William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
A SITUATION AND EXTERNAL RELATIONS
l Organs - cochlea and vestibular apparatus - sensitive
respectively to air vibrations (sound), and movement of the head and
its position relative to the gravitational field (balance), are
combined in the inner ear within communicating spaces - the bony
labyrinth - of the temporal bone. . Ear PowerPoint
2 Actually the receptors are enclosed in membranous tubes forming a
membranous labyrinth that lies within, but does not fill the bony
3 The two separate systems contain different fluids. The membranous
labyrinth is filled with endolymph and is a closed system, although it
extends a ductus endolymphaticus through the bone to end blindly by
the brain as an intradural sac involved in metabolic functions. This
sac can be drained surgically to relieve damaging excess endolymphatic
pressure - endolymphatic hydrops.
4 The space between the tubules of the membranous labyrinth and the bone is
occupied by perilymph, which is in communication via the aqueductus
cochleae and aqueductus vestibularis with the meninges and
with the CSF of the brain's subarachnoid space.
5 The fluid in the bony labyrinth can interact with the middle ear (and
indirectly with the external environment) by means of two soft areas in its
6 The stimulus in the environment that causes movement of the oval window
and pressure changes in the fluids is movement of air/sound, allowed a
little way into the head via the external ear.
- (a) The oval window/fenestra ovalis occupied by the moveable stapes
bone acting as a plunger.
- (b) The round window/fenestra rotunda covered with fibrous tissue
and epithelium, and moving passively as a pressure release for pressures
generated within the fluid system by movement at the oval window.
7 The fluids of the labyrinth are also subject to the gravitational force
and that accompanying movement of the head.
B DIVISIONS OF THE EAR
l External ear
l Auricle: core of elastic cartilage; lobule of adipose tissue; skin-covered.
2 External auditory meatus: lined with skin and stratified squamous
epithelium; has ceruminous (modified apocrine sweat) and sebaceous
glands; supported by cartilage and, further in, by bone.
3 Tympanic membrane/eardrum: inner limit of the external ear, core
of atypical collagen with thin epidermis externally, and a mostly simple
squamous epithelium internally; the manubrium of the malleus bone
inserts into the collagen. Elastin is present in the flaccid region.
2 Middle ear
l Epithelium-lined, air-filled, bony spaces of the tympanic cavity.
2 Communicates with the nasopharynx via the Eustachian/auditory/
pharyngotympanic tube, allowing equalization of air pressures on either
side of the tympanic membrane. The mucosa of the tube and middle ear has
several kinds of cell, and defensive systems.
3 Auditory ossicles articulate with one another - malleus,
incus and stapes. The malleus is vibrated by air moving the
tympanic membrane. This movement is then transmitted via the incus to the
stapes with its foot held in the oval window by the annular ligament.
4 Elastic membrane of the round window transforms the fluid pressures
generated in the inner ear into other forms of energy, thus acting as a
5 Fine skeletal muscles, (a) stapedius and (b) tensor tympani,
inserting into the stapes and malleus are protective, and influence sound
discrimination. Fine nerves pass to them.
3 Inner ear
The outer and middle ear thus have an exteroceptor function,
transmitting air vibrations (20-20000 cycles per second is the perceptible
range) to the perilymph fluid in the bone of the inner ear. Although the
resulting pressure changes involve all perilymph, the receptors sensitive
to the changes are localized in only one part of the labyrinth, the
cochlea, and lie in the inner endolymph-filled system. Elsewhere in
this inner system lie the intero- or proprioceptors for balance
and movement, located in the vestibular apparatus.
C VESTIBULAR APPARATUS
l Bony vestibule houses the membranous utricle and saccule.
2 The vestibule extends into three semicircular tubes or canals distributed
in three planes perpendicular to one another and containing the membranous
semicircular ducts/canals, each swelling out at one end into an
3 Movement of endolymph within the connecting membranous chambers stimulates
receptors in maculae and cristae - modified, small,
neuroepithelial areas of the lining membrane.
- (a) Macula, flat with:
- Basal lamina penetrated by
- nerve fibres from bipolar neurons of Scarpa's vestibular ganglion
passing to synapse with
- sensory hair cells, of two types, with their companion
- sustentacular cells, making a pseudostratified neuroepithelium.
- The 25 µm-long hairs (long microvilli and one cilium per cell)
of the hair cells project into
- the gelatinous otolithic membrane, in which are imbedded
- small crystalline otoconia/otoliths (weights subject to gravity).
- (b) Crista - rounded, crest-like prominence similar to a
macula in its components, but the gelatinous mass is called the cupola, and is
without otoliths. The crista detects the start and end of movement in the
plane of its duct.
4 Utricle (with a macula) is oriented in the plane of the base of the
skull, and the saccule (macula) in the sagittal plane. Both are
responsive to gravity and linear acceleration, thus giving information on
how the head is positioned.
5 Ampulla (with a crista) oriented in each of the horizontal, sagittal
and transverse planes; responsive to movement of the head in the plane of
that canal, thus furnishing information on the rate of angular acceleration.
7 The insensitive remainder of the vestibular membranous labyrinth is lined
by a simple squamous epithelium on CT, which is supported by collagen and
fibroblasts passing to the periosteum of the bony labyrinth, except on the
side it fastens to the bony wall.
l Structures and elements
The tube - the cochlear duct - containing the cochlear endolymph is not
surrounded by perilymph, but has it on two of its triangular sides. Thus,
three chambers are contained within the bony cochlea which spirals
for 2 1/2 turns around an axis of spongy bone, the modiolus. The
spiralling unit comprises:
- Scala vestibuli with perilymph and mesothelium-lined.
- Reissner's membrane (membrana vestibularis), thin;
- Cochlear duct/scala media with K+-rich endolymph made in the
stria vascularis; epithelium-lined, and containing the
- Organ of Corti (the actual receptor) on the
- basilar membrane, stretched from the tympanic lip of the bony
spiral lamina to the spiral ligament.
- Scala tympani with perilymph and mesothelium-lined,
separated by bone from the scala vestibuli adjacent in the spiral.
- Scalae vestibuli and tympani connect by a helicotrema at the apex
of the cochlea, whilst the cochlear duct ends blindly as the caecum
The cochlear duct at its base communicates with the saccule
via the ductus reuniens.
Fig. 5 Turn of the cochlea
# # # # # bone
# ( ( #
# ( #
# ( #
# ( SCALA #
# (* ~ VESTIBULI # b
# (* ~r # o
# (* SCALA ~m # n
# (* MEDIA ~ # # e
# (___________!! ~_# Modiolus
# (Basilar membrane # r~m Reissner's membrane
# ( #
# ( SCALA # !! Organ of Corti
# ( TYMPANI #
# ( # *** Stria vascularis
# ( #
# # # SCALA
# ( VESTIBULI of the next turn
2 Organ of Corti
l Rests on the tympanic lip and basilar membrane.
2 Internal border cells and internal hair cells (receptive).
3 Internal pillar cells lean outwards towards inwardly leaning
4 external pillar cells, thereby enclosing an inner tunnel.
5 Phalangeal cells/Dieter's supporting cells support
6 external hair cells, (50-l00 hairs per cell); contractile to amplify the response of the mechano-sensory
system; in three rows; damaged by loud sounds, streptomycin, cisplatin, etc.
7 Hairs (stereocilia of graded lengths) of outer cells go through a
reticular plate to attach to the overlying
8 tectorial membrane - a gelatinous body attached at the vestibular
lip to the CT limbus spiralis.
9 Nerve fibres derived from bipolar neurons of the spiral
ganglion/ganglion of Corti in the bony spiral lamina, passing through the
bone, serve the inner and outer hair cells.
(Centrifugal fibres also run from the brain stem to the outer
hair cells, to enhance the response.)
l0 The centripetal fibres of Scarpa's vestibular and Corti's cochlear ganglia
join to form the auditory/VIIth cranial nerve.
3 Organ of Corti's transducer function
Inner hair cells convert into neuronal discharges fluid pressure changes,
transmitted through the basilar membrane to the cochlear endolymph, from the
perilymph of the scala tympani. These changes originated at the oval window
in response to vibration of the auditory ossicles caused by air moving the
Discrimination of pitch (sound frequency) is based on different
cochlear regions responding preferentially to particular tones, with high
frequency received at the basal cochlea and lower ones apically where the
basilar membrane is broader.
4 Fluids and gelatinous bodies
Although these are lost or grossly distorted in the histological processing,
they are very important. The fluids transmit forces, and provide a metabolic
pathway and favourable ionic environment for the receptor and other cells
of the membranous labyrinth. In life, the gelatinous cupola and tectorial
and otolithic membranes are large, filling or almost filling their respective