Presbycusis
Loss of high frequency hearing
Normal with age
- starts around 20 kHz
Characteristics of sound
= pressure wave - expands spherically
Amplitude = intensity
- measure on logarithmic decibel scale (very wide range)
Frequency = pitch
- humans 20 Hz - 20 kHz, peaks at 2-3 kHz (speech)
- higher = dogs, bats, moths; lower = elephants, whales
Complexity = timbre (sum of multiple frequencies)
Structure of ear
External = auricle/pinna + meatus
- amplifies 30-100x
Middle - 200x amplification
- large tympanic membrane -> small oval window
- lever action of ossicles
- attenuation - tensor tympani (trigeminal), stapedius (facial)
- damage -> hyperacusis (ex Bell’s palsy)
- Eustachian tubes equalize pressure
Structure of cochlea
Scala media - endolymph (high K+) made by stria vascularis
Basilar membrane - different vibration frequencies along length
Organ of Corti = support cells, tectorial membrane
- hair cells (1 inner, 3 outer) - stereocilia and kinocilia (longest)
Scala vestibuli and tympani - perilymph
- connected at helicotrema
Tonotopy
Basement membrane - different vibration frequencies
- thin, rigid at base -> greatest intensity at high Hz (16 kHz)
- wide, flexible near helicotrema -> low frequency (500 Hz)
Each nerve fiber has greatest intensity at certain frequency
Maintained through cochlear nerve, auditory pathway
-> verticle bands in primary auditory cortex
Hair cell function
Basement membrane movement vs tectorial membrane -> Displacement of stereocilia -> mechanosensitive K+ channels
Towards kinocilium -> open K+ -> depolarize -> transmitter release
Away from kinocilium -> less transmitter release
“Transduction”
Relies on high electrochemical gradient from high K+ endolymph (125mV)
Cochlear amplifier
Outer hair cells = efferent innervation!
Superior olive -> Ach receptors -> hyperpolarize ->
Voltage sensitive motor protein = “prestin” ->
Lengthens -> less movement of basilar membrane (dec intensity)
(Reverse: depolarization -> shorten -> more membrane movement)
Protect from loud noises
Selectively dampen/enhance frequencies
Inhibited by furosemide
Auditory pathway overview
Cochlea -> spiral ganglion -> CN VIII -> cochlear nuclei
Superior olivary nucleus -> inferior colliculus -> medial geniculate nucleus (thalamus) ->
Primary auditory cortex
Cochlear nuclei
- dorsal - tonotopy
- postero and anteroventral - intensity
- ventral -> superior olivary (bilateral) -> localization
Sound localization
No way to distinguish front-back
Time delay - can detect 5 microseconds!, use up to 3 kHz
- medial superior olivary nucleus = coincidence detectors
- vary length of dendrites -> coincidence -> range of neurons respond to different delays
Intensity difference - used about 3 kHz
- lateral superior olivary nucleus
- contralateral input -> medial nucleus of trapezoid body -> inhibitory interneurons (vs stimulation by ipsilateral)
Phase difference - only very low frequency
Inferior colliculus
Integration with other sensory inputs
- > startle reflex
- > vestibulo-ocular reflex
- > filter out body sounds
- > auditory space map
Medial geniculate nucleus
Relay to cortex
Specific response to combinations of frequencies
Specific response to time differences
Auditory cortex
Primary = superior temporal aka Heschl’s gyrus
- vertical tonopy
Secondary aka “Belt areas” - combinations of sounds
- ventral stream via inferior frontal gyrus - pitch
- dorsal stream via superior frontal, superior parietal - location
Wernicke’s area - understanding speech
- both auditory and visual input
Components of language
Phonemes = sounds (ie letters) Lexemes = short groups, words
Auditory system must be able to distinguish frequency modulation
Echolocation
Bats emit range of frequencies
- delay = distance (1 ms = 17 cm)
- Doppler shift = change in frequency = movement (1 kHz = 3 m/s)
Speech areas
Broca’s area = production - projects to motor cortex
- aphasia - can’t produce speech
Wernicke’s area = comprehension - visual and auditory
- aphasia -> word salad (can’t understand their own)
- normally use both visual and auditory (ie loud room)
- McGurk effect - mismatch -> third related phoneme
Arcuate fasciculus = white matter tract
- aphasia = similar to Broca’s
Supramarginal gyrus - matches sounds to phonemes
(individual neurons for phonemes)
Angular gyrus - matches graphemes to phonemes
Music perception
No centralized area - inferior frontal, both hemispheres, inc Wernicke’s and Broca’s
Combination-sensitive neurons
Present at birth, improves with training
Pitch = frequency
- perfect pitch = labelling of frequencies, 1:10,000, critical period in development
- changes - R temporal cortex
- congenital amusia = tone deaf - can’t detect changes, wrong notes (inferior frontal)
Timbre = spectral and temporal envelopes, harmonic content
- R frontal
Rhythm - L hemisphere
Auditory agnosia
Can’t identify meaning of non-verbal sound (ie doorbell)
Conduction vs sensorineural deafness
Conduction = before cochlea
- ex wax blocking, ruptured membrane
Sensorineural - hair cells or nerve
Acquired hearing loss
Trauma
Infection
Drugs (antibiotics)
Presbycusis
vs genetic - complex, 50 genes - K channels, endolymph production, alignment of cilia
Rinne test
Bone conduction vs air conduction
Normal: air 2x > bone
Conductive loss: bone > air (bypassing external ear structures)
Sensorineural loss: air > bone but less than normal (compensation)
Tinnitus
Perception of sound in absence of stimulus
Temporary or permanent
Causes: Damage from loud sounds Wax buildup Antibiotics Age Vascular Cochlea, nerve, cortex
Acoustic neuroma
Schwann cell tumor
Begins on vestibular nerve -> cochlear involvement
Slow growing -> surgical removal
Meniere’s disease
Progressive, low frequency loss
Buildup of endolymph (? blockage?)
1:500 people
Tx with salt restriction, diuretics
Hair cell regeneration
Can be induced Epithelial stem cells differentiate - transcription factors - growth factors - transplant stem cells
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Audition25
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Vestibular11
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Chemical senses13
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Thalamus15
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Vision37
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Cortex14
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Motor systems27
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Basal and Degenerative34
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Cerebellum9
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Eye movement19
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Limbic17
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Localization and bx correlates23
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Sleep15
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Breathing6
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Pain10
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Coma/stupor7
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Headache and dizziness31
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Neoplasms15
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Motor disfunction/reflexes35
