The ear is divided into the external, middle, and inner ear. The external ear collects sound waves and directs them through the external auditory canal to the tympanic membrane. The middle ear contains the ossicles that amplify vibrations before passing them to the inner ear. The inner ear contains the cochlea for hearing and vestibular system for balance. Within these structures are specialized hair cells that detect mechanical stimuli and transduce them into electrical signals via stereocilia on their surfaces.

1. ORGANIZATION OF THE EAR
Dr N. Mutuku

2. IN THIS LECTURE WE WILL LEARN ABOUT
• The tissue organization of the external ear and its relation to function
• The tissue organization of the middle ear and its relation to function
• The tissue organization of the inner ear and its relation to function

3. The ear is contained in the temporal bone and contains
the receptors of balance and sound. It is divided into
external, middle and inner ear.

4. The external ear comprises the auricle and
external auditory meatus (EAM)
• The auricle is a single thin plate of
fibrocartilage with an attached lobule of
dense connective tissue
• Covered by skin
• Moulded to collect and conduct ear waves to the
tympanic membrane
• The external acoustic meatus consists of a
lateral fibrocartilaginous part and a medial
osseous part within the tympanic temporal
bone
• Subcutaneous tissue in the cartilaginous part
contains ceruminous glands
• Ceruminous glands are modified sweat glands of
the simple coiled tubular variety
• Cerumen protects the skin of the ear from drying
out, traps particles and prevents microbial
colonization

5. The middle ear (tympanic cavity) is a space in the petrous temporal bone between the external and inner ear
temporal bone and communicates with the pharynx and the mastoid air cells. It contains the bones of the
ossicular chain (malleus, incus and stapes) that perform mechanical coupling of the tympanic membrane to the
cochlea (via the round window) and amplify sound vibrations 15-20 fold.
• The mucosa of the tympanic
cavity contains respiratory
epithelium (pseudostratified
columnar with cilia and goblet
cells) resting on a lamina propria
• Continuous with mucosa of
pharynx and mastoid air cells
• Covers ossicles, muscles, nerves
and medial surface of tympanic
membrane
• Presence of keratinizing squamous
epithelium leads to cholesteatoma

6. Between the external and middle ear is the tympanic membrane, a thin, semi-transparent, ovoid disc of three
layers that is anchored in the tympanic sulcus at the medial end of the bony EAM by a fibrocartilage ring (the
anulus). The sulcus is deficient superiorly resulting in the tympanic membrane having 2 parts: one taut and one
flaccid. The lateral layer is cuticular (of skin from the EAM), the middle fibrous (having an lateral radial and
medial circumferential layer) and the medial mucous (from the tympanic cavity)

7. The inner ear is made up of a network of membranous
sacs and ducts contained, having a congruent shape
with and bathed inside fluid within the bony labyrinth
of the petrous temporal bone. The bony labyrinth
comprises the anterior cochlea, middle vestibule and
semi-circular canals in the posterior position. The oval
window (which the stapedial footplate covers) and the
round window (sealed by the secondary tympanic
membrane) abut the medial wall of the tympanic
cavity.
• The bony labyrinth is lined by
periosteum
• The bony labyrinth is filled with a fluid:
perilymph
• Derived from plasma
• Resembles CSF in composition: 5mM K+,
150mM Na+

8. The membranous labyrinth comprises the utricle and saccule (within the vestibule),
the cochlear duct (within the cochlea) and the semicircular ducts (within the
semicircular canals)
• The parts of the membranous labyrinth
contain specialized mechanosensory epithelia
• Utricular and saccular macules respond to linear
acceleration of the head, forward-backward
tilting and side-side tilting
• Utricular macule lies horizontally and responds to
horizontal linear acceleration of the head
• Saccular macule lies vertically and responds to
vertical linear acceleration of the head
• Ampullary crests of the semicircular ducts
respond to angular acceleration due to rotation
of head/body
• Ampullary crest of anterior duct responds to rotation
in the anterior-posterior axis of the head
• Ampullary crest of the lateral duct responds to
rotation in the vertical axis of the head
• Ampullary crest in the posterior duct responds to
rotation in the transverse axis of the head
• Organ of Corti in the cochlear duct transduces
sound

9. The membranous labyrinth comprises the utricle and saccule (within the vestibule),
the cochlear duct (within the cochlea) and the semicircular ducts (within the
semicircular canals) It is filled with endolymph produced by the stria vascularis of
the cochlear duct.
• The membranous labyrinth is filled with a
fluid: endolymph
• Produced by the stria vascularis of the cochlear
duct
• Contains 150mM K+, 2mM Na+ and is more
similar to plasma
• High endolymph K+ concentration leads to a
potential difference of charge across the
plasma membrane of +80mV termed the
endocochlear potential
• It is crucial to mechano-electrical transduction in
the inner ear, particularly the cochlea, and exerts
potential force on positively charged ions to
enter the cell
• Motion in the endolymph due to acceleration or
pressure waves (from sound) triggers opening of
K+ channels, leading to potassium influx and
resulting in depolarization of receptor cells

10. The receptor cells in the neuroepithelia of the ear are the hair
cells. They are found in a simple columnar epithelium together
with their supporting cells, which sometimes cup them. A hair
cell is characterized by the presence of stereocilia (with
microtubular cores) on the apical membrane. The stereocilia are
organized in rows of increasing height and are connected by
filamentous tip-link proteins that trigger a K+ channel when
stretched.
• Hair cells have a small influx of K+ at rest, so there
is some baseline activity in the afferent neurons.
• Bending the cilia toward the tallest one opens the
potassium channels and increases afferent activity.
• Bending the cilia in the opposite direction closes the
channels and decreases afferent activity.
• Bending the cilia to the side has no effect on
spontaneous neural activity.
• The bases of the hair cells are related to terminal
boutons of cochlear and vestibular nerve afferent
fibers

11. In the maculae of the utricle
(horizontal macule) and
saccule (vertical macule)
stereocilia of the hair cells
are embedded within a
gelatinous membrane
(otolithic membrane) having
small crystals of calcium
carbonate (otoconia/otoliths)
embedded just below the
membrane’s free surface.
• When linear displacement of
the head occurs, otoconia
experience inertia resulting in
displacement of the otolithic
membrane and thus motion
of the embedded stereocilia

12. The maculae of the utricle (horizontal macule) and saccule (vertical macule) both
have an imaginary midline called the striola. On either side of the striola the
direction of increasing height of the rows of stereocilia on the hair cells is inverted.
This allows for each macula to be sensitive in two directions of linear acceleration.
• In the utriclar macule,
the effect of this is that
the hair cells are most
sensitive to antero-
posterior linear
displacements of the
head
• In the saccular macule,
the effect of this is that
the hair cells are most
sensitive to supero-
inferior displacements of
the head

13. In the semicircular ducts the hair cells lie in a region called the ampulla. The ampulla is that part of
the membranous labyrinth found where each semicircular duct continues into the vestibule. In the
ampulla, the hair cells are arrayed in rows perpendicular to the long axis of the respective
semicircular duct, capped by a gelatinous membrane called the cupula. The structure thus formed is
called the crista ampullares.

16. The cochlear duct/scala media is the most anterior structure of the
membranous labyrinth and makes 2
3
4
turns around the bony modiolus
that contains the spiral ganglion (of 1o auditory neurons) and cochlear
nerve fibers. The duct is described as being bordered superiorly by the
scala vestibuli and inferiorly by the scala tympani. It is separated from the
scala vestibuli by Reissner’s membrane and from the scala tympani by the
basilar membrane. The neuroepithelium of the cochlear duct AKA the
Organ of Corti rests on the basilar membrane.
• The spiral ligament is a thickening of
endosteum opposite the modiolus/on
the outer wall of the cochlea
• The spiral lamina is a sharp lateral
projection of the modiolus
• The basilar membrane spans the two
• Reissner’s membrane passes from the
spiral limbus to the spiral ligament
above the stria vascularis of the
cochlear duct
• The tectorial membrane is a gelatinous
structure in which stereocilia of
cochlear hair cells are embedded
• It spans the spiral limbus and the outer
hair cells
• Endolymph fills the scala media;
perilymph fills the scala vestibuli and
tympani

17. The Organ of Corti is a
neuroepithelial structure
of hair cells and
supporting cells that
rests on the basilar
membrane and extends
the whole length of the
cochlear duct. Lying
centrally in the Organ is a
space, the tunnel of
Corti, that is flanked on
either side by inner and
outer pillar cells. Internal
to the inner pillar cells is
a single row of inner hair
cells; external to the
outer pillar cells are 3-4
rows of outer hair cells.

18. • Stereocilia of the
inner and outer hair
cells are embedded
in the tectorial
membrane
• Movement of the
basilar membrane in
response to sound
pressure waves in the
endolymph results in
shearing between BM
and TM leading to
stereocilia displacement

19. • The reticular lamina is an
impermeable barrier to ions
• Formed from the apical surface of
hair cells and apical processes of
supporting cells
• Impermeability assures the
electrochemical gradient that creates
the endolymphatic potential
• Endolympatic potential of +80mV
PLUS the haircell resting membrane
potential of -70mV creates a driving
potential/electrochemical gradient
of 150mV
• Positively charged ions can thus only
pass through the outer and inner hair
cells resulting excitation/inhibition of
a receptor potential i.e. transduction
of the mechanical activity caused by
sound pressure waves into electrical
impulses

20. • The stria vascularis is a
specialized stratified
epithelium overlying
the spiral ligament
between the basilar
membrane and
Reissner’s membrane
• 3 cell layers
• Responsible for the
maintenance of the unique
ion concentration of the
endolymph

21. The basilar membrane is
a frequency analyzer.
• As a sound pressure wave propagates
along the length of the basilar
membrane, the physical properties
(thickness and width) of the basilar
membrane are such that each
frequency of sound will cause
maximal vibration/displacement at a
specific point along the length of the
basilar membrane.
• Hair cells at this specific point will
experience maximum shear force and
depolarise resulting in transduction of
a specific sound tone.
• The basilar membrane responds to a
wide range of frequencies
• 20kHz at the base i.e high pitched tones
• 20Hz at the apex i.e low pitched tones

22. Inner hair cells are the true receptor for auditory stimuli and
synapse with up to 95% of all cochlear nerve afferents, having
a 1:1 relationship with cochlear nerve afferents.
• Outer hair cells receive up to 5% of
cochlear nerve afferents and have a
1:many (1:5-1:100) pattern of afferent
supply
• OHCs also receive cochlear nerve efferents
to which they respond in two ways
• Dynamic microtubules in the OHC respond to
efferent stimulation resulting in shortening of
the cells
• OHC stereocilia respond to efferent
stimulation by shortening
• The overall effect of efferent stimulation is
that the tectorial membrane is held closer
to IHC stereocilia
• This response is essential in amplifying low-
amplitude (audibility) sounds
• OHC therefore sensitize the Organ of Corti to
low amplitude sounds