 Let's talk about the circulation of the blood to the brain, because it's incredibly important. We have many cases of cerebral vascular accidents or strokes, and we have problems such as ischemic attacks, which are transitory. And so understanding the blood vessels that supply the inside of the brain is important because if you know what territory that vessel goes to, you can understand where and help to localize what part of the brain is involved. So we have two major blood supplies to the brain. First is the vertebral system. We call it the vertebral basilar system because the vertebral artery off the subclavian comes up through these foramina in the cervical vertebra and enters through the foramen magnum up into the cranial vault. The second major vessel is the internal carotid. This is the common carotid artery. This is the external carotid, which we're going to ignore, and this is the internal carotid, which is very, very important. And here where the two bifurcate at this junction is the carotid bifurcation easily involved in atherosclerosis and therefore an atherosclerotic plaque can embolize up and through the internal carotid artery. So it comes up through the base of the skull and enters through the cavernous sinus. So first let's look at the posterior circulation. Here are our two vertebral arteries coming up through the foramen magnum into the posterior fossa. The contents of the posterior fossa are the cerebellum and the three parts of the brain stem, medulla, pons, and midbrain and all the cranial nerves associated with it. So problems with the circulation coming off the vertebral system are very important. So the two vertebral arteries then fuse or join to form a midline basilar artery over the surface of the pons and then at the top of the basilar artery, the level of the midbrain, the posterior cerebral artery comes off and the superior cerebellar artery comes off. So that's the end of the basilar circulation. So recall then that one of the major of the three cerebral arteries, one of them, the posterior one, comes off this vertebral basilar system. The other that you can see here is the carotid system. The carotid is represented here by the two, pops out, two internal carotid arteries. Notice where they sit. They sit on the side of the optic chiasm. Here's the optic nerve. Here's the optic tract. And here's the optic chiasm. So right on the side sits the internal carotid artery. It comes up and it gives two major branches. The anterior cerebral artery here with a very tiny anterior communicating artery and a larger middle cerebral artery which comes off and goes into the lateral fissure and cannot be seen on this model. Let's review the cerebral circulation at the base of the brain that comprises the circle of willis. The circle of willis unites the posterior circulation with the anterior circulation. So the basilar artery ascends over the surface of the pons to branch into the posterior cerebral artery which surrounds either cerebral peduncle to supply the inferior temporal surface as well as part of the occipital lobe. Off of it comes the posterior communicating vessel. This is the link between the posterior and the anterior circulation. The anterior circulation begins with the internal carotid artery which branches into the middle cerebral artery which goes out into the lateral fissure to supply the lateral surface of the cerebral cortex and the anterior cerebral artery which ascends between the two hemispheres on the mesial surface after passing under the optic nerve. The anterior cerebrals are joined together very, very short anterior communicating artery which completes our circle of willis. Just because we have a circle does not mean we have alternate blood supplies from the different vessels. Each of the major cerebral arteries should be considered an end vessel and supply from other vessels does not normally occur in most cerebral vascular accidents or lesions. If we take this skull model with the vessels at the base of the brain and we put this different model into it and remove the right hemisphere here then if I turn it over onto the ventral surface this represents the internal carotid artery coming up and bifurcating into the anterior and into the middle cerebral arteries. So the two major cerebral vessels off of the internal carotid are the anterior which comes around and supplies all of this middle or mesial surface of the brain down to the corpus callosum including also some of the ventral surface of the frontal lobe. The middle cerebral artery branch comes along is actually deeper than it is on the model in this lateral fissure and supplies many large branches to the lateral surface of the hemisphere. We talked about the posterior cerebral coming off the vertebral basilar system so let's look at the other part of the model. On the other half of the brain model we can see the medulla, the pons, the midbrain. Remember I said at the top of the basilar artery off came the posterior cerebral artery. It wraps around and has two major branches one which supplies the inferior surface of this temporal lobe. This is the temporal lobe so it's supplying all of this inferior surface and then a big major branch which comes on over to the mesial or midline surface and supplies the cortex behind the parietal occipital notch here and all of this cortex happens to be visual cortex. So the posterior cerebral artery off of the vertebral basilar system supplies the inferior surface of the temporal lobe and the occipital lobe of the cortex. Now there is a concept called watershed and that is that the end vessels you can see them here. Here's a nice little branch and the end vessels coming from this side so it's like this. These have the lowest amount of oxygen and they have the least amount of blood flow and because they're end vessels. Thus this zone along here called the watershed zone because it's between the two cerebral artery branches the middle and the anterior this is a vulnerable area to anoxia and we sometimes get a condition called watershed infarcts. There are other watershed zones there should be a watershed zone between the occipital and the middle cerebral artery but this is less often involved in part because the posterior cerebral artery off the vertebral basilar is applying that and usually one or the other of these two major systems are intact. Now let's go to the gross specimen. On the ventral surface of this brain we have a very nice set of blood vessels so I would like to go over those same structures again. Here is one vertebral and here is the other vertebral artery and remember the vertebral artery and its branches are associated with the medulla and there's a bit of the top of the spinal cord on this nice brain. Alright, vertebral medulla. Then we come to the basilar artery from here to here and the basilar artery sits on top of the pons. So here we have the basilar artery and there's some little branches coming off at which you really can't see and then at the top of the basilar artery and this is so nice because we have the nerve still intact our landmark here is the third or ocular motor nerve associated with the midbrain below it is the superior cerebellar artery on each side and above the nerve is the posterior cerebral artery here is the posterior cerebral artery on the left side and the posterior cerebral artery on the other side and they are going to wrap around the side of the midbrain and supply the inferior surface of the temporal lobe which we cannot see all of because of the presence of the tentorium and the cerebellum but I'll pull it over to the side and I think you can imagine this surface here supplied by the posterior cerebral artery and then on around here to the occipital pole so very nice demonstrations now this is the optic nerve here we have the olfactory bulb and tract on the inferior surface of the frontal lobe I'll just sort of pat them down here and this is our optic nerve on either side and the optic chiasm so if I pull this back and put a little pressure on it I think you can see very nicely the two internal carotid arteries and off of the internal carotid into the lateral fissure here and here goes the middle cerebral artery and coming off the internal carotid and going between the two hemispheres I can see and I hope you can too the two anterior cerebral arteries now to see the lateral fissure with the internal portion of the internal carotid that branches to form the middle cerebral artery hope you can see this large vessel here entering the lateral fissure it's really big and I think you can see it right there and then it comes out here and branches and some of those branches come up to the surface of the frontal lobe and parietal lobe and other branches come over just the top edge of the temporal lobe so there you have the middle cerebral artery on that side but the distribution of the middle cerebral artery which is the most important one I think probably in terms of involvement in pathology is over the lateral surface of the hemisphere what does it supply? frontal lobe motor problems here motor problems on the opposite side of the body it also supplies the lateral portion of the parietal lobe what happens with problems here branches here supply somatic sensory cortex and you have distortion or loss of sensory information again from the opposite side of the body so we want to think of frontal as motor parietal as sensory I've already mentioned that the occipital pole this region here is supplied by the posterior cerebral artery and you want to associate that with vision the inferior surface of the temporal lobe also supplied by the posterior cerebral artery supplies an area on the inferior surface here involved with forming new memories so you want to associate the temporal lobe and the posterior cerebral artery with a formation of new memories and if I am on the left hemisphere in 97% of the people branches of these arteries supply the area for producing speech of the motor's speech areas and areas of the posterior superior temporal gyrus for comprehension or interpretation of speech so we have understanding language in the temporal lobe from the middle cerebral artery branches and we have the production of speech in the frontal lobe motor, frontal involved in expressing ourselves and language disturbances whether they are in understanding or in producing both can be caused by branches problems can be caused by branches of the middle cerebral artery different branches frontal motor temporal comprehension so language disorders we call aphasia so we can get aphasia we can get motor problems weakness or loss or paralysis we can get sensory problems from the opposite side of the body also and we can have visual loss from the opposite side of the visual field so understanding and remembering the distribution of the three cerebral arteries is very important