EARS: THE VESTIBULOAUDITORY
Tissues of the ear mediate the senses of equilibrium and hearing.
Each ear consists of three major parts:
1. The external ear, which receives sound waves;
2. The middle ear, in which sound waves are transmitted
from air to fluids of the internal ear via a set of small bones;
3. The internal ear, in which these fluid movements are
transduced to nerve impulses that pass via the acoustic
nerve to the CNS. In addition to the auditory organ, or
cochlea, the internal ear also contains the vestibular organ
that allows the body to maintain equilibrium.
Auricle or Pinna: irregular, funnelshaped
plate of elastic cartilage, covered by tightly adherent
skin, which directs sound waves into the ear.
Sound waves enter the external acoustic meatus
Near its opening
hair follicles, sebaceous glands, and modified apocrine sweat
glands called ceruminous glands are found in the submucosa. Cerumen, the waxy material found inside the ear that provides antimmicrobial properties.
Across the deep end of the external acoustic meatus lies a
thin, somewhat transparent sheet called the tympanic membrane
Sound waves cause vibrations of the
tympanic membrane, which transmit energy to the middle ear.
The middle ear contains the air-filled tympanic cavity, an
irregular space that lies within the temporal bone between the
tympanic membrane and the bony surface of the internal ear Anteriorly, this cavity...
communicates with the
pharynx via the auditory tube (also called the eustachian or
pharyngotympanic tube) and posteriorly with the smaller,
air-filled mastoid cavities of the temporal bone.
In the medial bony wall of the
middle ear are two small, membrane-covered regions devoid
of bone: the oval and round windows with the internal ear
The tympanic membrane is connected to the oval window
by a series of three small bones, the auditory ossicles, which
transmit the mechanical vibrations of the tympanic membrane
to the internal ear.
The three ossicles are:
for their shapes the malleus, incus, and stapes, the Latin
words for “hammer,” “anvil,” and “stirrup,” respectively.
Two small skeletal
muscles, the tensor tympani and stapedius, insert into the malleus
and stapes, respectively, restricting ossicle movements and protecting the oval window and inner ear from potential damage
caused by extremely loud sound.
The internal ear is located completely within the temporal
bone, where an intricate set of interconnected spaces,
the bony labyrinth,
houses the smaller membranous labyrinth, a set of
continuous fluid-filled, epithelium-lined tubes and chambers.
The embryonic otic vesicle, or otocyst, forms the membranous
labyrinth with its major divisions:
a. Two connected sacs called the utricle and the saccule,
b. Three semicircular ducts continuous with the utricle,
c. The cochlear duct, which provides for hearing and is
continuous with the saccule.
Mediating the functions of the inner ear, each of these structures
contains in its epithelial lining large areas with columnar
mechanoreceptor cells, called hair cells, in specialized sensory
a. Two maculae of the utricle and saccule,
b. Three cristae ampullares in the enlarged ampullary
regions of each semicircular duct,
c. The long spiral organ of Corti in the cochlear duct.
Structure and function of internal ear components.
1.The Vestibule is with the Utricle and Saccule.
It has the Macula which Detects linear movements and
static position of the head.
2. The Semicircular canals is with the Semicircular ducts and have the Cristae ampullares which: Detect rotational movements ofthe head.
3. The Cochlea is with the Cochlear duct and has the Spiral organ which:
The cochlea is about 35 mm long and makes 2¾ turns around
a bony core called the modiolus.
The modiolus contains blood vessels and surrounds the cell
bodies and processes of the acoustic branch of the 8th
cranial nerve in the large spiral or cochlear ganglion.
The bony and membranous labyrinths contain two different
fluids. The separation and ionic differences
between these fluids are important for inner ear function.
The two are:
1. Perilymph fills all regions of the bony labyrinth and has
an ionic composition similar to that of cerebrospinal fluid
and the extracellular fluid of other tissues, but it contains
little protein. suspends and supports the closed membranous labyrinth, protecting it from the hard wall of the bony labyrinth.
2. Endolymph fills the membranous labyrinth and is characterized
by a high-K+ (150 mM) and low-Na+ (16 mM)
content, similar to that of intracellular fluid.
The interconnected, membranous utricle and the saccule are
composed of a very thin connective tissue sheath lined with
simple squamous epithelium and are bound to the periosteum
of the bony labyrinth by strands of connective tissue containing
Each consists of
a thickening of the wall containing several thousand columnar
hair cells, each with surrounding supporting cells and synaptic
connections to nerve endings.
Within the Utricle and Saccule:
Hair cells act as mechanoelectrical transducers, converting
mechanical energy into the electrical energy of nerve action
has an apical hair bundle consisting of one rigid
cilium, the kinocilium, up to 40 μm long, and a bundle of 30-50
rigid, unbranched stereocilia.
Within the Utricle and Saccule:
The tips of the stereocilia and
kinocilium are embedded in a thick, gelatinous layer of proteoglycans called the otolithic membrane.
The outer region of
this layer contains barrel-shaped crystals of CaCO3 and protein
All hair cells have basal synapses with afferent (to the brain)
nerve endings but are of two types:
Type I hair cells have rounded basal ends completely surrounded
by an afferent terminal calyx.
The more numerous type II hair cells are cylindrical, with
bouton endings from afferent nerves.
Sensory information from the utricle and saccule allows the
brain to monitor the static position and linear acceleration
of the head. This information.
along with that provided visually
and by musculoskeletal proprioceptors, is important for maintaining
equilibrium and allowing the eyes to remain fixed on the
same point while moving the head.
Within the Utricle and Saccule:
When the hair bundle is deflected
toward the kinocilium,
protein fibrils called tip links connecting
the stereocilia are pulled and mechanically gated channels open
to allow an influx of K+ ions.
The three semicircular ducts extend from and return to the
wall of the utricle.
Each semicircular duct has one enlarged ampulla end containing
hair cells and supporting cells on a crest of the wall called
the crista ampullaris.
Cristae are histologically
similar to maculae, but the proteoglycan layer called the
cupola attached to the hair cells apically lacks otoliths and is
The hair cells of the cristae ampullares act as mechanoelectrical
Here the mechanoreceptors detect rotational
movements of the head as they are deflected by endolymph
movement in the semicircular ducts.
Neurons of the vestibular nuclei in the
CNS receive input from the sets of semicircular ducts on each
side simultaneously and interpret head rotation on the basis of
the relative transmitter discharge rates of the two sides.
This for perceiving movement
and orientation in space and for maintaining equilibrium or
The cochlear duct, a part of the membranous labyrinth shaped
as a spiral tube, contains the hair cells and other structures that
allow auditory function.
Held in place within the bony cochlea,
The cochlear duct itself forms the middle compartment, or
scala media, filled with endolymph. It is continuous with
the saccule and ends at the apex of the cochlea.
The larger scala vestibuli contains perilymph and is separated
from the scala media by the very thin vestibular
membrane called (Reissner’s membrane)
The scala tympani also contains perilymph and is separated
from the scala media by the fibroelastic basilar
The scalae tympani and vestibuli communicate with each
other at the apex of the cochlea via a small opening called the
The stria vascularis, located in the lateral wall of the
cochlear duct produces the endolymph with high levels of K+ that fills the entire membranous labyrinth.
The organ of Corti, or spiral organ, where sound vibrations
of different frequencies are detected, consist of 2 hair cell types:
Outer hair cells, about 12,000 in total, occur in
three rows near the saccule, increasing to five rows
near the apex of the cochlea. Each columnar outer hair cell
bears a V-shaped bundle of stereocilia.
Inner hair cells are shorter and form a single row of
about 3500 cells, each with a single more linear array of
The cell bodies of the afferent bipolar neurons constitute the spiral ganglion located in the bony core of the modiolus.
Two major types of columnar supporting cells are
attached to the basilar membrane in the organ of Corti:
Inner and outer phalangeal cells extend apical processes
that intimately surround and support the basolateral parts
of both inner and outer hair cells and the synaptic nerve
Pillar Cells: it is the inner tunnel, between the
outer and inner complexes of hair cells and phalangeal
cells. The stiff inner tunnel also plays a role in sound
On the outer hair cells the tips of the tallest stereocilia are
embedded in the gel-like tectorial membrane.
High-frequency sounds displace the basilar membrane
maximally near the oval window.
Sounds of progressively lower
frequency produce pressure waves that move farther along the
scala vestibuli and displace the spiral organ at points farther
from the oval window.
The sounds of the lowest
frequency that can be detected produce movement of the basilar
membrane at the apex or helicotrema of the cochlea.
Depolarization of the outer hair cells causes these columnar
cells to shorten very rapidly, an effect mediated by an unusual
80-kD transmembrane protein called prestin