4-3 Cranial Nerve VIII, Auditory Systems and Tracks Flashcards
What are the 3 parameters of sound that we can detect?
Frequency
Amplitude
Localization
Sound is transmitted in waves. What is the term for frequency of waves?
Pitch, can be measured in Hz (cycles/second)
How is intensity of sound measured?
By the bel scale, with log10 units
Most sounds are in decibels, 0 decibels as the threshold of hearing
Note: Intensity is not pitch, they are very different
From the pinna to the tympanic membrane, how does the external anatomy of the ear help us perceive sounds?
Pinna acts like a (symmetrical) funnel for sound
Sound is funneled down external auditory meatus to tympanic membrane
- big force is funneled into small area, like a fat guy on a scooter
How is sound transmitted from the tympanic membrane to the oval window?
Sound waves vibrate tympanic membrane
malleus attached to other side of TM is vibrated
vibration transferred to incus
vibration transferred to stapes
stapes is connected to oval window, and vibrates oval window
This compound lever/transformer setup will increase energy of sound through mechanical transduction 15x.
What 2 components make up the housing for the sensory apparatus for the inner ear?
Vestibule
Cochlea
What is the bony labyrinth in the inner ear called? What is it filled with?
Modiolus - filled with perilymph
What is the membranous portion of the inner ear labyrinth called? What is this filled with?
membranous labyrinth, which is inside modiolus
filled with endolymph, and sensory organs technically
What is Meniere’s disease? What are the 3 main symptoms?
Pathology caused by defective circulation or absorption of endolymph
Symptoms include:
Vertigo
Hearing loss
Tinnitis
What is endolymph high in? Perilymph? Which one resembles CSF?
Endolymph: High K+, low Na+
Perilymph: High Na+, low K+, like CSF
What makes up the turns of the cochlea? How many turns are there? What is at the top?
Spongy bone modiolus makes up scaffolding for cochlear ducts
- osseous spiral lamina will project through the cochlear duct form modiolus
2 3/4 turns, helicotrema up top
What is the purpose of the osseous spiral lamina?
Separates scala vestibuli from scala tympani
- except for helicotrema, which isn’t partitioned
What are the 7 major components of a cochlear duct? What fluid is in each compartment? What houses sensory organs/fibers?
- Scala vestibuli - perilymph
- Scala tympani - perilymph
- Scala media - endolymph, organ of corti
- Spiral ligament
- Basilar membrane
- Osseous spiral lamina
- Spiral ganglion
What are the 3 borders of the scala media?
Reissner’s membrane - separates scala media from scala vestibuli
- also physically separates endolymph from perilymph
Basilar membrane - continuation of spiral ligament
Spiral ligament - continuation of periosteum
What is on top of the basilar membrane? What does the basilar membrane separate?
Organ of corti
Separates scala tympani from scala vestibuli and scala media
What does the organ of corti, sitting on top of the basilar membrane and the spiral ligament, contain?
Contains 20-30k basilar fibers
attached at one end to modiolus
How do the fibers in the organ of corti respond to frequencies?
Fibers not fixed at distal ends, can vibrate
Lengths of fibers increase proximal to distal, and diameters decrease
- short stiff fibers at proximal end respond to high frequency
- long flexible fibers at distal end near helicotrema respond to low frequency
What does the organization of fibers within the organ of corti help create?
Tonotopic map
How many different types of hair cells does the organ of corti contain? What is the relative abundance of each? Do they do the same thing?
Inner hair cells
Outer hair cells - 3x as much
Inner hair cells transduce frequencies and intensities
Outer hair cells help modify input to inner hair cells
What does the tunnel of corti contain?
All the efferent axons for cochlear portion of VIII
Full of perilymph and outer hair cells
What is the primary sensory organ of the auditory system?
Organ of corti
Are the inner hair cells right next to outer hair cells?
No - inner hair cells are separated by tunnel of corti
What membrane is in direct contact with outer hair cells?
Tectorial membrane
What causes transduction in the organ of corti, from movement at the oval window to opening the tip link channels?
Oval window is moved
perilymph is moved through tunnel of corti
moving perilymph moves basement membrane
inner hair cells are moved against tectorial membrane, which causes shearing action
shearing works on stereocilia of inner hair cells
tip link channels opened
(‘Floor’ of system will move stereociliated inner hair cells into tectorial membrane)
What causes an inner hair cell to fire on to a spiral ganglion?
Shearing motion opens tip link channels
(bathed in endolymph)
K+ influx
Ca++ influx
Ribbon synapses - Glutamate released
What is the purpose of the round window? What is the clinical application?
Round window ‘catches’ the energy moving in the system due to inertia of perilymph and endolymph moving in a closed system. (Round window is elastic)
Movement of round window will often cause fibers to move in opposite direction, called optoacoustic emission. This can be measured in hearing tests for infants.
How is the basilar membrane tonotopic?
Detects both frequency and amplitude of soundwaves
Narrow base of basilar membrane is ‘tuned’ for high frequencies
Wider apex is ‘tuned’ for lower frequencies
Range of basilar fibers/inner hair cells can be stimulated with harmonics (many different frequencies coming together) or with high amplitude (enough energy to stimulate fibers of many different frequencies)
Brain then uses range of basilar fiber signals to determine intensity and loudness
Where else is there tonotopic organization?
Primary auditory cortex
Brain will still be able to detect different frequencies/pitch despite a high intensity signal due to this organization
What is the SSE component of CN VIII, functionally speaking? How does this function protect inner hair cells?
Outer hair cells
Not a sensory cell, but mechanical
- stimulus causes them to grow or shrink
Outer hair cells are attached to the tectorial membrane, and can grow or shrink to lift tectorial membrane away/towards inner hair cells and keep those cells protected
How is background noise filtered out?
Via outer hair cells raising or lowering tectorial membrane so that only ‘interesting’ stimulus makes it through, instead of background noise
What is the efferent input into outer hair cells?
medial olivocochlear system
efferents from medial superior olivary nucleus
What is the pathway from medial superior olivary nucleus to the outer hair cells (OHCs)?
Superior olivary nucleus in pons
lateral nucleus gets input from inner hair cells
medial nucleus innervates OHCs
Sensory input to a lateral nucleus on one side will cause medial nucleus on other side to affect OHCs, and sound will only be detected on one side
System allows for lateralization of sound
What is the pathway from inner hair cells to primary auditory cortex?
Inner hair cell sends afferents to cochlear nuclei in rostral medulla
Cochlear nuclei send fibers that ascend to pons
In pons, fibers can synapse in superior olivary nucleus -OR-
they can decussate through trapezoid body
Fibers ascend through lateral lemniscus to inferior colliculus
Synapse at inferior colliculus
Inferior colliculus sends fibers up through brachium of inferior colliculus
Fibers arrive at medial geniculate nucleus and synapse there
MGN sends fibers to auditory cortex
What happens to CN VIII at the ponto-medullary junction? (A more specific pathway question.)
Cochlear nerve/spiral ganglion afferent fibers arrive
Synapse in dorsal and ventral cochlear nuclei
Dorsal nucleus will send fibers to loop above inferior peduncle, called dorsal acoustic stria
Dorsal acoustic stria will b/l enter lateral lemniscus and ascend to inferior colliculus
Ventral nucleus will send fibers to loop below inferior peduncle, called ventral acoustic stria
Stria will head b/l to superior olivary nucleus
Ipsilateral sup. olivary nucleus will get signal first
Contralateral fibers will decussate in trapezoid body, innervate an inhibitory interneuron that synapses on contralateral sup. olivary nuc
What is the major function of the superior olivary nucleus? What are the differences between the medial and lateral divisions?
Major discriminator of system of spatial detection
Receives input from both ears
Medial:
Compare time lag
Send descending pathway of olivocochlear bundle to outer hair cells
Lateral:
Compare intensity
Other than laterality, what other way is sound localized?
Localized via intensity
Louder sounds go to superior olive, lateral and medial divisions
lateral divisions can potentially synapse on medial division, and cause innervation of descending pathway of olivocochlear bundle to outer hair cells (my notes were not clear on this, edits are welcome for add’l clarification)
Where does the lateral lemniscus terminate?
At the inferior colliculi
Ascending fibers from the dorsal cochlear nucleus come here and then hop on the brachium to head to the MGN to synapse
Where does auditory information go after synapsing in the MGN? What are some other names for this area?
Heads to primary auditory cortex
AKA - Heschl’s gyrus
Brodman’s area 41
located in transverse temporal gyrus
What are the belt and parabelt areas around Brodman’s area 41? What do they do?
Association cortices for auditory information
Help localize sound
You are sleeping on your right side and can hear your dog snoring loudly through your left ear. Which side of your brain/which auditory cortex processes this sound? Explain.
Both sides of your brain will process this noise
Neurons from dorsal cochlear nucleus will enter lateral lemnisci and will ascend bilaterally to inferior colliculus
Fibers will enter inferior colliculus b/l and will use ipsilateral brachium to head to ipsilateral MGN to synapse - both MGNs will get synapses
MGN from each side will send efferent fibers to primary auditory cortex and belt parabelt areas on both sides
On a physical/gross brain, where is Heschl’s gyrus?
Inferior portion of insula
What is the function of tensor tympani?
Tenses, pulls malleus medially, tenses tympanic membrane
Dampens noise produced by chewing
Innervated by trigeminal nerve
What is the action of stapedius?
Smallest skeletal mm in the human body
Pulls outward on the neck of the stapes to dampen vibration
What is the reflex pathway that protects the inner ear from high noise levels? What is the outcome of this pathway?
Ventral cochlear nucleus ⇒ bilateral superior olive ⇒ bilateral facial motor nuclei
In by VIII, out by VII
Stapedius and tensor tympani act together to make tympanic membrane more rigid and reduce sound transduction, protect inner ear from high noise levels
What is hyperacusis?
Defect in the reflex pathway that protects inner ear from high noise levels (and possibly dampens background noise)
Ipsilateral side will sound too loud
High sensitivity to sound and poor ability to filter background noise will result
What is the occurrence of deafness in adult population?
10%
What is central deafness?
Related to brain damage
Inability to process incoming auditory information
No deficits in conduction or transmission
What is conductive deafness?
Inability to conduct sound energy into mechanical energy
- otosclerosis - stapectomy
- OM
- outer ear origin
What is sensorineural deafness?
Inability to transduce mechanical energy into nerve stimulus
- cochlear damage
- high frequency hearing loss - presbycusis
- common with aging
