Download Auditory System

Auditory System
Physical nature of sound
1. Vibration of objects causes compression/rarefaction waves in air - sound waves
2. Humans with normal hearing can hear in the frequency range 20 Hz to 20,000 Hz
Mechanical structures of the ear
1. Pinna - visible part of ear - involved in localizing sounds in vertical plane
2. Auditory canal - tube connecting center of pinna with tympanic membrane –
channels sound into middle ear
3. Tympanic membrane (ear drum) - cone-shaped membrane that functions in
the reverse of a loudspeaker cone - converts sound to mechanical
vibration
4. Ossicles (malleus, incus, stapes) - three serial bones that conduct sound
vibrations from the tympanic membrane to the oval window of the
cochlea (footplate of stapes)
a. Ossicles amplify sound intensity by about 20x to go from air to
liquid in the cochlea
b. Attenuation reflex - two muscles connecting ossicles with walls
of the middle ear serve to change sound conduction from
the tympanic membrane to the cochlea
i. Stapedius muscle
ii. Tensor tympani
5. Eustachian tube - connection between middle ear and throat – equalizes
pressure on either side of tympanic membrane
Cochlea
1. Double-walled, fluid-filled tube curled up into a helix with 2 1/2 turns
2. Outer chamber - scala tympani/scala vestibuli - filled with perilymph
a. Footplate of stapes sits in the oval window which opens into scala vestibuli at
the base of the cochlea
b. At the apex, scala vestibuli communicates with scala tympani via a hole, the
helicotrema
c. At the base, scala vestibuli ends at the round window which is closed by the
round window membrane
d. Inward movement of the stapes footplate causes bulging of the round window
membrane because the fluid in the cochlea is incompressible
3. Inner chamber - scala media - filled with endolymph
a. Interfaces with scala vestibuli via Reissner's membrane
b. Interfaces with scala tympani via the organ of Corti
c. Third side is the stria vascularis which secretes endolymph
4. Perilymph is like cerebrospinal fluid, high Na, low K
5. Endolymph is very unusual, high K, low Na
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6. There is a voltage in scala media, the endocochlear potential of about +80mV relative
to perilymph
7. Frequency processing - base of cochlea processes high frequencies; apex, low
frequencies
Organ of Corti
1. Helical band that spans the space between the outer wall of the bony cochlea and the
inner bony covering of the modiolus (central axis of the helix)
2. Changes width along its length by 5 times - narrow at base and wide at apex
3. Layers
a. Basilar membrane - next to scala tympani, 100 times stiffer in base than in
apex
b. Rods of Corti, Hair Cells, Supporting Cells, Nerve Fibers - sandwiched
between the basilar membrane and the reticular lamina, 1 row of
Inner hair cells and 3-5 rows of outer hair cells
c. Reticular lamina - top rigid surface of cells that supports the stereocilia of the
hair cells
d. Tectorial membrane - gelatinous mass with internal fibers that sits on top of
stereocilia
Transduction
1. Sound input causes a traveling wave in the basilar membrane/organ of Corti
a. Upward movement of organ of Corti deflects stereocilia away from the
modiolus
b. Downward movement of organ of Corti deflects them toward the modiolus
c. This deflection is reflected in the receptor potential of the inner hair cells
2. Transduction in inner hair cells (IHC)
a. Cell body below reticular lamina sits in normal extracellular fluid, high Na, low
K
b. Top surface bearing stereocilia sits in endolymph, high K, low Na
c. In silence, mechanically gated potassium channels at tips of stereocilia are
partly open
d. Resting potential is about -70mV, but EK, the potassium equilibrium potential is
0mV because of the high concentration of potassium both inside the cell
and in the endolymph
e. Deflection of the stereocilia either fully opens or fully closes the potassium
channels
i. The mechanism is the mechanical springs (filaments) connecting the
stereocilia
ii. When open, depolarization results because inward rushing potassium
tends to move the membrane potential toward 0mV = EK
iii. When closed, hyperpolarization results
f. Depolarization opens voltage-gated calcium channels
g. Calcium mediates release of synaptic vesicles containing glutamate onto
auditory nerve neurites
h. Each IHC is innervated by about 10 auditory nerve fibers
3. Cochlear amplifier - outer hair cells (OHC)
a. Main purpose of OHCs is not to stimulate auditory nerve fibers, but to change
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the mechanical properties of the organ of Corti to affect
transduction in IHCs
b. Stimulation of OHC causes inward movement of potassium which contracts
motor proteins in the cell wall and shortens the cell - pulls reticular lamina
closer to basilar membrane and causes the stereocilia of the IHCs to
``````````bend more - cochlear amplifier
c. Blocking the action of the OHC motor proteins by drugs or sound damage
reduces the sensitivity of the cochlea
d. The actions of the OHCs can be modified by efferent nerve fibers from the
brain - the brain can modulate the sensitivity of the cochlea
e. Ototoxic effects of antibiotics occur because they damage the OHCs and
reduce the sensitivity of the cochlea. IHCs are not affected directly by
antibiotics.
Processing of auditory signals in the brain
1. Anatomy of central auditory pathways
a. Like vision, the anatomy is very complex - there are two major pathways, the
dorsal pathway and the ventral pathway - we are going to consider only
the ventral pathway
b. Ventral auditory pathway
i. Inner hair cells - synapse on neurites of auditory nerve fibers
ii. Spiral ganglion - spiral band of auditory nerve cell bodies in wall of
modiolus
iii. Auditory nerve - fibers enter modiolus and exit toward the brainstem
iv. Ventral cochlear nucleus - brainstem nucleus - ipsilateral
innervation,monaural response properties
v. Superior olive - each side is innervated from both ventral cochlear
nuclei - binaural response properties
vi. Lateral lemniscus - bundle of fibers from superior olive to inferior
colliculus
vii. Inferior colliculus - roof of mesencephalon behind superior colliculus all auditory pathways stop here before going to thalamus
viii. Medial geniculate nucleus - next to LGN, auditory thalamic relay
nucleus
ix. Acoustic radiation - fibers from MGN to A1 - auditory cortex
x. Auditory cortex - A1, Brodmann area 41, superior surface of temporal
lobe
xi. Secondary auditory cortices - e.g. Wernicke's area, etc. - more later
c. Tonotopic maps - throughout ascending pathways the map of sound frequency
from the basilar membrane in cochlea is preserved - like the retinotopic
map of visual system
2. Coding of sound intensity
a. Rate code - as sound intensity increases, the receptor potential in IHCs gets
larger and the auditory nerve fibers fire faster
b. Population code - as sound intensity increases, the deflections of the basilar
membrane stimulating IHCs broaden to that more and more IHCs are
activated
c. Together the number of neurons firing and the rate at which they are firing
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indicate to the brain the value of sound intensity - sensation of loudness
3. Coding of sound frequency
a. Place code - according to the tonotopic map, different frequency sounds cause
deflections of the basilar membrane at different places in the cochlea which IHCs are activated codes for what the sound frequency is
b. Phase locking - for sound frequencies below about 4,000 Hz, the timing of
action potentials in the auditory nerve is locked to parts of the cycle of
condensation and rarefaction in the sound wave - timing of action
potentials codes for sound frequency
c. For very low frequencies, below 200 Hz, only phase locking codes frequency
because there aren't dedicated fibers
d. For medium frequencies, 200-4000 Hz, both place code and phase locking
code sound frequency
e. For high frequencies, 4000-20,000 Hz, only place code indicates sound
frequency because phase locking stops
f. Together these two codes produce the sensation of pitch
4. Localizing sound in the horizontal plane
a. Interaural time differences
For frequencies 20-2000 Hz, the phase locking in firing patterns from the
two ears are compared and the difference is timing between them
specifies where the sound source is
b. Interaural intensity differences
For frequencies above 2000 Hz, the head produces a significant shadow
in the sound waves and the differences in intensity between the two ears
are compared to localize the sound
5. Localizing sound in the vertical plane
a. This task works as well with one ear as with two ears
b. Covering up the pinna eliminates this capability
c. Comparison of direct and secondary reflected sound paths from the wrinkles
on the pinna enables us to do vertical localization
Auditory cortex
1. Layers similar to visual cortex - 6 of them
2. A1 has a tonotopic map with low frequencies represented anteriorly and high
frequencies represented posteriorly
3. Most A1 neurons are sharply tuned for frequency
4. All are binaural
a. Some (EE) are excited by both left and right ears
b. Some (EI) are excited by one ear and inhibited by the other ear
5. Cortical module for auditory cortex
a. Each vertical column has cells sensitive to the same frequency
b. Adjacent columns in anterior-posterior direction change frequencies in order tonotopy
c. Adjacent columns in lateral-medial direction change from EE to EI to EE - like
ocular dominance columns
d. Analogous to cortical modules in Area 17
6. Cortical damage
a. Unlike the visual system, damage to auditory cortex often has little effect on
basic hearing - more often ability to understand speech or some other
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complex ability is lost
b. Damage to cochlea, auditory nerve, or cochlear nuclei are more typically
causes of deafness
Auditory disorders
1. Conduction deafness - blockage in sound conduction: wax in ear, disarticulated
ossicles, stiffening of insertion of stapes footplate into oval window. Usually
correctable with surgery
2. Nerve deafness - damage to hair cells or auditory nerve fibers from tumors, ototoxic
drugs, loud sounds, etc. No treatment for nerve deafness, but partial loss can be
compensated for by various hearing aids. Prevention is important.
3. Tinnitus - ringing in the ears is a common phenomenon
a. Caused by hyperactivity of cochlear amplifier
b. Oto-acoustic emissions - sounds actually produced in ear by cochlear amplifier
can be measured in auditory canal with microphones
c. The sounds of tinnitus are actually occurring in the cochlea and one is simply
hearing them - problem is that they originate in cochlea and mask
incoming external sounds
d. May indicate sound damage, cochlear disease or vascular abnormalities
e. Tinnitus after rock concerts is very common (and not healthy)!
============================= FINIS =============================
Dr Mahmoud Ahmad Fora
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