416
UNIT 3
Organ Systems
Figure 21-5.
Organ of Corti and associated structures.
H&E,
3
189
The
organ of Corti
is a band of specialized epithelial cells that rest on the
basilar membrane
, a thin sheet of F
brous connective tissue
that extends from the
osseous
(
bony
)
spiral lamina
to the
spiral ligament
. The surface of the basilar membrane in the scala tympani
is covered by a thin layer of vascularized connective tissue and elongated mesothelial cells. The organ of Corti contains the
audi-
tory hair cells
, the receptor cells for hearing, as well as several types of supporting cells. The hair cells have bundles of 50 to 100
stereocilia (hairs) protruding from their upper surfaces. The
transduction of sound waves
to nerve impulses is based upon changes
in the polarization of the hair cell membrane that occur when their apical stereocilia are bent during the vibration of the basilar
membrane by sound waves (see ±ig. 21-6A,B). The hair cells are in synaptic contact with
afferent
and
efferent nerve ± bers
of the
auditory branch of CN VIII (
efferent F bers not illustrated
). Auditory hair cells are divided into two groups:
inner hair cells
and
outer
hair cells
. In humans, there are about 3,500 inner hair cells arranged in a single row and about 12,000 outer hair cells arranged in
three or sometimes four rows (see ±ig. 21-7A). The hair cells are surrounded by a variety of
supporting cells
, including
pillar cells
,
phalangeal cells
,
border cells
, and
cells of Hensen
. The inner and outer hair cells are separated by
inner
and
outer pillar cells
. These
cells have long, thin processes that include dense bundles of microtubules and extend from the basilar membrane to the upper sur-
faces of the hair cells. Pillar cells surround a triangular, fl
uid-F
lled space, the
inner tunnel of Corti
. The basal and lateral aspects of
inner hair cells are surrounded by
inner phalangeal cells
. By contrast,
outer phalangeal cells
cup only the lower third of each outer
hair cell, whereas the upper two thirds of each outer hair cell is surrounded by a fl uid-F
lled space. The spaces between the upper
surfaces of the outer hair cells are F lled by processes of phalangeal cells. These processes form the
reticular lamina. Tight junctions
connect the phalangeal cell processes and the apical surfaces of the hair cells to form a barrier that separates the endolymph of the
scala media from the cells of the organ of Corti. Columnar epithelial cells called
border cells
mark the medial extent of the organ of
Corti; columnar epithelial cells called
cells of Hensen
mark its lateral extent. The
tectorial membrane
hangs over the organ of Corti
and defl
ects the stereocilia of the hair cells when sound waves move the basilar membrane. The tectorial membrane is a gelatinous
structure, containing F
ne F laments, which is secreted by columnar epithelial cells (
interdentate cells
) on the surface of the
spiral
limbus
. It is commonly distorted during tissue processing; its normal position is illustrated.
Bone
Bone
Basilar membrane
Organ of Corti
Basilar membrane
Modiolus
Tectorial membrane
Inner phalangeal
cell
Afferent
nerve fibers
Outer phalangeal
cells
Inner hair
cell
Border
cell
Outer hair
cells
Reticular
lamina
Outer
tunnel
Cells of
Hensen
Stria
vascularis
Vestibular
(Reissner)
membrane
Tectorial
membrane
Interdentate
cells
Spiral
limbus
Spiral
ganglion
Bone
Auditory
nerve
fibers
Bony spiral
lamina
Inner pillar
cell
Inner tunnel
of Corti
Outer pillar
cell
Spiral
Spiral
ligament
ligament
Spiral
ligament
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