Functional anatomy of the cochlea Flashcards
Name labels of the spiral organ of the cochlea
Cochlear duct: has the endolymph
-high K conc and low Na conc
-stria vascularis
-basilar membrane
-inner hair cell
-tectorial membrane
-outer hair cells create cochlear amplifier
Describe tonotopic organisation of the cochlea
-collagen fibres extend across the basilar membrane with weak longitudinal connectivity
-BM is narrow, thin and stiff at the base, wider, thicker and floppier at the apex
-hair cell anatomy is also ‘tuned’ to different frequencies
Describe the cochlea
-scala vestibuli
-cochlear duct
-scala tympani
-cochlear branch of the vestibulochochlear (VIII) nerve
-modiolus
Human hearing
-dynamic range - 1 trillion fold
-1picometer vibration of eardrum
-discrimination - 1/30th of a piano key interval
how do we discriminate frequency?
-Human cochlea has - 3500 inner hair cells and can resolve 1400 unique frequencies
-hair cell responses can replicate the sound waves
-but single afferents can’t fire fast enough at ‘follow’ even at 100Hz
-and oscillations disappear at higher freq
IHC afferent responses correlate with?
Basilar membrane displacement
OHC create________ of IHC signals
narrow tuning
-OHC amplify the effect of sound waves and increase sound by changing the size of amplification
Loss of OHC reduce….?
both sensitivity and frequencies resolution
difference between IHC and OHC
pick up signal and transmits to brain
vs
produce amplification of basilar membrane vibrations
what is peak in sensitivity due to?
cochlear amplifier
is the cochlear a passive structure?
NO - it amplifies sound waves
describe the cochlear amplifier in action
-with no assistance from the OHC no movement
-with sound-driven shortening of the OHC plus active movement of stereocilia
describe OHC ‘tug”
-optimally timed to increase resonance at this location
-effect is specific to activated location
Primary auditory pathway
- Superior olivary nuclei: discriminates sound direction
- cochlear nerve
- cochlear nuclei
- inferior colliculus
- medial geniculate nucleus
- temporal lobe
- primary auditory cortex
Describe afferent innervation of the spiral organ
-radial fibres
-spiral ganglion
-cochlear nerve
-95% of the afferent are type I
-5% of the afferents are type II
Describe the difference between T1 and T2 afferent
T1: big cells, thick axons, well myelinated, local connections
T2: small cells, thin axons, unmyelinated, long-ranging connections
Type I afferents
- each afferent contacts only one hair cells preserving frequency information
- synapses are fast and powerful preserving detailed timing information
- large well-myelinated axons transmit rapidly
- they vary in sensitivity so increasing loudness recruits additional afferents
- extending the dynamic range of the system
- to dorsal and venture cochlear nuclei
- each IHC drives up to 20 type I afferents (varies with location)
TI axons are the ‘true’ auditory afferents. explain
-the responses of t1 afferents are frequency-selective
-with a tightly tuned peak in sensitivity at their characteristic freq
Type II afferents
- afferents receive input from many (up to 30) hair cells covering a broad range of frequencies
- synapses are weak and slow so timing details are lost
- very slow axons which fire only if the entire pool of inputs is strongly activated
How T2 are “Strange and mysterious
- they rarely fire APs
- but their dendrites are both post- and pre-synaptic to OHCs
- and also make synapses with one another
- similar to horizontal cells in retina
- potentially an inhibitory signal?
- TII axons generate a laterally spreading feedback network but their role is obscure
Efferent innervation of the spiral organ
superior LOC and MOC
Medial olivocochlear (MOC) efferents
- respond well to sound
- have narrow frequency tuning
- feedback in humans - equally bilateral
- MOC inhibits OHCs
- synapse on OHCs
- projection tonotopic
- terminate in region slightly broader than their input
MOC Roles
- are a mechanism for adaptation to different sound levels
- increase signal to noise ratio in louder environments
- play a role in selective attention
- are protective against noise trauma
MOC activation reduces cochlear amplification adjusting responses to sound level
MOC inputs reduce activation of OHCs ⇒ responses of type I afferents ⇒ especially around their characteristic frequency
MOC activation improves signal-to-noise. True or False
TRUE
What is required to understand speech in noise co-varies with strength of MOC feedback
SNR
Lateral olivocochlear (LOC) efferents
- respond to sound but axons are thin and slow
- feedback loop is primarily ipsilateral
- some LOC efferents facilitate and others suppress transmission
- synapse on type I dendrites esp those that are least sensitivity
- heterogeneous
- use a variety of neurotransmitters
- are protective against noise trauma
- ‘balance sound’ in the two ears
- the axons are impossible to record from and the pathways use umpteen neurotransmitters with a range of effects - work in progress
Noise damage
LOC efferents may reduce noise-induced loss of afferents
