CN 3: Oculomotor Nerve (Scheme, Pathway, Clinical Relevance) | Neuroanatomy — Transcript

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1. 0:06What’s up, Taim Talks Med here.
2. 0:08Let’s continue our Cranial nerve series.
3. 0:11Cranial nerves are twelve pairs of nerves that exit the brain and the brainstem, and
4. 0:16in this segment, we’ll talk detailed about the third cranial nerve, which is the Oculomotor
5. 0:21nerve.
6. 0:22And we’ll do that by first making a quick scheme of the oculomotor pathway to get an
7. 0:26overview of it.
8. 0:28Then we’ll cover the very of Midbrain anatomy because this is where the oculomotor nerve
9. 0:33originates from.
10. 0:35After that, we’re going to cover the course of the oculomotor nerve and go detailed into
11. 0:41it’s pathway and which structures the oculomotor nerve goes through and while doing so we’ll
12. 0:47talk through the function of all the muscles that the oculomotor nerve innervates.
13. 0:52And at the end, we’ll talk a little bit about the clinical relevance around pathologies
14. 0:57related with the third cranial nerve pathway.
15. 1:00So, the oculomotor nerve is the third cranial nerve, and the name itself is a clear indication
16. 1:08of the function of the nerve.
17. 1:10Oculo means that it’s related to the eye, motor means producing movement.
18. 1:15Simply from the name then, it is easy to know that the oculomotor nerve will innervate muscles
19. 1:20that move the eye itself or components of the eye.
20. 1:24So, the oculomotor nerve allows movement of the eye muscles, constriction of the pupil,
21. 1:31and the position of the upper eyelid.
22. 1:33Let’s see how.
23. 1:35There are two nuclei for the oculomotor nerve that are both located in the midbrain, at
24. 1:41the level of the superior colliculi.
25. 1:42Those are called the nucleus of the oculomotor nerve and the accessory oculomotor nucleus,
26. 1:49which is known as Edinger–Westphal nucleus.
27. 1:53Those two nuclei are gonna give off fibers that are going to meet, and run together as
28. 1:58something called the oculomotor complex, which is a combination of somatic and parasympatethic
29. 2:05neurons.
30. 2:06This complex, the oculomotor nerve, is going to emerge from the midbrain, in a sulcus called
31. 2:13oculomotor sulcus, located in the interpeduncular fossa.
32. 2:16It penetrates the dura mater to run laterally in the wall of the cavernous sinus.
33. 2:23It enters the orbit through the superior orbital fissure, into a ring called the common tendinous
34. 2:29ring, and then it splits into a superior branch and an inferior branch.
35. 2:35The superior branch will innervate the levator palpebrae muscle, which is the upper eyelid
36. 2:41muscle, and the superior rectus muscle.
37. 2:45The inferior branch will innervate the Medial rectus, Inferior rectus and the inferior oblique.
38. 2:52The superior and the inferior branch are the somatomotor fibers of the oculomotor nerve
39. 2:59complex, meaning they innervate the extraocular muscles.
40. 3:03The remaining fibers are parasympathetic fibers coming from the Edinger Westphal nucleus.
41. 3:09The parasympathetic fibers run together with the inferior branch.
42. 3:14From the inferior branch, a nerve will go out towards the ciliary ganglion.
43. 3:20The ciliary ganglion is located posterior to the eyeball, and it gives off small ciliary
44. 3:26nerves which innervate the ciliary muscle, which changes the shape of the lens to focus
45. 3:32the eye on near objects, it’s called accommodation.
46. 3:36And it innervates the sphincter pupillae, which constricts the pupil, causes miosis.
47. 3:42Alright so this is the general overview of the oculomotor nerve.
48. 3:46Now what we’re going to do, is start from the beginning, where the oculomotor nerve
49. 3:51actually originates from.
50. 3:53And that is from the midbrain, so let’s go through the anatomy of the midbrain again.
51. 3:58If you take a look at this sideview of the brain, we can see the spinal cord here, medulla,
52. 4:03cerebellum, Pons, Mesencephalon and the diencephalon.
53. 4:07But if we remove the cerebellum, and focus only on the brainstem from the posterior side,
54. 4:12as you see here.
55. 4:13You’ll see the mesencephalon, Pons and the medulla.
56. 4:16So again the mesencephalon which is the midbrain is what we’re interested in now.
57. 4:22From the posterior view we can see the Cerebral Peduncles, as well as the tectal plate.
58. 4:28The tectal plate consists of the superior colliculi.
59. 4:32involved in incorporating environmental stimuli and coordinating gaze shifts involving eye
60. 4:38and head movements.
61. 4:39We can see the brachium of superior colliculus, which is a connecting arm between the superior
62. 4:45colliculus and lateral geniculate body.
63. 4:48And we got the inferior colliculi, which takes in sound information, and sends them further
64. 4:53up to the medial geniculate bodies through the brachium of the inferior colliculi.
65. 4:59From the anterior perspective of the Midbrain, we can see the Cerebral peduncles, and between
66. 5:05them we can see the interpeduncular fossa.
67. 5:08Whithin this region, we can find the posterior perforated substance, and we can see the oculomotor
68. 5:15sulcus of the Mesencephalon.
69. 5:17From here, that is where our oculomotor nerve will go out from.
70. 5:22So the oculomotor nerve goes out from the anterior side of the midbrain, from a sulcus
71. 5:28called the sulcus of the oculmotor nerve.
72. 5:31Alright so that was the external view.
73. 5:34Now what I wanna do, is take this model, and cut it right about here, at the level of the
74. 5:39superior colliculi.
75. 5:41Then we’re going to remove the upper part and look at it from this perspective, we’ll
76. 5:46see this.
77. 5:47Here we see the Superior colliculi, the Cerebral peduncles, the Interpeduncular space, and
78. 5:54the aqueduct of the midbrain, which connects thfe fourth ventricle to the third ventricle.
79. 5:59Now.
80. 6:00Within the midbrain, we can find the substantia nigra, we can find the superior colluculi.
81. 6:06We can see the periaqueductal gray matter, the reticular formation, the red nuclei which
82. 6:12take in impulses from the brain and the cerebellum, and give off rubrospinal tracts for muscle
83. 6:17movement coordination.
84. 6:19At this level, we can also find the nucleus of the oculomotor nerve which is what we’re
85. 6:25interested in, because this is where the oculomotor nerve starts.
86. 6:29The oculomotor nerve will travel towards the anterior side, and leave through the sulcus
87. 6:35of the oculomotor nerve on the anterior side of the midbrain.
88. 6:39The oculomotor nerve is a nerve that consists of somatic fibers and preganglionic parasympathetic
89. 6:45fibers.
90. 6:46The somatic fibers are the fibers you see here, coming from the nucleus of the oculomotor
91. 6:51nerve, which sipplies the extrinsic muscles of the eyeball.
92. 6:55It’s somatic, so it moved the skeletal muscles voluntarily.
93. 7:00The parasympathetic portion of the oculomotor nerve comes from the accessory nucleus of
94. 7:06the oculomotor nerve, which is also called Edinger Westphal nucleus.
95. 7:10They give off parasympathetic fibres that go together with the oculomotor nerve, forming
96. 7:16the oculomotor nerve complex.
97. 7:19Now.
98. 7:20Here we see the oculomotor nerve leaving the anterior brainstem.
99. 7:24The anterior midbrain, right?
100. 7:26When it leaves the anterior surface of the midbrain, the oculomotor nerve is then going
101. 7:32to penetrate the dura mater and run through the lateral wall of cavernous sinus.
102. 7:37Now what do do I mean by that?
103. 7:39That it runs through the lateral wall of the cavernous sinus?
104. 7:42The cavernous sinus is a part of the dural sinuses, right?
105. 7:46Carrying venous blood.
106. 7:48But there are several structures that pass through the cavernous sinus to enter the orbit.
107. 7:54And they’re usually sub-classified by whether they travel through the sinus itself, or through
108. 7:59its lateral wall.
109. 8:01And so the oculomotor nerve is one of the other structures that travel through the lateral
110. 8:06wall of the cavernous sinus.
111. 8:09After nerve leaves the cavernous sinus, it enters the orbital cavity through the superior
112. 8:14orbital fissure, as well as within a fibrous ring called the common tendinous ring.
113. 8:20Now.
114. 8:21If we zoom in, you can see that once the oculomotor nerve has entered the orbital cavity, it splits
115. 8:28into two divisions.
116. 8:30Right?
117. 8:31So let’s go ahead and now look at this model from an anterior perspective.
118. 8:36So here we can see the orbital cavity, right?
119. 8:39Imagine for a second that the eyeball is right here.
120. 8:42Around the eyeball, there’re gonna be muscles we call extraocular muscles.
121. 8:47And there are seven extraocular muscles in total.
122. 8:50There are a total of four rectus muscles, two oblique muscles, and the standalone levator
123. 8:56palpebrae superioris.
124. 8:58The four rectus muscles are the medial rectus, lateral rectus, superior rectus, and inferior
125. 9:05rectus.
126. 9:06The oblique muscles are the superior and inferior obliques.
127. 9:10The rectus and the oblique muscles are all involved in the different gaze position of
128. 9:15the eyeball.
129. 9:16The levator palpebrae superioris is primarily responsible for eyelid elevation.
130. 9:21Now.
131. 9:23As the oculomotor nerve goes past the common tendinous ring and enters the orbital cavity,
132. 9:29it’s going to divide into two branches.
133. 9:32A superior branch, and an inferior branch.
134. 9:35The superior branch innervates the superior rectus and levator palpebrae superioris
135. 9:42The inferior branch innervates the inferior and medial rectus and inferior oblique, as
136. 9:49well as a parasympathetic branch to make the lens globular, for close vision.
137. 9:53Alright.
138. 9:54Let’s do that one more time, just a little bit more detailed.
139. 9:58So here again we see the superior rectus, medial rectus, inferior rectus.
140. 10:04We see the superior oblique, inferior oblique.
141. 10:07And we see the .. Levator palpebrae superioris.
142. 10:10Now.
143. 10:12The oculomotor nerve is gonna go like this.
144. 10:15Then the superior branch passes medially over the optic nerve, and then supplies the superior
145. 10:22rectus and levator palpebrae superioris.
146. 10:27Superior rectus elevates the eye, causing the cornea to move superiorly.
147. 10:31While the levator palpebrae superioris raise the upper eyelid and maintains the upper eyelid
148. 10:38position.
149. 10:39The inferior branch of the oculomotor nerve is larger than the superior branch, and it
150. 10:45divides into three branches.
151. 10:48One passes beneath the optic nerve to the medial rectus.
152. 10:51Another, to the inferior rectus.
153. 10:53The third and longest runs forward to the inferior oblique.
154. 10:59The medial rectus is an adductor muscle, it works together with the lateral rectus to
155. 11:04allow the eyes to move from side to side.
156. 11:08Inferior rectus depresses the eye, causing the cornea and pupil to move inferiorly.
157. 11:14The inferior oblique is interesting.
158. 11:17The inferior oblique is actualy coming from the medial side or the nasal side, and coming
159. 11:22underneath the eyeball and attaches to the inferiorlateral portion of the eyeball.
160. 11:28And then when it contract it does something really interesting it pulls from that portion
161. 11:33and it pulls upwards.
162. 11:35And when it pulls it pulls the eyeball upwards and rotates it outwards.
163. 11:41So it elevates the eyeball and it rotates the eyeball laterally, so it does what’s
164. 11:47called superior and lateral rotation.
165. 11:50The superior oblique we’ll talk about it with he trochlear nerve.
166. 11:53A nice little acronym someone once told was like this.
167. 11:58LR6, SO4, and ATR3.
168. 12:01LR3 means lateral rectus, 6th cranial nevre.
169. 12:06Superior Oblique 4th cranial nevre.
170. 12:08And All The Rest, 3rd cranial nerve.
171. 12:11Ok so that’s that view.
172. 12:13So, the third cranial nerve has a superior branch and an inferior branch.
173. 12:18Now.
174. 12:20Together with the inferior branch, there’re gonna be parasympathetic fibres, coming from
175. 12:24Edinger Westphal nuclei in the midbrain, at the level of the superior colliculus.
176. 12:30Those fibers are called preganglionic parasympathetic fibers.
177. 12:35They’re pre-ganglionic so they go to a ganglia.
178. 12:39What is a ganglia?
179. 12:40A ganglia is a group of cell bodies located within the peripheral nervous system.
180. 12:46So those cell bodies there are postganglionic parasympathetic motor neurons.
181. 12:52From here they’re going to go out as postganglionic fibers within nerves called short ciliary
182. 12:58nerves.
183. 13:00And they’re going to supply the Iris, specifically the sphincter pupillae and causes the pupil
184. 13:05to constrict.
185. 13:06So it causes pupillary constriction.
186. 13:10And they’re going to supply the ciliaris muscle and cause the ciliaris muscle to contract
187. 13:15which alters the shape of the lens particurarly making the lens globular.
188. 13:20And when you make the lens globular that’s actually gonna be for near vision, for close
189. 13:24vision.
190. 13:25It’s called the accommodation reflex.
191. 13:26Alright, so that’s these.
192. 13:29Something else that I wanna talk about, with respect to the actual pathway of the oculomotor
193. 13:33nerve is.
194. 13:35Throughout its course, all the way from the nuclei of the midbrain to the orbital cavity,
195. 13:40it do have a number of vulnarble points.
196. 13:43If there’s a stroke or bleeding whithin the midbrain itself, that can damage the oculomotor
197. 13:49nuclei causing oculomotor nerve defect.
198. 13:53If there’s an increased pressure within the cavernous sinus, that can cause compression
199. 13:57of the oculomotor nerve damaging it.
200. 14:00Another thing that can happen is an aneurism of the posterior communicating artery up there
201. 14:06can compress the oculomotor nerve aswell.
202. 14:08You know, if there’s meningitis or paranasal sinus infection, that can lead to the infection
203. 14:14spreading over to the oculomotor nerve giving oculomotor nerve infections.
204. 14:19Trauma to the orbit or the superior orbital fissure can also compress these nerves damaging
205. 14:24it too.
206. 14:25Or an uncontrolled diabetes or a chronic hypertension can cause damage to the blood vessels arouns
207. 14:31the oculomor nerve aswell.
208. 14:33So how would that manifest?
209. 14:35How would we see if a person has an oculomotor nerve palsy?
210. 14:39Lets’s see.
211. 14:41Superior rectus and inferior oblique elevates the eyeball.
212. 14:45Medial rectus causes medially rotation.
213. 14:47As well as superior rectus and inferior rectus causes a little bit of medial rotation aswell.
214. 14:53Even tho the inferior rectus causes primarily the eye to rotate downwards it do help with
215. 15:00medial rotation to a certain extent.
216. 15:02Now.
217. 15:03If I can’t bring my eyeball upwards, where does it go?
218. 15:07Downward.
219. 15:08If I can’t bring my eyeball medially it’s probably gonna go..
220. 15:10Outwards.
221. 15:11So.
222. 15:12Let’s look at this video for a second.
223. 15:15This is a person undergoing a neurological examination, and notice how the left eye is
224. 15:20stuck laterally and inferiorly, because the oculomotor nerve is damaged.
225. 15:25He’s not able to elevate nor medially rotate the eyeball anymore because the medial, inferior
226. 15:30and superior rectus are damaged, as well as the inferior oblique.
227. 15:35BUT the lateral movement and inferior movement are spared, because the lateral rectus is
228. 15:41innervated by the 6th cranial nerve, the abducent nerve.
229. 15:44And the superior oblique is innervated by the 4th cranial nevre, the trochlear nerve.
230. 15:49When the two eyes aren’t able to coordinate the movement what happens?
231. 15:54You get double vision.
232. 15:56Notice also how the left eyelid is not as elevated as the right eyelid, it’s called
233. 16:01ptosis this condition.
234. 16:03Ptosis also happens with oculomotor nerve damage since the levator palpebrae superioris
235. 16:08is not functioning anymore.
236. 16:10So we get down and outwards gaze, as well as ptosis with oculomotor nerve damage.
237. 16:16What else do we get?
238. 16:18Remember the parasympathetic response constricts the pupil.
239. 16:21IF it can’t constrict the pupil what happens?
240. 16:25It dilates.
241. 16:27So their pupils are gonna be fixed and dilated, and it may not constrict in response to light
242. 16:33anymore.
243. 16:34The pupil is often affected when the cause is compression of the 3rd cranial nerve.
244. 16:39But when the pupil is not affected, the cause is often inadequate blood flow to the nerve
245. 16:46seen in microangiopathies often in diabetics or hypertension.
246. 16:51This is because when the vasa nervosum is affected it causes ischemia to the central
247. 16:56part of the nerve, which is somatomotor involvement only, while the peripheral parts of the nerve
248. 17:02is autonomic fibers.
249. 17:03Alright let’s recap once again.
250. 17:06The oculomotor nerve starts with two nuclei located in the midbrain at the level of the
251. 17:11superior colliculus.
252. 17:13Those are called the nucleus of the oculomotor nerve and the accessory oculomotor nucleus,
253. 17:18which is known as Edinger–Westphal nucleus.
254. 17:22Those two nuclei are gonna give off fibers that are going to meet, and run together as
255. 17:26something called the oculomotor complex, which is a combination of somatic and parasympatethic
256. 17:33neurons.
257. 17:34This complex, the oculomotor nerve, is going to emerge from the midbrain, in a sulcus called
258. 17:40oculomotor sulcus, located in the interpeduncular fossa.
259. 17:44It penetrates the dura mater to run laterally in the wall of the cavernous sinus.
260. 17:50It then goes through the superior orbital fissure, into a ring called the common tendinous
261. 17:55ring, and then it enters the oribal cavity and splits into a superior branch and an inferior
262. 18:01branch.
263. 18:02The superior branch will innervate the levator palpebrae muscle, which is the upper eyelid
264. 18:08muscle, and the superior rectus muscle.
265. 18:11The inferior branch will innervate the Medial rectus, Inferior rectus and the inferior oblique.
266. 18:17The parasympathetic fibers coming from the accessory oculomotor nucleus run together
267. 18:22with the inferior branch.
268. 18:24From the inferior branch, a nerve will go out towards the ciliary ganglion.
269. 18:30The ciliary ganglion is located posterior to the eyeball, and it gives off small ciliary
270. 18:36nerves which innervate the ciliaris muscle and cause the ciliaris muscle to contract
271. 18:41which alters the shape of the lens particurarly making the lens globular to focus the eye
272. 18:46on near objects, it’s called accommodation.
273. 18:47And it innervates the sphincter pupillae, which constricts the pupil, causes miosis.
274. 18:56So that was everything I had for the third cranial nerve.
275. 18:59The next video is going to be about the fourth cranial nerve, the trochlear Nerve.
276. 19:03Alright guys so that pretty much covers the oculomotor nerve.
277. 19:06Thank you so much for watching another one of my videos.
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