WEBVTT

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Now that you understand the characteristics of the sound wave

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we're ready to take another look at the auditory system.

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How does the ear convert the physical energy of the sound wave into neural impulses that the brain can interpret?

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Let's find out.

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When a sound wave reaches our head, the outer ear funnels the sound to our eardrum.

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The alternating bands or regions of high and low pressure cause the eardrum to vibrate

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back and forth at the same rate as the object that produced the sound wave.

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The eardrum's vibrations are passed through the middle ear bones, and on to the cochlea.

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Now let's take a closer look at the cochlea.

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The cochlea resembles a snail shell; a bony spiral filled with fluid.

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Let's turn the cochlea's spiral on its side and slice away a portion of the outer wall.

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Here we see that the area inside the cochlea is divided into two chambers.

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The double outer chamber is shown in pale yellow and the inner chamber is shown in brighter yellow.

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Each of the chambers is filled with fluid.

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A thin wall called the basilar membrane runs along the whole length of the cochlea

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separating the inner chamber from the outer chamber below it.

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The basilar membrane is covered by a collection of receptor cells called hair cells.

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Their job is to convert the sound waves into neural impulses that the brain can understand

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a process called transduction.

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At the large end (toward the middle ear) of each section of the outer chamber is an opening, or window

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covered with only a thin membrane that is similar to the eardrum.

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The third middle ear bone (the stirrup) is attached to the top (oval) window.

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When the vibration passes through the middle ear to the cochlea, the oval window moves in and out

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causing the pressure in the outer chamber to increase and decrease in a rapid sequence.

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The alternating high and low pressure in the outer chamber produces ripples in the basilar membrane.

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This movement bends the hair cells, causing them to generate neural impulses.

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As the ripple moves down the membrane, one region of the membrane is bent more sharply than the other regions.

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This region of activation, shown with a black rectangle, is different for each sound wave.

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Most of the neural impulses are generated in the region of greatest activation or bending.

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The neural impulses are picked up by the auditory nerve fibers and carried to the auditory cortex of the brain

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(in the temporal lobe of each hemisphere) for processing.

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Most of the information from the left ear crosses over to the auditory area in the right hemisphere

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but a small amount of the information goes directly to the auditory area of the left hemisphere.

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The reverse is true for the right ear.

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These areas of the cortex process the information and interpret it as a sound.

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So the process of hearing begins with a sound wave produced by a vibrating object

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and ends with the brain perceiving the sound and its meaning.