SENSE OF
HEARING
EXPERT VISION ACADEMY

What is Sound ?
 (Perception of) pressure waves generated by
vibrating air molecules.
 Sound is perceived when longitudinal
vibrations of the molecules in the external
environment, i.e. alternate phases of
condensation & rarefaction of the molecules,
strikes the tympanic membrane.

SOUND
SOUND WAVES

 Generally loudness of sound is correlated with the
amplitude of sound wave (subjective interpretation of
the intensity of a sound)
 Decibel - logarithmic scale to measure the intensity of sound
waves
 Human amplitude range - 0 dB - 120 dB (avg sound intensity
of human speech is 65 dB)
 Greater than 100 dB can damage to auditory apparatus &
greater than 120 dB can cause pain
 The pitch is correlated with the frequency ( no of
waves or cycles per second )
 The sound frequencies audible in humans - 20 Hz to 20000 Hz
 Normal pitch of male – 120 Hz
female - 250 HZ

THE EAR
 Houses two senses
 Hearing
 Equilibrium (balance)
 Receptors are mechanoreceptors

ANATOMY OF THE EAR
 The ear is divided into three areas
 External (outer) ear
 Middle ear (tympanic cavity)
 Inner ear (labyrinth)

ANATOMY OF THE EAR

THE EXTERNAL EAR
 Involved in hearing only
 Structures of the external ear:
Auricle (pinna), lobule, external auditory canal;
elastic cartilage
 External acoustic meatus (External
auditory canal)
Narrow chamber in the temporal bone
Lined with skin and ceruminous (wax) glands
Ends at the tympanic membrane
 Tympanic membrane- membrane that vibrates
in response to sound waves
 Ceruminous glands- wax secreting glands-
protects delicate lining of meatus and helps
prevent microorganisms from entering the ear

THE MIDDLE EAR (TYMPANIC
CAVITY)
 Air-filled cavity within the temporal bone
 Only involved in the sense of hearing
 Oval window- found on cochlea
 Round window- pressure window on cochlea
 Otitis media- inflammation of the middle ear; due to
bacteria or allergies, common in children whose
auditory tubes are short and horizontal

THE MIDDLE EAR
 There are several openings into the middle ear:
The opening from the auditory canal is covered by
the tympanic membrane
The Pharyngeotympanic auditory tube (Eustachian
tube) connects middle ear to pharynx
Allows for equalizing pressure
Two into the internal ear, the oval window and the
round window
 Clinical importance of these openings is that they
provide routes for infection to travel

BONES OF THE MIDDLE EAR
 Three small bones (ossicles) span the cavity
 Malleus (hammer)
 Incus (anvil)
 Stapes (stirrip)
 Function
help amplify sound
 Vibrations from eardrum move the malleus  anvil
 stirrup  inner ear

external
auditory canal
tympanic
membrane
Auditoy
tube
malleus
incus stapes
round
window
oval
window

INNER EAR
 Also termed labyrinth
 Consist of two main parts:
Bony labyrinth consisting of three parts:
 vestibule, cochlea, and semicircular canals
Membranous labyrinth
• Cochlea--Functions in hearing
• Semicircular canals--Functions in equilibrium
• Vestibule--Functions in equilibrium

INNER EAR
 Bony labyrinth is filled with Perilymph fluid,
similar to CSF
 Endolymph is the clear potassium-rich fluid that
fills the membranous labyrinth
 These fluids conduct sound vibrations

COCHLEA AND COCHLEAR DUCT
 Cochlea means snail
 In the center is a cone-shaped core termed
modiolus
 The modiolus houses the spiral ganglion that
has cell bodies of the first sensory neurons in
the auditory relay
 Cochlear ducts are the only part of the internal
ear concerned with hearing

ORGAN OF CORTI
 Contains hearing receptor cells (hair cells)
 Above the hair cells is a tectorial membrane
 On upper surface of basilar membrane
 Different vibration frequencies move different regions of
the basilar membrane and cause hair cells to shear
against the tectorial membrane
 Depolarization of hair cells transmits impulse along the
cochlear branch of the vestibulo-cochlear sensory
nerve

Note the hair cells and cochlear nerve.

How to discriminate the frequency
of the sound?

Vibration of Basilar Membrane and the
Traveling Wave Theory
• Sound wave entering at the oval window is to
cause the basilar membrane at the base of the
cochlea to vibrate
• Different frequencies cause vibrations at
different locations (places) along basilar
membrane
• Higher frequencies at base, lower frequencies
at top

25
MECHANICS OF HEARING
 sounds set up vibrations in air
 beat against the eardrum
 push a chain of tiny bones
 press fluid in inner ear against membranes
 set up forces that pull on hair cells
 stimulate neurons that send impulses to brain
 interpretation impulses
 hearing

THE PROCESS OF HEARING
Pathway of Sound
Vibrations

Transmission of sound:
Airborne sound → external auditory canal →
tympanic membrane →hammer, anvil, stirrup
→ oval window → vestubularcochlear nerve →
cochlear nuclei in medulla → superior olivary
nucleus → up the lateral leminiscus →
inferior colliculus → primary auditory cortex
in the temporal lobe

A. Perceiving Pitch (Frequency)
location of vibration on the basilar membrane
B. Perceiving Differences in Loudness (Intensity)
amplitude increases, more hair cells of the basilar
membrane (with same pitch) are activated
C. localizing Source of Sound
1. superior olivary nucleus - first point where sound from
both ears come together
2. relative intensity - the amplitude of sound waves hitting
the different ears
3. relative timing - the difference in timing in which a
sound reaches both ears
Processing of Auditory Information

Deafness
 Hearing loss- due to disease (ex. meningitus),
damage, or age related
 Conduction deafness- prevention or blocking
sounds from entering inner ear.
 Ear wax, ruptured ear drum, middle ear
inflammation (otitis media), and otosclerosis
(hardening of the ossicles of the ear)
 Sensoneural deafness- damage to the neural
structures from any point from the cochlear hair cells
to and including the auditory cortical cells
 Nerve deafness

TYPES OF DEAFNESS
Conductive Deafness Sensorineural Deafness
 due to impaired sound
transmission in external and
middle ear
 impacts all sound frequencies
 Causes
 plugging of the EAC with
cerumen of foreign bodies
 otitis externa and otitis media
 perforation of eardrum
 otosclerosis
 due to loss of cochlear hair cells
(common), problems with the
nerves or within central
auditory pathways (nerve
deafness)
 impairs the ability to hear
certain pitches (permanent)
 causes
 aminoglycoside antibiotics
(streptomycin and gentamycin)
 prolonged exposure to noise
 tumors and vascular damage

Deafness
 Tinnitus- ringing or clicking sound in the absence of
auditory stimuli
 1st symptom of cochlear nerve degeneration
 May result from inflammation of the inner or middle
ear
 Side effect from medicine
 Symptoms- vertigo, nausea, hearing loss

DIAGNOSTIC TECHNIQUES AND
PROCEDURES
 Audiometry
 The process of measuring how well an individual hears
various frequencies of sound waves
 Otoscopy
 The use of an otoscope to view and examine the
tympanic membrane and various parts of the outer ear