March 1, 2011

040 The Role of Hair Cells in Hearing

Do you ever wonder what happens to the hair cells inside our ears as we hear sound? What role do these tiny hairs have in hearing?

Watch this short movie as Leslie explains clearly and vividly enough for us to understand the main role of these tiny hair cells as sound enters our ears.


Transcript of Today’s Episode

Hello and welcome to another episode of Interactive Biology TV where we’re making Biology fun!  My name is Leslie Samuel and in this episode, Episode 40, I’m going to talk about the role of hair cells in hearing. So, let’s get right into it.

In Episode 39, we looked inside the cochlea to see what happened in response to sound. What we said was, in response to sound, the basilar membrane vibrated up and down, and this is the basilar membrane, which causes the Organ of Corti, which is this section here, to vibrate up and down, causing the tectorial membrane to move in a windshield wiper-like fashion that causes these hair cells to bend, the stereocilia, and the hair cells bend, causing a signal in the auditory nerve that then goes to the brain.  The brain says, “Okay that is sound,” and you hear it.

What we’re going to do today is we’re going to look specifically at what happens inside these hair cells, specially the inner hair cells which are directly responsible for the signal being sent to the brain that results in the sound that you are hearing.

So, let’s look at what happens inside those hair cells.  All right, so, I’m going to draw a hair cell.  Let’s say this is my hair cell right here. On the hair cell, I have stereocilia.  Now, this stereocilia occur in pairs: we have a long one and a short one.  In the short one, we have potassium channels so, that’s the potassium channel right here.   But, what’s interesting is that the long hair cell is mechanically connected to the short hair cell via that gate. Now, as you can imagine, when the tectorial membrane moves down on this hair cell, that causes the hair cell to bend.  So, let’s say this hair cell, the long hair cell, bends in that direction.  What is that going to do to these channels?  That’s going to cause these channels to open.  Now, these channels are mechanically-gated potassium channels.  They’re not extremely selective to potassium but, for this purpose, we’re going to look at what it does with the potassium ions.

Now, in the fluid that’s surrounding these stereocilia, we have endolymph. An endolymph is very rich in potassium ions.  So, let’s say we have potassium ions, K+, all around here.  When these mechanically-gated channels open, that is going to cause potassium ions to flow into the hair cells.  What is that going to do to the membrane potential (Em) ?  That is going to increase the membrane potential. Once the membrane potential increases, something else happens.  We have calcium ions that are also outside the cell. When that membrane potential increases, potassium is in here that’s going to cause calcium channels, voltage-gated calcium channels to open and calcium is going to rush into the cell.

Now, if you can remember when we spoke about neurotransmitter release, we said that calcium ions are the trigger that causes the neurotransmitter release in axon terminals.  This is the exact same thing that happens.  We have neurotransmitters in vesicles here and those neurotransmitters are then going to be released, and as I showed in the previous picture, this is connected to the auditory nerve, and that sends signals to the brain.

That’s all the content for this video.  I hope you learned a lot.  If you have any questions, go ahead and leave them in the comments below. That’s it for now, and I’ll see you in the next one.

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