Brainstem and related structures.
Hey everyone! This is Nicole from Kenhub, and in this tutorial, we're going to be looking at the brainstem and its surrounding structures.
So in this tutorial, we're going to be looking at this particular image which shows the dorsal aspect of the brain and very specifically for the first half of the tutorial, we're going to focus on this area that's highlighted – the dorsal aspect of the brainstem – and later on, we'll move on to talk about this region up here and then we'll finish with discussing the cerebellum and the cerebellar peduncles. Keep in mind too that in this tutorial, we're not going to be discussing the internal structures of the brainstem since these structures are not visible in this image.
So we're going to begin with taking a quick look at the brain in a midsagittal view, and as you can see, we have our brainstem highlighted in green. And the brainstem which you can see is shaped like a short stem is an important part of the brain as it connects the cerebral hemispheres to the spinal cord and the brainstem has several functions including the following: The brainstem helps with information relay between the cerebral hemispheres, the cerebellum and the spinal cord; the brainstem is also involved in regulating the sleep-wake cycle, consciousness and respiratory and cardiovascular control; and the brainstem also serves as the site of origin of most of the twelve cranial nerves of the brain.
And staying with our mid-sagittal section of the brain, we can see that structurally the brainstem is made up of three parts – the midbrain or the mesencephalon, the pons, and the medulla oblongata. However, the dorsal aspect of the brainstem does not necessarily corresponds to the ventral aspects of the brainstem. So we'll be looking at parts of the brainstem that may not always seem to connect. So as you can see in these next images, on the left is our image of the midbrain in midsagittal section and on the right is the part of the dorsal midbrain that we're going to be looking at in this tutorial. And the same goes for the pons except that the dorsal aspect of the pons is actually covered by a fluid-filled cavity called the fourth ventricle. So actually the region that we're going to be looking at dorsally is part of the floor of the fourth ventricle and the area that corresponds dorsally with the medulla oblongata is this area here on the right although as you can see, the upper part of the medulla oblongata makes up the floor of the fourth ventricle while the lower part covers the dorsal aspect of the medulla oblongata itself.
But first let's of course start our tutorial on the brainstem with the midbrain – the most superior part of the dorsal aspect of the brainstem. And the midbrain – often called the mesencephalon – is the most rostral part of the brainstem. And as you can see, the dorsal aspect of the midbrain is located between the thalamus which is this structure up here and the pons which we saw earlier. And as it's the location of the nuclei of cranial nerves three and four, its main function is to control the movements of the eye as well as process auditory and sensory information.
On its posterior aspect, there are several structures including the tectum posteriorly otherwise known as the quadrigeminal plate and the tectum can itself be divided into the superior colliculus and the inferior colliculus; and the cerebral peduncles anteriorly.
So as we mentioned the tectum or the quadrigeminal plate can be divided into superior and inferior colliculi and we're going to start with the superior one. And as you can see, the two superior colliculi which are rounded swellings of the midbrain are situated below the thalamus and they surround the pineal gland which is this structure located in the middle just here. And as you can see in this image, the superior colliculi are the relay station for reflex movements of the eye and pupillary reflexes receiving input from the retina and the visual cortexes of the eye. So clearly, the superior colliculi's function is primarily visual.
And just below the superior colliculi are the inferior colliculi, and the inferior colliculi are the central relay station of the auditory pathway and therefore its function is primarily auditory. And in this image, you can see the inferior colliculi and their relationship to the audiovisual pathways.
And below the inferior colliculi are the cerebral peduncles, and as you can see, there are two of them – one on the left and one on the right. The cerebral peduncles are collections of fibers that carry ascending and descending pathways between the brainstem and the thalami. And even though in this image, we're looking at the dorsal aspect of the brainstem, the cerebral peduncles are the most ventral part of the midbrain being the caudal extension of the internal capsule – a structure which is deep within the cerebral hemispheres – and they can therefore be considered to be part of the midbrain that connects the remainder of the brainstem to the cerebrum.
Within the region of the midbrain, there are a couple other structures that can be found which we'll also have a look at. And the first one of these is the superior medullary velum which is a transparent lamina of white matter found between the two superior cerebellar peduncles which are these structures just here. And the superior medullary velum is important to note as along with the superior cerebellar peduncles, it creates part of the roof of the fourth ventricle.
The superior medullary velum attaches to the rostral surface of the brainstem via a slightly raised band-like ridge called the frenulum of the superior medullary velum. And on the other side of the frenulum of the superior medullary velum, we can see another structure, the trochlear nerve also known as the fourth cranial nerve. And one thing that it's important to know about the trochlear nerve and its placement is that it's the only cranial nerve to exit the brainstem from the dorsal aspect. And the trochlear nerve is also an exclusively motor nerve that innervates the superior oblique muscle of the eye.
The next region we want to have a look at is the area that overlies the pons. Now if we remember our image from the beginning, this structure on the right here is the structure that overlies most of the pons that is the fourth ventricle and it's highlighted in green. And as we mentioned briefly, the floor of the fourth ventricle is called the rhomboid fossa. And as you can see, it's shaped like a rhomboid hence its name. So the tricky thing here is that part of the rhomboid fossa is actually formed by the roof of the pons and part of it is formed by the roof of the medulla oblongata. But for the purposes of the tutorial right now, I'm going to focus on the structures of the rhomboid fossa and as we go along, I'll point out which parts overlie the pons and which parts overlie the medulla.
So bearing that in mind, the three parts of the rhomboid fossa are the upper triangular part which is made up of the posterior part of the pons, and the triangular part consists of the medial eminences; the locus coeruleus, and the upper portion of the vestibular area. The rhomboid fossa also has an intermediate part which is marked on this surface by the medullary striae of the fourth ventricle and which is highlighted here in green and it also has a lower triangular part which is made up of the superior part of the posterior surface of the medulla which is also known as the open part of the medulla. And the structures that are found in this area are the lower portion of the vestibular area, the hypoglossal trigone, the vagal trigone, and the obex. And note that many of these regions contain elevations that are formed by cranial nerve nuclei and we'll talk about these cranial nerve nuclei in the following slides.
So the first part of the upper triangular part that we're going to be looking at in this slide are the medial eminences which are found on either side of the posterior median sulcus which itself divides the posterior aspect of the rhomboid fossa and the medulla oblongata into a right and left side. And the medial eminences which are longitudinal elevations of the fossa also contain the swellings of the facial colliculi which are caused by the motor nucleus of the facial nerve hooking around the internal genu of the facial nerve. And as you'll be able to see, the medial eminences are bounded laterally by a structure called the sulcus limitans. And the sulcus limitans provides the medial border for our next structure.
At the superior end of the sulcus limitans which we can't really see in this image is a flattening called the superior fovea and at the end of that lies a structure called the locus coeruleus which although highlighted in green is grey-bluish in color. The locus coeruleus receives its color from the melanin pigment in a group of cells called the substantia ferruginea and these cells are also the primary site of noradrenaline production therefore playing a significant role in the fight or flight response.
And as we mentioned, the rhomboid fossa covers both the pons and the medulla but there are also a couple of structures that overlie both, and there are two of them which I'm going to point out for you right now. And the first one of these is an area that's located immediately below the locus coeruleus and lateral to the medial eminence and this area is called the vestibular area. And this of course is mainly due to the fact that this is the area that overlies the vestibular nuclei. And the vestibular area straddles both the pons and the medulla.
The intermediate part of the rhomboid fossa is known as the medullary striae and these fibers refer to a strongly myelinated transverse bundle of nerve fibers which travel from the arcuate nucleus to the cerebellum and as the arcuate nucleus is located in the hypothalamus, it's therefore not visible in this image. And as we mentioned before, the medullary striae mark the end of the pontine portion of the rhomboid fossa and the beginning of the medullary portion so the following anatomical structures that we're going to describe are located in the medulla oblongata.
So now we're in the lower triangular part of the rhomboid fossa and we already mentioned the lower part of the lower vestibular area, so now we're moving on to looking at either side of the posterior median sulcus where we can see two small triangular bulges known as the hypoglossal trigone. And although it's not quite illustrated in this image, there's a medial part which overlies the hypoglossal nerve and a lateral part which overlies the hypoglossal intercalatus. And if we recall our sulcus limitans, we can see that it provides the lateral border of the hypoglossal trigone and the medial border for our next structure.
The vagal trigone, also triangular in shape, is highlighted here in green. And the vagal trigone is formed by the underlying dorsal nucleus of the vagus nerve as well as the glossopharyngeal and cranial accessory nerve nuclei. And finally, the rhomboid fossa closes with this structure seen here highlighted in green known as the obex. And this is an important physical landmark as the inferior apex of the fourth ventricle is the point where the fourth ventricle narrows to become the central canal of the spinal cord. And the posterior median sulcus which we saw before earlier in the tutorial passes through the obex as it continues down the spinal cord.
So as we've mentioned, the medulla oblongata is the most inferior portion of the brainstem and can be considered its terminal portion. Superiorly, it's connected to the pons and, inferiorly, it becomes the spinal cord at the foramen magnum. And as we mentioned before, the dorsal aspect of the medulla oblongata has two parts – an open part which is also considered the lower triangular part of the rhomboid fossa and a closed part which is the portion of the medulla that becomes the funiculi of the spinal cord. And the closed part itself can be divided into two columns – the dorsal column which has a gracile tubercle and fasciculus and a cuneate tubercle and fasciculus and the lateral column which contains the trigeminal tubercle and the lateral funiculus.
So the first pair of tubercles and fasciculi we're going to look at is the most medial pair surrounding either side of the posterior median sulcus – the gracile tubercle and its corresponding fasciculus, the gracile fasciculus. And the dorsal column also known as the medial lemniscus pathway is primarily a sensory pathway and is in fact the major sensory pathway of the nervous system and therefore it carries information for touch, pressure and proprioception of the lower body mainly from the legs and the trunk. The gracile fascicle is formed by the axons of the neurons located in the dorsal root ganglia of the seventh thoracic nerve and downward. And these axons then make its synapses in the nucleus beneath the bulge of the gracile tubercle and from there information is carried to the thalamus which integrates all the sensory information.
The second pair of structures in the dorsal column are the cuneate tubercles with their corresponding fasciculi. And as we can see, the cuneate tubercles and the fasciculi are located lateral to the gracile tubercles and fasciculi and they carry the same type of sensory information as the gracile fasciculi – touch, pressure and proprioception – but above the level of the seventh thoracic nerve.
And moving on to the lateral column, the tubercle found superior to the funiculus is called the trigeminal tubercle. Situated lateral to the cuneate tubercle, this tubercle marks the surface representation of the spinal trigeminal tract and its underlying nucleus. But we won't go too much into the lateral funiculus as the lateral funiculus is described in more detail in the tutorial on the spinal cord found on our website.
Now that we've taken a look at the detailed anatomy of the brainstem, I just want to give you a little bit of information about brainstem stroke syndromes. So as I mentioned at the beginning of this tutorial, the nuclei for most of the twelve cranial nerves are located in the brainstem. In addition to this, the most important ascending and descending pathways to and from the cerebral hemispheres pass through the brainstem. So, therefore, any occlusion of the arteries that supply the brainstem would result in a variety of symptoms depending on the affected cranial nerve nucleus and its affected pathway. And of course these syndromes are known as brainstem stroke syndromes.
The symptomatology of each syndrome, however, depends on the level of the affected artery. So if the affected artery is at the level of the midbrain, there are three syndromes that can result but the most common of these is the Weber syndrome. If the occluded artery is at the level of the pons, there are five syndromes that can come about as a result, however, the most important of these is the Foville syndrome. And if the blockage of the artery occurs in the medulla oblongata, this can result in another three syndromes but the most common of these is the Wallenberg syndrome. The characteristic clinical picture for all of these syndromes is that there is an ipsilateral cranial nerve palsy and a contralateral hemiparesis or hemiplegia and sensory loss.
So thanks so much for sticking with me so far, it's a big tutorial. And now that we've described the three parts of the brainstem – the midbrain, the pons and the medulla oblongata – it's now time to have a look at and describe some of the structures that are located close to the brainstem and we'll go through these from top to bottom like we did with the brainstem and if you're feeling a bit tired or you need to have a break, feel free to go away, make yourself a cup of tea.
Okay, so now you've come back, I'm going to talk about these three main areas that are the important parts of the surrounding structures of the brainstem and they are one, the basal ganglia which in this image is mainly the caudate nucleus; the diencephalon and the third ventricle – which is another fluid-filled cavity of the brain is located within this diencephalon; and the cerebellum which of course includes the cerebellar peduncles.
So as we just mentioned, the nucleus of the basal ganglia that is visible in this image is the caudate nucleus. And this nucleus is an elongated C-shape nucleus that follows the direction of the lateral ventricle and has three parts – the head, the body and the tail which you can see in this cross-section. And the tail terminates in the roof of the temporal horn of the lateral ventricle. Located in the angle between the caudate nucleus and the thalamus and covering the thalamostriate vein is the terminal stria and is a part of the limbic system. The terminal stria triggers emotional responses from the amygdala and also conveys olfactory signals.
So the diencephalon is the most dorsal aspect of the forebrain and it contains the hypothalamus, the thalamus, the metathalamus and the epithalamus. But in this image, we're only going to be able to see three parts – the thalamus which includes the metathalamus which is a composite part of the thalamus and the epithalamus. The third ventricle, as we mentioned, is a fluid-filled cavity which is enclosed by the diencephalon and we'll talk about that in just a few slides. And just before we launch into the thalamus, I very briefly want to mention the lamina affixa – a narrow white band found on the dorsal surface of the thalamus – and this is the site where the choroid plexus of the lateral ventricle is attached.
The thalamus is a large nuclei of the diencephalon. The two paired ovoid thalami can be considered the final relaying station before information is passed into the cerebral cortex as they relay and integrate a myriad of motor and sensory impulses between the cortexes and the periphery. The thalamus is comprised of grey matter and is made up of several nuclei but the ones that we can see on this image include the anterior thalamic tubercles, the pulvinar nuclei, and the two bodies that make up the metathalamus – the lateral geniculate bodies and the medial geniculate bodies.
The anterior thalamic tubercle is a small protuberance that corresponds with the anterior nucleus of the dorsal thalamus and it's found on the most rostral aspect of the thalamus. The role of the anterior thalamic nuclei is unclear but they may play a role in memory and learning. The most caudal part of the thalamus are the pulvinar nuclei seen here highlighted in green. And like the anterior thalamic nuclei, its functional significance is not fully understood but the pulvinar nuclei are made up of several nuclei that receive several intrathalamic inputs mainly from the lateral and medial geniculate bodies and don't receive extrathalamic input so therefore they're considered possibly to be an integration nucleus of visual and auditory information.
The lateral geniculate body – the most superior and lateral of the paired bodies of the metathalamus – is a processing station for the major pathway of the retina to the cortex. And in this image, you can see the bodies receiving information from the optic tract before giving rise to the geniculocalcarine radiation which is in turn received by the visual cortexes of the brain.
Medial and inferior to the lateral geniculate bodies are the medial geniculate bodies. The medial geniculate bodies are the thalamic relay station of the auditory pathway and the medial geniculate bodies receive afferent auditory axons from the ipsilateral inferior colliculi before giving rise to efferent fibers which travel from the medial geniculate bodies to the auditory cortex in the temporal lobe.
Moving towards the midline, we now find ourselves in the epithalamus and, specifically, at the habenular trigone – a small triangular area located at the posterior end of the third ventricle – comprising part of the stalk of the pineal gland. And the habenular trigone overlies the habenular nuclei and is considered part of the limbic system. The pineal gland, also known as the pineal body or epiphysis, is an endocrine gland. It is suspended from the habenula and lies in a groove between the two thalamic bodies and this small but important structure releases a hormone known as melatonin – a derivative of serotonin – which plays a role in the regulation of circadian rhythm.
And the final structure we're going to look at in the diencephalic region is the third ventricle. And as we mentioned earlier, the third ventricle is one of the four cerebrospinal fluid-filled ventricles of the brain and it's also a cavity of the diencephalon. The third ventricle is situated in the midline between the right and the left thalamus and extends from the lamina terminalis to the cerebral aqueduct, and you can see its length in this image. On either side, it communicates with the left lateral ventricle and the right lateral ventricle via the interventricular foramina of Monro which on this image is this little passageway just here. And posteriorly, the third ventricle connects with the fourth ventricle highlighted in green through the cerebral aqueduct.
So now we're going to move on to our final structure, the cerebellum, and again, thank you so much for sticking with me through this long tutorial. And although the cerebellum is not part of the brainstem, it is an integral part of the brain so we're going to spend a little bit of time talking about it and its peduncles. And as you've probably noticed before in our images of the cerebellum, the cerebellum overlies the rhomboid fossa at the level of the pons and the medulla oblongata. The term cerebellum comes from Latin and means "small brain". And if you look at our illustration of the cerebellum which shows part of it cut away, you can see that it looks just like a small brain. Inside the skull, the cerebellum is situated in the cerebellar fossa and just inferior to the occipital lobe of the brain.
There are several functions associated with the cerebellum mostly to do with the motor control and though the cerebellum does not initiate movement, it does play a role in modifying the motor commands of the descending pathways to make movements more adaptive and accurate. The cerebellum receives input from the sensory system of the spinal cord and other parts of the brain integrating these to fine tune motor activity, therefore, some of the functions the cerebellum is involved include maintenance of balance and posture, coordination of voluntary movements, motor learning and cognitive functions, for example, in language.
The cerebellar cortex consists of two laterally large hemispheres which are united in the midline by a structure called the vermis which we're now seeing on the right, and the view we're looking at here is dorsal. And just like the cerebral hemispheres, the cerebellar hemispheres are composed of an external layer known as the cortex and an inner layer which is the white matter. And the cerebellar cortex consists of three layers – a molecular layer which is the layer just beneath the surface and contains many stellate and basket cells; a Purkinje cell layer which is comprised, of course, of Purkinje cells and the Purkinje cells are the largest cells of the cerebellum; and the granular layer which is the densely packed cell-rich layer of the cerebellar cortex and in this layer we find granule cells and large Golgi cells.
And within the white matter of the cerebellar are masses of grey matter or nuclei. And there are four pairs of cerebellar nuclei but today we're just going to focus on one of them – the dentate nuclei. And the dentate nuclei which is the largest and most lateral of the cerebellar nuclei is particularly recognizable by its tooth-like serrated edge which we can see highlighted in green in the image. And the dentate nuclei receive input from the lateral hemispheres and is primarily associated with the planning, initiation and control of voluntary movements.
And the last set of structures I want to talk about are the cerebellar peduncles which are fiber bundles that carry input and output between the cerebellum and the brainstem. And they have a particularly important relationship with the brainstem as not only do they overlie the dorsal aspect of the brainstem but they also connect to the various parts of the brainstem and convey information. And of course there are three of them and they are the superior cerebellar peduncles, the middle cerebellar peduncles and the inferior cerebellar peduncles.
The most superior of the cerebellar peduncles is, of course, the superior cerebellar peduncles also known as the brachia conjunctiva. Primarily, the superior cerebellar peduncles provide an efferent pathway for fibers from the cerebellar nuclei mostly from the dentate nucleus to travel to the thalamus and it also contains an afferent fibers from the spinocerebellar tract. The superior cerebellar peduncles are connected to and provide the physical pathway for fibers running between the cerebellum and the midbrain.
The middle cerebellar peduncles also known as the brachia pontis are paired structures and are the largest of the three pairs of peduncles of the cerebellum. Unlike the superior cerebellar peduncles, however, the middle cerebellar peduncles contain only afferent fibers which arise from the pontine nuclei of the contralateral side. The middle cerebellar peduncles are connected to and provide the physical pathway for fibers running between the cerebellum and the pons.
And lastly but definitely not least, the inferior cerebellar peduncles, also known as the restiform bodies, are also paired structures that are shaped like thick ropes formed by the fibers of the posterior spinocerebellar tract and the axons of the inferior olivary nucleus which arise from the posterolateral aspect of the medulla oblongata. And the inferior cerebellar peduncles are primarily afferent conveying fibers from the medulla oblongata, however, they also convey some efferent fibers to the vestibular nuclei. And just to reiterate, the inferior cerebellar peduncles are connected to and provide the physical pathway for fibers running between the cerebellum and the medulla oblongata.
And that's the end of our tutorial on the brainstem and its surrounding structures. Thanks for watching!
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Now, good luck everyone, and I will see you next time.
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