Middle Ear

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From the course by The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

743 ratings

The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

743 ratings

Learn how the nervous system produces behavior, how we use our brain every day, and how neuroscience can explain the common problems afflicting people today. We will study functional human neuroanatomy and neuronal communication, and then use this information to understand how we perceive the outside world, move our bodies voluntarily, stay alive, and play well with others.

From the lesson

Hearing

The sound of birds chirping in the morning, a babbling brook or crashing waves on the beach, or warm conversation with the ones you love. The experience of all these things requires the ability to hear. Arguably the most important sense for human communication, it is also the most commonly impaired of our senses. In this module, you will learn how the human ear is artfully designed to enhance our ability to hear the human voice. You will follow sound waves as they travel from the external world, to the eardrum, through the bones of the middle ear, and to the cochlea that transduces sound information into neural impulses.

Hearing Pathways5:18

External Ear4:51

Meet the Instructors

Peggy Mason
Professor
Neurobiology

[MUSIC]

Okay, so let's talk about the Middle Ear.

The middle ear is where we're going to lose intensity.

We're going to lose the sound intensity.

We're not going to lose as much as, as as, one could, because of a couple of tricks.

Here in the middle ear, there are three small, ossicles.

One, two, and the final one is this piece right here.

Which looks like a s, a stirrup and it's called the stapes.

And these ossicles, they're, they're bones.

But they're reall,y really light little, little bones.

So we call them ossicles, baby bones.

And, they're so light that they are,

they're, they're moved by the tympanic membrane.

Now, going from air to actually having an air pushed on

a physical object to move it, you're going to lose energy,

if that's a, that's a losing proposition.

There are two ways in which we, in,

in which this is, modified so that you don't lose as much.

One is simply that these oscicles are so light, so

they're relatively easy to move, and the second way is because

the tympanic membrane is collecting sound energy from this much space.

It's going on to this, the stapes is actually going onto a,

an oval window, which is on the, an opening of the cochlea, which is this big.

So you're concentrating energy from this big a space to this big a space.

It's about 16, 15, 16 fold concentration,

and so, consequently you lose less energy.

Now the these, these bones,

turn out to have two muscles associated with them.

There are two middle ear muscles and these muscles are skeletal muscles,

they are of the same type as say my biceps.

They're of the same type as voluntary muscles.

But, we have no voluntary control over the middle ear muscles.

They are involuntarily in, controlled, but

they are fast and of the same, muscle structure as voluntary muscles.

So, one of them is called the tensor tympani.

And it does exactly what that may sound like, it tenses,

it pulls on the tympani, tympanic membrane.

So what would happen if you took a drum, and you tightened the drum?

What would happen?

Would sounds get louder or, or softer?

With the big effect would be that sound frequency would increase.

Okay? So, i, if you tighten a drum, the,

the frequency of the emanating sound will increase, and interestingly enough,

tensor tympani is engaged, it's contracted as a person is chewing.

So as you're chewing, you, automatically tighten the tympanic membrane.

What's the result?

Well, you're hearing chewing noises.

You're hearing yourself chew.

But, and that goes directly through the bone, to the cochlea, but

that's at a low frequency.

So, low frequency is chewing and

now incoming infor, incoming sound is,

is now at a, at a higher frequency.

So there's more of a separation between your chewing sounds and incoming, speech.

So, that e, enables you to understand or, or differentiate between

self-generated sounds, chewing sounds, and sounds coming from the outside.

The second middle-ear muscle is called the stapedius and

the stapedius is excited in response to unexpected loud sounds.

So the idea here I believe, is that if there's one loud sound,

there's going to probably be another loud sound shortly thereafter.

One crack of thunder begets another crack of thunder.

And so, what the stapes does is that, you can't do it before the first one,

because the, the first loud sound is unexpected.

But for the sec, subsequent loud sounds, the stapes is actually pulled back.

So it's as though I'm, I'm siting here drumming and

now somebody is pulling back on my arm, so, I can't drum as hard.

All right?

So, the stapedius is going to decrease the intensity of

the sound that's perceived from a given intensity sound wave.

Interestingly enough, this is if the, if the stapedius fails,

if that reflex fails, everything sounds too loud.

Sounds become too loud, and that's called hyperacusis.

The sounds are too loud, and Bauby, Jean Dominique Bauby,

who we discussed in the, in the first week, had hyperacusis.

Every, the TV sound was way too loud, and it's interesting to me, that,

that's one of the things that he keeps on coming back to.

It, it, it appears to me to be one of the most annoying features of

his locked-in existence, is this hyperaccusis.

Okay. So, we've got, we've reached the cochlea

and now we're going to go into the inner ear, in the next segment.

[MUSIC]

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