ANATOMY OF EAR

1. ANATOMY OF EAR
The three main regions of the ear
1. The external ear - collects sound waves and channels them inward.
2. The middle ear - conveys sound vibrations to the oval window.
3. The internal ear - houses receptors for hearing and equilibrium.
1. The external (outer) ear: it consists of auricle, external auditory canal and the
eardrum. The auricle (pinna) is a flap of elastic cartilage covered by skin. The rim is
the helix and inferior portion is the lobule. The external auditory canal is a curved
tube about 2.5 cm long that lies in the temporal bone and leads to the eardrum. The
tympanic membrane or eardrum is a thin, semitransparent membrane partition
between the external auditory canal and middle ear. It is covered by epidermis and
lined by simple cuboidal epithelium. Perforated eardrum – tearing of tympanic
membrane. The external auditory canal contains a few hairs and specialized sweat
glands called ceruminous glands that secrete earwax or cerumen which prevent the
external ear canal from water and insects.
2. Middle ear: It is a small air-filled cavity in the temporal bone lined by an epithelium.
Separated from the external ear by the tympanic membrane and from the internal ear
by thin bony partition containing 2 small membrane-covered openings (the oval
window and the round window). The 3 smallest bones in the body the auditory
ossicles connected by synovial joints attached to the tympanic membrane. They are
named bases on their shapes as malleus (hammer), incus (anvil) and stapes (stirrup).
The handle of malleus attached to the tympanic membrane and the head articulates

2. with the body of incus which in-turn articulates with the head of the stapes. The base
of the stapes fits into the oval window.
The ossicles are in position due the ligaments and the 2 tiny muscles. The
tensor tympani muscles limits movement and increases tension on the eardrum to
prevent damage due to loud noises. The stapedius muscle supplied by facial nerve; is
the smallest skeletal muscle in the human body dampens large vibrations of stapes
due to loud noises and protects the oval window but also decreases the sensitivity of
hearing. The anterior wall of the middle ear contains an opening that leads directly
into the auditory (pharyngotympanic) tube, commonly known as the eustachian tube.
It connects the middle ear with the nasopharynx (otitis media).
3. Internal (inner) rar: The internal ear is also called the labyrinth. It consists of 2
main divisions: an outer bony labyrinth that encloses an inner membranous labyrinth.
The bony labyrinth is a series of cavities in the temporal bone divided into three areas:
(1) The semicircular canals.
(2) The vestibule, both of which contain receptors for equilibrium, and
(3) The cochlea, which contains receptors for hearing.
The bony labyrinth is lined with periosteum and contains perilymph (fluid,
which is chemically similar to CSF) and membranous labyrinth contains endolymph.
The vestibule is the oval central portion of the bony labyrinth.
The membranous labyrinth in the vestibule consists of two sacs called the
utricle (little bag) and the saccule (little sac) connected by a small duct. Projecting
superiorly and posteriorly from the vestibule are the 3 bony semicircular canals, each
of which lays at approximately right angles to the other two. They are named as
anterior, posterior, and lateral semicircular canals based on their positions. At one
end of each canal is a swollen enlargement called the ampulla (saclike duct). The
portions of the membranous labyrinth that lie inside the bony semicircular canals are

3. called the semicircular ducts. These structures connect with the utricle of the
vestibule.
The vestibulocochlear (VIII) nerve consists of ampullary, utricular, and
saccular nerves. Cell bodies of the sensory neurons are located in the vestibular
ganglia.

4. Anterior to the vestibule is the cochlea (snail shaped), a bony spiral canal that
resembles a snail’s shell and makes almost three turns around a central bony core
called the modiolus.
Cochlea is divided into three channels as cochlear duct (scala media), scala
vestibuli, and scala tympani. The cochlear duct is a continuation of the membranous
labyrinth into the cochlea; it is filled with endolymph.
The channel above the cochlear duct is the scala vestibule, which ends at the
oval window and below is the scala tympani, which ends at the round window.
Both the scala vestibuli and scala tympani are part of the bony labyrinth of the
cochlea; these chambers are filled with perilymph.
The scala vestibuli and scala tympani are completely separated by the cochlear
duct, except for an opening at the apex of the cochlea, the helicotrema.
The perilymph in the vestibule is continuous with that of the scala vestibuli.
The vestibular membrane separates the cochlear duct from the scala vestibuli, and the
basilar membrane separates the cochlear duct from the scala tympani.
Resting on the basilar membrane is the spiral organ or organ of Corti. The
spiral organ is a coiled sheet of epithelial cells, including supporting cells and about
16,000 hair cells, which are the receptors for hearing.

5. There are two groups of hair cells: The inner hair cells are arranged in a single
row whereas the outer hair cells are arranged in three rows. At the apical tip of each
hair cell are 40–80 stereocilia that extend into the endolymph of the cochlear duct.
Stereocilia are actually long, hair-like microvilli arranged in several rows of
graded height. At their basal ends, inner and outer hair cells synapse both with first-
order sensory neurons and with motor neurons from the cochlear branch of the
vestibulocochlear (VIII) nerve.
Cell bodies of the sensory neurons are located in the spiral ganglion. The
tectorial membrane a gelatinous membrane covers the hair cells of the spiral organ.
The inner hair cells synapse with cochlear nerves that relay auditory information to
the brain. The 90% of motor neurons in the cochlear nerve synapse with outer hair
cells.