Arteries of the brain: anterior circulation

Origin and characteristics

The brain receives a dual supply of blood with numerous anastomoses. However, although there is a dual supply, they share a common origin. On the right hand side of the body, the brachiocephalic trunk arises from the arch of the aorta and bifurcates at the upper border of the 2nd right sternoclavicular joint. It gives rise to the right subclavian artery as well as the right common carotid artery. The left counterparts to these vessels are direct derivatives of the aortic arch.

Both the left and right common carotid arteries subsequently bifurcate between the third and fourth cervical vertebra (between the superior horn of the thyroid cartilage and the hyoid bone) to give the internal and external carotid arteries. The internal carotid arteries will serve as the anterior blood supply of the brain.

Parts

Traditionally, there were four described parts of the internal carotid artery. There is the cervical part in the neck, the petrous part in the base of the skull, the cavernous part within the cavernous sinus and the intracranial component.

However, the previous Cincinnati Classification (Bouthillier et. al., 1996) has classified the internal carotid artery into seven parts, namely:

• C1 – Cervical Segment
• C2 – Petrous Segment
• C3 – Lacerum Segment
• C4 – Cavernous Segment
• C5 – Clinoid Segment
• C6 – Ophthalmic (Supraclinoid) Segment
• C7 – Communicating (Terminal) Segment

In the other classification system, C2 and C3 were referred to collectively as the petrous part, and C5, C6, and C7 were referred to as the intracranial components.

Trajectory

The cervical internal carotid artery (C1) has no named branches along its cranial course in the neck. The vessel is slightly dilated at its origin; where it is referred to as the carotid sinus or bulb. It travels superiorly, anterior to the transverse processes of the upper three cervical vertebrae, towards the carotid canal where it becomes the petrous segment (C2).

The carotid canal traverses the petrous part of the temporal bone. Within the canal, the petrous segment travels superiorly before taking an anteromedial course; after which it travels along a superomedial course. This segment is said to have three parts – an ascending segment, a genu, and a horizontal segment. The internal carotid artery then travels over the cartilage occluding foramen lacerum to become the lacerum segment (C3). This is a short portion of the vessel that ends at the level of the petrolingual ligament (reflected periosteum spanning the petrosal process and lingula of the sphenoid bone). It should be noted that the lacerum segment of the internal carotid artery does not pass through the foramen lacerum; it simply travels above the foramen.

At the petrolingual ligament, the cavernous segment (C4) courses superiorly along the posterior clinoid process. Here it travels anteriorly adjacent to the cavernous side of the sphenoid bone before turning medially towards the anterior clinoid process. It finally emerges through the roof of the cavernous sinus. Within the cavernous sinus, the internal carotid artery travels superomedially to CN VI (abducens nerve) and medially to CN III (oculomotor nerve), CN IV (trochlear nerve) CN Va and CN Vb (ophthalmic and maxillary divisions of trigeminal nerve, respectively).

The artery exits the cavernous sinus at the proximal dural ring and becomes the clinoid segment (C5). It travels a short distance before becoming the ophthalmic segment (C6) at the distal dural ring. The artery travels parallel and horizontal in an inferolateral position with respect to the optic nerve. The ophthalmic segment ends at the origin of the posterior communicating artery. At this point, the internal carotid artery forms its terminal or communicating segment (C7). It bifurcates into its terminal branches before ending at the anterior perforated substance.

Branches

Main branches

Of the seven segments, three are without branches. These are the cervical, lacerum, and clinoid segments. The petrous part gives of the caroticotympanic and Vidian arteries.  The caroticotympanic artery occasionally occurs as a doubled vessel that travels through the tympanic cavity via the foramen within the carotid canal. It subsequently anastomoses with the anterior tympanic branch of the stylomastoid artery and the maxillary artery.

The cavernous segment gives numerous branches to the walls of the cavernous sinus and the surrounding nerves and dura mater. Of significance, the inferior hypophyseal artery originates from this segment of the internal carotid artery. Conveniently, the ophthalmic segment gives of the ophthalmic artery and the superior hypophyseal artery.

Finally, the communicating segment gives off several major vessels. The posterior communicating artery, which anastomoses with the basilar artery, arises from the C7 segment. The anterior choroidal (supplies mesencephalic, diencephalic, and telencephalic derivatives), anterior cerebral, and middle cerebral arteries also arise from this segment.

It is undoubtedly easier to remember the old classification. However, here is a quick mnemonic to remember the intracranial segments of the internal carotid artery: Please Let Children Consume Our Candy

• Please (Petrous – C2)
• Let (Lacerum – C3)
• Children (Cavernous – C4)
• Consume (Clinoid – C5)
• Our (Ophthalmic – C6)
• Candy (Communicating – C7)

Terminal branches

The middle cerebral artery is the largest terminal branch of the internal carotid artery. It travels through the Sylvian (lateral) fissure before coursing in a posterosuperior direction on the island of Reil (insula). It subsequently divides to supply the lateral cortical surfaces along with the insula.

The vessel gives numerous tributaries to both central and cortical regions of the brain. The central branches are relatively small and include the lenticulostriate arteries that pass through the anterior perforated substance to supply the lentiform nucleus and the posterior limb of the internal capsule.

The cortical branches include the frontal, orbital, parietal, and temporal branches. The frontal arteries perfuse the inferior frontal, middle, and precentral gyri. The lateral orbital parts of the frontal lobe as well as the frontal gyrus are supplied by the orbital branches. The inferior parietal lobe, the inferior part of the superior parietal lobe, and the postcentral gyrus receives blood from the parietal branch. Several temporal arteries then go on to perfuse the lateral aspect of the temporal lobe.

For clinical purposes, the vessel has been subdivided into four parts termed M1 – M4:

• M1 – also called the sphenoid segment, it continues from the termination of the internal carotid artery to the bifurcation (in some individuals, the vessel trifurcates) of the middle cerebral artery.  
• M2 – or the insular segment, travels in the Sylvian fissure.
• M3 – or the opercular portion, begins as the vessel leaves the Sylvian fissure.
• M4 – refers to the cortical segments of the vessel.

Compared to the middle cerebral artery, the anterior cerebral artery is a much smaller branch of the internal carotid artery. It begins at the terminal portion of the internal carotid artery (after the ophthalmic branch is given off) on the medial part of the Sylvian fissure. It travels in an anteromedial course, superior to CN II towards the longitudinal cerebral fissure. Here it anastomoses with the contralateral counterpart via the short anterior communicating artery. The paired arteries then travel through the longitudinal cerebral fissure in a posterior direction along the genu of the corpus callosum. Here, each vessel anastomoses with the ipsilateral posterior cerebral artery.

Like the middle cerebral artery, the anterior cerebral artery also gives off central and cortical branches. The cortical branches, which are named for the regions they supply, are responsible for the somatosensory and motor cortices of the lower limbs. Parietal branches perfuse the precuneus as the orbital branches supply the frontal lobe; olfactory cortex, medial orbital gyrus, and gyrus rectus; and the frontal arteries supply the paracentral lobule, medial frontal and cingulate gyri, and the corpus callosum. The anterior communicating artery has several anteromedial central arteries, which are responsible for supplying the cingulate gyrus, anterior columns of the fornix, hypothalamus lamina terminalis, optic chiasm, and the para-olfactory regions.

The anterior cerebral artery is also subdivided for clinical purposes into three divisions A1 – A3:

• A1 – from its origin to the merger with anterior communicating artery.
• A2 – extending from the branching of anterior communicating artery towards the callosomarginal artery (on the dorsal surface of the corpus callosum).
• A3 – extending onwards from the branching point of the callosomarginal artery.