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
Rarefaction
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
Wavelength Refers 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
Velocity Refers to the speed of travel of the sound wave. This varies between mediums and is also dependant on temperature (in air at 20°C 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 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 ,000 Hz
The speech frequencies; those frequencies most important for human hearing are from approximately 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

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 years of age, 15% of those 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 Pure tone audiometry (PTA)
Rinne’s test
Weber’s test
Pure tone audiometry (PTA)