physiology of external and middle ear function

1. HEARING
EXTERNAL & MIDDLE EAR
By :
Dr/ Nanees El-Malkey
Lecturer of medical physiology

2. Ilos:
• Structure and function of external ear
• Structure and function of middle ear

3. External Ear
auricle
External auditory canal
Tympanic membrane

4. Tympanic membrane
• Thin plastic, Conical
• Semi-translucent
• characters :
1. Aperiodic (has no natural frequency)
2. Resonator: vibrate in response to sound waves in
the same frequency
3. Highly damped:
stop vibration when sound waves stop

5. Function of ext. ear
1. Ear pinna ….
2. Auditory canal …..
3. Tympanic membrane:
Collect sound
• amplify sound pressure
• Keep humidity and temperature
• Wax….antiseptic & Lubricate the drum
• Hair……trap dust
• transmit sound to middle ear
• intensify sound pressure

6. Middle ear
• Air filled space in the temporal bone, contains:
3 ossicles (malles- incus- satepes)
2muscles (tensor tempani- stapedius)
2 nerves (trigeminal, fascial)
Austechian tube

7. Eustachian
tube

9. Function of middle ear
1- Accurate transmission of sound waves from the
tympanic membrane to the inner ear through the oval
window.
X

10. • 2-The tensor tympani muscle
• pulls inward the handle of the malleus
• which is attached to the tympanic membrane, this
keeps the membrane continuously tensed which help
optimum vibration and function.

12. 3-Magnification of sound waves about 22 times by:
1-Arrangement of the bony ossicles like a lever system amplify
the force of the stapes over the oval window which magnifies
the sound by about 1.3 folds.
2- Large area of the tympanic membrane in comparison to small
area of the oval window all force of vibration in tympanic
membrane is concentrated over the small area of the oval
window This leads to 17 times increase in sound intensity.
• The total increase in sound intensity caused by
the middle ear = 17 X 1.3 = 22 times

14. 4- The acoustic or tympanic reflex
• Stimulus ……………> loud sounds
• Response………………> contract of both muscles of middle
ear.
• Contraction of tensor tympani pulls on tympanic membrane
makes it more tense thus, reducing its vibration.
• Contraction of the stapedius prevents inward displacement of
the stapes inside oval window, so protect the delicate structures
of the inner ear from high sound.

15. exposure to gun shoots
• Failure of the reflex(due to the total reflex
time)
• causing damage to inner ear structures

16. 5-The Eustachian tube equalizes
the pressure across the two sides
of the tympanic membrane
This facilitates mobility of the
drum which is essential for the
hearing process.

17. The tube takes a curved course in adults
which prevents drinks from entering middle
ear, in children it is more horizontal and
wider

18. If the tube is continuously closed as in common cold, the air
in the middle ear will be absorbed and the tympanic
membrane will be sucked inside middle ear causing severe
pain and impairment of the hearing

19. • The Eustachian tube is normally closed but it opens
during swallowing, yawing or chewing which cause
renewal of the air in the middle ear
• If the Eustachian tube is continuously patent, the voice
of the person will cause great annoying sensation
during talking.
• During traveling by the airplanes chewing gums is
needed to open the Eustachian tube, otherwise,
rupture of the drum may occur.

20. 6. Impedance matching function
Ability of the air in the middle ear to overcome
the resistance of fluid in the inner ear
This is additional function of the magnification
of sound wave pressure (22 times)

21. *It is a flexible membrane connected to the scala tympani of the inner ear.
*This membrane moves with the movement of the basilar
membrane to absorb waves created in the perilymph due to
the movement of basilar membrane.
*Fixation of this membrane leads to hearing loss.
*

22. • Thank you