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
Inner vs Outer hair cells:
“<div><img></img></div>”
Define presbycusis
Presbycusis is the loss of hearing that gradually occurs in most individuals as they grow older
What are the afferent neurons of the cochlea?
Bipolar cells<br></br><br></br>(axons run in internal auditory meatus)
Where are the cell bodies of the bipolar cells?
Spiral ganglion (wraps around modiolus)
How much divergence occurs in auditory nerve fibres?
Lots<br></br>1 inner hair cell → 20 peripheral axons
How is intensity coded in auditory nerve fibres?
↑ sound pressure → ↑ firing rate<br></br><br></br>This saturates at 40dB > fibre threshold<br></br>(because fibres have different thresholds, overall range of intensities is 80dB)
What is a tuning curve?
A curve to show the amplitudes at which a specific frequency can be detected
Define characteristic frequency:
Each fibre shows a specific frequency at which it is more sensitive
Define two tone suppression:
When two tones are played at the same time, tone B (lying in specific regions of the tuning curve) will be suppressed to enhance the response to tone A (lying in favourable frequency domain)<br></br><br></br>This is useful at enhancing contrast in the frequency domain
Define phase locking:
Frequencies < 4kHz<br></br>Timing for APs in CN VIII are synchronized with individual cycles of the stimulus<br></br><br></br>At these frequencies there is dual representation of frequency in the discharge of auditory nerve (place code and periodicity code)
Why do frequencies >4Hz not have phase locking?
1) Cycles are shorter than refractory period<br></br>2) Receptor potentials are filtered off their periodic components by the time constant of the hair cell membrane
Define place code:
Each fibre has a narrow frequency range (by virtue of the place they arise from in cochlea)<br></br><br></br>Irrespective of temporal pattern, activation of a fibre signals the presence of a limited range of frequencies<br></br><br></br>At high frequencies, only place code works
Define periodicity code:
Temporal pattern for the discharge (phase-locking)<br></br>Also encodes frequencies <4kHz
What determines pitch of a pure tone?
Frequency
What is the missing fundamental?
When several frequencies are present in a complex sound, but the brain assigns a frequency (not originally present) as the fundamental frequency
What are the two efferent pathways from superior olive to cochlea?
<b>1) Crossed pathway</b><br></br>From contralateral superior olive<br></br>Ends on <b>Outer hair cells</b><br></br>Function - suppresses responses to lower level background noises to emphasise interesting aspects of a sound<br></br><br></br><b>2) Uncrossed pathway</b><br></br>Ends on afferent terminal CN VIII fibres beneath inner hair cells<br></br>Not been studied much
How many axons does the CNVIII have in humans?
30,000
Brief overview of central auditory pathways:
CNVIII → Cochlear nuclei (medulla) → Superior olive → Inferior colliculus → Medial geniculate nucleus → Primary auditory cortex (A1)
What are the three cochlear nuclei?
Dorsal<br></br>Posteroventral<br></br>Anteroventral
Where is the Primary auditory cortex?
On superior temporal gyrus
Where is Broca’s Area? What is it involved with?
Frontal cortex<br></br>Involved with speech production
Where is Werinke’s Area? What is it involved with?
Temporal cortex<br></br>Speech comprehension
Broca’s and Werinke’s Areas are _________ represented in the dominant (_______) hemisphere
Broca’s and Werinke’s Areas are unilaterally represented in the dominant (usually Left) hemisphere
What two angles are used for sound localisation?
<b>1) Angle of Azimuth</b><br></br>- Horizontal plane<br></br>- Can be localised to 1-2°<br></br><br></br><b>2) Angle of Elevation</b><br></br>- Mid-sagittal plane<br></br>- Can be localised to 10°
Define cone of confusion:
A theoretical problem that can arise in a completely symmetrical being where a sound source in a specific location may generate the same interaural time and intensity differences
Regarding sound localisation:<br></br><br></br>What does the medial part of superior olive (MSO) process?<br></br>What does the lateral part of superior olive (LSO) process?
MSO - interaural time difference<br></br><br></br>LSO - interaural intensity difference
What are co-incidence detectors?
Cells in MSO that only fire when EPSPs occur simultaneously from both inputs
