Hearing The Auditory Systems Chapter 8 Hearing The Auditory Systems

Reference P442-454 P651 - 662 P142 - 151

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

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

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

Part 2 Role of Middle Ear in Sound Transmission

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

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)

Transformer Action of Middle Ear Lever Action Fulcrum Effect pressure gain: 1.3 times

TRANSFORMER ACTION AMOUNT OF AMPLIFICATION 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)

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

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

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

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

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+

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

+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

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

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.

Two AC Potentials Cochlear Microphonic Potential Action Potential (AP) 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

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

Nerve and Conduction Deafness