1. The physiology of the ear involves sound being transmitted through the external ear to the middle ear, where it is amplified by the ossicles and impedance matched to the cochlea. 2. In the cochlea, the basilar membrane vibrates according to the sound frequency, generating nerve impulses via hair cells that are transmitted through the auditory pathway to the brain. 3. The auditory pathway involves several relay stations from the cochlear nuclei to the auditory cortex, allowing for analysis of sound properties like frequency and loudness.

1. Physiology of Ear
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
RDGMC

2. Learning objectives
 Describe the structure of the middle ear
 List middle ear functions
 Define impedance matching
 Describe the cochlea
 Describe the theory of transduction of sound
 Trace auditory pathway
 Describe the types of deafness

3. Introduction
 Sound is transmitted to the middle ear through
external ear.
 Sound is amplified in the middle ear
 Decibel: unit of loudness of sound
 Hertz: Number of cycles per second

4. Middle ear
 Tympanic membrane – eardrum
 Ossicles – conduct sound from the middle ear to cochlea
 Malleus – attached to tympanic membrane (handle of
malleus)
 Incus- malleus is attached to incus with a minute ligament
 Stapes – the incus articulates with stem of the stapes. Face
plate of stapes lies against the membranous labyrinth of
cochlea in the opening of oval window.

5. Impedance matching
 The amplitude of movement of stapes faceplate with each sound vibration
is only three-forth as much as amplitude of the handle of malleus.
 Ossicular system reduces distance and increase the force of movement
about 1.3 times
 In addition, surface area of tympanic membrane is 55 mm square
 Surface area of stapes 3.2 mm square
 17 fold difference
 This 17 fold difference and 1.3 fold ration increases the total force of lever
system on cochlear fluid about 22 times. Fluid has more inertia than air.
Increased force needed to cause vibration in fluid

6. Impedance matching
 Ossicular system and tympanic membrane
provides impedance matching between the sound
waves in the air and sound vibrations in the fluid.
 In absence of impedance matching also sound
waves can reach cochlea
 But the sensitivity of hearing is less

7. Attenuation of sound
 Transmission of loud sound through ossicular system to CNS
 Reflex contraction of stapedius and tensor tympani muscles.
 Latent period 40- 80 ms
 Tensor tympani muscle- pulls handle of malleus inward
 Stapedius – pulls stapes outwards
 Two opposite forces
 Increases rigidity in ossicular system
 Reducing the ossicular conduction of low frequency sound (cycles below
1000 cycles/ sec)
 Attenuation reflex

8. Attenuation reflex
1. Protect the cochlea from damaging vibrations
caused by excessively loud sound
2. To mask low frequency sounds in loud
environment
 Another function of stapedius and tensor tympani
is reducing persons hearing sensitivity to his own
speech

9. Transmission of sound through
bone
 Cochlea is embeded in bony cavity in the temporal
bone
 Bony labyrinth – bony cavity
 Vibration of entire skull can cause fluid vibrations in
the cochlea
 Placing a tuning fork on mastoid process – person can
hear sound
 Auditory function tests

10. Cochlea
 System of coiled tubes
 Three tubes coiled side-by-side
1. Scala vestibuli
2. Scala media
3. Scala tympani
 1, 2 - separated by Reissner membrane / vestibular membrane
 2, 3 are separated by basilar membrane
 On basilar membrane – organ of corti located – contains series of electro
mechanically sensitive cells – hair cells
 Auditory function tests

11. Cochlea

12. Travelling wave
 When the foot plate of stapes moves inward
against the oval window
 Round window must bulge outward
 The initial effect of sound wave that enters at oval
window is to cause the basilar membrane at the
base of cochlea to bend in the direction of round
window

13. Travelling wave
 Every wave is weak at the outset
 Become strong when it reaches the portion of basilar
membrane that has naturally resonant frequency equal
to respective sound frequency
 At this point, the basilar membrane vibrate back and
forth
 Consequently the wave dies at this point
 Fails to travel remaining part of basilar membrane

14. Travelling wave
 High frequency wave – travels only short
distance
 Medium frequency wave – travels half way
 Very low- frequency wave – travels entire
distance along the membrane

15. Travelling wave

16. Organ of corti
 Receptor organ lies on surface of basilar fibers and basilar membrane
 Generates nerve impulses in response to vibration of the basilar
membrane
 Actual sensory receptors in the organ of corti are two specialized nerve
cells – hair cells
 Number of hair cells – 3500
 The nerve fibers stimulated by the hair cells lead to the spiral ganglion of
corti that lies in centre of cochlea
 Spiral ganglion neuronal cells sends fibers into cochlear nerve and then
to CNS

18. Organ of corti
 Minute hairs or sterio cilia project outwards from hair cells
 Embeded in the surface gel coating of tectorial membrane
 Tectorial membrane is located in the scala media
 Hair cells are similar to hair cells in the vestibular apparatus
 Bending of hairs in one direction- excitation
 Bending of hair cells in opposite direction – inhibition
 This in turn excites the auditory nerve fibers synapsing with
their bases

19. Inner vs outer hair cells
 More outer hair cells than inner hair cells
 90% of auditory fibers are stimulated by inner cells
 10% of auditory fibers are stimulated by outer cells
 Damage of all outer hair cells – profound hearing loss
 Outer hair cells controls the sensitivity of inner hair
cells to different sound pitches
 This phenomenon is called tuning

20. Hair cell receptor potential
 Steriocilia are stiff structures
 Has rigid protein frame work
 Each hair cell has bout 100 steriocilia on its apical border
 These steriocilia become progressively longer on the side of the hair cells away from
modiolus
 Tops of the shorter steriocilia are attached by thin filaments to the back side of the
adjacent longer steriocilia
 When sterio cilia are bent in direction of longer hair cells
 Mechanical transduction
 Opening of 200-300 cation conducting channels
 Rapid movement of positively charged potassium ions from surrounding scala media
fluid
 Depolarization of hair cell membrane

21. Endocochlear potential
 Scala media is filled with a fluid – endolymph
 Scala vestibuli and scala tympani fluid – perilymph
 Scala vestibuli and scala tympani communicate
directly with subarachnoid space around the brain
 So, perilymph is almost identical to CSF
 Endolymph is opposite to peri lymph
 High conc of potassium and low conc of sodium

22. Endocochlear potential
 Between endolymph and perilymph there exist a
potential +80 mv
 Positivity inside the scala media
 Negativity outside
 This is called endocochlear potential
 Generated by continual secretion of positive potassium
ions into the scala media by the stria vascularis

23. Determination of sound
frequency – Place principle
 Low frequency sounds causes maximum activation of basilar membrane
near the apex of cochlea
 High frequency sounds activate basilar membrane near the base of
cochlea
 Intermediate frequency sounds activate the membrane at intermediate
distance between two extremes
 Spatial organization of nerve fibers from cochlea to cortex
 Specific brain neurons are activated by specific sound frequencies
 The mechanism used by the brain to detect different sounds is to
determine the area of basilar membrane that is maximum stimulated.

24. Determination of loudness
 Louder the sound – amplitude of basilar membrane vibration
increases
 Excitation of nerve endings with high rates
 More and more hair cells fires
 Spatial summation of impulses
 Transmission through many nerve fibers rather one
 Further, stimulation of outer hair cells occurs
 Appraisal of loud sound by CNS

25. Range of hearing
 Young person – 20 and 20,000 cycles/ second
 Old age – 50- 8000 cycles/ second
 Old age – frequency range is shortened

26. Auditory pathway
 Nerve fibers from spiral ganglion of corti enters the
dorsal and ventral cochlear nuclei located in the upper
part of the medulla
 At this point, all the fibers synapse and second order
neurons originates
 Second order neurons pass mainly to opposite side of
the brain stem and terminates in the superior olivary
nucleus.

27. Auditory pathway

28. Auditory pathway
 Few second order neurons pass to the superior olivary
nucleus on the same side
 From superior olivary nucleus, the auditory pathway
passes through the lateral leminiscus
 Some of the fibers terminate in the nucleus of
lemniscus
 But many fibers bypass this nucleus and travel to
inferior colliculus where all fibers synapse

29. Auditory pathway
 Few second order neurons pass to the superior olivary
nucleus on the same side
 From superior olivary nucleus, the auditory pathway
passes through the lateral leminiscus
 Some of the fibers terminate in the nucleus of
lemniscus
 But many fibers bypass this nucleus and travel to
inferior colliculus where all fibers synapse

30. Auditory pathway
 From there the pathway pass to medial geniculate
nucleus
 Here all the fibers synapse
 Finally, the pathway proceeds by way of the
auditory radiation to the auditory cortex
 Auditory cortex located in the superior gyrus of the
temporal lobe

31. Auditory pathway
 Signals from both the ears are transmitted through the
pathways of both sides of the brain, with predominance
of transmission in the contralateral pathway
 In three places of brain stem crossing over occurs
 Trapezoid body
 In the commissure between two nuclei of lateral
lemniscus
 In the commissure between two inferior colliculus

32. Auditory pathway
 Many collaterals from auditory pathway pass directly
into the reticular activating system
 This system projects diffusely upward in the brain
stem and downwards to spinal cord
 Activates entire nervous system in response to loud
sounds
 Other collaterals goes to cerebellum, which is activated
immediately in the event of sudden noise

33. Auditory cortex
 Primary auditory cortex – Directly excited by
projections from medial geniculate nucleus
 Secondary auditory cortex – excited secondarily by
impulses from primary auditory auditory cortex
 Destruction of both auditory cortices in human being
greatly reduces hearing sensitivity
 What happens if one side auditory cortex damaged?

35. Auditory cortex
 What happens if one side auditory cortex damaged?
 Slightly reduces hearing in the opposite ear
 It does not cause deafness in the ear because of many
cross over connections from side to side in the
auditory pathway
 However, the person ability to localize source of sound
decreases as localization need signals in both cortices

36. Blind-fold fight not possible after
damage of auditory cortex

37. Damage of auditory association
areas
 If primary auditory cortex not damaged, then person
hearing ability is normal
 But person can not interpret meaning of the sound
heard
 Example – Damage of Wernicke’s area
 Person can perfectly hear sound and can repeat also
 But can not interpret meaning of it

38. Hearing abnormalities
 Deafness is usually divided into two types
1. Caused by impairment of the cochlea, auditory
nerve or central nervous system circuits from the
ear – Nerve deafness
2. Caused by impairment of the physical structures
of the ear that conduct sound itself to the cochlea
- conduction deafness

39. Hearing abnormalities
 If either cochlea or auditory nerve is destroyed
1. Person becomes permanently deaf

40. Audiometer
 To determine nature of hearing disabilities
 Earphone connected to electronic oscillator
capable of emitting pure tones ranging from low
frequencies to high frequencies
 8-10 frequencies has to be tested
 Hearing loss determined for each frequency
 Plotted on a graph - audiogram

41. THANK YOU