Ear Development | Main Anatomy Index | UrogenitalDevelopment
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Last updated 30 March 2006
This page was contributed by David Boshell
The inner ear
- The inner ear consists of 2 labyrinthine compartments:
- The bony (osseous) labyrinth,
in the petrous portion of the temporal bone
- The membranous labyrinth, within the bony labyrinth
Spaces of the inner ear
- There are 3 fluid filled spaces in the inner ear:
- The endolymphatic spaces, within the membranous
- The perilymphatic space, in which the membranous
labyrinth is suspended
- The cortilymphatic space, lying within the organ of
The bony labyrinth
- The bony labyrinth consists of:
- The vestibule: the central space of the bony labyrinth,
containing the utricle and saccule of the membranous labyrinth
- The semicircular canals extending from the vestibule
- The cochlea, extending from the vestibule anteriorly
- The semicircular canals lie at about right angles to
each other in superior, posterior and horizontal planes, and each has an expanded ampulla at their lateral end.
- The cochlea is a conically shaped helix that spirals
about 2.5 turns around a bony core called the modiolus,
which contains the spiral ganglion of the
vestibulocochlear nerve (CN VIII).
The membranous labyrinth
- The membranous labyrinth, suspended within the perilymph
of the bony labyrinth, consists of:
The membranous cochlear duct, within the bony cochlea,
continuous with the saccule.
- The membranous semicircular ducts, within the
- The utricle and saccule,
contained in the vestibule, and connected by the utriculosaccular
- The semicircular ducts, utricle and saccule and part of the vestibular
system, concerned with balance and posture, whilst the cochlear duct is part of the auditory system, concerned with hearing.
Sensory regions of the membranous labyrinth
- There are 6 special sensory regions in the inner ear:
- 3 cristae ampullaris, in the ampullae of
the semicircular ducts, which are sensitive to angular acceleration
(turning) of the head
- 2 maculae of the vestibule: one in the utricle (macula
utriculi) and the other in the saccule (macula sacculi), both of which sense gravity, position and linear movement
- The organ of Corti, within the cochlear duct, that transduces sound vibrations into nerve impulses.
Hair cells of the vestibulocochlear system
- These non-neuronal mechanoreceptors are the common receptor cells in this system, that
function to initiate nerve impulses.
- All hair cells are epithelial, possess numerous stereocilia (sensory hairs), are associated with both afferent and efferent nerve
endings, and transduce mechanical energy into electrical energy.
- In the vestibular system, there are 2 types of hair
- Type I hair cells, piriform in shape with a rounded base and thin neck,
surrounded by an afferent nerve chalice and a few efferent fibres.
- Type II hair cells, cylindrical in shape, with afferent and efferent bouton
nerve endings the synapse basally.
- Both types of vestibular hair cell has one cilium called a kinocilium
- In the auditory system, hair cells lose their cilium in
development, leaving behind a residual basal body
- The hair cells of the inner ear function by the bending
of their sensory hairs:
- Bending of stereocilia
---> stretch plasma membrane ---> changed transmembrane
potential ---> conveyed to afferent nerves
associated with cell.
- In the vestibular system, the location of the kinocilium relative
to the bending stereocilia is important:
- Bending away from kinocilium ---> hyperpolarisation
of receptor cell
- Bending towards kinocilium ---> depolarisation
---> action potential
- Each crista is lined with the epithelium containing sensory hair
cells and supporting epithelial cells.
- The stereocilia and kinocilium of each hair cell are embedded in a gelatinous cupula that projects into the lumen of the ampulla, and is
surrounded by endolymph.
- During turning movements, the endolymph
tends to lag behind because of its inertia, thus swaying the cupula and bending
the sensory hairs that lie within, and generating nerve impulses.
Macula Sacculi and Macula Utriculi
The stereocilia and kinocilium of each hair cell are embedded in the gelatinous otolithic membrane, upon which crystalline particles called otoconia (otoliths) lie.
Again, the stereocilia are bent by gravity in the
stationary individual or linear movement in the moving
individual as the otolithic membrane drags on the stereocilia due to inertia.
- Like the cristae ampullaris, the maculae are innervated sensory
thickenings of the epithelium.
- The maculae are oriented at right angles to each other,
so that when a person in standing, the macula utriculi is
in a horizontal plane, and the macula
sacculi is in a vertical plane.
Organ of Corti
- The cochlear duct divides the cochlea into 3 scalae
- The scala vestibuli, above
- The scala tympani below, and
- The scala media, which, itself, is the cochlear duct, filled with endolymph, with the organ of Corti on its lower wall.
- The scala vestibuli, starting at the oval
window, and the scala tympani, ending at the round window, are filled with perilymph, and communicate with
each other at the apex of the cochlea through the helicotrema.
- The scala media is a triangular space with its acute
angle attached to the osseous spiral lamina that extends
from the modiolus.
- The upper wall, separating the scala vestibuli, is the vestibular (Reissners) membrane.
- The lateral wall is the stria
vascularis, lined by thick, vascular, pseudostratified epithelium that produces
- The lower wall, separating the scala tympani, is the basilar membrane.
- The organ of Corti rests of the basilar membrane, and
is overlain by the tectorial membrane.
- It is composed of:
- Inner hair cells in an single row, close to
the spiral lamina;
- Outer hair cells in a row 3-5 cells wide, farther from
the spiral lamina;
- Phalangeal (supporting) cells
for both rows of hair cells, preventing them from touching the basilar membrane.
- Inner phalangeal cells surround their
hair cells completely.
- Outer phalangeal cells only surround the basal part of
their hair cells, but have apical processes that covers the apical surface of the hair
cells, together forming a reticular lamina that separates
the endolymph-filled endolymphatic space from the cortilymph-filled cortilymphatic space.
- Pillar cells are "flattened" cells that rest
of the tympanic lip of the spiral lamina (inner pillar cells) and on the basilar membrane
(outer pillar cells), thus forming the tunnel of Corti
between the hair cell rows.
- The tectorial membrane, attached medially to the
modiolus, projects over the organ of Corti, attached to the stereocilia
of the hair cells.
- The basic mechanism of transduction of sound vibrations
is as follows:
- Sound waves ---> tympanic membrane vibrates --->
stapes moves at oval window ---> vibrations in perilymph of scala vestibuli, transmitted to scala media and
scala tympani ---> vibration of basilar
and tectorial membranes ---> shearing
of hair cells ---> generation of membrane potentials
---> afferents of spiral ganglion.