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The Auditory System: Properties, Transmission, and Imbalances in Hearing

This chapter explores the properties of sound, the role of the middle ear in sound transmission, the function of the organ of Corti, and homeostatic imbalances of hearing. It discusses how pitch, loudness, and timbre are determined, the mechanisms involved in sound transmission in the middle ear, the function of the organ of Corti in discriminating sound frequency, and common homeostatic imbalances in hearing.

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The Auditory System: Properties, Transmission, and Imbalances in Hearing

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  1. Chapter 8 Hearing The Auditory Systems

  2. Reference P462-472 UNIT X The Nervous System: B. The Special Senses P142 - 151

  3. Content • Properties of Sound • Role of Middle Ear in Sound Transmission • Function of Organ of Corti • Homeostatic Imbalances of hearing.

  4. Part 1. Properties of Sound Sound travels in waves as does light • 1. Pitch: determined by “frequency,” the number of cycles per second of a sound wave, measured in hertz (Hz) • 2. Loudness: determined by “amplitude” (height) of the sound wave, measured in decibels(dB) • 3. Timbre: determined by “complexity and shape” of the sound wave, gives each sound its unique quality

  5. Loudness of Sound • 0 dB = hearing threshold • 50 dB = normal conversation • 90 dB = danger zone • 120 dB = Rock concert • 130 dB = Pain threshold

  6. Part 2 Role of Middle Ear in Sound Transmission

  7. Mechanisms Involved in Transformer Process • Size difference between Tympanic Membrane and Stapes Footplate • Lever action

  8. First Component of Middle Ear Transformer Action • Size Difference • Tympanic membrane • .59 cm2 • Stapes footplate • .032 cm2 • Pressure formula • Pressure = force/area • Impact on sound transmission Pressure gain: 0.59/0.032 = 18.4 (times)

  9. Transformer Action of Middle EarLever Action • Fulcrum Effect pressure gain: 1.3 times

  10. TRANSFORMER ACTION AMOUNT OFAMPLIFICATION Pressure Gain Contribution from: 18.4 TM (Tympanic Membrane)to stapes footplate 1.3 Lever action 23.9 Total pressure gain (18.6 x 1.3)

  11. Part 3 Function of Organ of Corti • a structure rests atop the basilar membrane along its length • contains approx. 16,000 cochlear hair cells

  12. 1. How to discriminate the frequency of the sound? --- Traveling Wave Theory

  13. Vibration of Basilar Membrane and the Traveling Wave Theory • Sound wave entering at the oval window is to cause the basilar membrane at the base of the cochlea to vibrate • different frequencies cause vibrations at different locations (places) along basilar membrane • higher frequencies at base, lower frequencies at top

  14. As the pitch of a sound gets higher, displacements of the basilar membrane: occur closer to the oval window occur closer to the helicotrema occur uniformly throughout the membrane become greater in amplitude become smaller in amplitude

  15. 2. Electrical Potentials • DC vs. AC • Direct Current (DC) = stimulus doesn’t change with time, constant; i.e. battery • Alternating Current (AC) = always changing over time, looks like a sine wave

  16. Cochlea • Perilymph- • similar in composition to extracellular fluid. • High in Na+ and low in K+. • Endolymph- • found in the scala media. • Similar to intracellular fluid. High in K+ and low in Na+

  17. Two DC Potentials (EP) • Endocochlear Potential (EP) • +80 mV potential with respect to a neutral point on the body • due to the Stria Vascularis

  18. +80 mV Reticular Lamina -80 mV Two DC Potentials (IP) • Intracellular Potential (IP) or organ of corti potential (resting potential) • Recorded -80 mV inside cells of organ of corti

  19. Hair Cell in the Organ of Corti When the basilar membrane moves, a shearing action between the tectorial membrane and the organ of Corti causes hair cells to bend

  20. There are little mechanical gates on each hair cell that open when they are bent. K+ comes into the hair cell and depolarizes the hair cell. The concentration of K+ in the endolymph is very high so when it comes into the hair the positive ions come to the cell causing a depolarization.

  21. Two AC Potentials • Cochlear Microphonic Potential • Reproduces frequency and waveform of a sinusoid perfectly • Generated from hair cell • Action Potential (AP) • Electrical activity from the VIII Nerve • Can be measured from anywhere in the cochlea or in the auditory nerve

  22. Part 4 Homeostatic Imbalances of hearing. • Deafness. • Conduction deafness - • possible causes include: perforated eardrum, inflammation, otosclerosis • Sensineural deafness - nerve damage • Tinnitus - ringing in the ear • Meniere's syndrome - attacks of dizziness, nausea, caused by excess endolymph in the media canal

  23. Nerve and Conduction Deafness

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