CN 1: Olfactory Nerve (Scheme, Pathway, Clinical Relevance) | Neuroanatomy — Transcript

Full transcript

1. 0:06What’s up! Taim Talks Med here, let’s  start with our Cranial nerve series.
2. 0:10Cranial nerves are twelve pairs of nerves  that exit the brain and the brainstem,
3. 0:15and in this segment, we’ll talk detailed about the  first cranial nerve, which is the Olfactory nerve.
4. 0:20And we’ll do that by first making a quick scheme  of the olfactory pathway to get an overview of it.
5. 0:27Then we’ll talk detailed about the Olfactory  epithelium and nerve. Basically which striuctures
6. 0:32are involved in sensing particles of smell. Then we’ll follow the olfactory pathway and
7. 0:38look at which structures in the brain  are involved when you smell something.
8. 0:42And after that, we’ll do some clinical relevance  and examples of conditions that can alter the
9. 0:48normal functioning of this nerve. So, the olfactory nerve conducts
10. 0:53the sensation of smell to the brain  via the olfactory bulb and tract.
11. 0:57Let’s visualize this sentence. Here we got a funky  smell of some sort, let’s say someone opened a box
12. 1:04of tuna next to you, or maybe someone farted.  These gaseous odorants will eventually reach
13. 1:10your olfactory nerves. Axons of these olfactory  nerves are gonne come together, and run through
14. 1:16the cribriform plate, of the ethmoidal bone. So  we call the olfactory nerve a 1st order neuron
15. 1:23because this is where smell is first registered.  Once they go through the cribriform plate,
16. 1:29they will synapse with neurons in the olfactory  bulb. The olfactory bulb is an elongated structure
17. 1:36that lies just above the cribriform plate. It  continues posteriorly with the olfactory tract.
18. 1:42As the olfactory tract approaches the anterior  surface of the midbrain, it becomes wide, now
19. 1:48called olfactory trigone. Located just in front of  the anterior perforated substance. The olfactory
20. 1:54trigone now divides into striae. It divides into  the medial stria, the lateral stria, and in some
21. 2:01cases the intermediate stria, which extend into  the anterior perforated substance and ends in a
22. 2:08slight elevation called the olfactory tubercle.  The lateral stria terminates in the primary
23. 2:14olfactory cortex of the temporal lobe and we’ll  talk about that in details when we get to it.
24. 2:20The medial stria passes to the contralateral  olfactory tract and terminates in the limbic
25. 2:26structures to contribute to the emotional  responses brought on by the smell.
26. 2:31Alright so that is the general outline. Let’s  now cover this pathway in a little detail,
27. 2:37starting from the nasal cavity and then follow the  pathway to understand the mechanism behind smell.
28. 2:43So here we see the nasal cavity, along with the  pharynx, oral cavity and the hard and soft palate.
29. 2:50The sense of smell begins in a small area  on the roof of the nasal cavity. They’re
30. 2:56a specialized collection of cells that lines  the upper part of the nasal cavity in humans.
31. 3:02Let’s take a section of that specific  region and look detailed into the
32. 3:07structures that are responsible for  helping with the sense of smell.
33. 3:10The olfactory part of the nasal cavity consists  mainly of what is called olfactory epithelium.
34. 3:18The olfactory epithelium consists mainly of what  is called pseudostratified columnar epithelium and
35. 3:24it lies on top of a lamina propria. The cells that  make up the olfactory epithelium are Basal cells,
36. 3:31that act as stem cells and can differentiate into  olfactory epithelium. We have Sustentacular cells,
37. 3:38which support the olfactory neuron function. We  got an olfactory gland cell, or bowmans gland,
38. 3:44which produces mucin. Then we got olfactory  receptor neurons. These neurons are formed as
39. 3:51a bipolar neuron remember is a type of neuron  that has two extensions , one dendrite that
40. 3:58contains nonmotile sensory cilia going to the  epithelial surface, and one axon going upwards
41. 4:06that transmits signals to the olfactory  bulb after crossing the cribriform plate.
42. 4:11So what happens is, when we inhale, we create a  turbulent airflow whichn the the nasal cavity.
43. 4:18Odorants has two ways of coming in. One way is  through the nasal opening, and another way of
44. 4:24detecting smell is through posterior transfer of  odorants from the nasopharynx, which is important
45. 4:31for the detection of flavor during eating and  drinking. Remember taste is 80% dependent on
46. 4:37olfaction, so without the ability to smell, all  food and drink can only be sensed as one of those
47. 4:43five basic tastes, with no other differentiation  possible. So odorant comes in, what happens?
48. 4:51you know one of the most important things about  the nasal cavity is that there’s a lot of mucus.
49. 4:57There’s a lot of mucus in this area. And the  purpose of mucus is important because not only
50. 5:02is it good in basically helping to humidify  the incoming air, moisen the incoming air,
51. 5:07warming the incoming air, but also certain  particles. So certain particles will enter the
52. 5:13nasal cavity and diffuse into the mucus. They then  bind to certain proteins and gets transported to a
53. 5:20sensory cilia of the olfactory dendrite. Remember  cilia has specific olfactory receptors on them.
54. 5:27Each receptor is capable of binding to  several different odorants — some more
55. 5:32tightly than others. Each odorant is capable  of binding to several different receptors. This
56. 5:38provides the basis for combinatorial diversity.  So Odorant A binds to receptors on neurons #3,
57. 5:45#427, and #886. Odorant B binds to receptors  on neurons #2, #427, and #743. And the brain
58. 5:56then would interpret the two different  patterns of impulses as separate odors.
59. 6:01This mechanism allows us humans to be capable  of discriminating between a trillion different
60. 6:07mixtures of odorants. Dogs can detect up to  1000 to 10000 times more odorants than us.
61. 6:15So what happens is, an odorant bind to a  transmembrane protein, a "7-pass" transmembrane
62. 6:21proteins we call it, since it passes through the  membrane 7 times. And it’s bound to a G protein,
63. 6:28so it’s a G protein-coupled receptor. So  an odorant binds to a G-coupled receptor
64. 6:34and activates a G-protein. The activated  G-protein gets rid of GDP and binds to a GTP,
65. 6:42that then activated an adenylyl cyclace which  catalyzes the conversion of ATP to the "second
66. 6:48messenger" cyclic AMP. And when the concentration  of cyclic AMP increases within the olfactory cell,
67. 6:55it binds and opens ion channels. Now  we get an influx of positive sodium
68. 7:01and calcium ions and an efflux of negative  chloride ions. This neuronal depolarization
69. 7:07continues until the threshold potentially  occurs, firing a resulting action potential.
70. 7:13So the odorant has to be strong enough toinducee  an action potential in order for us to smell it.
71. 7:21Now the cilia is stimulated  and pass the signals up via
72. 7:25the dendrite and the cell body to the  axon on the olfactory sensory neuron.
73. 7:30Axons from the olfactory neurons form nerve  bundles, or fila olfactoria. Many of these
74. 7:36bundles combine to create the olfactory nerve,  which is enclosed by dura and arachnoid mater.
75. 7:42The olfactory nerve is the shortest fot he 12  cranial nerves, and is unique in that it can
76. 7:48partially regenerate. Which is good because  olfactory neurons are lost with exposure to
77. 7:54environmental toxins, and inflammation. But  they’re also said to be lost with age aswell.
78. 8:00Now as the axons pass through the cribriform  plate of the ethmoidal bone, they enter into
79. 8:06what’s called the olfactory bulb. Now in the  olfactory bulb, they will synapse with special
80. 8:12cells called mitral cells. And this synapse  between the olfactory and the mitral cells
81. 8:18is called glomeruli. SO they form glomeruli  between the axonal and dendritic branches.
82. 8:25Signals are passed through the mitral cells, and  it’s the mitral cells that carries information
83. 8:30of individual odors back to the brain. But we can  find other cells here in the olfactory bulb too.
84. 8:36There are Tufted cells, which are a minor  version of mitral cells. They’re different
85. 8:43than mitral cells in that they’re smaller, they  connect to several glomeruli whereas mitral cells
86. 8:49only connect to one glomeruli. And they’re many  more. We got more tufted cells and mitral cells.
87. 8:56Other cells that we can find here too are amacrine  granule cells, which are inhibitory interneurons,
88. 9:03allowing only the most excitatory impulses to  stimulate the mitral cells. So many impulses
89. 9:09come in, only the most excited ones are  allowed by the granule cells to stimulate
90. 9:15the mitral cells. We got also periglomerular  cells that are thought to do the same as
91. 9:22amacrine granular cells. So they function  by dampening the excitation of mitral cells.
92. 9:28So, axons from mitral and tufted cells leave  the olfactory bulb through the olfactory tract.
93. 9:36The tract passes underneath the medial fronal lobe  inside the olfactory groove. And then as the tract
94. 9:43gets close to the anterior perforated substance  of the midbrain, it becomes flatter and widens
95. 9:49as the olfactory trigone. From the olfactory  trigone, the tract is gonna continue as stria.
96. 9:55Three olfactory striae. There’s the lateral  olfactory striae, medial olfactory striae and
97. 10:02the intermediate olfactory striae. Cranial nerve 1  is one of two cranial nerve that doesn’t directly
98. 10:10enter the brainstem, the other is the optic nerve.  Let’s follow the lateral striae here for a minute.
99. 10:16The lateral stria continues on to an area  deep in the temporal lobe called uncus,
100. 10:22which is the anterior portion of the medial  parahippocampal gyrus. From here, information
101. 10:28are sent to the primary olfactory cortex. Now  what the heck is the primary olfactory cortex?
102. 10:34The term olfactory cortex is applied to all  areas of the cerebral cortex that receive
103. 10:41direct fibres from the olfactory  bulb. Kinda makes sense doesn’t it?
104. 10:45In the past it has been held that olfactory  pathways differ from other sensations in
105. 10:51that there is no relay in the thalamus.  Usually when you study the nervous system,
106. 10:56you see that all the sensory pathways usually  sends fibers through the thalamus. It has long
107. 11:02been said that the olfactory pathways differ in  that there’s no information sent to the thalamus.
108. 11:07This concept has now been changed  in that while most olfactory fibres
109. 11:12reach their cortical targets, some fibers  are now believed to reach the thalamus.
110. 11:18And because we now understand a little more  of the olfactory cortex than we used to,
111. 11:23we now divide the oflactory cortex into  primary and secondary cortical projections.
112. 11:29Primary olfactory projections are the areas  that first receive the information directly
113. 11:35from the olfactory bulb. And the main  regions receiving direct fibres from the
114. 11:40olfactory bulb are a whole lot of areas, we  got the piriform cortex, the semilunar gyrus,
115. 11:47the anterior part of the entorhinal area. You  don’t really need to know where these are located
116. 11:53specifically unless you’re researching about  them. But what’s good to know is that from here,
117. 11:58fibers are sent to what’s called the secondary  olfactory areas. Or secondary olfactory cortex.
118. 12:05Fibers are gonna go to an area called  the entorhinal area, or brodmann area
119. 12:1028. It’s partly considered a part of primary  olfactory cortex aswell because remember,
120. 12:16the anterior part of the entorhinal  area receives a few fibres directly
121. 12:22from lateral olfactory stria. But the  posterior part of entorhinal area is
122. 12:28secondary olfactory cortex because it  receives fibres from primary olfactory
123. 12:32cortex. The entorhinal cortex is concerned  with processing information about odours.
124. 12:39From the entorhinal area, information are sent to  the amygdala which is associated with the emotion
125. 12:45of fear associated with smell. And from the  amygdala, fibers are sent to the hippocampus.
126. 12:51And since fibers go to the hippocampus, then  you’re going to process the smell and form new
127. 12:58memory. Hippocampus is associated with memory. Another place that the olfactory cortex is
128. 13:04gonna send odor information out to are  portions of the hypothalamus and brainstem
129. 13:09that trigger autonomic responses involved in  appetite, salivation, and gastric contraction.
130. 13:16Another place that’s gonna be involved in  all of this is the oribitofrontal cortex.
131. 13:22Fibers are going to go from the primary  olfactory cortex to the medial dorsal
132. 13:27nucleus of the thalamus, and then to the  orbitofrontal cortex. The orbitofrontal
133. 13:32cortex is also secondary olfactory cortex. It  provides conscious perception of smell. It helps
134. 13:40you consciously understand what you’re smelling. So you see whenever you smell something,
135. 13:45through the primary olfactory cortex we gain  the ability to discriminate between odors,
136. 13:51we can process complex odorant mixtures. Odors  take a direct route to the limbic system,
137. 13:56including the amygdala and the hippocampus, the  regions related to emotion, memory and reward.
138. 14:02You’re consciously aware of the smells through  the orbitofrontal cortex. You know.. the olfactory
139. 14:08sensation has a very interesting pathway. So all of those parts of the brain are
140. 14:13involved because of the lateral olfactory striae. The medial olfactory striae are actually gonna
141. 14:20move around what’s called the paraolfactory area,  and it’s gonna go to a specific gyrus here called
142. 14:26the sub-colossal gyrus. The subcallosal gyrus is  also considered a part of limbic system of brain,
143. 14:32so the reward system, emotions,  memory and all of those things.
144. 14:37Now, we talked about the lateral olfactory stria  and the medial olfactory stria. Between them, the
145. 14:44intermediate olfactory striae it goes out towards  the anterior perforated substance of the midbrain
146. 14:49region, and ends as the olfactory tubercle. The  olfactory tubercle is a sensory processing centre,
147. 14:56it received input from olfactory bulb and also  from the amygdala and it’s believed to also play
148. 15:03a role in the reward system. It identifies  rewarded odors, and based on that you know
149. 15:09it affects your behaviour and decisions. So the  reward system represents behavioral responses.
150. 15:15One thing that’s also important is that  some of these fibers can cross to the
151. 15:19other side. So smell can be bilateral,  even though most fibers are ipsilateral,
152. 15:22some fibers do cross. So smell is bilateral. So up until now we talked about the olfactory
153. 15:30pathway, we talked about where it’s going  to in the cerebral cortex. Now I wanna say
154. 15:35what can happen if it’s not funcitonin correctly. Anosmia, is when you temporarily or permanently
155. 15:41lose the ability to smell. And there’re a lot  of conditions that can cause this. One of the
156. 15:47most common cause of anosmia is some type  of inflammation, or infections. Imagine if
157. 15:52there’re any sort of inflammation I this area.  Any type of situation like that, a lot fo mucus
158. 15:57builds up. Now the odor molecules has to travel  a farther distance, they have to be dissolved a
159. 16:03farther amount. Now there’re less molecules that  reach the olfactory receptors. You know what else
160. 16:09is important remember I told you that smell  is directly related with taste? If you ever
161. 16:14been really clogged, and have a nasal infection  a sinus infection. You probably also noticed that
162. 16:18your taste of food is also not very great, right?  That’s’ because a lot fot hese information from
163. 16:23the olfactory neurons are gonna be intertwined  with the cerebral cortex in certain taste centers.
164. 16:29Infalmmation can also damage supporting cells  and the actual olfactory dendrites which can
165. 16:36also cause anosmia. Blocking of the nasal passage  can cause anosmia. Trauma of the orbit or the
166. 16:42nasal cavity can cause anosmia, damaging the  ethmoidal bone, the cribriform plate of the
167. 16:47etmoidal bone. If there’s a pathological lesion in  the brain of any of those pathway we talked about,
168. 16:52that can alter the perception of smell.  Also, if there’s a degenerative CNS disease,
169. 16:59that can damage the neurons that take impulses  towards the brain, reducing the amount of smell.
170. 17:05Then we got something called Dysosmia.  Dysosmia is when you can smell,
171. 17:10but the smell perception is distorted. Where  some familiar objects may smell different or
172. 17:17unpleasant to you. They can further be divided  into Parosmia, where normal smell now taste or
173. 17:24smell unpleasant or disgusting. And Phantosmia,  where you detect smells that aren’t really
174. 17:30there in your environment. Terrible condiciton. Parosmia are usually caused by chronic conditions,
175. 17:36so chronic nasal congestion, chronic  sinusistis or prior head traumas can
176. 17:41also contribute to this. You know any  damage to olfactory nerve fibers can
177. 17:46occur as a complication of upper respiratory  tract infections. And in some cases a decrease
178. 17:51in the number of nerve fibers from these  infections mean that there are not enough
179. 17:57different fibers to accurately differentiate  odors, which you know result in parosmia
180. 18:03Phantosmia has been shown in people with inflames  sinuses aswell, but there are some medications
181. 18:10that can trigger olfactory hallucination,  specially some types of antipsychotics,
182. 18:15antimigraine medicine, and antiseizure drugs  aswell. Brain tumors can cause olfactory
183. 18:21hallusinations aswell depending on where the  lesion is. In some cases that can be a clinical
184. 18:26sign of a tumor whithin the CNS. Alright now a quick recap.
185. 18:31There’s a smell in your environment. These  gaseous odorants will eventually reach your….
186. 18:38olfactory nerves. Axons of these  olfactory nerves are gonne come together,
187. 18:43and run through the cribriform plate,  of the ethmoidal bone. So we call the
188. 18:48olfactory nerve a 1st order neuron because  this is where smell is first registered.
189. 18:53Once they go through the cribriform plate, they  will synapse with neurons in the …. olfactory
190. 19:01bulb. The olfactory continues posteriorly with  the olfactory tract. As the olfactory tract
191. 19:07approaches the anterior perforated substance,  it becomes wide, now called olfactory trigone.
192. 19:12The olfactory trigone now divides into striae. How  many stria does the olfactory trigone divide into?
193. 19:20Three striae, those are the medial stria,  the lateral stria, and in most cases the
194. 19:26intermediate stria. The lateral stria terminates  in the primary olfactory cortex of the temporal
195. 19:32lobe. The lateral olfactory stria contains  the largest number of fibers in the olfactory
196. 19:37tract and is responsible for the majority of  functional olfactory transmission. So it enter
197. 19:44the temporal lobe near the uncus, where the  primary olfactory cortex is. The medial stria
198. 19:50passes to the contralateral olfactory tract  and terminates in the limbic structures to
199. 19:55contribute to the emotional responses brought on  by the smell. The intermediate stria extend into
200. 20:02the anterior perforated substance and ends in a  slight elevation called the olfactory tubercle,
201. 20:07which plays a part in the reward system. So that was everything I had for the first
202. 20:13cranial nerve. The next video is going to be  about the second cranial nerve, the Optic Nerve.
203. 20:19Alright guys so that pretty much covers the  Olfactory nerve. Thank you so much for watching
204. 20:23another one of my videos. If you enjoyed, learned  something from it, please remember to like,
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