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

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17 Mar 2012
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Chapter 11 The Auditory ad
Vestibular System 375- 418
Introduction
- 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
cycles/second)
- 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
intensity
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
ear
- 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
membrane
- 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
damage
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
window-
- 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
stairway)
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
Corti
- 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
Na
- 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
Sound
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
THINK OF A PISTON
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
membranes
- 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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