Hearing- in a Nutshell

Managing hearing loss begins with understanding your hearing and how it works. Let’s talk about the hearing system. I call it a hearing system because a sound has quite a way to go once it enters your ear canal. We can divide the hearing system roughly into four parts; the outer ear, the middle ear, the inner ear, and the brain.

 

The Outer Ear

The outer ear has three parts. The pinna, the ear canal, and the eardrum. The part of the ear we can see on either side of our head, is called the pinna. It isn’t shaped the way it is by chance. The job of all those creases and crevices is to collect the sound and send it into the ear canal.

The sound travels through the ear canal and reaches the eardrum, which sits at the end of the ear canal. When the sound hits the eardrum it begins to vibrate. This initiates a series of events as the vibration travels to the middle ear.

 

The Middle Ear

The middle ear is an air-filled space that lies between the outer ear and the inner ear. It contains the ossicles- the smallest bones in the human body. These bones are called the malleus (hammer), incus (anvil), and stapes (stirrup). If you examine each one separately they resemble the objects they were named for.

The job of the middle ear is to amplify the sound and send it onward to the inner ear. The middle ear has to make up for the fact that the sound will be traveling from an air-filled middle ear to a liquid-filled inner ear.

The middle ear accomplishes this task by exerting a large amount of pressure on the opening to the inner ear using the middle ear bones. See the diagram below for an illustration of how the middle ear succeeds in increasing adequate pressure for the transfer of sound to the inner ear.

First, the structure of the bones creates a lever-action to apply a large force on the entrance to the inner ear.

Second, the pressure from the larger-sized eardrum, on one end of the ossicle chain, is exerted on the smaller-sized opening into the inner ear (oval window) on the other end of the chain. In order to understand how this second method works, imagine standing on the beach with your bare feet flat on the soft sand. Now imagine standing on your tippy-toes. You would sink further into the sand because you are putting more pressure on a smaller area than when your feet were flat. This is what happens in the middle ear.

 

 

The pressure from the middle ear moves the fluid inside the cochlea (snail-shaped structure) which sits in the inner ear.

 

The Inner Ear

The inner ear is home to three overlapping canals (semi-circular canals) and the cochlea. The semi-circular canals are part of the balance, not hearing mechanism, and will not be discussed here.

The cochlea (the snail-shaped structure) has three sections. The middle section of the cochlea contains about 15,000 tiny hair cells bathed in a liquid called endolymph, a fluid that is unique to the cochlea and does not exist elsewhere in the body.

As the last bone in the ossicle chain, the stirrup, moves in and out of the oval window, vibrations occur in the endolymph. The tiny hair cells lining the cochlea are stimulated. The hair cells located at the apex of the cochlea (the outer part) are attuned to high pitch sounds. The hair cells located at the base (inner-most part) of the cochlea are attuned to the lowest pitch sounds (bass). It’s similar to a piano keyboard with each piano key corresponding to one note.

This organization of hair cells gives us the ability to distinguish between pitches. That ultimately enables us to understand speech and enjoy music.

The job of the inner ear is to convert sound into electrical impulses which travel through the auditory nerve (8^th cranial nerve) to the brain.

 

The Brain

In the brain, the auditory cortex processes the sound you hear. But it doesn’t stop there. Brain functions like working memory and attention are vital for making sense of what we hear. For example, simultaneously hearing an address and entering it into your car’s navigation system requires overall brain function in order to make sound useful.

 

Changes in the Hearing System

What is happening in the hearing system that causes hearing loss? There are three types of hearing loss: conductive, sensorineural, and mixed.

1. Conductive Hearing Loss

Conductive hearing loss involves structures of the outer and middle ear and occurs when there is something along the hearing pathway that blocks the sound from traveling. Some examples are:

• Ear wax
• Foreign object in the ear canal
• Separation of the ossicular chain
  • Usually results from head trauma
• Ear infection
  • Middle ear air space fills with fluid
• Genetic or developmental disorders
  • For example, atresia is when a baby is born without an ear canal opening.

Most conductive conditions can be medically treated. Foreign objects can be removed; pressure equalization tubes can be placed in the eardrum to prevent fluid build-up in the middle ear. Surgical options are available for reconstructing certain parts of the outer ear.

Without surgery or medical intervention, a conductive hearing loss can sometimes become permanent. Conductive hearing loss can be quite significant causing up to a 60 decibel decrease in hearing in the affected ear.

2. Sensorineural Hearing Loss

Sensorineural hearing loss involves the structures in the inner ear (sensory) or beyond (neural-structures of the brain or 8^th cranial nerve). Some causes are:

• Aging
• Noise Exposure
• Genetic variants
• Benign tumor on the auditory nerve (called an acoustic neuroma, this is a rare condition)

Sensorineural hearing loss, by far more common than conductive hearing loss, is usually permanent and cannot be treated medically.

3. Mixed Hearing Loss

Mixed hearing loss is when two conditions exist at the same time. One causes a conductive hearing loss, and the other causes a sensorineural hearing loss.

• An older adult with underlying hearing loss from aging and ear wax would present with a mixed hearing loss.
• Sudden acoustic trauma can cause mixed hearing loss. Sometimes an extremely loud sound like an explosion near the ear can cause damage to the structures of all the parts of the hearing system at once; a hole in the eardrum and separation of the ossicular chain would result in the conductive component while damage to the hair cells in the cochlea would result in the sensorineural component.

 

*****

 

Now that you understand what is happening in your hearing system, learn about tests of hearing measurement and how to interpret them. Tune-in for my next post on this important subject.

 

Thanks for reading. For more information on this and other topics related to hearing and hearing management, please visit my website: www.experthearingadvice.com

For more information on this and other topics related to hearing and hearing management, please visit

See all hearing advice →