Quiz 4 CANS (CC+Efferent Path)

What and where is the corpus collosum?

It’s located within the center of the brain, under the cerebrum. It’s a white matter tract,

It’s a bridge that connects the two hemispheres. Roof of the lateral ventricle

What is the CC made of and what is its role

350 million nerve fibers, it’s like a cluster of wires highly myelinated to communicate motor, sensory, and cognitive info between hemispheres

What and where is the corpus collosum?

It’s located within the center of the brain, under the cerebrum. It’s a white matter tract,

It’s a bridge that connects the two hemispheres. Roof of the lateral ventricle

What is the CC made of and what is its role

350 million nerve fibers, it’s like a cluster of wires highly myelinated to communicate motor, sensory, and cognitive info between hemispheres

What’s the genu? Function? Role?

The thicker front portion of the CC, important for motor function. This connects the prefrontal cortex between the two halves

What’s the rostrum?

The part of the CC that extends down and backward in the front, also important for motor function

What’s the trunk/body Function? Role?

The main part in the middle, also good for somatory sensory info

The midbody interconnect the areas of premotor and supplementary motor regions

Posterior body communications somatosensory info between parietal lobe halves and visual center for occipital lobe

What’s the sulcus/isthmus?

It’s towards the back edge of the middle portion of the CC, before the splenium. it’s important for auditory information

What’s the splenium? Function? Role?

The most posterior portion of the CC, important for visual information. Interconnect premotor and supplementary motor regions and motor cortex

What are the age specifics of the corpus callosum and the connections and functions

The anterior region would be more matured by 3-6 years

• Frontal lobe / attention, organization, planning

The posterior region would be more matured by 6-15 years

• temporal occipital and parietal lobes / language and associative thinking, social cognition

Define Agenesis (ACC)

It is the underdeveloped or developmental absence of the corpus collosum, meaning partial or complete absence

What does isolated ACC mean vs complex ACC?

Absence with no other abnormalities / absence with other abnormalities

True or false - The incidence of ACC is 1/1000 births worldwide

False, the incidence is too difficult to estimate due to asymptomatic individuals. There are no large enough studies.

BUT IN CALIFORNIA the CBD Monitoring program marks 1.4 per 10,000 live births have ACC

What is the prevalence of associated brain abnormalities of agenesis of CC?

45.8% associated brain abnormalities will occur,

What’s the overall rate of chromosomal abnormalities?

17.8%

What are the two types of fibers of the CC?

Homolateral and heterolateral fibers?

What is the difference between homo and hetero lateral fibers?

Homolateral - a structure crosses to the other side to the same structure

Heterolateral - a structure crosses to the other side to a different structure

What’s the purpose of having heterolateral pathways?

It makes it possible to help brain sections connect and support each other

What’s the transcallosal auditory pathway (TCAP)

A bundle of interhemispheric neural fibers that connect the auditory regioons of the left and right cerebral hemispheres via the CC

• originates in primary and association auditory cortices

  • travels around lateral ventricles to contralateral hemisphere

What’s the dichotic listening paradigm?

Contralateral pathways dominate for dichotic listening. This requires an intact CC for interhemispheric transfers, especially for the left ear input because the ipsilateral pathway isn’t as strong. So the interhemispheric transfer is necessary for the signal from the left to get to the left hemisphere.

Simple words for CC

A big white matter tract that receives and sends info from each part of the brain

What are the different efferent pathways

Centrifugal pathways (brain, olivocochlear bundles, to CN and IC)

Corticofugal pathways (cortex to thalamus)

Pathways from inferior colliculus to lateral lemniscus, superior olivary complex, and CN

True or false - there are more fibers in the efferent pathway than the afferent

False, efferent system has less fibers than the ascending

How can the efferent pathway be divided?

Rostral (Auditory cortex → SOC)

Caudal (olivocochlear bundle, SOC → cochlea)

What are the feedback loops of the rostral efferent pathway?

1. Aud cortex, MGB, and IC

2. Aud cortex, posterior thalamus, and IC

3. Insular cortex, superior colliculus, aand MGB

What’s the AC/MGB/IC feedback loop order?

AC sends info to IC and MGB, IC sends info to MGB, MGB sends info back to AC

What’s the AC/PTG/IC feed back loop order?

AC sends info to the PTG and IC, IC sends info to PTG, PTG sends info back to AC

What’s the insular feedback loop order?

Insula sends info to SC, SC sends info to suprageniculate nucleus of MGB, then MGB sends info back to insula

What does the caudal efferent system include?

Pathways from SOC to cochlea

It’s also known as the olivocochlear bundle, can be dvided into lateral and medial components

What does the caudal efferent pathway do?

Sends neurotransmitters down to excite or inhibit signals

What are the synaptic terminals for the MOC?

MOC → contralateral outer hair cells directly on the base of themWh

What are the synaptic terminals of the lateral olivocochlear bundle (LOC)

LOC → the type 1 afferent nerve fibers of the IHC

Describe the LOC

The LOC is made of smaller unmyelinated fibers, and most fibers project ipsilaterally on to the type 1 fiber of the IHC. The origin is the lateral superior olive and surrounding areas

Describe the MOC

They’re made up of larger mostly myelinated fibers, projecting to the contralateral cochlea terminating on the base of the OHCs. they originate on the DMPO(?), MNTB, and VNTB

T or F - Myelinated fibers mean that the transmission of signal is faster

True

What are the types of cell bodies of the OCB?

Small cell bodies around the LSO, give rise to unmyelinated fibers → IHC (MORE LITTLE CIRCLES)

Large cell bodies around MSO, give rise to myelinated fibers → OHC (MORE LARGER TRIANGLES)

What are the contrbutions of the MOC and LOC cells for the diff components of the OCB?

Crossed OCB = crossing fibers of the crossed OCB?

IHCS receive mostly from IPSI LOC, some from Contra LOC

OHCS receive mostly from CONTRA MOC, some from IPSI MOC

T or F - LOC is mostly contralateral

FALSE - LOC IS MOSTLY IPSI, MOC IS MOSTLY CONTRA

Explain the tonotopicity of the OCB?

OCB is tonotopically organized and projects to the basilar membrane, most of the info is from the MOC bundle

MEDIAL - HIGH

LATERAL - LOW

What’s the physiology of the efferent system focused on?

It’s focused onto the MOC bundle since there’s more myelination and response properties are easier to measure

What are the fiber thresholds of the MOC?

Dynamic intensity ranges between 40-60dB, but thresholds can be as low as 10dB and as high as 80dB

What’s the spontaneous firing rate of the efferent system?

~80% of MOC neurons have little to no spontaneous activity, tuning curves are sharp and comparable to the type 1 fibers though

What are the effects of the OCB on ascending systems?

MOC reduce motility of OHC, reducing active amplification

MOC reduce gross evoked potential, shifting rate-intensity functions of auditory neurons by stimulating crossing fibers

OCB impact on auditory nerve?

low level electrical curent applied to MOC → inhibits auditory nerve

This is like damage control

How is the OCB activated?

It responds to higher level auditory areas to pay attention to sounds needed to be inhibited. Ensuring no damage to the peripheral system

What’s the significance of the OCB?

Improves signal detection in noise (sharpens rate-intensity function)

Protection for noise damage (temporary threshold shift) (dampens noise, reducting firing rate

Controls mechanical state of cochlea

plays a role in attention

What are the fast-acting effects in a quiet background for MOC?

Fast-acting means that bc the fibers are myelinated, there are faster transmissions. This reduces gain of the cochlear amplifier, showing a shift in rate-level function and elevation of tuning curve tips

DAMAGE CONTROL, OHCS TOO EXCITED? MOC CALMS THEM DOWN

What’s the cochlear amplifier?

The OHCs contracting and expanding, increasing amplitude of low-amplitude sounds, compressing sound applitude (large → smaller loudness), sharpening frequency resolution.

What’s the feedback to the OHC cochlear amplifiers?

The MOC nerve fibers send the signal back down to the OHC, turning down the cochlear amplifier. The auditory cortex sends fibers down to the brain stem to pay attention to specific frequencies and changing the amplitude of incoming sounds (intentions)

T or F - MOC has an unmasking effect on noise

True

Explain the fast-acting effets in a noisy background

This is related to MOC unmasking, where the MOC activation releases the noise, reducing cochlear amplifier gain and decreasing the ANF response.

In quiet vs in noise for MOC physiology

In quiet - elevating thresholds, decreasing sharpness of basilar membrane tuning

In noise - enhancing rseponse of ANF to signal at high levels

What about the slow acting effects of MOC?

Protects ear from damage due to intense long duration sounds

What’s the impact of MOC activation on OAEs

MOC → reduce gain → supresses OAE amp

Contra OAE suppression → shows effects of MOC activation

MOC reflex??

Increase discrim of transient sounds in noise, protects against damage, selective attention