1. THE EAR: HEARING AND BALANCE

2. The Ear: Hearing and Balance
Three parts of the ear
The External (outer) Ear
Pinna- Composed of the Helix (rim) and lobule (earlobe)
External acoustic canal (meatus)-short, curved tube leading to eardrum
Lined with ceruminous glands
Tympanic membrane- eardrum; CT boundary btwn. external and middle ear; vibrates in response to sound

3. (a) The three regions of the ear
Middle
ear
Internal ear
(labyrinth)
External
ear
Auricle
(pinna)
Helix
Lobule
External
acoustic
meatus
Tympanic
membrane
Pharyngotympanic
(auditory) tube
(a) The three regions of the ear
Figure 15.25a

4. The Middle Ear (tympanic cavity)
A small, air-filled, mucosa-lined cavity in the temporal bone; flanked laterally by the eardrum and medially by the oval and round windows
Three bones (ossicles)- Incus, Malleus,Stapes

5. Two tiny skeletal muscles prevent damage due to large vibrations
Middle Ear
Two tiny skeletal muscles prevent damage due to large vibrations
Tensor tympani-arises from wall of pharyngotympanic tube and inserts on the malleus
Stapedius muscle-runs from posterior wall of middle ear to the stapes

6. Middle Ear Pharyngotympanic (auditory) tubes
Connects middle ear to nasopharynx
Normally flattened closed; opens when yawning or swallowing
Equalizes pressure in the middle ear cavity with the external air pressure

7. Epitympanic recess Malleus Incus Superior Lateral Anterior View
Pharyngotym-
panic tube
Tensor
tympani
muscle
Tympanic
membrane
(medial view)
Stapes
Stapedius
muscle
Figure 15.26

8. Inner Ear Consists of the bony (osseous) and membranous labyrinth
Bony labyrinth
Tortuous channels in the temporal bone
Filled with perilymph
Three regions:
Vestibule-
Contains two sacs
Saccule-continuous w/ the cochlear duct
Utricle-continuous w/ the semicircular ducts
Cochlea
Semicircular canals

9. The Inner Ear Membranous Labyrinth
Series of membranous sacs and ducts contained w/in the bony labyrinth
Follows contours of bony labyrinth
Filled with endolymph

10. Superior vestibular ganglion
Inferior vestibular ganglion
Temporal
bone
Semicircular
ducts in
semicircular
canals
Facial nerve
Vestibular
nerve
Anterior
Posterior
Lateral
Cochlear
nerve
Cristae ampullares
in the membranous
ampullae
Maculae
Spiral organ
(of Corti)
Utricle in
vestibule
Cochlear
duct
in cochlea
Saccule in
vestibule
Stapes in
oval window
Round
window
Figure 15.27

11. The Maculae and Static Equilibrium
Sensory receptors for static equilibrium (monitor the position of head in space, respond to linear acceleration)
One in each saccule wall and one in each utricle wall
Each maculae contains:
ET hair cells and supporting cells
Otolithic membrane- gel like membrane that overlies hair cells
Otoliths- calcium carbonate crystals; increase weight and its inertia

12. Otoliths Kinocilium Otolithic membrane Stereocilia Hair bundle
Macula of
utricle
Macula of
saccule
Hair cells
Supporting
cells
Vestibular
nerve fibers
Figure 15.34

13. Steps of linear movement
Hair cells are always releasing neurotransmitter
When hair cells bend towards kinocilium they depolarize, neurotransmitter release is increased
When hair cells bend away from kinocilium they hyperpolarize, slowing release of neurotransmitter
This change in neurotransmitter release informs the brain of the changing of the position of the head in space

14. Otolithic membrane Kinocilium Stereocilia Hyperpolarization Receptor
potential
Depolarization
Nerve impulses
generated in
vestibular fiber
When hairs bend toward
the kinocilium, the hair
cell depolarizes, exciting
the nerve fiber, which
generates more frequent
action potentials.
When hairs bend away
from the kinocilium, the
hair cell hyperpolarizes,
inhibiting the nerve fiber,
and decreasing the action
potential frequency.
Figure 15.35

15. The Crista Ampullaris and Dynamic Equilibrium
Detected by:
Crista Ampullaris –
One in the ampulla of each semicircular canal
Major stimuli are rotatory movements

16. The Crista Ampullaris and Dynamic Equilibrium
Semicircular Canals
Three canals are located in each ear:
Located in all three planes of space
Anterior, posterior and lateral
Endolymph-fills the semicircular ducts
Ampulla- swellling at end of semicircular duct
Crista Ampullaris
Composed of hair cells and supporting cells
Structure and function of the crista ampullaris is basically the same as the hair cells of cochlea and maculae
Cupula – gelled mass that cilia of hair cells are embedded in

17. Hair bundle (kinocilium plus stereocilia)
Cupula
Crista
ampullaris
Endolymph
Hair bundle (kinocilium
plus stereocilia)
Hair cell
Membranous
labyrinth
Crista
ampullaris
Supporting
cell
Fibers of vestibular nerve
(a) Anatomy of a crista ampullaris in a
semicircular canal
Cupula
(b) Scanning electron
micrograph of a
crista ampullaris
(200x)
Figure 15.36a–b

18. The Crista Ampullaris and Dynamic Equilibrium
Steps of Rotational Movement
At rest the cupula stands upright
During rotational acceleration, hair cells are bent, they depolarize and impulses reach the brain faster
As movement slows, endolymph keeps moving, cilia are bent in opposite direction causing hyperpolarization and reduction of impulses to brain

19. At rest, the cupula stands upright. During rotational acceleration,
Section of
ampulla,
filled with
endolymph
Fibers of
vestibular
nerve
Cupula
Flow of endolymph
At rest, the cupula stands
upright.
During rotational acceleration,
endolymph moves inside the
semicircular canals in the
direction opposite the rotation
(it lags behind due to inertia).
Endolymph flow bends the
cupula and excites the hair
cells.
As rotational movement
slows, endolymph keeps
moving in the direction
of the rotation, bending
the cupula in the
opposite direction from
acceleration and
inhibiting the hair cells.
(c) Movement of the
cupula during
rotational
acceleration
and deceleration
Figure 15.36c

20. Equilibrium Pathway to the Brain
Vestibular nerve-Impulses travel to the vestibular nuclei in the brain stem or the cerebellum
Pathways are complex and poorly traced

21. Sound and the Cochlea Sound is detected by: the cochlea Cochlea
The cochlea is A spiral, conical, bony chamber and contains the cochlear duct
Cochlear duct- houses the spiral organ (of Corti)
Divides cochlea into three chambers:
Scala vestibuli-superior to cochlear duct (contains perilymph)
Scala tympani-inferior to cochlear duct; terminates at round window (contains perilymph)
Scala media (cochlear duct) -middle cavity; (contains endolymph)

22. Modiolus Cochlear nerve, division of the vestibulocochlear
nerve (VIII)
Spiral ganglion
Osseous spiral lamina
Vestibular membrane
Cochlear duct
(scala media)
Helicotrema
(a)
Figure 15.28a

23. Vestibular membrane Osseous spiral lamina Tectorial membrane Spiral
ganglion
Scala
vestibuli
(contains
perilymph)
Cochlear duct
(scala media;
contains
endolymph)
Spiral organ
(of Corti)
Scala tympani
(contains
perilymph)
Basilar
membrane
(b)
Figure 15.28b

24. Sound and the Cochlea
Oval window-an opening on the medial wall of the middle ear (foot of stapes rests at oval window)
Round window-an opening on the medial wall of the middle ear (scala tympani terminates at round window)
Vestibular membrane-roof of cochlear duct that separates the scala media from scala vestibuli
Basilar membrane- fibrous floor of cochlear duct

25. Sound and the Cochlea Organ of Corti Runs through center of cochlea
Has hair cells and supporting cells
Tectorial membrane- gel-like mass that cilia of hair cells are embedded in
Bending of the cilia: excites hair cells

26. Tectorial membrane Inner hair cell Hairs (stereocilia) Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
(c)
Figure 15.28c

27. Transmission of Sound to the Inner Ear
Sound Transmission
Transmission of Sound to the Inner Ear
Sound waves enter the external acoustic canal and cause tympanic membrane to vibrate
Ossicles vibrate and amplify the pressure at the oval window
Pressure waves move through perilymph of the scala vestibuli
Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane

28. Sounds with frequencies below hearing travel through
Auditory ossicles
Malleus
Incus
Stapes
Cochlear nerve
Scala vestibuli
Oval
window
Helicotrema
Scala tympani
Cochlear duct 2 3
Basilar
membrane 1
Sounds with frequencies
below hearing travel through
the helicotrema and do not
excite hair cells.
Tympanic
membrane
Round
window
Sounds in the hearing range
go through the cochlear duct,
vibrating the basilar membrane
and deflecting hairs on inner
hair cells.
(a) Route of sound waves through the ear
Sound waves vibrate
the tympanic membrane. 1
Pressure waves created by
the stapes pushing on the oval
window move through fluid in
the scala vestibuli. 3
Auditory ossicles vibrate.
Pressure is amplified. 2
Figure 15.31a

29. Sound Transmission
Resonance of the Basilar Membrane - fibers of the basilar membrane are “tuned” to a particular sound frequency
Vibrations of the basilar membrane causes cilia of hair cells to bend
Bending cilia towards kinocilium excites hair cells (increase neurotransmitter release)
Bending cilia away from kinocilium inhibits hair cells (slow release of neurotransmitter)

30. Sound Transmission
Impulses from the cochlea pass via the spiral ganglion to the cochlear nuclei of the medulla
Eventually impulses are sent to the primary auditory cortex (temporal lobe)

31. cortex in temporal lobe
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Midbrain
Cochlear nuclei
Medulla
Vibrations
Vestibulocochlear nerve
Vibrations
Spiral ganglion of cochlear nerve
Spiral organ (of Corti)
Figure 15.33

32. Deafness Hearing loss can be temporary or permanent
Common causes:
Middle ear infections
Conduction deafness
Can be caused by:
Impacted earwax
Ruptured eardrum
Middle ear inflammations
Otosclerosis

33. Deafness Nerve Deafness Can be caused by:
Gradual loss of hair cells throughout life
Single explosively loud noise
Prolonged exposure to loud noise
Degeneration of cochlear nerve, tumors in auditory cortex, etc.

34. Tinnitus
Ringing or clicking sound in ears in the absence of auditory stimuli
One of the first symptoms of cochlear degeneration
Can be caused by middle ear inflammation

35. Meniere’s Syndrome
Labyrinth disorder
Affects all three parts of the internal ear
Symptoms are repeated attacks of vertigo, nausea and vomiting
Balance is severely disturbed and hearing is ultimately lost