions. The upper division supplies the frontal lobe, the lower division supplies the parietal and temporal lobes and the midregion of the optic radiation. Named branches and their territories are listed in Table 5.2. Overall, the middle cerebral supplies two-thirds of the lateral surface of the brain.
|Stem||Frontobasal||Orbital surface of frontal lobe|
|Anterior temporal||Anterior temporal cortex|
|Upper division||Prefrontal||Prefrontal cortex|
|Central||Pre- and postcentral gyri|
|Postcentral||Postcentral and anterior parietal cortex|
|Parietal||Posterior parietal cortex|
|Lower division||Middle temporal||Midtemporal cortex|
|Temporooccipital||Temporal and occipital cortex|
|Angular||Angular and neighboring gyri|
The central branches of the middle cerebral include the lateral striate arteries (Figure 5.4). These arteries supply the corpus striatum, internal capsule, and thalamus. Occlusion of one of the lateral striate arteries is the chief cause of classic stroke, where damage to the pyramidal tract in the posterior limb of the internal capsule causes hemiplegia, a term denoting paralysis of the contralateral arm, leg, and lower part of face.
Figure 5.4 Distribution of perforating branches of the middle cerebral, anterior choroidal, and posterior cerebral arteries (schematic). The anterior choroidal artery arises from the internal carotid.
The two posterior cerebral arteries are the terminal branches of the basilar. However, in embryonic life they arise from the internal carotid, and in about 25% of individuals the internal carotid persists as the primary source of blood on one or both sides, by way of a large posterior communicating artery.
Figure 5.5 View from below the cerebral hemispheres, showing the cortical branches and territories of the three cerebral arteries. ACA, MCA, PCA, anterior, middle, posterior cerebral arteries; ICA, internal carotid artery.
Close to its origin, each posterior cerebral artery gives branches to the midbrain and a posterior choroidal artery to the choroid plexus of the lateral ventricle. Additional, central branches are sent into the posterior perforated substance (Figure 5.1). The main artery winds around the midbrain in company with the optic tract. It supplies the splenium of the corpus callosum and the cortex of the occipital and temporal lobes. Named cortical branches and their territories are given in Table 5.3.