Transcript Chapter 8
Chapter 8 Hearing The Auditory Systems Reference P142 - 151 P442-454 P651 - 662 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 2 .032 cm – 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 18.4 1.3 23.9 Contribution from: TM (Tympanic Membrane) to stapes footplate Lever action 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 – 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