Journey of Sound to the Brain


Have you ever wondered how sounds make their way from the source, [Trumpet playing]
all the way to your brain? Take a trumpet, for instance. When it’s played, it makes sound waves in the air. The outer ear catches the waves, which then travel through a narrow passageway called the ear canal. The sound waves reach the eardrum, which is a membrane roughly half the size of a dime. They make the eardrum vibrate, which in turn vibrates three tiny bones called the malleus, incus, and stapes. These bones amplify, or increase, the sound vibrations and send them to the cochlea. The cochlea is shaped like a snail and is the size of a garden pea. It is filled with fluid and the sound vibrations make this fluid ripple, which creates waves. Hair-like structures, called stereocilia, sit on top of hair cells and are grouped together as hair cell bundles inside the cochlea. The hair cells inside the cochlea ride these waves and the hair bundles are moved. The hair bundle on top of the hair cell turns these movements into electrical signals. As the hair bundles are moved, ions rush into the top of the hair cells, causing the release of chemicals at the bottom of the hair cells. The chemicals bind to the auditory nerve cells and create an electrical signal, which travels along the auditory nerve to the brain. Different hair cells respond to different frequencies of sound. The hair cells at the base of the cochlea detect higher pitched sounds, such as a piccolo or flute. [Piccolo playing] The hair cells toward the top of the spiral detect progressively lower pitched sounds, such as a trumpet or trombone. [Trumpet playing] At the very top, or apex, of the spiral, the hair cells detect the lowest pitched sounds, such as a tuba. [Tuba playing] The auditory nerve carries the electrical signal to the brain, which interprets the messages as sounds that we recognize and understand. [Trumpet playing]

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