CN 2: Optic Nerve (Scheme, Pathway, Lesions) | Neuroanatomy — Transcript

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1. 0:06What’s up!
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 second cranial nerve, which is the Optic
5. 0:21nerve.
6. 0:22And we’ll do that by first making a quick scheme of the optic pathway to get an overview
7. 0:26of it.
8. 0:28Then we’ll cover the very basic of eye anatomy, and talk a little bit about the visual field.
9. 0:33Afterwards we’ll go through the course of the visual pathway, and talk about the all
10. 0:38the steps that the optic nerve go through in order to make us perceive and understand
11. 0:43what we see.
12. 0:44We’re also going to shortly go through the visual pathway collaterals.
13. 0:49The optic nerve gives off collateral branches that aid with he pupillary light reflex, and
14. 0:54the pathway of convergance.
15. 0:56At the end we’ll talk a little bit about the clinical relevance around pathologies
16. 1:01related with the optic pathway.
17. 1:03So, I wanna start by saying that the olfactory and the optic nerves are not true nerves.
18. 1:10They are just extensions of the diencephalon.
19. 1:14They take the meninges with them as they extend towards the eyes.
20. 1:18They are not covered by Schwann cells like the peripheral nervous system are.
21. 1:23It’s covered by oligodendrocytes, like the Central nervous system is.
22. 1:27Now.
23. 1:28Another thing I want you know know about the optic nerve is that the optic nerve carries
24. 1:32visual information to the brain via the optic tract.
25. 1:36Let’s visualize this sentence.
26. 1:38Our vision starts at the eye, specifically the retina.
27. 1:42The visual pathway is what we call a four-neurone tract, meaning it consists of 4 different
28. 1:48neurons.
29. 1:50The first, second and third order neurons are located in the retina of the eyeball,
30. 1:54I’ll show you a scheme later when we get to it.
31. 1:58But it all starts with a visual stimuli from the three neurons in the retina.
32. 2:02The visual stimuli is then sent through the optic tract, which goes through the optic
33. 2:07canal of the spenoid bone, behind the eyes.
34. 2:10The optic nerve then crosses with the contralateral optic nerve of the other eye to then continue
35. 2:17as the optic tract.
36. 2:18So half the fibres go to the contralateral side, and half the fibres continue in the
37. 2:24ipsilateral side together with fibers from the other eye.
38. 2:28We will go through this in detail in a minute.
39. 2:31The optic tract then goes on and synapses with the fourth order neurones in the lateral
40. 2:37geniculate body of the metathalamus.
41. 2:40These neurones then take visual information from the retina to the primary visual area
42. 2:45in the occipital lobe.
43. 2:48Another thing that can happen, we won’t go in too much detail into this, but there
44. 2:52are some collateral branches that go from the thalamus to the midbrain aswell, that
45. 2:57are responsible for the pupillary light reflex, coordinated eye movements and contribute to
46. 3:03motor control of the whole body.
47. 3:06There are some collateral nerves going to the hypothalamus aswell, which have an important
48. 3:10effect on autonomic functions and circadian rhythm.
49. 3:15Our eyes control a lot of different functions whithin us.
50. 3:18Alright let’s now start with the very basic, beginning of the optic nerve.
51. 3:23Our eyes.
52. 3:24Let’s quickly do some eye anatomy.
53. 3:27The cornea is a circular transparent layer that covers the pupil, iris and anterior chamber
54. 3:34of the eye.
55. 3:35Back here we got the lens, which is held in position by small ligamentous bands that extend
56. 3:41from the ciliary process to the lens, called suspensory ligaments of the lens.
57. 3:48We can see the posterior chamber, as well as the ciliary muscles that change the tension
58. 3:52of the lens.
59. 3:54Then we have the sclera, which is the fibrous layer.
60. 3:57The sclera and the corena are both the fibrous layer of the eye.
61. 4:01We got the choroid, which is the vascular layer containing a lot of blood vessels.
62. 4:06Notice that the vascular layer consists of three parts that are continuous with each
63. 4:10other.
64. 4:11They are the choroid, ciliary body, and iris.
65. 4:15Then we got the Retina, which is the nervous layer.
66. 4:19Another thing we can see here is the vitreous body, or vitreous humor.
67. 4:23This is a gelatinous structure that contribute to the refraction of light, focusing the light
68. 4:28basically.
69. 4:30It has a channel in the middle that extends from the optic disc to the posterior pole
70. 4:34of the lens.
71. 4:36This channel is called the hyaloid canal and it serves to transmit the hyaloid artery in
72. 4:41fetal life, which supplies the lens in this period.
73. 4:45Then we got a budle of optic nerve and vessels, which is where the optic disk is.
74. 4:49But now what we’re going to do is have a little closer look into the Retina.
75. 4:54So let’s go ahead and take a small section of the eye, and zoom in.
76. 4:59What we can see here is the choroid, which contains a lot of blood vessels, and we can
77. 5:03see the retina.
78. 5:04The retina contains the neural component of the eye.
79. 5:08So what happens is, when light reaches the back of the eye, it enters the cellular layers
80. 5:13of the retina.
81. 5:15The cells of the retina that detect and respond to light, known as photoreceptors, are located
82. 5:21at the very back of the retina.
83. 5:23There are two types of photoreceptors: rods and cones.
84. 5:28Rods allow us to see in dim light, but don't allow for the perception of color.
85. 5:33Cones, on the other hand, allow us to perceive color under normal lighting conditions.
86. 5:39Throughout most of the retina, rods outnumber cones.
87. 5:44In one area called the fovea, however, there are no rods but many cones.
88. 5:49The fovea represents the area of the retina that provides our highest acuity vision, and
89. 5:55thus is at the center of our gaze.
90. 5:58Alright now let’s focus on the Optic nerve.
91. 6:01The visual pathway is what we call a four-neurone tract, meaning from start to end, it consists
92. 6:08of 4 different neurons.
93. 6:10The first, second and third order neurons are located in the retina of the eyeball.
94. 6:16And when you look at this image, you can see a clear demarcation between these three neurons.
95. 6:21The first order neurons are the Rods and cones.
96. 6:25They’re receptor cells that respond directly to light stimulation.
97. 6:29So when light hits photoreceptors, it interacts with a molecule called photopigment, which
98. 6:36begins a chain of events that serves to propagate the visual signal.
99. 6:41The signal is then transmitted to the second order neurons, called bipolar cells.
100. 6:46Bipolar cells take in stimuli from the rods and cones to the ganglion cells.
101. 6:51Ganglion cells are our third order neurons, and they’re the cells that form the optic
102. 6:57nerve.
103. 6:59The optic nerve is going to leave the eye in a large cluster at an area called the optic
104. 7:04disc.
105. 7:05Just a fun fact - The optic disc doesn't contain any photoreceptors, and so it represents an
106. 7:11area on the retina that can't process visual information, creating a natural blind spot.
107. 7:17However, we normally don't notice our blind spot.
108. 7:21The brain uses information from surrounding photoreceptors and the other eye to fill in
109. 7:26the gaps in images that are processed by the retina.
110. 7:30Alright.
111. 7:31Now we understand where we’ll find the three first neruons of the optic pathway.
112. 7:35They’re in the retina.
113. 7:37First order neurons are photoreceptos, so rods and cones.
114. 7:42Second order neurons are bipolar cells, and third order neurons are ganglion cells.
115. 7:47Now before we go over and thalk about the rest of the visual pathway, I wanna first
116. 7:53go through the visual field because you’ll understand a lot more of this pathway once
117. 7:57you get this concept.
118. 8:00When the head and eyes are maintained in a fixed position and one eye is closed, the
119. 8:05area seen by that eye constitutes the visual field for that eye.
120. 8:10Now, if the other eye is also opened, the area seen is more or less the same as was
121. 8:16seen with one eye.
122. 8:17So the visual fields of the two eyes overlap to a very great extent.
123. 8:22Now.
124. 8:23If an object is placed in the right half of the field of vision, its image is formed on
125. 8:29the left half of the retina and vice versa.
126. 8:32The two halves of the retina are usually referred to as nasal half, and temporal half.
127. 8:37Or the medial half and lateral halves.
128. 8:40We can also say that the region where the two visual fields overlap, they’re called
129. 8:45the binocular field of vision because both eyes are involved.
130. 8:50In the middle of the binocular field of vision, the light is focused towards both foveas,
131. 8:55which is responsible for sharp central vision.
132. 8:59Remember the fovea represents the area of the retina that provides our highest acuity
133. 9:04vision, and therefore is at the center of our gaze which is necessary for reading, driving,
134. 9:09and any activity where virual detail is important.
135. 9:13The other important thing with the fovea, is that it kinda splits the fibers into two
136. 9:19visual field fibers, here represented in red and blue.
137. 9:24The lateral fibers of the left eye will go to the ipsilateral side.
138. 9:28The medial fibers of the left eye will cross at the optic chiasm and cross to the contralateral
139. 9:34side.
140. 9:36Same thing with the other eye.
141. 9:37The lateral fibers will continue on the ipsilateral side.
142. 9:40The medial fibers of the right eye will cross at the optic chiasm and continue to the contralateral
143. 9:46side.
144. 9:47Alright.
145. 9:48Now that we got this concept, let us finally go through the actual course of the visual
146. 9:53pathway.
147. 9:55And to do so, we’ll use this scheme since it shows the complete pathway from start to
148. 10:00finish.
149. 10:01Remember, the visual pathway is a four-neurone tract.
150. 10:04I know I keep repeating this, but this is very very important.
151. 10:08The first-order neurons are photosensitive cells called rods and cones.
152. 10:13Visual stimuli are converted into neural impulses, the cells that react to light stimuli point
153. 10:19towards the choroid and is embedded into the pigmented layer of the retina.
154. 10:24Axons head towards the second-order neurons called bipolar cells, which continue into
155. 10:31the third-order neurons called retinal ganglion cells.
156. 10:36Axons converge to the optic disc and form the optic nerve.
157. 10:40Optic nerves will continue posteriorly and converge into something called an optic chiasma
158. 10:45or visual deccusation because this is where half the optic nerve fibers decussate.
159. 10:52The optic chiasma is located on the superior surface of the sphenoid bone.
160. 10:57What happens is, In the chiasma, the fibers from the medial half of each retina cross
161. 11:03the midline and enter the optic tract of the opposite side, whereas the fibers from the
162. 11:09lateral half of each retina pass posteriorly in the optic tract of the same side.
163. 11:15So the way this works is that the right optic tract conveys information from the right halves
164. 11:21of the retinae to from the left halves of the visual field.
165. 11:24While the left optic tract conveys information from the left halves of the retinae to from
166. 11:30the right halves of the visual field.
167. 11:33Kinda makes sense doesn’t it?
168. 11:35Alright so then, fibers of the optic tract terminate by synapsing with the fourth order
169. 11:42neuron within the lateral geniculate bodies of the metathalamus, on either side of each
170. 11:47thalami.
171. 11:48This is where we’ll go a little deeper into this topic.
172. 11:51The grey matter of the lateral geniculate body is split into six laminae.
173. 11:56Fibres from the eye of the same side end in laminae 2, 3, and 5; while those from the
174. 12:04opposite eye end in laminae 1, 4, and 6.
175. 12:08Now.
176. 12:09Remember I told you that that we got the temporal and the nasal parts of visual fields?
177. 12:14The visual field is actually subdivided into 4 quadrants depending on where the fibers
178. 12:20originate from within the eye.
179. 12:22The fibers that come from the fovea, which, you know constitute around 90% of all axons
180. 12:28leaving the eye since this is your point of focus, they’re called fovea fibers, or macular
181. 12:33fibers.
182. 12:34These fibers are going to end in the central and posterior part of the lateral geniculate
183. 12:40body, and this area is relatively large.
184. 12:43Fibres from the peripheral parts of the retina end in the anterior part of the lateral geniculate
185. 12:50body.
186. 12:51So, foveal fibers, in the central and posterior parts of the lateral geniculate bodies, and
187. 12:56peripheral fibers, going to the anterior parts of the lateral geniculate bodies.
188. 13:01So.
189. 13:02The upper half of the retina is represented laterally and the lower half of the retina
190. 13:07is represented medially.
191. 13:10Now you probably don’t need to know this in detail, but this is just to show you that
192. 13:14specific points on the retina project to specific points in the lateral geniculate body.
193. 13:20Now, from the lateral geniculate bodies.
194. 13:23What happens?
195. 13:24Fibres leave as the optic radiation.
196. 13:28The optic radiation ends in the primary visual area of the cerebral cortex which consists
197. 13:33of the cuneus and the lingual gyrus of the occipital lobe.
198. 13:38Also called Brodmann’s area 17.
199. 13:41These fibers go to the primary visual area in two different ways.
200. 13:46Fibers from the upper half of each retina, which remember is the lower field of vision.
201. 13:52Fibers from the upper half of the retina, they’re going to go from the lateral geniculate
202. 13:57bodies directly to the cuneus of the occipital lobe.
203. 14:01Fibers from the lower half of the retina, which remember is the upper field of vision.
204. 14:07Fibers from the lower half of the retina, they loop forward and downward into the temporal
205. 14:12lobe to swerve around the inferior horn of lateral ventricle before turning backward
206. 14:19to the occipital lobe, to terminate in the lower parts of the cuneus, called the lingual
207. 14:23gyrus.
208. 14:25This loop it makes before going to the occipital lobe is called Meyer’s loop.
209. 14:30This is how a point-to-point relationship is maintained between the retinae and the
210. 14:35visual cortex.
211. 14:37One interesting thing to remember here is that the cortical area for the macula is larger
212. 14:42than that for peripheral areas.
213. 14:45The macular area has dual blood supply, which is the posterior cerebral artery and branches
214. 14:52of middle cerebral artery.
215. 14:54All this because remember the macular fibers are important for our central vision.
216. 15:00The cortical area for the peripheral part of the retina is situated anterior to the
217. 15:05area for the macula.
218. 15:08So that was the course of the visual pathway.
219. 15:10Let’s do that one more time.
220. 15:13The retina has 1st, 2nd and 3rd order neurons.
221. 15:17The first-order sensory neurons being photosensitive bipolar rods and cone cells.
222. 15:23They synapse with second order neurons, which are bipolar cells.
223. 15:29Third order neurons are retinal ganglion cells, where axons of the retinal ganglion cells
224. 15:35converge to the optic disc and form the optic nerve.
225. 15:38The right and the left optic nerves join to form the optic chiasma, where fibres from
226. 15:44nasal half of retina cross to the opposite side and travel through the opposite optic
227. 15:48tract to terminate in the opposite lateral geniculate body.
228. 15:52The fibres from temporal half of each retina enter the optic tract of the same side to
229. 15:57terminate in the ipsilateral geniculate body.
230. 16:00The cell bodies of fourth-order sensory neurons are located in the lateral geniculate body.
231. 16:07Their axons form the optic radiation, which projects to the visual cortex.
232. 16:12Just a side note.
233. 16:14Some sources don’t count Rods and cones as 1st order neurons, which leaves the bipolar
234. 16:19cells being 1st order neuron, Ganglion cells being 2nd order neuron and the lateral geniculate
235. 16:25bodies being 3rd order neurons.
236. 16:27This is just some variations out there, but you know, keep this in mind in case your sources
237. 16:32say so.
238. 16:33Alright.
239. 16:34So that was the visual pathway.
240. 16:36Now the visual pathway has some collateral nerves as well, meaning that it gives off
241. 16:42nerves that go off to other parts outside the visual pathway.
242. 16:46There are fibers whithin the optic tract that instead of traveling to the lateral geniculate
243. 16:50nucleus, they diverge to the pretectal area in the midbrain, which is anterior ro the
244. 16:56cerebral aqueduct.
245. 16:58When they go to the pretectal area, the fibers then synapse the ipsilateral and contralateral
246. 17:04oculomotor nerve nuclei, which is the cranial nerve number 3, and it also synapses with
247. 17:06the Edinger-Westphal nuclei, which are accessory nuclei of cranial never number 3.
248. 17:11These nuclei, along with other cortical influences, are responsible for several visual reflexes
249. 17:17including accommodation, direct and consensual light reflex, and saccadic response of the
250. 17:24eyes.
251. 17:25Another thing that can happen, is that fibers can also bypass the lateral geniculate body
252. 17:31to go synapse with nuclei of the superior colliculi of the midbrain.
253. 17:36And from the superior colliculi, most fibers from here form the tectospinal tract, which
254. 17:41remember it deccusate and then run through Tegmentum of Pons, medulla oblongata, anterior
255. 17:48funiculus of the spinal cord, to synapse with motor nuclei of the anterior horn of the spinal
256. 17:54cord to supply skeletal muscles involved in the reflectoric visual pathway.
257. 17:59Meaning the tectospinal tract is responsible for reflectory movements according to unexpected
258. 18:05visual irritation.
259. 18:07Another thing that happens is, the superior colluculi is connected with MLF, or medial
260. 18:13longitudinal fasciculus, which is responsible for coordination of the movements of eyeball,
261. 18:19head and neck.
262. 18:20And so not only the optic nerve is involved int his, the medial longitudinal fasciculus
263. 18:25also communicates with the motor nuclei of the III, IV, VI and XI cranial nerves in order
264. 18:32to coordinate the neck and eye movements.
265. 18:36So that’s that one.
266. 18:37The last thing that I wanna mention here is clinical relevance.
267. 18:41Why is it important to know all of this detailed pathway.
268. 18:44The reason why is that based on the type of field defect, we can localize where the lesion
269. 18:50is.
270. 18:51Right?
271. 18:52Lesions involving the whole optic nerve cause complete blindness on the affected side, that
272. 18:58means damage at the right optic nerve causes complete loss of vision in the right eye.
273. 19:05A lesion involving complete optic chiasm, will disturb the axons from the nasal field
274. 19:10of both eyes giving visual field-bitemporal hemianopia.
275. 19:14Cuz remember the nasal part of the retina shoots vision to the temporal visual field,
276. 19:20and vise versa.
277. 19:21Causes can be many, specially compression by pituitary adenomas and meningiomas.
278. 19:26Lesion involving whole optic tract give homonymous hemianopsia.
279. 19:27So if there’s a lesion in the left optic tract, it’ll cause right-sided homonymous
280. 19:31hemianopsia, while a lesion in the right optic tract will cause left-sided homonymous hemianopsia.
281. 19:34Leasons can be caused by demyelinating diseases and infarction.
282. 19:39The lateral geniculate nucleus is the nucleus next to the thalamus that receives visual
283. 19:44information from the retina and sends it to the visual cortex via optic radiations.
284. 19:49A lesion of this nucleus produces the same outcome as lesions in the optic tract.
285. 19:55Isolated lesions of the lateral geniculate nucleus are rare, but some pituitary adenomas
286. 20:02can put pressure on this nucleus causing ti to degenerate.
287. 20:07Now, lesions in the optic radiation.
288. 20:09The optic radiation are axons from the neurons in the lateral geniculate nucleus to the primary
289. 20:15visual cortex, right?
290. 20:17If there’s an infarction caused by middle cerebral artery or the posterior cerebral
291. 20:22artery, this may affect the optic radiations, and can cause quadrantanopias.
292. 20:28If for instance there’s a lesion of the right temporal lobe damaging the meyer's Loop
293. 20:34of the optic radiation, that will lead to loss of the upper, outer quadrant of vision
294. 20:39on the contralateral side, known as homonymous superior quadrantanopia, or pie in the sky
295. 20:46disorder.
296. 20:47If there’s a lesion in the visual cortex itself, what happens?
297. 20:51The visual cortex is located in the occipital lobe.
298. 20:54If there’s any partial or complete visual deficit that is cased by damage to the cisual
299. 21:00cortex, we call that cortical blindness.
300. 21:03It’s most of the times permanent as a result of stroke, head injury or gunshot injuries.
301. 21:10Occipital cortex lesions tend to cause homonymous hemianopias of variable size, with or without
302. 21:17macular involvement.
303. 21:18It all depend on which parts of the brain is damaged.
304. 21:21Alright let’s recap once again.
305. 21:24Our vision starts at the eye, specifically the retina.
306. 21:27The retina has 1st, 2nd and 3rd order neurons.
307. 21:32The first-order sensory neurons being photosensitive bipolar rods and cone cells.
308. 21:38They synapse with second order neurons, which are bipolar cells.
309. 21:43Third order neurons are retinal ganglion cells, which takes their axons towards the optic
310. 21:50disc and form the optic nerve.
311. 21:53The right and the left optic nerves go through the optic canal, and then join to form the
312. 21:58optic chiasma, where fibres from nasal half of retina cross to the opposite side and travel
313. 22:04through the opposite optic tract to terminate in the opposite lateral geniculate body.
314. 22:09The fibres from temporal half of each retina enter the optic tract of the same side to
315. 22:15terminate in the ipsilateral geniculate body.
316. 22:18The cell bodies of fourth-order sensory neurons are located in the lateral geniculate body.
317. 22:25Their axons form the optic radiation, which projects to the visual cortex.
318. 22:31So that was everything I had for the second cranial nerve.
319. 22:35The next video is going to be about the third cranial nerve, the oculomotor Nerve.
320. 22:40Alright guys so that pretty much covers the Optic nerve.
321. 22:43Thank you so much for watching another one of my videos.
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