Week 4 - Audition Flashcards
what is the most common cause of hearing loss?
death of hair cells
how do cochlear implants work?
- cochlear prostheses bypass hair cells and excite appropriate nerve fibers
- general method is to pass multiple electrodes into Scala Tympani
- stimulating electrodes are connected to a sound or speech processor, which can determine which of the electrodes should be activated in accordance w/ its analysis of the sound waves being presented
- frequencies, durations, other variables defining particular sounds or speech patterns
how does the auditory system hear “sound”?
- responds to series of rapid fluctuations in air pressure brought about by compression and rarefaction (decompression) of air molecules
- for any simple tone (single frequency), a plot of the pressure as a function of time yields a sine wave
- max of sine wave corresponds to greatest density/pressure; min corresponds to least
- amplitude represents magnitude of pressure change (greater amplitude = greater physical magnitude of sound)
how is the physical magnitude of a sound-producing disturbance measured?
decibel scale
- compares physical magnitude of any given sound to a standard physical magnitude
- standard physical magnitude chosen is one near normal human threshold for tones to which humans are most sensitive (1000-3000 Hz)
what is the equation of decibels?
20 x log (test P / reference P)
-reference P always = 1, so if had to increase test P to 100, would mean 40 decibel hearing loss
what is the structure of the inner ear? how is “hearing energy” transmitted within?
contains auditory receptors as part of Organ of Corti, and filled with aqueous medium
- rarefactions and compressions are associated with molecules of air, and must be transmitted to molecules of aqueous medium
- at oval window, only 1/1000 of incident energy transmitted, meaning a 30 dB transmission loss
- to compensate for this loss, you need a 32-fold increase in pressure
where is the increase in pressure (to compensate for 30 dB loss) accomplished?
in the middle air, by principle of P = F / A
- effective area of tympanic membrane is 60 mm2, and the oval window is 3 mm2, meaning there’s a 20-fold increase in P at oval window
- lever action of ossicular chain supplies additional increase
- force at oval window is not equal to force at tympanic membrane (exceeds by 1.3)
- total increase in pressure by 26-fold
what is tympanometry? how does it work? what happens in conductive hearing loss?
clinical technique that measures impedance of middle ear to sound
- sounding sorce and microphone are introduced to ear canal
- amt of sound absorbed thru middle ear, or reflected from middle ear, is measured in microphone
- in conductive hearing loss, more sound is reflected than normal
what are pathologies that result in conductive losses?
otitis media, otosclerosis, and ear wax in external ear canal
what does the fact that “bone and soft tissue have impedances close to water” mean?
even without a tympanic membrane or middle ear ossicles, sound can be transmitted thru bone and soft tissues to head of cochlea
what does the analysis of frequency depend on?
the way sound waves of different frequency affect the basilar membrane in the cochlea
what are some properties of the basilar membrane? what does this allow? how does it change from the base to the apex? how are the hair cell cilia?
these allow different frequencies to affect it differently
- increases in width as it runs from the base to the apex of cochlea (.04 to .5 mm)
- concomitant decrease in stiffness (100 to 1)
- -for equally applied forces along membrane, regions of lower stiffness (wider apex) are displaced to a greater degree
- -decreased stiffness from base to apex causes an increase in phase lag from base to apex (more out of phase)
- hair cells are short/stiff at narrow/stiff base, and more than twice as long at apex
what happens when the basilar membrane is coupled to the traveling wave through the fluid media around it?
- stapes moves into oval window, compressing a volume of Scala Vestibuli
- compensation occurs by bulging out of round window
- downward bulge initially occurs near applied pressure at base
- stiff base provides restoring force that brings membrane back to neutral position, requiring that volume of compressed fluid is taken up elsewhere and shunted farther along membrane to less stiff region, etc.
- when stapes moves out, an upward bulge is produced, which follows downward bulge, producing a full traveling wave
what does the distance a wave progress depend on?
frequency of sound wave producing that traveling wave
what does maximum amplitude vary directly with?
frequency of stimulus
- greater the frequency of sound wave, the less distance the traveling wave will move along the basilar membrane
- lower the frequency, the farther along the basilar membrane will the max displacement occur, and only the lowest frequencies produce waves traveling the full length of the membrane
what is the character of the traveling wave reflected in?
including its points of max deflection, the character of the traveling wave is perfectly reflected in pattern of discharges transmitted by primary auditory neurons
how are auditory receptor cells arranged?
in an orderly manner along basilar membrane, and are mechanically stimulated by displacements of traveling wave
- template of mechanical events occurring on basilar membrane is transformed into primary neuron discharge
- -receptor cells at points along basilar membrane traversed by traveling waves respond in proportion to amplitude of traveling wave
- -“peak” will have max sound b/c max discharge
how wide is the peak displacement of the traveling wave?
as narrow as the width of a few hair cells
what does the organ of Corti house, and rest upon?
houses the auditory receptors, and rests upon the basilar membrane
what will displacement of the basilar membrane do?
cause movement of auditory receptor cells
what are the tectorial and basilar membranes hinged to? what does this mean? what happens if you push them up? if you push them down?
hinged to rigid limbus, which means they operate relative to each other
- if you push them up, it will put a shearing action on cilia in one direction (depolarize, increase rate of discharge)
- if push them down, shears cilia in opposite direction (hyperpolarize, decrease rate of discharge)
what does the electro-anatomy of the cochlea establish?
-what is the potential difference between scala media and ground? the inside of a hair cell? across reticular lamina?
a considerable battery for generation of receptor potential
- scala media: +80 mV
- inside of hair cell: -70 mV
- across RL: 150 mV
what does shearing action on cilia alter?
the permeability (resistance) of auditory cell membrane, allowing for a change in current flow -magnitude of change depends on magnitude of shearing force, which depends on magitude of basilar membrane displacements
are hair cells spontaneously active? how does their frequency differ?
yes, their membrane potentials oscillate
- frequency at which different hair cells oscillate differs according to position along basilar membrane
- characteristic frequency matches frequency at which it would be most responsive to mechanical stimuli
what does mechanical activation do to hair cell cilia?
amplifies spontaneous voltage oscillations
what is the role of the central auditory system? how are they organized?
interpret discharge patterns of auditory nerve fibers to produce perception of sound
-cochlear nuclei are first in chain of nuclei that operates in both parallel and hierarchial fashion
how is the central auditory system parallel?
each 8th nerve fiber terminates w/in cochlear nuclei by branching to each of its 3 divisions: dorsal, posteroventral, and anteroventral nuclei
what is the most important principle governing 8th nerve fiber terminations?
the full range of frequencies transudced by cochlea is represented in each of the 3 nuclei
-high frequencies are represented dorsally, low frequencies ventrally (tonotopic)
what is important about the ascending projections of auditory brainstem?
high degree of bilateral connectivity
-important for sound localization
what is the ultimate target of auditory information from the periphery?
primary auditory cortex (A1) in transverse temporal gyrus (largely buried in Sylvian/lateral fissure) and surrounding belt of auditory subregions
what happens within area A1?
neurons of similar best frequency are arrayed in a strip/belt-like structure that runs perpendicular (dorsoventrally for A1) to high-to-low frequency tonotopic axis
- auditory cortex is organized from cortical surface to white matter in columns
- transforms from 1D spatial frequency in cochlea to 3D spatial organization in cortex
what happens if one passes an electrode perpendicular to the cortical surface of a cat?
individual cortical cells with different response properties
- simplest shows narrow tuning around particular best frequency
- others are tuned more broadly and have variable number of multiple best frequencies
is the auditory cortex more involved with analysis of pitch patterns or pitch itself?
analysis of patterns
-this was discovered b/c after removal of entire auditory cortex, cats initially lost discrimination of frequencies, but could relearn it; however, they could not discriminate between frequency patterns, and could not relearn them
