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Physiology of Hearing

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Physiology of Hearing Processing of auditory signal Auditory nerve The place principle Intensity of the stimulus is coded as an increase in the frequency of action ... – PowerPoint PPT presentation

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Title: Physiology of Hearing


1
Physiology of Hearing
2
Sound
  • Sound is a form of energy
  • It is transmitted through a medium as a
    longitudinal pressure wave
  • The wave consists of a series of compressions
    and rarefactions of the molecules in the medium
  • The ear is capable of capturing this energy and
    perceiving it as sound information

3
Sound waves
Compression
the graph showing a sine wave refers only to
variations in pressure or compression, not to the
actual displacement of air
4
Properties of sound
  • The wave motion of sound can be described in
    terms of Amplitude, Frequency, Velocity and
    Wavelength 

5
Properties of sound
WavelengthRefers to the physical distance
between successive compressions and is thus
dependant on the speed of sound in the medium
divided by its frequency Amplitude (Intensity or
loudness) Refers to the difference between
maximum and minimum pressure Frequency
(pitch)Refers to the number of peak-to-peak
fluctuations in pressure that pass a particular
point in space in one second VelocityRefers
to the speed of travel of the sound wave. This
varies between mediums and is also dependant on
temperature (in air at 20C it is 343 m/s)
6
Loudness (or amplitude)
  • The intensity of sound is perceived as loudness
  • It is measured on a relational scale with the
    unit of measurement being the decibel (after
    Alexander Graham Bell)
  • Sound intensities require a standard sound level
    against which they are compared
  • The standard sound pressure level (SPL 0dB) is
    0.0002 dynes/cm2
  • The decibel is a numeric value that represents
    sound intensity with respect to the reference
    sound pressure level

7
Loudness (or amplitude)
  • sound pressure levels of common sounds
  • Sound intensity is measured on a logarithmic
    scale
  • An increase of 6 dB of sound pressure is
    perceived as double the intensity of the sound

SOUND dB SPL
Rocket Launching pad 180
Jet plane 140
Gunshot blast 130
Car horn 120
Pneumatic drill 110
Power tools 100
Subway 90
Noisy restaurant 80
Busy traffic 75
Conversational speech 66
Average home 55
Library 40
Soft whisper 30

8
Frequency (or pitch)
  • Frequency is perceived as the pitch of a sound
  • The higher the frequency, the higher the pitch
    and vice versa
  • The range of human hearing is said to be from 20
    - 20,000 Hz
  • The speech frequencies those frequencies most
    important for human hearing are from
    approximately 250 - 4000 Hz

9
Transmission of sounds through the ear
  • External ear
  • Mostly through air (External acoustic meatus)
  • Middle ear
  • Through solid medium - bone (ossicles)
  • Inner ear
  • Through fluid medium endolymph (cochlea)

10
Parts of the ear
11
Air and bone conduction
  • There are two methods by which hair cells can be
    stimulated
  • Air conduction
  • Sound stimulus travelling through the external
    and middle ear and activating the hair cells
  • Bone conduction
  • Sound stimulus travelling though the bones of the
    skull activating the hair cells
  • Whatever method it takes, the sound stimulus
    finally activate hair cells in the cochlea

12
External ear
  • Consists of
  • Pinna
  • External auditory meatus

13
Middle ear
  • composed of
  • the tympanic membrane
  • the tympanic cavity
  • the ossicles
  • Malleus
  • Incus
  • Stapes (connected to the oval window of the
    cochlea)
  • two muscles
  • the tensor tympani attached to the malleus
  • the stapedius muscle attached to the stapes
  • the Eustachian tube

14
Inner ear
  • Consists of two main parts
  • the cochlea (end organ for hearing)
  • the vestibule and semicircular canals (end organ
    for balance)
  • The inner ear can be thought of as a series of
    tunnels or canals within the temporal bone
  • Within these canals are a series of membranous
    sacs (termed labyrinths) which house the sensory
    epithelium
  • The membranous labyrinth is filled with a fluid
    termed endolymph
  • It is surrounded within the bony labyrinth by a
    second fluid termed perilymph
  • The cochlea can be thought of as a canal that
    spirals around itself similar to a snail. It
    makes roughly 2 1/2 to 2 3/4 turns

15
Cross section through cochlea
16
Cochlea
  • The bony canal of the cochlea is divided into an
    upper chamber, the scala vestibuli and a lower
    chamber, the scala tympani by the membranous
    labyrinth also known as the cochlear duct
  • The floor of the scala media is formed by the
    basilar membrane, the roof by Reissner's membrane
  • The scala vestibuli and scala tympani contain
    perilymph
  • The scala media contains endolymph

17
Endolymph and perilymph
  • Endolymph is similar in ionic content to
    intracellular fluid (high K, low Na)
  • Perilymph resembles extracellular fluid (low K,
    high Na)
  • The cochlear duct contains several types of
    specialized cells responsible for auditory
    perception

18
Cohlea
19
  • The sensory cells responsible for hearing are
    located on the basilar membrane within a
    structure known as the organ of Corti
  • This is partitioned by two rows of peculiar
    shaped cells known as pillar cells
  • The pillar cells enclose the tunnel of Corti
  • Situated on the basilar membrane is a single row
    of inner hair cells medially and three rows of
    outer hair cells laterally
  • The hair cells and other supporting cells are
    connected to one another at their apices by tight
    junctions forming a surface known as reticular
    lamina
  • The cells have specialized stereocilia on their
    apical surfaces

20
Organ of Corti
21
  • Attached to the medial aspect of the scala media
    is a fibrous structure called the tectorial
    membrane
  • It lies above the inner and outer hair cells
    coming in contact with their stereocilia

22
  • The fluid in the space between the tectorial
    membrane and reticular lamina is endolymph
  • Thus the endolymp bathes the stercocillia
  • But the body of the hair cells which lies below
    the reticular lamina is bathed by perilymph

23
Hair cells
24
(No Transcript)
25
  • Synapsing with the base of the hair cells are
    dendrites from the auditory nerve
  • The auditory nerve leaves the cochlear and
    temporal bone via the internal auditory canal and
    travels to the brainstem

26
Transmission of sound waves
  • The outer ear and external auditory canal act
    passively to capture the acoustic energy and
    direct it to the tympanic membrane
  • There, the sound waves strike the tympanic
    membrane causing it to vibrate
  • These mechanical vibrations are then transmitted
    via the ossicles to the perilymph of the inner
    ear
  • The perilymph is stimulated by the mechanical
    (vibrations) energy vibrations to form a fluid
    wave within the cochlea

27
Middle ear
  • The middle ear acts as an impendance-matching
    device
  • Sound waves travel much easier through air (low
    impedance) than water (high impedance)
  • If sound waves were directed at the oval window
    (water) almost all of the acoustic energy would
    be reflected back to the middle ear (air) and
    only 1 would enter the cochlea. This would be a
    very inefficient method.
  • To increase the efficiency of the system, the
    middle ear acts to transform the acoustic energy
    to mechanical energy which then stimulates the
    cochlear fluid

28
Middle ear
  • The middle ear also acts to increase the acoustic
    energy reaching the cochlea by essentially two
    mechanical phenomenon
  • The area of the tympanic membrane is much greater
    than that of the stapes footplate (oval window)
    causing the force applied at the footplate per
    square area to be greater than the tympanic
    membrane
  • The ossicles act as a lever increasing once again
    the force applied at the stapes footplate
  • Overall, the increase in sound energy reaching
    the cochlea is approximately 22 times

29
Cochlea
  • The cochlea consists of a fluid filled bony canal
    within which lies the cochlear duct containing
    the sensory epithelium
  • Energy enters the cochlea via the stapes bone at
    the oval window and is dissipated through a
    second opening (which is covered by a membrane)
    the round window
  • Vibrations of the stapes footplate cause the
    perilymph to form a wave
  • This wave travels the length of the cochlea
  • It takes approximately 5 msec to travel the
    length of the cochlea

30
Cochlea
  • As it passes the basilar membrane of the cochlear
    duct, the fluid wave causes the basilar membrane
    to move in a wave-like fashion (i.e. up and down)
  • The wave form travels the length of the cochlea
    and is dissipated at the round window
  • Due to changes in the mechanical properties of
    the basilar membrane, the amplitude of vibration
    changes as one travels along the basilar membrane

31
The place principle
  • Low frequency stimuli cause the greatest
    vibration of basilar membrane at its apex, high
    frequency stimuli at its base

Neurolab
32
  • As the basilar membrane is displaced superiorly
    by the perilymph wave, the stereocilia at the
    apex of each inner and outer hair cell, which are
    imbedded in the tectorial membrane undergo a
    shearing force (i.e. they are bent)
  • This shearing force causes a change in the
    resting membrane potential of the hair cell which
    is transmitted to its basal end
  • There a synapse is formed with a dendrite from
    the auditory nerve
  • The hair cell membrane potential change is
    transmitted across this synapse (? via
    acetylcholine) causing depolarization of the
    nerve fiber
  • This neural impulse is then propagated to the
    auditory centres of the brain

33
From the ear to the auditory cortex
34
Processing of auditory signal
  • Auditory nerve
  • The place principle
  • Intensity of the stimulus is coded as an increase
    in the frequency of action potentials
  • There is also recruitment of additional nerve
    fibres as the intensity increases
  • Cochlear nuclei
  • There is tonotopic organisation (neurons are
    arranged according to the sensitivity to each
    frequency)
  • Further processing happens

35
Processing of auditory signal
  • Superior olivary complex
  • Impulses from both ears are compared
  • This is necessary for the localisation of sound
  • Lateral leminscus, inferior colliculus, medial
    geniculate body
  • Further processing happens
  • Temporal lobe
  • Unique feature of cortical neuronal response to
    auditory stimulus is the brief duration of the
    response
  • Localisation of sound and sound discrimination
    based on the sequence of sounds in the stimulus
    occurs in the cortex

36
Perception of different characteristics of sound
  • Frequency
  • Starts at the basilar membrane and frequency
    sharpening occurs throughout the auditory pathway
  • Intensity
  • Starts at the hair cells (OHC are stimulated by
    weaker stimulus)
  • Frequency of impulses
  • Direction
  • Inter-aural time difference
  • Pattern recognition
  • Cortical function
  • Interpretation of speech
  • Complex cortical phenomenon

37
Hearing loss
  • Hearing can be defined as the ability to receive
    and process acoustic stimuli (i.e. sound)
  • Hearing is an important function for
    communication and provides people with
    pleasurable experiences such as listening to
    music
  • The loss of ability to hear has important
    consequences in ones day to day life and ability
    to function within the hearing culture (vs the
    deaf culture)
  • Hearing loss can be broadly defined as the
    decreased ability to receive or process acoustic
    stimuli

38
Hearing loss
  • It has several causes conduction, sensorineural,
    mixed, central or functional
  • Hearing loss is very common in our society
  • Its incidence is approximately 0.2 in those
    under 5 years of age, 5 in those 35-54 years of
    age, 15 of those 55-64 years of age and 40 (or
    more) in those over 75 years of age (in the west)
  • As one ages, the likelihood of hearing loss
    increases

39
Conduction deafness (or conductive deafness)
  • A conductive hearing loss exists when sound waves
    for any reason are not able to stimulate the
    sensory cells of the inner ear (i.e. cause a
    fluid wave within the cochlea)
  • Examples of conditions causing a conductive
    hearing loss include
  • impacted wax
  • external auditory canal atrecia
  • perforation of the tympanic membrane
  • ossicular discontinuity
  • Otosclerosis
  • Middle ear disease

40
Conduction deafness (or conductive deafness)
  • In a conductive hearing loss, the sound waves
    cannot be transformed into a fluid wave within
    the cochlea, thus the sensory cells receive
    decreased or no stimulation
  • The maximum conductive hearing loss is
    approximately 60 dB
  • Many conductive hearing loss can cured

41
Sensorineural deafness or nerve deafness
  • Sensorineural hearing loss occurs when the
    sensory cells of the cochlea (inner ear) or the
    auditory nerve fibers are dysfunctional
  • The acoustic energy (sound wave) is not capable
    of being transformed inside the cochlea to
    nervous stimuli
  • Reasons for this include
  • noise damage to the cochlea
  • aging (presbycusis)
  • ototoxic medications
  • tumours such as an acoustic neuroma

42
Sensorineural deafness or nerve deafness
  • Hearing loss can be in excess of 100 dB
  • Sensorineural hearing loss is, in general, cannot
    be cured
  • Cochlear implants are available as a method of
    treatment

43
Cochlear implants
44
Mixed Hearing Loss
  • Mixed hearing losses are simply the combination
    of a conductive and sensorineural hearing loss
  • For example, an elderly person with presbycusis
    plus impacted wax (cerumen)
  • or a heavy metal musician with noise induced
    hearing loss who develops a perforated tympanic
    membrane

45
Central Hearing Loss
  • Central hearing loss occurs in the auditory areas
    of the brainstem and higher levels (temporal
    lobe)
  • Very little information is known about lesions
    that cause this type of impairment
  • Persons with central hearing loss have normal
    hearing, but have difficulty with the processing
    of auditory information (word deafness)

46
Functional Hearing Loss
  • Persons with functional hearing loss have no
    physiologic basis for a hearing deficit
  • They are using their 'hearing loss' for secondary
    gain and are called malingerers
  • This is occasionally seen in adolescents or
    persons appying for pension benefits as a result
    of hearing loss

47
  • All of the different types of hearing loss
  • can be present at birth, i.e. congenital
  • or acquired later on in life

48
Diagnosis of Hearing Loss
  • The diagnosis of hearing loss can be relatively
    simple ("I can't hear from my right ear") to the
    more subtle (Sunil seems to have difficulty
    saying some words")
  • Auriscopic examination and identify the any
    structural defect in the ear canal
  • Tests of hearing need to be done

49
Tests of hearing
  • Tuning fork tests
  • Rinnes test
  • Webers test
  • Pure tone audiometry (PTA)
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