CK12 Life Science

(Marvins-Underground-K-12) #1
the ear.


  1. At the end of the ear canal, the sound waves strike the eardrum. This is a thin
    membrane that vibrates like the head of drum when sound waves hit it.

  2. The vibrations pass from the eardrum to thehammer.This is the first of three tiny
    bones that pass vibrations through the ear.

  3. The hammer passes the vibrations to the anvil, the second tiny bone that passes
    vibrations through the ear.

  4. The anvil passes the vibrations to thestirrup,the third tiny bone that passes vibra-
    tions.

  5. From the stirrup, the vibrations pass to theoval window. This is another membrane
    like the eardrum.

  6. The oval window passes the vibrations to thecochlea. The cochlea is filled with liquid
    that moves when the vibrations pass through, like the waves in water when you drop
    a pebble into a pond. Tiny hair cells line the cochlea and bend when the liquid moves.
    When the hair cells bend, they release neurotransmitters.

  7. The neurotransmitters trigger nerve impulses that travel to the brain through the
    auditory nerve.The brain interprets the sound and “tells” you what you are hearing.


No doubt you’ve been warned that listening to loud music or other loud sounds
can damage your hearing. It’s true. In fact, repeated exposure to loud sounds
is the most common cause of hearing loss. The reason? Very loud sounds can
kill the tiny hair cells lining the cochlea. The hair cells do not generally grow
back once they are destroyed, so this type of hearing loss is permanent. You can
protect your hearing by avoiding loud sounds or wearing earplugs or other ear
protectors.

Balance


Did you ever try to stand on one foot with your eyes closed? Try it and see what happens,
but be careful! It’s harder to keep your balance when you can’t see. Your eyes obviously
play a role in balance. However, your ears play an even bigger role. The gymnast inFigure
20.26may not realize it, but her ears—along with her cerebellum—are primarily responsible
for her ability to perform on the balance beam.


The parts of the ears involved in balance are thesemicircular canals. InFigure20.25,
the semicircular canals are the structures numbered 10. The canals contain liquid, and are
like the bottle of water inFigure20.27. When the bottle tips, the water surface moves up
and down the sides of the bottle. When the body tips, the liquid in the semicircular canals
moves up and down the sides of the canals. Tiny hair cells line the semicircular canals.
Movement of the liquid inside the canals triggers the hair cells to send nerve impulses. The
nerve impulses travel to the cerebellum in the brain. In response, the cerebellum sends
commands to muscles to contract or relax so the body stays balanced.

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