Central Auditory System Flashcards
What does Ipsilateral mean?
The pathways/structures that are all on the same side of the head
What does Contralateral mean?
The pathways/structures that are on the opposite side of the head
All the way through to the cochlea, everything is _________. After the cochlea, things can be _________.
Ipsilateral
Ipsilateral or Contralateral
What does Excitatory mean?
Increases in firing rate when there is a sound
What does Inhibitory mean?
Decreases in firing rate
What does Afferent mean?
It goes up to the brain
What does Efferent mean?
It comes down from the brain
What is Tonotopic Organization?
The preservation of the Place Frequency Code at all levels of the auditory system. The mapping of frequency to place is by location on the basilar membrane. Frequency that is close together are coded together (things that wire together fire together)
Why is important to study what happens in a synapse?
Every time there is a synapse, you can connect to other fibers (especially if they have different cell morphologies), which is an opportunity where more processing can happen
What is the order of structures that a sound travels through in the Central Auditory System?
1.) Cochlea
2.) Cochlear Nucleus
3.) Superior Olivary Complex
4.) Inferior Colliculus
5.) Medial Geniculate Body/Thalamus
6.) Auditory Cortex
What are First Order Afferent Fibers?
Auditory nerve fibers that leave the cochlea and go to the cochlear nucleus
What are Second Order Afferent Fibers?
Auditory nerve fibers that leave the cochlear nucleus and go to the Superior Olivary Complex
First Order Auditory Nerve Fibers are (Ipsilateral/Contralateral/Both)?
Ipsilateral
Auditory Nerve Fibers that leave the Cochlear Nucleus are (Ipsilateral/Contralateral/Both)?
Ipsilateral, contralateral, or both
Why do things go from the Auditory Cortex down to the cochlea?
We don’t really know.
Inhibition can help structures refine processing of the stimulus at different stages in the CAS.
Potential refining/sharpening/narrowing of tuning curves
Why are bipolar neurons important?
All afferent auditory nerves that leave the cochlea have this shape
Why are cell morphologies important?
Cell morphologies allow you to distribute signals in very different ways and allow for different types of processing
What are the functional differences between the Peripheral Auditory System (PAS) and the Central Auditory System (CAS)?
1.) All fibers that leave the cochlea PAS are excitatory. Cells in the CAS can be excitatory, inhibitory, or both.
2.) Response Type: Every afferent auditory nerve fiber that leaves the cochlea has the same post stimulus time histogram because all neurons are bipolar neurons. The CAS has more morphology, so there is a variety of response types (chopper, on, off, onset/offset, etc.)
What neurotransmitter is in the PAS?
Glutamate
What neurotransmitter is in the CAS?
Glutamate, GABA, glycine, acetylocholine AcH
What Central Auditory Nerve System fibers are related to the temporal grouping principle?
1.) On fiber
2.) Off fiber
3.) On-Off fiber
How do Tonic fibers behave?
It turns on when a sound starts and stays on for the duration. There is no real burst
How does an Inhibitory fiber behave?
It turns off when a sound starts and stays off for the duration of the sound
What is the fiber that is present in the PAS?
Acoustic Nerve
Why are there 3 representations of the cochlear nucleus (CN)?
The CN has tonotopic organization and is so tonotopic that it has 3 different representations. There is only one CN on each side of the head.
What are the 3 types of organization of the cochlear nucleus (CN?)
1.) Anteroventral CN (AVCN)
2.) Posteroventral CN (PVCN)
3.) Dorsal (DCN)
What is the role of the DCN?
When we measure tuning curves here, they are sharper than the ones you get from auditory nerve fibers. Frequency discrimination is better here.
What is the role of the AVCN?
It focuses more with the temporal/timing aspects of waveforms
What is the role of the PVCN?
It focuses more with monitoring changes in intensity
What are the 4 potential places a sound goes after leaving the Cochlear Nucleus?
1.) Some fibers are ipsilateral and some are contralateral and they go to the SOC
2.) Some fibers bypass the SOC and go to the IC
3.) From the AVCN and PVCN, fibers go both ipsilateral/contralateral to the SOC
3.) From the DCN, most fibers cross over and skip the SOC and go up to the IC
What are the 2 parts of the Superior Olivary Complex (SOC)?
1.) Medial SOC
2.) Lateral SOC
Why is the SOC important?
It is the first physiological location where in the APS to do binaural processing. The SOC is where you see inhibitory and excitatory coding. It takes in information based on phase and level differences
What does the medial SOC do?
It takes in information about interaural phase differences. You can tell where a sound is in azimuth if it is low frequency
What does the lateral SOC do?
It takes in information about interaural level differences
What is the main function of the SOC?
Comparing information from the two ears: binaural fusion
It provides protection from loud sounds with the acoustic reflex through efferent pathways in the facial nerve
How does the middle ear (ME) protect us from loud sounds?
The stapedius muscle pulls the stapes away from the oval window so the impact is less. This happens because there is an efferent arc that comes down from the SOC to the ME connected to the facial nerve that is connected to the stapedius muscle.
The SOC is responsible for the acoustic reflex
What are the main pathways out of the SOC?
Most fibers coming out of the lateral SOC go to contralateral to the IC
Most fibers coming out of the medial SOC go ipsilateral to the IC
Where is the Inferior Colliculus?
Midbrain
Where does information come from to the IC?
SOC and the bypass frequency info from the CN
What happens in the IC?
-Everything synapses here and it also has a lot of different cell morphologies
-Frequency, intensity, timing information/processing
-Possible critical band/tone-in-noise processing
-Additional processing of interaural phase & IA intensity
-Echo suppression
-Comparisons of high frequency information from the HRTF, precedence effect, elevation processing
-Tuning curves get even sharper here
Where do connections from the IC go to?
Ipsilateral and contralateral to the Medial Geniculate Body (Thalamus)
What happens in the Medial Geniculate Body/Thalamus?
Some cells only respond to certain sounds. so there is even more specificity coding here
Emotional response to sound processing happens here
Where do connections from the MGB go?
Some connections go to the amygdala, which regulates emotional responses to sounds.
Some pathways go to the auditory cortex.
Almost all connections are ipsilateral
Most connections go to A1 region of the PAC
What are interneurons?
Interneurons are a neuron which transmits impulses between other neurons. They are used in higher functions and only found in the auditory cortex
They are an additional neuron before the fibers that leave the MGB
How is the Primary Auditory Cortex divided?
A1 and A2 regions
What is the role of the A1 region?
It is incredibly tonotopic and has very good intensity, timing, and frequency response
What are multisensory areas?
Areas of the cortex that get input from multiple sensory areas.
McGurk Effect happens here
Where do sounds go after the PAC?
A1, A2, Wernicke’s Area, Broca’s area, multisensory areas of the cortex, frontal lobe, etc
The auditory cortex is the last place where ____________.
Things are purely auditory
How do you measure what’s going on in the auditory cortex?
Evoked responses through an electrocochleogram
What are evoked responses?
Responses from different paths of the auditory system measured through electrodes. You receive waveforms of electrical activity over time
What 2 things are you looking for with evoked responses?
1.) Presence or absence of peaks in the waveform
2.) Timing of those peaks
What do evoked responses tell us?
If you find a peak at a particular point in the waveform, it indicates that that structure in the auditory system is responding. Neurotransmitters released at each synapse point are electrochemical and indicate a particular structure is processing the stimulus
What do missing peaks tell us?
We see which structure is being most affected
What does a delayed/small peak tell us?
There is a problem in the transmission line/pathway between structures in the auditory system
Possibly an acoustic neuroma
How long is the transmission time after you present a signal in an electrocochleogram?
5ms
How long is the transmission time in an electrocochleogram with Auditory Brainstem Response?
0-10ms; includes the ECochG response
How long is the transmission time in an electrocochleogram with Middle Latency Response?
10-100ms
How long is the transmission time in an electrocochleogram with Long Latency Response?
100-500ms
How long does it take for the CAS to process sound?
500ms
What does Peak I tell us?
Measures response in the cochlea (response of hair cells, action potential of the auditory nerve fiber)
What does Peak II tell us?
Measures transmission along the primary auditory nerve fiber to the cochlear nucleus
Long bipolar neurons
What does Peak III tell us?
Measures activity in the CN
What does Peak IV tell us?
Measures SOC
What does Peak V tell us?
Measures IC
What does Peak VI and VII tell us?
Measures MGB of the thalamus
Why does the thalamus have 2 peaks?
Interneurons
What 3 peaks do you look for in the Middle Latency Response (MLR)?
-Negative and Positive going peaks
1.) Na is still the thalamus (interneurons)
2.) Pa is in the PAC; 25ms
3.) Nb is still the PAC, but also projections to other areas (A2, Wernicke’s, Broca’s)
What do the 4 peaks in Long Latency Response tell us?
-LLR reflects all cortical processing
1.) N1 reflects some activity in PAC and projections out of PAC
2.) P2 reflects activation in frontal lobe
3.) N2 also reflects activation in frontal lobe
4.) P3 represents/is only seen if you present good stimuli with a change or comparison. Bad stimuli is a constant sound
What do you do if evoked responses are not giving much/are inconclusive?
Refer to an ENT for an fMRI
Who do we use ABR for?
Useful for populations that are nonverbal or can’t make a voluntary response to audiograms. Usually sedated, but not seen much anymore unless it is a last resort
Sedated children/individuals give bad data
