Hearing: Hair Cells and Semicircular Channels

00:01 How hair cells work? These hair cells are important processes.

00:08 These hair cells allow for him to be their move from direction or the other.

00:14 They’re structures is very specific, and have very specific amounts of fluid that they’re filled in.

00:21 So the endolymph is associated with the top portions of the hair cells.

00:28 Perilymph is associate with the bottom portion of the cell.

00:33 What is in special about endolymph, is it's high potassium concentration.

00:39 It's high potassium concentration is different than other areas in the body where usually have the high potassium concentration inside the cell versus outside the cell.

00:50 Perilymph is more normal in nature, in which it has a low potassium concentration.

00:59 So, one of these two different potassium concentrations allow for then the signal process to work.

01:07 Well, a sound wave is going to be traveling down membrane, pushing on this tectorial membrane that will push down against the hair cells.

01:17 As it pushes down against the hair cells, the hair cells will deflect.

01:22 As the hair cells deflect, potassium is allow to rush into those hair cells.

01:29 Why does potassium wants to rush in? Because the endolymph has such high potassium concentration.

01:36 There’s a gradient between the endolymph and the cell itself.

01:41 Membrane potential depolarizes.

01:44 These opens up calcium channels.

01:46 These calcium influx causes synaptic vesicles which have glutamate in them to be released.

01:53 Therefore, these whole processes is set up by potassium entering into the cell to cause depolarization.

02:00 It is then the release of glutamate that is stimulatory to the next nerve that will transmit the information back to the brain.

02:09 You might ask, what happens to the potassium? The potassium exits the cell into the perilymph and then is recycled and brought back to the endolymph.

02:21 Sound Frequency Mapping.

02:25 So now that you understand how a hair cell works.

02:28 We can then bring in more information about how you hear different sounds.

02:33 And different sounds of course have different frequencies, from a high sound to a low sound.

02:40 These different sounds give you the insight into how a particular wave is coming through.

02:47 So, you have hair cells allocated from the base all the way to the apex.

02:54 These will now be responsive to different frequencies.

02:59 If you have a high frequency sound, this is going to stimulate the membrane closer to the base.

03:10 This high frequency sound is because of the greater amount of frequency not talking about amplitude here.

03:17 Frequency, so it's occurring through a large number of frequency waves will hit the hair cells closer to the base.

03:30 Sounds that are lower frequency have a slower wave.

03:35 Those who travel in further into the membrane before they will hit a hair cell.

03:42 That is how you will distinguish between a high pitch and a low pitch, is where the wave comes in to stimulate those particular hair cells associated with the depression in the membrane.

03:58 So we've discussed sound frequency mapping.

04:01 But let's add some richness to that information, and that is a particular frequency, although it primarily stimulates a certain area of the tectorial membrane.

04:12 It will spread a little bit further.

04:15 So let's take an example of that.

04:17 If we look at a frequency, let's say 8000 cycles/second.

04:22 You can see it stimulates a narrow band and if you think about it as the distance from the stay-bees.

04:29 If you take something like the cycle frequency of 400, you can see it stimulates a much wider area.

04:38 That wider area of stimulation is going to engage more hair cells, that gives us a little bit more ability to hear different sounds.

04:49 Besides the sound frequency mapping, the other component that we need to bring out is loudness.

04:56 For example, when you do a test of hearing, you will do it at different frequencies at the same decibel loudness level.

05:06 So how do you determine loudness? They usually is how many hair cells you're actually engaging.

05:13 So you have your outer hair cells that can be stimulated.

05:17 And if it's a loud enough stimulus, you can also stimulate your inner hair cells.

05:24 And the more of these hair cells that are engaged, the louder the sound will be perceived in your cerebral cortex.