11.50: Hearing and Balance

Feeling dizzy?

Have you ever gotten off a spinning ride at an amusement park and felt dizzy? Why does this happen? It actually all goes back to your ears! When you stop spinning, the fluid in the canals of your ears is still moving. Sensing the position of the liquid in the canals usually helps you keep balance, so spinning the fluid throws you off balance.

Hearing and Balance

What do listening to music and riding a bike have in common? It might surprise you to learn that both activities depend on your ears. The ears do more than just detect sound. They also sense the position of the body and help maintain balance.

Hearing

Hearing is the ability to sense sound. Sound travels through the air in waves, much like the waves you see in the water pictured below (Figure below). Sound waves in air cause vibrations inside the ears. The ears sense the vibrations.

Figure \(\PageIndex{1}\): Sound waves travel through the air in all directions away from a sound, like waves traveling through water away from where a pebble was dropped.

The human ear is pictured below (Figure below). As you read about it, trace the path of sound waves through the ear. Assume a car horn blows in the distance. Sound waves spread through the air from the horn. Some of the sound waves reach your ear. The steps below show what happens next. They explain how your ears sense the sound.

Figure \(\PageIndex{2}\): Read the names of the parts of the ear in the text; then find each of the parts in the diagram. Note that the round window is distinct from the oval window.

1. The sound waves travel to the ear canal (external auditory canal in the figure). This is a tube-shaped opening in the ear.
2. At the end of the ear canal, the sound waves hit the eardrum (tympanic membrane). This is a thin membrane that vibrates like the head of a drum when sound waves hit it.
3. The vibrations pass from the eardrum to the hammer (malleus). This is the first of three tiny bones that pass vibrations through the ear.
4. The hammer passes the vibrations to the anvil (incus), the second tiny bone that passes vibrations through the ear.
5. The anvil passes the vibrations to the stirrup (stapes), the third tiny bone that passes vibrations through the ear.
6. From the stirrup, the vibrations pass to the oval window. This is another membrane like the eardrum.
7. The oval window passes the vibrations to the cochlea. 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.
8. The neurotransmitters trigger nerve impulses that travel to the brain through the auditory nerve (cochlear nerve). The brain reads 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. But your ears play an even bigger role. The gymnast pictured below (Figure below) may not realize it, but her ears—along with her cerebellum—are mostly responsible for her ability to perform on the balance beam.

Figure \(\PageIndex{3}\): This gymnast is using the semicircular canals in her ears, along with the cerebellum in her brain, to help keep her balance on the balance beam.

The parts of the ears involved in balance are the semicircular canals. Above, the semicircular canals are colored purple (Figure above). The canals contain liquid and are like the bottle of water pictured below (Figure below). 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 causes the hair cells to send nerve impulses. The nerve impulses travel to the cerebellum in the brain along the vestibular nerve. In response, the cerebellum sends commands to muscles to contract or relax so that the body stays balanced.

Figure \(\PageIndex{4}\): This bottle of water models the semicircular canals in your ears. When you tip the bottle, the water moves up or down the sides of the bottle; when you tip your head, the liquid inside the semicircular canals moves up and down the sides of the canals. Tiny hair cells lining the canals sense the movement of liquid and send messages to the brain

Summary

1. At the end of the ear canal, sound waves hit the eardrum, which passes vibrations through a series of bones to the cochlea.
2. The parts of the ears involved in balance are the semicircular canals.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

• Hearing and Balance at http://www.youtube.com/watch?v=vTiGskc1o48 (1:02)

1. When sound enters your ear, it starts your eardrum vibrating. Where do the vibrations go next?
2. What part of your ear contains fluid?
3. Where are the nerve sensors that communicate with the brain located in your ear? What else is found in this same area of the ear?

Explore More II

• The Sense of Balance at http://www.youtube.com/watch?v=mmBB2bu1gEQ

1. What gives us our sense of balance?
2. What is a kinocilium? How is this involved in your sense of balance?

Review

1. How does sound travel through air?
2. Which structure in the ear changes sound waves in the air into vibrations?
3. What are the three tiny bones in the ear?
4. Which parts of the ear sense changes in the body’s position? How do they do this?
5. Why does death of hair cells in the cochlea cause hearing loss?