1. Hearing
Prof. K. Sivapalan

2. Sound Waves.
Longitudinal vibration- alternate phases of condensation and rarefaction of molecules.
Velocity in air- 344 m/s, 770 miles/h at 20°C, 1450m/s in water, faster in salt water.
Loudness- Sound energy- amplitude of the waves [A,B].
Pitch- frequency and ? other factors pitches can be identified by humans [B,C].
Timbre- character- different instruments playing at the same pitch can be identified.
Music- sound waves with patterns [D].
Noise- no pattern [E], ?Perception.
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3. Measurements.
Amplitude is measured in decibel scale- logarithm of ratio of a standard sound.
Standard sound- 0 decibel = dyne/cm2, threshold for hearing by an average human.
Pressure at see level- I bar [15 lb/in2].
Range of sound pressure from threshold to damage to cochlea is – 2000 μbar.
Audible frequency- 20 – 20,000 Hz.
Male voice- 120 Hz, female voice- 250 Hz.
Greatest sensitivity for hearing – 1000 – 3000 Hz.
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4. Structure of the Ear
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5. External ear.
Function is funneling sound through external auditory meatus to the tympanic membrane [ear drum]
Funneling can be improved by adding surface.
Wax secreted protects from insects but blocks when excessive.
Foreign bodies inserted can damage ear drum which is located in safe place.
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6. Tympanic Membrane.
Tympanic membrane divides the external ear from middle ear.
The membrane is pulled inwards by tensor tympani.
Tendon of the muscle is attached to the manubrium of the maleous which is attached to the membrane.
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7. Middle Ear. Air filled cavity in the temporal bone
Communicates with naso-pharynx through Eustachian tube.
The tube is normally closed.
The tube is opened during swallowing, chewing, and yawning.
This keeps pressure on either side of the ear drum equalized.
The cavity extends posteriorly as mastoid andrum.
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8. Three Ossicles.
Incus.
Malleus
Stapes.
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9. Attachments of ossicles.
Manubrium [handle of the malleus] is attached to tympanic membrane.
Short process is attached to incus – synovial joint.
Incus articulates with the head of the stapes – synovial joint.
The foot plate is attached to oval window.
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10. Ossicular conduction.
Motion of the tympanic membrane is imparted to malleus.
Malleus rocks in the axis of the junction between long and short processes.
Malleus transmits the movement to the head of the stapes.
Foot plate of the Stapes moves like a door hinged to the posterior edge of the oval window.
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11. Conduction in Ear.
Tensor tympani and Stapedius muscles keep the membranes tense, ready to vibrate.
Tympanic membrane acts as a resonator. [surface area is 55 mm2.
Leaver action multiplies the pressure 1.3 times. Surfaces of the oval window is 3.2 mm2 .[17 times less than Tympanic Membrane. This increases sound pressure 17 times at oval window]
There is a small loss because of resistance.
Even though some energy is lost, sound pressure is increased [1.3x17=22 times] to vibrate fluid in internal ear.
Bone conduction: Transmission through bones of the skull directly to inner ear.
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12. Tympanic Reflex
Loud sounds cause constriction of the middle ear muscles.
It protects the ear from high amplitude vibrations.
Reaction time is 40 – 160 ms.
This reflex cannot prevent the effects of gun shot, explosions or beginning of thunder.
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13. Inner ear [Labyrinth].
Bony labyrinth is a system of channels in the petrus part of the temporal bone containing perilymph.
Membranous labyrinth is inside the bony labyrinth, containing endolymph.
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14. Membranous Labyrinth It has two components.
Cochlea with organ of Corti for hearing.
Vestibule with semicircular cannels, utricle and saccule for sense of balance.
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15. Cochlea. Coiled tube, 35 mm long, makes 2¾ turns.
The bony core is the modiolus.
Spiral lamina spirals around the modiolus.
Spiral ganglion found in the modiolus contains the cell bodies of afferent nerves- cochlear nerve.
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16. Cochlea ctd.
Reissner’s membrane from the base of the spiral lamina maks the Scala vestibuli above.
Basilar membrane from the tip of the spiral lamina forms Scala tympani below.
Scala media is at the centre.
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17. Transmission in Cochlea.
Scala vestibuli and scala tympani communicate at helicotrema, contain perilymph.
At the base, scala vestibuli ends in oval window [stapes] and scala tympani ends at round window.
Scala media continues as membranous labyrinth into vestibule.
Movements of stapes set up waves in perilymph in scala vestibuli.
The Raisner’s membrane is the only flexible part of the scala vestibule and through it the vibrations are transmitted to the basilar membrane.
The waves reach maximum height at a certain point: high pitch near base and low pitch sounds near apex.
The displacements of basilar membrane are dissipated by the round window.
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18. Basilar Membrane.
Basilar membrane contains basilar fibers projecting from modiolus to outer wall.
Fibers are stiff, elastic, reed like.
They can vibrate like the reeds of harmonica.
The length increases towards the apex to 0.5 mm.
Diameter of the fibers decrease towards apex.
So, basal fibers vibrate best in high frequency and apical at low frequency.
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19. Organ of Corti. Located on the basilar membrane.
Extends from base to apex.
Hair cells – receptors.
One row of inner hair cells [3500] and three rows of outer hair cells [20000].
Thin, viscus and elastic tectorial membrane covers the hair cells. Hairs of the outer cells are embedded in the membrane.
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20. Perception in Organ of Corti.
Sound waves distort the basilar membrane and the location of maximum distortion is derermined by the frequency [pitch]
As the basilar membrane and the tectorial membrane are hinged in different axis, movements bend the hair in the organ of Corti.
Bending of the hair in one direction depolarizes the cell which sets up action potential in the axon.
Bending in opposite direction hyperpolarizes.
Inner hair cells are the primary sensory cells.
Role of outer hair cells not clear.
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21. Innervation.
Spiral ganglion found in the modiolus contains the cell bodies of afferent nerves.
90-95 % of the afferents innervate inner hair cells and the balance outer hair cells.
Most of the efferent fibers end on the outer hair cells.
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22. Afferents end in dorsal and ventral cochlear nuclei in the medulla.
Central connections are bilateral at all levels.
Inferior colliculi - auditory reflexes.
Medial geniculate body relays to auditory cortex.
Afferents to cerebellum and reticular system.
Olivocochlear fibers are the efferents.
Central connections.
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23. Cortical representation.
The primary auditory area is in the superior temporal gyrus.
Low frequency anteriorly and high frequency posteriorly.
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24. Determination of loudness:
Louder sounds vibrate basilar membrane more, causing more bending of hair, and higher frequency of action potential in the nerve.
Number of nerves conducting impulses- hair cells before and after the specific area for the frequency vibrate as the amplitude increases.
?outer hair cells may also be involved.
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25. Sound localization:
Difference in time between arrival in the two ears- 20μs difference can be detected.
Difference in loudness between the ears.
Turning the head to facilitate localization.
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26. Masking.
Presence of one sound decreases ability to hear another sound.
Middle ear muscles could play a role by altering the tension in membranes- especially low frequency sounds could be masked.
Efferents could inhibit hair cells.
Cortical selection of preferred sound.
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27. Deafness. Conduction deafness- defect in conduction up to middle ear.
Wax, damage to tympanic membrane, stiffness of joints of ossicles.
Nerve deafness- damage to haircells or nerves.
Infection, injuries.
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28. Tests for hearing. Tuning fork test-
Weber test- tuning fork at vertex.
Rinne test- tuning fork on mastoid process, when no sound before the ear.
Audiometry.
Pure tones produced at different frequencies and sent through ear phons.
The threshold amplitude is recorded for each frequency and plotted.
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