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Published byGilbert Anderson Modified over 9 years ago
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Wide range of sound pressure 20-20,000 Hz Differentiating small increments in frequency and intensity Listening to a signal embedded in background noise Extremely rapid sequences of sounds
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Outer ear collects sound and “shapes” its frequency components Middle ear matches the airborne acoustic signal with the fluid medium of the cochlea Inner ear performs temporal and spectral analyses on the ongoing acoustical signal Auditory pathway conveys and further processes the signal Cerebral cortex interprets the signal
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Collector of sound- localizes sound in space Pinna has ridges, grooves, and dished-out regions Excellent funnel for sound directed toward the head from the front or side Less effective for sound arising from behind the head No active/moveable elements- has a passive effect on the input stimulus. Pinna focuses the acoustic energy into the EAM EAM funnels sound to the TM The shape of the pinna and EAM boost the relative strength of the signal (approximately 20 Hz) The wax, oils and shape prevent foreign bodies
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Transmits acoustic vibrations from the tympanic membrane to the inner ear. Designed to increase the pressure approaching the cochlea Overcomes the (resistance to flow of energy=impedance) Uses the strategy of decreasing the area over which the force is being exerted Primary function is to match the impedance of two conductive systems- increasing the pressure of a signal as it travels from the outer ear to the cochlea
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Muscle contraction increases the stiffness of the ossicular chain. Tensor Tympani Innervated by a branch of the mandibular nerve of the trigeminal nerve Attaches to the manubrium of the malleus Stapedius Muscle Inserts on the posterior surface of the neck of the stapes Innervated by the stapedial branch of the facial nerve
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1 st Mechanicsm of Impedance matching 17 times larger than the Oval window Sound energy reaching the TM is “funneled” to the much smaller oval window which translates to an overall increase of 25 dB Pressure exerted by a lightweight individual with a spike heel vs. a piano mover in sneakers
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2 nd Impedance matching function Lever difference Length of the manubrium is 9 mm Long process of the stapes is about 7mm Overall gain of approximately 2 dB
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3 rd Mechanism of Impedance matching As the TM moves in response to sound, it buckles Arm of the malleus moves a shorter distance than the surface of the TM Reduction of displacement of the malleus Average increase of 4-6 dB increase in the signal
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All 3 mechanisms result in a signal gain of about 31 dB If the middle ear were removed, a signal entering the EAM would have to be 31 dB more intense to be heard. Any process that reduces the effectiveness of this function (otitis media) can have a serious impact on the conduction of sound to the inner ear.
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Extends downward, forward, and medially from tympanic cavity to the nasopharynx Lateral portion is osseous, medial portion is cartilage and other connective tissue Normally closed by elastic recoil forces to protect the middle ear from pathogens Equalizes pressure between the middle ear and external atmospheric pressure Allows tympanic membrane to operate efficiently Drains the middle ear cavity and aerates tissues.
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Semicircular canals respond to rotatory movements of the body All movements of the head can be mapped by combinations of outputs of the sensory components, cristae ampulares Activation of the sensory element arises from inertia As your head rotates, the fluid in the semicircular canals tends to lag behind The cilia are stimulated by relative movement of the fluid during rotation. The utricle and saccule sense acceleration of the head rather than rotation Major input serving the sense of one’s body in space
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Cochlea- structure would fit on the eraser of a pencil, fluid within it would be a drop on the table Extracts or defines the various frequency components of a given signal=Spectral analyses Sort out the frequency components Determine the amplitude Identify basic temporal aspects of the signal
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Sound is a disturbance in air The disturbance causes the TM to move TM moves in- stapes footplate in the oval window moves in; TM moves out- foot plate moves out Stapes compresses the perilymph of the scala vestibuli via the oval window Reissner’s membrane is pushed down toward the scala media
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Basilar membrane is pushed down toward the scala tympani Frequency of a sound is determined by the number of oscillations or vibrations per second- i.e. a 100Hz signal results in the footplate moving in and out 100 times per second Vibration is translated to the basilar membrane where it initiates a wave action=traveling wave
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BASILAR MEMBRANE Designed to support wave action that directly corresponds to the frequency of vibration High frequency sounds cause vibration of the basilar membrane closer to the vestibule Low frequency sounds result in a longer traveling wave that reaches the apex Basal end near the vestibule is “stiffer” than the apical end Becomes increasingly massive, from base to apex Becomes progressively wider from base to apex The 3 components- graded stiffness, mass and width combine to make the basilar membrane an excellent frequency analyzer.
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WAVE ACTION Wave roll in from the ocean- swell to a large amplitude as they break on the beach Point of maximum amplitude of the traveling wave on the basilar membrane is the primary point of neural excitation of the hair cells within the organ of Corti Only one true strong point of disturbance from the traveling wave Low frequency sounds cause the traveling wave to “break” closer to the apex Traveling wave can be stimulated in the absence of the middle ear mechanism (bone conduction testing) Always travels from base to apex
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Cilia of the outer hair cells are embedded within the tectorial membrane As the traveling wave moves along the basilar membrane, the hair cells are displaced relative to the tectorial membrane Produces a shearing action Inner hair cells are not embedded in the tectorial membrane Not subjected to the same forces as the outer hair cells
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Inner hair cells depend on fluid movement of the endolymph to excite them Traveling wave moves along the basilar membrane, fluid moves relative to the hair cell. Cilia are displaced by the fluid movement Outer hair cells are important for coding intensity Inner hair cells are essential for frequency coding
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Stimulation of hair cells permits the mechanical energy arriving at the cochlea in the form of movement of the stapes footplate to be converted into electrochemical energy Basilar membrane displaces towards the scala vestibuli, the hair cells are activated Basilar membrane is displaced towards the scala tympani, electrical activity of the hair cell is inhibited
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