Auditory system Flashcards
Sound can be detected in terms of
1) frequency, 2) amplitude, and 3) localization
vibrations create
energy carried through a medium (water, air)
Sound is a series of pressure changes in the air
Frequency
Frequency of sound is how quickly these waves oscillate
Differences are perceived as pitch
20-20,000 Hz, most sensitive at 3,000 Hz
amplitude
Amplitude of pressure changes are expressed as decibels (actually measure intensity)
Logarithmic scale
Reference point set at 0 (where threshold exists
outer and middle ear…
convey AND amplify vibrations in air to the inner ear
Pinna
is a funnel (humans are symmetrical)
Amplifies
Leads soundwaves down external auditory meatus, to tympanic membrane
Major amplfication
Vibration of tympanic membrane is transmitted to
ossicles
Malleus (hammer) is directly attached to tympanic membrane
Malleus is attached to incus (anvil)
Which is attached to the stapes (stirrup), connected to the oval window
bony and membanous labyrinth
Bony or osseous labyrinth (Modiolus) in the temporal bone
Perilymph
Inside the bony labyrinth is the membranous labyrinth, containing sensory organs
Endolymph
Meniere’s disease
Vertigo
Hearing loss
Tinnitis
Caused by defective circulation or absorption of endolymph
Cochlear organization: general anatomy
Modiolus: spongy bone inside
Osseous spiral lamina projects through cochlear duct from modiolus
Separates scala vestibuli from scala tympani (both perilymph)
Helicotrema, no longer partitioned
Cochelar duct contains scala media, (bordered by Reissner’s membrane and Basilar membrane) endolymph
Cavity within modiolus contains Spiral Ganglia
Cochlear organization: the scala media
Three borders-makes triangle Reissner’s membrane: physically separates endolymph from perilymph Basilar membrane Spiral ligament
Basilar membrane Structure
Basilar membrane separates the scala tympani from the scala vestibuli/scala media
On top of basilar membrane is organ of corti
Contains 20-30,000 basilar fibers attached at one end to the modiolus.
Not fixed at distal ends
Therefore can vibrate
Lengths of these fibers increase progressively from proximal to distal, diameters decrease
Short stiff fibers at proximal end: respond to high frequency
Long, flexible fibers at distal end (toward helicotrema): respond to low frequency
Tonotopic map
inner vs outer hair cells
Inner hair cells: Sites of auditory transduction
Outer hair cells: Sites of fine tuning
Organ of corti
Organ of corti is primary sensory organ of auditory system
Only one row of inner hair cells
Separated by tunnel from many rows of outer hair cells
Tectorial membrane sits on top of hair cells in the scala media
Direct contact with outer hair cells
Through diffusion from the basilar membrane, perilymph occupies tunnel, but not dorsal surface of organ where there are hair cells
Remember that perilymph is high Na+, low K+, like CSF
Scala media is filled with endolymph (high K+)
What causes transduction?
Tectorial membrane shearing against hair cells causes transduction
Stereocilia are arranged in gradient
There are no kinocilium
Sensory transduction similar to vestibular system
Shearing opens tip link channels:
- K+ influx
- Ca2+ influx
- Ribbon synapses
- Glutamate released
Basilar membrane is tonotopic:
detects both frequency and amplitude of soundwaves
Soundwaves don’t propagate through the entire membrane because of its fluidity: pebble on a pond
Amplitude detected by greater area of displacement
Tonotopic organization is maintained in
primary auditory cortex
Outer hair cell function
Physically change their shape and alter the structure of the organ of corti
Multiple rows of outer hair cells, but aren’t sending major sensory input to cochlear nerve
SSA
Instead, receive efferent input from medial olivocochlear system
Also an SSE
When loud noise, outer hair cells become contractile
Amplify movements of basilar membrane
Unmasks from background level noise
Need to be able to determine signal from noise. Like horizontal and amacrine cells in the visual system
Outer Hair cells are controlled by efferents of
medial superior olivary nucleus (in the pons)
Central auditory mechanisms: Taking the high road or the low road
Signals from both ears are transmitted bilaterally
Comparatively complex CNS wiring of auditory system is to figure out sound localization
There are 3 places with bilateral communication along pathway:
- Trapezoid body
- Lateral lemniscal commissure
- Inferior colliculi commissure
High road = dorsal cochlear nucleus, “Dorsal acoustic stira”
Low road = ventral cochlear nucleus, “Ventral acoustic stria”
At the ponto-medullary junction
Dorsal cochlear nuclei
- Loop above the inferior peduncle (Dorsal acoustic stria)
- Bilaterally to inferior colliculus, via lateral lemniscus
Ventral cochlear nuclei
- Loop below the inferior peduncle (Ventral acoustic stria)
- Bilateral to superior olivary nucleus
Contralateral fibers decussate in the trapezoid body
Superior olivary nucleus
Inputs from both ears
Major discriminator of system of spatial detection
Medial division
- Compare time lag
- Send descending pathway of olivocochlear bundle to outer hair cells
Lateral division
- Compare intensity
Outer Hair cells are controlled by efferents of
medial superior olivary nucleus
Inferior colliculi
- Lateral lemniscus terminates here
- Efferent fibers called “brachium”, travel to MGN of thalamus ipsilaterally
Heschl’s gyrus is
Primary Auditory area
Broadmann 41
“Transverse temporal gyrus”
Tonotopic map
Association cortices surround BA 41, (eg, 42, 22) organization called Core—Belt–Parabelt
