Blood supply of the brain

The Carotid Circulation Supplies:
• optic nerves and retina
• cortex and deep white matter of the frontal and parietal lobes, and lateral aspects ofthe temporal and occipital lobes
• all of the corpus callosum except its posterior regions
• most of the basal ganglia and internal capsule

The right common carotid artery originates from the bifurcation of the brachiocephalic trunk,while the left common carotid originates directly from the aortic arch. Each common carotid thenbranches to form the internal and external carotid vessels. After the internal carotid ascendsthrough the neck, traverses the temporal bone, and passes through the cavernous sinus it finallyreaches the subarachnoid space at the base of the brain.

As the internal carotid leaves the cavernous sinus it gives rise to its first intracranial branch, theophthalmic artery, which travels along the optic nerve into the orbit. There its branches supplythe retina and other structures of the eyeball itself, as well as other structures in and around theorbit. The internal carotid continues in a superior direction and usually gives off two additionalbranches: the posterior communicating and anterior choroidal arteries.

The posterior communicating arteries usually link the internal carotid to the posterior cerebralartery, and may be large or threadlike. However, in a number of individuals one or both of theposterior cerebral arteries retain their embryological state as direct branches of the internalcarotid artery itself. The anterior choroidal artery also varies a great deal in size and importancein different individuals, and may branch from the middle cerebral artery rather than the internalcarotid. For this reason, we will discuss it with the middle cerebral artery. Finally, the internalcarotid divides to form the anterior and middle cerebral arteries.

Anterior Cerebral Artery

The anterior cerebral artery (ACA) arises from the internal carotid at nearly a right angle. Itsends deep penetrating branches to supply the most anterior portions of the basal ganglia. It thensweeps forward into the interhemispheric fissure, and then runs up and over the genu of thecorpus callosum before turning backwards along the corpus callosum. As it runs backwards itforms one branch that stays immediately adjacent to the corpus callosum while a second branchruns in the cingulate sulcus (just superior to the cingulate gyrus

ACA supplies the medial and superior parts of the frontal lobe, and of the anterior parietal lobe.

These regions include the following key functional areas:
• septal area
• primary motor cortex for the leg and foot areas, and the urinary bladder
• additional motor planning areas in the medial frontal lobe, anterior to the precentral gryus
• primary somatosensory cortex for the leg and foot

ACA also supplies most of the corpus callosum except its posterior part. These callosal fibersenable the language-dominant hemisphere to find out what the other hemisphere is doing, and todirect its activities

The short anterior communicating artery joins the two anterior cerebral arteries. This vessel mayallow collateral flow into the opposite hemisphere if the carotid artery is occluded on either side

Middle Cerebral Artery
The middle cerebral artery (MCA) has a large diameter and branches at an acute angle fromthe internal carotid. The MCA passes laterally just underneath the frontal lobe, ultimately takingup a position between the temporal and frontal lobes in the Sylvian fissure. The initial part of theMCA is a single vessel called the stem or M1 segment. As it passes laterally, the stem gives off aseries of 6-12 long, small diameter penetrating vessels that travel directly upward to supply thebasal ganglia and much of the internal capsule. These are called the lenticulostriate arteries.

The lenticulostriate vessels are small diameter arteries that originate as right angle branchesfrom the MCA stem (a large diameter vessel with a brisk, high pressure blood flow). These smallarteries are particularly susceptible to damage from hypertension. They may either rupture(producing an intracerebral hemorrhage that is initially centered in the region they supply) orbecome occluded (producing a lacunar infarct in the tissue they supply). The lenticulostriatearteries are ‘end arteries’ and regions that they supply do not have significant collateral bloodsupply. Therefore occlusion of these vessels leads to stereotyped stroke syndromes.

In the case of the lenticulostriate vessels, hemorrhage may remain localized to the putamen (andcaudate), may involve neighboring structures like the internal capsule and other more distantwhite matter of the hemisphere, or may even rupture into the ventricular system. Lacunarinfarcts may have serious functional consequences if they involve motor or sensory fibers in theinternal capsule but may be ‘silent’ if they involve other small regions of white matter or thebasal ganglia.Once in the Sylvian fissure itself, the MCA stem divides into two or, in a smaller number ofcases, three main cortical branches that supply almost the entire lateral surface of the brain aswell as the insula. Large emboli carried up the carotid tend to be swept into MCA, and are proneto getting stuck at this branch point.

The superior (upper or suprasylvian) MCA branch gives rise to several arteries that supply muchof the lateral and inferior frontal lobe and the anterior lateral parts of the parietal lobe. Theinferior (lower or infrasylvian) MCA branch gives rise to arteries that supply the lateral temporallobe including its anterior tip and the amygdala, posterior parietal and much of the lateraloccipital lobe. Emboli can also lodge in one of these two major cortical branches, as well as inthe smaller arteries which each of them will subsequently form..

There is a tendency for atheromatous plaques to form at branchings and curves of thecerebral arteries. Thus in the carotid circulation the most frequent sites are in the internalcarotid artery at its origin from the common carotid, in the stem MCA or its bifurcation intosuperior and inferior divisions, or in the ACA as it curves backwards over the corpuscallosum.

Superior branches of MCA participate in supplying the following key functional areas:
• Primary motor cortex for face and arm, and axons originating in the leg as well asface and arm areas that are headed for the internal capsule as part of the corticobularor corticospinal tracts
• Broca’s area and other related gray and white matter important for languageexpression -- in the language-dominant (usually L) hemisphere
• Frontal eye fields (important for ‘looking at’ eye movements to the opposite side)
• Primary somatosensory cortex for face and arm • Parts of lateral frontal and parietal lobes important for 3-D visuospatial perceptionsof one’s own body and of the outside world, and for the ability to interpret andexpress emotions -- in the nondominant (usually R) hemisphere

Inferior branches of MCA participate in supplying the following key functional areas:
• Wernicke’s and other related areas important for language comprehension in thelanguage- dominant (usually L) hemisphere • Parts of the posterior parietal lobe important for 3-D visuospatial perceptionsperceptions of one’s own body and of the outside world, and for the ability tointerpret emotions -- in the nondominant (usually R) hemisphere
• Optic radiations, particularly fibers that represent information from the contralateralsuperior quadrants and loop forward into the temporal lobe (they are located anteriorand lateral to the temporal horn of the lateral ventricle) as they travel from the lateralgeniculate body to the striate cortex, located in the occipital lobe