Auditory Nervous System Flashcards
Where does the auditory nervous system begin?
At the hair cells of the cochlea
What is sound?
Energy imparted in waves through a medium
What are the 9 steps of sound transduction in the conductive auditory system?
1.) Sound travels through external ear and strikes TM
2.) Ossicles attached to TM move back and forth
3.) Stapes footplate moves in and out of the oval window
4.) Movement of stapes compresses the fluid inside the cochlea
5.) The fluid vibration causes the basilar membrane to move
6.) BM displacement deflects stereocilia on the HCs
7.) Movement of stereocilia opens ion channels
8.) Potassium ions flow from the positively-charged endolymph into the negatively-charged hair cells
9.) Inner hair cells depolarize and release glutamate
Where is perilymph found?
Scala vestibuli and scala tympani
What is the charge of perilymph?
0mV
Where is endolymph found?
Scala media
What is the charge of endolymph?
+80mV
What is another name for the charge of endolymph?
Endocochlear potential
Where is cortilymph found?
Under the reticular lamina/tectorial membrane
Which ion depolarizes hair cells?
Potassium (K)
What gradient is potassium flowing down?
Flowing down its electrical gradient
What is the charge inside of the hair cells?
-45-70mV
Why is the electrical gradient for K so powerful?
The endolymph is also very highly positive
Which structure recycles potassium?
Stria vascularis
What happens is stria vascularis cells die off?
It decreases the charge of the endolymph
Why are people who have decreased endolymph/endocochlear potential better hearing aid candidates?
They just need a higher intensity sound to tip the stereocilia
What hearing condition is common with stria vascularis dying off?
Presbycusis
What NT creates an EPSP to fire a charge?
Glutamate
Without IHC…..
You can’t hear complex sounds
Without outer hair cells…
Your thresholds are a lot higher
What is the internal auditory meatus?
Hole through petrous temporal bone
What is the cerebello-pontine angle?
Location between IAM and cochlear nucleus
What is contained in the IAM and CPA?
7th and 8th cranial nerves
What are the 3 fundamental pieces of information about a sound?
1.) Frequency
2.) Intensity
3.) Phase
What is a characteristic frequency?
The ideal frequency for eliciting response from a neuron; Requires the lowest stimulus level to elicit a response
Neurons will respond to stimuli other than CC, but…
The stimulus must be a higher level
What is a tuning curve?
The envelope of the thresholds required for provoking response across all the frequencies
Where are the low frequencies at on the auditory nerve?
Central core of the auditory nerve
Where are the high frequencies at on the auditory nerve?
Extend to outer layers
How is the auditory nerve organized?
Tonotopically
What does the similarity between the tuning curves of the auditory nerve and the BM tell us?
The auditory nerve faithfully represents the auditory processing of the cochlea and transmits that info to the brain
What is a rate-level function?
Increases in stimulus levels lead to increased neural discharge rate, but discharge rates eventually plateau
What does the plateau in the discharge rate tell us?
It is the point at which an increased stimulus level does not lead to increase in discharge rate
What is phase locking?
Action potential firing locks to the period of the stimulus
What are the two pathways of the central auditory system?
1.) Primary pathway is an ascending (afferent) parallel system
2.) Descending (efferent) systems
What does the auditory nerve synapse with?
Cochlear nucleus
Where is the tonotopic organization of the cochlea and auditory nerve preserved?
The cochlear nucleus
What does the cochlear nucleus synapse to?
Superior Olivary Complex
What is the importance of the SOC?
First site in CANS to receive binaural input
Where do auditory nerves connect to?
The cochlear nucleus on the ipsilateral side as the cochlea
Where do both CN connect to?
Both the ipsilateral and contralateral SOC
At what point in the CANS is info from both ears?
The SOC and above
What 2 mechanisms combine to provide info about localizing sound in azimuth?
1.) Interaural time difference
2.) Interaural level difference
Interaural time difference is effective for…
Low frequency sounds
Interaural level difference is effective for…
High frequency sounds
What are olivocochlear efferent tracts?
Connect from the SOCs to the cochleae
What are efferent tracts?
Descend from auditory cortex to lower CANS structures
What are the 2 pathways of the olivocochlear efferent system?
Medial and Lateral
What does the medial tract of the olivocochlear efferent system do?
Inhibits electromobility of the outer hair cells
What does the lateral tract of the olivocochlear efferent system do?
Induces IPSPs in the afferent dendrites of the spiral ganglion neurons
What does the SOC connect to?
Inferior colliculus through the lateral lemniscus
What are the 2 potential functions of the IC?
1.) Refines frequency processing of the CN
2.) Refines the binaural processing of the SOC
Use of these 2 things may aid in sound localization in 3 dimensions
Frequency analysis and binaural processing
What does the IC connect to?
Medial geniculate body of the thalamus
What does the MGB connect to?
Auditory cortex
Where is the auditory cortex?
Deep in the temporal lobe
Which of Brodmann’s areas make up the auditory cortex?
41 and 42
Populations of cells in the auditory cortex respond to…(3)
1.) Amplitude
2.) Frequency
3.) Phase
The auditory cortex has little reaction to…
Simple tonal stimuli
The cells in the auditory cortex work together to…
Process the complexities of high-level auditory stimuli like speech
What is subjective tinnitus?
Perception of hearing but without the evoked sound signal
What is objective tinnitus?
Perception of internal sounds produced by the body
What is subjective tinnitus often a consequence of?
Peripheral auditory system injury
How is the limbic system related to tinnitus?
1.) Amygdala is implicated in strong emotional responses to tinnitus
2.) Habituation of tinnitus may reduce activity in the parahippocampal gyrus region
