Unit 9 - Hearing Physiology Flashcards
Structures of ear
Function of ossicles
Function of eustachian tube
Ossicles transfer signal from eardrum to inner ear
Eustachian tube -> mouth - equalises pressure between oral cavity and middle ear
Contents of middle ear
Air filled (so signal is not reduced)
Eustachian tube equalises pressure
3 auditory ossicles - malleus, incus (anvil), stapes (stirrup)
Connected by tiny synovial joints - amplify vibrations from tympanic membrane to oval window
Why are younger children more susceptible to ear infections
Their face is small and the angle of eustachian tube might be too shallow to allow drainage
Relationship between external auditory meatus and malleus
EAM is flush with malleus
What structures does the oval window transmit vibrations to
To vestibule and on to the cochlea (scala vestibuli)
3 fluid filled compartments of the cochlea
Scala vestibuli
Scala media
Scala tympani
Scala vestibuli
Perilymph
Scala media
endolymph - unusually high K+ conc
Scala tympani
Perilymph
Compartment with high [K+]
Scala media - endolymph
What is the scala tympani connected to
Round window between inner and middle ear
Difference between perilymph and endolymph
Perilymph is more viscous than scala media
Where does the organ of corti sit
On top of basilar membrane
No of layers of hair cells
4, sometimes 5 but single row of INNER hair cells is crucial to our capacity to hear
Where do the cilia of hair cells project into
Scala media (endolymph)
Some are longer than others - keno cilium
Embedded in membranous membrane
Scala tympani - vibrations through it
Movement of reissner’s membrane
Reissners membrane is rigid - vibrations as they pass through scala vestibuli doesn’t do anything in scala media
Movement of basilar membrane
Basilar membrane is elastic - scala tympani underneath moves
Deflection of cilia caused by basilar and tectorial membrane moving but not together
What is found at the base of the cilia
Mechanoreceptors - ion channels
What does vibration of stapes cause
Vibration of perilymph through oval window
Causes vibration of elastic basilar membrane
Where are hearing receptors found
In organ of corti
Hair cells surmount the basilar membrane and are connected to tectorial membrane
Vibrations cause deflection of hairs
Depolarisation or hyperpolarisation
Organ of Corti - 16,000-20,000 hair cells (about 4 rows of 4,000)
What row is responsible for sending signal
Inner row of hair cells - 1
Function of outer hairs
Signal amplification role?
Focus on a particular aspect of sound?
Movement of membranes
Effect of movement of stereocilia
Opening/closing of K+ channels
When opened, K+ enters cells - depolarisation (endolymph has high [K+])
collagen fibres connect cilia
Unfurled cochlea (2 and 3/4 turns)
Sound frequencies seem to affect basilar membrane at different optimised places
How is frequency sensed
Range of freq heard by young vs old
Mainly by “place principle”
Young: 20-20,000 Hz
Old: 50-5,000 Hz
How is loudness sensed
By amplitude of response
Spatial summation
Activation of outer hair cells - some signal to brain
Low freq
Maximal vibration at tip (helicotrema)
High freq
Closer to base
Place principle
Basilar membrane is not of equal thickness along length of membrane
20-30,000 basilar fibres fixed at 1 end
Thickness & rigidity of membrane decreases from base → helicotrema (tip)
Each freq of stimulation excites maximal motion at a specific position (place) along the membrane
Deafness
Different types
Prolonged/frequent exposure to excessive loud noise can cause degeneration of hair cells, resulting in high freq deafness
SENSORINEURAL - Problems with neurons/brain - problems with hair cells and pathway that carries sound to auditory cortex
CONDUCTION - Problem is in ear up to point of hair cells - wax in outer ear, fluid in inner ear
Tuning fork on mastoid body can differentiate
MOA of hearing aid
- Under mastoid process is fluid filled inner ear - cochlear is there
- Sit it on mastoid process - amplifies signal
What do hearing tests examine
Air conduction - outer, middle, inner ear and auditory pathway function
Bone conduction - bypasses the outer and middle ear
Tuning fork tests & audiometry
Pure tone audiogram
Deafness severity
Neural processing within auditory pathway
Inner hair cells synapse on > 90% of ganglion cells - Glu NT
Eighth CN - vestibulocochlear
Signal has phasic and tonic components
Overview of auditory pathway
How do fibres travel in the auditory pathway
Majority decussate - contralateral side mostly
Some fibres from each ear go to both side of brain
Collaterals also innervate
- reticular activating system (sleep wake cycle)
- vermis of cerebellum (motor part - loud noise can knock you)
Inhibitory retrograde fibres from each level of auditory system back to cochlea - allows tuning in to specific sound (outer row of hair cells)
Medial superior olive
Distinguishes interaural delays of 10ms
Accurate sound location
Signal from 1 ear is insufficient to generate an AP - temporal summation needed
Temperotopic organisation
Map of sound source location - rudimentary
Each neuron is sensitive to a specific lag time (fractional head start)
Lateral superior olive
Interprets differences in intensity of signal from both ears
Direction of sound
Tonotopic organisation of SO
Different range of frequencies distinguishable
Primary auditory cortex
Tonotopic organisation
Neurons respond to only very specific frequencies of sound
Cells are responsive to input from both ears
Some are excited by signal from both ears (EE)
Some are excited by 1, inhibited by the other (EI)
Alternating columns
Contrast
Maps different aspects of sound - pitch, loudness, sound localisation, sound duration
What do lesions of auditory cortex result in
NOT complete deafness
Involvement of subcortical areas in sound perception
Loss of ability to discriminate sound patterns and localise sound
Complete deafness would only result from complete ablation of hair cells or auditory nerve prior to brainstem
Other surrounding cortical areas involved with elaboration of aspects of sound
Parietal - direction, temporal - meaning (sound of someone’s voice etc)
Parietal pathway and hearing
Direction
Temporal pathway and hearing
Meaning
Wernicke’s area
Comprehension of auditory and visual info
IQ test
Arcuate fasciculus
Connects Wernicke’s area with Broca’s area
Capacity to understand and articulate language
Broca’s area
Plans & co-ordinates vocalisation
Angular gyrus
Implicated in some dyslexias
Parts of brain associated with language implementation system
Frontal - logic - high level of thinking
Parietal - movement (sense)
Temporal - Emotional processing and memory
Hemisphere dominant in language
Left
Right hemisphere is dominant in
Spatial/temporal relations
Wada procedure
One hemisphere is anaesthesised
What is epilepsy
Excessive electrical activity
Focal region that is source of electrical discharge - can spread and affect the whole brain
Corpus callosum may be severed as treatment to reduce spread
Pathway when speaking a heard word
Auditory cortex → Wernickes → through arcuate to Brocas → motor cortex
Speaking a written word
Visual cortex → Angular gyrus → Wernickes → Brocas → motor cortex
Broca’s aphasia
Laboured slow speech in monotone
Impaired articulation
Comprehension is intact
Wernicke’s aphasia
Fluent, melodic speech
Errors in word choice
Difficulty in comprehending
Conduction aphasia
Intelligible simple speech and comprehension
Difficulty repeating sentences
conductive deafness
normal hearing
conductive deafness
sensorineural deafness
C
B
C
D
B
B
