A tour of the ear
How does our ear decode the sounds around us? Professor Brian Moore from Cambridge University took Georgia Mills on a guided tour of the ear...
Brian - Sounds enter the ear canal and travel down to the eardrum, which is like a drum of a drum, and that causes the eardrum to vibrate. In turn, those vibrations are transmitted through three tiny bones in the middle ear called the malleus, the incus and the stapes, and these are the smallest bones in the body.
Georgia - When I was in school we called them the hammer, anvil, and stirrup because of their distinctive shapes.
Brian - And a third of those bones - the stapes…
Georgia - That’s the stirrup.
Brian - … makes contact with an organ called the inner ear, which is also called the cochlear, the part that’s concerned with sound. The cochlear is encased in very rigid bone but there’s a small membrane covered opening, and the stapes rests on the top of that opening, so the sound is getting in through that small opening in the bone.
Now the magic starts to happen inside the cochlear; that’s where the analysis of sounds first takes place. The cochlear is filled with fluids and there’s a kind of ribbon that runs along the length of the cochlear that’s called the basilar membrane. The basilar membrane is kind of rather flexible and wide at one end, and it’s narrow and stiff at the other end, and because of those physical properties each place along the membrane is tuned to a different frequency. So the end that’s closest to the stapes is called the base of the cochlear and that responds best to high frequencies, and the other end is called the apex, and that responds best to low frequencies. There’s a kind of an analysis of the different frequencies that are present in the sound.
Georgia - So we have the sound banging the drum, the eardrum, travelling along those three tiny bones before being transferred into the cochlear. On most diagrams this cochlear looks like a snail shell, and inside is this basilar membrane, which separates out sounds to their different pitches. But before we even get to the brain our ear has another trick up its sleeve, one we didn’t know about until recently.
Brian - There’s a very wonderful thing that’s only really been discovered in the last 20 years, and that’s an active biological mechanisms that amplifies the vibrations on the basilar membrane. This active mechanism depends on the operation of specialised types of cells called outer hair cells.
These outer hair cells behave like miniature motors and the feed energy back into the basilar membrane and amplify the response, and they also sharpen up the tuning so each place is much more narrowly tuned to specific frequency than would be the case if you didn’t have this so called active mechanism.
This active mechanism is crucial for letting us hear very soft sounds, and the weakest sounds that we can detect produce a vibration of the eardrum that’s only the size of the diameter of a hydrogen atom. It’s really a very tiny vibration that we can detect and this all depends on the operation of these outer hair cells.
There are still many aspects of these processes that we don’t understand, including this active mechanism in the cochlear. Although we know that it’s there, exactly how it works is still not fully understood. But what we do know is that you can detect its effects relatively easily, and one remarkable thing is an effect called the cochlear echo. When you put the sound into the ear, as a result of the operation of this active mechanism, sounds actually come back out of the ear that you can measure in the ear canal.
Georgia - That’s incredible! So it’s not just our mouths making sounds, our ears are doing it in an albeit very quietly too?
Brian - That’s right. Our ears are actually generating sound. Another interesting thing is that this active mechanism is partly under the control of the brain. The brain actually sends signals down to the cochlear to control the operation of the active mechanism, so even the mechanical vibrations produced on the basilar membrane are partly influenced by the brain. No-one 20 years ago had even thought that that was a possibility that the brain is indirectly controlling what we hear by controlling the operation of the cochlear.
Georgia - So how does this information get transported into our brains?
Brian - Within the cochlear there’s another type of hair cell called the inner hair cells, which are like the microphones of the ear. They’re detecting these vibrations on the basilar membrane and converting them to an electrical signal, and that electrical signal, if it’s strong enough, can lead to what are called nerve spikes or action potentials in the auditory nerve. The more intense the sound is at a given place on the basilar membrane, the more action potentials you get. So information is signalled to the brain as a kind of digital code in terms of these patterns of action potentials in different neurons, and how rapidly those action potentials are occurring.