Auditory System Flashcards
Describe impedance matching
- Fluid has greater inertia so requires more energy to move
- Lever action from stapes
- Eardrum to oval window SA ratio is 20:1
- These two things cause a 26X pressure amplification
Describe role of middle ear
- Sound in air medium
- Vibrates ear drum
- Vibration causes Malleus to move the Incus to move the Stapes
- Stapes moves oval window
Describe the attenuation Reflex
When exposed to loud sound (>80dB)
- Stapes muscle and Tensor Tympani muscle stiffen the lever action
- Reduces energy transferred to inner ear
Describe the Cochlear
- 35mm spiral
- Top section is Scala vestibuli (perilymph)
- Middle is scala media (endolymph)
- Bottom is Scala tympani (perilymph)
Describe the organ of corti
- Inner (1 row) and outer hair cells (3 rows) (innervated by spiral ganglion to form auditory nerve)
- Sit in the flexible basilar membrane
- Tectorial membrane sits on top of bait cells
Describe the structure of hair cells
Inner:
- straight row of sterocilia (hair bundle)
- largely afferent innervation
Outer:
- V shaped rows of sterocilia
- largely efferent innervation
What are sterocilia formed off
- Actin
- Myosin
- Frimbrins
What are tip links
- Fine filaments between sterocillia
2. Made of cadherins (CDH23 and PCDH15)
How do tip links function
- Linked to transduction ion channels on sterocilia
- Non-specific cation channels (TMC1/2)
- Mutations in these cause deafness
Describe the cochlear fluids
Perilymph:
- resembles CSF
- bathes cell bodies in organ of corti
K+ 7mM
Na 140mM
Endolymph
- resembles extracellular fluid
- bathes sterocilia
K+ 145mM
Na+ 1mM
Describe the propagation of sound
- Movement of oval window displaces fluid
- Bends basilar membrane at base
- Traveling wave moves along basilar membrane
Describe the Basilar membrane
- Structure of basilar membrane varies along the length
- Position of maximal displacement depends on frequency of sound
- Base: narrow and stiff (higher frequency)
- Apex: wide and floppy (lower frequency)
What happens when the basilar membrane moves?
It takes the organ of corti with it- causing the tectorial membrane to move with it, pushing the inner and outer hair cells to bend
Describe how bundle stimulation opens ion channels (inner hair cells)
- When sterocilia move towards tallest, causes tension between the tip links
- Tip links attached to transducer channels and therefore open
- Causes K+ influx into the cell
Describe inner hair cell activation in vivo
- Sterocilia bathed in K+ rich endolymph (+80mV)
- -60mV in hair cell, so -140mV driving force)
- Depolarisation causes Ve-gated Ca+ ion channels to open
- Synaptic transmission to afferent neurones
What happens (in terms of receptor potential) when sterocilia are pushed in positive/ negative direction
Neutral: transducer current slightly +Ve
Negative: Ve- stimulus
Positive: Ve+ stimulus
Describe the relationship between nerve firing and receptor potential
Receptor potential responds to the wave pattern of said frequency
This creates a burst firing pattern in the nerve
Describe a step by step from sound to neuronal firing
- Sounds stimulus
- Bundle movement
- Changes in transducer current
- Graded changes in membrane potential
- Changes in transmitter release
- ## Changes in neuronal firing‘Phased locked’ - sound phases represented in neuronal firing
Describe the volley principle
- Neurones have max firing frequency of 1kHz
- Groups of neurones can respond to multiple cycles as one output
- Increases range of phase locking to 4kHz
How is frequency of up to 20kHz detected
- The structural variations in basilar membrane
- Neurones have a characteristic frequency by responding with the greatest frequency
- Maintains a tonotopic map over a range of frequencies
Describe isofrequency bands In the brain
Different frequency’s represented in tonotopic map in auditory cortex
Describe the role of the cochlea amplifier
- If cochlea was a passive structure we would see a viscous dampening of basilar membrane (would lose energy)
- Causes an application of inner hair cell stimulation
Describe the role of outer hair cells in the cochlear amplifier
- Inner hair cells depolarise
- Prestin (motor proteins) shorten up to 100nm
- This amplifies basilar membrane movement
- Modulated by efferent inputs
What are otoacoustic emissions
- Noise produced by the ear
2. Caused by the active mechanisms of the ear
Describe the afferent innervation of hair cells
Inner:
- Type 1 afferent
- 1:1 neurone to IHC
- Thick/ myelinated
Outer
- Type 2 afferent
- 1:50 neurones to OHC
- Thin and unmyelinated
How is sound amplitude encoded
- Populations of afferent nerves
- Fibres excited at low, medium and high levels of sound intensity
- Summation of these populations
From cochlear to brain
- Spiral ganglion
- Ventral cochlear nucleus
- Superior olive
- Inferior follicles
- Medial gelatinous nucleus
- Auditory cortex
What is conductive hearing loss
- Damage to outer/middle ear
- Defects in ossicles
- Damage to tympanic membrane
What is sensortineural hearing loss
- Damage or loss of sensory hair cells
2. Damage or loss of neurones
Two types of hair cell defects
- Loss of OHC
- increased threshold
- poor frequency discrimination
- hearing aid helps with intensity only - Loss of IHC and OHC
- complete deafness
- cochlear implant only current treatment
Describe hair cell defect- noise trauma
- 80-90 dB
- Disruption of hair bundle and breakage of tip links
- May be repairable
Describe severe noise trauma
- > 130dB
- Complete loss of hair cells due to build up for free radicals
- ## Scar formation fills voidAlso excitotoxicisty from excessive glutamate release can permanently damage synapse
From Cochelar to brain
- Spiral ganglion
- Ventral cochlear nucleus
- Superior olive
- Inferior colliculus
- MGN
- Auditory cortex
How is representation of both ears on each side of the brim achieved
Binaural neurones that bicussate allowing trigagulation of source of noise
How is intensity encoded
Populations of afferent nerves
- highest rate of firing at highest intensity
What are the two types of deafness
Conductive:
- outer/ middle ear
- defect of ossicles/ tympanic membrane
Sensorineural hearing loss
- damage to or loss of sensory cells
- damage to neurones
Result of loss of hair cells
OHCs:
- Increased threshold (50dB)
- poor frequency discrimination
OHCs and IHCs:
- complete deafness
- Cochlear implant
How to test OHC loss
Speech audiogram
- ability to discriminate words
Describe mild noise trauma
80-90dB
- disruption of hair bundle and breakage of tip-links
- may be repairable
Describe severe noise trauma
- complete loss of area of hair cells due to free radicals
- scar tissue fills void
