Anatomy of the blood supply to the brain

The human brain is one of the most important and sophisticated organs of the human body. In fact, every minute, it receives about 15% of the total blood pumped by the heart to our entire body! Cerebral circulation is a complex circulatory system, formed by the two internal carotid arteries, the two vertebral arteries, and their branches. The terminal branches of both the internal carotid arteries and the vertebral arteries lie in the subarachnoid space, which is a space between two meningeal layers called the arachnoid mater and pia mater. Anatomoses between these two arteries and their branches give rise to the Circle of Willis, which is a system of vessels at the base of the brain that helps to ensure adequate blood flow to this vital organ.

Let’s start off with the internal carotid arteries, or ICAs, which are the terminal branches of the common carotid arteries, and form the anterior part of the cerebral vascular system. The ICAs ascend on both sides of the neck to reach the base of the skull, where they enter a passageway in the petrous part of the temporal bone, called the carotid canal. Within the carotid canal, the ICA is close to venous plexuses as well as the carotid plexuses of sympathetic nerves. Within the canal, each ICA turns 90 degrees anteromedially, then another 90 degrees superiorly to exit the carotid canal and enter the cranial cavity.

Inside the cranial cavity, the ICA runs through the cavernous sinus, which is, in fact, a dural venous sinus. So the ICA, an artery, actually runs through a sinus filled with venous blood! Within the cavernous sinus, the ICA travels alongside the abducens nerve, and lies in proximity to the oculomotor nerve, the trochlear nerve and the ophthalmic and maxillary divisions of the trigeminal nerve. Then, the ICA emerges from the cavernous sinus and divides into the anterior cerebral artery, the middle cerebral artery and several smaller branches.

To simplify this complex course, the ICA can be divided into 4 parts: the cervical part, that extends from common carotid to the carotid canal; the petrous part, that extends from the carotid canal to the foramen lacerum; the cavernous part, meaning the part of the ICA within the cavernous sinus; and the cerebral part, after it exits from the cavernous sinus.

The cerebral part of the ICA ends by bifurcating into two major terminal branches: the anterior cerebral artery and middle cerebral artery. Now, the cerebral part of the ICA gives off several other collateral branches that supply various structures in the head - including the brain. Anteriorly, right after it exits from the cavernous sinus, it gives off the ophthalmic artery. This artery, intuitively, enters the orbit and gives off the central retinal artery which is the main blood supply to the retina. Posteriorly, the ICA gives off two branches: the posterior communicating artery and the anterior choroidal artery. The posterior communicating artery arises right before the terminal bifurcation of the ICA, and connects the middle cerebral and posterior cerebral artery in the Circle of Willis. The anterior choroidal artery, also arises close to the terminal bifurcation, and courses posteriorly to enter the lateral ventricle of the brain, to end in the choroid plexus, supplying it and several other cerebral structures along its course.

And now let’s use an image of the medial side of one cerebral hemisphere to look at the two terminal branches of the ICA: the anterior cerebral artery and the middle cerebral artery. From its origin, the anterior cerebral artery runs anteriorly and then turns superiorly and travels posteriorly arching over the corpus callosum. Throughout this course it supplies the medial aspect of the frontal lobe and the parietal lobe by giving off numerous small cortical branches. These cortical branches will also arch over to the lateral aspect of the hemisphere and supply the supero-lateral part of the frontal and parietal lobes. The two anterior cerebral arteries on each side are connected through the anterior communicating artery, forming the anterior part of the circle of willis.

Next, after branching from the internal carotid artery, the middle cerebral artery travels laterally along the lateral sulcus. While it traverses the lateral aspect, it gives off deep penetrating branches called central arteries which supply the deeper structures of the brain like the corpus striatum, which is made up of the caudate and lenticular nucleus. This is why these central arteries are often referred to as lenticulostriate branches of the middle cerebral artery.

After reaching the lateral surface, the middle cerebral artery provides a few branches that supply the lateral surface of the temporal lobe and part of the infero-lateral surface of the frontal and parietal lobe. Now, both the anterior and middle cerebral arteries are often referred to as terminal arteries as there is no collateral circulation in areas supplied by them. Thus any blockage in these arteries may prevent blood from reaching that part of the brain completely.

Okay, that was a lot! Let’s take a short break and see if you can recall the collateral and terminal branches of the internal carotid artery.

Alright! Now let’s switch gears and look at the vertebral arteries, which arise from the subclavian arteries and form the vertebro-basilar system. The vertebral arteries ascend through the transverse foramina of the cervical vertebrae to enter the skull through the foramen magnum, and then they pierce the dura mater to get into the subarachnoid space. Here, the vertebral arteries start their intracranial course, by moving forward, upward and medially over the medulla oblongata. At the ponto-medullary junction, which is where the pons and medulla meet, the two vertebral arteries merge with each other and form a single basilar artery - hence the term “vertebro-basilar” system.

Now, before the two vertebral arteries merge, they give off two important branches: the anterior spinal artery and the posterior inferior cerebellar artery. The anterior spinal artery is formed by two separate branches of the vertebral artery, which travel inferiorly and medially and subsequently form the singular anterior spinal artery at the level of the foramen magnum.

The anterior spinal artery then courses in the anterior, median aspect of the spinal cord, supplying its rostral anterior two-thirds.

The posterior inferior cerebellar artery, or PICA, on the other hand, originates from the vertebral artery and winds posteriorly around the upper part of medulla oblongata, travels along the inferior aspect of the cerebellum and divides into medial and lateral branches. The medial branch continues posteriorly, while the lateral branch supplies the inferior part of the cerebellum. Now, there is another branch called the posterior spinal artery which in one fourth of the population arises from the vertebral artery and in the other three fourths of the population from the PICA. The posterior spinal artery mainly supplies the posterior aspect of the spinal cord.