Exam 2 | Olivocochlear Efferent System

Where does cochlear efferent innervation originate?

The superior olivary complex.

What is the cochlear efferent pathway called?

The olivocochlear bundle (OCB).

What are the two divisions of the OCB?

Medial olivocochlear (MOC) and lateral olivocochlear (LOC) bundles.

Where does the LOC originate?

In or around the lateral superior olive (LSO).

Where does the MOC originate?

In or around the medial superior olive (MSO).

Where does the LOC primarily terminate?

In the ipsilateral cochlea.

Where does the MOC primarily terminate?

In the contralateral cochlea.

Which efferent pathway is also called the crossed OCB?

The MOC pathway.

What structure do MOC fibers cross in the cochlea?

The Tunnel of Corti.

Where do MOC fibers terminate?

Directly on the bases of outer hair cells.

Where do most LOC fibers terminate?

Alongside afferent fibers underneath the inner hair cells.

Which hair cells receive most efferent innervation?

Outer hair cells, about 95% of efferent fibers.

What is the cochlear amplifier?

The OHC contribution that enhances basilar-membrane motion.

What two major effects does the cochlear amplifier create?

Compressive nonlinearity and precise frequency selectivity.

What does MOC activation do to the cochlear amplifier?

Inhibits it and reduces its gain.

What happens to basilar-membrane motion when MOC fibers are activated?

BM motion produced by the cochlear amplifier becomes smaller.

What happens to OAEs when the MOC is activated?

OAE amplitudes decrease because cochlear amplifier activity is suppressed.

What can contralateral noise do to DPOAEs?

Lower DPOAE amplitude through MOC activation.

What activates the olivocochlear bundle?

Acoustic stimulation.

What is activation of the OCB by sound called?

The olivocochlear reflex.

In the MOC system, is the reflex mainly ipsilateral or contralateral?

Largely contralateral.

Why can cochlear amplification hurt signal-in-noise listening?

It can amplify low-level background noise that you do not want.

How can MOC activation improve signal-to-noise ratio?

It suppresses cochlear amplification of lower-intensity noise more than the stronger signal.

Is the MOC masking-release effect intensity dependent?

Yes. It more strongly suppresses low- or mid-intensity sounds.

Is the MOC masking-release effect frequency dependent?

Likely yes. It can reduce low-frequency noise that would mask higher-frequency signals.

Why is low-frequency background noise especially problematic?

It can spread basally and cause upward spread of masking.

How does MOC activation reduce upward spread of masking?

It suppresses amplification of low-frequency noise, reducing its basal spread.

Can the MOC turn a bad SNR into a good SNR?

No. It mainly makes an already decent/favorable SNR better.

What happens if both signal and noise are low intensity?

MOC activation may suppress both.

What happens if both signal and noise are very high intensity?

MOC has little effect because the cochlear amplifier contributes little at high levels.

Why is the MOC protective effect against hazardous noise relatively small?

Hazardous noise is high intensity, where the cochlear amplifier contributes little.

What can happen if the MOC is cut before noise exposure?

Greater temporary and permanent noise-induced hearing loss.

What do LOC fibers do to afferent auditory nerve activity?

Inhibit action potentials of afferent auditory nerve fibers.

How might LOC activity help signal-in-noise listening?

It may modify afferent firing depending on spontaneous activity and improve SNR.

What neurotransmitter from IHCs can cause excitotoxic injury if excessive?

Glutamate.

What is the hypothesized protective role of the LOC?

Reduce afferent firing and help prevent glutamate excitotoxicity.

Acoustic SNR vs functional cochlear SNR?

Acoustic SNR is the physical signal-to-noise relationship; functional SNR is the cochlear representation after processing such as MOC suppression.

What is the key MOC signal-in-noise chain?

MOC activation → cochlear amplifier suppression → less low-level noise amplification → less masking → improved functional SNR.