The Inner Ear

In the previous article we saw how the structure of the cochlea enabled it to separate sound into its component frequencies: waves travel along the basilar membrane until they reach their characteristic place: high frequencies resonating near the base, and low frequencies reaching further into the cochlea.

On the basilar membrane is the organ of Corti, and inside the organ of Corti there are rows of cells called hair cells. There is one row of inner hair cells (they are called inner because they are close to the axis of the spiral) and three rows of outer hair cells, each about seven micrometres in diameter. There are about 3500 inner hair cells and 12000 outer hair cells in one human cochlea. They are called hair cells because of the filaments called stereocilia (of the order of 0.2 micrometres in diameter) that protrude from the top of them (although they are not actually hairs). As the basilar membrane vibrates up and down, so does the organ of Corti and the hair cells.

The stereocilia of the inner hair cells are free to swing back and forth with the flow of endolymph across the top of the cells, whereas the longest stereocilia of the outer hair cells are embedded in another membrane above called the tectorial membrane. In both inner and outer hair cells, the deflection of the stereocilia tugs on filaments called tip links (of the order of ten nanometres in diameter) that connect adjacent rows of stereocilia. This opens ion channels in the stereocilia.This is where the differing composition of endolymph (at the top end of the hair cell) from perilymph (surrounding the hair cell) becomes important. This difference causes a difference in the potential (the voltage) inside and outside the cell. When the ion channels open, ions (particularly potassium) flow into the cell, and change the electrical potential inside the cell.

In the case of an inner hair cell, this change stimulates a neuron that commences in that cell, which sends a message to the brain so the brain can know that sound containing the frequency corresponding to this place is present. And that is how we detect sound.

Unlike the inner hair cells, the outer hair cells have very few afferent (towards the brain) nerves. We might wonder what could be their function. It has been found that the function of outer hair cells is to amplify sound.

There is a specialised protein (prestin) in the walls of outer hair cells. This protein responds to the change in electrical potential by contracting.

And this contraction exerts a force on the basilar and tectorial membranes that mechanically amplifies the movement that caused the deflection in the first place (the stereocilia themselves can also exert a force). This is a positive feedback mechanism. You’ll know from putting a microphone close to the loudspeaker that plays the amplified microphone signal, that positive feedback can easily result in instability and a deafening screech. However, there are myosin motors in the tip links that continually adjust their tension, so that the hair cells are poised right on the brink of instability, without actually tipping over the brink. (Sometimes they are a bit too close to the brink, and make small spontaneous oscillations that you can actually record if you put a microphone in someone’s ear, though the person owning the ear cannot hear them. These are called spontaneous otoacoustic emissions and are actually quite common in young people. They are quite different from tinnitus, which is not a sound at all, but is neurons firing when they shouldn’t.)

Earlier we compared the cochlear to a set of piano strings, but here there is another difference: the piano strings can only respond in a passive way, whereas the cochlea is active: it expends chemical energy to increase the energy of the resulting vibration to be greater than the sound energy entering the cochlea. It’s a bit like having a person dedicated to every piano string, waiting for their particular frequency: if they hear it, they pluck their string. Not only do the outer hair cells make the sound louder, but they also make the cochlea more finely tuned; able to distinguish sounds from each other, which enables speech to be understood more easily.

This amplification process enables us to hear very, very quiet sounds. Above a certain sound level, the sound energy entering our ear is greater than that provided by the outer hair cells, and so no amplification is necessary.

Both inner hair cells and outer hair cells deteriorate with age and with overuse, but outer hair cell damage is far more common. This is why hearing aids can work: a hearing aid can compensate for the lack of amplification from the outer hair cells (though it cannot provide the tuning capability that the outer hair cells provide). If the inner hair cells have deteriorated, hearing aids are less effective.

It is quite important to take care of your hearing. Those outer hair cells are quite easily damaged by loud sounds. Obviously those who work in loud industrial environments need to take particular care, and so do musical performers, particularly those in symphony orchestras, who are exposed to very loud sounds, but are often reluctant to wear hearing protection because of the precision and nuance that their playing requires.

Many clinicians who work in the hearing field are expecting an epidemic of hearing problems over the next few decades as the young people of today get older, because so many young people spend a lot of time listening to their personal audio devices. Many of us listen to a lot of Bible readings and Bible talks on these, and that can be a good thing, but if you find yourself turning up the volume to hear over the traffic noise, it might be a good time to turn it off, and start thinking about the last point you heard!

This brings us to the lesson of the cochlea, and of the outer hair cells in particular. Outer hair cells are poised and ready, and respond in a dramatic fashion to the sounds that stimulate them. But they only do so in a living cochlea, which is one reason why it has taken researchers such a long time to understand their behaviour.

God’s Word is powerful. When He said, “Let there be light”in Genesis 1:3, there was a response: there was light. What goes out of His mouth does not return void, but prospers in the thing whereto He sent it (Isa 55:11). God’s Word lasts forever (1 Pet 1:25).

God’s voice makes the hinds to give birth (Psa 29:9), and is the force that causes rebirth in a disciple (1 Pet 1:23; James 1:18), working powerfully in believers (1 Thess 2:13). His Word is living and active, able to divide soul from spirit, able to judge the desires and thoughts of the heart (Heb 4:12). Jesus says his words are spirit and life (John 6:63).

But even though God’s Word is powerful, it is possible—even easy—at an individual level to make it ineffective. For a start, we might think we’re hearing it when actually we’re hearing something else (2 Tim 4:3-4). Or more likely, we might hear, but not hear—as Jesus puts it in Matthew 13:13, alluding to Isaiah 6—we might love to hear the Word, but not love to do it (Ezek 33:30-32). When the Word is telling us things we don’t want to hear (such as the effects on others of our wealth acquisition, Zech 7:7-12), we can ‘deactivate’ our ears without even realising that that is what we are doing. The difference between the wise and foolish builders of Jesus’ parable is not what they hear—it is what they do with what they hear (Matt 7:24-27).

Jesus was responsive to the Word. When God opened his ear (Isa 50:4-5) he responded in obedience even though what was commanded was far from pleasant (v6), unlike the servant (Isa 42:18-20; 43:8; 48:8).

Let us be like those outer hair cells. Let us be like Jesus, ready to respond to each whisper of God’s Word that He sends to us.

Oh! give me Samuel’s ear,
The open ear, O Lord,
Alive and quick to hear
Each whisper of Thy word;
Like him to answer at Thy call,
And to obey Thee first of all.