Mesencephalon (Midbrain) - External & Internal structures + QUIZ | Anatomy — Transcript

Full transcript

1. 0:03What’s up. Meditay here. Let’s continue  the anatomy of the Central Nervous System.
2. 0:08In this segment, we’ll cover the complete anatomy  of the Midbrain, also known as the mesencephalon
3. 0:13So remember, the central nervous  system consists of two parts:
4. 0:16the encephalon and the spinal cord. The encephalon  is then further divided into specific parts.
5. 0:22We have the brainstem, which consists of  the Medulla, Pons, and the midbrain or the
6. 0:26mesencephalon. We have the cerebellum back here,  then the Diencephalon and the telencephalon.
7. 0:31Our focus in this video is going  to be the Midbrain, which is here.
8. 0:35So in this video, we’re first going to cover the  external surfaces of the midbrain. Basically,
9. 0:40look at its topography and what structures you’ll  find from an anterior view and a posterior view.
10. 0:45Then we’re gonna slice up the midbrain  and look at the internal surface.
11. 0:49Basically, see how the grey matter and  white matter are arranged within it.
12. 0:53Then I’ve made a little quiz at the end  which might help you if you need to memorize.
13. 0:57Alright, so we can start by replacing  this picture with a more realistic one.
14. 1:02From this view, we can locate Pons, the  Medulla, the cerebellum, and the spinal cord.
15. 1:07Now, if we remove a part of the cerebral Cortex,  you’ll be able to see the rest of the brainstem,
16. 1:12which is the mesencephalon, or the midbrain. So as you see from this picture,
17. 1:17the midbrain is located above Pons, in front  of the Cerebellum, and below the Diencephalon.
18. 1:23Now. Externally, your midbrain has two surfaces.  It has an anterior surface and a posterior
19. 1:30surface. Let’s now cover the typical morphology  of these two surfaces, starting with the anterior
20. 1:36surface first. And we’ll do that by looking  at the midbrain from an anterior view. So the
21. 1:41midbrain is here. The first thing we can do is to  remove the less significant surrounding structures
22. 1:47to make this easier. The majority of the anterior  surface of the midbrain is the Cerebral Peduncles.
23. 1:54These are two large peduncles that contain  tracts coming from the Cortex of your cerebrum
24. 1:59and are responsible for voluntary movement of  the body. I’ll show you later in this video
25. 2:04when we go through the cross-section. But  between these two peduncles, we have a pit
26. 2:10we call the interpeduncular Fossa. And within the  interpeduncular Fossa, you’ll find some structures
27. 2:16of the Diencephalon, which are not a part of your  midbrain, like the Hypothalamus and the Pituitary
28. 2:21gland. So again, they’re the Diencephalon and  our topic for the next video. But there are a few
29. 2:27structures here that are a part of the midbrain. And the first one is the posterior perforated
30. 2:33substance. This is a depression here on the  anterior surface of the midbrain, that contains
31. 2:38grey matter. And they contain small holes  here for blood vessels to go in and out from.
32. 2:44Then there is a groove called the  Ocuculomotor sulcus of the mesencephalon,
33. 2:48of which the third cranial nerve goes out, called  the oculomotor nerve. This nerve is called the
34. 2:54oculomotor nerve because it goes towards the eye  to innervate the extrinsic eye muscles that enable
35. 3:00the movement of the eyes. So that was all the  structures associated with the anterior surface.
36. 3:06Let’s now do the posterior surface. And to do  that, we’ll be looking at the midbrain from a
37. 3:11posterior view. Now from this view, we can  locate the Midbrain, Pons, and the Medulla.
38. 3:17So the first thing we can highlight is the  cerebral peduncles that we talked about earlier.
39. 3:22They are huge in comparison with the actual  midbrain, that is why you’re able to see them
40. 3:27peeking out from the sides from a posterior view.  But the most significant structures associated
41. 3:34with the posterior view, are the Tectal Plate, or  Lamina Tecti. And you’ll notice that on the tectal
42. 3:40plate, you’ll find four rounded structures. The  upper two are called the superior colliculus. And
43. 3:46this one Is associated with rapid and controlled  eye movements. Now how is it able to do so?
44. 3:52To understand that, we need to go through the  visual pathway. Within the retina of your eyes,
45. 3:57you have receptors for the 2nd cranial nerve,  the optic nerve. The fibers of the optic nerve
46. 4:02will go back, and then half of the fibers will  cross and form the optic chiasm. After that,
47. 4:08they will synapse with the Lateral geniculate  bodies, which is the metathalamus. We’ll go
48. 4:13through them when w go through the Diencephalon.  But From the Lateral Geniculate bodies, the fibers
49. 4:19will go back to the occipital lobe, which is  where you’ll find the primary visual Cortex.
50. 4:24When they go to the Cortex, that is when you’re  consciously aware of the things you see around
51. 4:29you. But fibers also go from the lateral  geniculate bodies to the superior colliculi,
52. 4:36through the Brachium of the superior colliculi,  which are these ones you see on the sides,
53. 4:40and then to the superior colliculi, which  will activate the tectospinal tract.
54. 4:44Which sends motor impulses for the  coordinated eyes and the neck muscles.
55. 4:49Then under that, we have the inferior colliculi,  Which is a part of the hearing pathway. So the
56. 4:55cochlear nerve receives impulses from the cochlea,  where sound is converted into nerve signals. The
57. 5:01cochlear nerve will then go to the cochlear nuclei  at the Rhomboid Fossa in Pons. And then they will
58. 5:07cross and form the trapezoid body, remember it’s  a structure of Pons. After that, fibers ascend,
59. 5:13they go up as the Lateral Lemniscus to synapse  with the inferior colliculi. From there,
60. 5:19impulses are sent through the Brachium of the  inferior colliculus to the medial geniculate body,
61. 5:24not the lateral this time. The lateral geniculate  body is for vision, medial is for hearing. From
62. 5:31the medial geniculate body, impulses go to  the primary auditory Cortex, which is in your
63. 5:36primary temporal gyrus. So that was these two. Another structure you’ll find on the posterior
64. 5:43surface is the Lateral sulcus  of the Mesencephalon. This is
65. 5:47the border between the cerebral peduncles  and the posterior surface of the midbrain.
66. 5:52Another thing you’ll find here is the trigone  of the lateral lemniscus, located in this area.
67. 5:58And then, below the inferior colliculus,  you’ll find the cranial nerve number 4 called
68. 6:03the trochlear nerve, which will turn towards the  anterior side to innervates the superior oblique
69. 6:08muscle of the eye. So that was everything  for he external surface of the midbrain.
70. 6:13Now let’s go ahead and cut the midbrain right  about here, and look at it from this perspective.
71. 6:18We’ll see this! This is at the level of the
72. 6:21superior colliculi. Structures are more or less  the same whether you make a cross-section at the
73. 6:26superior colliculus or the inferior colliculus,  but there are some slight differences. And I’ll
74. 6:32make sure to highlight them along the way. So anteriorly, we have the Cerebral Peduncles,
75. 6:38with the interpeduncular Fossa here in the middle.  And then there’s the aqueduct of the midbrain,
76. 6:43which remember is a part of the  ventricular system, connecting the
77. 6:46third ventricle with the fourth ventricle. So. The internal surface of the midbrain can
78. 6:51be divided into three regions. First is the  tectum of the midbrain, which contains the
79. 6:56colliculi. Then the tegmentum of the midbrain  in the middle. And then the cerebral peduncles.
80. 7:03Let’s now go through all the internal  structures, starting with the grey matter.
81. 7:07And just to remind you again, white  matter consist of myelinated nerve fibers.
82. 7:11So when we go through white matter, we talk about  tracts. Grey matter contain nuclei of neurons. So
83. 7:18structures in the grey matter are nuclei. So the  first grey matter structure we gonna talk about is
84. 7:25the red nucleus or nucleus ruber. These nuclei are  pale pink in color, due to the presence of iron,
85. 7:32as either hemoglobin or ferritin. These nuclei  are very characteristic of the midbrain. And
86. 7:39they contribute to the extrapyramidal pathway  of coordinating voluntary muscle control.
87. 7:44We’ll talk about this later in this video but  remember, from the lobes of the Cortex. The
88. 7:49corticopontine tract will descend and synapse  with the pontine nuclei of Pons. After that,
89. 7:55fibers will go to the cerebellum as the  pontocerebellar tract. Then fibers will go
90. 8:01from the cerebellum to the red nucleus as the  cerebellorubral tract. And then down towards
91. 8:07the spinal cord as the rubrospinal tract. To  coordinate and support voluntary muscle movements,
92. 8:13mainly flexor muscles. Tracts may even come  directly from the Cortex to the nucleus ruber,
93. 8:19but this right here is the most important  pathway when you talk about these nuclei.
94. 8:24After that, we have the Substantia Nigra.  Substantia nigra is actually made up of
95. 8:29two distinct regions. The substantia nigra pars  compacta and the substantia nigra pars reticulata.
96. 8:36When we talk about substantia nigra, we  generally mean pars compacta because that’s
97. 8:40the most significant one. The pars compacta  is very dark colors due to the large number
98. 8:46of dopamine neurons producing neuromelanin. The substantia nigra is considered a part of
99. 8:51your basal ganglia. The basal ganglia what that is  they’re a group of grey matter nuclei found within
100. 8:57your brain. We’ll talk about that when we talk  about the internal structures of the hemispheres,
101. 9:02but the basal ganglia consist of  the caudate nucleus, the Putamen,
102. 9:06Globus pallidus. There are two, external to the  left and internal to the right. Then the Thalamus,
103. 9:14but not the whole Thalamus, only the ventral  anterior and ventral lateral part of the Thalamus
104. 9:19is considered a part of your basal ganglia. Then  there are the subthalamic nuclei and then the
105. 9:24substantia nigra as well. You might find other  names when you talk about the basal ganglia,
106. 9:29like the striatum. Whenever you’re talking about  striatum then you’re talking about the Putamen
107. 9:35and the Caudate nucleus together. And when you  put the Putamen and the Globus Pallidus together
108. 9:40you’re talking about the Lentiform nucleus. Now. You’re probably wondering what the
109. 9:44heck I’m talking about and why I’m mentioning  them. The basic motor function, is coordinated
110. 9:50by the cerebral Cortex, right?  Primarily the primary motor area.
111. 9:54Whenever you decide to consciously  move a limb, your primary motor cortex
112. 9:59will send motor tracts along the spinal cord to  engage muscles necessary to do so through the
113. 10:05corticospinal tract. But in order for this  motor plan to be able to go to the muscles,
114. 10:10you need to kind of have a  communication with the basal gangion.
115. 10:14SO imagine for a second we’ve combined  all the basal ganglia structures into
116. 10:19a purple bulb here. So the primary motor  area have to communicate their motor plan
117. 10:25with the basal ganglia. The basal ganglia take  that motor plan and modify it in a particular way,
118. 10:31and send it back to the cerebral Cortex to send  now the proper motor plan to start movement,
119. 10:38stop movement or modulate the movement. Beautiful.  Now let’s get back to the substantia nigra. So
120. 10:44the Substantia nigra is therefore a part of a  larger group of structures that start movement,
121. 10:50stop movement and modulate movement, through the  nigrostriatal pathway using dopamine. I won’t
122. 10:56go in detail into that, but if for instance  a disease happen that causes the substantia
123. 11:01nigra to have less dopamine neurons. What do you  think will happen? You’ll get Parkinsons Disease.
124. 11:07And this will give the typical TRAP symptoms,  which are Tremor, Rigidity, Akinesia and Postural
125. 11:14instability. All of those, because the basal  ganglia is not able to function well. Awesome.
126. 11:20I hope the substantia nigra gave a little more  sense now. Understanding that will give you a good
127. 11:24starting point in studying its physiology. Alright Our next grey matter nuclei depend on which
128. 11:31layer you’re looking at. If we make a  cross-section at the superior colliculi,
129. 11:35we’ll see the nucleus of the oculomotor nerve and  the posterior accessory nucleus of the oculomotor
130. 11:41nerve. So the oculomotor nerve will travel  towards the anterior surface towards the eye,
131. 11:47to innervate the extraocular muscles to move your  eyes. And the posterior accessory nucleus will
132. 11:53send fibers together with the oculomotor nerve  to give a parasympathetic innervation to the
133. 11:58eyes. So that is at the superior colliculi level. Then at the Inferior Colliculi level, there’s the
134. 12:04nucleus of the trochlear nerve. Remember, we have  the trochlear nerve going out at the posterior
135. 12:09surface, which turns anteriorly, goes towards the  eyes, and supplies the superior oblique muscle.
136. 12:16Another nucleus you’ll find at the level of the  Inferior colliculus is the mesencephalic nucleus
137. 12:21of the Trigeminal Nerve. Ok, let’s repeat the  nuclei of the trigeminal nerve at the Rhomboid
138. 12:26Fossa. These are the Mesencephalic nucleus of  the trigeminal nerve. So here is the trigeminal
139. 12:32nerve. The mesencephalic nucleus takes in sensory  information from mainly muscles of mastication.
140. 12:39Other nuclei are the Principal nucleus of  the trigeminal nerve, which senses touch
141. 12:44and vibration. Spinal nucleus which senses  pain and temperature, and motor nucleus of
142. 12:49the trigeminal nerve, which provide motor  innervation for the muscles of mastication.
143. 12:54These are the nuclei of the trigeminal nerve, but  the mesencephalic nucleus of the trigeminal nerve
144. 13:00is the only one extending upwards to the  inferior part of the midbrain, and that’s why
145. 13:06you see it at that level. So that’s these. Then we have the reticular formation,
146. 13:12which is essential for the vital functions and  balance. And we have the periaqueductal grey
147. 13:18substance, or the central grey substance, which is  associated with eliminating, or decreasing pain.
148. 13:24So that was all for the grey matter of the  midbrain. Now let’s do the White matter of
149. 13:29the midbrain. White matter in the CNS is  distributed as either ascending tracts,
150. 13:36which are sensory tracts. And descending tracts,  which are motor tracts. We’ll do the ascending
151. 13:41tracts first and then do the descending ones. The first one is the medial lemniscus. The medial
152. 13:47lemniscus is an ascending tract you’ll  find throughout the whole brainstem.
153. 13:51So here is the cross-section of  the spinal cord, the medulla,
154. 13:55and Pons. Do you remember that sensory  fibers came from the Lower parts of the body,
155. 14:01which ascend as Gracile fascicle, and sensory  fibers that came from the upper parts of your
156. 14:06body, which ascend as Cuneate fascicle? They sense  conscious proprioception and mechanoreceptors. And
157. 14:13I say conscious because they take their fibers  all the way up to your cerebral Cortex. If it
158. 14:19were unconscious, then it would take its fibers  to the cerebellum or other subcortical structures.
159. 14:24They will ascend to the gracile and cuneate nuclei  in the medulla. Then fibers there will leave as
160. 14:30either the external arcuate fibers or internal  arcuate fibers. The internal arcuate fibers
161. 14:36cross to the other side. Then they will ascend as  the medial lemniscus, which is what you see here
162. 14:42in the midbrain. They will ascend and go to the  primary somatosensory area in the cerebral Cortex.
163. 14:48So that is the Medial Lemniscus. Next to the medial lemniscus,
164. 14:53we have the spinal lemniscus. Ok. So again  here’s the cross-section of the medulla and Pons.
165. 14:59On the cross-section of Medulla, you’ll  find two tracts called the anterior
166. 15:04and the lateral spinothalamic tracts. At some  point between the Pons and the Medulla, these
167. 15:10tracts will join together and form the spinal  lemniscus. And the spinal lemniscus will ascend
168. 15:16through Pons and the mesencephalon and then to  the primary somatosensory area in the Cortex.
169. 15:22They’re responsible for conscious  sensory input of Pain temperature,
170. 15:26Pressure, and touch. So that is this one. Next, we have the Trigemnical Lemniscus.
171. 15:32The trigemnical lemniscus comes from the  trigeminal ganglion, which is a part of the
172. 15:37trigeminal nerve, the 5th cranial nerve. It  receives sensory input from the facial area,
173. 15:42and send their axons towards Pons. In  Pons, these fibers will cross to the
174. 15:47other side and ascent through the midbrain to  the primary somatosensory area as well. Awesome.
175. 15:55Then we have a tract we went through a little  earlier in this video, the Lateral Lemniscus.
176. 15:59Which was part of the hearing pathway. So the  cochlear nerve receives impulses from the cochlea,
177. 16:05where sound is converted into nerve signals.  The cochlear nerve will then go to the cochlear
178. 16:11nuclei in Pons. Then they will cross and form the  trapezoid body of Pons. After that, fibers ascend,
179. 16:19they go up as the Lateral Lemniscus to synapse  with the inferior colliculi. From there,
180. 16:24impulses are sent through the Brachium of the  inferior colliculus, to the medial geniculate
181. 16:29body, and then to the primary auditory Cortex,  which is in your superior temporal gyrus. Cool.
182. 16:36That was all of our sensory  tracts in the mesencephalon.
183. 16:40Now let’s do all the descending tracts we find  here. The descending tracts of the mesencephalon
184. 16:46are arranged along the Cerebral peduncles and the  tegmentum of the Midbrain. We’ll do the tegmentum
185. 16:51first and then go over to the cerebral peduncles. The first one is a tract that comes from the
186. 16:57superior colliculus. And that tract will decussate  as the posterior tegmental decussation, to then
187. 17:04descend as the tectospinal tract. Remember when  we went through the superior colliculus, where we
188. 17:09said that the superior colliculus is a part of the  visual pathway? Where you have receptors for the
189. 17:14optic nerve at the eyes. The fibers of the optic  nerve will go back, and then half of the fibers
190. 17:20will cross and form the optic chiasm. After that,  they will synapse with the Lateral geniculate
191. 17:26bodies, which is the metathalamus. From the  Lateral Geniculate bodies, the fibers will go
192. 17:30back to the occipital lobe so that you’ll be able  to consciously perceive what you see around you.
193. 17:36But fibers also go from the lateral geniculate  bodies to the superior colliculi. And from the
194. 17:42superior colliculi, fibers are sent down as the  tectospinal tract. And when the superior colliculi
195. 17:48are engaged, it usually just engages the neck  muscles based on what you see. So that’s this one.
196. 17:55So, we talked about the posterior  tegmental decussation. And since we
197. 17:59have a posterior tegmental decussation, we’ll  also find an anterior tegmental decussation!
198. 18:05Which is formed by the rubrospinal  tract just as it leaves the red nuclei.
199. 18:11Remember, from the Cortex, the corticopontine  tract will synapse with the pontine nuclei of
200. 18:16Pons. After that, fibers will go to the cerebellum  as the pontocerebellar tract. Then they will go to
201. 18:23the nucleus ruber as the cerebellorubral  tract. And then, from the nucleus ruber,
202. 18:28the rubrospinal tract will cross and then descend  towards the spinal cord and then to muscles to
203. 18:34coordinate and support voluntary muscle movements. Another tract you’ll find in the midbrain is,
204. 18:40it’s sort of logical, but at some point in the  midbrain, you’ll find the cerebellorubal tract
205. 18:46as it goes towards the nucleus ruber. So  we might as well include this one too.
206. 18:51Then after that, we have the reticulospinal  tract, which is part of the balance and posture
207. 18:56system. They come from the reticular formation  inside the brainstem. The Reticular system is
208. 19:01responsible for Sleep, alertness, cardiovascular  control, breathing, and all of those vital things.
209. 19:07But they’re also responsible for motor control  like your balance and posture through the
210. 19:13reticulospinal tract. So that is this one. Then we have a tract called the medial
211. 19:18longitudinal fasciculus, which descends  towards the spinal cord and is present only
212. 19:24in the cervical segments. This tract coordinated  involuntary movements of the head, neck, and eyes
213. 19:31through synapses between the cranial nerves 3, 4,  6, and 11. So that was all the descending tracts
214. 19:39in the tegmentum of the Midbrain. Now let’s do the  descending tracts within the Cerebral Peduncles.
215. 19:45These tracts are very easy to recognize  as they all come from the cerebral Cortex.
216. 19:50Hence the name cerebral peduncles. So the first descending tract is
217. 19:55our famous Corticospinal tract. The corticospinal tract originates from
218. 20:00the pyramidal cells of the primary motor area. And  since they come from the pyramidal cells, we call
219. 20:05this a pyramidal motor tract which descends down  to the spinal cord to innervate skeletal muscles.
220. 20:13Alongside the corticospinal tract, you’ll  find corticonuclear tracts as well.
221. 20:18It descends in the same areas as the corticospinal  tract. But the corticonuclear tracts are
222. 20:24responsible for the voluntary control  of muscles located in the head and neck.
223. 20:29Then we have the corticopontine tract. And I’ve  mentioned this like two other times in this video,
224. 20:34but it’s important to know it.  Remember the corticopontine tract
225. 20:38descends within the cerebral peduncle to synapse  with…. The Pontine nuclei of Pons. After that,
226. 20:45they go to the cerebellum as the pontocerebellar  tract and then back to the mesencephalon as the
227. 20:53cerebellorubral tract to  synapse with the nucleus ruber.
228. 20:56From which the rubrospinal tract will descend to  skeletal muscles to support voluntary movements.
229. 21:02The term corticopontine tract is a little  inaccurate to use in this sense. Because one tract
230. 21:09will arrive from each lobe of the brain and then  descend. They’ll all have the same pathway, it’s
231. 21:15just the origin point that differ. So if the tract  starts from the Cortex of the frontal lobe, it’s
232. 21:20called frontopontine tract. If it starts from the  occipital lobe or parietal lobe or temporal lobe,
233. 21:27it’ll be called occipitopontine, parietopontine,  or temporopontine tract. Notice that none of these
234. 21:33come from the pyramidal cells of the primary  motor cortex, that’s why they’re referred
235. 21:38to as extrapyramidal tract. They don’t initiate  movement, but they support the voluntary movement.
236. 21:45So, that was all the descending tracts  of the cerebral peduncles. That was all
237. 21:49the white matter of the mesencephalon. Now, I made this table with all the grey
238. 21:54and white matter we just went through, along  with a little description here on the right.
239. 21:58I am going to make all the names of the nuclei and  tract disappear. And can you, based on the picture
240. 22:04and the description, tell m what is the name of  number 1? What is the name of the number 7 and
241. 22:10so on. If you can do that, then you pretty much  have a good understanding of the mesencephalon.
242. 22:14If you found this video helpful, please  put a like, comment, share, whatever
243. 22:18you find convenient to you. My next video will be about the Diencephalon.