Hearing Mechanism

Hearing is one of the most important of the human senses. It is essential for the location of sounds that may warn us of danger, the enjoyment of pleasant sounds such as music and the natural environment and, most importantly for humans, the development of speech and language for communication.

The hearing mechanism is our only sense which never shuts off and which receives input from all directions, giving us a continual connection with and warning of our surroundings. Unlike vision, which can be shut off by simply closing our eyes, hearing can only be partially protected from continual high intensity noise by complex central nervous system (CNS) mechanisms. Our ears just weren’t designed to be able to cope with some of the noises, which have been introduced to our environment since the industrial revolution. The result for many people is partial deafness.

How we hear:

This slide shows a cross section of the ear. Many of you will be very familiar with how the normally hearing ear works but I thought Id go through this briefly as it will lead into my explanation of how the implant works. The ear is divided into three sections the outer ear, the middle ear and the inner ear. The outer and middle parts of the ear are the mechanical parts of the ear and the inner ear or cochlea is the electrical part of the ear.

So how does the normally hearing ear hear?

Sounds waves travel into the outer ear and cause the ear drum to vibrate. This causes the three small bones in the middle ear to vibrate which causes the fluid in the inner ear to vibrate. Here you can see the hearing nerve endings where they are embedded into the cochlea.

In the cochlea there are tiny hair cells which at one end are embedded in the nerve and at the other end are suspended in the fluid. The movement of the fluid in the cochlea causes the tiny hairs to vibrate which in turn causes stimulation to the nerve.

So essentially this is where the mechanical energy is changed into electrical energy it is the the mechanical-electrical interface.

Once the hearing nerve is stimulated the electrical pulses travel along the nerve up to the brain and we hear a sound.

If there is a hearing loss in the outer or middle ear, it can often be treated or can be helped by the use of a hearing aid.

If there is a hearing loss in the cochlea the usual cause is damage to the hair cells. Either the hair cells are damaged or missing completely. There may be a few hair cells left but usually not many. The effect of this is that we lose the interface where the mechanical energy is changed into electrical energy. So the mechanical energy enters the inner ear but stops there and is not transmitted to the nerve.

We can put a hearing aid on to the ear which will make the sound louder and the vibration stronger but with the hearing aid we are still relying on the remaining hair cells to change this louder vibration into electrical energy.

The hearing aid makes the sound louder which will be effective for some hearing losses but when the hearing loss is severe to profound the hearing aid may not be enough. That is when you will consider a CI.

What the cochlear implant does is bypass the damaged hair cells i.e. the CI directly stimulates the hearing nerve so it does not matter if the hair cells are damaged or missing.