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Hearing

The Outer (External) Ear

Adaped from Physiology 1999-2000 Student presentations given by by Jennifer Lau, Pency Mak, and Susana Leung, Spring 2000.

The Ear's outer part has two parts:

  1. Pinna - The appendage on the side of the head surrounding the opening of the external auditory canal.
  2. Canal - A curving tube about 2.5 cm in length. It extends from the temporal bone and ends at the tympanic membrane (the eardrum).

The skin of the auditory canal, expecially the outer 1/3, contains many short hairs and creuninous glands that produce a waxy substance called cerumne that may collect in the canal and impair hearing by asorbing or blocking the passage of sound waves. . . .

SOUND WAVES travel throught he external auditory canal, strikes the tympanic membrane and causes it to vibrate . . . .

The Middle Ear

After sound has traveled through the outer ear and hits the tympanic membrane, the sound wave moves from one ossicle to another through the middle ear.

The middle ear has three bones (aka ossicles), which are named for their shapes.

  1. Tympanic Membrane
  • Separates the outer and middle ear.
  • Vibrates when sound hits it.
  1. Mallus (hammer)
  • End of the mallus is attached to the tympanic membrane.
  • Head of the mallus is attached to the incus.
  1. Incus (anvil)
  • Incus is attached to the stapes.
  1. Stapes (stirrup)
  • Stapes is attached to the oval window.
  • Oval window separates the inner ear from the middle ear.
  • The ossicles amplify sound waves, and transmits them to the inner ear.
  • Auditory tube leads to the throat. It helps the ear pop at different levels of pressure, equalizing pressure in the middle ear.
  • Movement of the stapes against the oval window causes fluid to be released.

The Inner Ear

Hearing: the activation of specialized mechanoreceptors in the inner ear generates nervous impulses that result in hearing.

 

Mechanoreceptors - receptors that are mecahnical in nature (e.g. balance).

Bony Labyrinth Perilymph: Watery fluid that fills bony space of ear.

The three spaces in the ear:

    1. Vestibule
    2. Semicircular canal
    3. Cochlea

The Vestibular is a small round chamber about 1/2 inch long. Its bony walls connect the semicircular canals and cochlea.

The cochlea detects sounds. Lying within the cochlea is the organ of Corti (the organ of hearing). The organ of corti is surrounded by a cochlear duct, which is filled by endolymph. When the endolymph is bent by sound waves, specialized hair cells on the organ of Corti generates nerve impulses, which is sent to the brain through the Cochlear nerve.

The Semicircular canal is located in the inner ear, containing specialized receptor cells called crista ampullars that generates a nerve impulse on movement of the head. The hair cells detects sounds, then they send the information through the cochlear nerve to the brain.

The inner ear also detects changes in your balance and head position. The heart of this detection system is the semicircular canal, which is filled with fluid. It is the movement of this fluid that detects changes in the position of the head, and contributes to the sense of balance.

The semicircular canal consists of 3 canals, each forming 2/3 of a circle. These canals is a fluid filled duct that contains 20,000 hair-like cells, called crista ampullaris.

When the head moves, that movement moves the fluid in the semicircular canal bending the crista ampullaris. When the crista amphularis bends, it sends a signal to the brain through the the vestibular nerve.

Hearing Impairments

Conductive disorders: The way the ear transmits sound is impaired; these disorders are generally correctable through surgery or medication.

Sensorineural disorders: The auditory nerve, by which we receive and process signals from sound, is impaired; these disorders are generally not correctable through amplification or hearing aids.

Middle Ear Deafness:

The Rinne test is used to check for middle-ear deafness. This test works because sound can be conducted (travel) through bones. In a person with middle-ear deafness the malleus, incus, and stapes are often at least partially fused and do not move properly when sound waves traveling through air hit the tympanic membrane. Such an individual will not be able to hear sound through their typmpanic membrane, but can hear sound that travels through their skull.

To conduct the Rinne test, do the following:

(From Malasanos, L., Barkauskas, V., Moss. M., and Stolen berg-Allen, K.: Health assessment, ed. 2, St. Louis, 1981, The C.V. Mosby Co.)

  1. Activate a tuning fork by striking it against your hand. (Note: striking a tuning fork against a hard surface can damage the tuning fork.)

 

  1. Touch the handle (base) of the vibrating tuning fork firmly to the mastoid process of your temporal bone.

(From Thompson, J.M., and Bowers, A.C.; Clinical manual of health assessment, St. Louis, 1980, The C.V. Mosby Co.)

  1. Now check for the individual's ability to hear through air conduction.