Physiology of hearing ppt

physiology of hearing

1. PHYSIOLOGY OF HEARING BY: Dr.Humra shamim

2. IS HEARING IMPORTANT? Communication: hearing is essential to language Localisation : determination of location of unseen sound sources

3. What is required for normal hearing? Adequate stimulus (sound) Conduction of stimulus to sensory organ of hearing Sensory transduction of stimulus at organ of hearing Neural transmission of the signal Central auditory processing of the signal at brain

4. SOUND Sound is a form of energy that propagates in the form of waves The speed of sound depends on the medium through which the wave passes. Speed of sound in air is 343m/s in water is 1482m/sec The sound frequencies audible to humans range from about 20 to 20,000 cycles per second (cps, Hz). sound intensity is expressed by taking the logarithmic ratio of two sound intensities (the numerator being the sound intensity of interest, and the denominator being a reference sound intensity) and multiplying by 10. dB = 10 log 10  J/ Jr , where J is the intensity of the sound of interest, and Jr is the intensity of reference

5. Technical jargon:

6. intensity Intensity is defined as the power transmitted by sound wave a unit area. Intensity is dependent on pressure and velocity average taken over whole cycle Intensity =peak pressure x peak velocity/2 Displacement produced by sound waves vary with frequency if the intensity is constant Low frequency vibrations produce greater displacements

7. Simple harmonic motion. Simple harmonic motion is a periodic motion that undulates around a null point with equal amplitudes. The amplitude is the maximum amount of displacement from the null point in one direction. The frequency of a simple harmonic motion is the number of cycles per second, and is measured in Hertz (Hz). The period of a cycle is the inverse of its frequency (1/ f), and represents the duration of a single cycle

8. hUman auditory field The human ear is sensitive to sound over wide range of amplitudes:0.0002—200 dyne/cm2 It can detect the difference between two sounds occuring 10micro seconds apart in time.

9. EAR ACTS AS A TRANSDUCER

10. Natural resonant frequency EXTERNAL AUDITORY CANAL--------------- 3000Hz TYMPANIC MEMBRANE----------------------- 800-1600Hz MIDDLE EAR---------------------------------------- 800Hz OSSICULAR CHAIN------------------------------ 500-2000Hz

11. Functions of external ear:

12. External ear Act as a resonator It increases the pressure at the ear drum in a frequency sensitive way Helps in localisation of direction of sound

13. Sound collection Pinna- concha system catches sound over large area and concentrate it to smaller area of ext. auditory meatus . This increases the total energy available to the tympanic membrane

14. Features of external canal Open on one end only The impedance of ear drum is about 3-4times more than air 30% of incident energy gets reflected from external canal Efficient in conducting sound in frequency range of 3-5kHz Cuts off unwanted frequency helping in better speech discrimination

15. Pressure increase by EAC If a tube which is closed at one end and open at other is placed in a sound field then pressure is low at open end and high at closed end. This phenomenon is seen in EAC at 3kHz frequency , and at concha at 5kHz The two main resonance are complementary , and increases sound pressure in range of 2-7kHz.

16. Sound localisation : Because of its shape , the pinna shield the sound from rear end,change timbre,and helps to localize sound from in front or back Cues for sound localization from right/left Sound wave reaches the ear closer to sound source before it arise in farthest ear Sound is less intense as it reaches the farthest ear because head act as barrier Auditory cortex integrates these cues to determine location

17. TOTAL GAIN The total effect of reflection of sound from head,pinna and external canal resonances is to add 15-20dB to sound pressure, over frequency range of 2-7kHz.

18. Functions of middle ear: Couples sound energy to the cochlea Impedance matching Attenuation reflex Physically protects the cochlea Phase differential effect :Couples sound preferentially to only one window ,thus producing a differential pressure between the windows required for the movement of cochlear fluid

19. Impedance transformer Impedance is defined as the resistance offered by a medium for transmission of sound middle ear acts as impedance transformer Cochlear fluids have an impedance equall that of sea water (1.5X10 N.sec/m3)

20. Impedance transformer Impedance is defined as the resistance offered by a medium for transmission of sound Middle ear ossicles are suspended by ligaments Axis of rotation of ossicles and axis of suspension by ligaments virtually coincides with their centre of inertia At low frequencies the ligaments play an important role in maintaining ossicular positions(elastic effect) Middle ear converts the low pressure high displcement vibrations of ear drum into high pressure low displacement vibrations this is suitable to drive cochlear fluids

21. Impedance mismatch IF THERE WAS NO MIDDLE EAR SYSTEM ,99% OF SOUND WAVES WOULD HAVE REFLECTED BACK FROM OVAL WINDOW MIDDLE EAR BY ITS IMPEDENCE MATCHING PROPERTY ALLOWS 60% OF SOUND ENERGY TO DISSIPATE IN INNER EAR

22. Impedance efficiency Only 60%of sound energy from TM gets transmitted &absorbed in the cochlea Without the middle ear only 1%of sound energy will be absorbed by the cochlea

23. Low frequency sound dampeners Middle ear efficiency is the best at 1kHz There is transmission loss of low frequency sounds due to elastic stiffness of middle ear ligaments(annular ligament is the most important) Air inside middle ear cavity also dampens low frequency sound transmission Grommet insertion improves transmission of low frequency sounds

24. “Impedance Matching” by the middle ear System

25. A) Area of the tympanic membrane relative to oval window Total effective area of tympanic membrane 69mm2 Area of stapes footplate is 3.2mm2 Effective areal ratio is 14:1 Thus by focusing sound pressure from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased

26. b) Lever action of ear ossicles Handle of malleus is 1.3 times longer than long process of incus Overall this produces a lever action that converts low pressure with a long lever action at malleus handle to high pressure with a short lever action at tip of long process of incus

27. c) Shape of the tympanic membrane TM buckles as it moves to and fro This reduces malleolar movement TM thus acts as a mechanical lever This causes high pressure low displacement system

28. HYDRauLIC ACTION OF TYMPANIC MEMBRANE The most important factor in the middle ear's impedance matching capability comes from the “area ratio” between the tympanic membrane and the stapes footplate Total area of tympanic membrane 90mm2 Functional area of tympanic membrane is two third (69mm2).Area of stapes footplate is 3.4mm2.So , Effective areal ratio is 14:1 Thus by focusing sound pressure from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased

29. action of tympanic membrane Eustachian tube equilibriates the air pressure in middle ear with that of atmospheric pressure,thus permitting tympanic membrane to stay in its most neutral position. A buckling motion of tympanic membrane result in an increased force and decreased velocity to produce a fourfold increase in effectiveness of energy transfer

30. Total transformer ratio=14x1.3=18.2:1 The combined effects of the area ratio and the lever ratio give the middle ear output a 28-dB gain theoretically. In reality, the middle ear sound pressure gain is only about 20 dB; this is mostly due to the fact that the tympanic membrane does not move as a rigid diaphragm

31. PHASE DIFFERENTIAL EFFECT Sound waves striking the tympanic membrane do not reach the oval and round window simultaneously. There is preferential pathway to oval window due to ossicular chain. This acoustic separation of windows is achieved by intact tympanic membrane and a cushion of air around round window This contributes 4dB when tympanic membrane is intact

32. ROLE OF MIDDLE EAR MUSCLES: TENSOR TYMPANI MUSCLE ATTACHES TO THE HANDLE OF MALLEUS.IT PULLS THE DRUM MEDIALLY. STAPEDIUS MUSCLE ATTACHES TO THE POSTERIOR ASPECT OF STAPES CONTRACTION OR THESE MUSCLE INCREASES THE STIFFNESS OF OSSICULAR CHAIN THUS BLUNTING LOW FREQUENCIES DECREASES A PERSON’S SENSITIVITY TO THEIR OWN SPEECH

33. Protective functions of middle ear muscles Stapedius contraction can reduce transmission by upto 30dB for frequencies less than 1-2 kHz. for higher frequencies this is limited to 10dB. Only the stapedius muscle contracts in response to loud noise in humans The whole stapedial reflex arc has 3-4 synapses Stapedial reflex latency is 6-7ms

34. Attenation reflex When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of only 40 to 80 ms to cause contraction of the stapedius muscle and the tensor tympani muscle The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low frequency sound

35. Damaged middle ear scenarios Damaged middle ear can cause loss of transformer mechanism Differntial pressure levels between the two windows could not be maintained Scala vestibuli is more yielding than scala tympani .differential movements of fluid is still possible . Small compliance of annular ligament in comparison to much larger compiant round window could again cause differential pressure

36. Bone conduction Normal route for hearing some component of one’s own voice Useful in cases of severe conductive losses Can be used as a diagnostic tool

37. Bone conduction inner ear factors: Intrinsic detection of distortional vibrations of cochlear bone Differential distortion of bony structures of cochlea( s.vestibuli is larger than s.tympani )could cause movement of cochlear fluid Direct vibration of osseous spiral lamina Direct transmission of vibrations from the skull via CSF to the cochlear fluids Leaving one window open improved sound conduction

38. Bone conduction middle ear factors Vibration of the skull faithfully transmitted to the ossicles of middle ear cavity Inertia of middle ear ossicles doesn’t coincide with their point of attachment Middle ear acts as a band pass filter with peak transmission around 1kHz This accounts for carharts notch though at a slightly higher frequency

39. Bone conduction external ear factors: Bone vibrations are conducted through the external canal and the air within it Vibrations can escape externally if the canal is open Occlusion of external ear increases bone conduct ion External radiation of sound is best for low frequencies, hence change with occlusion Is greatest for these frequencies.

40. INNER EAR PHYSIOLOGY The two important functions of the inner ear are HEARING and BALANCE. The portion of the inner ear that deals with hearing is the cochlea , and that deals with balance is collectively known as the vestibular organs (semicircular canals, utricle, and saccule ) . COCHLEA acts as a TRANSDUCER that translates sound energy into a form suitable for stimulating the dendrites of auditory nerve.

41. Structure of cochlea: The cochlea is a fluid-filled space with three compartments: scala tympani, scala media, and scala vestibuli The scala tympani and the scala media are separated by the basilar membrane, and the scala media and the scala vestibuli are separated by Reissner's membrane. The scala media contains the organ of Corti which contains inner and outer hair cells

42. The inner hair cells are flask-shaped cells,3000 approx in number and arranged in a single row the outer hair cells are cylindrical-shaped,12000 approx in number arranged in 3-4 rows The hair cells derive their names from having hairlike projections on their apical surface. These hair like projections are stereocilia , which play an important role in the signal transduction properties of the hair cells

43. endolymph Formed by stria vascularis Endolymphatic sac maintains homeostsis of endolymph It has a high sodium and low potssium content Endolymph has positive potential gradient +50-120mv( endocochlear potential) Na k ATPase is responsible for this gradient.

44. Perilymph : Site of production is controversial ?CSF Occupies perilymphatic space. continuous between vestibular &cochlear divisions Ionic concentration resembles extracellular fluid Perilymph from s.vestibuli originates from plasma ,while perilymph from s.tympani originates from plasma and CSF Electric potential of s.tymapani is 7mv and s. vestibuli is +5mv

45. Basiliar membrane Separates s.media from s .tympani Length’s of basilar membrane increases progressively from oval window to the apex (0.04mm near oval window and 0.5mm at helicotrema )12 fold increase Diameters of basilar fibres decrease from oval window to helicotrema The stiff short fibres near the oval window vibrate best at very high frequency,while long limber fibres near the tip of cochlea vibrate best at a low frequency.

46. Schematic cross-sectional view of the human cochlea. The scala media (cochlear duct) is filled with endolymph , and the scala vestibuli and tympani are filled with perilymph . The endolymph of the scala media bathes the organ of Corti , located between the basilar and tectorial membranes and containing the inner and outer hair cells. Hair cells contain stereocilia along the apical surface and are connected by tip links. In response to mechanical vibration of the basilar membrane, deflection of stereocilia , displacement of tip links, and opening of gated potassium channels. Epithelial supporting cells ( connexin channels, red ) allow for the flow of potassium ions

47. The scala vestibuli and the scala tympani are filled with perilymph , which has a low potassium concentration. The scala media is filled with endolymph , which has a high potassium concentration. The unique electrolyte composition of the scala media sets up a large electrochemical gradient, called the endocochlear potential, which is about +80 mV relative to perilymph . The maintenance of such a large electrochemical gradient is performed by the stria vascularis

48. Endocochlear potential The importance of is that the tops of hair cells project through the reticular lamina and are bathed by the endolymh of the scala media ,whereas perilymph bathes the lower bodies of the hair cells. further more the hair cells have a negative intracellular potential of -70mv wrt the perilymphbut -150mv wrt endolymph at their upper surfaces where the hair cells project through the reticular lamina and into the endolymph

49. Cochlear mechanics: Mechanical travelling wave in the cochlea is the basis of frequency selectivity The travelling wave reaches a peak and dies away rapidly As the wave moves up the cochlea towards its peak ,it reaches a region in which the membrane is mechanically active. In this region the membrane stars putting energy into the wave .the amplitude raises rapidly only to fall rapidly.

50. Travelling wave theory The movements of the footplate of the stapes set up a series of traveling waves in the perilymph of the scala vestibuli High-pitched sounds generate waves that reach maximum height near the base of the cochlea; low-pitched sounds generate waves that peak near the apex The basilar membrane is not under tension, and it also is readily depressed into the scala tympani by the peaks of waves in the scala vestibuli

51. Schematic showing sound propagation in the cochlea. As sound energy travels through the external and middle ears, it causes the stapes footplate to vibrate. The vibration of the stapes footplate results in a compressional wave on the inner ear fluid. Because the pressure in the scala vestibuli is higher than the pressure in the scala tympani, this sets up a pressure gradient that causes the cochlear partition to vibrate as a traveling wave. Because the basilar membrane varies in its stiffness and mass along its length, it is able to act as a series of filters, responding to specific sound frequencies at specific locations

52. Hair cells: The hairs ends of the OUTER HAIR CELLS are fixed tightly in a rigid structure composed of a flat plate, called the reticular lamina, supported by triangular rods of Corti, which are attached tightly to the basilar fibers. The hairs of the INNER HAIR CELLS are not attached to the tectorial membrane, but they are apparently bent by fluid moving between the tectorial membrane and the underlying hair cells .

53. Inner hair cells Makes large no. of synaptic contact with afferent fibres of auditory neve 95% of afferent auditory nerves make contact with inner hair cells Detects basilar membrane movement Tips of inner hair cells are not embedded in the tectorial membrane as outer hair cells They fit loosely into a groove called “ henson’s groove” They are driven by viscous drag of endolymph inner hair cells respond to the velocity rather than displacement of basilar membrane

54. Outer hair cells Very few outer hair cells synapse with auditory nerves Inside of outer hair cells have -70 mV They serve to amplify basilar membrane vibration They increase the sensitivity and selectivity of cochlea Cochlear microphonics are derived from these cells

55. Resting potential of hair cells Each hair cell has an intracellular potential of (-70mV) with respect to perilymph . At upper end of hair cell the potential difference between intracellular fluid and endolymph is (-150mV) This high potential difference makes the cell very sensitive. Tip links The tops of the shorter stereocilia are attached by thin filaments to the back sides of their adjacent longer stereocilia

56. The basilar fibers, the rods of Corti , and the reticular lamina move as a rigid unit Upward movement of the basilar fiber rocks the reticular lamina upward and inward toward the modiolus. Then , when the basilar membrane moves downward, the reticular lamina rocks downward and outward. The inward and outward motion causes the hairs on the hair cells to shear back and forth against the tectorial membrane.Thus , the hair cells are excited whenever the basilar membrane vibrates

57. Schematic showing the role of tip links in hair cell signal transduction As the stereocilia is deflected toward the direction of the tallest row, it causes the tip links to stretch. The stretch of the tip links causes the opening of stretch-sensitive cationic channels located on the stereocilia The opening of these stretch-sensitive cationic channels on the stereocilia causes a large influx of cationic current, which leads to hair cell depolarization. As the stereocilia is deflected away from the tallest row, it causes a relaxation of the tip links, which decreases the probability of ion channel opening. This leads to hyperpolarization of the hair cell

58. Depolarization/activation When the cilia are bent in the direction of the longer ones, the tips of the smaller stereocilia are tugged outward.This causes a mechanical transduction that opens 200 to 300 cation -conducting channels, allowing rapid movement of potassium ions from the surrounding scala media fluid into the stereocilia , which causes depolarization of the hair cell membrane

59. The influx of potassium inside the cell causes activation of calcium channels This calcium drags the neurotransmitter filled vesicle to fuse with cell membrane at base of cell. Neurotransmitter (glutamate)releases and excites the dendrites of afferent nerve fibres .

60. Central auditory pathway Inputs from auditory nerve drive multiple cell types in different subdivisions of the cochlear nucleus, with each cell type projecting centrally to different targets in the superior olivary complex, lateral lemniscus nuclei, and inferior colliculus Cochlear nucleus is the critical first relay station for all ascending auditory information originating in the ear, and is located in the pontomedullary junction its major subdivisions: the dorsal cochlear nucleus, the anterior ventral cochlear nucleus, and the posterior ventral cochlear nucleus

61. nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus the superior olivary nucleus,the auditory pathway passes upward through the lateral lemniscus .

62. Some of the fibres terminate in the nucleus of lateral lemniscus ,but many bypass this nucleus and travel on to the inferior colliculus,where all or almost all the auditory fibres synapse From there the pathway passes to the medial geniculate nucleus,where all the fibres do synapse Finally the pathway proceeds by way of the auditory radiations to auditory cortex.

63. The lateral lemniscus is formed by the three fiber tracts from the cochlear nucleus The inferior colliculus located in the midbrain just caudal to the superior colliculus. receives projections directly from the cochlear nucleus and information about interaural time and amplitude differences from the medial superior olive and lateral superior olive processes the information it receives and sends fibers to the medial geniculate body of the thalamus. . Functional magnetic resonance imaging showing the ascending pathways of auditory processing from the auditory brainstem to the auditory cortex

64. THE MEDIAL GENICULATE BODY is the thalamic auditory relay center that receives auditory information from the inferior colliculus. It has three divisions: ventral, dorsal, and medial. Plays an important role in sound localization and processing of complex vocal communications, such as human speech AUDITORY CORTEX The main auditory portion of the cerebral cortex resides in the temporal lobe, close to the sylvian fissure The primary auditory cortex is located on the superior surface of the temporal lobe ( Heschl's gyrus ). This is also known as area A1, and corresponds to Brodmann's area 41.

65. The auditory association cortex is also known as area A2, and corresponds to Brodmann's areas 22 and 42. The primary auditory cortex is directly excited by projections from medial geniculate body,whereas the auditory associaton area are excited by impulses from primary auditory cortex as well as some projections thalamic association areas adjacent to MGB the primary auditory cortex is tonotopically tuned, with high frequencies being represented more medially, and low frequencies being represented more laterally FUNCTIONS integrating and processing complex auditory signals, including language comprehension the auditory association cortex plays an important role in speech perception auditory association cortex is located lateral to the primary auditory cortex, and it is part of a language reception area known as Wernicke's area

66. FUNCTIONS OF AUDITORY CORTEX Perception of sound Judging the intensity of the sound Analysis of different properties of sound

67. PECULARITIES OF AUDITORY PATHWAY First ,signals from both ears are transmitted through the pathways of both sides of the brain ,with a preponderance of transmission in the contralateral pathway Second ,many collateral fibres from the auditory tracts pass directly into the reticular activating system of the brain stem Third ,a high degree of spatial orientation is maintained in the fibre tracts from the cochlea all the way to the cortex

68. Determination of Loudness Determined by the auditory system in at least three ways. First, as the sound becomes louder, the amplitude of vibration of the basilar membrane and hair cells also increases, so that the hair cells excite the nerve endings at more rapid rate Second, as the amplitude of vibration increases, it causes more and more of the hair cells on the fringes of the resonating portion of the basilar membrane to become stimulated, thus causing spatial summation of impulses. Third, the outer hair cells do not become stimulated significantly until vibration of the basilar membrane reaches high intensity, and stimulation of these cells presumably apprises the nervous system that the sound is loud.

69. Determination of Sound Frequency— The “Place” Principle There is spatial organization of the nerve fibers in the cochlear pathway, all the way from the cochlea to the cerebral cortex Specific brain neurons are activated by specific sound frequencies The major method used by the nervous system to detect different sound frequencies is to determine the positions along the basilar membrane that are most stimulated. This is called the place principle

70. Auditory nerve fibres : Inner hair cells excite auditory nerves Sound stimulus, transmittor release and action potential generation occur in synchrony (phase locking) Commonly seen at low frequencies

71. Frequency coding at auditory nerve Phase locking Temporal properties (timing of action potential) Frequency selectivity(place coding)

72. Theories of hearing Place theory of Helmholtz Temporal theory of Rutherford Volley theory of Wever Place theory of Lawrence Travelling wave theory of Bekesy

73. PLACE THEORY Acc to helmholtz basilar memebrane has different segments that respond to different frequencies Sharply tuned resonators dampen slowly this could cause after ringing cessation of stimuli This theory fails to explain why a stream of clicks of frequencies ranging from 1220,1300 and 1400 Hz is heard as 1000 Hz

74. Telephonic theory Rutherford proposed that entire cochlea responds as a whole to all freqquencies instead of being activated on a plate by place basis. Here the sound of all frequencies are transmitted as in a telephone cable and frequency analysis is done at a higher level(brain) Damage to certain portion of cochla can cause preferential loss of hearing certain frequencies i.e. like damage to the basal turn of cochlea causing inability to hear high frequency sounds This cannot be explained by telephonic theory.

75. Volley theory Proposed by Wever Several neurons acting as a group can fire in response to high frequency sound even though none of them could do it individually

76. Place volley theory Proposed by lawrence Combines both volley and place theory This theory thus attemps to explain sound transmission and perception

77. Travelling wave theory Proposed by bekesy This theory proposes frequency coding to take place at the level of cochlea. High frequencies are represented towards the base while lower frequencies are closes to apex

78. Tuning by outer hair cells Tuning of sound in basilar membrane requires local addition of mechanical energy There are efferent fibres from crossed olivocochlear bundle supplying the outer cells The inputs from these bundle causes contraction of outer cells located close to maximum of travelling wave give rise to extra distortion of basilar membrane This provides an extra gain of 40-50dB to the system

79. Centrifugal innervation of cochlea Cochlea recieves centrifugal or efferent nerve supply,i.e . olivococchlear bundle It reduces the magnitude of travelling wave ,and possibly protects the ear against moderate level of noise damage Reduces the masking effect of background noise in complex tasks

80. Cochlear echoes/ otoacoustic emissions Energy produced by outer hair cell motility serves as an amplifier within the cochlea, contributing to better hearing OAEs are produced by the energy from outer hair cell motility that makes its way outward from the cochlea through the middle ear, vibrating the tympanic membrane, and propagating into the external ear canal

81. Thank you