Lecture 7 Flashcards
In the innervation of the cochlea, what are the afferent and efferent pathways?
What do they do?
Afferent (up): goes first into cochlear nucleus; pathway up to the cortex. Transmits info to the brain for important functions
Efferent (down): Superior olivary complex, lateral olivary nuclei, and medial nuclei. Higher auditory centres may exert control over lower centres, influencing the processing of sounds at early stages.
What are the Type I/II neurons in the afferent system?
Type I: thick (large diameter), communicate info fast in the system - myelinated
Type II: not really myelinated (OHC have these type of nerves)
What is the role of the medial geniculate body?
Sends projections to the OHC, synapse on OHC, hyperpolarize IHC. (mainly inhibitory)
What is the role of the lateral geniculate body?
Sends projections (inhibitory) afferent fibre from the IHC. Synapse on afferent fibres (inhibit auditory nerve fibres) Projections to cochlea
In the ascending auditory system pathways, what is the sequence of structures from the cochlea to the brain?
Ipsilateral cochlea > right cochlear nucleus > superior olivary complex > lateral lemniscus > inferior colliculus > medial geniculate body > auditory cortex.
Bilateral redundancy - input from each cochlea. Systems in parallel - rapid processing of info.
What is the role of the superior olivary complex?
Lateralizing information from the environment (voice). Ears both getting input, there is crossover to show that you are moving from one location to the next - to localize your voice.
In the ascending auditory pathway, what important factor happens at the inferior colliculus?
Crossing fibres.
What happens if there is a lesion in the first portions of the ascending auditory pathway?
Challenge is localizing. Still receive information from the crossover - parallel processing.
What is happening at each stage of the ascending auditory pathway?
Information about the sound is processed by a network of neurons, with parallel projections taking different types of info up to the brainstem.
Each nucleus may contain many different types of neurons, with varying properties.
What are the components of the Cochlear Nucleus?
Nerve branches innervate 3 divisions of the CN in the medulla:
Dorsal
Anteroventral
Posteroventral
What is the role of the inferior colliculus?
Integration centre, tonotopically organized, pitch perception
What is the role of the superior olive?
Integration of information from two ears - sound localization.
What is the role of the auditory nerve?
Bundle of neurons coming from cochlea.
What is the role of the primary auditory cortex?
Where is it?
First cortical receiving area for auditory information. Tonotopically organized. Integrates information about space, time and frequency.
Putting together things at higher complexity - sound perception.
Buried in the sylvian fissure, on top (superior temporal plane)
What is the medical geniculate nucleus?
Obligatory relay station on route to the cortex.
What is the role of the central auditory system?
Takes sound apart at a peripheral level; preserves mapping all the way up the chain.
What does the cortical tissue do?
Reserved/used for hearing sound and understanding speech and complex sounds.
IMPORTANT FOR COMMUNICATION
Tissue integrates info that’s useful, meaningful to us.
How is cochleotopy maintained at the cortex?
Good representation of the cochlea - nerve fibres synapse into cochlear nucleus - map of low to high frequency (almost 1:1 relationship)
Each band responds to one frequency - fibres of neurons synapse on band
Get higher in centres - info becomes more specific. Reach excitatory chain = lateral inhibition. More discrete perception of pitch as tuning of cochlea occurs.
In descending pathways, what are the types of interconnections?
Interconnections may be inhibitory or excitatory.
Only the direct connections with cochlear are termed “efferent”
Attention to cortex and cortical descending control to the cochlea. What you have going up is the same going down. How these things affect localization, masking, bilateral movement
In the efferent system of cochlear innervation, what process is going on?
Descending information is coming down form the superior olivary complex; the lateral olivary complex synapses on afferent fibres of the IHC
The output of the cochlea is changed.
IHC/OHC inhibition occurs; inhibits post-synaptic potential, hyperpolarize to stop them from adding energy to the cochlea by adding double bounce = less stereocilia tilt, less afferent info out from OHC (suppressing loud noise = no double bounce)
Why are auditory evoked potentials important?
Testing for brain response when you fire a sound in the ear. Transduction of energy that can be picked up by recording electrical activity on surface of skull; electricity transmitted through cells.
Tests function of the auditory system for those who cannot reliably respond - aphasia, low IQ.
What occurs with recorded event-related potentials?
Measuring the voltage difference from the top of the head to the side - measuring potential difference. The average - stimulus causes nerve fibres to fire to cortex in time with stimulus. Sensory cognitive**
Early responses are the auditory brainstem responses; also middle and late latency responses.
P3 response - attend and unattend to info. Look at attention based on auditory stimuli spoken words - specific to semantics
When the evoked potential are averaged over many trials, what is the response?
Average of many trials to get noise down - the signal stays the same going to the cochlea.
Average over many trials the amplitude goes down, the peaks average to 0. The RMS goes down over time - complex wave amplitude of signal.
Where are ABR, MLR and CAEP measuring in the auditory evoked potentials?
What are the resulting affects of attention?
ABR - 8th nerve and brainstem - no effect of attention
MLR - thalamus and auditory cortex - very small effect of attention
CAEP - auditory cortex - moderate to large effect of attention
What is the reasoning behind attention for cortical AEPs?
When someone attends to a set of beeps, you see the difference in two waveforms when the stimulus changes. There is a difference negativity, and you can tell if you are paying attention base on the brain waves. We are not good at paying attention to two things at a time - have to understand auditory selection. We are good at switching quickly but cannot attend to something simultaneously.
What is the result of gap-evoked cortical auditory evoked potentials?
What is this test helpful for?
These help identify if a person has a gap-detection problem (if temporal processing in the auditory system is working).
When gaps are presented in noise, there is the onset and then gaps are made shorter between the two noises until the person cannot hear the gap. Your responses begin to disappear as the gap gets smaller and smaller
The average person has a gap detection of 6ms.
Helpful for those who have trouble responding reliably/behaviourally (infants, development disorders)
What are cochlear and 8th nerve potentials useful for telling us diagnostically?
Oscillations represent the steady state of hair cells going back and forth. Getting the blip in the graph shows the action potential from the auditory nerve fibre - how the nerve is actually responding.
Need to see the blip (dip)
What are the important pieces of information to gather from auditory brainstem responses?
Put in a click or a tone, need to present quickly at 2000 of them, getting the wave I-V complex.
I - auditory nerve fibre responding
II - auditory nerve fibre to cochlear nucleus
III - somewhere between
IV - inferior colliculus
V - used as threshold (for hearing level)
This can be used to screen for tumours - if info from the auditory nerve fibre takes to long to get to the inferior colliculus, the tumour may be preventing the sending of information
How can ABR be of use in clinical practice?
Used to detect infants with hearing loss - diagnose. If child gets detected at 2 years old - the ability to detect your own languages’ phonemes closes out by 12 months - want to screen and diagnose by 6 months.
If language delays are present, this may be evidence for hearing loss.
Want to see normal threshold responses at various frequencies - helps provide prescription for elevated thresholds to set hearing aids.(bilateral/unilateral, conductive, sensorineural)
What is the importance of evoked related potentials and semantic violations? (The N400)
This is a measure/indication in speech and language assessments to see if someone understands incongruity when presented normal/obscure sentences.
Looks at the time auditorally to get info up to the auditory cortex, process the speech, categorize it as the word and figure out whether it matched expectations. (Violation of expectation = N400)
If the construct is not part of your grammar structure you get the N400
