wide range of sound pressure 20-20,000 hz differentiating small increments in frequency and...
TRANSCRIPT
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
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
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
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
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
1st 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
2nd 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
3rd 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
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.
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.
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
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
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
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
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.
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
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
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
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