EAR

I.   INTRODUCTION
*  hearing (audition): detecting sound waves and converting them to nerve impulses

*  equilibrium (balance): detect position of head

II.  EXTERNAL EAR

A.  Auricle (pinna):  outer mass of tissue
*  function:

B.  External Auditory Canal:  between auricle and tympanic membrane
*  function:

C.  Tympanic Membrane (Eardrum):

*  Function:

III. MIDDLE EAR
*  lateral (external) boundary is the tympanic membrane
* medial (internal) boundary is oval window and round window of inner ear

A. Auditory Ossicles (ear bones):

1.  Malleus:  (hammer)

2.  Incus: (anvil)

3.  Stapes: (stirrup) - connects with oval window of the inner ear
**  the oval window transmits sound waves to the inner ear

 

Function of the ossicles:

B.  Auditory (pharyngotympanic, Eustachian) Tube:  connects middle ear with the nasopharynx

Function:

IV.  INNER EAR

1)  bony labyrinth:  SPACES/cavities inside temporal bone
2)  membranous labyrinth: membranous sacs/tubes that rest inside the bony labyrinth

–  between bony and membranous labyrinth is a fluid called perilymph

–  inside the membranous labyrinth is a fluid called endolymph

–  Bony Labyrinth consists of

1)  Vestibule: oval central portion
2)  Semicircular Canals:  three circular spaces/canals
3)  Cochlea:  spiral chamber

–  Membranous Labyrinth consists of

1)  Utricle:  sac inside vestibule

2)  Saccule:  another sac inside vestibule

3)  Semicircular Ducts:  three membranous tubes in semicircular canals
4)  Cochlear Duct (Scala Media):  membranous tube inside the cochlea

–  utricle, saccule, and semicircular ducts responsible for interpreting equilibrium (balance)
*transmit this information via the vestibular branch of the vestibulocochlear nerve (CN VIII)

V.   COCHLEA & COCHLEAR DUCT

–  The cochlear duct (scala media) splits the cochlea into two chambers:

1)  scala vestibuli: above cochlear duct

*  oval window

 

*  vestibular membrane

 

2)  Scala Tympani: below cochlear duct

*  round window

 

*  basilar membrane

–  cochlear duct contains a structure called the spiral organ (Organ of Corti)
*  spiral organ rests on basilar membrane

*  spiral organ contains hair cells, which have stereocilia

–  When sound waves enter via the scala vestibuli, they “bounce” a small portion of the basilar membrane

–  When the basilar membrane moves, the sterocilia of the hair cells touch the tectorial membrane
*  bending of the stereocilia generates a nerve impulse
*  nerve impulses are sent to the brain via the cochlear branch of the vestibulocochlear nerve (CN VIII)

–  Remaining sound waves are transferred to the scala tympani and exit the inner ear via the round window

VI.  BRIEF REVIEW OF HEARING

1)  Sound waves are collected by auricle, travel in external auditory canal, sent to tympanic membrane

 

2)  Vibration of tympanic membrane causes vibration of auditory ossicles, which in turn vibrates the oval window

 

3)  Sound waves travel through scala vestibuli, and “bounce” a portion of the basilar membrane

4)   Basilar membrane vibrates, and stereocilia on hair cells bend against tectorial membrane
-- nerve impulse generated

5) Nerve impulse is sent to brain via the cochlear branch of the Vestibulocochlear nerve (CN VIII)

6) Remaining sound waves transferred to the scala tympani and leave the inner ear through the round window

All of the following structures are located in the inner ear EXCEPT:

a.  vestibule

b.  cochlea

c.  auditory tube

d.  semicircular canal

e.  utricle

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