1. Miguel B. Sebastian Jr. M.D., D.P.P.S.
Department of Physiology
OLFU

2.  The Human Ear

6.  Function of the muscles and ossicles
 Functions
◦ Protects the cochlea from damaging vibrations by
excessively loud sounds
◦ Mask low frequency sounds in loud environments
◦ Decrease persons sensitivity to his or her own
speech
◦ Latency – 40 to 80 milliseconds
◦ Contraction of tensor tympani and stapedius
dampens the movement of ossicles and
decreases the sensitivity of acoustic apparatus
◦ Reduces intensity of sound transmission by 30 –
40 dB

7.  Impedance Matching Device (tympanic
membrane and ossicles)
 Acoustic impedance higher in fluid-filled
cochlea than in air
 Depends on the following
◦ Ratio of the surface area of large tympanic
membrane to that of the smaller oval window
◦ Mechanical advantage of the lever system formed
by the ossicle chain

8.  Eustachian tube – equalizes pressure
differences between external ear and middle
ear
 Infection: fluid collects in middle ear  E.T.
blocked  pressure differences 
pain, displacement of the Tympanic
Membrane  rupture

14.  Sound
◦ Produced by compression and decompression
waves that are transmitted in air or in other elastic
media such as water

15.  Sound Frequency
◦ Measured by cycles per second – hertz (Hz)
◦ Travels more slowly than light
 Light – 300,000 km/sec (186,000 miles/sec)
 Sound – 0.331 – 0.334 km/sec (0.2 miles/sec) – at 20
oC at sea level (↑ with temperature and altitude)
- 335 m/sec in air (Berne & Levy)
 Speed of sound: Solid > Liquid > Air

16.  Sound wave
◦ Expressed as Sound Pressure Level (SPL)
◦ Decibel (dB)
◦ SPL = 20 log P/Pr
P= sound pressure
Pr = reference pressure (0.0002 dyne/cm2)
- the absolute threshold for human
hearing at 1000 Hz

17.  Normal Human Ear
◦ Sensitive to pure tone with frequency of 20 to
20,000 Hz
◦ >100 dB – damage the auditory apparatus
◦ >120 dB – pain and permanent damage
* As people age, their threshold at high frequency
rises thereby reduces the ability to hear such tone
(presbycusis)

20.  Sound waves  cause T.M. to oscillate  Oscillations
transmitted to the scala vestibuli  creates a pressure
difference between the S.V. and S.T.  displaces the basilar
membrane  stereocilia of the hair cells bend.
 Upward displacement bends the stereocilia toward the tallest
cilium (away from the modiolus) depolarizes the haircells.
 Downward deflection – bends the stereocilia away -
hyperpolarized

22. • apex is wider than the
base
• tension is higher at
the base than at the
apex
• base vibrate at higher
frequency than the
apex (frequency
analyzer)

23. • length of the fibers is
greater at the apex than at
the base
• fiber diameter is greater at
the base than at the apex
• base -- shorter and
wider
• apex – taller and slender
• high –frequency resonance
(base), low frequency
resonance (apex)

25.  Ossicular Conduction
◦ Main pathway for normal hearing
 Air Conduction
◦ Unimportant for normal hearing
◦ Mediated by vibration of round window
 Bone Conduction
◦ Involves skull bone
◦ Plays a role in transmission of extremely loud
sounds

26.  Presence of one sound decreases an
individual’s ability to hear other sounds
 Due to the relative and absolute
refractoriness of previously stimulated
auditory receptors and nerve fibers to other
stimuli

27.  Brodmann’s area 22
◦ Concerned with the processing of auditory signals
related to speech
◦ Right side – melody, pitch and sound intensity
 Planum Temporale
◦ Portion of posterior superior temporal gyrus
◦ Involved in language-related auditory processing

30.  Conductive Deafness
◦ Due to impaired sound transmission in external and
middle ear
◦ Impacts all sound frequencies
◦ Causes:
 Plugging of the EAC with cerumen or foreign body
 Otitis externa and otitis media
 Perforation of eardrum
 osteosclerosis

31.  Sensorineural Deafness
◦ Due to loss of cochlear hair cells
◦ Problems with CN VIII
◦ Lesions within the Central Auditory Pathway
◦ Impairs the ability to hear certain pitches
(permanent)
◦ Causes:
 Aminoglycosides
 Prolonged exposure to noise
 Tumors and vascular damage

32.  Audiometer
 Tuning Fork

34. Thanks!!!