Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear. The academic field concerned with hearing is auditory science. Sound may be heard through solid, liquid, or gaseous matter.

1. Physiology of Hearing

2. Sound
 Sound is a form of energy
 It is transmitted through a medium as a longitudinal
pressure wave.
 The wave consists of a series of compressions and
rarefactions of the molecules in the medium.
 The ear is capable of capturing this energy and
perceiving it as sound information


3. Compression
Compression
Compression
Rarefaction
Rarefaction
Rarefaction
Sound waves

4. Properties of sound
 The wave motion of sound can be described in
terms of Amplitude, Frequency, Velocity and
Wavelength

5. Properties of sound
Wavelength
Refers to the physical distance between successive compressions and is thus dependant
on the speed of sound in the medium divided by its frequency
Amplitude (Intensity or loudness)
Refers to the difference between maximum and minimum pressure
Frequency (pitch)
Refers to the number of peak-to-peak fluctuations in pressure that pass a particular
point in space in one second
Velocity
Refers to the speed of travel of the sound wave. This varies between mediums and is
also dependant on temperature (in air at 20°C it is 343 m/s)

6. Transmission of sounds through the ear
 External ear
– Mostly through air (External acoustic meatus)
 Middle ear
– Through solid medium - bone (ossicles)
 Inner ear
– Through fluid medium – endolymph (cochlea)

7. Parts of the ear

8. Air and bone conduction
 There are two methods by which hair cells can
be stimulated.
– Air conduction
 Sound stimulus travelling through the external and
middle ear and activating the hair cells
– Bone conduction
 Sound stimulus travelling though the bones of the skull
activating the hair cells
 Whatever method it takes, the sound stimulus
finally activate hair cells in the cochlea

9. External ear
 Consists of
– Pinna
– External auditory meatus

10. Middle ear
 composed of
– the tympanic membrane
– the tympanic cavity
– the ossicles
 Malleus
 Incus
 Stapes (connected to the oval window of the cochlea)
– two muscles
 the tensor tympani attached to the malleus
 the stapedius muscle attached to the stapes
– the Eustachian tube

11. Inner ear
 Consists of two main parts
– the cochlea (end organ for hearing)
– the vestibule and semicircular canals (end organ for balance)
 The inner ear can be thought of as a series of tunnels or canals within the
temporal bone
 Within these canals are a series of membranous sacs (termed labyrinths)
which house the sensory epithelium
 The membranous labyrinth is filled with a fluid termed endolymph
 It is surrounded within the bony labyrinth by a second fluid termed
perilymph
 The cochlea can be thought of as a canal that spirals around itself similar to
a snail. It makes roughly 2 1/2 to 2 3/4 turns

13. Cross section through cochlea

14. Cochlea
• The bony canal of the cochlea is divided into an upper
chamber, the scala vestibuli and a lower chamber, the scala
tympani by the membranous labyrinth also known as the
cochlear duct.
• The floor of the scala media is formed by the basilar
membrane, the roof by Reissner's membrane.
• The scala vestibuli and scala tympani contain perilymph.
• The scala media contains endolymph.

15. Endolymph and perilymph
 Endolymph is similar in ionic content to
intracellular fluid (high K, low Na)
 Perilymph resembles extracellular fluid (low K,
high Na)
 The cochlear duct contains several types of
specialized cells responsible for auditory
perception

16. The sensory cells responsible for hearing are located on the basilar
membrane within a structure known as the organ of Corti.
This is partitioned by two rows of peculiar shaped cells known as
pillar cells.
The pillar cells enclose the tunnel of Corti,
Situated on the basilar membrane is a single row of inner hair cells
medially and three rows of outer hair cells laterally.
The hair cells and other supporting cells are connected to one
another at their apices by tight junctions forming a surface known
as reticular lamina.
The cells have specialized stereocilia on their apical surfaces

17. Organ of Corti

18.  Attached to the medial aspect of the scala
media is a fibrous structure called the tectorial
membrane
 It lies above the inner and outer hair cells
coming in contact with their stereocilia

19.  The fluid in the space between the tectorial
membrane and reticular lamina is endolymph
 Thus the endolymp bathes the stercocillia
 But the body of the hair cells which lies below
the reticular lamina is bathed by perilymph

21. Hair cells

22.  Synapsing with the base of the hair cells are
dendrites from the auditory nerve
 The auditory nerve leaves the cochlear and
temporal bone via the internal auditory canal
and travels to the brainstem

23. Transmission of sound waves
 The outer ear and external auditory canal act passively
to capture the acoustic energy and direct it to the
tympanic membrane
 There, the sound waves strike the tympanic
membrane causing it to vibrate
 These mechanical vibrations are then transmitted via
the ossicles to the perilymph of the inner ear
 The perilymph is stimulated by the mechanical
(vibrations) energy vibrations to form a fluid wave
within the cochlea

24. Middle ear
 The middle ear acts as an impendance-matching device
 Sound waves travel much easier through air (low impedance)
than water (high impedance)
 If sound waves were directed at the oval window (water)
almost all of the acoustic energy would be reflected back to the
middle ear (air) and only 1% would enter the cochlea. This
would be a very inefficient method.
 To increase the efficiency of the system, the middle ear acts to
transform the acoustic energy to mechanical energy which then
stimulates the cochlear fluid

25. Middle ear
• The middle ear also acts to increase the acoustic
energy reaching the cochlea by essentially two
mechanical phenomenon.
• The area of the tympanic membrane is much greater
than that of the stapes footplate (oval window)
causing the force applied at the footplate per square
area to be greater than the tympanic membrane
• The ossicles act as a lever increasing once again the
force applied at the stapes footplate.
• Overall, the increase in sound energy reaching the
cochlea is approximately 22 times

26. Cochlea
 The cochlea consists of a fluid filled bony canal within which
lies the cochlear duct containing the sensory epithelium
 Energy enters the cochlea via the stapes bone at the oval
window and is dissipated through a second opening (which is
covered by a membrane) the round window
 Vibrations of the stapes footplate cause the perilymph to form
a wave
 This wave travels the length of the cochlea
 It takes approximately 5 msec to travel the length of the
cochlea

27. Cochlea
 As it passes the basilar membrane of the cochlear
duct, the fluid wave causes the basilar membrane to
move in a wave-like fashion (i.e. up and down)
 The wave form travels the length of the cochlea and is
dissipated at the round window
 Due to changes in the mechanical properties of the
basilar membrane, the amplitude of vibration changes
as one travels along the basilar membrane

28. The place principle
 Low frequency stimuli cause the greatest
vibration of basilar membrane at its apex, high
frequency stimuli at its base

29.  As the basilar membrane is displaced superiorly by the perilymph wave, the
stereocilia at the apex of each inner and outer hair cell, which are imbedded
in the tectorial membrane undergo a shearing force (i.e. they are bent)
 This shearing force causes a change in the resting membrane potential of
the hair cell which is transmitted to its basal end
 There a synapse is formed with a dendrite from the auditory nerve
 The hair cell membrane potential change is transmitted across this synapse
(? via acetylcholine) causing depolarization of the nerve fiber
 This neural impulse is then propagated to the auditory centres of the brain

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