Ear anatomy

The ear is one of the most sensitive and complicated parts of our body. The earlobe is just the part we can see; the ear actually contains three sections: external, middle, and inner.

The external ear consists of the pinna and the outer ear canal, both of whose size and shape vary from person to person. The pinna is made of cartilage covered with skin; the outer ear canal is a tunnel 2.5 to 3 cm long.

The middle ear starts with the eardrum (tympanic membrane), a small, elastic membrane at the end of the outer ear canal. This is followed by the ossicles, which are three small bones—the hammer (malleus), the anvil (incus), and the stirrup (stapes)—located in the tympanic cavity. The hammer and anvil act as levers and beat against the stirrup. The tympanic cavity connects to the nasal cavity via the Eustachian tube.

The inner ear, or labyrinth, is the most complicated part of the ear. It consists of the following two sections, each with a different function:

• The semi-circular canals, which form the rear labyrinth and help maintain balance; and
• The cochlea, which forms the front labyrinth and helps with hearing. Its name comes from the Latin word for “snail” in reference to its coiled, snail-shell shape. It is filled with a watery liquid and contains the Organ of Corti (a group of hair cells whose motion is communicated to nerve cells via neurotransmitters).

How we hear

Sound waves enter the outer ear canal and strike against the eardrum, causing it to vibrate. This vibration causes the hammer and anvil to rock back and forth, passing the vibration on to the stirrup. The stirrup in turn passes the vibrations to the inner ear and the liquid inside the cochlea, which sets the hair cells of the Organ of Corti into motion. These hair cells convert the acoustic energy (vibrations) into nerve impulses. Fibres in the cochlear nerve then transmit the impulse to the auditory cortex of the brain.

Every day we are bombarded by sounds from all sides, without even realizing it. Fortunately our auditory system can sort these various sounds based on their different frequencies, so that our neurons can process them separately. Our ears play a crucial role in letting our brains distinguish between sound and noise, so that we “hear” only what might be important to us. In other words, our ears help us screen all the information around us, thereby enabling us to pay attention to only what concerns us at a given moment; this is called the discrimination function.