Circle of Willis and Saccular Aneurysm

00:01 Now, I want you to understand the arterial supply to the brain. Here, we see the inferior view of the brain. These are the frontal lobes. Here is the temporal lobe. The occipital lobe is here, for example. Here is our arterial circulation. It's very elaborate and also very important.

00:30 The arterial supply to the brain is going to be via internal carotid arteries which we see here.

00:41 The other is unlabelled over here as well as vertebral arteries. One is labelled here.

00:51 Since these are paired, here’s the one on the opposite side. Now, this particular slide is identifying the connections that exist between your internal carotid arteries and your vertebral arteries. There is a very important circle that forms as a result of how these vessels connect to one another. This is the circle of Willis. So, we’re going to explore the circle of Willis.

01:22 We’re also going to explore the various branches of the arterial supply to the brain.

01:28 They’re all shown here in this particular image. Our circle of Willis, if we start there is shown in through here. We have our internal carotids here and here. Inferior in the image here on the lower part your screen, we have our vertebral arteries that we've mentioned before.

02:00 So, they’re coming in through the foramen magnum. Just below here and out of view of the actual image which would be a branch coming off of each vertebral artery. This particular branch would be your posterior inferior cerebellar artery. It goes by the acronym of PICA.

02:25 The vertebral arteries then continue joined together and form a very prominent structure called the basilar artery. Shortly, after the basilar artery forms, you’ll give rise to the anterior inferior cerebellar artery. So, here’s the one on this side of the image and one on the opposite side as well. The acronym here is AICA. The basilar artery continues here upwards in the the image giving rise to small branches to the pons. Then at this point, we see superior cerebellar arteries coming off the basilar. Then the basilar artery bifurcates or ends by becoming the posterior cerebral arteries that we see here and here. Connecting your posterior cerebral arteries to the internal carotid circulation are posterior communicating arteries. Going back to our internal carotids on either side, internal carotids will give rise to the middle cerebral arteries that we see going in this direction on this side and in this direction on the opposite side. Then we have coming from the internal carotids, anterior cerebral arteries. These will travel on the medial aspect of each cerebral cortex. Then your anterior cerebral arteries are connected to one another at this point. That connection is the anterior communicating artery. Then your circle of Willis would be everything that you see in through here. So your posterior cerebral, posterior communicating, to internal carotid, to your anterior cerebral, anterior communicating artery coming over to the opposite anterior cerebral, opposite internal carotid, opposite posterior communicating artery, opposite posterior cerebral. Then you can continue around the circle of Willis. Now, the internal carotid arteries and the vertebral arteries do provide blood supply to an anterior circulation and to a posterior circulation. The internal carotids shown here provide the anterior circulation to the brain through their branches.

05:03 So, you have a lot of territory being supplied then by the internal carotids in this anterior circulation. The vertebral arteries provide blood flow to the posterior circulation.

05:17 So, you’re looking more into these areas here in the branches providing that posterior circulation of blood flow. Within the circle of Willis, this is a common site for the development of saccular or berry aneurysms. Anatomically, they’re slightly different but nonetheless, these areas represent weaknesses in the wall of the circulation. 40% of these areas, the weakness are going to occur at the junction of the anterior cerebral and the anterior communicating arteries. So that would be at this particular point here. So, at any branching point, that is the area that can have a structural weakness and produce an aneurysm or dilatation of that involved segment. Not too far behind, 34% of these aneurysms will occur at the bifurcation of the middle cerebral artery. So where there is a bifurcation point as we do here, you could have an aneurysm developed there.

06:34 So, 34%of the time, that’s where that is. Less frequently, an aneurysm will develop at the point of bifurcation of the internal carotid and the posterior communicating arteries.

06:49 That would be in this area where you see the posterior communicating artery branching off the internal carotid. Certainly, here on the opposite side, this is the same branching point as well. 20% occur at that level of branching. Then 4% of aneurysms will develop where the basilar artery bifurcates into the posterior cerebral arteries. If an aneurysm would rupture, you would have a subarachnoid hemorrhage or bleed as a result.