20.4 Audition Flashcards
What are the 3 principle sections of the ear and what are their roles?
- outer/ external ear → amplification + source preference
- Middle ear → Amplification + impedance matching (managing air-fluid transition)
- Inner ear → Transducing sound into nerve firing pattern
What are the 2 muscles found in the middle ear?
Muscles → protection from loud, sustained noises.
*tensor tympani - innervated by cranial nerve V,
*Stapedius - innervated by cranial nerve VII. (Smallest skeletal muscle)
Describe the structure of the cochlea.
Coiled structure (around modiolus) w/ 3 fluid-filled chambers (scala vestibuli/ media/ tympani) separated by Reisner’s/ Basilar membranes.
What is the function of the cochlea?
transduces vibrations from sound waves into neural impulses
Which membrane separates the scala vestibuli and the scala media?
Reissner’s membrane
Which membrane separates the scala media and scala tympani?
Basilar membrane
On which membrane does the organ of Corti sit?
Basilar membrane
What is the overlying membrane on the organ of Corti called?
Tectorial membrane
What is the stria vascularis?
a capillary loop in the upper portion of the spiral ligament (the outer wall of the cochlear duct or scala media). It produces endolymph for the scala media in the cochlea.
What is the composition of the perilymph and where is it found?
Perilymph (↑Na+ ↓K+) → in vestibuli/ tympani.
What is the composition of the endolymph and where is it found? How is it maintained?
Endolymph (↑ K+ ↓Na+) → in media
Maintained by Na/K-ATPases in stria vascularis
How many rows of inner and outer hair cells are there usually?
One row of inner hair cells
Three rows of outer hair cells
Name drugs that can have ototoxic effects on the sensitive inner ear cells? What else do they decrease?
Gentamicin (aminoglycoside)
Furosemide
Aspirin
Quinine
↓cochlear amplification + sharp tuning.
What is the organ of corti?
where auditory transduction takes place. On basilar membrane
Rows of specialised auditory receptors (OHCs + IHCs) + support cells
What are inner hair cells?
Inner hair cells - afferent fibres → convey auditory info
10 sound transducers (95% of afferent fibres innervate IHCs).
single row of 3500 form ribbon synapse (large divergence, many IHCs to fibre) → rapid + sustained release of glutamate (which reflects sound intensity)
What are outer hair cells?
Outer hair cells - efferent innervation → amplify basilar membrane motion (electromotility)
12000 in 3 rows
(are motors)
How does transduction take place?
- In/outward movement of oval window
*Ossicles transmitting pressure of sound waves - Round window counters motion (bulges in opposite direction) → pressure grad. in perilymph
- Triggers travelling wave of basilar membrane from base → apex
*Inward movement of oval window → pressure s.v>s.t → basilar membrane moves ↑ - Shearing motion caused between tectorial membrane + stereocilia - displacing B.membrane.
What happens if the basement membrane is displaced upwards?
B.membrane displaced ↑ → stereocilia bend towards kinocilium → ↑ tension between tip-links → mechanosensitive ion channels open → K+ influx (driven by endolymphatic potential) → depo → basolateral VGCa2+C open → glutamate exocytosis → AMPA receptors on afferent fibres → ↑ firing rate.
What happens if the basement membrane is displaced downwards?
B.membrane displaced ↓ → ↓ tension in tip-link → mechanosensitive channels close → HCs hyperpolarise
How does audiometry work?
Individual puts on headphones and sounds are played from higher to lower volume until they are reported as inaudible
What are the two main types of hearing loss?
Conductive - affects external/middle ear
Sensorineural - affects inner ear/auditory nerve
What is frequency resolution?
The ability of the ear to discriminate a sound at one frequency from a sound at a different frequency (heard at the same time)
How does Rinne’s test reveal conductive hearing loss?
It compares air conduction and bone conduction thresholds
Normally a much more intense sound is needed to perceive sound by bone conduction, but with hearing loss the difference is reduced
How does Weber’s test reveal unilateral hearing loss?
The patient reports which ear the sound is heard louder - with a positive result sounds are consistently heard louder on one side
What are the two main ways we are able to localise sound?
- Binaural timing: sound arrives at left and right ear at different times
- Intensity differences: different sound intensity for each ear
What is the spiral ganglion and where is it located?
A group of neuron cell bodies in the conical central axis of the cochlea
Where does the auditory (/cochlear) nerve project to from the spiral ganglion?
To the cochlear nucleus in the pons
What is the role of the medial superior olive in hearing?
Estimating interaural time differences
How does the medial superior olive detect interaural time differences?
There is an MSO on each side, which compare activity given the time taken for the action potential to arrive at each MSO (faster to left = sound is more towards left)
–> able to find the ITD
What is the role of the lateral superior olive in hearing?
Processing interaural intensity differences
How does the lateral superior olive detect interaural intensity differences?
LSO receives excitatory input from the ipsilateral cochlear nucleus but inhibitory input from the contralateral MNTB (medial nucleus of the trapezoid body)
Overall: ipsilateral excitation, contralateral inhibition
Where do the MSO and LSO pass info to?
Inferior colliculus
Where are the MSO, LSO and MNTB located within in the brain?
In the pons
How is the cochlear nucleus split up?
Ventral and dorsal cochlear nuclei
Ventral contains antroventral and posteroventral as well
What is the role of having three different structures in the cochlear nucleus?
Each form a tonotopic map preserved from the cochlea
Responsive to certain characteristic frequencies
Starts to integrate tones
Where do some signals travel to from the cochlear nuclei to decussate?
Trapezoid body
Where do signals travel to - from either the superior olive (i.e. MSO/LSO) or the trapezoid body?
Inferior colliculus
Where do signals from the inferior colliculus travel to?
Medial geniculate nucleus of the thalamus
Which gyrus is the auditory cortex in?
Superior temporal gyrus
How is the auditory cortex organised?
In columns, tonotopically organised so that the mapping reflects the frequency grading in the cochlea
Though the mapping is not identical to the cochlea - there are some distortions/non-representations/multi-peak frequency response areas
What structural feature of the basilar membrane means that different frequencies cause displacement at specific points on it?
It varies in stiffness from one end to the other
(so lower frequency waves move further along the cochlea before the membrane starts to move while higher frequency waves displace membrane closer to the base of the cochlea)
What is phase locking?
The phenomenon of the auditory nerve impulses being synchronised to the phase of the stimulus waveform
Why does phase locking occur?
Because the hair cells are depolarised or hyperpolarised according to whether the basilar membrane moves up or down with the phase of the sound wave - so the auditory nerve impulses match up to this
What is an auditory nerve fibre tuning curve? What does it show?
The response of the nerve fibre to a certain frequency.
At lowest point, shows the frequency at which it is most sensitive (i.e. has the lowest threshold for)
What are the three main mechanisms to produce speech?
- Respiration at the lung
- Phonation at the larynx
- Articulation in the mouth
How is pitch increased by the vocal cords?
Increased tension
How is volume increased by the vocal cords?
Increased separation
How does spectrotemporal processing allow us to perceive specific linguistic speech sounds?
Acoustic signals are transformed into pattern representations to determine linguistic structure
Based on frequency and timing
What is the role of Wernicke’s area?
Sensory comprehension of speech/language
What is the role of Broca’s area?
Motor production of speech
What are the characteristics of Wernicke’s aphasia?
Makes it hard to understand meanings of words
= receptive aphasia
What are the characteristics of Broca’s aphasia?
Difficulty forming words and sentences
= expressive aphasia
Which hemisphere of the brain is thought to be specialised for speech production and language?
Left
What is acoustic neuroma?
- Benign tumour on the either/or the cochlear and vestibular branches of the vestibulocochlear nerve
- Compression can lead to hearing and/or balance loss
