Malformations in Cerebral Development

by Peter Ward, PhD

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    00:01 So things that can go wrong during cerebellar development include the chiari malformation.

    00:06 Now, chiari malformation has a variety of subtypes but the main thing to remember is that there´s too little space in the posterior cranial fossa.

    00:14 So the cerebellum normally fits in that posterior cranial fossa very nicely but with too little space, it has to go somewhere.

    00:21 And in type I and type II chiari malformation, the cerebellum partially herniates inferiorly through the foramen magnum.

    00:30 In particular, the cerebellum tonsils tend to extend down the foramen magnum and they put pressure on the medulla that´s located anterior to it.

    00:39 Now, that pressure on the medulla can result in difficulty with organizing your respiratory and heart activity because your parasympathetic cardiorespiratory nucleus is located just in that area.

    00:54 So this is tied to some problems with regulating breathing and heartbeat.

    00:59 In addition, having that pressure can back up the flow of cerebral spinal fluid causing hydrocephaly or even syringomyelia which is going to be a cyst that develops in the central canal of the spinal cord as fluid is unable to exit the central canal, it builds up and creates a cyst in the upper cervical region.

    01:19 And again, that is syringomyelia.

    01:21 Now, another problem involving the cerebellum is called Dandy-Walker Malformation.

    01:26 And it´s a series of problems that basically boil down to there being too little cerebellum.

    01:32 Agenesis of the cerebellar vermis, the midline region of the cerebellum can result in an expansion of the fourth ventricle.

    01:40 If there´s no cerebellum there, then, the space that it used to occupy will now be filled with cerebrospinal fluid of the fourth ventricle.

    01:47 And as that fluid builds up, it´s gonna cause a ballooning of the dura mater that´s just superior to where the cerebellum´s going to be located.

    01:57 So altogether, that´s known as Dandy-Walker Malformation and this can result in post-natal hydrocephaly and has a variety of other problems that need to be diagnosed radiologically and this is a fairly good example of what that´s going to look like in sagittal cross section.

    02:14 Proceeding further superiorly, we´re gonna now enter the midbrain.

    02:21 And just like before, it´s easy to get confused by all of the nuclei present in the midbrain but try to think of whether they´re motor or sensory and it will make a little more sense.

    02:31 Early on, the alar plate sends some sensory nuclei very far dorsally to become the superior and inferior colliculus and the basal plate sends some motor nuclei ventrally to become the substantia nigra.

    02:50 So we take a look at the inferior portion of the midbrain or the mesencephalon and we can see the inferior colliculus which is tied to sensory activity of hearing.

    03:01 It´s a relay for hearing and other sensory activity coming from the vestibular and cochlear nuclei.

    03:08 Then we have some motor nuclei located inferior to that.

    03:13 In particular, the trochlear nucleus is found in the inferior midbrain and the substantia nigra is present very far ventrally close to the motor fibers coming from the brain down to the spinal cord, the crus cerebri.

    03:29 Go a little further up and it looks very much the same.

    03:33 Take a look at this picture and this picture.

    03:36 They look very similar.

    03:37 One thing that distinguishes the superior midbrain from the inferior midbrain is that we have a superior colliculus for visual activity relay and we´ve got the oculomotor nucleus for moving the eyes.

    03:54 But we have one more motor nucleus located in between the substantia nigra and the oculomotor.

    04:00 This is called the red nucleus and it´s a motor nucleus that´s very important in some animals.

    04:05 It´s a little bit less pronounced in humans but that nucleus is still present.

    04:09 Now, we move a little further up and we come to the diencephalon or the thalamic nuclei.

    04:17 This is where that easy breakdown of alar and basal plate somewhat breaks down and we have an epithalamus or pineal gland developing off the very superior most portion of the alar plate, the thalamic nuclei which receive a great deal of sensory information from all over the body develop form the alar plate and the hypothalamus develops from the basal plate.

    04:43 Now, if you remember what the hypothalamus does, it directs a lot of our very basic motor activity.

    04:49 Our basic drives of appetite, weightfulness, thermoregulation are all located in the hypothalamus.

    04:57 So you can think of it as a type of motor nucleus but it´s a bit more sophisticated than just an easy motor versus sensory breakdown.

    05:05 So the hypothalamic nuclei are gonna be present there whereas the thalamic nuclei are present coming from the alar plate just above that hypothalamic sulcus.

    05:16 Now, the cerebral cortex is gonna have its own lecture altogether.

    05:20 We´re going to talk really quickly though about a small portion of the hypothalamus that blebs downward and meets a very strange structure called rathke´s pouch.

    05:31 Rathke´s pouch is a little ballooning of the epithelium of the oral cavity and it stretches upward to meet a down growth, an infundibulum of the diencephalon.

    05:45 And this weird two part structure is gonna form our pituitary gland.

    05:51 So rathke´s pouch grows up from the oral cavity.

    05:54 The infundibulum goes down from the diencephalon and eventually, the rathke´s pouch completely disassociates from the oral cavity and wraps around the infundibulum and this is gonna be our pituitary gland.

    06:08 And the anterior lobe of the pituitary gland is from rathke´s pouch oral epithelium whereas the posterior lobe or infundibulum are coming from the diencephalon.

    06:19 So very strange embryologic development creating that single gland from two separate sources.

    06:27 And finally, the pituitary gland sits inside a little saddle of bone called the sella turcica, completely separate from the oral cavity but interestingly, if you need to do a pituitary surgery, you will actually somewhat replicate its pathway of migration by going through the upper nasal pharynx to crack through that bone and get to the pituitary by following more or less the original pathway of rathke´s pouch.

    06:54 Now, let´s take a step back.

    06:55 We´ve discussed how the alar plate gives rise to sensory and the basal plate to motor neurons.

    07:02 When we think about the cranial nerves, 1 through 12 and their extreme complexity, one thing that can help make sense of them is the fact that any motor activity of a cranial nerve must be tied to a derivative of the basal plate.

    07:15 Likewise, any sensory activity associated with the cranial nerve has to be tied to a nucleus originating from the alar plate.

    07:23 So if you look at where those plates originate and where the final sensory and motor nuclei are located in the brain stem, it can help make sense of where the different nuclei are located when you´re trying to look through a cross sectional image and localize a lesion inside the brain stem.

    07:41 Now, the sensory ganglia that are associated with the cranial nerves 5, 7, 9, and 10, originate from the same place that the posterior root ganglia originate.

    07:52 They come from neural crest cells that migrate and then, extend axons out to the skin or other sensory structures and back into the central nervous system to inform the brain about what they´re sensing.

    08:05 So a cranial nerve 5 is going to be innervating the face, cranial nerve 7, part of the ear and taste for the tongue, and cranial nerves 9 and 10 innervating the pharynx, palate, larynx, and other structures inside the head.

    08:20 Now, we´ve left out cranial nerves 1, 2, and 8.

    08:24 They are very special structures that are going to be given the rest of the olfactory, the optic, and the vestibular cochlear apparatus in particular and we´re gonna have separate discussions on each one of those because they´re very special in the way that they develop.

    08:40 To take a step back into the lowest part of the body, initially, our spinal cord extends all the way to the very tip of our developing vertebral canal and during early development as those spinal nerve roots come out, they just immediately exit the intervertebral foramina that are associated with them.

    09:01 But for some reason as we grow, our spinal cord does not grow at a corresponding length and it remains tethered to the base of the sacrum and coccyx.

    09:12 But the whole cord extends, leaving behind a small little cord of material called the filum terminale and the cord "ascends".

    09:23 It doesn´t literally move upward but it does not elongate as quick as the rest of the body.

    09:27 So as it gets pulled more superiorly, we wind up with the nerve roots being stretched out behind it and that creates a structure called the cauda equina.

    09:38 That translates to horse´s tail and it really does look like a horse´s tail.

    09:43 You have a variety of nerve roots filling the space inside the vertebral canal and then, exiting at their designated spinal level.

    09:52 You generally find the tip of the spinal cord, the conus medullaris located at about the L1 to L2 region in most mature people.

    10:00 But by the time we´re born, it has already ascended quite a long way.

    10:05 And if we have to do a spinal tap on someone, we´re generally gonna aim to enter the canal at some place below L1 so that we´re not gonna risk hitting the actual cord but can push those nerve roots out of the way and pull cerebrospinal fluid out of a nearby space.

    10:21 Alright, thank you very much and I´ll see you at the next talk.

    About the Lecture

    The lecture Malformations in Cerebral Development by Peter Ward, PhD is from the course Development of the Nervous System, Head, and Neck.

    Included Quiz Questions

    1. Foramen magnum
    2. Foramen rotundum
    3. Foramen ovale
    4. Foramen spinosum
    5. Foramen lacerum
    1. Inferior colliculis
    2. Superior colliculus
    3. Trochlear nucleus
    4. Trigeminal nucleus
    5. Substantia nigra
    1. Rathke's pouch
    2. Infundibulum
    3. Diencephalon
    4. Hypothalamus
    5. Epithalamus
    1. Filum terminale
    2. Conus medullaris
    3. Cauda equina
    4. Coccygeal ligament
    5. Pia mater
    1. Conus medullaris
    2. Cauda equina
    3. Filum terminale
    4. Coccygeal ligament
    5. Sacral ligament

    Author of lecture Malformations in Cerebral Development

     Peter Ward, PhD

    Peter Ward, PhD

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