Friday Sprog Blogging: hear hear!

Dr. Free-Ride: What do you guys want to discuss this afternoon?

Younger offspring: The human body.

Elder offspring: Yeah, how the human body works.

Dr. Free-Ride: Um, you guys know that "how the human body works" is a huge subject that we will never get through before dinner, right? You're going to have to settle on a particular system or body part.

Younger offspring: The skeleton!

Elder offspring: The ear!

Dr. Free-Ride: Is there any room for compromise here?

Elder offspring: Well, the ear contains the smallest bone in the body.

Younger offspring: If it has a bone, I agree to the ear.

Dr. Free-Ride: Thank goodness!

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Dr. Free-Ride: Tell me what you know about the ear.

Younger offspring: Well, the ear is what you use to hear things.

Elder offspring: When something makes a sound, vibrations float through the air.

Dr. Free-Ride: That's right, sound waves.

Elder offspring: I think the ear is shaped like the way it is to catch the sounds and send them into your brain.

Dr. Free-Ride: How does the ear catch the vibrations? It doesn't send vibrations into the brain does it?

Younger offspring: Does sound make the brain vibrate?

Elder offspring: No, the brain doesn't vibrate. The ear sends messages into the brain about what sound it is, if it's too loud or too soft.

Younger offspring: Or too horrible.

Dr. Free-Ride: What kind of messages does the ear send?

Elder offspring: Messages along your nerves.

Younger offspring: What is a message along the nerve like?

Elder offspring: That's another system. I don't really know how nerves work.

Dr. Free-Ride: That's OK. Let's see what we can find out about how the ear goes from the sound waves to the message to the nerve, and we won't worry about how the nerve signal gets sent to the brain.

Elder offspring: Here, they have it in the cross-section book. The sound waves make the eardrum vibrate. Then the eardrum vibrating makes the malleus, incus, and stapes vibrate.

Younger offspring: Those are the tiny bones!

Elder offspring: Let's see ... it says that then the stapes presses on the "oval window", and those vibrations make the fluid in the cochlea vibrate.

Dr. Free-Ride: And then?

Younger offspring: There's more?!

Elder offspring: The vibrations moving through the cochlea make some tiny hairs wiggle, and those start the nerve sending the signal from the ear to the brain.

Dr. Free-Ride: So basically the whole ear is set up to turn sound waves into vibrations, and those into other vibrations, and it's not until you get the special little hairs involved that the vibrations are converted into nerve impulses.

Younger offspring: And the nerve tells the brain about the sound?

Elder offspring: Then maybe the brain sends a signal back to the ear saying, "OK, I got the message!"

Dr. Free-Ride: Really? Do you think the ear needs to get word from the brain that the message was received?

Elder offspring: Actually, maybe the ear is more like a TV station that broadcasts the show even if we don't call to tell them the show is on our TV.

Younger offspring: Some people can't hear, though. If you're deaf, you can't hear the vibrations, but maybe you can feel them.

Elder offspring: If you're deaf, maybe the hairs get stuck.

Dr. Free-Ride: Or maybe there's a problem with some other part of the system.

Elder offspring: That's why deaf people use sign language. But they can read and see perfectly well... unless they're blind.

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Younger offspring: If it has a bone, I agree to the ear.

That is now one of my favourite sprog quotes.

By Donalbain (not verified) on 27 Jul 2007 #permalink

Elder offspring: Then maybe the brain sends a signal back to the ear saying, "OK, I got the message!"

Dr. Free-Ride: Really? Do you think the ear needs to get word from the brain that the message was received?

Your sprog is right on, actually. Outer hair cells receive efferent innervation from the auditory cortex which can serve to enlongate or compress the cell bodies in response to sound. This is theorized to have the effect of dampening some frequencies and enhancing others (think of the crowded room syndrome...)

Your sprog is right on, actually.

I will inform the elder offspring!

Clearly, the signal from the brain back to the hair cells was beyond the scope of the cross-section book (which gives us the opportunity to discuss more and less complete accounts of the same system).

you should tell them that they can hear better than you. Most adults can only hear up to about 10 or 12 KHz. Children can hear all the way up to 20 KHz. Your hearing peaks at about age 12, then it falls off from there. Age does something to those bones in your head such that they no longer resonate at those high frequencies. Spending too much time near aircraft, at rock concerts, or mowing the lawn without hearing protection hurries the process along.

Age does something to those bones in your head such that they no longer resonate at those high frequencies.

Noise and drug damage (called ototoxic stimuli) kill off heair cells, starting from an early age and progressing our whole lives. We're born with all the hair cells we'll ever have, so yes, children do hear higher ranges of frequencies that adults since the hair cells in the high frequency regions are most suspectible to ototoxicity.

When they're a tad older you can set them off on this quest, to learn how and why mammals have three bones in the middle ear while reptiles and birds only have one. One clue, it all started with a hole in the head.

you should tell them that they can hear better than you. Most adults can only hear up to about 10 or 12 KHz. Children can hear all the way up to 20 KHz. Your hearing peaks at about age 12, then it falls off from there.

The linked story describes one practical application of this phenomenon that received some media coverage last year.