AVS CANS Flashcards
How is the BM tonotopically organized?
high frequencies to low frequencies (base to apex)
How are the CN VIII tonotopically organized?
high on external part of bundle - lows wrapped around the cores
How is the A1 tonotopically organized?
Lows laterally to highs medially
what is the main thing we know about the Cochlear Nucleus?
The CN is the first station for processing auditory information in the brainstem (monoaural/ipsilateral at this level)
What is the gold standard for symptoms of Schwanomas?
MRI with enhancement agent (gadalidium)
Is CN VIII a part of the peripheral nervous system
yes
What is a bundle of myelinated axons ascending in the brainstem
lateral leminiscus
Are there more fibers ipsilaterally and or more going contralaterally?
Contralaterally (stronger of the two pathways)
why do we say as have built in redundancy?
info comes in from one cochlea to go to both sides and crosses over in different places and a lesion in one spot and a tone is played, the information from that cochlea would make it up to the brain because it has other ways to go up and bypass the lesion
so many pathways it can take that it will still reach the cortex for perception due to so many commissures and ipsilateral and contralateral pathways for it to get to the cortex through one of those pathways.
Why is redundancy important?
knowing our environment and for our safety
communication
helps us do really well
causes problem as audiologist because a simple test is not possible because the signal can still get to the brain so it is hard to find lesions or tumors
in order to assess a lesion or understand by audiogram have to do speech testing
if words or sentences are too easy, we could do CAP (put sentences in noise or split the sentences up between the two ears)
How do vestibular Schwanoma’s form?
from myelin (Schwann cells) of the superior and inferior vestibular nerves (of cranial nerve VIII)
Where do Schwanoma’s form?
In the medial internal auditory canal (IAC) or lateral cerebellopontine angle (CPA)
Schwanoma’s make up _____ of CPA tumors and _______ of all intracranial tumors
80%, 7-8%
What is the hallmark symptom of Schwanoma?
Slowly progressive unilateral sensorineural hearing loss
What are the other symptoms of Schwanoma’s?
include unilateral tinnitus, dizziness and dysequilibrium, headache, sensation of pressure or fullness in the ear, otalgia, trigeminal neuralgia, and numbness or weakness of the facial nerve
How can we test for Schwanoma’s and what do they show?
An audiogram is the first test performed (see ). It usually reveals an asymmetric sensorineural hearing loss and a greater impairment of speech discrimination than would be expected for the degree of the hearing loss. Acoustic reflex decay, the absence of waveforms, and increased latency of the 5th waveform in auditory brain stem response testing are further evidence of a neural lesion. Although not usually required in the routine evaluation of a patient with asymmetric sensorineural hearing loss, caloric testing demonstrates marked vestibular hypoactivity (canal paresis).
What are the 3 major divisions of the cochlear nucleus?
anterior ventral cochlear nucleus (AVCN)
Posterior ventral cochlear nucleus (PVCN)
Dorsal cochlear nucleus
What part of CANS is the only auditory brainstem nuclei to receive only ipsilateral input?
cochlear nucleus
Is the Cochlear Nucleus Tonotopically Organized?
yes
what are stria and the 3 divisions of cochlear nucleus?
dorsal stria, intermediate, ventral acoustic stria
DCN, PVCN and AVCN
the first place to get binaural info is here (ISPI AND CONTRA)
SOC
What are the two major divisions of SOC?
lateral superior olive
medial superior olive
describe the LSO
Largest of the nuclei of the SOC in cats, smaller in humans
S-shaped in cat and more like a boxing glove in other species
Receives input from the ipsilateral AVCN and contralaterally from the AVCN and PVCN (some fibers via the medial nucleus of the trapezoid body)
timing is disrupted
describe the MSO
Smaller
Disc-shaped
Receives direct fibers from the CN of both sides
timing and integrity is maintained because of no extra synapse
what are the functions of the LSO
Predominantly receives and processes high frequency information
Localization of sounds based on interaural intensity differences
Why is sound intensity different at the two ears? (interaural attenuation)
Head shadow
Body baffle
Intensity decreases with distance
cannot rely on time fully because some of the neurons run through synapse and reaches ears differently based on presentation
sound decreases over distance and head shadow (sound doesn’t get around head as well and absorbed on body)
what are the functions of MSO
Receives and processes predominantly low-frequencies
Localization of sounds based on temporal cues
Stimuli reach the ear at different times and at different phases
what receives direct input from both CN with no other synapses that would interfere with analysis of timing information?
MSO
