Audiology Week 3 - Lecture notes Auditory System PDF

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Audiology Lecture Notes Week 3 The Auditory System  Anatomy: The form structure of the human / animal body  Physiology: The functioning/how the body functions  Pathology: diseases When things go wrong=  Pathophysiology : How things work when something goes wrong = The basic anatomy of the human ear The 3 parts of the human ear  Outer ear  Middle ear  Inner ear The mechanisms of hearing Air conduction Hearing  The primary pathway of hearing  The sound energy goes through the air to the ear canal through the middle ear then it will be encoded by the inner ear  The air conduction pathway - where the sound passes through air Bone Conduction Hearing  Applying a vibrator directly on the skull  when it vibrates it sets the skull in vibration and directly stimulates the inner ear- the cochlea Comparing air conduction and Bone conduction gives us knowledge about different pathologies The Audiogram The description of hearing loss = how to understand and translate what the audio gram tells us How much energy is there in Sound?  The energy that strikes the tympanic membrane that then we perceive as hearing is tiny  The air molecules that move that reach the human ear via a pressure wave  These movements are very small, yet we can perceive them because the human ear is very sensitive.  Remarkably! are able to hear these small waves and more so  The range we can hear is also very remarkable: The difference between the softest and the loudest sound we can hear is about 100 000 000: 1  Humans can perceive a big range of frequencies Young normal hearing adult can hear from a range of 20Hz to 20 000 Hz.  As humans age the sensitivity to the higher frequencies declines  We can also measure/ discern very small differences in pitch i.e. in 1000Hz we can detect change in pitch of 3Hz Frequency = physical correlate of pitch

The ear is very sensitive and has smart mechanism to be able to perceive the small movement of air molecules The anatomy of the human Skull

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The cochlea is housed in the temporal bone of the skull That’s why the bone conduction is possible since the cochlea is housed in the skull Via Bone Conduction we are able to set the skull in vibration and this can be perceived by the cochlea as it is housed in the skull The Anatomy of the human ear

We can divide the peripheral organ of the ear into 3 parts 1 Outer ear:  consists of the ear canal and the tympanic membrane (eardrum)  The tympanic membrane divides the outer ear from the middle ear 2 The middle ear:  filled with air (Tympanic cavity)  has three tiny bones:

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The malleus (hammer) the incus (anvil) and the stapes (stirrup) The stapes is in contact with the Oval window The oval window acts as the door to the inner ear – it sets things in motions for the inner ear to function

3 The inner ear:  the cochlea (hearing organ) and the semicircular canals (the balance organ )  Audiologists assess hearing and balance because inner ear has the hearing organ and the balance organ.  Some pathologies that can affect the cochlea can also affect the balance  The Auditory Nerve (HEARING) and the Vestibular Nerve (BALANCE) together from the Eighth Cranial Nerve The Outer Ear

The function of the outer ear  capture sound  indicate direction of sound  The shape of the outer ear creates a certain maxima and minima in the spectrum of sound which gives cues on the direction – where the sound is coming form top/ bottom  The Pinna captures sound and directs it to ear canal  The outer ear has a cartilaginous part (outer) and a bony part that (the inner 2 thirds)  Has S shape- it is about 2.5 cm long  Has glands that secretes cerumen to protect the ear

The middle Ear

The middle ear contains:  Malleus- Incus -Stapes  It starts at the tympanic membrane  The stapes makes contact with the Oval window  The oval window is the entrance to the inner ear – has a membrane and another window beneath called the Round window  The Eustachian tube located under -connected to nasal cavity The Tympanic membrane – Ear drum Has different layers of tissue (3 layers):  Outer Epithelial  Middle: Fibrous tissue  Inner: Mucosal tissue Malleus, Incus and Stapes (smallest bones in the body)  Malleus bone (Handle)makes contact with the tym. Mem. (attached to ear drum)  The footplate of the Stapes is attached to the Oval window (which is the entrance to then cochlea)  The Ossicles form a chain (Ossicular chain) across the middle ear  There are 2 muscles attached to the ossicles : The Stapeduis & The Tensor Tympani Mechanisms of the Ossicular Chain: The middle ear is filled with AIR the cochlea filled with FLUID When sound hits ear drum – ear drum vibrates – the vibration then sets ossicular chain in motion If there no Ossicular Chain- ear drum vibrates the air in mid. Ear starts moving – that movement would set the membrane in Oval window in motion- however this would be inefficient to deliver the energy because this would create an impedances mismatch- as the air moves differently through air or water/ fluid Example: when swimming, if the person under the water is called most of the sound waves would be reflected and not absorbed so the person under the water would not be able to hear the sound (water is harder to move through than air)

The Role of the Ossicular Chain  act as an impedance matching device i.e. when the ossicles vibrate they set the cochlea in motion (they amplify the sound, so it can reach the cochlea) they increase the energy so the fluid in the cochlea is set in motion the ossicles do this with minimal loss of energy There are 2 mechanism to increase the energy 1. The typ. Mem. Is larger than oval window So a smaller surface creates more pressure 2. The malleus is longer than incus – so it acts as a lever These 2 mechanisms combat the miss-match between air and fluid The ossicular chain will improve the energy delivered to the cochlea The Eustachain Tube  Connects mid ear to the nasal pharynx (Cavity behind the nose)  Equalises the pressure between outer ear and middle ear For Tympanic Mem. To move optimally the pressure in ear canal has to be the same / Equal to the Mid. ear Under normal circumstances  Pressure in ear canal is environmental pressure – from surrounding space  Pressure in nasal pharynx is also environmental  When the Eustachian tube opens – pressure in the middle ear is equalised to the same as the surrounding air pressure When Flying: The Eustachian tube isn’t always open – i.e. opens when swallowing The air pressure changes – air pressure in ear canal is different tot the middle ear  Ear drum cannot move optimally- so the sounds will be attenuated –  When Eustachian tube opens as you swallow – it equalises the pressure in ear-(ear popping) The Eustachian tube undergoes developmental change as the head grows  Children: shorter- more horizontal- reason middle ear problems / infection more common- fluids trapped not easily to clear out  Adult: 3.5cm long -angles downwards Acoustics Reflexes The ossiclular chain has 2 muscles – Stapeduis - Tensor Tympani  The Stapeduis muscles: contacts to very loud sounds (Acoustic Reflex)  contraction limits the movement of the Ossicular chain – Upon contraction the stapes will move from the oval window (a different position)  Change of movement of the ossicular chain creates a less efficient way to transmit the energy to the cochlea- thought to be a protective mechanism- (Theory- no evidence yet) - The reflex is used clinically –  Reflex happens when (the Auditory) eighth Nerve VIII and (Facial) the seventh VII are functioning normally  Audiologist can determine certain pathologies

i.e. When middle ear infections /fluid in mid. ear – so the sound presented is can be attenuated- so it won’t reach the cochlea as intense hence this reflex will be absent  indication of ear pathology- site of lesion- hearing loss- problems of Auditory nerve- cochlea deficit The Inner ear  The cochlea (sensory hearing organ) and the semicircular canals (the balance organ )  Both are joined and the same fluid runs through them The cochlea  filled with fluid – snail shape- 2 ½ - 2 ¾ turns – 5mm across  3 ducts scala vestibuli – scala media – scala tympani  Scala vestibuli - Scala tympani contain fluid called (Perilymph= thin- water like - High in sodium(NA-) Low Potassium (K+)  Scala media – Contains fluid called (Endolymph= thicker viscous fluid- High Potassuim (K+) Low Soduim (NA-)  These levels are important for normal functioning of cochlea – Perilymph has mostly negatively charged ions – Endolymph mostly positively charged ions  The ducts are separated by the Basilar mem. & Riessner’s mem. The Organ of Corti – on the Basilar mem. –

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houses the sensory hair cells that detect sound (the movement of the cochlear fluid) (Inner Hair cells IHCs 1 row – Outer Hair Cells OHCs 3 strands/ rows ) have different functions

Auditory Nerve pathways Afferent Pathway From the cochlea to the brain  An Inner hair cells has more afferent neurons than the outer hair cells- there is more info. Going to the brain form the IHC than the OHC Efferent pathway Goes from the brain to the cochlea Ascending Pathway Info sent to the Audiotory cortex in the brain through the ascending auditory pathway  staring at the cochlea  cochlear Nucleus  Superior Olive

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At the Superior Olive There are many nerve fibres crossing form one ear to the other – There is interaction between the 2 ears The info is integrated for the brain to compare the input form one ear to the other it uses this info to know where the sound is coming form

How do we actually hear?  Sound (movement of air) travels along the ear canal to the ear drum. • The ear drum moves back and forth. Its frequency of movement matches the frequency of the sound. • The magnitude of the movement matches the intensity of the sound. • The movement of the ear drum sets the ossicles into vibration (mechanical energy) • The footplate of the stapes pushes against the oval window.  

Sound travels through air to ear canal  Tympanic mem. Tympanic mem. Moves back and forth movement frequency matches frequency of sound  Then Ossicles are set into vibration  the Stapes pushes and pulls at the oval window (stimulates movements of the fluids in the cochlea)  When the Stapes are pushed in against Oval window the Round window will be pushed out  Travelling Wave = when sound vibrations travel along the cochlea along the basilar membrane.  When the Stapes cause deflection at a point on the basilar membrane PUSH IN = basilar membrane moves Down PUSH OUT = Basilar membrane moves UP High Frequency sounds will cause maximum deflection at the BASE Low Frequency sounds will cause maximum deflection at the APEX • The movement of the stapes footplate in the oval window creates a travelling wave inside the cochlea.

• The place at which the travelling wave reaches its maximum depends on the frequency of the sound. • High frequencies are detected at the basal end of the cochlea. • Low frequencies are detected at the apical end. TONOTOPIC organisation of the cochlea The Basilar mem.: o Narrow and stiff at the base = more susceptible to higher frequencies o Wider and more flexible at apex = more susceptible to detecting low frequencies

As a wave travels through the cochlea the shearing forces of the tectorial membrane cause the stereocillia of the sensory cells to bend. • There are two types of sensory cells, i.e. outer and inner hair cells • Outer hair cells • Inner hair cells ― Biological amplifiers ― They do not response to low levels of ― Stimulation of the outer hair cells makes sound. them expand and contract. ― They need the amplification of the outer ― By expanding and contracting, they hair cells in order to detect low level boost the displacement of the basilar sounds. membrane to quiet sounds ― By expanding and contracting, they also sharpen the displacement of the basilar membrane. •

• Inner hair cells ― The bending of the steriocillia causes the opening of mechano-sensitive channels allowing K+ to flow into the cell leading to cell depolarisation ― This opens calcium (Ca2+) channels at the base of the cell which leads to vesicular transmitter release stimulating the action potential. K+ flows out of the cell at the base leading to repolarisation ― The inner hair cells create an action potential that travels along the VIIIth nerve to the brainstem. • The sound then travels along the nerve pathways to the cortex where it is perceived. Bone Conduction

• A travelling wave can also be initiated in the cochlea by applying sound to the temporal bone. • This is referred to as bone conducted hearing. • Audiologists make use of bone conducted hearing to determine the location of a hearing loss. Classification of Hearing Loss • Air Conduction • Bone Conduction • Type of Hearing Loss ―Conductive Hearing Loss ―Mixed Hearing Loss ―Sensorineural Hearing Loss • Degree of hearing Loss ―Mild ―Moderate ―Moderately Severe ―Severe ―Profound Air Conduction Bone Conduction • Air conduction is the usual method through which we hear • Sound travels from the environment through all the structures of the ear to the brain

Conductive Hearing Loss • A conductive hearing loss occurs when there is a problem in the Outer or Middle Ear, however cochlear function is normal. • Literally, the sound cannot be conducted through the ear. • A conductive hearing loss is diagnosed when hearing through air conduction is Worse than bone conduction.

• Bone conduction bypasses the outer and middle ear and stimulates the cochlea directly • However, without a process called masking we can not determine which cochlea is responding • This is called cross-hearing

Sensorineural Hearing Loss • Sensory- the cochlea • Neural- the nerves • A sensorineural hearing loss is diagnosed when both the air conduction and bone conduction thresholds are below normal. • In some cases it is difficult to diagnose if the hearing loss is sensory of neural

Mixed Hearing Loss • A mixed hearing loss is a combination of conductive and sensorineural hearing losses. • That is, there is a problem in the outer/middle ear AND in the cochea/nerve • Air conduction thresholds are below normal • Bone conduction thresholds are below normal • There is an Air-Bone gap: Air conduction thresholds are worse

than bone conduction thresholds

Frequency • Frequency refers to the rate at which air particles vibrate • We use the measurement Hertz (Hz). 1 Hz is one cycle per second. 1000Hz is 1000 cycles per second. • Pitch is the subjective correlate of frequency. • The higher the frequency- the higher we perceive the pitch to be. Decibels • The intensity of a sound is measured in decibels (dB). • The decibel scale is a logarithmic scale. • For Audiometry we mostly use dBHL (or HTL in some books) • This is dB Hearing (Threshold) Level. • 0dBHL is the average threshold of normally hearing people. • 0dBHL does not mean an absence of sound. The Pure Tone Audiogram

• We use pure tone audiometry to determine the softest sound a person can hear in each ear, at ear frequency through both air and bone conduction. • We use this to classify the Type and Degree of hearing loss. • Type of hearing loss is: ―Conductive, Sensorineural or Mixed • Degree of hearing loss is: ―Mild, Moderate, Moderately Severe, Severe or Profound

Classification of Hearing Loss -10 to 20 dB HL Normal 20 to 45dB HL Mild 45 to 60 dB HL Moderate 60 to 75dB HL Moderately Severe 75 to 90 dB HL Severe 90 dB HL > Profound...