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Sound And The Structure Of The Auditory System Notes

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Lecture 35, 38 & 40 Sound and Structure of the Auditory System

Properties and perception of sound o Amplitude or 'loudness' (dB) o Frequency or 'pitch' (Hz) Conductive pathway for sound o Conductive- outer ear
 External auditory canal
 Tympanic membrane (eardrum) o Middle ear
 Malleus; Incus; Stapes (in the oval window)
 Round window (covered by secondary tympanic membrane) o Sensory-Inner ear
 Semicircular canal (vestibular branch of VIII)
 Cochlea (cochlea branch of VIII)
 Auditory tube The ear- structure and function o Middle ear function- impedance matching
 More energy required to move fluid than air
 Ossicles have lever action
 Eardrum to round window surface area= 20:1 in humans
 Results in 20x pressure, enough to move fluid
 Without impedance matching only 0.1% energy transfer o Middle ear- ossicular reflex
 Protective reflex against loud sounds (>70dB)
 Stiffens lever reducing energy conduction (done by tensor tympani muscle & stapes muscle)
 No role above 1 or 2kHz in man
 Helps discrimination where lots of low freq. Noise
 Ineffective for impulse noise (50-100ms delay) o Inner ear
 1, 7 & 8= Vestibular system (semi-circular canals)
 10= Oval window moved by ossicles
 11= Round window
 13= Cochlea
 5= Basilar membrane inside cochlea o Cochlea
 Spiral structure
 Fluid filled, compartmentalised

Cochlear fluids

• Perilymph o Resembles CSF o Bathes cell bodies of organ of Corti o K 7mM; Na 140mM; 0mV potential

• Endolymph o Resembles ICF o Sealed in tight compartment o Bathes surface of organ of Conti o Maintained by stria vascularis o K 145mM; Na 1mM; +80mV potential
 In mammals, organ of Corti=organ of hearing

• Located on flexible basilar membrane

• Inner & outer hair cells- sensory

• Spiral ganglion nerve cells

• Associated with supporting and non-sensory cells

• Hair cells held rigidly The specialist hair cells o Hair cells- sound to nerve impulse
 Inner (IHCs- 3,500)) and Outer (OHCs- 12,000) hair cells
 Afferent & efferent connections
 Sensitive to damage and disease (not replaced in mammals)
 Mechano-electrical transduction o The sensory hair cells

Stereocilia (hair bundle)

• Mechanosensing organelles of hair cells

Respond to fluid motion for various functions (hearing and balance)

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