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Lecture: 5
The Sense of Hearing
Dr. Eyad M. Hussein
Ph.D of Neurology
Consultant in Neurology Department,
Nasser Hospital,
Assistant Professor,
Faculty of Medicine, Islamic University
Faculty of Dentistry, University of Palestine
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‫الصامت الرجاء تحويل الجوال إلى وضع‬
‫مع الشكر‬
Is Hearing Important?
1. Communication: hearing is essential to
Language.
2. Localization: determining the location of unseen
sound sources.
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The Sense of Hearing
 It is the ability to detect sounds.
 The most important function of hearing is involved in
language and social communications.
 Sound is a stimulus (energy) for hearing.
 In air sound travels at about 330 m/sec. While in water
sound travels at about 1500 m/sec.
 The ear with all its parts is located in the temporal bone.
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What needed for normal hearing?
1. Adequate stimulus (Sound).
2. Conduction of stimulus to sensory organs of
hearing.
3. Sensory transduction of stimulus.
4. Neural transmission of the signal.
5. Central auditory processing of the signal.
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The Peripheral Organ of Hearing
The peripheral organ of hearing is the ear, which has
three divisions:
1. The external ear.
2. The middle ear.
3. The inner ear.
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The External Ear
1. Auricle = Pinna.
2. External auditory canal (meatus).
The auricle: elastic cartilage except the lobule (fat).
The auricle consist of:
• Lobule.
• Concha.
• Helix.
• Antihelix.
• Tragus.
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Relations of the External Auditory Canal

Anterior: the temporo-mandibular joint.

Posterior: the mastoid.

Superior: the temporal lobe of the brain.

Inferior: the parotid gland.
Functions of the External Ear
1. Collection of sound stimuli.
2. Increase the sound pressure at the tympanic
membrane.
3. Sound localization.
4. Protective function: skin of the auditory canal secretes
wax (a bactericidal effect).
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The Tympanic Membrane
Eardrum
It is a thin membrane about 1 cm in diameter located
between the external and middle ear.
Function:
1. Separates the external auditory canal from the
tympanic cavity (middle ear).
2. Transmits vibrations from outside to middle ear.
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The Tympanic Membrane
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Nerves Supply of the External Ear and
Tympanic Membrane
1.
Greater auricular nerve: from C2, C3 roots.
2.
Lesser occipital nerve: from C2 root.
3.
Auriculotemporal branch of the mandibular
nerve.
4.
Posterior auricular branch of the facial nerve.
5.
Auricular (Arnold) branch of vagus nerve.
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The Middle Ear
 It is an air filled cavity within the temporal bone.
 The middle ear opens through Eustachian tube into
the nasopharynx.
The middle ear consists of:
1. Tympanic cavity.
2. Mastoid air cells.
3. Eustachian tube.
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Structures in the Middle Ear
 Ossicles.
 Stapedius
 Tensor
 Facial
muscle.
tympani muscle.
nerve and branches (chorda tympani and
nerve to stapedius muscle).
 Jacobson
nerve (tympanic plexus).
 Eustachian
 Mastoid
tube.
air cells.
The Auditory Ossicles
These are the chain of bones in the middle ear that
connect the eardrum to the inner ear.
1. Malleus: the malleus is the bone embedded in the
eardrum.
2. Incus: the incus is a small bone which connects the
malleus to the stapes.
3. Stapes: the stapes is the last bone in the chain. It
connects to the fluids of the inner ear.
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Function of Middle Ear
1.
Conduct sound from the tympanic membrane through
the middle ear to the cochlea.
2.
Physical protection of the cochlea.
3. The malleus with tensor tympani muscle keeps the
tympanic membrane tensed.
4. The stapedius muscle reduce excessive vibration and
sound transmission to the inner ear.
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Muscles of the Middle Ear
There are two middle ear muscles:
1. The tensor tympani muscle: inserts on the malleus.
Action: dampens down vibrations of tympanic membrane.
2. The stapedius muscle: inserts on the neck of stapes.
Action: dampens down vibrations of stapes.
Function:
1. They regulate the range of movement in the tympanic
membrane and ossicles when sound vibrations strike the
membrane.
2. Filter function: Reduce excessive vibration and sound
transmission to the inner ear.
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The Eustachian Tube
(Auditory Tube or Pharyngotympanic Tube)

It begins in the anterior wall of the tympanic cavity and
ends in the nasopharynx.

It is 18mm at birth and 35 mm in adults.

The Eustachian tube is normally closed.

During swallowing, the action of the tensor veli
palatini muscle opens the tube.
Functions of the Eustachian Tube
1.
It connects the middle ear with the nasopharynx: the
tube is closed at rest, but it opens during swallowing.
2.
Equalization of pressure in the tympanic cavity.
3.
Drainage and ventilation of the middle ear.
4.
Protection of the middle ear.
5.
Drainage of middle ear secretion.
Eustachian Tube
Closed
Open
Mastoid
 Mastoid
 Lies
antrum:
behind the middle ear in the petrous part of the
temporal bone.
 Mastoid
 A series
air cells:
of communicating cavities that are
continous with the antrum and the middle ear.
The Inner Ear (Labyrinth)
The inner ear is divided into two parts:
1.
Osseous (Bony) labyrinth.
2.
Membranous labyrinth.
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Bony Labyrinth
The hardest bone in the body!!!
1.
Vestibule.
2.
Semicircular canals.
3.
Cochlea.
4.
Oval window.
5.
Round window.
It is filled with a fluid called the perilymph (between
the bony and the membranous labyrinth).
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The Membranous Labyrinth

It lies within the bony labyrinth.

3 semicircular ducts: superior, posterior, lateral.

Utricle

Saccule

Cochlear duct.

Endolymphatic duct and sac.

It is filled by a fluid called the endolymph.
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The Inner Ear
◦ The cochlea (end organ for hearing).
◦ The vestibule and semicircular canals (end organ
for balance).
The Internal Auditory Canal
Contains:

The facial nerve,

The acoustic nerve.

The superior
vestibular nerve.

The inferior vestibular
nerve.

Labyrinthine artery.
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The Vestibulo-Cochlear Nerve (VIII)
(Auditory or Stato-Acustic Nerve)
 It is a special sensory (hearing and balance) nerve.
 It has two divisions:
1. Cochlear division.
2. Vestibular division.
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The Vestibulo-Cochlear Nerve (VIII)
(Auditory or Stato-Acustic nerve)
 It is a special sensory (hearing and balance) nerve.
 It emerges from the anterolateral surface between the
pons and medulla oblongato with the facial nerve.
 It has two divisions:
1. Cochlear division.
2. Vestibular division.
The Pathway of Hearing
Receptors: the organ of the corti in the cochlea of inner
ear.
First order neuron: cells of the spiral (cochlear)
ganglion.
Carry hearing sensation from the corti in the cochlea of
inner ear → cochlear nerve enters the brain stem
between the pons and medulla oblongata → ventral and
dorsal nuclei of the pons and upper medulla.
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Cross Section Through Cochlea
Second order neuron: cells of cochlear nuclei:
Few axons ascend in the lateral lemniscus of the same side.
Most of the axons of these cells form the trapezoid body →
decussate to opposite side and joined by few fibers from the
cochlear nuclei of that side ascends as lateral lemniscus →
inferior colliculus → medial geniculate body.
Third order neuron: cells of medial geniculate body →
posterior limb of internal capsule → auditory sensory area
in superior temporal gyrus (area 41 & 42) where
hearing is bilaterally represented.
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Steps in Hearing
The auricle → the external auditory meatus → vibration of
the tympanic membrane → vibration of the malleus →
incus → stapes → vibration of the perilymph of the inner
ear → vibrationo of the endolymph causes → vibration of
the basilar membrane → hair cells in the organ of Corti
→ cochlear nerve → auditory center of the brain the
CNS (area 41 & 42 in the superior temporal lobe).
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Lesion of the Vestibulo-Cochlear Nerve
A. Lesion of the Cochlear Division:
1. Tinnitus in irritative lesion.
2. Deafness in destructive lesion.
B. Lesion of the Vestibular Division:
1. Vertigo.
2. Spontaneous nystagmus.
3. Ipsilateral incoordination.
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Causes of Deafness
1. Conductive Deafness: otitis media, cholesteatoma,
tympanic membrane rupture, carcinoma.
2. Sensorineural Deafness:
• Congenital
• Infection
• Trauma
• Drugs
• Meniere’s disease
• Cerebellopontine angle tumor
• Brain stem disease
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