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	The Ear

The ears contain structures for both the sense of hearing and the sense of balance. The eighth cranial nerve (vestibulocochlear nerve made up of the auditory and vestibular nerves) carries nerve impulses for both hearing and balance from the ear to the brain.

Hearing (Audition) Sound waves cause the tympanic membrane (eardrum) to vibrate. Humans can hear sounds waves with frequencies between 20 and 20,000 Hz. The three bones in the ear (malleus, incus, stapes) pass these vibrations on to the cochlea. The cochlea is a snail-shaped, fluid-filled structure in the inner ear. Inside the cochlea is another structure called the organ of Corti. Hair cells are located on the basilar membrane of the cochlea. The cilia (the hair) of the hair cells make contact with another membrane called the tectorial membrane. When the hair cells are excited by vibration, a nerve impulse is generated in the auditory nerve. These impulses are then sent to the brain. (By the way...the stapes is the smallest bone in the human body. It is only 0.25 to 0.33 cm long [0.10 to 0.13 inches] and weighs only 1.9 to 4.3 milligrams.)

Loudness is measured in decibels (dB) - this is the force of sound waves against the ear. The louder the sound, the more decibels. Here are approximate decibel levels for some everyday sounds: Sound Intensity (db) - Ticking of a Watch 20 Whisper 30 Normal Speech 50-60 Car Traffic 70 Alarm Clock 80 Lawn Mower 95 Chain Saw 110 Jackhammer 120 Jet Engine 130

Did you know?

At a temperature of 68oF (20oC), sound travels at 1,125 feet/sec (343 meters/sec). This is the same as traveling at 756 miles/hr (1,217 km/hr). Also, as the temperature rises, the speed of sound gets faster.

Hearing Loss Loud noises, infections, head injuries, brain damage and genetic diseases may cause people to lose some or all of their ability to hear. Hearing loss is also common in older people. There are several types of hearing loss: Conductive Hearing Loss: occurs when sound vibrations from the tympanic membrane to the inner ear are blocked. This may be caused by ear wax in the auditory canal, fluid buildup in the middle ear, ear infections or abnormal bone growth. Sensorineural Hearing Loss: occurs when there is damage to the vestibulocochlear (auditory) nerve. This type of hearing loss may be caused by head injury, birth defects, high blood pressure or stroke. Presbycusis: occurs because of changes in the inner ear. This is a very common type of hearing loss that happens gradually in older age. For more information about presbycusis, see: Presbycusis Information Tinnitus: people with tinnitus hear a ringing or roaring sound. The cause of this ringing cannot always be found. Some cases of tinnitus are caused by ear wax, ear infections or a reaction to antibiotics, but there are many other possible causes of this disorder. For more information about tinnitus see: OHSU Tinnitus Clinic American Tinnitus Association

Take a short on-line, interactive quiz about the ear. Try some experiments that test your hearing. Lesson Plan about hearing with teacher and student guides. Visit Dangerous Decibels for information, games and activities to protect your sense of hearing. There is a good animated demonstration from KidsHealth showing How the Ear Works. The virtual tour of the ear provides some great information about the ear and hearing disorders. Don't forget that your sense of hearing can be damaged by loud noises...you can read about this "invisible hazard" at the Centers for Disease Control. Audiology Net and the American Academy of Audiology also have basic information about hearing and ear health.

Hear IT!	Cochlea	Decibel	Incus	Malleus
	Stapes	Tympanic Membrane	Vestibulocochlear Nerve

Hearing Ranges
Animal	Low Frequency Limit	High Frequency Limit
Elephant	17	10,000
Human	20	20,000
Cow	23	35,000
Horse	55	33,500
Dog	60	45,000
Monkey	110	45,000
Rat	650	60,000
Mouse	1,000	90,000
Bat	3,000	120,000
(Source: Schiffman, H.R., Sensation andPerception. An Integrated Approach, New York: John Wiley and Sons, Inc., 2001)

Balance (Vestibular Sense) Balance depends on visual information, "feedback" from muscles and from an inner ear structure called the semicircular canal. The semicircular canal is a set of 3 fluid-filled canals that are aligned at right angles to each other. Within parts of the semicircular canal are hair cells. When the head is moved, the fluid in the canals moves the hair cells and a nerve impulse is generated in the vestibular nerve. Don't try this. The world record (according to the Guinness Book of Records, 1996) for balancing on one foot is 55 hr. 35 min.

Did you know?

Of course you know that if you spin around in circles you will get dizzy. But do you know why? When you spin, fluid in the semicircular canals of your ear moves around. This stimulates the hair cells. When you stop spinning, the fluid still moves a bit. Because the fluid is still activating hair cells, your brain stills gets a message that you are moving and you feel dizzy.

Did you know?

200,000 people in the United States are deaf; 3 million people in the US have serious hearing problems. Gallaudet University in Washington, D.C. is an undergraduate institution for deaf and hard of hearing undergraduate students.

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