Auditory System Flashcards
Label the anatomy of the outer ear
Describe the inner ear: including the oval window, bony and membranous labyrinth
Oval window = window between the Middle + Inner ear
Inner ear contains the Bony + Membranous labyrinth
Bony: a convoluted channel within the Temporal bone
Contains Perilymph = ECF (high [Na], low [K])
Membranous labyrinth runs w/in the bony labyrinth, through the cochlea. Contains Endolymph = ICF (low [Na], high [K])
The cochlea is the auditory part of the ear with 3 compartments. Describe these…
What innervates Hair cells in the Spiral organ?
3 compartments: Upper (scala vestibuli) + lower (scala tympani) - both r filled w Perilymph
Vestibular + Basilar membranes separate the 3rd compartment aka the cochlear duct (contains endolymph)
The cochlear branch of Vestibulocochlear nerve (VIII) innervates hair cells within the spiral organ
Label this cochlea
What does the spiral organ inside the cochlea contain?
The spiral organ contains hair cells - the surface of these hair cells have stereocilia
Stereocilia tips are embedded in the Tectorial membrane
The stereocilia are arranged into rows by height
Glycoproteins link shorter stereocilia to taller ones; hearing depends on these tip links.
3 things which cause damage to the ears?
Ototoxic agents eg Aminoglycoside antibiotics (Gentamycin) + Pt based cancer therapies (Cis-platin) damage hair cell receptors
High amp vibrations of the Basilar + Tectorial membranes damage the stereocilia + kill hair cells
Overstimulated hair cells kill their afferents = Excitotoxic damage from excess Glutamate
The afferents that respond to loud sounds die first, so affected individuals keep turning their music up!
Describe how sound waves enable us to hear
Sound waves →pinna→ Ear canal→ vibrates tympanic membrane which vibrates hammer → Anvil → Stirrup - transmitting the vibrations to fluid in Inner ear
Vibrations in the Inner ear fluid move through the Cochlea→ Scala Vestibuli→ Scala Tympani, which causes the Basilar + Tectorial membranes to vibrate
This tilts the stereocilia back and forth
What happens when there is a failure to transmit sound waves? + 4 causes of this
Failure to transmit sound waves = Conductive hearing loss. Due to:
* Blockages
* Fusion of Joints between the Ossicles
* Fusion of head of Stirrup + Temporal bone
* Ruptured Tympanic Membranes
Relate pressure to the tympanic membrane
The Tympanic membrane only vibrates effectively if the Outer + Middle ear are at equal pressures
The Eustachian tube opens in the Middle ear when we swallow to equalise this pa.
What is the importance of the sterocillia?
Basilar and tectorial membranes have separate hinge points which tilts the stereocilia back and forth
Tilting towards the taller tip link pulls it taught
Tilting to the shorter stereocilia lets the tip link go slack
This alternating tightness and slackness transduces the vibration signal
What happens when the stereocillia are tilted back and forth?
The shorter stereocilia tip has a mechanically gated channel. The tip link pulls it open
Endolymph is +80mV, but hair cells have a RMP of -40mV
This creates a 120mV electrical gradient to drive cations into the hair cell= depolarises it
When the gated channel is closed, hair cell repolarises
K efflux at rest takes membrane potential back to resting level
What is endolymph produced + reabsorbed by?
What can happen if there are issues in reabsorption?
Endolymph: constantly produced+reabsorbed by stria vascularis
It exchanges ions w blood→ forms high K, low Na fluid
Endolymph flows thru cochlear duct -> central cochlea, where its reabsorbed
Drainage rate = production to maintain constant pa!
If production>removal, increased pa w/in cochlea duct damages cochlea hair cells=Endolymphatic Hydrops (occurs in Meniere’s)
Diff frequencies activate diff sets of hair cells , explain how this is used to encode different sounds in the cochlea - Base vs Apex?
what do we lose as we age and why
At base of spiral organ, the basilar membrane is narrow + stiff→ good for high Hz but not low ones
At Apex, the basilar membrane is broad + floppy→ Low Hz!
High pitched sound is more energetic, so more damaging to hair cells. As we age, we lose High frequency hearing for this reason= Presbyacusis
Describe the Primary Auditory Pathway for discriminative hearing
Where does the base vs apex of the cochlea send inputs?
Spiral organ afferents→ Cochlear nerve and then cochlear nuclei
The 2nd order neuron decussates + goes up to Inferior Colliculus→ MGN in Thalamus→ Superior Temporal Lobe → Primary Auditory Cortex (PAC).
Base of Cochlea sends input to caudal part, (High Hz sound)
Apex of cochlea sends input to ant. part of the cortex (Low Hz sound)
What would happen if there would be damage to the primary auditory cortex?
We have 2 ears + 2 PACs, w ‘bypass routes’ between them - therefore even bilateral PAC damage won’t cause deafness! - Instead affected ppl can detect sound but can’t discriminate; sound will have no Meaning to them
Wernicke’s area in Sup. Temporal Gyrus is the Comprehensive Language Area
Damage leads to poor comprehension of language
Wernicke’s Aphasia= fluent, nonsensical speech
