Auditory system

Question 1

Sound can be detected in terms of

Answer 1

1) frequency, 2) amplitude, and 3) localization

Question 2

vibrations create

Answer 2

energy carried through a medium (water, air)

Sound is a series of pressure changes in the air

Question 3

Frequency

Answer 3

Frequency of sound is how quickly these waves oscillate
Differences are perceived as pitch
20-20,000 Hz, most sensitive at 3,000 Hz

Question 4

amplitude

Answer 4

Amplitude of pressure changes are expressed as decibels (actually measure intensity)
Logarithmic scale
Reference point set at 0 (where threshold exists

Question 5

outer and middle ear…

Answer 5

convey AND amplify vibrations in air to the inner ear

Question 6

Pinna

Answer 6

is a funnel (humans are symmetrical)
Amplifies
Leads soundwaves down external auditory meatus, to tympanic membrane
Major amplfication

Question 7

Vibration of tympanic membrane is transmitted to

Answer 7

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

Question 8

bony and membanous labyrinth

Answer 8

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

Question 9

Cochlear organization: general anatomy

Answer 9

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

Question 10

Cochlear organization: the scala media

Answer 10

Three borders-makes triangle
Reissner’s membrane: 
physically separates endolymph from perilymph
Basilar membrane
Spiral ligament

Question 11

Basilar membrane Structure

Answer 11

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

Question 12

inner vs outer hair cells

Answer 12

Inner hair cells: Sites of auditory transduction

Outer hair cells: Sites of fine tuning

Question 13

Organ of corti

Answer 13

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+)

Question 14

What causes transduction?

Answer 14

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

Question 15

Basilar membrane is tonotopic:

Answer 15

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

Question 16

Tonotopic organization is maintained in

Answer 16

primary auditory cortex

Question 17

Outer hair cell function

Answer 17

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

Question 18

Outer Hair cells are controlled by efferents of

Answer 18

medial superior olivary nucleus (in the pons)

Question 19

Central auditory mechanisms: Taking the high road or the low road

Answer 19

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”

Question 20

At the ponto-medullary junction

Answer 20

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

Question 21

Superior olivary nucleus

Answer 21

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

Question 22

Outer Hair cells are controlled by efferents of

Answer 22

medial superior olivary nucleus

Question 23

Inferior colliculi

Answer 23

• Lateral lemniscus terminates here

- Efferent fibers called “brachium”, travel to MGN of thalamus ipsilaterally

Question 24

Heschl’s gyrus is

Answer 24

Primary Auditory area

Broadmann 41
“Transverse temporal gyrus”
Tonotopic map
Association cortices surround BA 41, (eg, 42, 22) organization called Core—Belt–Parabelt