The way in which the ear is shaped enables people to process sounds. Therefore, there are many different parts in the ear, each with its own function.

The structure of the ear is as follows:

Outer Ear

• Pinna = auricle: collects sounds

The shape of the pinna helps us hear better when sounds are coming at us from the front. The shape of the pinna also enables us to localise sounds.

• Auditory canal : reflects the sound waves

 Middle Ear

• Eardrum
• Ossicles
  - Malleus = hammer
  - Incus = Anvil
• Stapes = stirrup
  - The bottom of the stapes is called the footplate, and moves in and out of the oval window.

Inner Ear

• Oval window
• Cochlea = cochlea
  - Here, the vibrations of sound waves are converted into electrical signals that can be processed by neurons.

In order to transmit sound waves in the ear, the signals need to be amplified. This is because the cochlea is filled with fluid, which means that small vibrations are not passed on properly. The ossicles provide this amplification of the waves by acting as a lever. As a result, the signals entering the ear are amplified approximately 20 times between the eardrum and the oval window.

There are two muscles attached to the ossicles:

• Tensor tympani muscle: attached to the hammer
• Stapedius muscle: attached to the stirrup

When these muscles contract, the ossicles become much less movable, making it more difficult to amplify the signals. The contraction of these muscles is called the attenuation reflex, and aims to protect the auditory system.

The cochlea is a spiral-shaped structure that is filled with fluid inside. At the beginning of the cochlea we find two membranes, the round window and the oval window.

When a sound reaches the oval window, the basilar membrane starts to move. You can compare this to the swinging of a rope. When the frequency of a sound is high, it does not travel far across the basilar membrane. Low frequency sounds can travel to the end of the cochlea.

The basilar membrane contains auditory receptors. The place where these receptors are located is called the organ of Corti. On the organ of Corti there are hair cells. Each hair cell has about 100 protrusions, which are called stereocilia. When the basilar membrane moves, the stereocilia of the hair cells move too. The stereocilia of the different hair cells move as a unit, i.e. all at the same time and in the same direction. The hair cells are designed to depolarize when they move in one direction and hyperpolarize when they move in the other direction.

Hair cells have connections to spiral ganglion cells. These ganglion cells transmit their axons to the auditory nerve. The auditory nerve projects to the ventral and dorsal regions of the cochlear nucleus. From there, the ventral cochlear nucleus sends signals to the superior olive, and from the olive, the axons go to the inferior colliculi. From the inferior colliculi, the signals go to the medial geniculate nucleus (MGN) of the thalamus, and from there, of course, to the auditory cortex.

We can localise sound in different ways:

1. Time difference between ears (for sounds from 20 to 2000 Hz): it takes about 0.6 ms for a sound wave to travel from one side of the head to the other. If a sound comes directly from the right or directly from the left, it is thus received 0.6 ms earlier in one ear than in the other.
2. Intensity difference between ears (for sounds from 2000 to 20000 Hz): your head blocks part of the sound waves. When a sound comes directly from the right, there will be a difference in volume between the sound that enters the right ear and the sound that enters the left ear (in the right ear, the sound will be somewhat louder).

Comparing the input of the two ears (using the time difference or intensity difference) is useless to locate a sound vertically. However, it is possible for humans to localise sound in this way. This is possible because of the way our ear is shaped. The ear reflects sound waves in a specific way, depending on the angle at which the sound enters the ear.

Author: Myrthe Princen (translated by Thomas von Rein )
Image: Marcel Loeffen