Auditory Flashcards
Describe sound pressure waves
alternating compression and rarefaction of air
What causes amplitude/intensity to increase?
more forceful air compression
What is the equation for determining amplitude?
dB SPL = 20 log [P1/P2]
P2 = standard reference pressure P1 = pressure of tested sound
what is the sound amplitude threshold associated with permanent hearing loss?
> 120 dB
What is frequency range compatible with human hearing?
20-20,000 Hz
What are the units of frequency?
measured in Hz = cycles/sec
Describe presbycusis
loss of HIGH frequency hearing, trouble hearing fricative consonants (t, p, s, f)
Describe auditory threshold
smallest amplitude (dB SPL) a person can just detect
How does the middle ear alleviate acoustic impedance mismatch?
P = F/A
Surface area of ossicles = 1/20 surface area of tympanic membrane.
Ossicle orientation –> levering action –> larger force
What is sensorineural hearing loss?
Damaged/lost hair cells and/or nerve fibers.
List some causes of sensorineural hearing loss
Excessive loud sounds, ototoxic drugs, age (presbycusis)
What is conductive hearing loss?
degraded mechanical transmission of sound energy through the middle ear
List some causes of conductive hearing loss
otitis media, otosclerosis (impeded movement of ossicles), atresia, perforated tympanic membrane, static pressure in middle ear
How would you distinguish conductive vs sensorineural hearing loss on exam?
Compare audibility of 512Hz tuning fork heard in air vs against skull.
Positive result for conductive loss: tuning fork held against bone –> sound transduction via bone –> overcome conductive hearing loss
Describe the physical setting of the basilar membrane
spans the length of the cochlea; direct contact with CN VIII axons; sandwiched between scala media and scala tympani
How does sound affect the basilar membrane
sound –> oval window compression –> oval window bulges into scala vestibuli –> fluid compression –> downward movement of basilar membrane –> bulging round window into middle ear
The basilar membrane has a tonotopic map. Where is it sensitive to HIGHER frequencies?
Near the round and oval windows. (thinner, more rigid BM)
The basilar membrane has a tonotopic map. Where is it sensitive to LOWER frequencies?
Apex of the cochlea. (Flexible, wide, thick BM)
What is the primary stimulus attribute mapped along the cochlea?
sound frequency and intensity
How are we able to discriminate among different frequencies?
Each hair cell responds best to a certain frequency
What is the relationship between BM tonotopic map and hair cell frequency sensitivity?
Low frequency-sensitive BM contains low frequency-sensitive hair cells; high frequency-sensitive BM contains high frequency-sensitive hair cells
what is the normal membrane potential of a hair cell?
-50 mV
What is the hair cell response to bending of stereocilia?
change in membrane potential (mechanosensitive ion channels).
What happens when stereocilia are bent in the direction of LONGEST stereocilia?
“tip links” pull the tops of stereocilia –> mechanical opening of ion channels –> depolarization
What happens when stereocilia are bent in the direction of the SHORTEST stereocilia?
“tip links” push tops of stereocilia –> mechanical closing of ion channels –> hyperpolarization
Describe the physical environment of stereocilia
Apical: bathed in endolymph from scala media (High K, low Na; maintained by active pumps on scala media)
Basal: bathed in perilymph (similar ionic composition to blood)
What ion is responsible for deoplarization of hair cells?
K+ influx
What is the endocochlear potential?
+80 mV (leads to K influx driving force of -130 mV)
What happens with collapse of endocochlear potential?
Sensorineural deafness (due to loss of driving force for transduction)
Name a major cause of congenital deafness
mutation of gap junction connexin 26. (leads to collapse of endocochlear potential due to loss of active K pump on scala media)
What is a major difference between outer and inner hair cells?
OHCs = poorly innervated by auditory nerve fibers (ANFs), therefore they are not transducers
What is the role of outer hair cells?
Cochlear amplifiers. OHCs attach to basilar membrane. Efferent neurons –> change in OHC length –> BM pulled toward/away from tectorial membrane –> changed mechanical frequency selectivity of BM
What is the intensity contribution of OHCs?
50 dB. Damage –> sensorineural deafness
How do ototoxic antibiotics lead to deafness?
blockage of transduction channels –> loss of cochlear amplifier
How do spiral ganglion cells respond to high frequency sounds?
high frequency sounds activate fibers sensitive to specific frequency. Greatest sensitivity –> maximal action potential firing rate
How do spiral ganglion cells respond to low frequency “pure tones”?
Auditory nerve fiber AP phase lock (APs only fire at compression OR rarefaction of sound wave) –> temporal pattern of action potential firing
Describe the auditory pathway
cochlea –> spiral ganglion –> auditory nerve –> rostral medulla –> cochlear nuclei on dorsal/lateral aspects of ICP –> trapezoid body in mid pons (some decussation) –> superior olivary complex –> ascend in lateral lemniscus –> inferior colliculus (some decussation) –> medial geniculate of thalamus –> primary auditory cortex at sup temporal gyrus
Why would a unilateral lesion rostral to the cochlear nuclei NOT lead to unilateral deafness?
Some axons decussate, joining the contralateral lateral lemniscus, while the rest remain at the ipsilateral lateral lemniscus
What are the main mechanisms involved in sound localization?
Interaural time delays (physical separation of ears),
Interaural level differences (high frequency sound –> head creates “acoustic shadow” for far ear)
Monaural spectral shape (pinna -> localization of elevation and front/behind)
Where and how are ITDs encoded?
Medial superior olive (MSO). ANFs –> excitatory input to anteroventral cochlear nucleus (AVCN) –> excitatory inputs to MSO. Differences in neural path lengths to MSO from L/R ears –> differences in conduction times –> offset by physical ITD cue
MSO neurons are also known as:
coincidence detectors. Respond maximally when they receive simultaneous, bilateral AVCN stimulation
MSO neurons encode ITDs produced by sounds from which side of the body?
Contralateral. ITD can compensate for longer conduction time from contralateral ear
Where and how are ILDs encoded?
Lateral superior olive (LSO)
Ipsilateral ear –> ANF –> AVCN –> excitatory projection to ipsilateral LSO
Contralateral ear –> ANF –> AVCN –> decussation –> medial nucleus of the trapezoid body (MNTB) –> glycinergic calyx of Held –> inhibitory effects at LSO
From what side of the body is sound represented in the inferior colliculus?
Contralateral
How would a unilateral lesion to the inferior colliculus present?
contralateral deficit of sound source localization (MSO, LSO, DCN cues re-converge at inferior colliculus)
Would a unilateral lesion to the inferior colliculus result in unilateral deafness?
No! IC is heavily innervated by neurons from both ears
After passing the inferior colliculus, where do auditory fibers travel next?
medial geniculate body –> amygdala (auditory fear conditioning) or auditory cortex in sup temporal gyrus
Describe the tonotopic cortical map
Brodmann’s area 41 (primary auditory cortex): Low frequency = anterior; high frequency = posterior
Describe Wernicke’s area
Within secondary auditory cortex (surrounding primary auditory cortex). responsible for comprehension/processing of spoken language
