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CMSD5280 Audition ll > Cochlear Physiology V: Efferent > Flashcards

Cochlear Physiology V: Efferent Flashcards

1
Q

What are the aspects of Centrifugal innervation? (3)

A
  1. Ascending to descending, and exerts top-down processing.
  2. Optimizes the encoding of ascending signal processing
  3. Best investigated in the cochlear control by lower brainstem Olivo-cochlear neurons in the Superior olive complex (SOC)
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2
Q

What are the descending pathway of efferent in CAS and its characteristics? (3)

A
  1. A1 – MGB – IC – SOC –CN – COCHLEA
  2. there is no direct innervation efferent pathway from A1 to cochlea
  3. stronger pathway from A1 to MGB than A1 to IC
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3
Q

What is the difference between AR and Middle Ear AR?

A

AR is different than Middle Ear AR
At the level of the brainstem, Whole loop circuit

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4
Q

What are the major circuit pathways for efferent acoustic reflexes? (3)

A

PMT: pontomesencephalic tegmentum.
1. Starts from cochlea, ascending to AVCN PVCN but PVCN(Major projection that send to SOC part of efferent system) Involved in the AR
2. PVCN project bilaterally to MSO cross middle line (more contralaterally)
Efferent from LSO not sure the afferent towards it not sure of its role in the reflex circuit
3. MSO cross the middle line (main pathway) goes back to the starting cochlea

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5
Q

What are the projections of the descending pathways? (3)

A
  1. AC—MOC N (purple line)—cochlea (green lines)
  2. AC—IC—MOC N (purple line)—cochlea (green lines)
  3. AC—IC—CN and LL
  • Our focus: MOC N contralateral cross middle line back to the cochlea
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6
Q

What are the projections of the descending pathways? (2)

A
  1. SGN—PVCN—MOC neurons
    - SGN-AVCN-IC-MOC (less clear)
  2. SGN—DCN(?)—LOC neurons
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7
Q

In the ascending pathway, which part of the cochlear nucleus is stronger?

A

In the ascending:
Cochlea – PVCN AVCN (stronger AVCN)

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8
Q

What can we see in these images related to the Olivocochlear efferent pathway? (3)

A
  1. In the descending pathway from SOC, there are many crossings in the middle line to the MOC
  2. Medial Fibers are Thicker and can be accessed more easily than lateral fibers since many cross over.
  3. MOC Efferent have larger synapses under the OHC MORE IMPORTANT, LOC efferents synapse at the terminal of the auditory synapse of IHC which we do not know their function
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9
Q

Describe the location, grouping, crossing, and targets of the cochlear efferent innervation from the brainstem.

A

Location of efferent neurons: in perinuclei of SOC
Grouping: medial versus lateral
Cross (Medial many cross) versus uncross: related to accessibility
Targets: synapse at afferent terminals under IHCs versus synapse at OHC cell bodies

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10
Q

The perinuclei of SOC is divided into the ______ and ______

A

MSO
LSO

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11
Q

LOC nuclei go to -> ____________ -> ________________ (afferent/efferent) terminals at ______ (Hair Cells)

The nerves are _______ (thickness) and ____________________ (myelination). They mostly got to _________________ (lateralization) cochlea.

A

LOC nuclei go to -> UOCB (uncrossed olivocochlear bundle) -> afferent terminals at IHC

The nerves are thin and unmyelinated. They mostly got to the ipsilateral cochlea.

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12
Q

MOC nuclei go to -> ____________ -> ________________ (afferent/efferent) at ____________ (Hair Cells) lateral basal surface.

The nerves are _______ (thickness) and ____________________ (myelination). They mostly got to _________________ (lateralization) cochlea.

A

MOC nuclei go to -> COCB -> OHC bodies lateral basal surface.

The nerves are thick and myelinated. They mostly go to the contralateral cochlea.

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13
Q

What effect does the double loop crossing have on the middle ear acoustic reflex?

A

The whole loop double-crosses the midline which causes stronger ipsilateral middle ear AR effect

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14
Q

What are the reasons why we know more about the MOCB than the LOCB? (2)

A
  1. MOCB is more accessible (stimulation and recording at brainstem surface below cerebellum (4th ventricle)
  2. MOCB fibers are thick and myelinated, producing stronger action potential, easier to be recorded.
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15
Q

What are the characteristics of the efferent circuit of acoustic reflex (EAR)? (2)

A

The efferent acoustic reflex (EAR) is part of the AR
It is distinguished from AR from the fact that the AR is via the middle ear muscle

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16
Q

What is the major loop pathway of the EAR?

A

Cochlea-PVCN-MOC-Cochlea

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17
Q

What is the Target of the EAR?

A

OHC

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18
Q

What is the function of the EAR on OHC? (2)

A

To provide motility and change of active cochlear gain

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19
Q

What proportion of MOC and LOC are crossed and uncrossed?

A

MOC: crossed 26%
uncrossed 11%
Total MOC: 37%

LOC: uncrossed 48%
crossed 15%
Total: 63% we are unsure of the function
Data from Cat

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20
Q

What are the characteristics of the LOC circuits in monkeys and cats? (4)

A

LOC efferent is more numerous
LOC efferent is more ipsilateral (from LOC to cochlea)
In cats: 48:15 (uncrossed:crossed)—total 63%
In monkey: 55:20—total 75%
CN source for LOC efferent is not well known

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21
Q

Explain the characteristics of the PVCN to MOC pathway for the feedback loop to OHC. (3)

A
  1. There is a STRONG innervation from PVCN to contralateral MOC neurons and WEAK innervation from PVCN to ipsilateral MOC neurons
  2. Since MOCB is mainly crossed, there is a double cross for ipsilateral MOCB
  3. Therefore, the contralateral MOC reflex is 1/3 as strong as the ipsilateral MOC reflex
    Even though MOC contra is much weaker, it is easier to test using OAE suppression of contra OAE
22
Q

What does this image show?

A

This is a depiction of the Binaural organization in the feedback loop

  1. Efferent feedback to one cochlea can be initiated by acoustic stimuli to either ear.
  2. PVCN to MOC n,
    Double cross for ipsilateral reflex
    The contralateral reflex is weaker
23
Q

What does this graph show? (3)

A
  1. This is a depiction of the Efferent density along cochlea
  2. We can see that uncrossed vs crossed both are significant even though uncrossed is smaller in terms of number however the crossed is much smaller and can be ignored
  3. Stronger efferent innervation occurs in the middle frequencies region
24
Q

What does this image show?

A

This is a tonotopic feature in the EAR loop: the point of cochlear stimulation which receives the feedback. The feedback will come back to the same side/point that was stimulated at the BM.

25
Q

What can we see from the efferent innervation pattern of IHC vs. OHC?

A
  1. Efferent Synapses on IHC bodies also exists. (not shown)
  2. There are synapses from MOCB to LOC fibers, and vice versa, not only under HCs but also in the tunnel of Corti.
  3. LOC synapses on Afferent terminal of IHC and MOC synapses on OHC bodies.
26
Q

What is the name of the area where MOC fibers synapse on the OHC body?

A

Efferent MOC to OHC synapse to the Synaptic cistern

27
Q

What NT play a role in the Efferent system? (2)

A
  1. NT to MOC (clear): acetylcholine (ACh)
  2. NT to bilateral efferent LOC (unclear):
    ACh—the major one, clear function
    Dopamine (DA)1: modulate input to SGns: protection against intense noise
    GABA2
    Dynorphin3
28
Q

What are the NT in the MOC- OHC synapse? (3)

A

ACh***
GABA
CGRP

29
Q

What are the functions of MOC activation? (3)

A

Depress BM motion
Depress IHC receptor potential
Depress CAP

30
Q

What are the potential mechanisms of the MOC activation? (3)

A
  1. not directly on IHCs
  2. Control of OHC motility
  3. Control of cochlear gain
31
Q

What does this experiment show?

A

This experiment shows the OCB stimulation and IHC potential

Site of stimulation: underneath 4th ventricle, target: MOC crossed

32
Q

What is the MOC stimulation impact on the CAP?

A

Amplitude reduction occurs at a low level: suggesting the effect via OHCs

33
Q

What does this graph show?

A

The effect of shock rate suggests the role of MOCB

Peak at 300/s, only MOC can respond because refractory period is longer for LOC

34
Q

Give a quick summary of the efferent stimulation by electrical shocks:

A
  1. There is an impact IHC potential via OHCs
  2. Suppresses CAP and RLFs of ANFs, large shifts at the low sound level
35
Q

What is the evidence for the contribution of OHCs in Efferent stimulation? (4)

A
  1. MOC stimulation is effective
  2. MOC innervates OHCs
  3. The peak shock rate corresponding to the refraction time of MOC f.
  4. CAP shifts at low sound level (the range of OHC active amplification)
36
Q

What is the efferent impact on ANFs (efferent stimulation to MOCB)? (3)

A
  1. Depends on ANF categorization
  2. For high SR ANFs, rate-level function changes in a similar way to IHC DC potential change: the main effect at low sound levels
  3. For lower SR ANFs, there is a plateau depression
37
Q

What does this graph show?

A
  1. Inhibition on high SR fibers
  2. There is a shifting at low levels which suggests depressed active mechanism
38
Q

What is the efferent effect we can see on RLF of SR groups?

A

3 SR groups shown
You don’t see plateau depression of High SR only art medium and low SR
No difference in level-shift between the 3

Right shifting by efferent stimulation
A, B and C: the averaged RLF from all ANFs with CFs between 3-27 kHz in each of the SR groups
D: the threshold elevation at CFs and SR categories

39
Q

What does this graph show?

A

The impact of efferent activation on the maximal firing rate.

The reduction in maximal firing rate is only seen in L/M SR ANFs, no reduction of High SR ANF

40
Q

What are the 3 main aspects of the Inhibition on low SR fibers?

A
  1. Depression occurs at the whole range of sound level
  2. The reason for that is probably related with LOC innervation
  3. But not clear how because eliminating LOC decreases SR, instead of increasing it.
41
Q

What does this graph show? (3)

A

MOC stimulation changes the tuning curve: elevating the threshold, narrowing the frequency range

Thin lines = tuning curve when efferent is activated, Threshold elevation, narrow frequency range on the response

We have a clear thought that the efferent inhibition of the cochlea is related to OHC

42
Q

The efferent effect on OHC can be demonstrated as the ____________________ which is induced by _____ as the result of more __ channel opening causing an outward ____ flow (increase in conductance)

A

The efferent effect on OHC can be demonstrated as the Hyperpolarization—induced by ACh as the result of more K channel opening causing an outward K flow (increase in conductance)

43
Q

The efferent effect on OHC causes an increase in ________________ and ______ response is increased, ____________________, and _________________ effect. The overall effect is _________________.

A

The efferent effect on OHC causes an increase in CM amplitude and AC response is increased, hyperpolarization, and shunt effect. The overall effect is suppression.

44
Q

Pay attention to PHCs and Endochochlear potential. What can we see from this image?

A

Stria vascularis: provide the driving force for MET current
GMOC =Conductance induced by stimulation
The hypothetical voltage across IHC at afferent synapse.
(B) Becomes more negative
(C) Recorded in scala medius

45
Q

What does this green arrow shows?

A

It shows how the efferent activation adds a new pathway (Reff)

Because of this new pathway, the OHC becomes more electrically conductive in response to the same signal at the same level

46
Q

What does this graph shows?

A
  • Response level function of CF = When OCB is stimulated, CM is increased even though MOCB normally inhibits everything
47
Q

What can we conclude from the MOC activation to OHC motility? (3)

A
  1. MOC activation decreases OHC motility
  2. MOC stimulation depresses DPOAE
  3. MOC stimulation depresses CAP: more shift at a low level which involves an active mechanism that suggests reduced OHC motility
48
Q

What are the effects of ACh on OHC motility recorded in Vitro? (3)

A
  1. Increases conductance, MET current gets increased
  2. Reduce stiffness, hair cell become softer
  3. Increase motility (in vitro) pointing to the fact that Ach depresses motility
49
Q

What is contradictory between in vivo and in vitro data on ACh effect on OHCs?

A

Contradictory to in vitro data, in vivo data you can see an CAP and OAE suppression and reduction in BM motion

50
Q

How does MOC on OHC reduce the active part of BM motion even though it is not fully understood? (3)

A
  1. The CM increase due to the shunt effect
  2. In vitro studies show reduced stiffness and increased motility of OHCs.
  3. Big contradiction with the depression of CAP, IHC receptor potential, and OAEs, all pointing to a reduction in OHC motility (in vivo)
51
Q

What do we know about the efferent effects on SOAE?

A
  1. Controversial issue
  2. There is a lack of animal data since it is difficult to measure (they can’t report the sound compared to humans)
  3. In humans, contralateral stimulation is used to evoke an efferent effect.
  4. However, we need to rule out the role of acoustic reflex via the middle ear
51
Q

What do we know about the efferent effects on SOAE?

A
  1. Controversial issue
  2. There is a lack of animal data since it is difficult to measure (they can’t report the sound compared to humans)
  3. In humans, contralateral stimulation is used to evoke an efferent effect.
  4. However, we need to rule out the role of acoustic reflex via the middle ear

CMSD5280 Audition ll (19 decks)

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