Hearing Basics & Deafness
THE ANATOMY OF THE EAR

The ear consists of 3 sections:

i) the outer ear
ii) the middle ear
iii) the inner ear
The outer ear

The outer ear comprises the auricle and the external auditory canal.

The auricle is the part we can see. It has the job of collecting sound waves and conducting them to the canal. The external auditory canal leads from the auricle to the eardrum. Special glands can pump wax into the canal to trap specks of dirt or insects, and a few hairs are present to help.
 
 

The middle ear

The middle ear is a small cavity. In many respects it is like a narrow oblong house. Its outer wall is formed by the ear drum. The inner wall is mainly bone but it has two windows - the oval window, and the round window. It also has two openings, the Eustachian tube from the nose, and a link with air cells in the skull.

The Eustachian tube connects the middle ear to the outside air. This should ensure that the air pressure is the same on each side of the eardrum. However, if you have a cold, the tube can block and the unequal pressure can cause temporary deafness. The temporary deafness or "blocked ears" experienced at high altitudes is also due to a pressure difference.

The Eustachian tube usually opens when you chew, swallow, yawn etc., which activates the muscles around the tube. So at high altitudes when the atmospheric pressure is low, it requires a lot of swallowing-type action to open the Eustachian tube, release the air, and thereby equalise the pressure each side of the eardrum.

There are three very important bones in the middle ear. They are called the malleus (or hammer bone), the incus (or anvil bone)and the stapes (or stirrup bone).
 
 

The bones of the middle ear

The handle of the 'hammer' is firmly attached to the eardrum, and the stapes is connected to the oval window of the inner ear.

Sound waves make the ear drum vibrate. The three bones of the middle ear then transfer the sound to the inner ear by vibrating themselves.
 
 

The inner ear

The oval window separates the middle ear from the inner ear. Inside the oval window is the vestibule or entrance. This leads into the cochlea (little shell), and the semi-circular canals.

The cochlea looks like the shell of a small snail. It contains the organ of hearing that connects to the auditory nerve leading to the brain.

The semi-circular canals are three canals set at right angles to each other. The movement of fluid in these canals sends messages to the brain helping you to keep your balance.

The whole inner ear, including the vestibule, cochlea and semi-circular canals are filled with fluid. The vibrating bones in the middle ear make ripples in this fluid, which in turn stimulates the nerves.

There is a great site with lots of information and animation about how we hear.
 
 

CAUSES OF DEAFNESS

There are two main types of deafness:

Conductive deafness -   caused by some blockage of sound to the inner ear. Infections of the middle ear give rise to many cases of conductive deafness. Surgery, medication and/or the use of a hearing aid can often help this type of deafness.

Nerve deafness (or sensioneural deafness)occurs in the Inner ear and involves the area of transmission of sound waves to the brain. This is a far more complicated problem than conductive deafness. Surgery or medication cannot correct it. Cochlear implants are sometimes used to for this type of deafness.

Sometimes children are born deaf. This is usually called congenital deafness.

Some of the cause of congenital deafness are:-

1. Rubella, or German Measles. When contracted by women during pregnancy it can have tragic consequences for the child, including deafness, mental retardation, blindness and spasticity. WOMEN CAN BE IMMUNIZED AGAINST RUBELLA. The Immunization should last several years, but a blood test should be taken 3 months before pregnancy to be sure of that immunity.

2. Hereditary deafness is caused through genetic factors. Most hereditary deafness results in the inner ear not developing properly.

3. Influenza, mumps and other infectious diseases may also contribute to congenital deafness.

4. Some drugs, such as quinine, may also contribute to the problem.

5. The Rh factor in blood also causes congenital deafness. It can be avoided by prior injection, and corrected by blood transfusions before and after birth.
6. Jaundice, and the supply of too much oxygen to aid recovery at birth, have also been implicated.
Deafness causes after birth is called adventitious deafness. Some diseases that cause adventitious deafness are meningitis, mumps and chicken pox.

Other causes of deafness after birth, are: Injury, Age, and Excessive noise