Ear Anatomy | How You Actually Hear (Special Senses) — Transcript

Full transcript

1. 0:00Alright, so… have you ever stopped to  think about how sound actually works?
2. 0:03I mean, how does a random vibration in  the air - like music, someone talking,
3. 0:07or even the noise of traffic - somehow get turned  into something your brain understands as sound?
4. 0:13It feels automatic, but the process behind  it is incredibly precise—and honestly,
5. 0:18kind of mind-blowing. Because your ability to  hear depends on this small, complex system deep
6. 0:24inside your head, where tiny bones vibrate, fluid  waves travel, and microscopic hair cells convert
7. 0:30mechanical energy into electrical signals. And if any part of that chain fails - even
8. 0:35slightly - you start to lose your ability to hear. So in this video, we’re going to go through
9. 0:41exactly how that process works. We’ll  look at the external ear, the middle ear,
10. 0:46the inner ear - and by the end of this video,  you’ll have a full understanding of how your
11. 0:51body lets you hear the world around you. What’s up everyone, my name is Taim. I’m
12. 0:55a medical doctor, and I make animated medical  lectures to make different topics in medicine
13. 0:59visually easier to understand. If you’d like a PDF  version or a quiz of this presentation, you can
14. 1:03find it on my website, along with organized  video lectures to help with your studies.
15. 1:07Alright, let’s get started. So we’ll begin with the external ear, and
16. 1:11this part is made up of three main components: the  auricle, or pinna, the external acoustic meatus,
17. 1:17and the tympanic membrane. Let’s go through  each of these, starting with the auricle.
18. 1:22The auricle is the visible part of the ear  that sticks out from the side of your head.
19. 1:26It’s made up of elastic cartilage which  gives it the flexibility to bend forward,
20. 1:31upward, or twist in any direction,  and still recoil back into position.
21. 1:36Now, externally, we divide the  auricle into several anatomical
22. 1:40landmarks. And I recommend you to feel  those structures on your own ear as we
23. 1:44go through them to activate more areas of  your brain for stronger memory formation.
24. 1:48The outermost rim is called the helix. Just  inside of that is a ridge called the antihelix.
25. 1:54Between the two, there’s a narrow groove known  as the scapha. Closer to the center of the ear,
26. 1:59we’ve got the concha of the auricle. This is that  bowl-shaped depression that leads directly into
27. 2:04the ear canal, and it helps collect and  funnel sound. Then we’ve got the tragus,
28. 2:09that small protrusion just in front of the ear  canal opening. Across from that is the antitragus,
29. 2:15and between them is a little dip called the  intertragic notch. And at the very bottom,
30. 2:20we have the lobule of the auricle -  the soft earlobe, which is unique in
31. 2:24that it doesn’t contain any cartilage. These structures are actually clinically
32. 2:28useful too. For example, the tragus is often  used in exams for signs of otitis externa,
33. 2:34that is inflammation of the external ear,  where pressing on the tragus can cause
34. 2:38pain. That’s one of the classic findings. Right beneath the skin of the auricle,
35. 2:43we have a few more important structures. There are the extrinsic auricular muscles,
36. 2:47which attach the ear to the skull. Then the intrinsic auricular muscles,
37. 2:51which are small muscles within the auricle itself,  but these don’t have a real function in humans.
38. 2:57You’ll also find the auricular cartilage making up  the core of the pinna, and nearby runs the facial
39. 3:03nerve, or cranial nerve VII, which provides  motor innervation for the muscles of facial
40. 3:07expression, including some around the ear. One more thing I want to mention here is
41. 3:12sensory innervation, because the external  ear has a pretty interesting nerve supply.
42. 3:16At the top portion, we have the  anterior auricular branches of the
43. 3:20auriculotemporal nerve, which is a branch of  the mandibular division of cranial nerve V.
44. 3:24The back side of the auricle is innervated by the  lesser occipital nerve, from the cervical plexus.
45. 3:29Then, down at the bottom, there’s the great  auricular nerve, also from the cervical plexus.
46. 3:34And the rest is covered by the  auricular branch of the vagus nerve,
47. 3:37which is why stimulation of the external auditory  canal can sometimes cause a vagal response,
48. 3:42like coughing. As weird as it sounds. So that was the auricle.
49. 3:47Now let’s go inside the ear and look  at the external acoustic meatus.
50. 3:51The external acoustic meatus is the ear canal - it  runs from the outer ear to the tympanic membrane.
51. 3:57And anatomically, we divide it into several parts. At the opening, we have the external acoustic
52. 4:03aperture. Just after that is the  cartilaginous portion of the canal,
53. 4:07which makes up the outer third. Then, deeper inside, is the bony part,
54. 4:11which makes up the inner two-thirds of the canal. These two regions also differ in sensitivity—the
55. 4:17bony part is more sensitive to pain,  which is important to remember when
56. 4:21examining the ear with an otoscope. Next to the tympanic membrane,
57. 4:25we find the tympanic sulcus and notch, which help  anchor the eardrum to the walls of the canal.
58. 4:30Now, along the skin of the canal,  especially in the cartilaginous portion,
59. 4:34you’ll find ceruminous glands. These are modified sweat glands that
60. 4:38produce something called cerumen, which is also  known as earwax. The earwax is slightly acidic
61. 4:44and what it does is that it’s specialized to trap  things like dust, bacteria, and other particles,
62. 4:49helping to protect the middle and inner ear. So that was the external acoustic meatus.
63. 4:55Now let’s cover the last segment of the external  ear, which is the tympanic membrane, or eardrum.
64. 5:01And it’s easiest to understand if you look at  it from this direction, and then isolate it.
65. 5:05This is what your eardrum looks like. Now,  around the eardrum, we have something called the
66. 5:10fibrocartilaginous ring. This is a thickened ring  that basically holds the eardrum in place within
67. 5:16the tympanic sulcus. Now, the tympanic membrane  itself is divided into two main components,
68. 5:22we have the pars tensa, which is a very very  heavily dense fibrous irregular connective tissue,
69. 5:27the other one is called pars flaccida, which is  the upper, loose part of the tympanic membrane.
70. 5:32And this is important because if you go and stick  a Q-tip into the ear and you accidentally hit the
71. 5:37pars flaccida too hard you can puncture the  tympanic membrane. And that would affect the
72. 5:41middle ear pressure as well as open up for certain  types of microbes to come in. Now, another thing
73. 5:46we see is that we can actually see an outline  of the malleus, which is one of the ossicles
74. 5:51located in the middle ear, but what it does is  that it’s pushing towards the tympanic membrane
75. 5:56and forming an umbo of the tympanic membrane. It  forms the mallear stria, and a mallear prominence.
76. 6:03Imagine now that you’re going to assess the  eardrum, so what you do is shine a light into the
77. 6:08external acoustic meatus, you should be able to  see all of those structures, and as you do this,
78. 6:13you should be able to see a reflection a light  reflex, or cone of light. Which basically is a
79. 6:18reflection that once it’s visible, it indicates  a healthy eardrum during an otoscope examination.
80. 6:25So that was basically all three parts of the  external ear. We finished the external ear,
81. 6:29now let’s do the middle ear. So let’s zoom in. When you look at the middle ear, the first thing
82. 6:35to understand is that it contains three tiny  ossicles, arranged in a very specific way to
83. 6:41help transmit sound from the eardrum into  the inner ear. These are the smallest bones
84. 6:46in the human body, and without them, we  wouldn’t be able to perceive sound at all.
85. 6:50And the first one of those three ossicles is  the malleus. The malleus has a head, a neck,
86. 6:56and then a long process called the handle of the  malleus, which is the part that actually attaches
87. 7:01to the tympanic membrane. That’s the structure  we saw earlier forming the mallear prominence,
88. 7:05the mallear stria, and the umbo. So every  time the tympanic membrane vibrates,
89. 7:10that handle of the malleus is the very  first thing to pick up that motion.
90. 7:14From the malleus, the vibration continues to  the next ossicle, the incus. The incus consists
91. 7:20of a body, a short limb, and a long limb. And  that long limb is important because it directly
92. 7:25articulates with the third ossicle, the stapes. The stapes has a head, an anterior and posterior
93. 7:31limb, and then a flat portion called the base  of the stapes. And what the stapes does is that
94. 7:36it sits right on top of the oval window,  which is the entrance into the inner ear.
95. 7:41So right now, as you're hearing my voice—or  any sound, for that matter—the soundwaves
96. 7:46of that sound is traveling through the external  acoustic meatus, striking the tympanic membrane,
97. 7:51and causing it to vibrate. Those vibrations  are then passed from the tympanic membrane to
98. 7:56the malleus, then to the incus, and then to  the stapes. And each ossicle amplifies that
99. 8:01vibration slightly before the stapes finally  pushes on the oval window and sets the inner
100. 8:06ear fluid into motion. We’ll talk about that  fluid movement in detail later in the video,
101. 8:10because that’s where sound, or mechanical  energy is transformed into electrical signals.
102. 8:15But before we move on, there’s also an important  protective mechanism here. Imagine for a second
103. 8:20that the sound is suddenly very loud—like a  sharp, unexpected noise. That kind of strong
104. 8:26amplitude would cause significant vibration of  the tympanic membrane, and if nothing helped
105. 8:31regulate it, the membrane could actually be  damaged. To prevent that, we have a small
106. 8:35muscle called the tensor tympani muscle, which  attaches to the malleus and tightens the tympanic
107. 8:40membrane to reduce the force of the vibration. Now. The middle ear is a cavity, right? And this
108. 8:47cavity has walls. On the lateral wall, we have the  tympanic membrane, the one we just talked about.
109. 8:52On the medial side, we’ve got the inner ear,  and anteriorly, we find the auditory tube. Now,
110. 8:58the thing is, the middle ear is actually  an essential part of the temporal bone,
111. 9:02and what makes it even more important is that  many different structures pass through it.
112. 9:06If we look at it from this angle, and then  remove the facial bones, we’ll be able to
113. 9:10see the temporal bone. And coming off from this  bone on the anterior side, is the auditory tube.
114. 9:16This auditory tube has two distinct parts,  there’s a lateral bony part, which is rigid,
115. 9:22and then medially, we have the cartilaginous  part, which is a bit more flexible. This tube
116. 9:27extends all the way into the nasopharynx,  and its job is to equalize pressure between
117. 9:32the middle ear and the outside environment. Now, the ear structures are located deep within
118. 9:37the petrous part of the temporal bone, As you  see here, we can see parts of the external ear,
119. 9:42the middle ear, and the internal  ear. Let’s zoom in for a moment.
120. 9:46Can you see the middle ear structures now?  You can see the tympanic membrane forming
121. 9:50the lateral wall, and behind it, we have the  malleus, the incus, and the stapes. You can
122. 9:56also see the tensor tympani muscle, originating  from the cartilaginous part of the auditory tube,
123. 10:01and inserting on the handle of the malleus. So. This is the middle ear. I know I’m
124. 10:06overemphasizing this, but I’m doing it for a  reason. So imagine this, let’s say you casually
125. 10:11walk across the external acoustic meatus. What are  you looking at now? You’re looking at the tympanic
126. 10:17membrane, right? And remember, that membrane is  the lateral wall of the middle ear. Makes sense?
127. 10:23Now. Let’s go ahead and remove the tympanic  membrane - imagine we’ve taken it out
128. 10:28completely. That means now you’re looking  directly into the middle ear cavity. What
129. 10:33you’re seeing is the cavity formed by six walls.  You’ve got the medial wall, the superior wall,
130. 10:38the posterior, anterior, and the inferior wall. The medial wall is called the labyrinthine wall,
131. 10:44because it’s the one that’s directly facing  the inner ear. The superior wall is called
132. 10:48the tegmental wall, because it’s formed by the  tegmen tympani, which is a thin plate of bone
133. 10:53separating the middle ear from the cranial cavity  above. The posterior wall is the mastoid wall,
134. 10:58which connects to the mastoid air cells. The  anterior wall is known as the carotid wall,
135. 11:03because the internal carotid artery runs right in  front of it. And the inferior wall is called the
136. 11:08jugular wall, because that’s where the jugular  bulb of the internal jugular vein lies just
137. 11:13beneath it. So that whole cube you’re looking  at, that entire space, is the tympanic cavity.
138. 11:19So I’ve removed all the ossicles just  temporarily, so we can see everything
139. 11:22more clearly. And what I want to do next  is take you through each wall one by one,
140. 11:27and talk about all the landmarks and openings  related to them. Because if you understand these,
141. 11:32it becomes way easier to mentally navigate the  tympanic cavity, and it’ll help you recognize
142. 11:37what goes in, what comes out, and what’s at  risk during infections or surgical procedures.
143. 11:42Let’s start with the labyrinthine  wall, which again is the medial wall,
144. 11:46facing the inner ear. First, there’s the  promontory, which is a rounded bulge created
145. 11:51by the first turn of the cochlea. Then, just  above that, we have the oval window, which is
146. 11:57the opening where the base of the stapes sits.  Just below the promontory is the round window,
147. 12:02which is covered by a flexible membrane called  the secondary tympanic membrane. These structures
148. 12:06will make sense a little later in this video. From the inferior wall of the tympanic cavity,
149. 12:11there’s a small opening called the  tympanic canaliculus, and through that,
150. 12:15we have the tympanic nerve, which is a  branch from the glossopharyngeal nerve,
151. 12:19cranial nerve IX. This tympanic nerve enters  the middle ear through the canaliculus and
152. 12:24then spreads out over the promontory and forms  a nerve network called the tympanic plexus.
153. 12:29Now, from that plexus, we get a branch called  the lesser petrosal nerve. This nerve exits the
154. 12:35middle ear through its own canal, the canal  for the lesser petrosal nerve, and continues
155. 12:39its path toward the otic ganglion, where it  eventually synapses and contributes to the
156. 12:44parasympathetic innervation of the parotid gland. There’s another set of fibers that come from the
157. 12:49internal carotid plexus, which surrounds  the internal carotid artery called the
158. 12:54caroticotympanic nerves. These fibers enter the  tympanic cavity through the caroticotympanic
159. 12:59canaliculi and join the tympanic plexus,  contributing sympathetic fibers to it.
160. 13:05So, if we look at it in perspective, we can  actually see the glossopharyngeal nerve giving
161. 13:09off the tympanic nerve, which enters the  middle ear and forms the plexus, from which
162. 13:14the lesser petrosal nerve takes off heading up  toward the otic ganglion. And alongside this,
163. 13:19we’ve got the internal carotid plexus, which  surrounds the internal carotid artery, giving
164. 13:24off those caroticotympanic nerves, supplementing  the plexus with sympathetic innervation.
165. 13:30Now let’s look at the posterior wall of the  middle ear, we can see the facial nerve—cranial
166. 13:35nerve VII—running in its facial canal. As it  travels, it gives off a small branch to the
167. 13:40stapedius muscle. That branch exits through a  small projection called the pyramidal eminence,
168. 13:46and it’s within that little hollow that the  stapedius muscle sits. This muscle inserts onto
169. 13:52the neck of the stapes, and when it contracts,  it pulls the stapes away from the oval window to
170. 13:57dampen the vibration—basically acting as a second  layer of protection against loud sounds. That
171. 14:03contraction is controlled by the facial nerve. Now while we’re talking about the facial nerve,
172. 14:08another branch it gives off is the chorda tympani  nerve. That nerve enters the tympanic cavity
173. 14:14through the posterior canaliculus, crosses  over the tympanic membrane and malleus,
174. 14:19and exits through the anterior canaliculus  to go toward the infratemporal fossa,
175. 14:23where it joins the lingual nerve. It carries  taste sensation from the anterior two-thirds
176. 14:29of the tongue and parasympathetic fibers  to the submandibular and sublingual glands.
177. 14:34If we now look at the anterior  wall of the tympanic cavity,
178. 14:38one structure we see here is the tensor tympani  muscle. Remember this one originates from the
179. 14:43cartilaginous portion of the auditory tube and  the greater wing of the sphenoid, and inserts
180. 14:48onto the handle of the malleus. Again, helping  to reduce the amplitude of sound vibrations.
181. 14:54Right next to it on the anterior side, we  also find the opening of the auditory tube, or
182. 14:59Eustachian tube, which connects the middle ear to  the nasopharynx. This tube helps equalize pressure
183. 15:05between the middle ear and the environment. So that was everything for the tympanic cavity,
184. 15:10and with that, we’ve covered the middle ear.  Alright, now let’s finally do the inner ear,
185. 15:15and figure out how we actually convert sound into  electrical signals that our brain understands.
186. 15:21The internal ear is represented by the  vestibulocochlear organ. Let's zoom in on
187. 15:26it. This is such a beautiful little organ. I wanna  switch over to a 3D model for a moment to see it
188. 15:31in detail. The inner ear is divided into three  parts—the semicircular canals, the vestibule,
189. 15:37and the cochlea. This organ has two functions.  The semicircular canals and the vestibule are
190. 15:42responsible for maintaining balance, while the  cochlea is responsible for hearing. Just before
191. 15:48I continue, remember what these two holes were  called? The oval one is called the oval window,
192. 15:54and the round one is called the round window.  Again here we see the eardrum, with the malleus,
193. 15:59incus, and stapes, and it’s the stapes  that’s pushing against the oval window.
194. 16:04The round window has a membrane, and that’s  why we call it the secondary tympanic membrane.
195. 16:09Anyways, the round and oval windows will be  important very soon when we talk about the
196. 16:13hearing mechanism, but let's cover some more  structures of the inner ear first. Now, the
197. 16:17vestibulocochlear organ is divided into two parts.  The outer shell is called the osseous labyrinth,
198. 16:24and inside that, there's a membranous  labyrinth. The osseous labyrinth is bony,
199. 16:28and the membranous labyrinth is a  fluid-filled sac. Makes sense so far?
200. 16:33The three parts we mentioned earlier—the  semicircular canals, the vestibule, and
201. 16:37the cochlea—are regions that contain the osseous  labyrinth. Now let's fade the osseous labyrinth
202. 16:44and focus on the inside for a moment. Inside  the semicircular canals, we find semicircular
203. 16:50ducts. We have an anterior, a posterior, and a  lateral duct. And there’s also a common membranous
204. 16:56limb that connects some of them together. We can also see the utricle, the saccule,
205. 17:02and the cochlear duct. And then we have the  endolymphatic duct, which leads into the
206. 17:06endolymphatic sac. These lie deep to the dura  mater in the posterior cranium, and one thing
207. 17:12to keep in mind is that the membranous labyrinth  is a closed system, it's a poutch. And the fluid
208. 17:17within it is removed by reabsorption via  the epithelium of the endolymphatic sac.
209. 17:23These are all part of the membranous labyrinth.  Other things we can see is the ampulla, which is
210. 17:28the widened base of each semicircular duct. All  of these fall under the membranous labyrinth.
211. 17:34So that’s generally how we divide the  bony and the membranous labyrinth.
212. 17:38From personal experience, it makes so  much more sense to go through all of
213. 17:41the structures of the inner ear before we talk  about the actual function. Now, this is where
214. 17:47the interesting part of this video starts. Ready? Between the osseous labyrinth and the membranous
215. 17:52labyrinth, we’ve got something called perilymph.  So, perilymph covers all of this area between the
216. 17:59two structures. Within the membranous labyrinth,  we’ve got endolymph, which fills the inner
217. 18:04compartments. These two fluids never mix, and this  is really important because even their chemical
218. 18:10composition is different. The perilymph is similar  to extracellular fluid—so it’s high in sodium and
219. 18:17low in potassium. Endolymph, on the other hand,  is similar to intracellular fluid or cytosol—so
220. 18:23it’s low in sodium and high in potassium. This is super super important to know
221. 18:28because it’ll come into play in just a  minute when we start talking about the
222. 18:31function of the hair cells. And again,  remember: these two fluids do not mix.
223. 18:37Alright, i’m gonna make you an expert in the  hearing system by the end of this video. Now,
224. 18:41we divide the inner ear into a vestibular system  for balance, and a cochlear system for hearing.
225. 18:47Let’s do the vestibular system first.  The vestibular system is made up of two
226. 18:52functional parts: the three semicircular canals,  which detect rotational movements of the head,
227. 18:57and the otolithic organs—the utricle and  saccule—which detect linear acceleration and head
228. 19:03position relative to gravity. The semicircular  canals are arranged roughly in three perpendicular
229. 19:09planes. The anterior canal is responsible for  detecting nodding motions like when you say yes,
230. 19:15the posterior canal detects movements like  tilting your head towards your shoulder,
231. 19:20and the lateral canal detects horizontal  head turns, like shaking your head no.
232. 19:25Each canal has an enlarged end called the  ampulla, and inside the ampulla, we have the
233. 19:30crista ampullaris. That’s the sensory organ that  responds to rotation. It consists of the cupula,
234. 19:37a gelatinous structure that bends with endolymph  flow; hair cells, which are the actual sensory
235. 19:42receptors; supporting cells; and fibers of  the vestibular nerve, which carry signals
236. 19:46centrally. When the head turns, the endolymph  inside the canal lags behind due to inertia
237. 19:53and pushes against the cupula. This deflects the  cupula, bends the stereocilia on the hair cells,
238. 20:00and if deflected in the excitatory direction,  that allows potassium-rich endolymph to enter
239. 20:05the hair cells. That depolarizes the cells and  sends nerve impulses through the vestibular
240. 20:10nerve to inform the brain about the movement. Below the semicircular canals, we have the utricle
241. 20:17and the saccule. The utricle is responsible for  horizontal acceleration—like when you accelerate
242. 20:22forward in a car—and the saccule responds to  vertical acceleration, like when you go up or
243. 20:27down in an elevator. Together, these are called  the otolithic organs. They each contain a sensory
244. 20:33area called the macula. On top of the hair cells  in the macula lie tiny calcium carbonate crystals
245. 20:40called otoconia, or otoliths. These are suspended  in a gelatinous layer and add weight and inertia.
246. 20:47When the head accelerates in any of those planes,  the otoliths shift and drag the gel with them,
247. 20:52bending the embedded hair cells. That bending  again opens channels, potassium flows in from
248. 20:58the endolymph, depolarizing the cell and sending  signals through the vestibular nerve. If these
249. 21:03crystals jump out of their place and enter the  semicircular canals, they can cause a condition
250. 21:08called benign paroxysmal positional vertigo,  or BPPV, which leads to episodes of dizziness.
251. 21:14So that’s the vestibular system, and signals  from here are carried to the brain via the
252. 21:19vestibular nerve. Makes sense? That’s how  balance is perceived by the inner ear. Let’s
253. 21:24now do the auditory system. This one is such  an interesting system. The auditory system is
254. 21:29built within the cochlea. It’s divided into the  scala vestibuli, scala tympani, and scala media.
255. 21:35These are wrapped around a central axis called the  modiolus, which is the bony core of the cochlea
256. 21:41that houses the spiral ganglion and blood vessels. Now, if we take a cross-section of the cochlea,
257. 21:47we’ll be able to see those three compartments  clearly. The scala vestibuli and scala tympani
258. 21:53both contain perilymph, while the scala media  contains endolymph. Why does this matter?
259. 21:59Well, when sound waves enter your external  acoustic meatus, they vibrate the eardrum,
260. 22:04which then transmits that vibration through the  malleus, incus, and stapes. The stapes pushes
261. 22:10into the oval window, transmitting that mechanical  energy into the perilymph of the scala vestibuli.
262. 22:16As the stapes pushes inwards, it sends  a pressure wave through the perilymph,
263. 22:21which is separated from the endolymph. That wave  travels up the spiral of the cochlea toward its
264. 22:27apex. The end point where that fluid shifts to the  other side, is called the helicotrema. From there,
265. 22:33the wave continues down the scala tympani and  eventually dissipates at the round window.
266. 22:39The round window has a secondary tympanic  membrane that bulges outward to accommodate
267. 22:44the pressure of the incoming fluid wave. Now, as the perilymph moves through these
268. 22:48chambers, it exerts pressure on the membranes  that separate the scala media. The area above
269. 22:54scala media is the scala vestibuli and  below is the scala tympani. Alright,
270. 22:59we’ve come this far. So now you may ask, so Taim,  how does this system ‘’understand’’ sound? I’m
271. 23:05glad you asked. And in order to understand  that we need to go through some structures.
272. 23:09The vestibular membrane separates the scala  vestibuli from the scala media, and the basilar
273. 23:15membrane separates the scala tympani from the  scala media. Vibrations in the perilymph cause
274. 23:21these membranes to oscillate, and the most  important of these is the basilar membrane,
275. 23:26because there’s a very important organ  attached to it, it’s the organ of Corti.
276. 23:31Let’s zoom in on the organ of Corti, which is  the actual structure responsible for turning
277. 23:36vibrations into electrical signals,  this is where hearing really happens.
278. 23:40The main sensory cells here are the hair cells,  divided into inner and outer hair cells. The inner
279. 23:46hair cells are the ones that detect sound and  send information to the brain. The outer hair
280. 23:51cells help amplify the signal and fine-tune  the response. These hair cells are supported
281. 23:57by different types of supporting cells — internal  supporting cells next to the inner hair cells, and
282. 24:02external supporting cells next to the outer ones.  Between them, we see the pillar cells, which form
283. 24:08a triangular space called the tunnel of Corti.  This is important for maintaining the structural
284. 24:13integrity of the organ and helping guide how  vibrations travel across the basilar membrane.
285. 24:18It also plays a role in isolating and organizing  the movement of the hair cells during stimulation.
286. 24:24Sitting just above the hair cells is the tectorial  membrane, a gelatinous membrane that extends over
287. 24:29the organ of Corti. And projecting from the  top of each hair cell are fine structures
288. 24:35called stereocilia, which make contact with,  or lie close to, the tectorial membrane.
289. 24:40When sound vibrations travel through the  cochlea, they cause the basilar membrane
290. 24:45to move up and down. This movement  causes the stereocilia to bend. Now,
291. 24:50these stereocilia are suspended within endolymph,  which is high in potassium and low in sodium,
292. 24:56a composition that's very different from the  perilymph that surrounds the rest of the cochlea.
293. 25:01When the stereocilia bend in a certain direction,  potassium channels open, allowing potassium from
294. 25:06the endolymph to enter the hair cell. This causes  depolarization, which leads to the release of
295. 25:12neurotransmitters at the base of the cell. That  stimulates the cochlear nerve fibers underneath,
296. 25:17and those signals are sent through the cochlear  nerve. So again, the basilar membrane vibrates,
297. 25:22the stereocilia bend, potassium enters  the hair cell, the cell depolarizes,
298. 25:27and a signal is transmitted through the cochlear  nerve, which passes the cochlear ganglion and
299. 25:32then continues on to the auditory pathway. Now, what if I’m speaking in a low volume,
300. 25:38or what if I’m speaking in a high pitch?  How does the system discriminate that?
301. 25:43This is where everything is going to make sense. Now, we got the scala vestibuli, scala media and
302. 25:50scala tympani. Imagine for a moment that we  roll out the cochlea, we're just rolling out
303. 25:55the whole cochlea. What we're going to see  is the basilar membrane. Above the basilar
304. 25:59membrane is the scala vestibuli, below  is the scala tympani. Makes sense so far?
305. 26:05Imagine now that a sound is detected in  the external environment. That soundwave
306. 26:09will eventually cause the stapes to push towards  the perilymph of the scala vestibuli. That wave
307. 26:15of impulse is going to continue along the scala  vestibuli, and then loop around the apex. Which
308. 26:21is called what? It’s called the helicotrema. The  wave impulse will continue along the scala tympani
309. 26:28and then eventually reach the round window, which  basically bulges outward to release the pressure.
310. 26:33Now. As the wave impulse travels along the  perilymph, some of that fluid is going to
311. 26:38oscillate, or vibrate the basilar membrane and  cause it to react to that sound. But our body is
312. 26:44so smart at discriminating the frequency  and amplitude of that sound. And it does
313. 26:49that because in reality, the basilar  membrane looks more or less like this.
314. 26:53The base of the basilar membrane, the  base closest to the round and oval window,
315. 26:58is the narrowest. And not just that,  the base is also the stiffest. And this
316. 27:03is perfect because the basilar membrane  here is tuned for high-frequency sounds.
317. 27:08Towards the apex, the basilar membrane is the  widest and is very flaccid, and it’s tuned for
318. 27:13low-frequency sounds. Now, humans can detect sound  frequencies ranging roughly from 20 to 20,000 Hz.
319. 27:21We don’t commonly use the extremes of this  range, but we’re capable of perceiving them.
320. 27:26For example, if the sound has a frequency  of 1600 Hz, the wave will cause the basilar
321. 27:32membrane to vibrate at a certain location that  is tuned to 1600 Hz. If the frequency is lower,
322. 27:38say 400 Hz, the vibration will occur further  along the membrane, closer to the apex.
323. 27:44This is how the cochlea  separates pitch, or frequency.
324. 27:48And now you might be asking, but Taim, what  if the person is speaking really loud? How
325. 27:52does loudness affect the basilar membrane? Loudness is determined by amplitude, and that
326. 27:58amplitude determines how strongly the basilar  membrane vibrates at that specific pitch. The
327. 28:04louder the sound, the more intense the vibrations  will be at that specific pitch. The lower the
328. 28:09loudness, the weaker the basilar membrane  will vibrate at that same pitch. Makes sense?
329. 28:14So loudness is determined by amplitude.  And normal amplitude hearing for humans
330. 28:19is around 0 to 120 decibels. We don’t usually  experience the whole spectrum in normal life.
331. 28:25If you look here, a normal speaking voice  is going to be around 60 dB. And prolonged
332. 28:30exposure to loudness of about 85 dB or more  is damaging to the ear long term, because it
333. 28:36can wear out or even destroy those hair cells.  And once they’re gone, they don’t regenerate.
334. 28:41So in summary, how strongly the basilar membrane  vibrates—that’s loudness. Can be low volume or
335. 28:47high volume, measured in decibels. Where on  the basilar membrane it vibrates—that’s pitch,
336. 28:52or hertz. Can be high-frequency  sounds or lower-frequency sounds.
337. 28:56So. Balance from the vestibular system is carried  through the vestibular nerve. Hearing is carried
338. 29:02through the cochlear nerve. They join together  to form the vestibulocochlear nerve—the eighth
339. 29:07cranial nerve—which goes to specific areas of  the brainstem. And this path onwards is covered
340. 29:12in the video about the vestibulocochlear nerve. And with that, we’ve now covered the detailed
341. 29:17anatomy of the human ear, the organ of hearing  and balance. We’ve covered the external ear,
342. 29:22the middle ear, and the inner ear. I  really hope you found that helpful.
343. 29:26I’ve made free courses for other topics  here on YouTube if you wanna keep learning,
344. 29:29otherwise if you want a handmade PDF  version of this lecture or take a quiz
345. 29:32to test your knowledge, or access  an organized list of all my videos,
346. 29:35you can find everything on my website.  Thanks for watching! See you in the next one.