Chapter summary
Arterial supply of the forebrain
Study guidelines
- 1.
On simple outline drawings of the lateral, medial, and inferior surfaces of a cerebral hemisphere, learn to shade in the territories of the three cerebral arteries.
- 2.
Identify the main sources of arterial supply to the internal capsule.
- 3.
Become familiar with carotid and vertebral angiograms.
- 4.
Be able to list the territories supplied by the vertebral and basilar arteries.
- 5.
Identify the two blood – brain barriers. Be able to understand why, for example, shallow breathing following abdominal surgery may tip a patient into coma.
Because interpretation of the symptoms caused by cerebrovascular accidents requires prior understanding of brain function, Clinical Panels on this subject are placed in the final chapter.
A Clinical Panel on blood–brain barrier pathology is placed in the present chapter because the symptoms are of a general nature.
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 carotid and vertebral arteries. The arterial oxygen tension is controlled by a medullary chemosensitive area that monitors respiratory gas levels in the internal carotid artery and in the cerebrospinal fluid (CSF). 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.
Arterial supply of the forebrain
The blood supply to the forebrain is derived from the two internal carotid arteries and from the basilar artery ( Figure 5.1 ).
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 .
The choroid plexus of the lateral ventricle is supplied from the anterior choroidal branch of the internal carotid artery and by the posterior choroidal branch from the posterior cerebral 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. (These designations refer to both the location on the ventral surface of the brain and the small perforations that appear when the numerous small penetrating arteries that supply these areas are pulled away from their points of entry.) These small perforating arteries 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 (lenticulostriate) branches of the anterior and middle cerebral arteries.
Anterior cerebral artery ( Figure 5.2 )
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 arterial blood supply of the internal capsule and the head of the caudate nucleus. 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.
| Branch | Territory |
|---|---|
| Orbitofrontal | Orbital surface of frontal lobe |
| Polar frontal | Frontal pole |
| Callosomarginal | Cingulate and superior frontal gyri; paracentral lobule |
| Pericallosal | Corpus callosum |
* The term cortical is conventional. Terminal is better, because these arteries also supply the underlying white matter.
Middle cerebral artery ( Figure 5.3 )
The middle cerebral artery is the main continuation of the internal carotid, receiving 60 to 80% of the carotid blood flow. It immediately gives off important central branches and 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 and parietal lobes; 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.
| Origin | Branch(es) | Territory |
|---|---|---|
| Stem | Frontobasal | Orbital surface of frontal lobe |
| Anterior temporal | Anterior temporal cortex | |
| Upper division | Prefrontal | Prefrontal cortex |
| Precentral | Premotor areas | |
| 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 neighbouring 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 a cause of a classic stroke syndrome (pure motor hemiplegia) , where damage to the corticospinal tract in the posterior limb of the internal capsule causes contralateral hemiplegia, a term denoting paralysis of the contralateral arm, leg, and lower part of the face.
Note: Additional information on the blood supply of the internal capsule is provided in Chapter 35 .
Posterior cerebral artery ( Figures 5.2 and 5.5 )
The two posterior cerebral arteries are the terminal branches of the basilar artery. 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 .
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 .
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