The Inner Ear

In the previous article we saw how sound (the vibration of air on the eardrum) was converted by the middle ear to vibration of the stapes’ footplate on the oval window of the cochlea. Now we move onto considering the cochlea itself. The cochlea is where these vibrations are converted into messages to be sent via the auditory nerve to the brain.

The cochlea is extraordinarily fascinating and complex. It has taken more than 150 years of dedicated research for scientists to understand its intricacies, and even now, this understanding is incomplete; so you might want to read slowly, and take the time to find the various parts in the diagrams.

The word ‘cochlea’ is the Latin for snail, which describes its shape: a coiled tapered tube. If the tube could be uncoiled it would be about 35mm long. In its coiled state, the cochlea is about as big as your thumbnail. This tube is divided by two membranes into three thinner tubes. The oval window and the round window connect at the cochlear base to the outer two of these tubes, which are called the scala vestibuli and the scala tympani, and which are filled with a fluid called perilymph. At the far end (the apex) of the cochlea, these two tubes are connected to each other. The inner of the three tubes is called the scala media, and is filled with a different fluid called endolymph.

A structure on the outside wall of the cochlea, called the stria vascularis, ensures that the perilymph and the endolymph have different concentrations of various ions.

The membrane between the scala media and the scala vestibuli is called Reissner’s membrane. This is very thin, and its only function appears to be simply to be a barrier between the endolymph and the perilymph.

The membrane between the scala media and the scala tympani is much more interesting. It is called the basilar membrane. On the basilar membrane is a structure called the organ of corti. The basilar membrane itself includes fibres running across it like the strings of a piano. These fibres are short (about 0.1mm), stiff and tight near the base of the cochlea, whereas they are longer (about 0.5 mm) and more floppy near the apex (even though the cochlea itself is narrower at the apex). Like piano strings, these fibres can vibrate, with higher frequencies resonating near the base, and lower frequencies near the apex.

If you’ve ever held down the damper pedal of a piano and shouted at the strings, you’ll have experienced a resonance which is a bit like what happens in the cochlea.

However, unlike piano strings, these fibres are strongly coupled to each other by the fluid on both sides of the membrane. When a tone vibrates in the oval window, this sets up a wave which travels along the basilar membrane, growing in amplitude as it gets closer to the position (the ‘characteristic place’) where the frequency of that tone resonates the most. Beyond this position, the wave decays away rapidly.

The fact that different frequencies resonate at different places in the cochlea is called its ‘tonotopic’ property. Nerves convey messages to the brain from every place along the cochlea, and inform the brain of how strong the signal is at all of these places, and thus tell the brain how much of each frequency is present. The travelling wave design is a very efficient design for separating out the various frequencies in the sounds we hear: much more efficient than having a set of filters (one for each frequency that we might want to distinguish).

The travelling wave design does have some drawbacks however. One is that since all the energy has to go through the high-frequency section of the cochlea, this section can tend to get overworked. It might surprise you to know that you can damage the high frequency range of your hearing by being exposed to very loud bass sounds.

Another disadvantage is that there can sometimes be ambiguity regarding the origin of a vibration. Suppose your ear is presented simultaneously with a loud low frequency tone, and a quiet high frequency tone.

The low frequency wave has to travel through the region of your cochlea where the high frequency wave resonates. Even though both resonances occur, your brain will decide that the energy in the high frequency region was simply the result of the low frequency energy passing through the high frequency region, and you won’t hear the higher frequency at all. This phenomenon is called frequency masking.

So far we’ve seen how the cochlea separates sound into its component frequencies, so that the brain can know how much of each frequency is present. In the next article we’ll discuss how hair cells respond to sound—sending messages to the brain, and also amplifying the sound. We’ll find that their readiness to respond provides a good lesson on responsiveness to God’s Word.