NROC64H3 Chapter Notes - Chapter 11: Stria Vascularis Of Cochlear Duct, Cochlear Duct, Thalamus

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17 Mar 2012
Chapter 11 The Auditory ad
Vestibular System 375- 418
- 2 systems that have different functions but have
similar structures and mechanisms:
1. Audition - hearing
2.Vestibular System balance, internalized
process that informs where and how we move
- Hearing is important while balance is something
we rarely think of
The Nature of Sound
- Sounds are audible variations in air pressure,
almost anything can move air molecules when
objects move toward patch of air, it
compresses air = increase density of
molecules - produce variations
- Frequency of sound is number of
compressed patches of air that pass by
ears hertz/Hz (number of
- Whether sound is perceived high or low,
or pitch, is determined by frequency
- Intensity: Difference in pressure b/w
compressed and rarefied patches of air,
sound intensity determines the loudness
we perceive loud sounds have higher
Box 11.1: Of Special Interest: Ultrasound
and Infrasound p.378
The Structure of the Auditory System
- Visible part of ear has cartilage + skin,
funnel called pinna (Latin: “wing”)
collects sound from wide area, more
sensitive to sound form ahead
- Auditory canal entrance to internal
- Tympanic Membrane/eardrum, series
of bone connected to medial surface is
ossicles (Latin: Little bones)
- Ossicles transfer movement 2nd
membrane called oval window (has cochlea contains apparatus for
transforming physical motion of oval window membrane)
- Pinna to tympanic membrane = outer ear
- Tympanic membrane and ossicles middle ear
- Apparatus medial to oval window = inner ear
- Neural response to sound from inner ear transfer and process in brain
stem (thalamus medial geniculate nucleus/MGN primary auditory
cortex/A1 @ temporal lobe)
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The Middle Ear
- Variations in air pressure are
converted into movement of ossicles
Components of the Middle Ear
- Composed of tympanic
membrane, ossicles and 2 tiny
muscles that attach to ossicles
- Conical w/ cone extending into
cavity of middle ear
- 3 ossicles:
1. Malleus (Hammer)
2. Incus (Anvil) - Form rigid
connection w/ Malleus
3. Stapes (Stirrup) Form flexible
connection w/ incus, flat bottom
portion of stapes, the footplate,
moves in and out like piston at oval
window to transmit sound vibrations
to fluid of cochlea
- Air in middle ear is continuous w/
air in nasal cavities via Eustachian tube (usually closed by valve)
Sound Force Amplification
-Sound waves move tympanic membrane and ossicles move
another membrane at oval window
- Pressure on membrane force pushing it/ surface area, will
be greater at oval window than tympanic membrane if:
1. Force of oval window > force of tympanic membrane
2. Surface area of oval window < surface area of tympanic
- Force is greater at ossicles act like levers (small by stronger
vibrations), s.a is smaller at oval window
The Attenuation Reflex
- Tensor tympani muscle is anchored to bone in cavity of
middle ear at 1 end and attaches to the malleus at other end
- Staedius muscle extends form fixed anchor of bone and
attaches to stapes
- When muscles contract, chain of ossicles becomes rigid and
sound conduction is greatly diminished
- Attenuation reflex: Onset of loud sound triggers neural
response to cause muscles to contract
- May help with allow ear to adapt to continuous sound at
high intensities, protect inner ear form loud noises to prevent
The Inner Ear
- Inner ear comprised of cochlea, and labyrinth (part of vestibular system)
Anatomy of the Cochlea
- Cochlea (Latin-snail) has spiral shape, hollow tube is made of bone
- Central pillar of cochlea is modiolus
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- Base of cochlea are 2
membrane-covered holes:
oval window and round
- Tube of cochlea has 3 fluid-
filled chambers:
1. Scala vestibuli
2. Scala media
3. Scala tympani
The scalae wrap around
inside cochlea like spiral
staircase (scala-Latin for
Ressiner`s membrane
separates scale vestibuli
from scala media
Basilar membrane separate scala vestibule
form scala media
- Organ of Corti: sits on basilar membrane,
contains auditor receptor neurons
- Tectorial membrane: hangs over organ of
- Helicotrema
- Perilymph: Fluid in scala vestibule and
tympani, ionic contract similar to CSF (low K
and HIGH K)
- Endolymph fluid for scala media, ionic
concentration similar to ICF (HIGH K and low
- Difference in ion content is generated by active transport at stria vascularis, endothelium lining 1 wall of scala
media reabsorbs Na and secrete K against concentration gradient endocochlear potential) endolymph has
electrical potential |80mV . perilymph = ENHANCES AUDITORY TRANSDUCTION
Physiology of Cochlea
- Some structures in cochlea aren`t rigid
basilar membrane is flexible and bends in
response to sound
The Response of the Basilar Membrane to
Basilar membrane has 2 structural properties
that determine the way it responds to sound
1. Membrane is wider at apex than at base by
factor of 4
2. Stiffness of membrane decreases from base to apex, the base being 100x stiffer
Much info comes form George con Bekesy
The Organ Corti and Associated Structures
Auditory receptor cells convert mechanical energy into change in membrane polarization @ organ of Corti
- Auditory receptors are hair cells b/c each one has 100 stereocilia extending from its tops transduction os
found into neural signal is bending of these cilia
- Reticular lamina: thin sheet of tissue where hair cells are sandwiched, rods of corti span these 2
- Inner hair cells: between modiolus and rod s of Corti
- Outer hair cells: Cell rather out of rods of Corti
Basilar base of organ of Corti tectorial forms roof over structure, RETICULAR = middle
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