Functional anatomy of the cochlea

Question 1

Name labels of the spiral organ of the cochlea

Answer 1

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

Question 2

Describe tonotopic organisation of the cochlea

Answer 2

-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

Question 3

Describe the cochlea

Answer 3

-scala vestibuli
-cochlear duct
-scala tympani
-cochlear branch of the vestibulochochlear (VIII) nerve
-modiolus

Question 4

Human hearing

Answer 4

-dynamic range - 1 trillion fold
-1picometer vibration of eardrum
-discrimination - 1/30th of a piano key interval

Question 5

how do we discriminate frequency?

Answer 5

-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

Question 6

IHC afferent responses correlate with?

Answer 6

Basilar membrane displacement

Question 7

OHC create________ of IHC signals

Answer 7

narrow tuning
-OHC amplify the effect of sound waves and increase sound by changing the size of amplification

Question 8

Loss of OHC reduce….?

Answer 8

both sensitivity and frequencies resolution

Question 9

difference between IHC and OHC

Answer 9

pick up signal and transmits to brain
vs
produce amplification of basilar membrane vibrations

Question 10

what is peak in sensitivity due to?

Answer 10

cochlear amplifier

Question 11

is the cochlear a passive structure?

Answer 11

NO - it amplifies sound waves

Question 12

describe the cochlear amplifier in action

Answer 12

-with no assistance from the OHC no movement
-with sound-driven shortening of the OHC plus active movement of stereocilia

Question 13

describe OHC ‘tug”

Answer 13

-optimally timed to increase resonance at this location
-effect is specific to activated location

Question 14

Primary auditory pathway

Answer 14

1. Superior olivary nuclei: discriminates sound direction
2. cochlear nerve
3. cochlear nuclei
4. inferior colliculus
5. medial geniculate nucleus
6. temporal lobe
7. primary auditory cortex

Question 15

Describe afferent innervation of the spiral organ

Answer 15

-radial fibres
-spiral ganglion
-cochlear nerve

-95% of the afferent are type I
-5% of the afferents are type II

Question 16

Describe the difference between T1 and T2 afferent

Answer 16

T1: big cells, thick axons, well myelinated, local connections
T2: small cells, thin axons, unmyelinated, long-ranging connections

Question 17

Type I afferents

Answer 17

• 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)

Question 18

TI axons are the ‘true’ auditory afferents. explain

Answer 18

-the responses of t1 afferents are frequency-selective
-with a tightly tuned peak in sensitivity at their characteristic freq

Question 19

Type II afferents

Answer 19

• 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

Question 20

How T2 are “Strange and mysterious

Answer 20

• 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

Question 21

Efferent innervation of the spiral organ

Answer 21

superior LOC and MOC

Question 22

Medial olivocochlear (MOC) efferents

Answer 22

• 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

Question 23

MOC Roles

Answer 23

• 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

Question 24

MOC activation reduces cochlear amplification adjusting responses to sound level

Answer 24

MOC inputs reduce activation of OHCs ⇒ responses of type I afferents ⇒ especially around their characteristic frequency

Question 25

MOC activation improves signal-to-noise. True or False

Answer 25

TRUE

Question 26

What is required to understand speech in noise co-varies with strength of MOC feedback

Answer 26

SNR

Question 27

Lateral olivocochlear (LOC) efferents

Answer 27

• 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

Question 28

Noise damage

Answer 28

LOC efferents may reduce noise-induced loss of afferents