The Cerebellum

In this lecture we’ll discuss mainly anatomy of the cerebellum, which will be covered again in the Lab. - Anatomy Atlas is for need only ^.^ First, where are we now?? We are in the area below the big boss, yet we’ll meet his most trusted assistant that has all the country’s secrets! Our dear cerebellum lies in the posterior cranial fossa below the cerebrum, separated from it by a fold of dura mater called “tentorium cerebelli”, why is it called so? Because it’s like a tent covering the cerebellum. In front of it we’ll find the Brain stem with its famous three parts “Midbrain, pons, medulla oblongata”, and Figure 1: mid-sagittal section of the brain between the brain stem and the cerebellum we’ll see the last cavity in the brain before we reach spinal cord’s central canal, which is the 4th ventricle! 1|Page Get to know the cerebellum more … Send warm greetings to 3 views of it! Nodule Flocculus Anterior notch 1ry fissure V E Tonsil Cerebellar Hemisphere R M I S Cerebellar Hemisphere Posterior Notch Inferior surface Folia Superior surface Posterior view Figure 2: Views of the cerebellum. Left; inferior, Right; superior, Bottom; posterior 2|Page I guess you have noticed that the cerebellum somehow shares the same feature as the cerebrum in having 2 HEMISPHERES. These 2 cerebellar hemispheres are linked together by a middle structure called the “VERMIS”. Vermis; a worm-like structure. Anterior notch 1ry fissure V E Cerebellar Hemisphere R M I S Cerebellar Hemisphere If you referred to figure 2, you can notice that the Vermis can be seen on the superior and inferior surfaces; so Vermis is the midline Posterior structure of the cerebellum. Note carefully Notch which words we used; always keep in your Figure 3: Superior surface mind that Vermis is ,with the same consistency, NOT a group of commissural fibers of white matterthat connect the hemispheres together like corpus callosum did to the cerebral hemispheres. - Notches of the cerebellum. Notch; A depression or indentation on an organ. (Anatomy Atlas – 1) 1. Anterior Notch: Occupied by the brain stem. 2. Posterior Notch (Vallecula): Occupied by falx cerebelli “Falx; a sickle which is afold of dura between the 2 cerebellar hemispheres; Part of it can be seen in figure 3 and it continues then in the inferior surface. Falx cerebelli is attached to the internal occipital protuberance and crest from inside between the 2 occipital bones. (Anatomy Atlas –2). Vallecula; Latin for Valley, a depression below the general surface level of a part. - Surfaces of the cerebellum. 1. Superior surface (Figure 3) 2. Inferior surface “as if you’re flipping the cerebellum to see this view” - Fissures of the cerebellum. Since we have cerebellar lobes “will be discussed later”, we must have got things that separate them from each other, and these are what we call fissures “deep sulci”; as the cerebral sulci separated between the Gyri, these fissures will separate between parts of the cerebellum we call Folia “leaf-like structures”. We can see that we have manytransverse ridges separated by the fissures, so if you cut the part between 2 fissures,this elevation is what we call a Folium; 3|Page White If you took a section of a folium to see under the microscope (figure 4), you’ll notice that gray matter lies outside while white matter is observed inside “similar to the cerebrum”. The gray matter represents nuclei that from it efferent cerebellar axons go to the white matter; the inner core white matter represents the axons getting out of the cerebellum or coming to it “E.g. Spino-cerebellar tracts”. Gray Figure 4: The cerebellum under microscope We have 4 MAIN fissures which separate the cerebellum into lobes: 1. Primary fissure “Fissure prima”. It’s a V-shaped sulcus which separates between the anterior & posterior lobes (figure 5). So we can see that lobes were assigned as posterior or anterior . The anterior lobe represents the anterior third of the cerebellum while the posterior lobe represents the remaining 2 thirds. Anterior lobe Superior vermis Primary fissure Posterior lobe We can notice from figure 2 that the anterior lobe lies Figure 5: Superior surface superiorly, so if we flipped the cerebellum to see the inferior surface, we won’t be able to see the Anterior lobe clearly; instead, we’ll be able to see the large posterior lobe “whole green part” which extended from the superior surface all the way to the inferior surface. We can also see that Vermis also got separated in a way that its anterior third is within the Anterior lobe superiorly, and its posterior 2 thirds are within the posterior lobe superiorly and inferiorly. TO SUM UP: The 1ry fissure separated the cerebellar substance into (1) anterior lobe + anterior vermis in the superior surface, and (2) posterior Lobe + Posterior vermis which are mainly in the inferior surface but are also seen in the superior. 4|Page Another important lobe is the lobe in blue seen in the inferior surface; we call this lobe the “flocculo-nodular lobe”, why is it called so? Because it’s formed by 2 strucutes; the 2 flocculi laterally “butterfly wings”, and the nodule medially “which is part of the vermis and it’s the most anterior part of the inferior vermis”. This tiny Butterfly shaped lobe is the one responsible for ! Can we see it from the superior view? 100% absolutely not!!!!! This lobe can only be seen in the inferior surface. Horizontal fissure MCPs Nodule Flocculus Horizontal fissure Tonsil Retro-tonsillar fissure Figure 6: Inferior surface Floccus; flock of wool, Nodulus; a small knot 2. Postero-lateral fissure: Separates the flocculus & nodule totally from the rest of the cerebellum. This fissure can ONLY be seen on the inferior surface; it separates the FN Lobe totally from the rest of the cerebellum in a way that you can grab it from both sides and separate it easily from the cerebellum; it’s called so because it lies posteriorly “hence Postero-“ and laterally “lateral” to the FN Lobe. 3. Retro-tonsillar fissure: separates tonsil from the rest of cerebellum. It’s less important; as the name implies, it lies behind “Retro” the cerebellar tonsils. Remember Arnold-Chiari Malformation “Remember Lab #3”, which in it herniation of the tonsils happened through foramen magnum and pressed on the medulla (Anatomy Atlas4)??? Well, you’ve finally met the movie star!! Tonsils are structures of the cerebellum itself “same consistency; same folia exist”, and they are situated just lateral to the inferior vermis so they are located in the inferior surface “and sure cannot be seen from superior” (figure 6). Normally, tonsils along with the rest of cerebellum exist above foramen magnum but never normally pass through it; yet, in cases of increased intracranial pressure, whether sporadic or congenital, herniation of tonsils might be the next to expect! And since a really important structure pass through this foramen – of course we’re talking about medulla oblongata, the chance of pressing on a respiratory center to cause respiratory failure, or pressing on cardiovascular center to cause cardiac arrest does exist! And this is what we actually see. 5|Page 4. Horizontal “transverse” fissure (figure 7): Extends between the middle cerebellar peduncles. If you observed the posterior view of the cerebellum, you’ll notice clearly a deep fissure extending horizontally on the lateral surface of the cerebellum from a side of the anterior notch, circling around the whole lateral surface, passing through the posterior notch A.K.A the vellecula, getting back to circling until reaching the anterior notch again from the other side (see also figure 6). Figure 7: Posterior surface It’s of no functional significance, but what it does is that it simply separates the cerebellum into superior part “above” and inferior part “below”. Of course we must conclude that Vermis also got separated by it into superior part and inferior part. Since it reaches the anterior notch from 2 sides, it’s predictable that it has a relation somehow with what’s occupying it, which is the brain stem for sure! So it can be seen “as in figure 6” extending between Middle cerebellar peduncles posteriorly. (Anatomy atlas -3) We have separated the cerebellum into posterior, anterior, flocculo-nodular lobes based on the fissures; what if we unfolded the cerebellum, what extra things would we see & add?? Unfold Paravermis Figure 8: folded Cerebellum Figure 9: Unfolded Cerebellum 6|Page - Unfolding of the cerebellum The cerebellum is folded like a hedgehog, if you tried to unfold it like a sheet, first you’ll have to grab its tips, and in this case its tipsare the most anterior parts ofsuperior and inferior surfaces whichare the closest to the brain stem.They are the most anterior part ofthe anterior lobe superiorly, which Unfold corresponds to the most anterior part of the superior vermis (figure 5), and the flocculonodular lobe Paravermis inferiorly- specifically the nodule which represents the most anterior part of the inferior vermis (figure 6, 8). What are your observations? 1. You can see that in the , Figure 10: Unfolded Cerebellum; Vertical classification “whether of the cerebellum it was superior or inferior vermis in folded shape”. 2. The fissures are seen clearly “horizontal, primary” 3. The nodule is seen in tip of the inferior vermis “so it’ll be the most Anterior upon folding” 4. Beside the Vermis, 1cm to the right and 1 cm to the left you can see an area which we’ll call “para, beside”. 5. The rest of the posterior lobe is called “ ”: Why to bother ourselves in unfolding and vertically classifying the cerebellum,wouldn’t it be enough to just classify it horizontally to lobes as we did?? For sure eachvertical part has a separate function “which will be discussed in details next lecture!” In the cerebellum, we said that we have outer gray matter, and inner white matter, but like father like son, as the cerebrum has deep basal nuclei “Lentiform, caudate, substantia nigra, subthalamic nucleus, Amygdalla”, the cerebellum will also have ones, which we put together in the famous sentence “Don’t eat greasy food”. 7|Page - Deep cerebellar Nuclei Note: Don’t worry; all the connections with these nuclei will be taken next lecture. 1. (having tooth like projections) nucleus: Most lateral; connected to the Red nucleus in Midbrain. 2. : Emboliform “oval; to lateral” and Globose “rounded; to medial”. 3. (Fastigatus; narrowing at the top) nucleus: Most medial. Figure 11: Deep cerebellar Nuclei All in a sentence: "Don't Eat Greasy Food" What is the importance of this order?? It’s very very important in their connections with the cerebellar hemisphere “Remember first that they’re deep nuclei inside, so we’re relating to their sites anatomically in the outer parts of the cerebellum we see”. How?? The Dentate nucleus is in the rest of cerebellar hemisphere, interposed nuclei are in the paravermis, and the fastigial nucleus is in the vermis. So sure these nuclei are connected to cerebellar parts they exist in, how? They’re connected in a way that they share functions of the cerebellum. Figure 12: Cerebellum vertical parts and corresponding functions 1. (central part on superior and inferior surfaces) represents head, neck, trunk, shoulders and hips so by default the proximal muscles of the body; it controls these body parts to “it’ll prevent you from swaying to right or left while standing”; so its fibers will sure project to . 8|Page 2. (lateral to vermis) represents muscles lateral to the midline; we mean by that distal parts of the upper and lower limbs. Its fibers will sure project to and Nuclei. - Vermis and Para vermis “and a part of the anterior lobe” are called the SPINO-cerebellar part of the cerebellum; since they control body’s proximal and distal muscles, this will make sense right??! It receives proprioception “remember that it’s the unconscious proprioception” from the spinal cord through anterior and posterior SPINO-cerebellar tracts “and a tract called cuneocerebellar tract coming from accessory cuneate nucleus; Anatomy Atlas - 5”. Remember: Conscious proprioception was through PCML tract. Emboliform and globose nuclei receive these afferent fibers and then send efferent fibers to go the spinal cord and control muscle tone. 3. has fibers projecting to function? Planning and Coordination of the motor movement. Nucleus. What’s their E.g. In a tennis game, the player will always try to plan how to stand and place his legs and bend his trunk in the ball direction, he’ll plan how to place his hand and rotate his wrist in a way that he’ll succeed in hitting the ball. All these coordination plans are the function of dentate and rest of cerebellar hemispheres; they represent sites of the cerebellum that the cerebrum consulted for planning of movement; of course they were consulted along with the basal ganglia which are also important in these situations. Besides (1) controlling proximal and distal muscles’ tone, (2) planning and coordination, the last function of the cerebellum is (3) balance, which was the function of the flocculo-nodular lobe “we’ll get to know in the next lecture its association with the vestibular nuclei which also play the most important role in balance”. - In this figure, you can see the pink colored nodule “which we said it’s a part of the inferior vermis” attached to the pink colored flocculi “A.K.A: flocculo-nodular lobe”. In embryology, this lobe is called Archicerebellum, which means the most old and primitive part of the cerebellum. As we called the vermis and paravermis functionally the spinocerebellum, we also call this lobe functionally the Vestibulocerebellum. Makes sense right?? 9|Page - We can see that the lingula as well, which is the most anterior part of the superior vermis, is included embryonically in Archicerebellum. 1ry fissure - This picture seems to be a bit old, so in fact we should see the whole vermis “including the yellow midline structures”, the paravermis, and the anterior lobe “anterior to primary fissure” all coming from the same embryonic origin -here in green- which is paleocerebellum “paleo; old”. These parts formed the spinocerebellum which controlled “along with interposed and fastigial nuclei” the body muscles’ tone. Figure 13: the embryological divisions of the - The rest of the cerebellum- seen here colored cerebellum in yellow- comes from the neocerebellum “neo; new”. This part is called functionally the cerebrocerebellum, thus along with dentate nucleus it’s responsible for movement coordination and planning. - ALL IN ALL: You’ll have to know the function “balance, tone ...”; anatomical “post, Ant, Flocculo-nodular”, functional “vestibule, cerebro, spino…”, and embryonic “paleo, archi, neo” classification of each lobe. Now coming to a beautiful view of the cerebellum. The view of “tree of life”♥!! Arbor Vitea 10 | P a g e Arbor vitae If we took a sagittal section like in figure 14, we’ll notice that the cerebellum looks like a tree with branches going everywhere; this appearance is called Arbor Vitea or This tree is of white matter after all, so all the afferent fibers coming to the cerebellum and all the efferent fibers going from the cerebellum to their destinations “cerebrum, spine, vestibular nuclei” will be found there. Can you see how in this view the cerebellum is Figure 14: Mid-sagittal 4th ventricle divided further into small branches? Think with me section in the brain brilliant; we did a cut in the midline of the cerebellum didn’t we, and which structure was representing the Medline? Yes you’re absolutely true! And why was it a worm like structure, what made it look like that? Because it was separated into what we call “lobules”. When cutting in the midline, we would by that cut the vermis of course, and the vermis isn’t continuous like the anterior or posterior lobe. It is rather further divided into small lobules. Explain please: In the anterior and posterior lobes we had only transverse folia but here between each certain group of folia we have a thing that separates them from the other groups, that’s why this gave us cerebellar lobules; each lobule, however, preserved the characteristic of having gray matter outside and white matter inside. This made the scientists figure out that each has different function, so they named them, and connected each vermis’ part with a part of the cerebellar hemisphere beside it as a lobule. (Figure 15) IMPORTANT: all the details related to Figure 15: Cerebellar lobules; unfolded function of each lobule are not required, nor cerebellum the names of all lobules; what is required is only to know that we have a superior vermis on the superior surface, and an inferior vermis on the inferior surface, and some names of important parts of vermis which we’ll mention just in a second. 11 | P a g e Important Vermis parts “refer to figure 15 please”: 1. In Superior vermis: A. Lingula. The most anterior part of the superior vermis; it forms a part of the spinocrebellum. B. Central C. Culmen. “culmen; ridge” By that we covered the whole superior vermis which is anterior to the 1ry fissure. 2. In inferior vermis: a) Nodule. The most anterior part of the inferior vermis; it forms a part of the vestibulocrebellum which is important for balance. b) Uvula “Diminutive of Uva; grape ” c) Pyramis “also called Pyramid”. - Functional classification of the cerebellar lobes. Today we’ll only discuss the function of the flocculo-nodular lobe; the rest will all be discussed in the upcoming lecture. We’ve already known that it maintains the balance, but also it participates in what we call “vestibulo-ocular reflex”. What’s that?? The movement of your eyes in the opposite direction of your head rotation is what this reflex is all about. This happens because there’s a connection between the vestibular part of the 8th C.N. and this lobe. (B) (b) Note: in each connection we’ll discuss the afferent fibers coming to the cerebellum and the efferent fibers getting out from it. So, if we got back to remember, which spinal tract was mainly responsible for controlling balance? Vestibulo-spinal tract; which enhanced axial extensors “anti-gravity muscles”. (a) (A) Figure 16: Vestibulo-ocular 1. Afferent fibers: they come by two ways; reflex either (a) directly from the vestibule to the FN lobe, or (b) from the vestibule “the vestibular apparatus in the internal ear ”, fibers synapse in vestibular nuclei then go as vestibulocerebellar tract to FN lobe. 12 | P a g e We have many vestibular nuclei “lateral, medial, superior and inferior vestibular nuclei”; all exist in the floor of the 4th ventricle “back of pons and upper part of medulla” on the lateral side (Anatomy atlas 6). They say that the most related nucleus to this reflex is the lateral vestibular nucleus; they even say that the lateral vestibular nucleus was originally a part of the cerebellum but during development it was partially laterally displaced to finally be situated in the floor of the 4th ventricle. 2. Efferent fibers get out of the FN lobe to (A) affect extrapyramidal vestibulo-spinal tract. It also (B) sends impulses to the vestibular tract related to 3rd, 4th, and 6th cranial nerves “vestibule-occular tract”, which forms the medial longitudinal fasciculus responsible for coordinating the head movement with eyes movement. So over all, we can affirm the FN lobe role in balance and preserving the posture by knowing that it affects the vestibulo-spinal tract, and we can affirm its role in the vestibulo-ocular reflex by affecting the vestibulo-ocular tract. Note: the rest of tracts will be discussed in the next lecture :D Finally, Test yourself with these 3 pictures discussed in the lecture, answers are in next page! On the right below: Notice here how the flocculo-nodular lobe is removed! **Not all numbers are supposed to be known, what you’ve learnt today would be more than enough :D Cerebellar lobules: Up; posterior, left; superior, right; inferior 13 | P a g e Anatomy Atlas 1. Dura mater folds; Falx cerebelli and tentorium cerebelli 2. Sites of Falx cerebelli attachment in the posterior cranial fossa. - Answers for Test-your-self pictures 14 | P a g e 3. Horizontal fissure and middle cerebellar peduncles. Horizontal fissure **Sure you can notice how they are in the same horizontal plane, and between the MCPs extending from Pons to cerebellum, you’ll find in between posteriorly the horizontal fissure. 4. Arnold- Chiari Malformation 5- Cuneo-cerebellar tract 6. Vestibular nucleus complex 15 | P a g e