Auditory System Physiology Flashcards
Energy Transducer
-Energy has to be converted from acoustic energy: TM is transducer
-Transduction of sound from acoustical to mechanical energy: outer to middle ear
-Transduction of mechanical energy into hydraulic (fluid in scala vestibuli) energy: middle to inner ear, oval window transducer
-Transmission and interpretation of electrochemically produced signal: inner ear to hair cells in scala media
Generally: What does the outer ear do?
collects sound
Generally: What does the middle ear do?
-provides match between low impedance of air and high impedance cochlear fluid:
-air is less dense than fluid
-hearing underwater is how we would hear if we didn’t have the middle ear
Generally: What do the hair cells do?
convert hydraulic energy to chemical-electrical energy
Generally: What does the organ of corti do?
-break sound into frequency, intensity, and temporal components
-At the organ of corti is when sounds start to get broken down and further refined
Generally: What does the VIII nerve and brainstem pathways do?
-initiates processing and transmits sounds
Generally: What does the cortex do?
-puts individual components together
-provides speech interpretation and linguistic processing
Outer Ear: Pinna
catches sound and directs sound down canal to tympanic membrane
Outer Ear: Resonant Frequency
-the frequency that is best conducted through a space or medium based upon its physical characteristics
-Concha: 5000 Hz
-Canal: 2700 Hz, high frequencies provide the understanding of speech sounds
Middle Ear
-2 cubic cm
-Impedance (resistance) matcher between air (outer ear) and fluid (inner ear)
-Without Middle Ear all have conductive HL
Remember: Middle Ear
-Pars tensa responsible for transmitting sound: tenser things transmit sound better like a drum
-make up 2/3 of TM: bottom part
-TM approximately 90 mm squared
-approx. 60 mm squared of TM are responsible for processing sound
Overcoming Impedance Mismatch: Aerial Difference
-Size difference between TM and oval window
-Same amount of sound pressure hitting the TM has to get squished down into the oval window resulting in increased force/pressure
-TM
-60 mm squared
-Oval window
-3 mm squared
-Approx. 20 times difference
-Intensity increase between 14-25 dB
Overcoming Impedance Mismatch: Lever Principle
-Lever developed between manubrium of malleus and long crus of stapes
-Manubrium approx 2X longer than long crus of stapes: fulcrum where incus and malleus connect
-Results in doubling of sound pressure
Impedance Matching Equation
-2 (lever principle) X 15 (aerial difference) = 30 dB increase
Inner Ear
Stapes displaces fluid in cochlea like hands makes waves in water
Stapes and Round Window
-Cochlea encased in bone and placed in temporal bone petrous bone
-Without round window stapes would not move: fluid unable to move
-Round window bulges out when stapes pushes on oval window and moves fluid, push (oval window)/pull (round window)
-Reciprocal action between oval and round window
-Stapes sitting in oval window converts mechanical energy to hydraulic energy
-Rocks in oval window creating waves in cochlear fluid
Stapes and Characteristics of Fluid Waves
-Characteristic of waves dependent on how stapes moves
-lower frequencies the stapes moves further than for higher frequencies
-greater force higher amplitude but frequency is not changing
-Determines location wave raise or lower on Reissner’s membrane: high frequencies fall lower on membrane because frequency is greater
Fluid Displacement
-Reissner’s membrane movement displaces fluid in scala media which then moves basilar membrane and displaces fluid in scala media
-Fluid movement in scala media moves organ of corti, tectorial membrane, basilar membrane
-Basilar membrane movement displaces fluid in scala tympani
-move round window membrane
Cochlear Hair Cells
-Outer: 3 rows
-Inner: 1 row
Cilia Sheared
-Tectorial membrane and basilar membrane move at different times because the tectorial membrane is a gel and lags behind, which causes the cilia to bend
-The way the cilia moves depends on what they do
Outer Hair Cell Movement
-embedded in tectorial membrane
-sheer relative to basilar membrane
Inner Hair Cell Movement
-not embedded in tectorial membrane so moves because of fluid
-turbulence at point of max excursion causes cilia to bend like seaweed in ocean
Hair Cell Excitatory Function
-Cilia bend toward tallest one
-initiates transmission along auditory pathway: opens door, positive ions in, cell stimulated
-influx the positive ions: depolarize
Hair Cell Inhibitory Function
-pumping out more positive ions making the cell more negative.
-Further from depolarization stage
-reduces transmission along auditory pathway
-pumps out positive ions
-hyperpolarize: more negative than resting
Tip Links
-Cilia connected
-Pulls all cilia when bend toward tallest
-open sodium potassium ports to depolarize cell
Sound Waves
Sinuosoidal Wave Phase
Sound Waves: Compression
-regions where molecules are closer together
-pushes stapes into cochlea
-results in fluid moving
Sound Waves: Rarefaction
-Regions where particles spread apart
-Pulls stapes out of cochlea
-Results in fluid and basilar membrane moving up bending towards tallest cell, excitation
Sound can begin in
compression or rarefaction
Movement of Organ of Corti and Tectorial Mmebrane
dependent on phase of sound (compression/rarefaction)
Excitation of Auditory Nerve
-activated by upward basilar membrane movement
-Rarefactions stimulus: bend towards larger cilia, depolarizes, signal is sent
-Inhibition of auditory nerve activated by downward basilar membrane movement
Rarefaction Stimulus
-produces outward movement of TM and Stapes
Organ of Corti, Tectorial Membrane, Basilar Membrane Move Upward
-causing sterocilia to bend toward tallest stereocilia
-Produces excitatory response sending signal to auditory cortex
Organ of Corti, Tectorial Membrane, Basilar Membrane Move Downward
-Causing stereocilia to bend away from tallest stereocilia
-Produces inhibitory response reducing signal to auditory cortex
Compression Stimulus
-bend towards smaller cilia
-hyperpolarizes
- prevents signal from being sent off
Outer Hair Cell Purpose
-Awareness and amplify
-5-10% neural stimulation
Inner Hair Cell Purpose
-discrimination, fine tuning
-90-95% of neural stimulation
Traveling Wave
-point of maximum perturbation: produces maximum excitation
-bends stereocilia toward apex: excites
-bends stereocilia toward base: inhibits
Resonance Characteristics of Sound
-Low freqs: more mass less stiff, longer
-High freqs: less mass, more stiff, shorter
Basilar Membrane Resonance Characteristics
-governed by
-width gradient
-thickness gradient
-stiffness gradient
Basilar Membrane Width Gradient: Osseous Spiral Lamina
-width decreases moving from base to apex
-basilar membrane attached to osseous spiral lamina
Basilar Membrane Width Gradient: Basilar Membrane
-width increase moving from base to apex
-wider
-more mass
-resonance freqs lower
Basilar Membrane Thickness Gradient
-basilar membrane
-thinner at base
-thicker at apex
-thicker=more mass
Basilar Membrane Stiffness Gradient
-basilar membrane
-more stiff at base
-less stiff at apex
Tonotopic Organization
-basilar membrane is arranged by freq
-result of physical characteristics
-freq specific filters
-frequencies processed by basilar membrane
-20 Hz apex
-20,000 Hz base
Signal Conduction
-hair cells depolarize sending the signal to neuron under cells
-neurons travel along basilar membrane to center of cochlear to spiral ganglion
-exit cochlea via modiolus
-not complete VIII nerve until vestibular neurons intertwine with cochlea neurons
-low freqs hit the core of VIII nerve and high freqs are on the outer parts
