auditory vestibular system - central auditory nervous system Flashcards
what does the middle ear serve as
impedance matcher
-remember the area difference between TM and stapes footplate, lever action and buckling of TM
what stimulates the organ of corti
traveling wave
is the ear canal the same length at the floor and ceiling
no, the floor is longer
how many layers of tissue is the TM
technically 4 layers
-continuous of skin from canal, radial fiber mesoderm, concentric circular fiber mesoderm, and continuous with mucous membrane of middle ear
how many row’s are there of each hair cell’s within the organ of corti
single row of inner, multiple of outer
how do the hair cells get stimulated
by the sterocilia getting stimulated
structures of the CANS
8CSLIMA
-from CN 8 to the cortex
what structures are part of the 8CSLIMA
cranial nerve 8, cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate nucleus, A1 (primary auditory cortex)
CN 8
goes into brainstem
-enters at cerebellopontine angle (CPA)
-starting point of the CANS
cochlear nucleus (CN)
first station for processing auditory information in the brainstem
-monaural or ipsilateral at this level
-gets information from CN8 and begins processing
3 division of the cochlear nucleus
anterior ventral cochlear nucleus (AVCN), posterior ventral cochlear nucleus (PVCN), and dorsal cochlear nucleus (DCN)
what division is superior?
dorsal cochlear nucleus
3 fibers going off the CN divisions
ventral acoustic stria (down), intermediate stria (middle), dorsal stria (top)
-DIVe down (for direction)
direction of the ventral acoustic stria
from AVCN to join with the contralateral SOC and LL nucleus
direction of the intermediate stria
from PVCN to contralateral SOC
direction of the dorsal stria
from DCN to contralateral SOC
what does the cochlear nucleus do in terms of the cochlea?
preserves frequency mapping (tonotopic organization)
how are frequencies arranged within the cochlear nucleus
low are ventrolateral, high are dorsomedial
superior olivary complex (SOC)
gets information from the stria’s
-first level of the CANS to receive binaural input
-localization
2 divisions of the superior olivary complex
lateral and medial
lateral superior olive
received and processes high frequency information, localization of sounds based on interaural intensity differences
-high frequency and intensity
how does the lateral superior olive receive input
from ipsilateral AVCN and contralaterally from the AVCN and PVCN
how does the lateral superior olive have impacted time integrity
due to an extra synapse at the nucleus of trapezoid body
medial superior olive
receives and processes predominantly low frequencies, localization of sounds based on temporal cues
-low frequency and timing
how does the medial superior olive receive input
directly from the CN of both sides
lateral lemniscus (LL)
lateral bundle of axons
-from CN to IC
-tonotopicity
-commisure of probst
commisure of probst
connection across midline between both of the nuclei of lateral lemniscus
2 nuclei of the lateral lemniscus
ventral nucleus (VNLL) and dorsal nucleus (DNLL)
reticular formation
center core of the brainstem
-LL adds redundancy when it goes through here
inferior colliculus (IC)
obligatory relay station of the ascending auditory pathway (it has to stop here)
-largest of the brainstem nuclei
what are the inferior colliculus visible as
rounded humps
how does the inferior colliculus receive input
from contralateral IC
two major nuclei in each IC
central nucleus (core) and pericentral nucleus (belt)
what information does the central nucleus (core) carry
purely auditory fibers
what information does the pericentral nucleus (belt) carry
somatosensory (sensations from the body) and auditory fibers
brachium of the inferior colliculus
large tract projecting ipsilateral to the medial geniculate body in the thalamus
-fibers going into the thalamus from the inferior colliculus
medial geniculate body (MGB)
group of nucleus in the thalamus
-last station in CANS prior to areas in the cortex
-gudden’s commissure
-leaves through auditory radiations
3 divisions of the MGB
ventral, dorsal, and medial
ventral MGB
responsive primarily to acoustic stimulus
-transmits directly to A1
dorsal MGB
response to somatosensory and acoustic information
-projects to association areas of A1
medial MGB
responsive to somatosensory and acoustic stimulation
-multisensory arousal system
internal capsule
within the thalamus and is where items pass through
-all fibers are coming up into the thalamus and are trying to pass though
-all fibers gets packed tightly here
-corona radiate are where axons leave and radiate around the crown
gudden’s commissure
connects the two thamalus geniculate bodies
A1
primary auditory cortex
why do auditory systems have built in redundancies
information comes up and crosses over in multiple places so if there was a lesions in one areas, there would still be the potential to continue getting to the cortex
-so many ipsilateral and contralateral pathways/commissures that it is bound to get to the cortex in one way or another
how do redundancies impact audiologic testing
it makes our testing harder because the redundancy of the ear causes pure tones and word recognition to get through easier, which means there has to be an increase of difficulty in our testing
vestibular schwannoma (acoustic neuromas)
a tumor formed from myelin of the superior and inferior vestibular nerves
-tumor can sit and be impactful of CN 7/8
-vascular supply can be impacted as well as nerve functions based on what nerves are impacted
what is used during an MRI to find a vestibular schwannoma
an enhancement agent
-makes the tumor light up so we can tell where it is
blood supply to the AVS
labyrinthine artery branches from AICA or the basilar artery
labyrinthine artery
divides into common cochlear artery and anterior vestibular artery
pathway of the common cochlear artery
comes from the labyrinthine artery to the common cochlear artery to both the posterior vestibular artery and the main cochlear artery
-posterior vestibular artery goes to PS
-main cochlear artery goes to the cochlea
pathway of the anterior vestibular artery
comes from the labyrinthine artery to the anterior vestibular artery and goes to LSU
LSU on top PS
lateral ampullary nerve, superior ampulalry nerve, utricle nerve, posterior ampullary nerve, and saccule nerve
-where artery goes to
temporal bone fractures facts
this bone is a tough pyramid shaped bone but can fracture with force
-can be bilateral
-3:1 ratio male to female
-longitudinal, transverse, and oblique
longitudinal fracture
parallels the long axis of the petrous portion of the bone
-from lateral to medial (long way)
-tears in skin of canal and TM
-CHL due to middle ear/ossicular disruption
transverse fracture
perpendicular down through the petrous portion
-vertical direction
-SNHL
-TM could show blood in it
oblique fracture
mixed, but most similar to longitudinal fracture
new classification of temporal bone fractures
otic capsule sparing, otic capsule disrupting
-otic capsule refers to the bony labyrinth of the inner ear
otic capsule sparing
more outer and middle ear involvement
-temporoparietal blow, FN paralysis, CHL or mixed, CSF leak not likley
otic capsule disrupting
damage to labyrinth, IAC, or other internal aspect
-occipital blow, FN paralysis, SNHL, CSF leak more likely
battle sign with temporal bone fracture
bruising behind the ear
-good sign of a fracture
-post auricle bruising
diagnosis of temporal bone fracture
-look at reflexes and facial nerve function
-inspection of ear and temporal bone
-bloody otorrhea (ear drainage) and hemotympanum (blood or bruise within TM) are common)
-CT or high resolution CT scan
-check for vertigo, dizziness, and presences of nystagmus
treatment of temporal bone fracture
surgery, decompression, corticosteroids, bed rest and not straining
-will treat HL and remaining balance issues later
extradural hematoma
blood or bruising that is outside the dura (between dura and bone)
-looks more like the shape of a lemon due to the dura being forced away from the bone
subdural hematoma
blood or bruising that is lower than the dura (subarachnoid where CSF is)
-looks more like a banana as it is making the space wider and following the curve of the bone
transduction in the hair cells
sterocilia move as a whole bundle, special tip links stretches and deflection occurs towards the tallest sterocilia, opens the channels, endolymphy flows inward and depolarizes the cell, causes voltage gated calcium channels to open, triggers release of NT on the CN 8 ending
hair cells with their sterocilia are ______________
specialized mechanoreceptors
inner hair cells
main sensory receptor
-receives around 90-95% of CN 8 fibers
-1 IHC synapses to many neurons CN 8 fibers (diverges)
outer hair cells
amplifiers
-receives only around 5-10% of innervation from afferents
-many OHC synapses to one neurons of CN 8 fibers (convergent)
unipolar neuron
one cell body and one part coming off of it
bipolar neuron
one cell body and two parts coming off of it
-piece coming off each side
-CN 8 is this type
multipolar
multiple dendrites, an axon, and various branches
pseudo-unipolar
mix between unipolar and bipolar in a way
-dendrite side and axon side
-splitting and going in two places
CN 8 firing
when stimulated by sufficient NT, AP is generated, sodium influx to depolarize based on opening of voltage gated ion channels, followed by potassium outflux to repolarize
AP key factors
voltage gated ion channels, all or non depolarization, self propagating, moves forward only, jumps from node to node
summary of encoding by CN 8 (how does it encode frequency and intensity?)
frequency : tonotopic organization and phase locking
intensity : rate of firing of AP, how many fibers are firing, which fibers are firing
description of rate of firing
AP’s are all or none, so therefore there are no small or large AP signals. this leads to intensity being encoded by a soft sound equalling a slower AP firing rate and a louder sound having a faster AP firing rate
description of which fibers are firing
each fiber fires for a certain range as not every fiber can fire for 0-120 dB
description of how many fibers are firing
the louder a sound is leads to the stimulation of more BM so more hair cells will get stimulated and a larger bunch of nerves will fire
where within CANS does tonotopic organization occur
superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate body, A1
what CANS pathway is dominant?
contralateral is more dominant
what are the auditory ganglia?
spiral ganglia
what are the vestibular ganglia?
scarpa’s ganglia
how does a sound get to the cortex
-acoustic energy meets pinna meets ear canal as resonator
-turns into a mechanical vibration in ME with impedance matching
-pushes on oval window which sends hydraulic wave through cochlea with pressure release in round window
-receptor potential : sterocilia will be sheared, short to tall will excite causing potassium to rush in to depolarize, calcium flows in and triggers NT to dump out
-action potential : opens voltage gated ion channels, sodium rushes in to depolarize spot on nerve, potassiums channels open and cell will reset itself
-CANS pathway will occur upon stimulation of CN 8
-8CSLIMA
