1. The Ear-Hearing and Balance
Dr.Spandana Charles

2. Anatomy The ear is divided into three major areas- External ear-
The Pinna
External acoustic meatus
Middle Ear-
Tympanic membrane
Auditory ossicles
Malleus
Incus
Stapes
Inner Ear-
Cochlea-Hearing
Semicircular canals-Balance

4. Cochlea

5. Top of the Reissner’s membrane lies the scala vestibuli containing ‘perilymph’
Between the Reissner’s membrane & the basilar membrane lies the scala media containing ‘endolymph’
Below the basilar membrane the perilymph filled cavity called scala tympani
The organ corti rests on the basilar membrane

6. Scala Vestibuli & Scala Tympani-Peri lymph Ultra filtrate of Blood
Resembles ECF-Rich in Na
Scala Media- Endo lymph-
Secreted by Stria Vascularis.
Rich in Potassium

7. Basilar Membrane
Fibrous membrane separating scala media & scala tymphani
Contains 20,000 – 30,000 basilar fibres
Stiff, elastic reed like structures, fixed at basal ends
Length  from base to apex (0.04 – 0.5 mm)
Diameter tapers towards apex
Stiff short - high frequency –base
Long limber - low frequency - apex

8. Organ of Corti The end organ of hearing Rests on the basilar membrane
Contains 2 rows of hair cells- inner and outer
Hair cells contain Stereocilli
They are capped by a membrane called tectorial membrane
At the bottom of the hair cells , nerve fibers emerge and later on form the cochlear division of the 8 th nerve

9. Tiplink
Animation Ear

10. Route of sound waves through the ear
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Slide 1
Auditory ossicles
Malleus
Incus
Stapes
Cochlear nerve
Scala vestibuli
Oval
window
Helicotrema 4a
Scala tympani
Cochlear duct 2 3
Basilar
membrane 4b 1
Sounds with frequencies below hearing travel through the
helicotrema and do not excite hair cells. 4a
Tympanic
membrane
Round
window
Route of sound waves through the ear
Sound waves vibrate the tympanic membrane. 1
Auditory ossicles vibrate. Pressure is amplified. 2
Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli. 3
Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and
deflecting hairs on inner hair cells. 4b
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11. © 2013 Pearson Education, Inc.
Figure The auditory pathway.
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary
nucleus (pons-
medulla junction)
Midbrain
Cochlear nuclei
Medulla
Vibrations
Vestibulocochlear
nerve
Vibrations
Spiral ganglion
of cochlear nerve
Bipolar cell
Spiral organ
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12. © 2013 Pearson Education, Inc.
Auditory Processing
Pitch perceived by impulses from specific hair cells in different positions along basilar membrane
Loudness detected by increased numbers of action potentials that result when hair cells experience larger deflections
Localization of sound depends on relative intensity and relative timing of sound waves reaching both ears
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13. Equilibrium and Orientation
Vestibular apparatus
Equilibrium receptors in semicircular canals and vestibule
Vestibular receptors monitor static equilibrium
Semicircular canal receptors monitor dynamic equilibrium
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14. © 2013 Pearson Education, Inc.
Figure Structure of a macula.
Macula of
utricle
Macula of
saccule
Kinocilium
Otolith
membrane
Otoliths
Stereocilia
Hair bundle
Hair cells
Supporting
cells
Vestibular
nerve fibers
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15. © 2013 Pearson Education, Inc.
Maculae
Sensory receptors for static equilibrium in utricle and saccule
Monitor the position of head in space, necessary for control of posture
Respond to linear acceleration forces, but not rotation
Contain supporting cells and hair cells
Stereocilia and kinocilia are embedded in the otolith membrane studded with otoliths (tiny CaCO3 stones)
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16. The Crista Ampullares (Crista)
Sensory receptor for rotational acceleration
One in each semicircular canal
Major stimuli are rotational movements
Each crista has supporting cells and hair cells that extend into gel-like mass called ampullary cupula
Dendrites of vestibular nerve fibers encircle base of hair cells
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17. © 2013 Pearson Education, Inc.
Figure 15.35a–b Location, structure, and function of a crista ampullaris in the internal ear.
Ampullary cupula
Crista
ampullaris
Endolymph
Hair bundle
(kinocilium
plus stereocilia)
Crista
ampullaris
Hair cell
Membranous
labyrinth
Supporting
cell
Fibers of vestibular nerve
Anatomy of a crista ampullaris in a semicircular canal
Scanning electron micrograph
of a crista ampullaris (200x)
Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
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 rotation. Endolymph flow bends the cupula in the opposite direction from acceleration and inhibits the hair cells.
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Movement of the ampullary cupula during rotational acceleration and deceleration

18. (cranial nerve XI nuclei and vestibulospinal tracts)
Figure Neural pathways of the balance and orientation system.
Input: Information about the body’s position in space comes from
three main sources and is fed into two major processing areas in the
central nervous system.
Somatic receptors
(skin, muscle
and joints)
Vestibular
receptors
Visual
receptors
Vestibular
nuclei
(brain stem)
Cerebellum
Central nervous
system processing
Oculomotor control
(cranial nerve nuclei
III, IV, VI)
(eye movements)
Spinal motor control
(cranial nerve XI nuclei
and vestibulospinal tracts)
(neck, limb, and trunk
movements)
Output: Responses by the central nervous system provide fast
reflexive control of the muscles serving the eyes, neck, limbs, and
trunk.
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19. Applied aspects Motion Sickness Conduction deafness
Sensorineural deafness
Menierre s disease
Nystagmus

20. Properties of Sound Waves
Frequency
Number of waves that pass given point in given time
Pure tone has repeating crests and troughs
Pitch
Perception of different frequencies
Normal range 20–20,000 hertz (Hz)
Higher frequency = higher pitch
Quality
Most sounds mixtures of different frequencies
Richness and complexity of sounds (music)
Amplitude - loudness
Subjective interpretation of sound intensity
Normal range is 0–120 decibels (dB)
Severe hearing loss with prolonged exposure above 90 dB
Amplified rock music is 120 dB or more
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21. Thank you

22. Dissection