Arteries of the brain II

Hey everyone! This is Nicole from Kenhub, and welcome to our tutorial on the arteries of the brain as seen from a basal view. You may notice that the vascular anatomy of the brain is beautifully complex, but that doesn’t mean it should be difficult to learn. The vascular anatomy of the brain has been a subject of study since late antiquity with centuries of research leading to scarce results, mainly because most anatomists would choose to study the vasculature of the brain from a difficult perspective. It wasn’t until the late seventeenth century that a British physician named Thomas Willis decided to study the base of the brain, preparing specimens in a manner that would leave the base of the brain intact. By doing this, Willis was able to identify a pattern in this vascular complexity and this same pattern is what we will be discussing in the rest of our tutorial.

As we discuss the arteries of the brain, we’ll be discussing them as seen from the basal view of the brain which is basically the bottom or the underside of the brain. We’ll break down the arteries that supply the brain into two circulations according to which part of the brain these arteries supply, namely the arteries of the anterior circulation which supply the anterior part of the brain and these include the internal carotid artery, the posterior communicating artery, the anterior choroidal artery, the anterior communicating artery, the middle frontobasal artery, and the lateral frontobasal artery; and the arteries of the posterior circulation which, as you may already have guessed, supply the posterior part of the brain, and these include the vertebral arteries, the anterior spinal artery, the posterior inferior cerebellar artery, the basilar artery, the anterior inferior cerebellar artery, the labyrinthine artery, the pontine artery, the superior cerebellar artery, and the posterior cerebellar artery.

We’ll also look at the anastomotic connective between these two circulations known as the circle of Willis. So, let’s begin.

As you can see, the arterial vasculature is highlighted in two different colors, and that’s because Willis managed to break it down into two simpler circulations namely the anterior circulation which you can see highlighted in pink and the posterior circulation which you can see highlighted in blue. The main difference between these two circulations is their origins. So if we look at the anterior circulation, we’ll see that it arises from the internal carotid arteries in the front while the posterior circulation arises from the vertebral arteries.

The two circulations ultimately anastomose forming a circle around the hypothalamus and optic nerve at the base of the brain and this circle came to be known as the circle of Willis, honoring the contributions of Thomas Willis to the vascular anatomy of the brain. So now let’s take a closer look at each of these circulations.

So, here we can see the internal carotid arteries highlighted in green and the internal carotid artery arises in the neck at the bifurcation of the common carotid artery. It gives off nerve branches in the neck, enters the skull through the carotid canal and emerges in the cranial cavity after a torturous course in the carotid canal. In this illustration, most of the intracranial portion of the internal carotid artery is removed except for the part which serves as an origin point for the anterior circulation.

The anterior carotid artery gives off two branches which constitute the anterior circulation – a smaller branch for the brain parenchyma and an anastomosing branch to communicate with the posterior circulation – and the two major branches are the anterior cerebral artery and the middle cerebral artery. The smaller branch is the anterior choroidal artery and the communicating branch is the posterior communicating artery.

And now let’s move on to examine each of these branches in greater detail. We’ll start examining these arteries in the order in which they arise from the internal carotid artery.

The first artery of cerebral circulation to be given off the internal carotid artery is the posterior communicating artery. This branch serves as a bridge between the anterior and posterior circulation and you may be wondering whether this artery is really important to know. Well, occlusion of this artery will have little clinical impact if neither the anterior nor the posterior circulation are affected, however, aneurysms in the posterior communicating artery may gradually present with symptoms.

Notice that the posterior communicating artery courses very close to a cranial nerve. If an aneurysm becomes big enough to press on that cranial nerve, we can expect that the function of that nerve to decrease and leads to clinical findings. And the nerve in question is the oculomotor nerve, and the findings we may see include oculomotor nerve palsy, a condition characterized by a dilated pupil and limited motion in the eye ipsilateral to the lesion.

The second artery of cerebral circulation to arise from the internal carotid artery is the anterior choroidal artery. And this tiny artery may, at first glance, seem insignificant, however, it is a very important vessel as it provides blood supply to many deep brain structures such as the basal ganglia and the internal capsule. An occlusion of this vessel may therefore lead to contralateral hemiplegia. And contralateral hemiplegia is basically the absence of both movement and reflexes on the side of the body opposite to the side of the lesion.

The next branch of the internal carotid artery that we’ll look at is the anterior cerebral artery seen here highlighted in green, and this is a major branch of the anterior circulation as it irrigates the entirety of the frontal medial surface of the brain. Due to this fact, we need to know and remember its abbreviation which is ACA and we’ll now switch to a slide of the middle view of the brain just to see the extent of the cerebral cortex irrigated by the anterior cerebral artery.

Here we see the right anterior cerebral artery as seen from the medial view of the brain. So, take a good look at the anterior cerebral artery and its branches and try to imagine the extent of the cerebral cortex irrigated by them. So this area of context include motor regions, sensory regions as well as regions associated with will and behavior. So, should an occlusion occur early in the anterior cerebral artery, all the surface of cerebral cortex may die. So, the clinical findings of an occlusion of the anterior cerebral artery will surely vary based on the location of the occlusion, but they may include motor and sensory deficits of the lower limb and the pelvis, in addition to more complex motor function and personality changes.

Okay, so let’s now have a look at a small artery which connects the two anterior cerebral arteries to each other just as they reach the longitudinal fissure of the brain. And this artery forms an anastomosis between the right and left anterior cerebral arteries, and as you can see, it also forms part of the circle of Willis. The anterior cerebral artery gives off one more important branch in the basal surface of the brain and that’s the middle frontobasal artery seen here highlighted in green, and it supplies the orbital part of the frontal lobe, which is also known as the orbitofrontal cortex. And this region is involved in the cognitive process of decision-making, so an infarct in the medial frontobasal artery would lead to problems with decision making.

So we’re going to conclude our discussion on the anterior circulation with the middle cerebral artery and its branches, and you may also encounter this artery abbreviated as MCA. If you take a close look at the illustration, you may wonder where the internal carotid artery stops and where the middle cerebral artery begins. And this transitional point is the origin of the anterior cerebral artery. In other words, the middle cerebral artery arises from the internal carotid artery and it’s the largest terminal branch of the ICA. The middle cerebral artery irrigates the lateral surface of the brain including regions of the frontal lobe, the temporal lobe, and the parietal lobe.

So we’ll finally be looking at a major branch of the middle cerebral artery, and that would be the lateral frontobasal artery, and this artery branches out anteriorly, superiorly and laterally to vascularize the inferior frontal gyrus.

So let’s now move on to the posterior circulation, and this is the part of the circulation that we can see highlighted in blue. And as we’ve already discussed, this part of the brain circulation arises from the vertebral arteries. So let’s take a closer look.

The vertebral artery which you can see highlighted here is a paired vascular structure which arises from the neck as a branch of the subclavian artery, and it enters the skull via the foramen magnum. And in this illustration, you could see the intracranial part of both the right and the left vertebral arteries and within the skull, these arteries travel along the sides of the medulla oblongata.

So there are some key points to know about the vertebral arteries. The first is that they fuse just below the pons and form the basilar artery, the second is that they provide blood supply at the medulla oblongata and spinal cord, and the last is that they supply the lower part of the cerebellum. Notice how each vertebral artery gives off a small branch and that these small branches fuse to form a single artery, and this artery is the anterior spinal artery. And as you can see, it supplies parts of the medulla and the spinal cord and this is not the only spinal artery. In fact, there are also two posterior spinal arteries, but our aim in this tutorial is to study the blood supply of the brain, so we’ll leave the blood supply of the spinal cord for another tutorial.

Okay, so finally, let’s have a look at the branches that go to the lower part of the cerebellum which you can see highlighted here, and these are the posterior inferior cerebellar arteries, abbreviated as PICA. And the posterior inferior cerebellar arteries are responsible for the supply of the lower parts of the cerebellum.

And now let’s move on to the basilar artery. And here we see the basilar artery highlighted in green. In addition, we’re also reminded that the two arteries contributing to its formation are the vertebral arteries formed from the union of the vertebral arteries. The basilar artery forms at the level of the pontomedullary junction and courses upward and anteriorly in the middle of the base of the pons. It gives off many important branches along its course and finally it divides into the right and left posterior cerebral arteries at the level of the pontomesencephalic junction. In that level which is now shown with an arrow is the level where the surface of the midbrain ends and the pons begins.

As you can see, the basilar artery gives off many branches which we will cover from bottom to top.

So let’s start with the anterior inferior cerebellar artery, and this artery is also known as AICA. And as you can see, this is the first branch of the basilar artery after its formation, and this artery supplies the lateral pons and the cerebellum. Infarction of this artery can lead to facial paralysis and sensory loss in the distribution of the trigeminal nerve, and that’s because the respective nuclei receive their supply from this artery.

So moving upward, we’re now looking at the labyrinthine artery which can be a branch of either the basilar artery or the anterior inferior cerebellar artery, AICA. And this artery accompanies the vestibulocochlear nerve into the inner ear and supplies the inner ear. And you may come across some literature where this artery is referred to as the internal auditory artery or simply as the auditory artery.

The branches that we’re looking at right now are known as the pontine arteries, and these tiny arteries originate from either side of the basilar artery and they, of course, supply the pons.

Alright let’s now look at the superior cerebellar artery. And this artery is also abbreviated as SCA, and the superior cerebellar artery is the last branch of the basilar artery before it divides into two posterior cerebral arteries and this artery supplies the superior surface of the cerebellum and anastomosis with branches of the AICA and the PICA. It also gives off some branches to the pineal body and the anterior medullary vellum.

The final artery of the posterior circulation we’ll discuss is the posterior cerebral artery, and as you can see it arises as the basilar artery as the basilar artery bifurcates. The posterior cerebral artery is abbreviated as the PCA, and it supplies the inferior and the backside of the brain and mainly regions of the occipital and inferior temporal cortex. In addition to this cortical regions, the PCA also supplies some deep brain structures on the backside of the brain.

So thromboembolic obstruction of the posterior cerebral artery results in occipital and inferior temporal infarcts, and occipital cortex infarcts usually clinically present as contralateral homonymous hemianopsia. And this means that the patient will lose the same half of the visual field from both eyes. On more extended cases, occipital lesions can result in blindness which in this case can be called cortical blindness and inferior temporal lesions may result in problems in naming and discrimination of visual elements, memory problems, and spatial orientation.

Okay, so at the beginning of this tutorial, we looked briefly at the arterial circle of Willis, and we’re going to end our tutorial by summarizing the cerebral arterial circle. So let’s take a look at its components. And as we discussed earlier, the cerebral arterial circle or circle of Willis is an anastomotic circle where the anterior circulation meets the posterior and vice versa, and the vessels contributing to its formation are the following. So from the anterior circulation, we have the anterior cerebral artery and the anterior communicating artery while from the posterior circulation, we have the posterior cerebral artery and the basilar artery, and the two circulations unite via the posterior communicating artery. Note that the internal carotid artery and the middle cerebral artery are not considered parts of the cerebral arterial circle.

Alright, so that’s all we have for you today. I hope you enjoyed this tutorial and have a better understanding of the brain. So thanks for watching and happy studying!