4.0 Auditory System Flashcards
What is the range of frequencies that the ear can respond to?
20Hz - 20kHz
What is the intensity of sound measured in?
Decibels (usually expressed in log scale)
What is the formula for the amplitude of sound (decibels sound pressure level [dB SPL]) ?
dB SPL = 20log₁₀P/P₀
What is the minimal audible sound pressure detectable by normal human ears?
20μPa
Define auditory threshold:
Sound pressure at which pure tone is just heard
Define pain threshold:
Sound pressure at which sound becomes painful
Structures of external ear:
1) Pinna<br></br>2) EAM
Structures of middle ear:
1) Tympanic membrane<br></br>2) Ossicles<br></br>3) Middle ear cavity (temporal bone)
Structures of inner ear:
1) Vestibular apparatus (balance)<br></br>2) Cochlea (hearing)
Function of middle ear?
Electrical transformer
How does middle ear match the low impedence in air to the high impedence in cochlear fluid?
1) Area of eardrum = 14x times the area of footplate of stapes, therefore the pressure at stapes is 14x the eardrum<br></br>2) Ossicular chain acts a lever
What are the muscles of the middle ear?<br></br>What is their function?
1) Tensor tympani (attatches to malleus)<br></br>2) Stapedius (alters angle of stapes at oval window)<br></br><br></br>These two muscles reduce sound transmission (thus offer protection)
At what sound levels do the muscles of the middle ear contract (reflex)?
80dB SPL
How many dBs can the muscles of middle ear reduce sensitivity by?
30 - 40 dB SPL
What is the purpose of the eustachian tube?
Equalizes the pressure in the middle ear with atmosphere
Function of helicotrema?
Prevents partition from vibrating at subauditory frequencies. Only allows non damaging sounds
Define tonotropic map:
Different sound frequencies are mapped along the partition. This is not linear (logarithmic increments)
What is the travelling wave:
Caused by vibration of the partition. The partition executes a travelling wave that moves from base to apex. Each point undergoes a sinusoidal vibration with the amplitude being highest at its characteristic place on the tonotropic map.<br></br><br></br>Phase lag (where more apical points lag behind stapedial ones) = proof that this is not a standing wave
Where is the Organ of Corti?
On the basilar membrane along the whole length of the partition
What is the gelatinous mass covering the Organ of Corti?
Tectorial membrane
How many rows do outer hair cells have?<br></br>How many rows do inner hair cells have?
outer hair cells = 3 longitudinal rows<br></br>inner hair cells = 1
What is the type of fluid in the following?<br></br><br></br>1) Scala vestibuli<br></br>2) Scala tympani<br></br>3) Scale media
1) Scala vestibuli = perilymph<br></br>2) Scala tympani = perilymph<br></br>3) Scale media = Endolymph (more potassium + 100mV more positive)
What secretes endolymph?
Stria vascularis
How does mechano-electrical transduction occur in the streriocilia?
Steriocilia = microvilli<br></br><br></br>Movements of steriocilia are linked to opening and closing of <b>cation channels</b><br></br><br></br>Movement towards longest steriocilia = opening<br></br>Movement towards shortest = closing
(axons run in internal auditory meatus)
1 inner hair cell → 20 peripheral axons
This saturates at 40dB > fibre threshold
(because fibres have different thresholds, overall range of intensities is 80dB)
This is useful at enhancing contrast in the frequency domain
Timing for APs in CN VIII are synchronized with individual cycles of the stimulus
At these frequencies there is dual representation of frequency in the discharge of auditory nerve (place code and periodicity code)
2) Receptor potentials are filtered off their periodic components by the time constant of the hair cell membrane
Irrespective of temporal pattern, activation of a fibre signals the presence of a limited range of frequencies
At high frequencies, only place code works
Also encodes frequencies <4kHz
From contralateral superior olive
Ends on Outer hair cells
Function - suppresses responses to lower level background noises to emphasise interesting aspects of a sound
2) Uncrossed pathway
Ends on afferent terminal CN VIII fibres beneath inner hair cells
Not been studied much
Posteroventral
Anteroventral
Involved with speech production
Speech comprehension
- Horizontal plane
- Can be localised to 1-2°
2) Angle of Elevation
- Mid-sagittal plane
- Can be localised to 10°
What does the medial part of superior olive (MSO) process?
What does the lateral part of superior olive (LSO) process?
LSO - interaural intensity difference
