Now that you understand the characteristics of the sound wave
we're ready to take another look at the auditory system.
How does the ear convert the physical energy of the sound wave into neural impulses that the brain can interpret?
Let's find out.
When a sound wave reaches our head, the outer ear funnels the sound to our eardrum.
The alternating bands or regions of high and low pressure cause the eardrum to vibrate
back and forth at the same rate as the object that produced the sound wave.
The eardrum's vibrations are passed through the middle ear bones, and on to the cochlea.
Now let's take a closer look at the cochlea.
The cochlea resembles a snail shell; a bony spiral filled with fluid.
Let's turn the cochlea's spiral on its side and slice away a portion of the outer wall.
Here we see that the area inside the cochlea is divided into two chambers.
The double outer chamber is shown in pale yellow and the inner chamber is shown in brighter yellow.
Each of the chambers is filled with fluid.
A thin wall called the basilar membrane runs along the whole length of the cochlea
separating the inner chamber from the outer chamber below it.
The basilar membrane is covered by a collection of receptor cells called hair cells.
Their job is to convert the sound waves into neural impulses that the brain can understand
a process called transduction.
At the large end (toward the middle ear) of each section of the outer chamber is an opening, or window
covered with only a thin membrane that is similar to the eardrum.
The third middle ear bone (the stirrup) is attached to the top (oval) window.
When the vibration passes through the middle ear to the cochlea, the oval window moves in and out
causing the pressure in the outer chamber to increase and decrease in a rapid sequence.
The alternating high and low pressure in the outer chamber produces ripples in the basilar membrane.
This movement bends the hair cells, causing them to generate neural impulses.
As the ripple moves down the membrane, one region of the membrane is bent more sharply than the other regions.
This region of activation, shown with a black rectangle, is different for each sound wave.
Most of the neural impulses are generated in the region of greatest activation or bending.
The neural impulses are picked up by the auditory nerve fibers and carried to the auditory cortex of the brain
(in the temporal lobe of each hemisphere) for processing.
Most of the information from the left ear crosses over to the auditory area in the right hemisphere
but a small amount of the information goes directly to the auditory area of the left hemisphere.
The reverse is true for the right ear.
These areas of the cortex process the information and interpret it as a sound.
So the process of hearing begins with a sound wave produced by a vibrating object
and ends with the brain perceiving the sound and its meaning.