Development of the Midbrain and Thalamus

00:00 Proceeding further superiorly , we're gonna now enter the midbrain and just like before it’s easy to get confused by all the nuclei present in the midbrain, but try to think of whether they are motor or sensory, and it will make a little more sense.

00:16 Early on, the alar plates sends some sensory nuclei very far dorsally to become the superior and inferior colliculus and the basal plate sends some motor nuclei inferiorly or pardon me, ventrally, to become the substantia nigra.

00:35 So we take a look at the inferior portion of the midbrain or the mesencephalon and you can see the inferior colliculus which is tied to sensory activity of hearing.

00:46 It’s a relay for hearing and other sensory activity coming from the vestibular and cochlear nuclei, then we have some motor nuclei located inferior to that 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.

01:15 Go a little further out and it looks very much the same.

01:18 Take a look at this picture, and this picture; they look very similar.

01:23 One thing that distinguishes the superior brain, pardon me, 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 but we have one more motor nucleus located in between the substantia nigra and the oculomotor, this is called the red nucleus and it’s a motor nucleus that’s very important in some animals, is a little bit less pronounced in humans, but that nucleus is still present.

01:54 Now we move a little further up and we come to the diencephalon or the thalamic nuclei.

02:02 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 most, very superior most portion of the alar plate.

02:18 The thalamic nuclei which receives a great deal of sensory information from all over the body developed from the alar plate and the hypothalamus develops from the basal plate.

02:28 Now if you remember what the hypothalamus does? It directs a lot of our very basic motor activity, our basic drives of appetite, wakefulness, thermoregulation are all located in the hypothalamus.

02:42 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.

02:50 So the hypothalamic nuclei are gonna be present there whereas the thalamic nuclei are present coming from the alar plate just above the hypothalamic sulcus.

03:00 Now the cerebral cortex is gonna have its own lecture all together.

03:06 We're gonna talk really quickly though about a small portion of the hypothalamus that blebs downward and meets a very strange structure called Rathke’s Pouch.

03:16 Rathke’s Pouch is a little, what you call, ballooning of the epithelium of the oral cavity and it stretches upward to meet a down grove an infundibulum of the diencephalon.

03:30 And this weird two part structure is gonna form our pituitary gland.

03:36 So Rathke’s Pouch grows up from the oral cavity, 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.

03:53 In the anterior lobe of the pituitary gland is from Rathke’s pouch, oral epithelium, whereas the posterior lobe and infundibulum are coming from the diencephalon, so very strange embryologic development creating that single gland from two separate sources.

04:12 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 it’s pathway of migration by going through the upper nasal pharynx to the crack through that bone and get to the pituitary by following more or less the original pathway of Rathke’s Pouch.

04:39 Now let’s take a step back.

04:40 We've discussed how the alar plate gives rise to sensory and the basal plate to motor neurons.

04:47 When we think about the cranial nerves I through XII, 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.

05:01 Likewise, any sensory activity associated with the cranial nerve has to be tied to a nucleus originated from the alar plate, so if you look at where those plates originate and where the final sensory and motor nuclei located in the brainstem it can help make sense of where the different nuclei are located when you're trying to look through a cross section image and localize a lesion inside the brainstem.

05:26 Now the central ganglia that are associated with cranial nerves V, VII, IX, and X, originate from the same place that the posterior root ganglia originate.

05:37 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, so cranial nerve V is going to be enervating the face, cranial VII part of the ear and taste for the tongue, and cranial nerves IX and X enervating the pharynx, palate, larynx and other structures inside the head.

06:05 Now we've left out cranial nerves I, II, and VIII.

06:10 They're very special structures that are going to be giving rise to 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 a way that they develop.

06:25 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 the spinal nerve roots come out they just immediately exit the intervertebral foramena that are associated with them.

06:46 But for some reason, as we grow, our spinal cord does not grow in corresponding length and it remains tethered to the base of the sacrum and coccyx but the whole cortex stands leaving behind a small little cord of material called filum terminale, and the cord “ascends” it doesn’t literally move upward but it doesn’t elongate as quick as the rest of the body. 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, that translates to horse’s tail.

07:27 And it really does look like a horse’s tail.

07:29 You’ll have a variety of nerve roots filling the space inside the vertebral canal and then exiting at their designated spinal level.

07:37 You generally find the tip of the spinal cord, the conus medullaris located at about the L1 - L2 region in most mature people, but by the time we're born it is already ascended quite a long way and if we have to do a spinal tap on someone, we're generally gonna aim to enter the canal at someplace 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 the nearby space.

08:06 Alright, thank you very much and I’ll see you at the next talk.