functional anatomy of the cochlea

Question 1

label diagram

Answer 1

A: basilar membrane
B: inner hair cells
C: tectorial membrane
D: stria vascularis- creates endolymph
E: cochlear duct- high in potassium and low in sodium that has the endolymph

Question 2

describe the tonotopic organisation

Answer 2

highest frequency at the base(closest to the oval window, where the stapes connects to cochlear) to low frequency at the apex (tip of cochlear spiral), along the basilar membrane, whereby the inner hair cells are positioned along and tuned to different frequencies according to its sensitivity

Question 3

what amplifies the frequencies in the cochlear

Answer 3

the outer hair cells

Question 4

what is the role of the amplifier?

Answer 4

is to exert great force against the fluid within the inner ear to amplify the sound wave

Question 5

describe the cochlear amplification in action

Answer 5

Vibrations tilt the stereocilia such that when it is tilted towards the longer hair cells, the prestin in the outer hair cell membrane will shrink, causing it to pull down on the tectorial membrane. This depolarizes the membrane of the hair cells as it forces open the potassium channels at the tip link, which then lead to the non-specific ion channels opening, resulting in calcium influx, which in turn closes those channels, and that also pulls back on the membrane, helping the amplification system.

This is the amplification system.

Question 6

what happens if the outer hair cells are destroyed?

Answer 6

you lose the peak sensitivity and frequency resolution that originally created the cochlear amplification of a frequency

Question 7

How do we
discriminate
frequency?

Answer 7

We have place-coding, which allows the cerebral (hearing) cortex to know what frequencies are present within a sound. This is based on the location of specific inner hair cells that are tuned to specific frequencies along the basilar membrane in the cochlear.

Question 8

what are the three fluid-filled chambers within the cochlea in order from top to bottom? and what structure is adjacent to it

Answer 8

scala vestibuli
scala media (cochlear duct)
scala tympani

the vestibulocochlear nerve 8

Question 9

how do we discriminate frequencies?

Answer 9

place coding

Question 10

what is the spiral ganglion

Answer 10

where they keep their cell bodies and the axons are spread out to different inner hair cells located along the basilar membrane

Question 11

what are the two types of afferents and which hair cell do they get innervated from

Answer 11

type 1 (innervation by inner hair cell) and type 2 afferents (innervation by outer hair cell)

Question 12

which type of afferent is most populated

Answer 12

type 1 with 95%

Question 13

describe the difference between type 1 and 2 afferents

Answer 13

type 1:
bigger cell with well-myelinated axon
transmitting signal rapidly
and is in direct contact to each inner hair cell

type 2:
smaller cell with thinner and unmyelinated axon
transmitting signal much slower
They do not synapse to just one outer hair cell but receive inputs from a broad range of hair cells, responding to wider range of frequencies

Question 14

what type of afferent is called the true afferent

Answer 14

type 1, as they carry signals that discriminate frequencies and thus discriminate hearing

they synapse onto one hair cell, preserving frequency information, being faster (due to well-myelinated axons), and having powerful synapses (depolarizing and hyperpolarizing quite rapidly too)

Question 15

what type of afferent is most populated and why

Answer 15

type 1 afferents
95% of afferents are type 1 afferents because each inner hair cell synapses onto multiple type 1 afferents contacting it.
The reason being that there are different types of type 1 afferents, ranging from low thresholds to high thresholds.
So some only require a small amount of neurotransmitter release to depolarize it, but it has a low threshold, so it will saturate quickly. However, that’s not a problem, as other synapses are gradually less sensitive, having higher thresholds, so they activate when picking up a signal and report its changes where they are decoded in higher centres.

Question 16

what does each peak in a sound correspond to (refer to what happens to neurons)

Answer 16

neurotransmitter release

Question 17

what are the two parts of superior olivary nucleus?

Answer 17

medial superior olivary nucleus
lateral superior olivary nucleus

Question 18

what efferents dampen down the cochlear amplification in response to loud sounds that may be damaging

Answer 18

MOC efferents
medial olivocochlear efferents

Question 19

what is the advantageous mechanism of MOC (medical olivocochlear efferents) for when there’s a really loud sound and when you talk to someone in a really noisy background

Answer 19

loud, damaging sounds:
when type 1 afferents send signals from inner hair cells to the medial superior olivary nucleus, it will in turn send inhibitory feedback signals back down to the outer hair cells to reduce its activity by hyperpolarizing it in order to reduce cochlear amplification and minimise damage caused by vibration on outer hair cell from loud sounds

when talking to someone in a really noisy background:
the way the cochlear works too is that when you listen to a tone or sound, your system adapts so it doesnt saturate and therefore becomes less sensitive

when the MOC efferents are stimulated, it can dampen down the noisy background to the point where you still receive the signal stimulus but just reduce the response to that signals that you can hear over it

Question 20

What is the comparison between older and younger people in terms of the strength of their MOC feedback?

Answer 20

Younger adults have strong MOC feedback, such that we can hear over background noises quite well, but as you get older, our hearing goes down, such that it becomes harder to hear people over loud background noises, and this is also as the strength of MOC feedback goes down too.

Question 21

which hair cells is likely to get damaged from high oscillations and why

Answer 21

outer hair cells as they are attached to tectorial membrane so they get pulled aggressively with higher vibrations and can cause distortions, damaging it

Question 22

what is hidden hearing loss?

Answer 22

is when not only does loud sound damage outer hair cells but also afferents. This is hidden hearing loss, as it is not so obvious.
the louder the sound as well as pronlongness, the more likely it is to being a permanent loss of function of afferents

Question 23

what happens if you lesion MOC efferents

Answer 23

it can lead to hearing loss
-due to damage to stereocilia or loss of outer hair cells
-damage to the less sensitive afferent that responds to those high frequencies

ultimately reducing sensitivity to high frequencies as you damage

Question 24

what are the two components of the inner ear and describe placement of the endolymph

Answer 24

• vestibular system
  (helps with balance)
  (endolymph is found within semicircular canal that sends information to brain about head movement that aids in balance.)
• cochlear
  (involved in hearing)
  (endolymph fills spiral structure called scala media that is crucial for converting sound vibrations into neural signals)