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Structures seen from a basal view of the brain with the brainstem removed, showing the cut surface of the mesencephalon.
Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the basal view of the brain. Essentially, what we’re going to be doing here on this tutorial is looking at the structures that can be seen when we look at the brain from an inferior aspect or the base of the brain like you’re seeing right now on this image on the screen. We will be highlighting and discussing the structures that are identifiable from this aspect and also talk a bit about their function.
Before we start describing the several structures that we see in this basal view of the brain, just for orientation purposes, I want to show you that in the base of the brain, we see also these structures that I just added here. The yellow structures are then the cranial nerves but we also see here the cerebellum – which is the structure here – and you also see here this structure which is then the brainstem.
Now, this area of the brain is then protected by bone, specifically, the base of the skull as you can see here on this image on the right side. So, the most anterior portion of the brain which is this part here will be then located in this area of the base of the skull which is known as the anterior cranial fossa. Then you see here that the temporal lobes are going to be located in these or in this area which is known as the middle cranial fossa, and then on this area, you can see that this structure will fit in perfectly – so you can see that the cerebellum is going to be found on the posterior cranial fossa. So you can see that the base of the skull is perfect because it allow the brain to really be accommodated.
Now, we’re going to remove the cerebellum and the brainstem to then be left with just the structures that we find on the basal view of the brain, and right now we’re looking at the first highlight, the first structure that we’re going to be talking about which is the longitudinal cerebral fissure. Now, this structure is very prominent and clearly definable and it is also known as the median longitudinal fissure. This deep groove separates the two hemispheres of the brain. In the depth of the longitudinal fissure lies the corpus callosum which you can see a little bit here, and inside the fissure is then located the falx cerebri.
So, one structure we can see here is then one that I already mentioned on the previous slide, this is the genu of the corpus callosum. So, when we pull apart the longitudinal cerebral fissure – as you can see here on this image with this tool – we do so at the frontal lobe and we can see the corpus callosum from the inferior view of the brain specifically the genu of the corpus callosum seen here highlighted in green. Now, the term genu comes from the Latin meaning “knee”. The genu is the anterior part of the corpus callosum which you can now see here from this mid-sagittal section of the brain – this is the corpus callosum – and this is where you should find the genu of the corpus callosum.
Next, let’s take a look at the most anterior lobe of the brain which is the frontal lobe and we’re highlighting this structure which is known as the frontal pole. Now, here in our illustration, you can see this structure highlighted in green which is the most anterior end of the frontal lobe. You can also see it here from this lateral view of the brain – so you can see here the frontal pole highlighted in green.
Immediately posterior to the frontal pole, we find this structure here highlighted which is known as the straight gyrus. The straight gyrus is continuous with the superior frontal gyrus on the medial aspect of the brain and is located between the longitudinal fissure – as you can see here on this image highlighted – and also between the olfactory sulcus which happens to be the next structure that we’re going to be talking about, the olfactory sulcus. And as you can see in our illustration, the olfactory sulcus is a groove that is located between the straight gyrus and the medial orbital gyrus. The olfactory tract rests within this groove and you can see here the olfactory tract here on the left side and, on the right side of the brain, we cut it here to then expose the olfactory sulcus.
As we saw on the previous slide, the olfactory tract sits within the groove at the base of the brain known as the olfactory sulcus – and now we’re highlighting then the olfactory tract. The olfactory tract is made up of axons of mitral and tufted cells from the olfactory bulb. This tract connects the olfactory bulb to the cerebral cortex and is part of the first cranial nerve, the olfactory nerve. On this image as well, we can then see this structure here that we’re now highlighting, the olfactory bulb, which is located at the beginning of the olfactory tract – you see then this knob-like enlargement known as the olfactory bulb.
Now, within the olfactory bulbs, the olfactory nerves coming from the nasal mucosa and passing through the perforations of the cribriform plate which you see here – this is the cribriform, cribriform plate – and notice here that this is a zoomed-in image of this cut here, and notice the cribriform plate and how there are a few perforations here that we just mentioned. Notice here on this image as well that we’re highlighting the portion of the olfactory – the highlight of the olfactory bulb. Now, to say that the olfactory nerve synapse with the mitral cells whose axons project to the olfactory cortex.
On the next slide, we’re going to be highlighting two structures here. On the left side, we find the orbital gyri and, on the image on the right side, we see then the orbital sulci. We’re still looking at the anterior part of the brain from a basal view and notice here that on the orbital gyri, we find then these structures which are known as the orbital sulci. This is an inferiorly located part of the frontal lobe which rests on the orbital plate of the frontal bone. The orbital gyri are divided into the anterior, the medial, lateral and posterior portions by H-shaped orbital sulci as you can see from our illustrations. And you can see even here anterior, a posterior, a medial, and lateral portions of the orbital gyri.
Next, we’re going to be showing you here this nerve that you can still see on this image. This is known as the optic nerve. So, medial to the optic gyri, we see this nerve here, and the optic nerve which is the second cranial nerve – if you remember well. It is paired nerve that will be transmitting visual information from the retina to the brain. And this nerve will be then originating from the bipolar neurons of the retina and leaves the orbit entering then the skull through the optic canal. The two nerves then run posteromedially and then arrive at the optic chiasm which happens to be the next structure that we’re going to be highlighting here on this image, this is the optic chiasm.
Now, this is located at the base of the brain just below the hypothalamus. The term chiasm comes from the Greek word meaning “crossing” and in the optic chiasm, the fibers of the optic nerve that come from the nasal sides of each retina cross over to the opposite side of the brain while the fibers from the temporal side of the retina remain uncrossed. After the decussation of the nasal or medial fibers of the optic nerve at the optic chiasm, the fibers continue as the right optic tract and the left optic tract, so we see here then the optic tracts and there is then a right optic tract and a left one. Now, the fibers of the optic tract terminate in the lateral geniculate nucleus, the pretectal nuclei, and the superior colliculus. Please note that these visual informations then conveyed from the contralateral half of the visual field to which the optic tract is then located.
The next structure we’re going to be highlighted is known as the anterior perforated substance. Lateral to the optic chiasm and posterior to the olfactory trigone, you see this area here highlighted in green which is the anterior perforated substance – this is how we call it – and it contains perforations for the passage of small branches of the anterior and middle cerebral arteries. If there is an anterior perforated substance, there should be then the posterior one – as you see now highlighted on the image – which is located in the interpeduncular fossa extending from the anterior border of the pons up to the mammillary bodies. Well, in the same way as the anterior perforated substance, the posterior one contains perforations for the passage of branches of, this time, the posterior cerebral artery.
On the next image, we’re going to be highlighting the lateral sulcus. So far, we have been covering structures of the frontal lobe that are seen on the basal view of the brain as well as some other important structures. Now, we’re going to be moving on and looking at structures visible on the temporal and occipital lobes from the basal view. Now, we begin by first looking at this deep fissure that separates the frontal lobe from the temporal lobe and that is then the lateral sulcus. To be clear here, this is the frontal lobe and the temporal lobe and how it separates – the lateral sulcus is separating these two lobes.
You can also see here the division happening from this lateral view of the brain where you can still see here the temporal lobe and the frontal lobe being separated by the lateral sulcus. The lateral sulcus is one of the earliest developing sulci during fetal development of the brain and also known as the Sylvian fissure. The lateral sulcus is one of the most prominent sulci in the brain.
The next structure we're going to be highlighting here is known as the temporal pole. Now, this is the anterior most part of the temporal cortex which is then the temporal pole. Now, the temporal pole is one of the four poles of the cerebral hemispheres. If I show you here also an image of the lateral view of the brain, you can see the temporal pole highlighted in green, to say that this is the most anterior end of the temporal lobe.
Next structure, we're going to go back to the inferior view of the brain to be highlighting the inferior temporal sulcus. So, while we are on the subject of the temporal lobe, let's look at four sulci that begin from/or near this lobe. Now, firstly, seen here from an inferior view, we see the inferior temporal sulcus which is a fissure between the medial and inferior gyri. As you can also see on our illustration here especially on this lateral view, it extends from near the occipital pole to within a short distance of the temporal pole.
The next structure we're going to be highlighting here is known as the occipitotemporal sulcus. Now immediately medial to the inferior temporal sulcus, we see this one here that we're now highlighting. Now, this furrow divides then the medial and lateral occipitotemporal gyri and it is located on the inferior aspect of the brain lateral to the collateral sulcus which happens to be the next highlight here, this is then the collateral sulcus which extends from the temporal lobe to the occipital lobe. It is bordered by the lingual gyrus posterosuperiorly and it lies between the parahippocampal gyrus and the medial part of the fusiform gyrus anteriorly.
Next, we're going to be highlighting this structure here which is known as the rhinal sulcus which is a continuation of the collateral sulcus. It is located lateral to the uncus at the anterior part of the parahippocampal gyrus separating it from the fusiform gyrus. The sulci that we described previously border some important gyri in the basal surface of the brain. Now, this one that we see now highlighted in green is known as the inferior temporal gyrus. Now, this gyrus is located between the inferior temporal sulcus and the occipitotemporal sulcus. This gyrus, from the functional point of view, is critical for then visual object recognition.
Just laterally to the inferior temporal gyrus, we see this one which is called the lateral occipitotemporal gyrus or fusiform gyrus. It is located between the occipitotemporal sulcus and the collateral sulcus, and as you can see clearly, this gyrus is part of both the temporal and the occipital lobe. This part of the brain is responsible and helps with recognizing faces. Now, a really strange and cool thing to remember is that when the patient has brain damage in this specific gyrus, the patient is not able to recognize faces and this clinical entity is then called prosopagnosia or face blindness. One of the most famous cases of documented prosopagnosia is the story described in the book "The Man Who Mistook His Wife For A Hat". Now, in this book, the renowned neurologist, Oliver Sacks, interviewed Dr. P who had this disorder and he described the symptoms of this patient.
Now, the next structure we're going to be highlighting here is known as the medial occipitotemporal gyrus. Now, going back to the basal view of the brain here, the next gyrus we see here in this area is the short gyrus known as then the medial occipitotemporal gyrus, and as the name suggests, this gyrus is located medial to the previously described lateral occipitotemporal gyrus. It is implicated in the processing, or involved in the processing, of visual stimulation.
In the most medial part of the temporal lobe, the next gyrus that we see here surrounds the hippocampus and is aptly named the parahippocampal gyrus. It is part of the limbic system and plays an important role in the encoding and retrieval of memory.
The next structure we're going to be highlighting is known as the uncus, which is located at the anterior end of the parahippocampal gyrus. Now, this hook-like structure is part of the rhinencephalon. On the next image, we see here now, the lingual gyrus, which is then a continuation of the uncus and the parahippocampal gyrus. It is named as such because it was thought that its shape resembled that of a shape of a tongue. It is located between the calcarine sulcus and the collateral sulcus although this is best seen from the medial view of the brain. Now, this gyrus is thought to play a role in encoding of visual memories.
Now, closer to the midline, we see this structure here highlighted which is known as the isthmus of the cingulate gyrus. Now, the isthmus of the cingulate gyrus, as you can see in this illustration showing the, now, mid-sagittal section of the brain where you see now highlighted then the cingulate gyrus. Now, the isthmus of the cingulate gyrus is the posterior most part of the cingulate gyrus – so this portion around here. It is the part that connects the cingulate gyrus to the lingual gyrus and the parahippocampal gyrus. Now, they form the major part of the limbic system.
Immediately posterior to the isthmus of the cingulate gyrus, we see a small portion of the cuneus here from the basal view of the brain. Now, when we look at the medial aspect of the cerebral hemisphere as you can see from this view, you have then a full view of – or almost full view – of the cuneus, which is located between the calcarine sulcus and the parieto-occipital sulcus. This is a small lobule of the occipital lobe containing then the Brodmann areas 17, 18 and 19 and plays a role in basic visual processing.
The next structure we can also see here from the base of the brain is then the calcarine sulcus that we talked about before. Now, below the cuneus, we see this very important fissure, which is known as then the calcarine sulcus. And as you can see from the medial view of the brain now on this image, this sulcus starts near the occipital pole running forward to just below the splenium of the corpus callosum. Anteriorly, the calcarine sulcus meets the parieto-occipital sulcus at an acute angle which you see here as well – this is the parieto-occipital sulcus – and just as I'm showing you here, this is the corpus callosum. You have to remember that the calcarine fissure corresponds to Brodmann area 17 and it is the primary visual cortex.
The next structure where going to be highlighting here is a pole known as the occipital pole. Now, this is the most posteriorly located part of the occipital lobe which you can also see here from this lateral view of the brain. In the illustration here that you see below, you can better visualize the occipital pole as seen from the lateral view of the cerebral hemisphere.
Now that we have covered all the structures of the cerebral hemispheres, let's move on to discuss the structures of the diencephalon seen from the basal view of the brain. And first we're going to look at this structure here that you see highlighted in green, the pituitary gland, which is not actually part of the diencephalon but stems from the diencephalon. Now, the pituitary gland, also known as the hypophysis cerebri, is a small pea-sized bulb that extends from the hypothalamus at the base of the brain which you can also see here from this view, the pituitary gland highlighted in green. Now, this is an endocrine gland that regulates activity of the six other major endocrine glands that release hormones integral for growth and development. Now, this gland rests in the hypophyseal fossa of the sella turcica in the superior part of the sphenoid bone and this small gland is comprised of two lobes to be specific. One that is known as the anterior pituitary or adenohypophysis which regulates several physiological processes through the synthesis and secretion of hormones. There's also a posterior pituitary known as neurohypophysis which then stores and secretes hormones and is functionally connected to the hypothalamus via the infundibulum.
Immediately posterior to the pituitary gland, we see these two round elevations on the floor of the diencephalon known as the mammillary bodies – so each one is known as a mammillary body. Now, they are part of the limbic system and they consists of two groups of nuclei, the medial mammillary nuclei and the lateral mammillary nuclei. The mammillary bodies are connected with the thalamus and the midbrain.
I'd like to take this opportunity and make a clinical note associated to the mammillary bodies. Now, the mammillary bodies, they demonstrate characteristic pathology in a clinical condition which is called the Wernicke-Korsakoff syndrome or alcoholic encephalopathy. Now, this syndrome is very common among alcohol abusers and it is actually a manifestation of vitamin B1 or thiamine deficiency. Now, the patients who suffer from these disease, they demonstrate changes in mental state, amnesia and ataxia. The most prominent pathological finding in this patients is the marked degeneration of the mammillary bodies. The symptoms of this entity can be explained by the multiple connections of the mammillary bodies with the limbic system and the midbrain.
Next, we're going to be highlighting this structure here which is known as the lateral geniculate nucleus, also known as the lateral geniculate body, which is a nucleus of the ventral thalamus – and you can also see it here from this view of the thalamus and brainstem. This small ovoid nucleus contains six layers of neurons. The main function of the lateral geniculate nucleus is to act as a relay center for the visual pathway in the thalamus. There's also here what you see now known as the medial geniculate nucleus. Now, these are the other pair of thalamic nuclei that we see in this basal view of the brain and are then the medial geniculate nuclei or bodies. Now, the medial geniculate nuclei which you can also see here on this image, so you can see that on this image we removed the cerebral cortex, to add here that the medial geniculate nuclei are part of the auditory pathway.
The next structure we're going to be highlighting here is known as the pulvinar of the thalamus – so this is another thalamic structure that we can see here from the basal view of the brain. The pulvinar can be divided into several nuclei and it occupies the posterior of the thalamus – and you can clearly see here on this last image that we just added how it's occupying the posterior part of the thalamus. It makes a reciprocal fiber connections with the parietal and temporal lobes. It also receives afferent fibers from the lateral geniculate body and possibly from the medial geniculate body as well. The pulvinar is connected to the visual cortex as well as the optic and acoustic control centers.
The next structure we're going to be highlighting here is known as the splenium of the corpus callosum. Finally, in this part of the brain we see the posterior end of the corpus callosum, which is known as then the splenium. And, as you know, the corpus callosum is a structure at the base of the longitudinal fissure of the cerebrum comprised of massive transverse fibers that connect the left and right cerebral hemispheres. Now, the splenium is the thickest part of the corpus callosum and it overlaps the tela choroidea of the third ventricle.
Next, we're going to be highlighting then the cerebral peduncle and, on this last part of the description of the basal view of the brain, we will see some structures of the midbrain or the mesencephalon. We're going to continue on with the cerebral peduncle which are then caudal continuations of the internal capsule. The medial surface of each cerebral peduncle borders the mammillary bodies of the hypothalamus and the posterior perforated substance as you can see here in our illustration. Now the cerebral peduncles form the most ventral part of the midbrain and then are located where most of the descending and ascending corticospinal pathways pass through.
Dorsal to the cerebral peduncle, we see these structures here that we are highlighting which is known as the substantia nigra or black nucleus which is considered to be the mesencephalic gray matter portion of the basal ganglia. This part of the basal ganglia is black because it is composed of cells that contain melanin. It can be divided into two parts based on the arrangement of the neurons. The substantia nigra compacta which produces then the inhibitory transmitter dopamine and whose neurons are arranged very close to one another. There's also the substantia nigra reticulata whose neurons are arranged further apart from one another. Now, one quick clinical note in Parkinson's disease, the cells of the substantia nigra that produce dopamine are damaged and that causes most of the motor symptoms of this disease.
Next, we're going to be highlighting here what is known to be as the red nucleus which is located between the substantia nigra and the central gray matter. It is called the red nucleus due its pale pink color as a result of being rich in iron. This structure is comprised of two parts: the parvocellular part and the magnocellular part. Now, this nucleus is involved in motor coordination.
Finally, the last structure we're going to be highlighting here – this really tiny structure that you can see highlighted in green – this is known as the cerebral aqueduct. Now, this is a narrow cerebrospinal fluid-filled canal in the midbrain known as the cerebral aqueduct then, also known as the aqueduct of Sylvius. The cerebral aqueduct connects the third ventricle located in the diencephalon to then the fourth ventricle of the mesencephalon. This small canal allows for the flow of CSF between these two ventricles of the brain.
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