Auditory System Flashcards
(45 cards)
Sounds above what SPL can result in permanent hearing loss?
120dB
What is the frequency range of human hearing?
2,000 - 20,000 Hz
Auditory threshold
The smallest dB sound that a subject can just detect
Different for each ear and at each frequency
Impedence mismatch
Because fluid is much more resistant to movement than air, most of the sound energy reaching an air-water interface will be reflected back (99.9%)
How does the middle ear alleviate impedence mismatch?
- Decreased area of the stapes footplate relative to the tympanic membrane - augments force transmitted to the inner ear
- Orientation of the middle ear bones confers a levering action resulting in greater force
Conductive hearing loss
Caused by degradation of mechanical transmission of sound energy through the middle ear; due to:
Filling of middle ear with fluid Impaction of ear canal with wax Otosclerosis - arthritic bone growth impeding movement of ossicles Malformations of ear canal Perforation of TM
Sensorineural Hearing Loss
Caused by damage to or loss of hair cells and/or auditory nerve fibers; due to:
Exposure to excessively loud sounds
Ototoxic drugs
Age
Presbycusis
Gradual, bilateral, high frequency sensorineural hearing loss due to age
What do excessively loud sounds and ototoxic antibiotics have in common?
Damage to outer hair cells causing sensorineural hearing loss
Cochlear amplifier
OHCs receive efferent innervation from the central auditory system; prestin, a voltage-sensitive motor protein, responds by changing the length of the hair cell which pulls the BM toward or away from the tectorial membrane thus changing the mechanical frequency selectivity of the BM;
Contributes up to 50dB of cochlear sensitivity to sound
Type I ANFs
95% of all ANFs
10-30 Type I ANFs innervate a single IHC
Myelinated
Type II ANFs
5% of all ANFs
Each Type II ANF innervates ~10 different OHCs
Not myelinated
What is the most common form of congenital hearing loss?
Mutation in gap junction protein Connexin-32, found in the stria vascularis
Decreased K+ concentration of the endolymph causes reduced driving force on K+ to enter the IHCs; diminished depolarization leads to less effective sound signal transduction
Otoacoustic Emissions (OAEs)
Spontaneous movements of the OHCs which set the BM in motion, causing the tympanic membrane to act as a loudspeaker
Lack of normal OAEs in infants can indicate sensorineural hearing loss
Medial Olivocochlear neurons (MOC)
Efferent neurons which innervate OHCs; MOCs sense the context of the sound environment (frequency and intensity) and provide feed-back control to change cochlear sensitivity via the OHCs
How is sound intensity encoded?
Rate of firing of IHCs (frequency specific)
Recruitment of neighboring IHCs and ANFs
Important for perception of loudness
Phase-locking
The property by which auditory nerve fibers fire action potentials in response to a particular phase of the sound waveform; this temporal pattern of action potentials in ANFs is used to code the “pitch” of sounds with frequencies below ~1,000Hz
Most important for low frequency sounds which cause phasic release of NT from low frequency sensitive IHCs
Important for perception of pitch
Interaural Time Differences (ITDs)
Caused by the direction-dependent differences in path length that sounds must travel to reach each ear from a source, generating different times of arrival of the sound at the two ears
Interaural Level Differences (ILDs)
For high frequency sounds with wavelengths on the order of the diameter of the head, the head creates an “acoustic shadow” for the far ear as sound waves are reflected off of the near side of the head; therefore, the sound arising at the far ear is relatively attenuated, creating a direction-dependent difference in amplitude
ILDs are small in magnitude for low frequency sounds and increase in magnitude for high frequency sounds
Monaural spectral shape
Results from direction and frequency dependent reflection and diffraction of sound pressure waveforms by the pinna, causing spectral “shapes” that encode information about location (including elevation, front/behind)
What is the tonotopic arrangement of the basilar membrane?
The basilar membrane is thinner, narrower, and more rigid at the base of the cochlea near the oval and round windows; this region vibrates in response to higher frequency sounds
The BM is wider and more flexible at the apex; here it vibrates in response to lower frequency sounds
Where are inner hair cells located?
In the Organ of Corti, which sits in the scala media, on top of the basilar membrane
How do IHCs participate in signal transduction?
The apical surface of hair cells project an array of stereocilia of various lengths; bending of the stereocilia toward the longest fiber opens a voltage insensitive, non-specific cation channel which allows influx of depolarizing K+ from the endolymph into the IHC
Endolymph
K+-rich fluid bathing the IHC stereocilia within the scala media
