The receptors
for equilibrium and hearing are located within fluid-filled
chambers and tubes that form the membranous labyrinth.
The fluid of the membranous labyrinth is called endolymph
and differs from extracellular fluid in having a high
potassium and low sodium concentration. |
The membranous
labyrinth is surrounded and protected by a shell of bone called
the bony labyrinth. The contours of the bony labyrinth
closely resemble the membranous labyrinth. The space between the
membranous and bony labyrinth is filled with a fluid called
perilymph that closely resembles CSF. |
The bony
labyrinth can be divided into three regions the vestibule,
semicircular canals and cochlea. The bony wall of the
bony labyrinth is solid except at two locations at the base of
the cochlea:
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1. Round window - is a
round opening in the bone that is spanned by a thin,
flexible membrane. |
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2. Oval window - is an
oval opening that is sealed by the foot plate of the
stapes which is held in position by an annular
ligament that attaches the outer edge of the
footplate to the edge of the oval window. |
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Vestibular Complex and
Equilibrium
The
vestibular complex is the part of the inner ear that
preserves physical equilibrium by detecting rotation,
gravity and acceleration.
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Semicircular Canals
The anterior, posterior and
lateral semicircular canals are
designed to detect rotation. Each
semicircular canal contains a
semicircular duct. Each semicircular
canal has an expanded portion which
contains within it an expanded portion
of the semicircular duct called the
ampulla which contains the hair
cells. |
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The hair cells are the receptor cells.
They are associated with supporting
cells in a raised portion of the
lining of the ampulla called a
crista. The apical surface of the
hair cell has long microvilli
called stereocilia (hence, "hair"
cell) and one cilium called a
kinocilium. The kinocilium and
stereocilia are embedded in a gelatinous
substance called the cupula which
nearly fills the space within the
ampulla. |
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Rotational movements in different planes
are detected by the mechanical
distortion of the stereocilia as
fluid moves within the semicircular
ducts. |
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Utricle and Saccule
The utricle and saccule are interconnected membranous
sacs of the membranous labyrinth found in the vestibule.
The endolymph of the utricle is confluent with that of
the semicircular ducts and the endolymph of the saccule
is confluent with that of the cochlear duct. The utricle
and saccule are interconnected by a narrow
endolymphatic duct that ends in a blind pouch called
the endolymphatic sac. |
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Both the
utricle and saccule contain hair cells similar to those
found in the semicircular canals. In each sac the hair
cells are concentrated on a oval spot in the wall called
a macula ("spot"). The kinocilia and stereocilia
of the hair cells are embedded in a gelatinous mass that
has crystals of calcium carbonate embedded on its
surface. This gelatinous mass with its crystals is
called an otolith and the crystals are called
statoconia. |
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The
difference in density between the crystals and the
gelatinous matrix causes a mechanical distortion of the
stereocilia of the hair when the head is tilted or the
body experiences acceleration. Hence, the hair cells
detect the position of the head in space and linear
acceleration. |
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Cochlea and Hearing
The
cochlea coils about 2.5 turns around a central hub
called the modiolus. The sensory neurons that
form the cochlear nerve have their cell bodies in the
modiolus in a ganglion called the spiral ganglion.
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The
cochlear duct is present within the cochlea flanked
by the vestibular duct and the tympanic duct
which contains perilymph. At the base of the cochlea
the oval window is in contact with the vestibular
duct and the round window is in contact with the
tympanic duct. The vestibular and tympanic ducts are
confluent at the tip of the cochlea at a place called
the helicotrema. |
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Organ of Corti
The hair cells that are responsible for hearing
are within the organ of Corti, or
spiral organ. The organ of Corti rests on a
basilar membrane which separates the
cochlear duct from the tympanic duct. The hair
cells are arranged in an inner row and outer
rows that follow the turns of the cochlear duct.
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The stereocilia (a kinocilium is lacking) of the
the hair cells are in contact with an
overhanging tectorial ("roof") membrane
that is attached to the inner wall of the
cochlear duct. |
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Reversed
Image
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Sound Detection
Sound is detected when a pressure wave of a
frequency between 20 - 20,000 Hz strikes the
tympanic membrane causing it to vibrate. The
vibration is transferred to the oval window by
the ear ossicles and creates a pressure wave
within the fluid of the cochlea because of the
presence of the round window. The pressure wave
moves the basilar membrane relative to the more
rigid tectorial membrane causing the stereocilia
to bend. |
The basilar membrane is relatively narrow and
stiff at the base and wide and loose at the tip
of the cochlea. As a consequence, the maximum
vibration any location along the length of the
basilar membrane depends upon the frequency of
the sound waves. By comparing the relative
movement of stereocilia along the length of the
basilar membrane the brain perceives frequency,
or pitch. The loudness of a sound is perceived
when the greater force of a louder sound causes
an increase in the range of movement of the
stereocilia of the hair cells. |
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